Impact of pre-crystallization process on structure and product properties in dark chocolate
The following is a summary of the stated research mentioned above. The content summarized here, including the figures and tables, all belong to the researchers (unless otherwise indicated). The summary attempts to stay as close to the original paper as much as possible with some adjustments in regards to jargon, length, or to focus on bean to bar aspects.
Abstract
Dark chocolate was tempered by seeding with βvi (beta 6) seeding method, or with conventional seeding method. Different microstructures were created, with various degrees of temper, and were evaluated according to how well they kept their preferred properties over time. The structure and strength of the cocoa butter was analyzed, as well as fat bloom, and fat/moisture migration over time.
The two different pre-crystallization processes (βvi vs conventional seeding), created significantly different cocoa butter structures with different storage stabilities. Well tempered βvi pre-crystallization resulted in a dense homogeneous structure, and retarded fat bloom and fat migration. However, a too high structure density generated heterogenous structures with reduced ability to retard fat bloom. A lower structure density exhibited optimal resistance against moisture migration.
Introduction
Plain dark chocolate is essentially cocoa particles and sugar crystals suspended in a phase of cocoa butter. Filled chocolates (bonbons or truffles) are more complicated in structure. The outside shell is the same as the plain dark chocolate, but inside is a filling often containing liquid fat or water components. These components have the potential to migrate toward and into the shell and causing it to change structurally (cracking of shell, hardening of filling, softening of shell, fat bloom) due to less stable TAGs (triglycerides) or moisture from the filling.
Cocoa butter is polymorphic, and has six different crystalline forms. Each form has a different thermodynamic stability and melting point, which translates into differences visually and texture wise as well. The βv form is the most desired by those who work with chocolate, due to its appearance (shiny/homogenous in colour), strong firm texture, ideal melting point, stable microstructure and retardation of fat migration.
conventional method
Traditionally, this ideal βv polymorph is achieved through a well-defined temperature program under the action of shear (practically speaking, cooling it down quickly with constant movement). This is what will be referred to as “conventional” pre-crystallization. This process includes using about 1-3% volume wise of seed crystals from which the rest of the liquid fat will grow (crystalize) from.
βvi-seeding method
On the contrary, the βvi-seeding method using (introduced in 1999) about 0.05-1% of cocoa butter crystals in the βvi form mixed into pre-cooled (32-34°C) chocolate, which results in a large number of small well-defined crystal nuclei. Even though the seeds themselves are in the βvi form, the surrounding chocolate grows in the preferred βv form.
Either method requires a correct amount of stable crystal nuclei to be formed, otherwise the chocolate becomes over- or under-tempered.
over tempered
When chocolate is over-tempered, too many stable crystal nuclei develop in the liquid chocolate, and causes the chocolate to crystalize at too high a temperature (above 30°C)
under tempered
Not enough stable crystal nuclei are produced. Therefore more cooling is required to move the crystallization along. However, by the time that happens, many less stable polymorphic forms are generated in the chocolate.
Microstructure
The microstructure of under-, over-, and well-tempered chocolate has been established. Recent work has shown that the contraction of chocolate as it solidifies differed depending on the degrees of temper. This supports the idea that a decrease in volume of the chocolate causes a more dense structure (closer packing of the fat crystals).
Objective
The objective of this research was to understand more deeply the impact of pre-crystallization process on the storage stability, characterized by physical and chemical properties that are related to chocolate structure.
Materials and Methods
Traction tests and DSC evaluation were used to estimate the structure density. Figure 1 gives an overview of this experiment in this research paper.
Sample Preparation
Obtained from Buhler Group AG (Uzwil, Switzerland) and consisted of 39.6% cocoa mass, 10% cocoa butter, 50% sugar, and 0.4% lecithin. The total fat content was 32%. This chocolate was then pre-crystallized with either the conventional tempering or βvi-seeding.
Conventional Tempering
A tempering machine was used (Model AMK 10, Aasted Mikroverk A/S, Farum, Denmark). Chocolate was heated to 50°C for 2 hours to erase pervious crystal memory. The chocolate temperature in 3 zones was adjusted to 31°C (zone 1), 24°C (zone 2), and 29-31°C (zone 3). The last zone had a range of temperatures in order to achieve the under-, over-, and well-tempered chocolate.
βvi-seeding
The βvi seeds were produced using the SeedMaster Cryst (Buhler, AG, Uzwil, Switzerland). This cocoa butter was allowed to solidify, stored at 15°C until used for pre-crystallization. This cocoa butter was analyzed via DSC melting curves, which showed a distinct second peak at 34°C (results not shown) which correspond to polymorph βvi.
