Volatile Compounds Of Oxidized Cocoa Butter
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.
Note By Geoseph
This is a research summary I mentioned in a video I posted a while back regarding the question of whether cocoa butter can oxidize and go rancid. Some people in the bean-to-bar world hold the belief that cocoa butter is not able to go rancid. The fact is, all fats go rancid. Some can oxidise faster than others. But all fats can oxidise. This brings up some interesting ideas, especially in regards to how cocoa butter or chocolate products are stored especially in regards to sunlight and heat which can increase the change of off-flavours. Keep in mind here that in this paper both light and heat was used to fast-track the oxidation process so to speak. Oxidation can occur slowly or quickly depending on how much heat or how much light is exposed to it. This brings up a good point of how sometimes chocolate is stored for long periods of times (weeks) in melters where fat and cocoa solids separate and are exposed to higher than room temperatures for prolonged periods of time.
A chunk of solid cocoa butter broken from a larger piece. Photo by Bean To Bar World
Introduction
Oxidation of lipids create off-flavours in foods containing fat, such as chocolate which is a fat-based product. Environmental factors such as heat, oxygen exposure, and sunlight can cause these fats to oxidise. Autoxidation is a natural process, but lipid oxidation is a problem faced by many within the food industry. Oxidation of fat causes both volatile and nonvolatile compounds to degrade and form other compounds which will impact taste, odor, and nutrition. We can analyze the quality and flavour of these foods by analyzing the volatile (aroma) compounds produced from this oxidation. The analyzing of the compounds can be achieved via gas chromatography and mass spectrometry.
What is oxidation of fats
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Oxidation of unsaturated fatty acids is a chain reaction. Lipids first react with oxygen in the presence of a catalyst (light or metal ions such as copper or iron). From this a chain reaction occurs and produces what are called free radicals (a molecule that is highly reactive). The free radicals then also react with oxygen to produce hydroperoxide and more free radicals. And the cycle repeats. The hydroperoxide molecules break down to produce strongly flavoured oxygenated molecules, resulting in what we refer to as rancidity. This can be avoided by good practise or the use of antioxidants to slow down the oxidation process.
The isolation and identification of the aromas from cocoa butter have been described by Rizzi (1967) and Carlin et al. (1986). The purpose of this research: to assess oxidation of cocoa butter and evaluate the aromas produced. The addition of antioxidants (α-tocopherol, copper, and iron) and their impact was also studied.
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Materials & Methods
Both refined and non-refined cocoa butter was purchased from Cacao Barry (France) and 100g samples were placed in glass open bottles. And the samples were oxidized in the following ways:
In an oven at 90°C
Daily light at room temperature for 12 weeks
The refined cocoa butter had the following added:
100 ppm of α-tocopherol (a type of Vitamin E)
1000 ppm α-tocopherol (a type of Vitamin E)
5 ppm of CU+2
5 ppm of Fe+3
All the samples were then oxidized for 12 weeks, with analysis taken weekly. Steam distillation-extraction was used to extract the volatile compounds from the cocoa butter. The compounds were separated by capillary column gas chromatography. The identification was confirmed using gas chromatography-mass spectrometry.
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Results & Discussion
Non-Oxidized refined and un-refined cocoa butter samples
Figure 1 displays the volatile compounds (and their amounts) extracted from the non-refined (AKA deodorized) and refined (deodorized) cocoa butter samples before oxidation. Table one lists in detail the volatile compounds in both non-refined and refined cocoa butter. The numbers on Figure 1 and 2 can be linked to the numbers on Table 1 and Table two.
As expected, the refined cocoa butter contained less volatiles than the non-refined cocoa butter. This is due to the deodorization process in refined cocoa butter processing, which removes low molecular weight fatty acids and other volatiles (Bergman and Nauman, 1986).
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In the non-oxidized samples, a total of 37 compounds were identified in the non-refined cocoa butter and 29 in the refined cocoa butter (mostly at much lower levels). Fifteen of those compounds were aldehydes/ketones, 11 were heterocycles, 6 others (ester, alcohol, acid, or other).
Oxidized cocoa butter
Figure 2 displays a chromatogram of the volatile compounds identified in the oxidized refined cocoa butter, and table two lists these compounds and their concentrations. There were 40 compounds identified in the oxidized cocoa butter: 25 aldehyde/ketones, 4 alcohols, 3 lactons, 4 acids and 3 heterocycle compounds.
By comparing figures 1 and 2 as well as Tables 1 and 2, one can see that all cocoa butter samples contained the same type of major volatile compounds such as aldehydes (hexanal, heptanal, octanal, nonanal, and decanal), however, there was a higher concentration of these in the oxidized cocoa butter samples. As well, their concentration in the oxidized cocoa butter increased with time of oxidation or storage time.
Hexanal is a compound used often to indicate lipid oxidation in foods. It was found to be the most abundant aldehyde in this study. It is usually formed from hydroperoxides of omega 6 fatty acids such as 13-hydroperoxide of linoleic acid (Hallberg and Lingnert, 1991).
“Side Note: Hexanal is described by some as having the aroma of freshly cut grass, but also described as leafy, fatty, fruity, hay, woody, and sometimes as an off-flavour. Nonanal can have positive aromas associated (floral, fruity) but also unfavorable such as rancid, deep-fried, and fatty.”
In Figure 3 and 4 (Oxidation via light and oxidation via heat) we can see which aldehydes were found in the greatest concentrations in both non-deodorized (unrefined) and deodorized (refined) cocoa butter samples. These aldehydes were most likely produced via lipid oxidation. In oxidation via light we see that the hexanal was the aldehyde in the highest concentration. In oxidation via heat, we see nonanal was the aldehyde at the highest concentration.
Figure 5 displays the results of oxidized deodorized cocoa butter samples in the presence of various antioxidants. Figure 5a (oxidized via daylight) shows the results of oxidized cocoa butter with 100ppm of the antioxidant α-tocopherol (Vitamin E), which had lower concentrations of the aldehydes compared to the the cocoa butter sample without Vitamin E. Figure 5b (oxidized via heat) we also see a lower concentrations of the aldehydes compared to cocoa butter without Vitamin E, but not as low as in the sample oxidized via daylight. It is suggested that Vitamin E can be used as an antioxidant because it is a hydrogen donor.
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A very interesting point is that increasing the amount of Vitamin E from 100ppm to 1000ppm actually accelerated the oxidation of the cocoa butter as seen in Figure 5c. The higher concentration of Vitamin E in the sample had a weaker not stronger impact on the oxidation impact. This confirms results by Karahadian and Lindsay (1989) where a high concentration of α-tocopherol may react as a prooxidant and not as an antioxidant product.
Figures 6-9 we see a timeline of the oxidation of each of the samples. In Figure 6 we see the impact of both iron and copper (used as antioxidants) in samples oxidized by day light (a and b) and oxidized by heat (c and d). Again, the levels of aldehydes produced were greater in the samples oxidized by heat at 90°C versus oxidized by day light (in both copper and iron samples). However, comparing Figure 6 where copper and iron were added to Figures 3 and 4 where samples were oxidized without the addition of these trace metals, we see that the trace metals did not greatly reduce the concentrations of aldehydes as did α-tocopherol and in some cases seemed to increase the concentrations of some aldehydes.
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In conclusion, we see that both prolonged heat and daylight will oxidize cocoa butter, with prolonged heat having a greater impact on the increase of aldehydes (off-flavours). The addition of α-tocopherol at the right concentration appears to have a buffering effect on oxidation. Therefore, measures can be taken in order to inhibit the oxidation of cocoa butter either through better storage or other means.
