Milk fat can help eliminate chocolate fat bloom, claims study

Replacing some of the cocoa butter with milk fat can control fat bloom in chocolate, according to new research from Canada.
Fat bloom frequently results in significant product losses for confectionary manufacturers as, although it does not affect the taste, the tell-tale sign of the bloom - a white frosting - is unacceptable to consumers.

Fat bloom is a consequence of changes in the fat structure of the substance and can be caused by inadequate cooling processes whereby tiny temperature fluctuations as small as plus or minus 2°C can cause cocoa butter crystals to melt and then recrystallise, forming large cone-like structures that scatter light giving a dull appearance.

Food science professor at Ryerson University, Derick Rousseau, was the lead researcher in a study published in Food Chemistry that found that the partial replacement of cocoa butter (five per cent) with milk fat (MF) in chocolate manufacturing reduces the incidence of large surface crystals and the number and diameter of cones.

The results also show that the addition of MF decreases the surface roughness and the rate of surface coarsening.

The amount of milk fat is crucial, claims Rousseau, with the study demonstrating that the addition of 7.5 per cent MF made for a chocolate too soft to handle with a lack of snap and poor melting properties.

“The addition of five per cent MF was a suitable compromise between maintaining the sensory properties of chocolate and robustness to temperature cycling,” he continued.

He said that the results show that milk fat reduces the initial solid fat content, and slows the rate of change in whiteness index.

Rousseau also found that the fat crystal growth is accelerated by repeated temperature-cycling compared to isothermal conditioning but that cone hardening occurs more rapidly when isothermally-stored.

Irrespective of fat composition and storage conditions, fat crystal growth, welling and ultimately fat bloom begin only at specific locations on the chocolate surface, he added.

He said that this suggests that chocolate’s microstructural heterogeneity is responsible for distinct surface fat crystallisation pathways.

Source: Food Chemistry
Published online ahead of print: DOI:10.1016/j.foodchem.2009.06.031
Title: Controlling fat bloom formation in chocolate – Impact of milk fat on microstructure and fat phase crystallisation
Authors: S Sonwai, D Rousseau

Control fat bloom in chocolate.

Fat bloom is that thin gray layer of fat crystals that can cover the surface of chocolate. But now scientists at Switzerland's Buhler-Bindler have developed a seed crystallization process--called the SeedMaster System--that keeps chocolate products free from fat bloom longer.

In the process, you create a cocoa butter crystal suspension system. The liquid cocoa butter is partially crystallized in a shear crystallizer. The microscopically small crystals undergo additional processing in a conversion tank in which they are converted to their most stable form.

Next, it is necessary to cool the chocolate mass that is to be precrystallized, but this must be done without causing crystallization to occur. To do this, researchers developed a system in which they use static or dynamic heat exchangers. These units make it possible to cool the chocolate at elevated water temperatures.

Then the cocoa butter crystals are added at a controlled rate to the chocolate mass that has been cooled. The crystals and chocolate are uniformly mixed in a dosing and mixing unit. A static mixer performs a gentle mixing. It is not required to preform the crystals throughout the product. All that is necessary is to precool the individual ingredients while additional crystallization occurs within a short time after the seed crystal suspension is added. You might do this as part of a continuous mixing and forming process.

As the chocolate mass is seeded directly with stable cocoa butter crystals, the processing temperature could be up to 4 C greater than that for conventional masses that are precrystallized in tempering equipment. Because of the greater temperature, high-viscosity masses are easier to process because the heat reduces the viscosity. It's also possible to reduce fat levels in low-viscosity masses.

Seed recrystallization better resists fluctuations in temperatures due to the presence of crystals that have a maximum melting temperature. This leads to a more uniform precrystallization state. Seed precrystallization is especially helpful in chocolate-like masses whose continuous fat phase contains such fats and oils as hazelnut oil or almond oil and hardened vegetable fats. The stable cocoa butter crystals have several crystallization nuclei that make for fast recrystallization of the remaining fat. The high number of crystal nuclei creates a dense fine crystalline structure that retards fat migration.

Further information. Buhler-Bindler, CH-9240 Uzwil, Switzerland; phone: +41 71 955 31 36; fax: +41 71 955 35 82; URL: www.buhlergroup.com.

The effects of storage and process conditions on fat bloom formation in chocolate

Effects of processing conditions and storage temperature on fat bloom formation in
chocolate were investigated in this study. Samples, chocolate coated cream filled with crispy rice, were produced due to different temper indexes, (TI 5, 6, 7), shell thickness’ (1.5, 2.5, 3.5 mm), moulding rates (9, 12, 15, 18 mould/min) and storage temperatures(18 and 28 °C). Samples were analysed along for 40 weeks due to acidity % as oleic acid, peroxide value as meq O2/kg and crystal structure peaks at differential scanning calorimetry (DSC) twice a month. Next; solid fat contents at nuclear magnetic resonance (NMR) and fatty acid compositions as % at gas chromatography/mass spectroscopy (GC/MS) in order to compare initial and final results to achieve verification with other periodic analysis.

Finally; shelf life of samples was determined.
Storage conditions were effective for delaying bloom formation on chocolate surface.
Storing chocolate samples at 28 °C instead of 18 °C caused a decrease in their shelf life from 8 months to 5 months due to bloom formation. Slowly cooled chocolates in
chocolate production were more resistant to fat migration compared to fast cool-ones.
The shelf life of products with a cooling rate of 9 mould/min was observed to be 4
months higher than the chocolates with 18 mould/min. Shell thickness in chocolates
coated products was observed to be effective in delaying bloom formation. It was easier to migrate cream oil from inner to outer surface for thin shell (1.5 mm) products, whereas thicker one (3.5 mm) reached later to chocolate surface.

CHANGES IN PHENOLIC PROFILES AND ANTIOXIDANT ACTIVITIES FROM COCOA TO CHOCOLATE DURING THE CHOCOLATE PROCESSING

THE NATURE OF FAT BLOOM ON LAURIC COATING

Geoff Talbot and Kevin W. Smith
Consultants to Loders Croklaan

Introduction

The pressure on confectionery manufacturers to move away from high trans-containing compound coatings to either lower trans or completely non-hydrogenated fat bases is increasing as more becomes known about the effects of trans fatty acids on health. But what are the alternatives? Essentially there are four alternatives:

• Move to ‘real’ chocolate – but this is more expensive and also requires sophisticated tempering and processing equipment
• Move to a ‘supercoating’ – a coating whose fat base is essentially a cocoa butterequivalent – less expensive than real chocolate but sophisticated tempering and processing equipment is still needed.
• Move to one of the new low-trans or no-trans compound coatings – the lowtrans compound coatings still have an hydrogenated fat base and still contain some trans; the no-trans compound coatings overcome this issue but are still in the stage of being evaluated by the industry.
• Move to a lauric compound coating – this is the subject of this paper and one which we will therefore explore in more detail.

Lauric compound coatings

Lauric compound coatings are generally based on palm kernel oil, usually in the form of either a palm kernel stearine (PKS) or a fully hydrogenated palm kernel stearine (HPKS).
Although the latter would need to be declared as hydrogenated, the fact that it is fully hydrogenated makes it effectively ‘trans-free’. The advantages of lauric compounds over the other options available are (a) their price and (b) their ease of use. The disadvantage, however, is their lack of tolerance to cocoa butter. This means that formulations should have cocoa butter levels of less than 5% of the total fat phase.
This in turn means that any cocoa element in the recipe must be a low-fat cocoa powder, not a higher-fat cocoa mass. If the cocoa butter level is higher than 5% there is an increasing risk of bloom forming on storage. It is the composition and nature of this bloom that can form on storage that we have been studying.

A quick re-cap

In a previous paper (1) we reported on what was the composition of the bloom on lauric compounds. In this paper we go a stage further and look in more detail at the nature of the bloom, in particular, its thermal properties and crystal structure. But, it’s useful to first summarise the results we found on the composition of the bloom.

Two compound coatings were used, both of which had the same basic recipe (Table 1)

These deliberately had a higher level of cocoa butter (CB) than would be recommended in order to promote bloom formation. Bars were moulded and stored at 15°C, 20°C or25°C for 12 months before evaluation. After this storage time the bloom was carefully removed at 20°C and analysed.
The results showed that:

• At 15°C the bloom in both the PKS and HPKS systems is considerably enriched in cocoa butter compared with the bulk composition.
• At 25°C the bloom is almost completely composed of lauric fat.
• At 20°C the bloom is cocoa butter rich but this is more predominant in the PKS coating than in the HPKS coating.
• There is a general trend of the bloom being more enriched in cocoa butter the lower the storage temperature.

