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.

Introduction

Roasting

Roasting is crucial to flavour development of cocoa beans as well as their appearance. The roasting temperature, time, and roasting humidity depends on the quality of the cocoa beans and the goals of the chocolate maker. Roasting has an impact on both the physical and chemical processes within the nib [1].

Evaporation of acids

Roasting at high enough temperatures will cause the evaporation of volatile acids (acidic aromas), which will reduce the nib acidity and in turn reduce how sour and even bitter the cocoa nib will be.

Maillard Reaction

During fermentation, chemical reactions within the cocoa bean occur to produce free amino acids, short-chain peptides, and reducing sugars [3, 4]. During roasting and through the Maillard reaction, these amino acids, peptides, and sugars interact and form flavour compounds (aroma molecules) [5-8] such as:

pyrazine
alcohols
esters
aldehydes
ketones
furans
thiazoles
pyrones
acids
imines
amines
oxazoles
pyroles
ethers

The specific flavour profile of the cocoa bean will be dependent on many factors such as the origin of the beans, period of harvesting, postharvest treatment, variety, and the roasting conditions [4,9-14].

The end product of the Maillard reaction depends on:

roasting time
roasting temperature
pH
concentrations of the reactants (amino acids, peptides, reducing sugars)
water activity

Colour Development

During roasting, colour development occurs and is mediated by:

3-deoxyxylose
3,4-dideoxyxylose-3-en [15]

These two substances along with other substances participate in polymerization reactions. These reactions create brown polymers containing:

carbonyl
carboxyl
amino
amide
indole
pyrrole
ester
ether
methyl
hydroxyl groups [15]

Temperature appears to be the main factor impacting pigment formation during roasting [16-18]. The dynamics of pigment formation during roasting depends on the temperature gradient [17].

Krysiak W [19] showed that roasting using a convection oven at 135°C to 150°C (275°F to 300°F) and at a high air humidity (5.0%) resulted in desired colouration for dried cocoa beans that were not pulp pre-conditioned.

Pulp pre-conditioning, Fermentation, & Drying on Colour

These 3 stages also appear to impact brownness of cocoa beans [11].

During fermentation, polyphenols and anthocyanins undergo different reactions that lead to the creation of flobafens. Flobafens contribute to the characteristic brown colour of roasted cocoa beans.

Polyphenols oxidation and polymerization, degradation of proteins, Maillard reactions and dextrinization of starch create other brown pigments [11, 20-22].

Afoakwa et al. [23] indicated that pulp pre-conditioning through pod storage improves the fermentation outcome such as degree of brownness (fermentation index) in Ghanian cocoa beans. However, the extent to which pulp pre-conditioning by pod storage impact browning still remains unknown.

Therefore, this research was aimed at evaluating the changes in browning index and colour properties during roasting of pulp pre-conditioned (storing) and fermented cocoa beans.

Materials and Methods

Raw Materials

Cacao was retrieved from fully ripe mixed hybrid variety from the cocoa plantation at the Cocoa Research Institute of Ghana (CRIG), New-Tafo in the Eastern Region of Ghana.

Pods were harvested only when fully ripe and good appearance (no bruises), and stored in 4 treatments (0, 3, 7, and 10 days). They were then fermented using the basket technique. The beans were mixed every 48 hours during fermentation. The beans were then sun dried and stirred four times a day. At 5.5-6% moisture level they were immediately packaged in air tight bags before roasting.

Roasting was done according to methods by Owusu M et al. [24] with some modifications. They were roasted using hot air ovens at temperatures of 120°C for 0, 15, 30, and 45 minutes.

Analytical methods

This third party site might help give you a better understanding the following analytics: L*, a*, and b* values.

Browning Index

The browning of the nibs was determined by using the method done by Krysiak W (2006) [25] with some modifications.

About 0.1 g of ground cocoa bean was mixed with 10 ml of methanol, and hydrochloric acid was added. The mixture was placed in a vortex (spun really fast) for 2 minutes. This mixture was left alone for 20 hours at 6°C. Absorbance was taken at wavelengths of 460 nm and 525 nm using UV/Visible spectrophotometer. The browning index was calculated using the formula OD460/OD525.