2 kg of chocolate was heated to 50°C for 2 hours, then dropped to 33°C. The solid βvi seeds were ground into a powder, and then added to this 33°C molten chocolate. The amount of βvi seeds was adjusted to either 0.2%, 0.7%, or 2% of the weight to obtain under-, well, and over-tempered chocolate.
Data Collection
The degree of tempering was determined by a MultiTherm Tempermeter TCTM (Buhler Group AG, Uzwil, Switzerland) and adjusted to give a temper index (TI) of 3, 5, or 6 for under-, well-, and over-tempered chocolate. The temper index values can be seen in Figure 2. A summary of the different samples analyzed can be seen in Table 1.
Each pre-crystallization process was repeated 3 times, and 10 samples were collected from each run for physical/chemical characteristics, and 25 from each run for storage stability tests, for a total of 525 samples.
Results and discussion
Structure characterization right after Solidification (setting/cooling)
Traction tests were performed by applying a longitudinal force to the sample. The tensile strength of the βvi-seeded (Seed-) vs conventionally (Conv-) pre-crystallized chocolate samples (Table 1) is presented in Figure 3. Both well- and over-tempered βvi-seeded samples required the greatest force to break as compared to the conventional samples and under-tempered βvi sample.
There was found to be a correlation with density and a homogeneous fat crystal network with high tensile strength.
The Seed-WT and Seed-OT samples contained a cocoa butter crystal network that was stronger and more developed, and had a higher structure density.
An important point to note is the Seed-OT (over-tempered) sample had a greater degree of standard variation between replicates, which corresponded to a more heterogeneous structure. The researchers suggest this is due to assemblies of seed particle agglomerates, which resulted in spots of lower mechanical strength.
Both under-tempered samples (Seed-UT and Conv-UT) had the lowest density, and weakest fast crystal structure.
The results from the tensile strength tests (Fig. 3) were further analyzed with DSC melting curves right after the chocolate solidified (Fig. 4), and differences between βvi-seeded and conventional are evident.
Figure 4 illustrates the melting properties of the samples. Researchers used a Mettler Toledo, DSC 1 to record the findings.
The conventional samples (b and d in Figure 4) had a broader melting curves, meaning, each replicate varied a greater deal than did the replicates in the βvi-seeded samples (a, c, and e). And so the CONV- samples were also less dense and more heterogeneous in structure, making the fat crystal network weaker (Fig 3).
The authors state that this shows us the βvi-seeded samples had less distribution of polymorphic forms than the conventional samples, meaning more of the cocoa butter in the βvi-seeded samples were in the βv form.
By combining results shown in Fig. 3 and Fig. 4, we see that Seed-WT (well tempered sample using the βvi-seeded method) resulted in chocolate samples with high structure density, and strong and homogeneous fat crystal network due to even distribution of fat crystal nuclei.
The Seed-OT was also had a strong overall crystal network, but connections between the crystals was weaker due to it being more heterogeneous (different mix of crystals, which don’t pack/connect as well together due to different shapes and sizes).
Overall, it appears that a chocolate which has a homogenous structure (made up of mostly the same crystal polymorph), with crystals packing densely and evenly distributed, appeared to have the strongest structure.
Structure characterization during storage (week 1 vs week 12)
Figures 5 shows us DSC melting curves for the 5 samples at either one week (5a) or 12 weeks (5b). We can see that in week 1, the curves are much more closely packed, meaning the samples had similar melting properties to one another versus what happens at week 12 (5b) where we see a shift in the melting properties (some samples more than others).
Comparing Figure 5a to the curves in Figure 4, suggest that during the first week of storage, the crystal structure in all the samples continued to develop due to post-crystallization, making all the samples appear to have a similar polymorphic distribution. However, as time went on (Figure 5b) the differences became more apparent again as in Figure 4.
Traction tests were also performed at 1 week and 12 weeks of storage. After only one week, the tensile strength was increased in all the samples from the tensile strength test done right after they cooled. This means the crystal network developed a higher structure density over that week, making all the samples a little stronger than they were 1 week prior right after cooling.
By the end of the storage test period (12 weeks) the βvi-seeded samples (which had the highest tensile strength after cooling) diminished a great deal. The Seed-WT sample strength diminished 34%, Seed-OT 28%, and Seed-UT 37%. After 12 weeks, all samples showed a more comparable tensile strength.
The authors state that the DSC melting curves didn’t show any indications for βvi crystals after 12 weeks in the Seed-WT sample, only in the Seed-OT sample (see dotted line in Fig. 5b).