Knowing what the bloom was in chemical compositional terms we turned our attention to the nature of the bloom in physical and structural terms. This is the subject of the rest of this paper.

Rate of bloom formation

Perhaps the first question to answer is – how fast does this bloom develop? This is very much temperature dependent. Chocolate manufacturers are used to the fact that chocolate itself blooms more rapidly the higher the storage temperature. In lauric compound coatings, however, the exact opposite is true. The PKS coatings bloomed after 4 weeks at 15°C but after 23 weeks at 20°C and 25°C. The HPKS coatings showed bloom after 10 weeks at 15°C and, again, after 23 weeks at 20°C and 25°C.


Physical characteristics of bloom

The physical characteristics were measured using a Perkin Elmer DSC-2 differential scanning calorimeter. Not only were the bloom samples themselves evaluated but we also looked at the underlying compound by sampling from the centre of the bars.
Samples were first cooled in aluminium DSC sample pans to -20°C so that each had a specific and constant starting temperature. The melting curves were measured during heating from -20°C to 60°C at a heating rate of 5°C/minute. The melting curves are shown in Figures 1a-1d. Figures 1a and 1b relate to the samples of the compounds themselves, whereas Figures 1c and 1d relate to the corresponding bloom samples. The first thing that is immediately obvious, simply from the shapes of the curves, is how much sharper are the melting peaks for the bloom compared to those for the compound. Looking firstly at the samples stored at 20°C and 25°C, the melting points of the bloom samples (as defined by the DSC peak onset temperature) are 2-3°C higher than the melting points of the underlying compound. Coupling this with the much sharper peaks found with the bloom samples means that the temperatures at which the DSC peak maxima occur are much the same in bloom and compound. At 15°C, however, the whole thing turns round in that the melting points of the bloom and the compound are much closer together but, again because of the sharpness of the bloom peak, the peak maximum of the bloom sample is lower than that of the corresponding compound.

A further observation that we can make relates to peaks at temperatures below those of the main peak. In the bloom samples there really are no such peaks, whereas they are quite apparent in the compound samples. This shows that the amount of liquid oil in the bloom is almost zero.
So, we can draw two fundamental conclusions from this:

• The bloom is very sharp-melting and much sharper-melting than the underlying compound
• The bloom is almost completely solid.
Structural characteristics of the bloom

The structure of the bloom was studied by means of X-ray diffraction (XRD). We expected to find some complexities in this because of the large differences in the stable crystal forms of palm kernel stearines and of cocoa butter. As is well known in the industry, cocoa butter can crystallise in a number of polymorphic forms, the most stable of which are the two β-forms, Form V and Form VI. Each of these crystal forms pack in a triple-chain configuration, that is to say the crystal ‘layers’ are three fatty acid chain lengths long. Palm kernel stearines, on the other hand, crystallise in a β’-form with a double-chain configuration, i.e. the crystal ‘layers’ are two fatty acid chain lengths long.

Clearly with both fats present there is scope for considerable interaction and the formation of mixtures of polymorphs, especially in the bloom. One of the problems with performing XRD on compounds, chocolates and even, to some extent, on bloom is the contamination of the fat with sugar. Sugar also shows XRD peaks in the same region as fat. Indeed there was some evidence of sugar being present in the bloom samples taken from storage of PKS compound at 15°C and 20°C, but not in the other bloom samples. To overcome this problem and to look at the basic fat phase we moulded bars containing only the lipid components of the formulations (i.e. PKS or HPKS plus CB and lecithin) and measured X-ray diffractograms after 24 hours at 20°C. Both were in a β’-2 configuration (i.e. β’ polymorphic form with a double-chain configuration). Assuming the same thing happened in the compounds themselves then this is our start point.
As already indicated, the bloom samples themselves showed a much more complex crystal structure (Table 2).

During storage, therefore, there has been a polymorphic transition from β’-2 to a mixed β’ + β configuration. This is consistent with observations made by Noorden (2), Rossell (3) and Timms (4). Whilst we may be able to relate this transition to the mechanism of bloom formation, we are unable to say whether it occurred before, during or after the formation of the bloom.
In our earlier paper relating to this subject (1) we defined in detail the concentrations of specific triglycerides in the bloom as have been summarised above. The major triglycerides in the compositions are LLL (trilaurin from PKS and HPKS) as well as POP, POSt and StOSt (from cocoa butter). Interestingly, although we found that the fat phases of these compounds crystallised in the β’-2 form, each of these triglycerides individually are β-stable. This, then, could be the key to presence of β crystals along with β’ crystals in the bloom samples. Whilst we cannot assume that polymorphic changes always result in bloom formation, nor can we assume that they are even necessary for bloom to occur, it does appear from this work, that the formation of bloom is linked to a polymorphic change from β’ to β of triglycerides that themselves are β stable.

Control of bloom

So now we know both what the bloom is composed of in triglyceride terms and we can define a mechanism by which it is being produced. But how does this help us in terms of being able to slow down or prevent altogether the formation of bloom in lauric compounds?
Because the main triglycerides in the system are all β stable the compounds themselveswhich crystallise in a β’-2 form are effectively metastable. Two routes therefore suggest themselves for minimising bloom formation:

• Preserve the crystal structure in the metastable form throughout storage
• Promote crystallisation in the stable form before storage.

The latter is, with our present technology at least, unlikely to be successful. Various attempts have been made, mainly by additions of specific triglycerides, to retard the onset of bloom (5, 6). In these the addition of LLL (trilaurin) accelerated bloom formation whereas others (LML – lauric-myristic-lauric; LPL – lauric-palmitic-lauric; SSS – tristearin) did retard the formation of bloom.
Perhaps a more successful route would be to try to preserve the metastable structure throughout storage. Here, both temperature and composition can play a role. From a temperature point of view bloom occurred much more quickly when the bars were stored at 15°C than when stored at 20°C or 25°C. Ensuring that the storage temperature is not too low is therefore of importance. There is, of course, a balance to be made here between not having the temperature so low that bloom will occur within a short time and not having the temperature so high that the compound will start to soften. This is where the aspect of composition comes in.
Using the higher melting HPKS as a basis for the compound rather than PKS has two effects. Firstly it increases the temperature at which the coating will begin to soften and so allows a higher storage temperature to be used. Secondly, we saw that bloom formation, certainly at lower temperatures, was slower with HPKS than with PKS. A second, and perhaps even more important, aspect of composition is that of the level of cocoa butter used in the formulation. In the compounds studied in this work we deliberately used a much higher level of cocoa butter in the compound formulation in order to accelerate and exaggerate the effects. There was about 10% cocoa butter in the fat phases of the compounds we studied compared with a recommended maximum of 5%. Despite using this high level we saw differences and effects that can be important in defining the optimum composition and storage for an enhanced bloom-free shelf life.

Summary

From the studies reported both here and in our previous paper (1) we can make the following recommendations to maximise the bloom-free shelf life of lauric compounds:
• Ensure that the level of cocoa butter used in the compound is no more than 5% of the total fat phase – and, ideally lower consistent with any cocoa solids materials used in the formulation
• Where possible, use HPKS in preference to PKS. This may not always be possible because, whilst HPKS is effectively free of trans fatty acids, consumers may make a mental link between ‘hydrogenated vegetable oil’ on labels and the presence of trans.
• Use as high a storage temperature as possible within the limitation, of course, of not unduly softening the coating on storage. Ideally sub-20°C storage temperatures should be avoided.

References

1. Talbot, G. and Smith K.W. The Composition of Fat Bloom on Lauric Compound Coatings. ……………………….
2. Noorden, A.C. Fat Bloom – Causes and Preventions when using Lauric Hard Butters. Susswaren Technik Wirtschaft 1982, 26, 318-322
3. Rossell, J.B. Fractionation of Lauric Oils. J. Am. Oil Chem. Soc. 1985, 62, 385- 390
4. Timms, R.E. Physical properties of oils and mixtures of oils. J. Am. Oil Chem.Soc. 1985, 62, 241-248
5. Kawada, T.; Suzuki, S.; Kamata, F.; Matsui, N. The Study of Lauric Hard Butter.II. Fat Bloom. J. Jpn. Oil Chem Soc.: Yukagaku 1971, 20, 332-335
6. Kawada, T.; Suzuki, S.; Kamata, F.; Matsui, N. Studies on the Lauric HardButter. III. Fat Bloom (2). J. Jpn. Oil Chem Soc.: Yukagaku 1971, 20, 807-810

PALM BASED CHOCOLATE PRODUCTS

CHOCOLATE gives people great pleasure when eaten. It also makes a suitable gift for many occasions. The fat content of chocolate varies from 28% to 35% depending on its intended use. Cocoa butter obtained from cocoa beans or specialty fats obtained from vegetable oils and fats are the usual sources of fats for chocolate manufacture.

