Flavor Development of Cocoa during Roasting - ACS Symposium

Chapter 28

Flavor Development of Cocoa during Roasting

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: January 15, 2000 | doi: 10.1021/bk-2000-0754.ch028

L . Hashim Laboratoire Biophysicochemie et Technologie Alimentaires, Université de Technologie de Compiègne, B.P. 20529, 60205 Compiègne, France

The desired flavor of cocoa results from the compositional changes that occur in the beans principally during roasting. The roasting process of cocoa beans is necessary to develope the final chocolate flavor. The flavor of cocoa is highly complex in character, and unfortunately not yet fully understood, due to the very large number of different substances present. Maillard reaction or non-enzymatique browing is one of the most important and complex reaction involving flavor development during the roasting process. Sugars and amino acids are the components that undergo the most significant changes during roasting and are suited for use as monitors of compositional changes occuring during the process. There are around 500 volatile compounds which have been already identified in cocoa aroma. The roasting process, heating the cocoa beans to 120-140°C during 20-40 minutes in general depending on cocoa bean type, is needed to develope the final flavor desired.

Chocolate manufacturers are very concerned with the sensory properties of cocoa beans when assessed as finished chocolate (1). Flavor in cocoa beans is influenced by several variables including the type of beans used and processing in chocolate factory. Cocoa beans are transformed into chocolate by a combination of many techniques such as fermentation, roasting and conching. The roasting technique is one of the most important factors in determining the organoleptique properties of cocoa beans and eventually chocolate. With chocolate the chemical complexity of flavor development is evident when one realizes the numerous parameters which influence its development (2). The controlles high-temperature heating step of roasting that magnifies the complex interaction between the flavor precursors and the results in chocolate flavor. Prior to roasting, the beans may taste astringent, bitter,

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In Caffeinated Beverages; Parliment, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.


277 acidy, musty or nutty. Roasting reduces the acidity as evidenced by the significant decrease in the concentrations of volatile acids, especially acetic acid (3; 4). The roasting process, normally heating the beans to 110-140°C for 20-40 minutes to develope the final flavor desired. The generation of flavor, the types of reaction and the influencing factors during roasting are very complex fields of study. The development of new techniques have given the chocolate manufacturer a wide choice of methods of carrying out cocoa beans roasting. It is essential for each manufacturer to use one which will produce the appropriate flavor. Cost is also important and considerable differences exist between the capital and running costs of the different procedures (5). During roasting, one of the most important and complex reactions involving cocoa flavor is called the non-enzymatic browning, Maillard browning or the carbonyl-amine reaction. These reactions involove two major precursors namely the free amino acids and reducing sugars which develop during fermentation (6; 7). Modern analytical techniques such as capillary gas chromatography and mass spectral data of the volatile aroma fraction have contributed to the understanding of chocolate flavor. There are about 500 compounds which have been already identified in chocolate. The precursors of cocoa flavor Work on cocoa aroma started many years ago. The characteristic of chocolate arises as a result of two principal stages in the processing of cocoa bean. The first is called fermentation, which is affected where the cocoa is grown. The second stage is accomplished in the factory, where the fermented beans are roasted. In the absence of either process, no cocoa aroma is observed. Roasted unfermented cocoa beans do not generate cocoa aroma but produce an excessively bitter and astringent taste instead (8). Investigations into the origine of the characteristic aroma of cocoa beans have essentially followed two different ways. The first sought to concentrate the volatile aroma from fermented and roasted cocoa beans and tried to characterize its components. The second was the indirect approach, which attempted to isolate from unroasted beans the compounds which gave the characteristic aroma when heated, the so called flavor precursors which have developed during the fermentaion of cocoa beans. Free amino acids (leucine, alanine, phenylalanine and tyrosine), peptides and reducing sugars have been shown to be precursors of cocoa flavor and undergo Maillard reactions to produce cocoa flavor during roasting (9; 10). During fermentation, free amino acids and peptides are formed by proteolytic enzymatiquc reactions, whereas the reducing sugars, such as, fructose and glucose are the products of invertase hydrolysis of sucrose (8). During the fermentation process, the biochemical process will result in the production of flavor precursors, which later will develop into cocoa-specific flavor and aroma upon roasting. During fermentation, the total amino acids concentrations showed an almost linear increase with increasing days of fermentation, up to day 4, after which the values remained relatively constant as the fermentation proceeded



