Steering the Flavour of Chocolate through Pod Storage & Roasting techniques.

Cover Photo Credit: Geoseph Domenichiello
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This is a summary of the research by Hinneh et al. This research paper looked at the impact of pod storage and roasting temperature on the aroma profiles, or overall flavour, of various dark chocolates.  All tables and figures belong to the researchers involved in this paper.


Pod Storage & Roasting

Chocolate is made from the seed of the plant, Theobroma cacao.  We also refer to this seed as a cocoa bean, or cacao. The flavour potential of this seed dictates to a degree the flavour of the chocolate which is made from it.  The better tasting the seed, the better tasting the chocolate. The less bitter the seed, the less bitter the chocolate. However, there are many stages in the process of curing the cacao and making the chocolate which impact and alter the overall aroma profile of the product.  Aroma molecules (also known as volatiles) give foods their distinct characteristics we refer to as flavour beyond taste (that is, sweet, salty, sour, bitter, savoury).

In this study, Hinneh et al. looked at two specific processes which impact flavour, pod storage (PS) and roasting temperature (RT).  Making chocolate includes picking the cacao pods, fermenting the fruit and seeds together, drying the seeds, roasting them, and grinding these seeds up into chocolate.

Pod storage is a step some farmers take between harvesting the pods and fermenting the seeds.  It is said to reduce acidity and improve the overall flavour of the cacao beans. This is a technique farmers can utilize for free, and if this means improving the flavour or quality of their cacao without having to invest more money, it might allow them to make more of a profit.  

Roasting is the step which actually creates the flavour of chocolate within the cacao beans.  Roasting post fermented and dried cacao beans builds all the aroma molecules (pyrazines, esthers, aldehydes, etc.) we associate with chocolate. Altering the temperature a few degrees or the time of roasting a few minutes can drastically alter the aroma concentrations of the cacao beans, which in turn will alter the flavour of the chocolate.  Roasting is often a challenge for the maker, who needs to decide how long and how high or low to roast their cacao. The more they understand about the mechanisms behind roasting, the less (costly) trial and error they would have to go through.  

The Experiment:

Cacao pods were harvested in Ghana, and stored for 0, 3, or 7 days.  From these pods, the seeds were harvested, fermented, dried, roasted, and turned into chocolate liquor.  Chocolate liquor is basically 100% ground cacao seed kernels. Think peanut butter, but with cacao beans instead of peanuts.  The seeds from the 3 pod storage treatments were also each roasted at 100, 120, 135, 140, and 160*C each for 35 minutes. They ended up with various chocolate liquors of differing combinations of roasting temperatures and pods storage treatments. Of all these samples, the authors chose 10 chocolates with the most distinct and diverse aroma profiles. See Table 1 Below.

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All of these samples from the pod storage/roasting experiment were then compared to 6 other standard commercial chocolate liquors from different countries, including Ecuador, Ghana, Ivory Coast, Madagascar, Venezuela, and Vietnam (see Table 2 below). The pod storage, fermentation, and roasting specifications for these commercial chocolate liquors were not known. The idea here was to see if these commercial chocolates contained differing concentrations of aromas, and how these compared to the 10 chocolates in the pod storage and roasting experiment.

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After collecting all these 16 various chocolates, they were all analyzed using headspace solid-phase microextraction-gas chromatography-mass spectrometry.  Basically what that means is that the actual aroma molecules (which give each chocolate its characteristic flavour) were identified in each of these 16 chocolate liquor samples .  Charts below show which aroma molecules were identified and the amount of each aroma molecule present within the samples.

Aroma Molecules Identified

Table 3a below displays the 69 major aroma molecules identified within the first 10 chocolate samples.  These molecules are organized according to the group or type of aroma they belong to. At the top of the chart you can see how many days the pods were stored, as well as the temperature of the roasting for that batch.  The concentrations of all the aroma molecules are in nanograms per gram of cocoa liquor. Each molecule listed corresponds to a specific flavour we perceive. However, the table doesn’t include this information, so unless you google the names of the aroma molecules, you likely won’t know what they represent. But understand that these various molecules represent aromas such as fruity, caramel, cocoa, nutty, smoky, etc.

What can you take from this table? Notice how the concentrations of each aroma is different from each of the 10 chocolates. This difference in concentration of aromas is due to how long the pods were stored and the temperature of roasting. Its very interesting to notice how these two aspects, especially roasting, can alter the aroma profile of the chocolate.

For instance, look at 2-Phenylethyl alcohol, number 9 on the list. This volatile aroma molecule is associated with a floral scent, and can be found in roses. Notice how drastically the amount of this changes from treatment to treatment. The first sample (0PS - 100 *C) where the pods were not stored and roasted at a lower temperature, the concentration is about 383 ng/g of cocoa. But in sample 2 (7PS - 100 *C) the concentration is only 98 ng/g of cocoa. The only difference between sample 1 and 2 is that the pods in sample 2 were stored for 7 days before fermentation, where in sample one the pods were not stored at all. Remember, all these chocolates were made from the same cacao. Now compare both of these to sample 9 (0PC - 160 *C) where the concentration is 1083 ng/g of cocoa. The only difference between all of these samples was how long they were stored before opening the pods, and the temperature of the roasting. Check out other aromas listed, google to see what aroma they represent, and have fun comparing the different concentrations.

