Although heterocyclic compounds are present only in minute amounts in foods, they constitute the most important character impact compounds because of their low threshold values. They are extremely important as compounding ingredients and in the development of new flavors. Almost half of the 5000 flavor compounds identified till now are heterocyclic in nature (16).
Furans are mainly associated with caramel-like, sweet, fruity, nutty, meaty, and burnt odor impressions. Because of their olfactory properties, many furans are commercially important flavoring chemicals. Furans are formed in food by thermal degradation of carbohydrates and by the Maillard reaction. They are almost present in all food aromas and essential oils. The most abundant furans are 2- and 2,5-disubstituted, such as 2-methylfuran, which is found in coffee aroma. The 2-substituted furans with aldehyde, ketone, or alcohol functional groups generally have fruity aromas with the mild flavor of caramel when added in small amounts to nonalcoholic beverages and ice creams. 3-Acetyl-2,5-dimethylfuran is used in imitation nut flavors (Fig. 13).
Thiophenes can significantly contribute to the sensory properties of foods, but they are not as numerous as furans; they have been detected in boiled and canned beef, cooked chicken, asparagus, leeks roasted peanuts, popcorn, rice, bread, and coffee. Of the 46 sulfur-containing compounds identified in pressure-cooked lean ground beef, 20 were thiophenes. The most frequently found thiophenes are the parent compound itself, 2-alkylated, 2- and 3-acylated derivatives; they are described as being pungent and green-sweet. 2-Acetyl-3-methylthiophene (Fig. 14) is described in a patent as imparting a honeylike flavor to syrup bases, and 3-acetyl-2,5-dimethylthiophene is recommended in another patent for improving the aroma of tobacco and perfumes.
Pyrroles are among the most widespread heterocyclic compounds found in food flavors such as coffee, roasted peanuts, popcorn, and tobacco smoke. Few pyrroles have also ch3
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