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to day 6. At the end of the fermentation the concentration of leucine, phenylalanine, tyrosine, valine and isoleucine increased significantly (11). These amino acids accumulated during cocoa fermentation, contribute positively to the formation of cocoa flavor. In unfermented cocoa beans, sucrose is the only sugar present in significant concentration (more than 90% of the total sugars), fructose and glucose present in trace amounts. During fermentation, the sucrose decreased significantly. In contrast, fructose and glucose concentration increased. Reducing sugars and amino acids form addition compounds during Maillard reactions, which in turn form glycosylamines or fructosylamines, depending on the initial reducing sugar. One of the key reactions is the rearrangement of these glycosylamines into isomerization products. If glucose is the starting sugar in the reaction, it would be converted to aminated fructose. The intermediate reactions involove the previous compounds as well as sugar and amino acid degradation (Strecker). They are essential for the proper flavor of a cocoa product and involve interactions of numerous compounds. Odors from mixtures of leucine and glucose, threonine and glucose, and glutamine and glucose, when heated to 100°C, have been described as 'sweet chocolate', 'chocolate' and 'chocolate' respectively. Valine and glucose, upon heating to 180°C, have been described as 'penetrating chocolate'. A change in temperature elicits a change in description, when heated 20°C higher, the leucine-glucose mixture was described as 'moderate breadcrust'. The isomerization products formed during the initial phase are primarily addition compounds formed from amino acids and sugar. During the intermediate stage they are then dehydrated, fragmented and transaminated, forming complex compounds, depending on temperature and pH. On the acid side, generally hydroxymethylfurfural and other furfural products are formed. If the pH is neutral, the result of the reaction is reductones. In the end a host of compounds, depending on the substrates and the pH, will polymerize and in turn contribute to the final cocoa flavor. Some of the most important compounds are pyrazines, pyrroles, pyridines, imidazoles, thiazoles, and oxazoles. As important as the carbonyl-amine reactions are, one must realize that it is unlikley that a food system will consist of only amino acids and sugars. Other compounds such as peptides, proteins, vitamins, fats and their oxidation products and other derivatives can enter the reactions and influence the final product. With the many compounds found in chocolate, it is virtually impossible to identify all reactions and pathways needed to produce chocolate flavor (2). Roasting conditions The development of cocoa flavor, effective roasting temperatures are necessary depending on the origin of cocoa. The optimum roasting times applied industrially depend mainly on the heat transfer and the temperature gradient in the products. If the maximum temperature and time conditions are exceeded signs of over roasting become noticeable which diminish the



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quality of final product. The greatest problem in roasting cocoa beans is controlling a uniform temperature throughout the product. There are three different roasting procedures to produce cocoa mass from cocoa beans. The first is the roasting of whole cocoa beans which has several disadvantages, among them the size of the beans varing according to the source and the growth season. The degree of roasting which each receives will depend upon its size. Cocoa beans vary somewhat in size and shape with 1g being a representative weight (12). If the roasting time is optimized for medium-sized cocoa beans, the small beans are inevitably overheated as a result. This has an unfavourable effect on the taste of the cocoa liquer. In addition the large cocoa beans are not roasted sufficiently, which is also deterimental to the final chocolate. The temperature differences between the outer skin and the center of the bean that exist during roasting may be 12°C. This allows the conclusion to be drawn that in the normal course of events over roasting of the outer layer cannot be avoided while the inner parts of the beans may still contain raw components, resulting unequal roasting of the whole bean (13). The advantage of whole beans roasting however is that it loosens the shell making it easier to remove, in order to aid the winnowing of unroasted beans. The second is Nib roasting which has the advantage to remove the shell before beans roasting, the major problems of fat migration, off-flavors from burning impurities and extra energy use are overcome. In addition the particles being roasted are of a more uniforme size, resulting more uniforme roasting intensity of beans. The main disadvatage of this type of roasting procedure is that it is necessary to remove the shell from unroasted beans. The third is the Mass roasting, in this technique the main problems of bean roasting again are overcome, and a uniformity of heat treatement is ensured. The heat input is definitely more efficient during mass roasting in which the homogeneous mass is kept in constant mouvement instead of the slowly turned solid mass of beans or nibs (14). The beans could be roasted in thin layer, mass roasted in this way under optimum temperature conditions reaches a high flavor intensity without any of the defects arising out of over roasting. This could be explained by a negligibly small temperature gradient, good heat transfer. This procedure, however, requires both the winnowing and the grinding of raw beans. All these methods are currently in use and favoured by different cocoa processors. The quantity of reducing sugar and amino acids consumed during cocoa bean roasting is variable depending on the type of cocoa beans and timetemperature of the heat treatement. Figure 1 shows the results obtained for roasted cocoa beans (Ivory Coast) during 25 minutes at 110°C to 170°C (15). It is obvious that the quantity of reducing sugars consumed are in linear relation with the roasting temperature. At 140°C the totality of reducing sugars are already consumed in the Maillard reaction, and only 40% of amino acids are still non consumed. This would indicate that the addition of reducing sugars is efficient to improve or to change the flavor of the final product. It is now possible to control the process of cocoa roasting by adding reducing sugars, amino acids, or a mixture of both, depending on the characteristics of the original cocoa beans. Knowing the exact specification desired, and by testing the raw material, it is possible to control the Maillard