Specialty fats include cocoa butter equivalent (CBE), cocoa butter substitute (CBS), general purpose coating fats and toffee fat. Palm oil and palm kernel oil are ideal raw materials for the production of specialty fats due to their excellent physico-chemical properties. They can be further modified to extend their range of utilisation. CBE are specialty fats which contain symmetrical unsaturated triacylglycerols similar to that of cocoa butter (Table 1).
CBE are often regarded as fully compatible to cocoa butter. Generally, CBE is formulated with palm mid fraction (PMF) blended with illipe and shea fats. CBS are classified as lauric- and nonlauric- based. Lauric CBS are derived from the two major lauric oils namely palm kernel oil and coconut oil. Palm kernel oil can be fractionated to give palm kernel stearin with similar physical properties to that of cocoa butter. The stearin, with or without hydrogenation, is an excellent.

CBS suitable for the manufacture of solid or hollow-moulded chocolate products. Non-lauric CBS are made from oils such as palm, soybean, cottonseed and peanut oils. They have to be hydrogenated in order to bring their consistency to appropriate levels. These products have excellent uses in compound coating for biscuits, enrobed products and chocolateflavoured baking chips. In products where price consideration is important, this type of CBS is a good alternative.

Advantages of Using Palmbased Fats in Chocolate Products

Do not require tempering
Chocolate based on CBS crystallises rapidly into stable there is no need for tempering. This saves processing time.β’ crystal form, thus form stable crystals and are less subject to bloom
Chocolate made with CBS has a good resistance to fat bloom, thus the productlooks shiny and attractive.
Have rheological properties suitable for modern and highly flexible production lines
The same production plant for pure cocoa butter chocolate can be used.
Stable in hot climates
The melting properties of the fat are higher than those of cocoa butter due to the stearic-oleic-stearic (StOSt) triglyceride content. It is more stable in hot climates than cocoa butter.
Consistent quality
CBS has a low iodine value (IV), indicating low levels of unsaturated fatty acids. Thus, palm-based CBS are stable against oxidative deterioration.
Reliable supply
Palm oil and palm kernel oil can be
obtained at all times of the year. The oils especially from Malaysia, which are of high quality, are readily available to be processed for CBE and CBS production.
The fats are bland in flavour. They can be used for chocolate-flavoured end products, white compounds and pastels. The fats are also compatible to a certain degree with milk fat and other non-lauric oils.

Manufacturing Process

Economical

The price of these fats is highly competitive than that of cocoa butter.
Costs can be reduced by replacing part of the cocoa butter with CBE or by replacing the total amount of cocoa butter with CBS.
Plain chocolate contains sugar, cocoa mass, 10% cocoa butter and 5% CBE. Milk chocolate contains sugar, full cream milk powder, cocoa mass, 15% cocoa butter and 5% CBE. In chocolate pastel, the cocoa is omitted. The colour of chocolate pastel is basically white and colour can be added to make it more attractive.

Processing

The ingredients are pre-mixed. Melted fat is added and mixed until smooth. It is then refined, conched, tempered, moulded, cooled and demoulded. The chocolates are then stabilised to complete crystallisation.

1. Pre-mixing

aw materials, i.e. cocoa liquor, sugar and milk powder, are mixed with an adjusted fat content (26 ±1%) before the mass is refined.

2. Refining




It is important to refine chocolate paste in order to obtain the required smoothness. The mass is ground until 80% of particle size distribution is between 20-30μ . A roll refiner is commonly used for this purpose.

3. Conching



This process is characterised by chemical and physical change in the product mix. Flavour develops during the process and water content is reduced to less than 1%. A homogenous product mass is produced in an efficient homogeniser and this is a crucial step. During conching, good temperature control is essential. Temperature for plain chocolate is 65 oC- 90oC and for milk chocolate 50oC-65oC. Time for conching depends on the composition of the mass but it is normally more than three hours and often longer for plain chocolate.


4. Tempering



Tempering of the chocolate mass based on CBS is carried out by heating the mass at 50 oC the quoted slip melting point of the fat. For CBE-based chocolate, it is necessary to form the right amount of stable crystal in the chocolate mass to enable moulding.
The mass is heated at 50 to 26.5 - 25.5 oC-60oC and cooling it to at least 2oC aboveoC-60oC and cooledoC-27.5oC for plain chocolate andoC-26.5oC for milk chocolate.

5. Moulding


Temperature of the mass is raised to 31 oC. The chocolate is then cooled in a cooling cabinet set at 5 oC-33oC (plain chocolate) and 29oC- 31oC (milk chocolate) respectively before moulding. Temperature of the mouldshould be 2oC-5oC lower than the moulding temperature of the mass. The chocolate mass is filled into the mould, tapped and strapped of excess chocolate.oC-12oC.

6. Cooling and Demoulding

A suitable cooling cabinet with air circulation and sufficient cooling capacity should be used. Cooling temperature is usually between 5 45 minutes (normally 15-20 minutes). For easy demoulding, the shrinkage should besufficient. Difficult demoulding may be caused by
- mould not being sufficiently cleaned
- chocolate not being properly tempered
- temperature of the mould not being appropriate oC-12oC and time should not exceed

The finished products are stabilised at 18 crystallisation. Plain and Milk Chocolate Formulation Using CBE oC-20oC for at least 24 hours to complete

FAT BLOOM AT CHOCOLATE

1.Introduction
Fat bloom is a common problem in the confectionery industry. It is most often seen on chocolate, but can also appear on the surface of biscuits. The problem can cause very significant product losses. This is not because there is a contamination or specific quality issue. It is mainly because the visual characteristics become unacceptable, due either to loss of gloss or to the appearance of a white "frosting" at the surface of the product. The white frosting (Figure 1) is sometimes mistaken for mould growth but quite definitely is not.
It is a surface re-crystallisation of fat caused generally by migration.








Figure 1: White "beta-form" bloom ("frosting") on the chocolate of a half-coated biscuit
A more severe situation in chocolate can occur after some time, where the chocolate becomes grainy, crumbly and with no gloss or sensory appeal (Figure 2). This happens throughout the bulk of the chocolate material and is not just a surface phenomenon.





Figure 2: Severe bloom caused by Form VI crystal growths on the surface of chocolate
From what we have said above it becomes plain that there are at least two bloom effects.The bloom that is a characteristic of chocolate (Figures 2 and 3) may be termed Form VI bloom. The bloom shown in Figure 1 on the surface of chocolate can also appear on the surface of biscuits and even in semi-solid fats such as butter and fat spreads. This type is caused by a re-crystallisation of fat and is often accompanied by a change in crystal morphology. This type may be loosely called "beta-form" bloom. It is important to understand also that there is more than one mechanism at work. Food products are not static entities, they are quite dynamic and this creates partly the phenomenon of shelf life.
The properties that we have to consider are, in the main, thermodynamic ones.
Multiplephase foods, such as butter-cream-filled biscuit bars with a chocolate coating, create conditions where thermodynamic change is positively driven. Entropy is not a term just for physicists, because in making such foods the processes must address free energy change if a stable, long shelf life is to be achieved. The manufacturing process must attempt to achieve a stable low energy state for the components. If it does not then you end up, for instance, with partially crystallised fat in the insulated environment of palletised finished goods. Why is this a problem? We will explain this in the following text.
Figure 3: Form VI bloom in chocolate causes major structural change

2. Fat Crystallisation

All natural fats are mixtures of triacylglycerols (TAG). Each pure individual TAG will have a different crystallisation temperature. However, when mixed this causes the fat to have a wide range of temperature over which crystallisation occurs. Thus you obtain the phenomenon of solid/liquid ratios in fat mixtures. These ratios can be determined using pulsed nuclear magnetic resonance (pNMR) and some data is shown in Figure 4. It can be seen from this data that, if a product is made using one or more of the fat mixtures and then
displayed for sale, the temperature of the display area will affect the state of the fat in the product. Thus as temperature cycles, so does the liquid/solid fat ratio. More importantly, the ratio will often be different in each fat phase at the given temperature, creating imbalances across the different food matrixes, e.g. biscuit dough, cream filling and chocolate coating in a biscuit snack. We shall come back to this situation.






