Deep Fat Fried Flavor

FIGURE 5.7 Proposed mechanism for the formation of volatiles from heated animal fats. (From Ohnishi, S., T. Shibamoto, J. Agric. Food Chem., 32(5), p. 987,1984. With permission.) 

A second distinguishing factor of thermal oxidations is their more random nature than typical room temperature oxidations. Very high temperatures make more sites available on the fatty acid for oxidation to occur. Thus a wider range in end products (volatile flavor components) will occur. So even though the same chemical mechanisms are involved in flavor formation in deep fat fried foods, the flavor developed is unique to this process. 

Chang et al. [73] and Nawar et al. [74] have identified many of the volatiles formed during deep fat frying. They have found numerous acids, alcohols, aldehydes, hydrocarbons, ketones, lactones, esters, aromatics, and a few miscellaneous compounds (e.g., pentylfuran and 1,4-dioxane) as products of deep fat frying. More recently, Wagner and Grosch [75] have studied the key contributors to French fry aroma. The list of key aroma compounds in French fries includes 2-ethyl-3,5-dimethylpyrazine, 3-ethyl-2,5-dimethylpyrazine, 2,3-diethyl-5-methylpyrazine, 3-isobutyl-2-methoxypyrazine, (E,Z), (E,E)-2,4-decadienal, trans-4,5-epoxy-(E)-2-decenal, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, methylpropanal, 2- and 3-methylbutanal, and methanethiol. If one examines this list, it is obvious that the Maillard reaction (pyrazines, branched chain aldeydes, furanones, and methional), and lipid oxidation (nnsaturated aldehydes) are the primary sources of this characteristic aroma. 

---

J. Pokomy, Flavor Chemistry of Deep Fat Frying, Flavor Chemistry of Lipid Foods, American Oil Chemists Society Publication, Champaign, Illinois, 1989. 

Chin-chin is crisp, slightly sweet, golden brown knots of pastry. These deep fat fried cakes are made in different shapes and styles. Chin-chin is often flavored with vanilla extract, nutmeg, caraway seed, orange or lemon rind. The African panel preferred the chin-chin with no substitution for all characteristics except appearance. The non-African panel preferred chin-chin without any substitutions for all characteristics. The soybean flour was the most desirable substitution at 30% of the all purpose flour. 

Heterocyclic compounds are primarily formed through non-enzymatic browning reactions. Recent studies of deep-fat fried food flavors led to the identification of pyrazines, pyridines, thiazole, oxazoles and cyclic polysulfides which had long-chain alkyl substitutions on the heterocyclic ring. The involvement of lipid or lipid decomposition products in the formation of these compounds could account for the long-chain alkyl substitutions. Model systems were used to study the participation of lipids in the formation of pyrazines, pyridines, thiophenes and cyclic polysulfides. 

Alkylpyrazines have been recognized as important trace flavor components of a large number of cooked, roasted, toasted and deep-fat fried foods (3). As a rule, alkylpyrazines have a roasted nut-like odor and flavor. Formation pathways for alkylpyrazines have been proposed by numerous researchers (4, 5, 6). Model studies suggest that they are minor products of the Maillard reaction. 

In some recent research on flavor formation during deep-fat frying at Rutgers University, a number of heterocyclic compounds with long-chain alkyl substituents were found the volatiles of fried chicken (15) and fried potato (16). These included pyridines, thiazoles, oxazoles, trithiolanes and a pyrazine. Only the involvement of lipids or lipid degradation products in the formation of... 

This paper discusses the formation and aroma characteristics of selected classes of heterocyclic compounds important to the flavor of foods, especially deep-fat fried foods. 

Hexanal and 2,4-decadienal are the primary oxidation products of linoleate. The autoxidation of linoleate generates 9- and 13-hydroperoxides of linoleate. Cleavage of 13-hydroperoxide will lead to hexanal and breakdown of 9-hydroperoxide will lead to 2,4-decadienal (9). Subsequent moisture-mediated retro-aldol reaction of 2,4-decadienal will produce 2-octenal, hexanal, and acetaldehyde (10). 2,4-Deca-dienal is known to be one of the most important flavor contributors to deep-fat fried foods (11). 

The oxidation products of lipids include volatile aldehydes and acids. Therefore, lipids are one of the major sources of flavors in foods. For example, much of the desirable flavors of vegetables such as tomatoes, cucumbers, mushrooms, and peas (Ho and Chen, 1994) fresh fish (Hsieh and Kinsella, 1989), fish oil (Hu and Pan, 2000) and cooked shrimp (Kuo and Pan, 1991 Kuo et al., 1994), as well as many deep-fat fried foods such as French-fried potatoes (Salinas et al., 1994) and fried chicken (Shi and Ho, 1994), are contributed by lipid oxidation. LOX-catalyzed lipid oxidation produces secondary derivatives, e.g., tetradecatrienone, which is a key compound of shrimp (Kuo and Pan, 1991). The major difference between the flavors of chicken broth and beef broth is the abundance of 2,4-decadienal and y-dodeca-lactone in chicken broth (Shi and Ho, 1994). Both compounds are well-known lipid oxidation products. A total of 193 compounds has been reported in the flavor of chicken. Forty-one of them are lipid-derived aldehydes. 

Tempeh, or tempe in some literature, is made by fermenting dehulled and cooked soybeans with mold, Rhizopus sp. Freshly prepared tempeh is a cake-like product, covered and penetrated completely with white mycelium, and has a clean, yeasty odor. When sliced and deep-fat fried, it has a nutty flavor, pleasant aroma, and crunchy texture, serving as a main dish or meat substitute. In recent years, tempeh has been found to provide some health benefits, including antimicrobial and antioxidant effects and protection against diarrhea (Hachmeister Fung, 1993 Nout Kiers, 2005). 

Figure 9. Effects of browning reaction products for the autoxidation of potato chips at 50°C. Potatoes were sliced with deep-fat-fried in soybean oil with our without browning reaction products. Symbols are o — o, POV and o — o, CV of soybean oil in potato chips (control), — , POV and -—, CV of soybean oil browning reaction products (10%) in potato chips. Arrows indicate the development of oxidized flavor.

Volatile Flavor Components of Deep Fat-Fried Soybeans. J. Agric. Food Chem. 

---