Chemical compounds, coffee flavor

Numerous reviews on coffee flavor have been published over the past few years (1, 2, 3 ). So far, more than 700 components have been characterized in roasted coffee. Therefore, approximatly 20 % of the 4000 chemicals reported in the "List of Volatile Compounds in Food" edited by TNO (4 ) may be consumed by drinking coffee. 

The major advantage of the seunpling technique developed, was that some trace chemicals could be trapped tind described for the first time as Black Truffle aroma constituents. In particular, some compounds, important flavor contributors, generally appearing in small concentrations, such as benzaldehyde, propanal, ethyl acetate, anisole or dimethyl disulfide - previously identified in Shiitake mushrooms (9) - could be characterized. This was also the case for three aromatic compounds, toluene, xylene and ethyl benzene, well known as raw vegetable constituents (1 ). In addition, two aliphatic esters, isopropyl and sec-butyl formates, and one cyclic sulfur compound (2-formyl thiophene) previously reported respectively in plums and apples (W) and in coffee and bread products (n) were identified. 

The authors observed that an exhaustive list of all the chemicals present in coffee flavor had not yet been compiled, but they believed they had identified the components that are present at the higher ratio of weight, and those which principally control the odor note. Most of the substances identified were well-known compounds present in other roasted products as well, for instance in caramel sugar, cocoa, baked bread and—partially—even in wood tar. However, some of the chemicals detected were new and, obviously, characteristic of roasted coffee. Traces of methyl mercaptan, which was already known at that time and which smells even worse, were also detected in coffee aroma. Commenting on this observation, Reichstein and Staudinger note that it is generally known that many popular raw materials and synthetic perfume compounds owe their characteristic note, which is extremely pleasant to the olfactory sense, to their content of small quantities in additives which carry a rather unpleasant odor in themselves but prove very attractive in thinned solutions and in admixture with other oils. The authors tried to reconstitute coffee aroma, and only by combining over 40 of the substances extracted from coffee... 

The extremely elusive nature of the chemical basis of Cheddar-like flavors supports the hypothesis that an unstable aroma compound is involved in the flavor of Cheddar cheese, and this view deserves thorough investigation. The circumstantial evidence in favor of the existence of a Cheddar-like aroma also includes considerations relating to the sensitivity of Cheddar flavor to heat and oxygen, and the fact that the redox of cheese is quite low. Additionally, sulfury or sulfide-like defects as well as brothy flavor-like defects are often encountered in Cheddar cheeses of various compositions and origin. These flavors could reflect either production of excessive amounts of certain sulfur compounds or the absence of certain essential compounds that are initially required to allow formation of a Cheddar compound. While attempts to date have not resulted in the isolation of such a compound, this could reflect the very unstable nature of the proposed compound. Other similar circumstances appear to occur in freshly roasted coffee and nuts where transient... 

Nishimura and Mihara (1990) investigated 2-hydroxy-2-cyclopenten-l-ones in a steam-distilled coffee extract and identified ten compounds of this class, seven of them being reported for the first time in a natural flavor and some even being original chemicals. Procedures used for their syntheses are described. All the 2-hydroxy-2-cyclopenten-l-ones were also identified in a model reaction, when sucrose, the major component of green coffee, was heated with alkali in aqueous solution. Some had been previously found in maple syrup or tobacco. We are reminded that Gianturco et al. (1963) mentioned for the first time the presence of these typical, sweet, caramel-like and burnt compounds in a roasted coffee extract. 

The thermal generation of flavor is a very essential process for the "taste" of many different foodstuffs, e.g. cocoa, coffee, bread, meat. The resulting aromas are formed through non-enzymatic reactions mainly with carbohydrates, lipids, amino acids (proteins), and vitamins under the influence of heat. Thiamin (vitamin B ) and the amino acids, cysteine and methionine, belong to those food constituents which act as flavor precursors in thermal reactions. The role of thiamin as a potent flavor precursor is related to its chemical structure which consists of a thiazole as well as a pyrimidine moiety. The thermal degradation of this heterocyclic constituent leads to very reactive intermediates which are able to react directly to highly odoriferous flavor compounds or with degradation products of amino acids or carbohydrates. 

However, because of the initial concern, researchers sought alternative methods for extracting caffeine from coffee beans. Extraction by CO2 at supercritical temperatures and pressures was found to be a better method because it extracts caffeine without simultaneously extracting some of the flavor compounds, as dichloromethane does. This was one of the first green (environmentally benign) commercial chemical processes to be developed. After the caffeine has been removed, the CO2 can be recycled, whereas dichloromethane is not a substance that should be released into the environment (Section 7.12). 

In addition, the chemical formation of aroma compounds for the synthesis of desired flavors and prevention of off-flavors is considered. Chapters 15, 21, 26, 33, and 34 discuss how light and heat can affect the formation of both desirable and undesirable flavors. Microorganisms and aging are key in off-flavor formation in dairy products, wine, and grain degradation. Potent flavor compounds are released during the processing of coffee. 

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