Figure 4: Solid Fat Content by pulsed NMR - typical Values

Crystallising fat systems are usually polymorphic. In most cases this is represented by 3 crystal forms termed _ (alpha) , _' (beta prime) and _ (beta), in increasing thermodynamic stability and increasing melting point. Not all fats can move freely from _ to _ because this depends on the ability of molecules to pack closer together. Some can, like cocoa butter; some cannot, like Salatrim (Benefat) which stays in the _ form. There is further complexity in that certain TAG types (cocoa butter is very rich in these symmetrical TAGs which have
an unsaturated fatty acid at the middle position) create the possibility for up to 6 crystal forms. The chocolate tempering process is designed to stimulate cocoa butter to crystallize in Form V. This process involves maintaining the temperature at around 29C-31C after a controlled cooling process.
Crystallising fat gives up heat energy (its heat of crystallisation). As the process progresses to more stable forms a certain amount of heat energy might be required to begin the process. Thus if a fat has reached _' and is then stored at, say, less than 20C, it is unlikely to progress to _ form, unless it is heated to above 25C. This point brings us back to two issues mentioned above: cycled temperature storage and palletised storage.
Cycled temperature storage applies significant energy stress to a product. This often results
in a change of crystal form. In addition, during cycling temperatures, the crystalline fat can begin to separate from the liquid phase and undergo individual changes. For palletized storage of poorly cooled product, the crystallising fat yields heat that cannot escape due to the insulating nature of palletised boxed products. This excess heat can raise the internal temperature by 2-3C. This can be sufficient to cause fat/chocolate melting and recrystallisation.

3. Liquid Fat Migration

In a crystallised fat, the liquid fat component is dispersed in and around the solid crystal clumps. The mobility of this liquid will to some extent depend upon the three-dimensional structure of the solid crystal network. At a given temperature, the liquid component will have a certain composition. As the temperature rises the amount of liquid fat will increase and its TAG composition will change.

In a single-matrix system, the consequence of this change may be nothing. Alternatively, it may trigger certain TAGs to separate or fractionate from the main bulk of the fat. This will lead to the growth of larger crystals over time that may be in the _ form. In a system with more than one matrix, each containing a different fat, the picture becomes complex. There are different liquid/solid ratios, maybe different total fat contents and different TAG compositions. All these points apply their own thermodynamic pressure for the liquid fat components to move or migrate between the matrixes. Although this fat migration may not be a problem, it usually does create problems.

As the liquid fats move between matrixes they mix with other liquid fat phases and thus change their composition. This may cause a change in the solid fat solution, i.e. more solid fat might dissolve at a given temperature, causing softening. In addition, the balance of the solution composition might change sufficiently to make other TAGs less soluble, causing them to crystallise out. The effect of temperature cycling must also be superimposed on this process. Different effects can be seen in some products when they are stored at different
constant temperatures (e.g. 22C or 25C) because the composition of the liquid fat phase varies with temperature.

4. Bloom Types

We have described above a number of dynamic processes that happen in products containing significant amounts of TAG. So how are these related to fat bloom? The visual effect of fat bloom is caused by crystal growth or a change in the crystal morphology after the product has been made (i.e. during shelf life). Let us now consider the Form VI and beta-form types of fat bloom.
Figure 1 shows a white frosted, beta-form bloom growth on the surface of chocolate. The crystals that are growing look (in close-up) very like diamond clusters. The finer structure is shown in Figure 5. This contrasts greatly with the leaf-like structures of classic Form VI bloom shown in Figure 6. This beta-type bloom is migration related and can be particularly bad where the migrating components come from a palm oil based dough fat. This type of bloom is rich in symmetrical TAGs of the POP and POSt type (P = palmitic acid, O = oleic
acid, St = stearic acid). Form V to Form VI inhibitors will not stop this bloom occurring because it is a shift in solid solution caused by the migrating palm oil TAGs. It is interesting to note that if the chocolate-coated product is stored at a constant 22C this bloom is predominant. However, if the storage temperature is a constant 25C then Form VI bloom occurs.




Figure 5: Beta form bloom crystals on surface of chocolate (these can also appear on biscuits)

The white frosting can be generated directly on the surface of high-fat baked goods, such as biscuits through fat separation and crystal growth. These biscuits can show the effect whether half coated with chocolate or not. The effect is more prevalent where palm oil is a major part of the fat blend in use. It is a consequence of the TAG composition of palm oil, which contains significant amounts of POP/POSt and PPP. Palm oil readily fractionates under certain conditions to yield fast growing crystals in the _ form. This problem can be
caused by over-cooling (shock-cooling) the product as it leaves the oven and packaging point. A certain amount of _ crystal is formed and an unstable situation exists.
Transformations over the next 24 hours often yield a bloomed surface. This bloom is not always permanent and can sometimes be removed by warming the product to 25C for a short period of time. A similar effect can be caused in the biscuit where a chocolate coating or inclusion (e.g. chocolate chips in cookies) may act as a very efficient "sink", via migration, for the liquid fat fraction. The consequence in time is a bloomed visual effect as the more solid TAGs recrystallise with growth of crystal size.
Another effect caused by overheating the surface of, for example a biscuit, is that the fat melts and liquid fat is drawn to the surface. This then cools quiescently and forms large crystal clumps that appear as bloom. This bloom is usually permanent. Such effects are caused, for example, by the hot plate, pack, end sealers overheating the packaging. The end biscuits in a pack then exhibit a "staining" effect, caused by large fat crystals.

Figures 2 and 3 show a particularly severe case of Form VI bloom in chocolate. The chocolate bar had a fat-cream filling and had been exposed to temperatures cycling between 18C and 28C over a period of 8 weeks. The chocolate has completely destabilized and the surface is covered with crystal growths. More particularly the whole structure has transformed to Form VI (confirmed using X-ray diffraction) and has become open, fragile and powdery. This type of bloom can be reduced, but not always eliminated, using Form V to VI inhibitors.







Figure 6: Leaf-like structures of classic Form VI bloom

5. Bloom Prevention

The title of this section is easy to write, the doing of it is not so easy. As we said earlier, foods are dynamic systems and undergo significant change during shelf life. If we are to prevent bloom then we have to address thermodynamic change. That means either
ensuring the food product has a low free energy state (i.e. is stable to further physical change), or, introducing some means of interrupting change by perhaps blocking it. The former method is used extensively by the fat spreads and chocolate industries. The product is made under optimum temperature conditions to ensure that the correct fat crystal form is generated. The product is then given sufficient time to "temper" under controlled temperature storage before it is fully packaged, boxed and palletised for delivery. This regime ensures that, for spreads the _' fat crystal form predominates and is stabilised, while for chocolate, the latent heat of crystallisation is removed below 25C to ensure a stable Form V crystal network is created. This approach does, to a large extent, assume that the product will not undergo large, post-production, temperature fluctuations during transport, sale and use/storage by the consumer.

However, it is a fact of life that time costs money and any means of speeding up product flow and increasing volume is enthusiastically sought by manufacturers (or at least by their management!). In addition, environmental temperature for product storage can be advised but not always ensured. In these situations some means of blocking crystal change can be advantageous. Milk chocolate contains a significant amount of butterfat and, because butterfat creates a strong eutectic with cocoa butter, the resultant product is much softer than plain chocolate. Milk chocolate rarely if ever undergoes Form VI bloom. The reason for this is the composition of butterfat contains a large range of TAG structures and molecularweights. This is due to the presence of significant amounts of fatty acids from C4 to C14. These serve to block the move from Form V to Form VI because, as they co-crystallise with cocoa butter TAG, the packing density of the crystals does not permit the thermodynamic change. The butterfat TAG also cause a change in the solid solution and thus a softer product. Both these effects reduce the incidence of bloom. This action of butterfat can (and is) harnessed to reduce the occurrence of bloom in plain chocolate. Butterfat can be added at a low level where softening of the chocolate is not significant but the effect on crystal form change is very significant.

