Effects Of Fermentation Intensity On Polyphenols And Antioxidant Capacity Of Cocoa Beans
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
Cocoa pod on tree. Image by @isuru18.
Cocoa beans are actually seeds of the fruit of the tree Theobroma cacao. West Africa produces more than two-thirds of the world’s supply of cocoa. A major step in processing the cocoa beans in order to make chocolate is fermenting the raw seeds. Raw cocoa beans are astringent and unpleasant to eat. Steps in processing such as fermenting, drying, and roasting them allows for the characteristic cocoa flavour.
Cocoa beans and products made from it (chocolate, cocoa powder) are rich in phenolic compounds. Dreosti (2000) reported that 60% of these phenols in cocoa are flavanol monomers (epicatechin and catechin) and procyanidin oligomers, which are stated to combat free radicals in our body. Studies have suggested cocoa or chocolate reduce risk of cardiovascular disease (Keen et al., 2001), and extracts of cocoa powder/beans were shown to exhibit antihyperglycaemic effect (lowering blood glucose) on streptozotocin-induced diabetic rats.
Fermentation Process Overview
Fermentation is a crucial step which occurs right after cocoa pods are harvested. The fermentation processes produces aroma precursors which are important for producing the full aroma of chocolate. Cocoa beans are most often fermented spontaneously by microorganisms originating from surfaces such as the workers hands, containers used, tools such as knives, and the outside of the cocoa pod itself. These microorganisms inoculate the pulp around the cocoa bean which is the substrate for the fermentation process.
Fermenting cocoa beans usually takes 5-7 days. The actual fermentation process occurs in the pulp surrounding the cocoa beans. This is because the fruit pulp is rich in sugars (glucose, fructose, and sucrose) and the pH of fresh pulp ranges from 3.3-4 due to high levels of citric acid (1-3%). There are external and internal fermentation stages. External fermentation involves the catabolism (breakdown) of the sugar pulp by microorganisms. The internal fermentation encompasses biochemical changes within the cocoa bean itself. Yeast begins the process, which grows in the high sugar substrate and limited oxygen availability within the fermentation mass. The yeast primarily produce ethanol from the carbohydrates within the pulp along with degradation of pectin (which allows for the fruit to slough off the seed).
Open cacao pod with segments of cacao fruit pulp. Within each segment is the seed we refer to as a cocoa bean. It is the cocoa bean we use to make chocolate. Image by @kristianapinne
Chocolate produced from unfermented beans have no chocolate flavour and is quite bitter and astringent. However, fermentation of the cocoa beans has also been attributed to a loss of polyphenols (Kim & Keeney, 1984). In fact, the oxidation of polyphenols into insoluble tannins is responsible for some of the flavour precursors (Jinap & Dimick, 1990). Catechin has been described as having a bitterness followed by a sweet aftertaste, or simply bitter and astringent (Bonvhi & Coll, 1997). Catechins (which include epicatechin, catechin, procyanidin B2, procyanidin B5, and procyanidin C1, are compounds responsible for bitterness and astringency in roasted cocoa beans (Stark et al., 2005). Between days 2-3 of fermentation, epicatechin decreased and may indicate it is being used up for the formation of large tannins or lost when the fluids of the mass drain (Kim and Keeney, 1984).
Due to pressures from cocoa buyers, some cocoa farmers do not allow their beans to ferment long enough before drying and selling them. Some are fermented only a few days, while some batches are not fermented at all. Since fermentation is important to flavour and nutritional value, this work here is to evaluate the effect of various fermentation durations on polyphenol content as well as antioxidant capacity of beans.
Materials & Methods
Ripe Amelonado cocoa pods were collected at the Cocoa Research Institute of Nigeria, Ibadan. The beans for sampling were wrapped in polyethylene nets and inserted into the heap so that they could be easily removed but also to ensure there was enough heat for properly ferment the beans. The beans were removed at each day and sun-dried for seven days. The beans were then analyzed for the mineral analysis, polyphenol content, and antioxidant capacity.
Results And Discussion
Cocoa beans contain an array of nutrients. Table 1 shows the level of nutrients observed with the cocoa beans in this study. Most values are on average with other beans such as those reported by Olaofe et al. (2006), except for a lower level of magnesium. This is likely due to low levels of magnesium in the soil the trees grew in. Reports by Wollgast & Anklam (2000) and Misnawi et al. (2002) polyphenol content from 120-180 g/kg. Three groups of polyphenols have been identified: catechins (or flavan-3-ols) at 37%, anthocyanins at 4%, and proanthocyanidins at 58%. Polyphenols along with alkaloids, some amino acids, peptides, and pyrazines contribute to the bitter and astringency of cocoa products. Polyphenol levels also decreased during high drying temperatures. The processes involved in cocoa products (such as chocolate) is a result of fermented, drying, and roasting (which also reduces polyphenol levels) cocoa beans.
Figure 1 clearly shows the decrease in total phenol content of the cocoa beans over 6 days of fermentation. The total content of polyphenols started at 16.11% wt/wt. The weights decreased from days 2-6 the polyphenol values decreased to 15.5%, 10.7%, 8.2%, 7.6% and 6.01%. During fermentation, the polyphenols are involved in biochemical changes through polymerization and complexation with proteins. This is also why there is also a decrease in solubility and astringency (Bonvehi & Coll, 1997).
Figure 2 shows the levels of epicatechin specifically, which also decreased from about 12 mg/g to a reduction of about 5 mg/g or 60% of the initial value. The reduction may also be due to fermentation sweating (Kim and Keeney, 1984), which means being carried away with the juices as the fruit pulp breaks down and drains. Caligiani et al. (2007) saw a decrease in epicatechin and catechin from unfermented beans (slaty color) to partly fermented (violet color) and then at the lowest amount in the fully fermented beans (brown in color). High tempertures and prolonged processing times also contributes to lower catechin levels. In this study, catechin levels (0.17 mg/g) did not change during fermentation, but epicatechin decreased about 60%. It’s suggested this decrease in epicatechin is due to diffusion out of the cocoa bean as well as polyphenol oxidation.
Total antioxidant activity
The antioxidant capacity of the cocoa bean samples was measured for each of the 6 days of fermentation using the DPPH method (a free radical that can measure the scavenging activity of antioxidants) which measured the inhibition percent (IP%). From day 1 to 6 the inhibition activity of the polyphenols in the cocoa beans decreased from 96% to 79%. The trend in the reduction of antioxidant capacity parallels the reduction of total polyphenols. Therefore, this correlation suggests that the polyphenols are responsible for the antioxidant capacity. Steinberg et al. (2002) also confirms that the antioxidant capacity of chocolate is related to the polyphenol content.
Conclusion
Polyphenols are the major compounds responsible for the antioxidant capacity of cocoa beans. This means that a reduction in polyphenols due to processing the cocoa beans, including fermentation, reduces the cocoa bean’s (and products made from this cocoa) antioxidant capacity. Catechin levels did not decrease, while epicatechin levels did. However, concentration of polyphenols in these beans (even after decreasing during fermentation) still contained a high antioxidant capacity.