Colour Measurement

Hunter Lab Miniscan™ XE Clolormeter Model 45/0 LAV was used to measure the colour of the nibs. L* (luminance, black to white); a* (green to red); and b* (blue to yellow) values were calculated.

Statistical Analysis

All the data was analyzed using Statgraphics. ANOVA was used to determine the relationships between the studied attributes. Significance was established at a probability of p<0.05.

Results and Discussion

Effect of pod storage and roasting on browning

Roasting is the main factor to impact not only colour development, but also flavour, and texture. During the Maillard reaction, reactions lead to an increase in brown pigment formation, which increases the (OD460/OD525) values (browning index).

The browning of the dried fermented beans increased with increasing pod storage (Table 1). The browning index for pods stored for 3 days was 1.007 versus 1.144 for 10 days. This was consistent with previous studies [26] which might be due to the degradation of polyphenols into brown pigments (tannins) during fermentation [27].

Perhaps not surprisingly, increasing in roasting time also increased browning indices of the nibs. The increase in browning over time during roasting was likely due to pigment formation from Maillard reactions, thermal oxidation and polymerization of polyphenols to form tannins [28] and Strecker degradation reactions [22,25].

Pod storage and roasting time had a significant (p<0.05) influence on browning (Table 1).

Effect of pod storage and roasting time on colour Measurements

Colour of the nibs is used as an indicator of the quality of a fermentation on the cacao. Fermentive index (AKA browning index) and visual appearance by cut tests are used to ascertain the quality of the cacao.

Changes in L* - values of the cocoa nib

Both increased pod storage and roasting time decreased the L* value (100 being white, 0 being black). Decreasing L* value means darker, closer to black values.

The longer the beans were stored, the darker the browning (L* value) such as from around 25.6 to 24.05. However, pods stored at 10 days (and at 45 minutes of roasting) had the most reduced L* value at 20, which indicates that they would be too dark (darker than “chocolate brown”) and therefore unacceptable.

Aside from the beans stored for 10 days prior to roasting, the L* values for all were positive, indicating the beans were lighter even after roasting for 45 minutes.

Changes in a*- values of the cocoa nib

The a* values decreased from 12.5 in unstored pods to 9.22 in the 10 day stored pods. (Figure 3). They suggest the reason a* values decreased with pod storage could be due to leaching out of the polyphenols and anthocyanins from the bean druing pod storage [27].

The longer the beans were roasted, the lower the a* values for all pod stored treatments. The authors suggest the decrease in a* values of the nibs with increase roasting time was due to thermal oxidation of the anthocyanins in the nibs.

The interaction between pod storage and roasting time had a significant influence (p<0.05) on the a* values.

The a* values were all positive before roasting, means they were more red and magenta in colour. They suggest this may be due to the presence of purple beans, which indicate less fermented cacao.

As well, the a* values were all positive after roasting too, meaning not all the anthocyanins were degraded, giving the beans a more red colour post roasting.

Changes in b*- values of the cocoa nib

As pod storage time increased, so did the b* values (going from more yellow than blue) of the cocoa nib. Beans from unstored pods had a b* value of 6.91, which increased to 8.65 for pods stored for 10 days.

Generally, b* values in nibs from all treatments increased with roasting time. This might be due to thermal oxidation of polyphenols as well as Maillard reactions. There again was a significant relationship between time of roasting and increased b* value.

As well b* values for all stored beans before and after roasting were all positive, meaning they were more yellow, which is favorable to cocoa beans. Blue is not a favorable colour in post fermented or roasted beans.

The increase in b* value with pod storage suggests pod storage may enhance the appearance of fermented cocoa beans.

Conclusion

This study showed us that both increased pod storage and roasting time impact the browning index and colour properties of cacao beans. Pods stored for 10 days produced beans with the highest browning index, while those from unstored pods showed the lowest.

L* and a* values decreased with increased pod storage and roasting time (more black, less white; more red, less green respectively), and b* values increased (more yellow, less blue) with increased pod storage and roasting time.

Beans from pod stored for 3 to 7 days produced the most acceptable BI and L* values after roasting for 30 minutes at 120°C (~250°F).

References

Effect of Roasting on the Browning Index and Appearance of Pre-Conditioned and Fermented Cocoa Beans.