The blocking of V to VI crystal form change in chocolate with butterfat is accompanied by a strong eutectic; in addition butterfat currently costs more than cocoa butter. Partly in response to this, some of the special (confectionery) fat suppliers have designed bloom inhibitors as vegetable fat additives to chocolate. It is important to recognise that these additives have to form part of the current legislated limit of 5% added vegetable fat in chocolate. However, the composition of these fats is such that a range of TAG structures containing fatty acids from C10 to C18 is introduced into the cocoa butter. These act to inhibit the re-crystallisation of cocoa butter TAG in Form VI.
Form V to Form VI inhibitors such as described above do not stop the beta-form bloom.
This is because beta-form bloom is mainly a consequence of re-crystallisation and crystal growth following transformation from _' fat crystal. This type of bloom is much more difficult to address and arguably is much the most prevalent. Prevention here is one of understanding the product makeup, the process temperatures and times, and the compatibility of adjacent fats where the product has more than one component. This last point is imperative when considering shelf life of the product ex factory. Compatibility of adjacent fats is necessary because fat migration will take place. Noncompatibility will cause changes in the fat phase balance resulting often in recrystallisation at the product surface (as described above). Migration can be limited by the use of sprayon barriers or the use of fats that are highly nucleating on cooling. In the former case, spray-on barriers can be useful for example in chocolate shells that are to be filled with nut praline. In the latter case, the fats (sometimes called fractal fats), when cooled from the melt, create many nuclei that grow to relatively small dimentions, rather than fats that form few nuclei but larger crystals. The effect is to form a very efficient three- dimensional structure that holds liquid fat very efficiently, reducing the rate of migration. These fats must be _' stable.
Process considerations must include cooling the product at an appropriate rate so that resulting crystal forms are stable and not prevalent to transformation. Thus cooling a baked product too rapidly (say from 30C to below 14C) often results in retention of some _ crystal.This will move to _' crystal on standing, a condition that can cause temporary surface bloom. Alternatively, packing the product too hot (>25C) and then providing an insulated environment, such as on a pallet, can cause slow crystallisation with large crystal growth and separation of solid and liquid fat phases.

6. Conclusion

In conclusion, fat bloom is an expensive problem for food manufacturers and significant care has to be exercised to reduce its occurrence. Thermodynamic change will happen, but fat bloom is not inevitable if care is taken over the selection of fats that have to be adjacent in a product. Care must also be taken in process control and this has to include consideration of all aspects from ingredient mixing, through packaging, transport and storage.
We have attempted to describe types of bloom and the way in which they can occur in fat based products. We have discussed the relative complexity involved in trying to limit fat bloom. We believe that it is important to generate such understanding of products because bloom issues are time-related. Current food industry structure can involve products being transported over large distances and to variable ambient climatic conditions. These can include high temperatures and also high humidities. To these times are added the turnover time of the product, which may be quite different for the large and small retailer. Getting the
thermodynamic balance right is a challenge for the producer, but the prize is worth the effort in terms of quality and loss.

SUGAR AND FAT BLOOM

It is not uncommon for a chocolate bar or box of chocolate confections to be opened and the chocolate is no longer shiny. Instead, it looks as if the chocolate has spoiled and is covered with white or light brown colored splotches. While it may look as if the chocolate has spoiled, the chocolate is in fact edible.

What has occurred is that the chocolate has undergone a process called "bloom." There are two main types of chocolate bloom. The first is sugar bloom and the second fat bloom. Each has different causes. However, no matter the type of bloom, the surface of the chocolate will become unappealing and will have a mottled or hazy look. If fat bloom is present, it is likely that the texture of the chocolate may have changed from when it was originally molded.

Sugar Bloom

Sugar bloom is caused by moisture coming into contact with the chocolate. Chocolate is composed of ground cocoa beans and sugar, and sometimes vanilla and lecithin. While you may not see the sugar crystals present in chocolate, they are there. They simply are too small to see. Water when it comes in contact with the chocolate, dissolves the sugar on the surface of the chocolate. As the water dries, the dissolved sugar crystallizes and precipitates onto the surface of the chocolate. The resulting small sugar crystals give the chocolate a dusty appearance.
The sugar bloom may have occurred in a number of ways. The most obvious of is that water was inadvertently spilled on the chocolate, or the chocolate came in contact with or was placed on something wet. Sugar bloom may occur in other not so obvious ways. For example, if the chocolate was placed in the refrigerator where it became cold and then removed and placed in open air, the cold chocolate will condense moisture from the air, and the condensation will cause the sugar bloom. Sugar bloom may also occur if the chocolate has been in an environment with too high a humidity.

The best way to avoid sugar bloom is to store your chocolate in an area of low humidity and stable temperature so as to avoid condensation. If the chocolate is cold, such as when it has been stored in the refrigerator, it should be covered (perhaps with a towel) so that it will warm slowly and air circulation is minimized.

A Simple Test

One way you can easily check to see if a piece of chocolate has undergone sugar bloom or fat bloom is to lick your finger and touch it to the chocolate. If the dusty appearance disappears, then it is sugar bloom. (The moisture on your finger dissolved the sugar crystals on the chocolate.) If the bloom remains, then it is fat bloom.

Fat Bloom

Fat bloom, unlike sugar bloom, is not always caused by a simple set of circumstances, such as the chocolate becoming wet. Fat bloom is more complicated, and oftentimes it may be more difficult to discover the actual source of the problem.
Fat bloom typically appears as lighter color spots on the chocolate. As the name implies, the bloom is composed of fat, in this case the naturally occurring fat that comes from the cacao bean -- cocoa butter.

When discussing the reasons for fat bloom, it is important to note that when
cocoa butter hardens, it forms crystals. Some of the crystals are stable, but other crystals are not and will actually change form over time. During chocolate manufacturing, a process called tempering is used to ensure that only stable crystals form, while the chocolate hardens. Fat bloom is caused by the interaction of the various types of crystals or the tempering process (or lack thereof).






An example of severe fat bloom in completely untempered chocolate






If chocolate is not tempered, the unstable forms of cocoa butter crystal will form, most notability the Beta Prime and Alpha forms. After the cocoa butter hardens, these unstable forms will slowly change their forms to the stable Beta form. The Beta crystals are slightly smaller than the Beta Prime or Alpha forms, so that when this transition occurs, the chocolate contracts. The new stable Beta crystals then form, projecting above the surface of the chocolate, visible as bloom. If the chocolate is stored in a room where the temperature fluctuates near the melting temperature of the stable Beta crystals, two additional types of fat bloom may form. In the first, some of the Beta crystals melt. When they recrystalize, they recrystalize slowly, since the ambient temperature is close to that of the chocolate. This allows the crystals to grow much larger than the original small, compact crystals. In addition to projecting above the surface of the chocolate, these larger crystals may displace cocoa butter, forcing it to the surface. The second type of bloom is created when the crystals have softened instead of melted. It is during this period that cocoa butter that has slightly melted migrates toward the surface. When it breaks the surface, it pools ever so slightly, and when it cools the cocoa butter appears as spots.







An example of fat bloom on a chocolate bar





Many people are surprised to learn that fat bloom also occurs in cocoa powder. Cocoa powder contains between 12-20% cocoa butter. Since some cocoa butter is present, it must be tempered during manufacturing, just as chocolate is. Cocoa powder that has been improperly tempered or undergone temperature fluctuations may cause bleaching of the cocoa powder and may cause clumping as the cocoa butter helps the particles of the cocoa powder adhere to each other. As with chocolate, when bloom occurs it does not affect the edibility of the cocoa powder but may have an aesthetic impact.

Studies on fat bloom indicate that the bloom consists of large, single cocoa butter crystals or collections of crystals of the stable Beta form of cocoa butter. Other forms of cocoa butter crystals are not present in fat bloom.

Fat blooming actually occurs in a third process. This case affects not so much the chocolate industry directly but the ancillary confectionary industry. When chocolate is used to coat nuts or fillings that contain oils or fats (such as nut butters) that are incompatible with chocolate, the oils may actually seep into or through the chocolate over time. This is called fat migration. As the oils displace the cocoa butter, cocoa butter may seep onto the surface of the piece of confectionary and recrystalize as bloom. When this occurs, the manufacturing process needs to be examined or the confectionary reformulated.

If fat bloom is present and the chocolate is not newly cast, then temperature fluctuations should be the first thing looked at. If an air conditioner is in use, the outlet may be too close to the chocolate (causing temperature fluctuations as the air conditioner turns on and off), or it may simply be undersized for the room to be cooled. If the chocolate was recently molded, the temper of the chocolate is suspect, and adjustments to the temper procedure may be required.

