Water activity, browning, nonenzymatic

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. 

T. P. Labuza and M. Saltmarch, The nonenzymatic browning reaction as affected by water in foods, in Water Activity Influences on Food Quality, L. B. Rockland and G. F. Stewart (eds), Academic Press, New York, 1981, 605-650. 

Over time, diagrams were developed relating water activity with enzyme activity, dormancy of stored seed, loss of dry product crispness by moisture absorption, pigments and vitamins degradation, nonenzymatic browning, and fat oxidation. Response curves generally are not linear, and readers working with food or feed formulations are referred to the technical literature about their products. 

Roos (1995) has used a combined sorption isotherm and state diagram to obtain critical water activity and water content values that result in depressing Tg to below ambient temperature (Figure 1-25). This type of plot can be used to evaluate the stability of low-moisture foods under different storage conditions. When the Tg is decreased to below ambient temperature, molecules are mobilized because of plasticization and reaction rates increase because of increased diffusion, which in turn may lead to deterioration. Roos and Himberg (1994) and Roos et al. (1996) have described how glass transition temperatures influence nonenzymatic browning in model systems. This deteriorative reaction... 

There are various chemical reactions that proceed and may be accelerated at low values of water activity. Maillard reactions leading to lysine loss and brown color develop peaks at aw values around 0.5-0.8. Nonenzymatic lipid oxidation increases rapidly below aw = 0.4. Enzymic hydrolysis decreases with water activity to aw= 0.3, after which, it is negligible. 

Nonenzymatic browning (NEB) results from a series of reactions starting with an amino-carbonyl condensation. It represents an important cause of quality loss in foods, and has also been reported as a cause of damage of pharmaceutical products (Kumar and Banker, 1994). Its mechanism and kinetics have been extensively studied in real foods and in model systems. These studies have shown that the rate of the NEB reaction is strongly dependent on the material composition, temperature, water content, water activity, and pH (Hodge, 1953 Labuza et al., 1970 Labuza and Baisier, 1992). 

Monsalve et al. [236] studied the degradation of dehydroascorbic acid and chlorogenic acid, at pH 6, and at variable water activities and temperatures in model systems containing cellulose, dehydroascorbic acid and/or chlorogenic acid. They found that at 45°C nonenzymatic browning of dehydroascorbic acid-cellulose model system increased linearly through a certain time and then it... 

The presence or activity of water in foods may also enhance the rate at which deteriorative chemical reactions occur. Some products may become rancid through free radical oxidation even at low humidities and thus become unacceptable. Labile nutrients such as vitamins and natural color compounds are oxidized more rapidly when stored at low moisture levels. Enzyme-mediated hydrolytic reactions may reduce the quality of the food product. Other reactions such as the Maillard type of nonenzymatic browning may be enhanced by the presence of higher levels of water. On the other hand, water content is crucial for the textural characteristics and the sensory perception of foods. A food may be found unacceptable by consumers simply because it does not satisfy their textural (sensory) anticipation. 

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