Browning nonenzymatic

The off-flavors due to the Maillard reaction are most typically labeled as being stale in character. Stale is one of the more nebulous terms used in sensory evaluation. Stale is a rather nondescript term denoting a lack of fresh character. Benzothiazole and 0-aminoacetophenone are two compounds believed to be formed via Maillard browning, which are responsible for this stale flavor. These compounds were found in stale, dry milk by Parks et al. [88]. 0-aminoacetophenone along with some furans were also found to be partially responsible for the gluey flavor of old casein [89]. There is also some work in this area related to fruit juices. 

Fruit juices generally develop undesirable color changes and a stale flavor during storage [90]. While off-flavors due to the Maillard reaction are quite conunon, few studies other than those noted have been successful in identifying the components responsible for staleness. 

Some quite unique off-flavors may be produced via photo-catalyzed reactions in foods. Skunkiness in beer and burnt feathers in dairy products are two of the more unique examples of off-flavors in foods due to light exposure. 

Amino acid degradation is catalyzed by light and riboflavin as is shown  

While all amino acids undergo this degradation, the extremely low sensory thresholds of compounds arising from the sulfur-containing amino acids (particularly methionine) are believed to be most responsible for this defect. Patton [92] and Allen and 

---

Amino acids play a role in food processing in the development of a cooked flavor as the result of a chemical reaction called the nonenzymatic browning reaction (228). 

Manzocco L, Calligaris S, Mastrocola D, Nicoli MC and Lerici CR. 2001. Review of nonenzymatic browning and antioxidant capacity in processed foods. Trends Food Sci Technol 11 340-346. 

Lievonen, S.M. and Roos, Y.H. 2002a. Nonenzymatic browning in amorphous food models Effect of glass transition and water. J. Food Sci. 67, 2100-2106. 

Schebor, C., Buera, M.P., Karel, M., and Chirife, J. 1999. Color formation due to nonenzymatic browning in amorphous, glassy, anhydrous, model systems. Food Chem. 65, 427-432. 

Karmas, R., Buera, M.P., and Karel, M. 1992. Effect of glass transition on rates of nonenzymatic browning in food systems. J. Agricult. Food Chem. 40, 873-879. 

Dyer DG, Blackledge JA, Thorpe SR and Baynes JW (1991b) Formation of pento-sidine during nonenzymatic browning of proteins by glucose identification of glucose and other carbohydrates as possible precursors of pentosidine in vivo. J Biol Ghem 266,11654-11660. 

As part of extensive studies lasting over 30 years on the structures of chromophores involved in nonenzymatic browning reactions, two intensely orange, previously unknown, compounds have been identified (2R,8aR)-l and ZS, 8aR)-4-(2-furyl)-7-[(2-furyl)methylidene]-2-hydroxy-2//,7//,8a//-pyrano[2,3-3]pyran-3-one <1998CAR215>. Additional studies on the single Maillard reaction products of these compounds have also been reported <1998JFA3912>. 

Dworschak, E. 1980. Nonenzymatic browning and its effect on protein nutrition. CRC Crit. Rev. Food Sci. Nutr. 12, 1-40. 

Ferretti, A., Flanagan, V. P. and Ruth, J. M. 1970. Nonenzymatic browning in a lactose-casein model system. J. Agri. Food Chem. 18, 13-18. 

Two other broad areas of food preservation have been studied with the objective of developing predictive models. Enz.yme inactivation by heal has been subjected to mathematical modeling in a manner similar to microbial inactivation. Chemical deterioration mechanisms have been studied lo allow the prediction of shelf life, particularly the shelf life of foods susceptible to nonenzymatic browning and lipid oxidation. 

Kato, H. Chemistry of nonenzymatic browning phenomena. "Shokuhin no Henshoku to Sono Kagaku (Food Discoloration and Its Chemistry)" Nakabayashi, T., Kimura, S. and Kato, H., Eds. Korin Shoin Tokyo, 1967 pp. 223-89. 

Many studies of nonenzymatic browning have been carried out using model systems of monosaccharides and amino acids. Given the ease of release of arabinose from the pentosan, there is the possibility that Maillard reactions could take place if the temperature is appropriate. 

In this article, we will review studies on both nonenzymatic glycosylation and nonenzymatic browning in vivo. Structural, functional, analytical, and kinetic aspects will be discussed. Clinical aspects have been reviewed recently elsewhere (12,13,14) and will therefore not be considered in this chapter. 

Although nonenzymatic glycosylation may affect practically every protein in vivo, it is likely that nonenzymatic browning will occur only in proteins that have a slow turnover or none at all, such as lens crystallins, collagen, elastin and proteoglycans. In some tissues, these proteins are, in effect, "stored" for a lifetime and undergo some characteristic changes, many of which have been observed in stored and processed foodstuffs (Table II). 

The potential for the occurrence of nonenzymatic browning in collagen is supported by the recent demonstration by Schnider and... 

Hidalgo, F. J., and Zamora, R. (2000). The role of lipids in nonenzymatic browning. Grasa... 

Our study also investigated the effect of water activity (a ) on the kinetics of the formation of pyrazines. water activity is defined as the ratio of partial pressure of water in a food to the vapor pressure of pure water at a given temperature. Nonfat dry milk (NFEM) was chosen as a model system for this study since NFEM and lactose/casein systems which had undergone nonenzymatic browning were found to contain pyrazines (21. 22). The current study investigates the effect of increasing product over the range of 0.32 to 0.85 on the rate of formation of pyrazines. 

S. M. Lievonen, T. J. Laaksonen, and Y. H. Roos, Nonenzymatic browning in food models in the vicinity of the glass transition effects of fructose, glucose, and xylose as reducing sugar, J. Agric. Food Chem., 2002, 50, 7034-7041. 

---