Fat Bloom as Quality Indicator

Fat Bloom is a good indicator that the chocolate may not be in good condition. It is not uncommon for chocolate that has bloomed to undergo other changes. For example, it may have lost its temper. When chocolate has properly crystallized, it will have a shiny finish, have a nice snap when broken, and will melt at approximately 95 degrees Fahrenheit (35 C). If chocolate is stored in a room where the temperature has fluctuated or has become too hot, the chocolate will recrystallize. When this happens, the cocoa butter crystals will regrow in an uncontrolled fashion and will likely result in fat bloom. While fat bloom may be only an aesthetic problem, chocolate where fat bloom is present should be examined to ensure that its temper remains intact. If the chocolate will be melted and then remolded or used in baking, neither sugar nor fat bloom will appreciably affect the quality of the final product. The one exception to this is where fat migration has occurred, such as may happen in the confectionary industry.

In an effort to eliminate bloom, some manufacturers will add a variety of fats and their derivatives (most notably stearins) to the chocolate prior to molding. This provides only a limited amount of protection from bloom, though as of yet, there is no bloom-free chocolate. Of course, at Amano Chocolate, we do not use additives to prevent bloom, nor do we encourage their use. Instead, at Amano we choose to watch our manufacturing practices with added diligence. Chocolate simply means too much to us to adulterate it.

EFFECTS OF TEMPERING AND FAT CRYSTALLISATION BEHAVIOUR ON MICROSTRUCTURE MECHANICAL PROPERTIES AND APPEARANCE IN DARK CHOCOLATE SYSTEMS

Fat crystallisation behaviours in dark chocolates from varying particle size distribution (PSD) (25, 35 and 50 over-temper and under-temper), and their effects on mechanical properties and appearance evaluated.Microstructures of derived products were determined using stereoscopic binocular microscopy. Wide variations in mechanical properties and appearance were noted in products from different particle size and temper regimes. Particle size (PS) was inversely related with texture and colour, with the greatest effects noted in hardness, stickiness and lightness at all temper regimes. Over-tempering caused significant increases in product hardness, stickiness with reduced gloss and darkening of product surfaces.Under-tempering induced fat bloom in products with consequential quality defects on texture, colour and surface gloss. Micrographs revealed variations in surface and internal crystal network structure and inter-particle interactions among tempered, over-tempered and under-tempered (bloomed) samples.Under-tempering caused whitening of both surface and internal periphery of products with effects on texture and appearance. Thus, attainment of optimal temper regime during pre-crystallisation of dark chocolate was central to the desired texture and appearance as both over-tempering and under-tempering resulted in quality defects affecting mechanical properties and appearance of products.

1. Introduction

Tempering is a technique of controlled pre-crystallisation employed to induce the most stable solid form of cocoa butter, a polymorphic fat in finished chocolates. The process consists of shearing chocolate mass at controlled temperatures to promote crystallization of triacylglycerols (TAGs) in cocoa butter to effect good setting characteristics, foam stability, demoulding properties, product snap, contraction, gloss and shelf-life characteristics. Time–temperature protocols and shearing are employed to induce nucleation of stable polymorphs with the formation of three-dimensional crystal network structure influencing the microstructure, mechanical properties and appearance of products. The crystal network organisation and the polymorphic state of the TAGs crystals as affected by the crystallisation conditions are major factors determining rheological and textural properties of crystallised TAGS systems (Herrera and Hartel, 2000; Narine and Marangoni, 1999;).Lipp and Anklam, 1998; Segall). These occur as symmetric triacyglycerolsChevalley,; Beckett, 2000; Afoakwa et al., 2007b; Do et al., 2007; Afoakwa). Smaller particles improve sensory propertiesZiegler et al., 2001) but plastic viscosity and yield stress increasedistribution and ultimately sensory character in consumption and product identification.

Instrumental measurements can act as complements for sensory evaluations (Lawless and Heymann, 1998) with statistically significant correlations (Mohamed et al., 1982; Meullenet et al., 1997; Rosenthal, 1999; Ali et al., 2001; Bourne, 2002). Appropriate strategies can objectively assess features of texture and appearance such as gloss, colour, shape, roughness, surface texture, shininess, and translucency (Leemans et al., 1998; Jahns et al., 2001; Hatcher et al., 2004; Briones and Aguilera, 2005; Briones et al., 2006; Altimiras et al., 2007; Afoakwa et al., 2008a). Knowledge of tempering effects on product texture and appearance attributes can have significant commercial implications.

With recent innovations and growth in chocolate confectionery industry, understanding the factors influencing chocolate microstructure, texture and appearance would be of value in predicting changes in quality. This study was therefore aimed at investigating effects of tempering and fat crystallizations behaviors on microstructure, mechanical properties and appearance in dark chocolates varying in particle size distribution.

2. Materials and methods

2.1. Materials

Cocoa liquor of Central West African Origin was obtained from Cargill Cocoa Processing Company (York, UK); sucrose (pure cane extra fine granulated) from British Sugar Company (Peterborough, UK); pure prime pressed cocoa butter and soy lecithin from ADM Cocoa Limited (Koog aan de Zaan, Netherlands) and Unitechem Company Ltd. (Tianjin, China),respectively. The recipe, formulation and production of samples have been described previously (Afoakwa et al., 2007b). Chocolates were formulated with total fat of 35% (w/w) from sucrose, cocoa liquor, cocoa butter and lecithin. Experimental samples (5 kg batch for each formulation) were produced by mixing sucrose (40.8%) and cocoa liquor (53.7%) in a Crypto Peerless Mixer (Model K175, Crypto Peerless Ltd, Birmingham, UK) at low speed for 2 min and then at high for 3 min, then using a 3-roll refiner (Model SDX 600, Buhler Ltd., CH-9240 Uzwil, Switzerland) to a specified particle size (D90:18 ± 1 lm, 25 ± 1 lm, 35 ± 1 lm and 50 ± 1 lm) conducting particle size analysis, during refining, to ensure D90 values. The refined chocolates were melted at 50–55 _C for 24 h and the chocolate mass conched in a Lipp Conche (Model IMC-E10, Boveristr 40-42, D-68309, Mannhein, Germany) at low speed for 3.5 h at 60 _C. Lecithin (0.5%) and cocoa butter (5%) were added and then conched at high speed for 30 min to effect adequate mixing and liquefaction. Samples were kept in sealed plastic containers at ambient (20–22 _C) and moisture and fat contents determined using Karl Fischer and Soxhlet methods (ICA, 1988) and (ICA, 1990).

2.2. Determination of particle size distribution

A MasterSizer_ Laser Diffraction Particle Size Analyzer equipped with MS 15 Sample Presentation Unit (Refractive index 1.590) (Malvern Instrument Ltd., Malvern, England) was used. About 0.2 g of refined dark chocolate was dispersed in vegetable oil (Refractive index 1.450) at ambient temperature (20 ± 2 _C) until an obscuration of 0.2 was obtained. The sample was placed under ultrasonic dispersion for 2 min to ensure particles were independently dispersed and thereafter maintained by stirring during the measurement. Size distribution was quantified as the relative volume of particles in size bands presented as size distribution curves (Malvern MasterSizer_ Micro Software v 2.19). PSD parameters obtained included specific surface area, largest particle size (D90), mean particle volume (D50), smallest particle size (D10) and Sauter mean diameter (D[3,2]).

2.3. Tempering experiment

Samples were incubated at 50 _C for 4 h for melting and tempered using Aasted Mikrovert laboratory continuous three-stage tempering unit (Model AMK 10, Aasted Mikroverk A/S, Farum, Denmark). Chocolate was pumped through the multi-stage units and a worm screw drove the product through the heat exchangers.

Sensors located at specific points in the equipment measured the temperature of both the chocolate and the coolant fluid at each stage. Based on our earlier work modelling temperature controls to study tempering behaviour (Afoakwa et al., 2008b), the temperature of each of the coolant fluids (Zones 1:2:3) were thus set as 26:24:32 _C, 21:19:32 _C and 18:16:32 _C, respectively for attaining the under-tempered, optimally-tempered and over-tempered regimes. The degree of pre-crystallisation was measured using a computerized tempermeter (Exotherm 7400, Systech Analytics, Neuchâtel, Switzerland) and a built-in algorithm provided the tempering curves and temper readings in chocolate temper index (slope), corresponding to optimal temper (slope 0), undertemper (slope 1.0) and over-temper regimes (slope _1.0). The principle of this method has been described by Nelson (1999).