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Table 3b below also depicts 69 aroma molecules identified in the 6 commercial liquors from the 6 different countries. Just as in table 3a, compare the various concentrations. In this table, you have the privilege of having a description in the far right column of what each aroma is associated with.

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The author also included data on odor activity values (OAV). Basically, these values indicate how important the specific aromas are to the overall flavour profile of the food. However, not to over complicate things here, I decided not to dwell on these findings for the purpose of this summary. You are free to look over this data in Tables 4 a and b. These values correspond with the concentrations in Tables 3 a and b.

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Analysis of The 6 Aroma Types Within The Chocolates as it relates to pod storage and roasting

1. Acids

Acids are formed during fermentation. In regards to pod storage, higher concentrations were found in chocolate made from unstored pods. It has been documented that a high amount of fruit during fermentation leads to higher acidity levels. Some farmers will squeeze excess fruit from the pulp before fermentation to reduce this acidity.

In this instance of pod storage, as the pods are held the fruit inside is reduced due to decomposition, which likely could be why acidity decreases with pod storage. However, prolonged pod storage saw an increase in acidity. This is suggested to be due to fermentation-like reactions occurring within the pod, raising acidity levels.

In regards to temperature, higher temperatures were insignificantly linked to higher acidity levels, except for and very high temperatures. Although it’s been said by many that roasting reduces acidity, here the authors suggest that roasting at higher temperatures may even “unlock” these acidic aromas from the cacao, making them more available. This could be why the acetic acid concentration was greater in chocolates with very high roasting temperatures.

The commercial chocolates all had relatively lower levels of volatile acids, with chocolate made from Ivorian and Venezuelan cacao being the lowest.

2. alcohols

Alcohol type volatiles can give chocolates their flowery and candy aroma notes. Similar to acids, the longer pods were stored, the less alcohols they contained, expect for pods stored at 7 days, for which there was a rise in alcohol volatile concentrations. Again, this is likely due to fermentation-like processes (which obviously result in alcohol production). Therefore, the authors suggest that using cacao with no pod storage is optimal for achieving greater concentrations of alcohols. That said, at very high roasting temperatures (160 *C or higher) high levels of alcohols can also be achieved. The commercial chocolates had relatively lower levels of alcohols present.

3. aldehydes and ketones

These are important for good cocoa flavour as stated by the author, and are formed from the Strecker degradation reaction. In regards to pod storage, no storage saw higher levels of these volatiles.

Roasting appeared to increase the levels of these volatiles, until about 140 *C or higher where there was a decrease, likely due to thermal degradation. Aprotosoaie et al. (2016) stated that higher temperatures decreased the content of aldehydes.

It appears that except for at a temperature of 160 *C, roasting temperature and pod storage had no significant impact on the concentration of ketones. At 160 *C, the highest concentration of ketones were observed. Ghanaian commercial chocolate contained the highest concentration of ketones.

4. pyrazines

Pyrazines are formed through heterocyclization reactions proceeding the Strecker degradation reaction during roasting. Pyrazines are key to chocolate’s distinct flavour, and are associated with aromas such as cocoa, chocolate, and nutty notes.

Pyrazines increased in concentration with prolonged pod storage, likely due to the pod storage facilitating the formation of more precursor molecules, which then formed more pyrazines during roasting.

Most pyrazines were formed during roasting via the Strecker reaction, except for 2 that were partly formed during fermentation, including Tetramethylpyrazine (cocoa, coffee roasted) and trimethylpyrazine (cocoa, roasted nuts, sweet, smoky). Tetramethylpyrazine was the most dominate pyrazine, and its highest concentration was found in the sample where pods were stored the longest and cacao was roasted at the highest temperature, however, its Odour Active Value (OAV) was low and therefore contributed very little to the overall flavour of the chocolate. This point highlights the complexity of aromas, where a high concentration doesn’t necessarily mean that aroma contributes a great deal to the overall flavour of the chocolate.

Some pyrazines appeared to be absent in the chocolate samples, likely due to roasting and refining procedures which eliminated them from the chocolate. Many pyrazines were absent in the commercial chocolate samples, but Ghanian and Vietnamese chocolates contained the highest pyrazine concentration among them.

5. esters, terpenes and terpenoids

Esters are associated with fruity and floral notes, and are said to be the second most important group of aroma compounds in cocoa nibs after pyrazines. Pods stored the longest and roasted the highest were observed to contain the highest concentration of most esters.

Most esters are microbial metabolites which are formed during the fermentation process, but the roasting temperatures and pod storage still appeared to influence their concentrations.

