Effect of water activity

Increase in water activity has an opposite effects on activity and stability of whole cells. Both for R. eiythropolis and X. autotrophicus an increase in dehalogenation was noticed on raising water activity [14, 15]. Interestingly, a minimum water activity of 0.4 was required in both cases to observe hydrolysis of 1-chlorobutane, while Dravis et al. [47] showed that the isolated haloalkane dehalogenase of R. 

The same behavior has already been observed with whole dehydrated cells of Saccharomyces cerevisiae [11] or immobilized ADH in the gas phase [45]. It seems also that the cellular matrix increases the need for hydration to perform catalysis. 

In our case, raising water activity logically increases the rate of hydrolysis. This can simply be explained by the fact that water is one of the substrates, but it could also be partly due to an increase of molecular mobility with the increase of hydration. 

For both bacteria, the increase in activity was concomitant with the decrease in the catalyst s stability. At high temperatures, this phenomenon can be explained by thermal denaturation. At low temperatures (lower than 40 °C for R. erythropolis and 30 °C for X. autotrophicus) the drop in activity could be attributed to the accumulation of acid, more rapidly produced at high water activity. 

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Bell, L.N. and Hageman, M J. 1994. Differentiating between the effects of water activity and glass transition dependent mobility on a solid state chemical reaction Aspartame degradation. J. Agric. 

In order to study the effects of water activity on an enzymatic reaction, there is a need for practical methods to adjust the water activity in the reaction mixture. Likewise, it is highly desirable to keep this parameter close to the optimal value during large-scale conversions. A range of water activity control methods have been developed [11], and which one to choose depends on the scale of reaction, the quantities of water to be removed or added, and availability of equipment. 

When water molecules interact with an enzyme, it is natural that conformational changes can occur, which in turn can cause changes in the selectivity of the enzyme. Since enantioselectivity of enzymes is of major importance for many applications, it is a common task to investigate how to choose reaction conditions providing the maximal enantioselectivity. As might be expected, because water can interact with enzymes in many ways, it is difficult to generalize the effects. In some studies of lipase-catalyzed esterification reactions, no effects of water activity on enantioselectivity were observed [30]. In a similar study, no effects were observed in most cases, while the enantioselectivity of one lipase-catalyzed reaction decreased... 

In addition to its direct influence via the water activity in the system, the amount of water also influences the activity coefficients of the other components in the mixture, and therefore equiUbrium constants like K0 can vary with the water activity in the system (Table 1.5) [29, 63, 64]. This can be seen as a solvent effect on the equilibrium constant The tendency in esterification reactions is that increases with decreasing water activity, which means that it is favorable to use low water activity because of both the direct effect of water activity on the equilibrium and the influence of water on K0. 

Figure .6 (a) Effect of water activity on the relative dehalogenase activity of dehydrated R. erythropolis cells maintained at 40°C (D)or (b) X. autotrophicus cells maintained at 30°C ( ) in a solid/gas reactor. The maximal dehalogenase activity obtained with 0.8 water... 

Effect of Water Activity. A preliminary study was done to determine the a at which encapsulated orange peel oil was the most stable to oxidation. Figure 1 summarizes the results of this study. The formation of the limonene oxidation product, limonene oxide, was the slowest for the powder adjusted over Mg(NO3)2 (a 0.536). While the levels of oxidation product do not follow in exact order of a, it is evident that better storage stability correlates with a higher a of the powder. This relationship was not anticipated. Literature on lipid oxidation (2, 2) indicates that there is an optimum a for product... 

Figure 1. Effect of water activity on shelf life of encapsulated orange peel oil.

Figure A2.2.1 Water activity stability map (adapted form Labuza, 1970). A representation of a typical sorption isotherm for food materials and of the effects of water activity on the relative reaction rates of several chemical processes, as well as the growth of microorganisms, in foods are shown.

R. Bovara, G. Carrea, G. Ottolina, and S. Riva, Effects of water activity on Vm3X and Km of lipase catalyzed transesterification in organic media, Biotechnol. Lett. 

Michor, H. Marr, R. Gamse, T. Enzymatic Catalysis in Supercritical Carbon Dioxide Effect of Water Activity. High Pressure Chem. Eng. 1996a, 12, 115-120. 

Table IV summarizes the effects of water activity. In this system, there does not appear to be the usual biphasic dependence on water activity. Fluorescence production rises monotonically with water activity to saturation. In a freeze-dried immobilized system, Eichner (30) has found a similar dependence for initial rate of reducing power development.

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. 

The effect of water activity on the rate of formation of pyrazines was investigated in NFEM systems over the aw range of 0.32 to 0.84. Rates of formation were found to increase with aw, up to aw 0.75. A linear relationship was found to exist for the logarithms of the rate of formation of these pyrazines versus aw. 

Most enzymes are inactive when the water activity falls below 0.85. Such enzymes include amylases, phenoloxidases, and peroxidases. However, lipases may remain active at values as low as 0.3 or even 0.1 (Loncin et al. 1968). Acker (1969) provided examples of the effect of water activity on some enzymic reactions. A mixture of ground barley and lecithin was stored at different water activities, and the rates of hydrolysis were greatly influenced by the value of a (Figure 1-27). When the lower a values were changed to 0.70 after 48 days of... 

The effect of water activity on oxidation of fats is complex. Storage of freeze-dried and dehydrated foods at moisture levels above... 

Labuza, T.P. 1980. The effect of water activity on reaction kinetics of food deterioration. Food Technol. 34, no. 4 36-41,59. 

Figure 8-45 The Three Regions of the Sorption Isotherm Related to the Textural Properties of Food Systems. Source Reprinted with permission from M.C. Bourne, Effects of Water Activity on Textural Properties of Food, in Water Activity Theory and Applications to Food, L.B. Rockland and L.R. Beuchat, eds., p. 76,1987, by courtesy of Marcel Dekker, Inc.

Figure 8-46 Relationship Between Water Activity and Crispness of Popcorn. Source. Reprinted with permission from E.E. Katz and T.P. Labuza, Effect of Water Activity on the Sensory Crispness and Mechanical Deformation of Snack Food Properties, J. Food Sci., Vol. 46, p. 403, 1981, Institute of Food Technologists.

Bourne, M.C. 1987. Effects of water activity on textural properties of food. In Water activity Theory and applications to food, ed. L.B. Rockland and L.R. Beuchat. New York Marcel Dekker. 

Katz, E.E., and T.P. Labuza. 1981. Effect of water activity on the sensory crispness and mechanical deformation of snack food properties. J. Food Sci. 46 403-409. 

Figure 10.18. Top Typical force-displacement curves of agglomerated instant coffee when dry and after moisture sorption. Bottom. The effect of water activity on the stiffness and brittleness of agglomerated instant coffee. Notice the sharp drop of both at about the same water activity. From Gerhard et al. (1998).

The viscoelastic properties of polymers make them valuable for suppression of sound and vibration. A comprehensive, useful understanding of the viscoelastic damping inherent in these systems can come only from studies of mechanical properties over wide ranges of time (frequency) and temperature. If materials are moisture sensitive, the effects of water activity also should be determined. A similar rule holds for plasticizers and solvents. 

Ishibashi, N., Tatematsu, T., Shimura, S., Tomita, M., and Okonogi, S., Effect of water activity on the viabiltiy of freeze-dried bifidobacteria and lactic acid bacteria, in I.I.E.-I.I.R-Commission Cl, Tokyo, Japan — 1985/1,227-32 (1985). 

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