Chocolate from the three regimes were moulded using plastic moulds: 80 mm length; 20 mm breadth; and 8 mm height. The final products were allowed to cool in a refrigerator (5 _C) for 2 h before de-moulding onto plastic trays and conditioned at 20 ± 2 _C for 14 days before analysis. Triplicate measurements were taken for each product composition and the mean values recorded.

2.4. Texture measurements

Mechanical properties of chocolates (hardness and stickiness) were measured using TA-HD Plus Texture Analyzer with a penetration probe (needle P/2) attached to an extension bar and a 50 kg load cell and a platform reported by Afoakwa et al. (2008a). Maximum penetration and withdrawal forces through a sample (80 _ 20 mm, depth 8 mm) were determined with 8 replications at a pre-speed of 1.0 mm/s, test of 2.0 mm/s, post speed of 10.0 mm/s, penetrating 5 mm at 20 _C, converting mean values of the penetration force exerted by the 50 kg load cell into hardness (g force) and the withdrawal force with time into stickiness (g force s) data, respectively using XT.RA Dimension, Exponent 32 software (Stable Micro Systems, Godalming, Surrey, UK).

2.5. Colour and gloss measurements

HunterLab MiniscanTM XE Colorimeter Model 45/0 LAV (Hunter Associates Inc., Reston, VA) calibrated with white ceramic reference standard was used. Colour images of chocolate surfaces were converted into XYZ tristimulus values, which were further converted to CIELAB system: L*, luminance ranging from 0 (black) to 100 (white); and a* (green to red) and b* (blue to yellow) with values from _120 to +120. Information was obtained using a software algorithm (Matlab v. 6.5; The Math-Works, Inc., Natick, MA): hue angle (h_) = arctan (b*/a*); chroma (C*) = [(a*)2 + (b*)2]½. Mean values from five replicate measurements and standard deviations were calculated. Gloss of chocolate surface was measured using the multiple angle Tricor Gloss meter (805A/806H Gloss System, Elgin, IL). Reflectance was measured at an incidence light angle of 85_ from the normal to the chocolate surface, in accordance with ASTM method D523. A polished black glass plate with a refractive index of 1.567 was used as standard surface (ASTM, 1995) and given a gloss value of 200. Gloss was reported as gloss units (GU) based on determinations (in triplicate) at six positions along a chocolate sample. As a reference, a surface with a gloss value less than 10 GU is considered a low gloss surface (BYK, 1997; Briones et al., 2006).

2.6. Image acquisition and capture

A colour digital camera (Canon Powershot, Model A70, MA, USA) was mounted on a stand inside a large box with internal black surface impervious to light. Images of the optimally tempered, under- tempered and over-tempered samples were captured before storage and after 14 days in storage. The iris was operated in manual mode, with the lens aperture at f = 8 and speed 1/20 (no zoom, no flash) to achieve high uniformity and repeatability. The camera was gray balanced before each imaging session. Uniform diffuse lighting was used to illuminate the samples. The lighting system consisted of four CIE source D65 lamps (60 cm length and 18 W; Model TLD/965, Philips, Singapore) placed above the sample at a 45_ angle to maximize diffuse reflection responsible for colour. The angle between the camera lens axis and the sample was around 90_ to reduce gloss. A Kodak gray card with 18% reflectance was used as a white reference to standardize the illumination level. The gray-level image (1600 _ 1200 pixels) of this card was divided into 192 blocks, each one of 100 _ 100 pixels. After calibration, samples were placed in the field of view of the camera and an image of 1600 _ 1200 pixels (approximately covering the whole area of the tablet) was acquired and stored in JPEG (joint photographic experts group, a standard for compressing digital photographic images) format of high resolution and superfine quality.

2.7. Microstructural determinations

Chocolate samples were characterised using stereoscopic binocular microscope (Nikon, SMZ-2T, Tokyo, Japan) equipped with a variable removable lens. Micrographs (coloured images) were captured using a digital camera (Model 2.1 Rev 1, Polaroid Corporation, NY, USA) and observed using Adobe Photoshop (Version CS2, Adobe Systems Inc. NJ, USA). Triplicate experiments were conducted capturing 6 images per sample, and micrographs representing the surface of each temper regime captured and presented.Samples were then sectioned (cut) into two pieces using a knife and the internal microstructures observed.

2.8. Experimental design and statistical analysis

Two experimental variables comprising temper regime and PSD were used. Other variables including refiner temperature and pressure, conching time and temperature were held constant. A 3 _ 4 factorial experimental design was used comprising:
(i) Temper regime: optimal temper, under-temper and overtemper. (ii) PSD (D90): 18, 25, 35 and 50 lm. Statgraphics Plus 4.1 (Graphics Software System, STCC, Inc,Rockville, USA) examined mechanical properties (hardness and stiffness) and appearance (colour [L, C*, h_] and gloss) using twoway analysis of variance (ANOVA) and multiple comparison tests to determine effects of factors and their interactions. Tukey multiple comparisons (95% significance level) determined differences between levels. All experiments were conducted in triplicates and the mean values reported.

3. Results and discussion

3.1. Particle size distribution of dark chocolates

These findings (Fig. 1), previously reported (Afoakwa et al., 2008a), show volume histograms consisting of narrow (18 lm PS) and wide (25 lmPS) bimodal and narrow (35 lmPS), and wide (50 lm PS) multimodal size distributions. This PSD range 18– 50 lm using D90 values (>90% finer) covers optimum minimum and maximum sizes with direct effects on texture and sensory character in manufacture (Ziegler and Hogg, 1999; Beckett, 2000). Data from the PSD as previously described (Afoakwa et al.,2008a) showed variations in specific surface area, mean particlevolume D(v,50), Sauter mean (D[3,2]) and mean particle diameter (D[4,3]) with increasing D90 particle sizes. Specific surface area (SSA) was inversely correlated with the different component of PSD. Similar inverse relationships of SSA with all the other components of PSD have been reported (Beckett, 1999; Ziegler and Hogg, 1999; Sokmen and Gunes, 2006). Beckett (1999) concluded largest particle size and solids specific surface area are the two key parameters for chocolate manufacture. The former determines chocolate coarseness and textural character, the latter with desirable flow properties. Fat contents of the products were 35 ± 1% and moisture within the range of 0.90–0.98%.

3.2. Fat crystallisation behaviours during tempering of dark chocolate

Four different temper regimes (untempering, under-tempering, over-tempering and optimal tempering) were characterised (Fig. 2) each with its unique characteristic crystallisation behaviour. In optimal tempering, the temperature of the chocolate remained constant for sometime during cooling, to initiate formation of stable fat crystals. The crystallisation heat released was then balanced by an equal amount of cooling energy causing the growth of stable crystal nuclei in adequate amounts, which during post-tempering conditioning mature to effect shelf stability of the product. The temperature of the chocolate dropped further when the liquid cocoa butter was transformed into solid crystals resulting in solidification of the products (Fig. 2). Beckett (2000) reported that properly tempered chocolate shows formation of Form V, the most desirable polymorphic form which confers appropriate product snap, contraction, gloss and shelf-life characteristics.

Under-tempering (insufficient tempering) was caused by the relatively higher temperatures released between the multi-stage heat exchangers during tempering. The process caused development of more crystallisation heat within the product during solidification, effecting quick cooling, as more liquid fat was transformed quickly into solid form, resulting in the formation of very few stable fat crystal nuclei (Fig. 2). Distinct increase in temperature was observed at the beginning of the crystallisation, which declined again after reaching a maximum point where most of the stable crystals formed were re-melted prior to cooling. Untempered chocolate, produced no stable fat crystals as the heat exchange system generated higher crystallization heat during cooling, resulting in quick cooling of the completely melted product with no inflexion point for stable fat crystal formation (Fig. 2). Beckett (2000) explained that the crystallisation processes in both untempered and under-tempered chocolates lead to the formation of unstable Form IV polymorph, which later transforms into more stable Form VI polymorph during storage. Preliminary studies showed that untempering and under-tempering regimes exhibits different crystallisation behaviours but results in similar unstable fat crystal nucleation and growth, with similar associated storage polymorphic transformations and defects in products. Storage of the under-tempered products under ambient temperature (20–22 _C) for 14 days of conditioning induced blooming in samples, effecting various quality changes in the products as reported in this study. Products from under-tempering regime were used in this study.
Over-tempering occurred when relatively lower temperatures were exchanged between the multi-stage heat exchangers of the tempering equipment, causing significant part of the liquid fat to withdraw from the continuous phase of the chocolate, and transformed
into solid form when less liquid fat was available for pumping the product. The process released little crystallization heat during cooling, rendering a rather flat and slow cooling curve (Fig. 2). This crystallisation process results in too many small stable seed crystal formation leading to reduced strengths in the polymorphic stabilities of the fat crystals formed during the process (Talbot, 1999). As a substantial part of the phase transition (from liquid to solid) took place before the chocolate reached the mould, less contraction occurred in the mould, leading to demoulding problems with defects in final product quality and storage characters (Hartel, 2001; Lonchampt and Hartel, 2004).