Fat Migration
Analysis of the fat migration was done by soaking filter papers with liquid sunflower oil with high oleic content, and was placed under the chocolates in a Petri dish. The weights of the chocolate before and after were recorded. They were left at 20°C at 50% relative humidity for 13 weeks. The uptake of the sunflower oil was measured every second week gravimetrically.
This analysis is important in regards to filled chocolates (truffles and bonbons). Liquid fats in the ganache over time get absorbed by the hard chocolate shell. The more or faster this happens, the sooner the shell can get soft and misshapen or crack.
The conventional samples absorbed more liquid oil than the βvi-seeded samples. The Seed-WT and Seed-OT exhibited the most resistance to fat migration (Figure 7) followed by Conv-WT. Therefore, the dense structure observed in these same samples (shown in Fig. 3 and 4), also correlate to better resistance to fat migration.
Due to the weaker structure of the more heterogenous under-tempered samples, it is likely that not only did the liquid from the sunflower disrupt the crystal lattice of the chocolate, but that some low-melting polymorph TAGS within the chocolate itself migrated out, further enhancing the fat migration.
This also correlates with other findings that saw under-tempered chocolate to have a higher saturation of liquid cocoa butter within it.
Therefore, the temper influences the migration of liquid oil through the chocolate. The better the temper (stronger homogenous polymorph structure) the better at retarding fat migration, and in turn keeping a better texture, shape, and appearance.
It’s important to point out that the researchers in this paper point out other studies that didn’t see a difference in migration rate between over- and well-tempered milk chocolate, or well- and under-tempered dark chocolate.
Moisture Migration
The absorption of moisture into the chocolate samples was accomplished by placing the chocolate in contact with a gelatin-gel for a period of 7 weeks.
The results are interesting. The samples in Figure 7 that saw more resistance to fat migration, were not as successful at moisture migration. It appears that the conventionally seeded chocolate samples were more resistant to moisture migration. As well, under-tempered samples seemed to have a better resistance than well-tempered samples.
The mechanism behind the transport of moisture through the chocolate appears to be by diffusion. Capillary flow through pores in the chocolate has also been suggested by Ziegleder (2008). This capillary action flow might be the reason why the well-tempered more dense chocolate samples saw a greater uptake.
The researchers suggest the high structure density after production, with extensive contraction, might make way for micro-pores/cracks to form within the chocolate. Since water has a higher surface tension and lower viscosity than oil, it will generate a stronger capillary force. However, more work is required to understand the mechanisms of this finding.
Fat Bloom
The fat bloom development was quantified using the Digital Color Imaging System (DigiEye). It was used to capture images of the chocolate surface during storage. The software allowed them to scan the surface of each chocolate sample, and essentially calculate the percentage of a certain color within the image.
S1 represents surface original colour, S2 represents light fat bloom, and S3 represents heavy fat bloom.
The surface appearance of the chocolate samples during storage are shown in Figure 9. The Conv-UT and Seed-OT were the first to lose the original colour (S1), and developed fat bloom the fastest.
Seed-WT samples had the best resistance to fat bloom, and first observed after 13 weeks of storage. βvi-seeded samples had the best resistance to fat bloom vs conventional seeding.
The Seed-OT (over-tempered) chocolate sample had a very poor resistance to fat bloom. They suggest the reason for this is due to the heterogeneous structure caused by overconcentration of βvi-seeds, which form agglomerates and obstructing the high-melting TAGS for being incorporated into the fat crystal network. Therefore, a larger amount of liquid fat was available in the Seed-OT sample, and eventually migrated to the surface to create fat bloom. A similar suggestion was made by Smith et al., 2007.
Conclusion
It appears that the Seed-WT (well-tempered) sample had the strongest structure, most resistant to fat bloom and fat migration, but not so resistant to moisture migration. The Seed-OT sample had a strong structure as well, but was made up of a more heterogeneous mix of crystal polymorphs, and so overall was less stable than the Seed-WT, and also most susceptible to fat bloom.
During storage, post-crystallization occurred in all the samples. Initially they all samples were more dense after one week than after initial cooling, but eventually the various samples had a high degree of variance in their structure at 12 weeks of storage. This was due to their fat crystal structure, density, and whether they were made up of homogenous or heterogenous crystals.
When it comes to moisture migration, the opposite seems to be true, with the weaker, heterogeneous samples being able to resist it more than the well-tempered samples.
A better understanding of the chocolate structure (fat crystallization) the better one can improve storage stability.