Of all terpenes and terpenoids, linalool (floral and sweet) was found in most samples. It appeared pod storage had no significant impact on total concentrations, and a slight increase in regards to roasting temperature. The commercial chocolates, except for Ghanaian chocolate, contailed low amounts of terpenes and terpenoids.

6. Furans, furanones, pyrans, pyrones, pyrroles and others

The concentrations of these aromas tended to increase with increased roasting temperatures. There appeared to be no correlation with pod storage and increased concentrations, except for at 7 days of pod storage at high roasting temperatures. Just as with acids and alcohols, the extra days of pod storage may have helped break down more of the precursor molecules necessary to build these aromas.

Of the commercial chocolates, Madagascan, Ecuadorian, Vietnamese and Ghanaian chocolates contained the most concentrations of these aromas.

It’s interesting to note that two unfavorable volatiles, 2-methoxyphenol (smoky) and dimethyldisulfide (sulfurous), are formed at very intense heating processes, and were odor-active with cacao roasted at 160 *C.

In this figure, you can see an overview of all 16 of the chocolates (10 experiments and 6 commercial chocolates) and the quantity of aromas found within them organized into aroma groups. You can see clearly that on the far right, most of the samples with higher roasting temperatures also have the highest number of aroma molecules. On the far left are commercial samples from various countries with a very limited array of aromas.

In this figure, you can see an overview of all 16 of the chocolates (10 experiments and 6 commercial chocolates) and the quantity of aromas found within them organized into aroma groups. You can see clearly that on the far right, most of the samples with higher roasting temperatures also have the highest number of aroma molecules. On the far left are commercial samples from various countries with a very limited array of aromas.

Comparing chocolates based on aroma profiles

The following two diagrams Agglomerative Hierarchical Clustering (AHC) was used to basically compare and contrast chocolates with similar profiles. If you start at the base of the diagram, you’ll see all 16 of the chocolates sampled in this study. Think of it as a phylogeny, where those chocolates grouped closer together shair the most traits. In this case, these chocolates closer together have the most in common in regards to aroma profile.

For instance, in the first diagram, You can see that on the right, all the samples with low roasting temperatures have the most in common with the commercial chocolates. If you remember from earlier when analyzing the aroma concentrations, we found that the commercial chocolates as well as chocolates made from cacao with low roasting temperatures didn’t have as high a concentration of most of the aromas.

In this diagram, the AHC clustered chocolates according to similar aroma volatiles identified, including chocolate produced from the experiment and commercial chocolates.

In this diagram, the AHC clustered chocolates according to similar aroma volatiles identified, including chocolate produced from the experiment and commercial chocolates.

In this diagram, the chocolates are clustered according to their odor activity values as stated in Table 4 a & b.

In this diagram, the chocolates are clustered according to their odor activity values as stated in Table 4 a & b.

Conclusions:

Pod Storage:

A decreasing effect was observed, where the longer the pod was stored, the lower the concentration of volatile aromas. However, with pods stored for around 7 days, there was an increase in volatile concentration, likely due to fermentation-like activity going on within the pod and making more aroma precursor molecules available for later processes. Chocolate made from pods stored for 3 days (as opposed to no storage or 7 days) recorded the lowest volatile concentrations.

Roasting:

Roasting appeared to have a greater impact than pod storage on the aroma concentrations. Chocolate made from cacao roasted between 133 - 160*C seemed to have consistently high levels of volatile aroma concentrations. The authors state that this is due to “unlocking” of aromas within the matrix of the cacao, or from forming new aroma molecules due to Maillard reactions during roasting.

Comparing Various Roasting & Pod Storage Treatments:

High Roasting Temperatures

Unstored Pods: Higher in alcohols (fruity/floral), aldehydes (cocoa/chocolate), and ketones (fruity/creamy).

Prolonged Pod Storage: Higher in pyrazines (cocoa/nutty/roasted), acids (sour), esters (fruity/floral/sweet), furans (roasted/cheesy/green), and pyrroles (caramel). But also higher levels of undesirable volatiles such as dimethyldisulfide (sulfurous) and 2-methoxyphenol (smoky).

Low Roasting Temperatures

Unstored Pods: High in alcohols (fruity/floral), acids (sour), and aldehydes (cocoa/chocolate).

Prolonged Pod Storage: Higher in pyrazines, and terpenes/terpenoids with more fruity/floral notes.

This study has shown that the same cacao can produce chocolates with varying concentrations of aromas due to how long the pods were stored before fermenting, and the roasting temperature of the post-fermented cacao. This idea offers valuable insight into ways to improve the aroma profile “bulk” cacao for those farmers who have no other choice but to grow this type of cacao.

There is no one approach to cacao and chocolate that will optimize its flavour and quality. The more we understand about the impact specific processing techniques have, the more control we can have over dictating the flavour of our chocolate and cacao. The more we can dictate where we want the flavour to go, the less we have to rely on trial and error to come to a delicious conclusion.