3.3. Effect of temper regime and PSD on mechanical properties

Hardness showed an inverse relationship with particle sizes, with significant reductions at all temper regimes, and greatest in the under-tempered (bloomed) products (Fig. 3). Hardness of the optimally-tempered products decreased from 5318 g with 18 lm PS to 4259 g at 50 lm. Similar trends in hardness were noted with the over-tempered samples, decreasing from 6064 g with 18 lmPS to 4651 g at 50 lm, and from 6533 g with 18 lm PS to 5459 g at 50 lm in the bloomed products (Fig. 3), suggesting differences in hardness with varying PS at all temper regimes. Particle sizes have been noted as an important parameter in the hardness of fat crystal networks in many confectionery products (Narine and Marangoni, 2002; Campos et al., 2002; Marangoni and Narine 2002; Pérez- Martínez et al., 2007). Earlier studies showed inverse relationships of hardness in tempered dark chocolates with particle sizes at varying fat and lecithin levels (Afoakwa et al., 2008a), attributed to the relative strengths of their particle-to-particle interactions (Campos et al., 2002; Afoakwa et al., 2008c). Do et al. (2007) also reported consistent reductions in hardness (texture) of milk chocolates with increasing particle sizes.



The results showed that the under-tempered products had the greatest hardness (texture), attributable to the re-crystallisation process undergone by the fat in the under-tempered chocolates resulting in intense hardening of products. This trend in hardness was followed by the over-tempered samples with the optimal tempered products possessing relatively lesser hardness levels, suggesting over-tempering of dark chocolates leads to increased hardness of samples at all PS as compared to their respective optimally-tempered products.
Chocolate stickiness showed an inverse relationship with particle sizes at all temper regimes, and the greatest trends were noted in the over-tempered products (Fig. 4). Stickiness of the optimallytempered products decreased consistently from 380.67 g with 18 lm PS to 325.25 g at 50 lm. Likewise, the levels of stickiness in the over-tempered samples decreased from 447.92 g with 18 lm PS to 365.10 g at 50 lm, and from 336.86 g with 18 lm PS to 309.20 g at 50 lm in the bloomed products (Fig. 4), explaining that the over-tempered products had the greatest stickiness levels, followed by the optimally tempered products with the bloomed samples having the least. Narine and Marangoni (2001) noted that stickiness of confectionery gives information about deformability related to oral sensory characters. Analysis of variance (ANOVA) suggested significant differences (P < href="http://img03.blogcu.com/images/a/c/a/acarserpil/3_1243391246.jpg">

3.4. Effect of temper regime and PSD on colour and gloss

Lightness (L*), chroma (C*) and hue (h_) followed similar trends with varying PS at all temper regimes (Table 2). Significant (P <>

As well, the blooming caused great reductions in C* and h_ in the under-tempered products at all PS (Table 2). Hutchings (1994) stated that L*, C* and h_, respectively represent food diffuse reflectance of light, degree of saturation and hue luminance, which are dependent on particulate distribution, absorptivity and scatter-ing factors or coefficients. In a densely packed medium, scattering factor is inversely related to particle diameter (Saguy and Graf, 1991). Chocolates with varying particle sizes differ in structural and particulate arrangements influencing light scattering coefficients and thus appearance (Afoakwa et al., 2008a).

Similar decreasing trends in L* were noted in both tempered and over-tempered samples with increasing PS. However, the over-tempered samples had relatively lower L* values at all PS as compared to their corresponding optimally tempered products (Table 2).
These suggest that over-tempering reduces the degree of lightness in dark chocolates, effecting product darkening and thus affecting quality. However, no noticeable effect on C* and h_ were observed among the tempered and over-tempered products (Table 2). Thus, changes in colour in dark chocolates were primarily dependent on PS and temper regime. Bloomed dark chocolates tend to scatter more light, appear lighter and less saturated than over-tempered and optimally tempered products. The blooming process resulted in higher scattering coefficients, with subsequent paleness (whitening) - higher L* values. Hartel (1999) reported that the whitish haze in bloomed chocolate is caused by the dispersion of light of fat crystals. Similar effects of PS on the degree of whitening during blooming have been reported (Altimiras et al., 2007).Colour of foods may be affected by various optical phenomena among them scattering and surface morphology, therefore an accurate understanding of the influence of appearance on measured colour is essential.

Gloss relates to capacity of a surface to reflect directed light at the specular reflectance angle with respect to the normal surface plane (ASTM, 1995). Significant (P <>

ANOVA showed that PS and temper regime both significantly (P <>3.5. Effect of temper regime on product image

Digital images of dark chocolates (18 lm PS) were assembled to show surface appearances of optimal, under- and over-tem pered products before and after the 14 days conditioning (Fig. 5).

Initially surface appearances were similar and smooth but after 14 days, clear differences were apparent. Optimally and overtempered chocolates maintained their characteristic glossy appearance and dark brown colour but the under-tempered samples had bloomed, with appearance of surface whitish spots, rendering them dull and hazy in colour (Fig. 5). Similar increases in whiteness in under-tempered (bloomed) chocolates have been reported (Lonchampt and Hartel, 2004, 2006; Altimiras et al., 2007). Hartel (1999) explained this phenomenon as re-crystallisation of fats from a less stable Form IV to a more stable Form VI polymorph, with changes in light dispersion on small surface fat crystals (>5 lm), consequently impacting on both appearance and textural attributes. Fat bloom development, mechanisms and effects on chocolate appearance, quality and marketability has been extensively studied (Bricknell and Hartel, 1998; Ali et al., 2001; Hartel, 2001; Timms, 2003; Walter and Cornillon, 2001, 2002; Lonchampt and Hartel, 2004, 2006; Altimiras et al., 2007; Smith et al., 2007).

3.6. Effect of temper regime on microstructure

Microstructural examination using stereoscopic binocular microscopy after the 14 days conditioning showed clear variations in both surface and internal peripheries of products from varying temper regimes (Fig. 6). Over-tempered products had relatively darker surfaces and internal appearances than optimally tempered confirming the reported relative differences in L* (Table 2). Undertempered products showed both bloomed surface and internal periphery with large whitish, and distinct smaller brown spots (Fig. 6). The observed whitish appearance on surfaces and internal peripheries appear to be mixtures of re-crystallised fat and sugar crystals, and the small brown spots, cocoa solids. Lonchampt and Hartel (2004, 2006) suggested these whitish spots were primarily sugar crystals and cocoa powder and nearly devoid of fat. This difference in interpretation is the subject of further studies.

4. Conclusion

Fat crystallisation behaviour during tempering of dark chocolate play vital roles in defining the structure, mechanical properties and appearance of products. Wide variations in mechanical properties and appearance occurred in products from different PS and temper regimes. Particle size was inversely related with texture and colour, with the greatest effects noted with hardness, stickiness and lightness at all temper regimes. Over-tempering caused increases in product hardness, stickiness with reduced gloss and darkening of product surfaces. Under-tempering induced fat bloom in products with consequential quality defects in texture, colour and surface gloss. Micrographs revealed clear variations in surface and internal crystal network structure and inter-particle interactions among tempered, over-tempered and under-tempered (bloomed) samples. Blooming caused whitening of both surface and internal periphery of products with consequential effects on texture and appearance. Hence, attainment of optimal temper during tempering (pre-crystallisation) of dark chocolate is vital to the desired texture and appearance of products, as both over-tempering and under-tempering result in quality defects affecting mechanical properties and appearance of products.

Acknowledgements

This study was co-funded by the Government of Ghana and Nestlé Product Technology Centre (York, UK). The sponsors are gratefully acknowledged for the Research Support. We also wish to thank Drs. Steve Beckett, Angela Ryan, John Rasburn and Angel Manez (Nestlé PTC, York) for useful technical discussions.