Figure 1. Influence of roasting temperature on the precursors of cocoa flavor


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reactions during the roasting of the cocoa beans, so the taste of the final product could be exactly tailored to demand. Figure 2 shows the process of producing cocoa mass from cocoa beans. Aroma compounds formed


The flavor of cocoa has been the subject for intense research during the last three decades. Many researches have been realized to review all volatile components in cocoa beens. There are almost 500 different components (15) divided over 20 different chemical classes. Table 1 : Number of identified compounds in cocoa Compound

Number

Hydrocarbons Alcohols Aldehydes ketones Esters Lactones Amines Pyrroles Pyridines Pyrazines Sulfurs Thiazoles Thiols Phenols Furans Oxazoles Acetals Ethers Halogens Nitriles Bases Acids

48 28 21 27 61 6 36 15 15 95 9 9 4 8 23 15 6 4 2 3 12 53

The principal components are : pyrazines which represent about 20% of the identified compounds, then the esters 15%, the acids and hydrocarbons 10%, the amines 8%, the alcohols and ketones 6%, the furans and aldehydes 4%. Roasting control Many working parties have studied the possibility to control cocoa beans roasting. The question is very complex, only because cocoa beans contain a


Figure 2




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large number of substances that have detected which contribute positively to the typical cocoa flavor. For example, the aldehyde has an important role in cocoa bean roasting. The evolution of isopentanai during bean roasting has shown that a concentration of this molecule equal to 60 ppm would give a normal roasting of the beans and a concentration equal to 70 ppm shows an over roasting of the beans (13). The total quantity of aldehydes could be used as indicator of cocoa roasting (15). Figure 3 shows the evolution of the total aldehyde during cocoa beans roasting. At 120°C the concentration of aldehydes was in its maximum. Their concentration slow down from 130°C. At this temperature the quantity of reducing sugar is practically non existent (Figure 1). That would suggest the importance of reducing sugars to develop aldehydes in cocoa beans during roasting. The 2,3-dihydro-3-hydroxy maltol was detected in roasted cocoa by HPLC technique (17). It was shown that, the concetration of this compound showed a linear correlation with respect to the extent of roasting and therefore it can be used as an indicator substance for the determination of the degree of cocoa beans roasting. The pyrazines principal components of cocoa beans flavor, and their ratios could be used as indicator for the degree of cocoa beans roasting. Their concentrations are in a linear correlation with the time-temperature cocoa bean roasting (13; 15; 18). Figure 4 shows the formation of certain methylpyrazines during cocoa beans roasting. The ratio between tetra-/2,5diand tetra-/tri- decreased at high temperature, over roasting of cocoa beans. Figure 5 shows the same observations when cocoa beans roasted at 150°C at different times. The recent application of the electronic nose to determin the degree of cocoa beans roasting has shown the possibility to predict the quality of cocoa beans and to monitor the cocoa aroma (19).

Figure 3 : Influence of roasting temperature on the quantity of total aldehyde



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Figure 4 : Formation of certain methylpyrazines in laboratory roasted cocoa beans (Ivory Coast) during 30 min.

Figure 5 : Formation of certain methylpyrazines in laboratory roasted cocoa beans (Ivory Coast) at 150°C.


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Flavor Development of Cocoa during Roasting - ACS Symposium

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