the handbook of food engineering practice crc press chapter 10 ...

or food systems. Enzymatic and microbial activity exhibits an optimum pH range and
limits above and below which activity ceases, much like the response to temperature (Figure
6). The functionality and solubility of proteins depend strongly on pH, with the solubility
usually being at a minimum near the isoelectric point (Cheftel et al.,1985), having a direct
effect on their behavior in reactions.
Examples of important acid-base catalyzed reactions are nonenzymatic
browning and aspartame decomposition. Nonenzymatic browning of proteins shows a
minimum near pH=3-4 and high rates in the near neutral-alkaline range (Feeney et al.,
1975; Feeney and Whitaker, 1982). Aspartame degradation is reported at a minimum at
pH=4.5 (Holmer, 1984), although the buffering capacity of the system and the specific
ions present have significant effect (Tsoumbeli and Labuza, 1991). Unfortunately very few
studies consider the interaction between pH and other factors e.g temperature.
Such
studies (Bell and Labuza, 1991and1994; Weismann et al., 1993) show the significance of
these interactions and the need for such information for the design and optimization of real
systems. Significant progress in elucidating and modeling the combined effect to
microbial growth of factors such as T, pH, a w or salt concentation has been achieved in the
field of predictive microbiology ( Ross and McMeekin, 1994; Rosso et al., 1995)
Gas composition also affects certain quality loss reactions. Oxygen affects
both the rate and apparent order of oxidative reactions, based on its presence in limiting or
excess amounts (Labuza, 1971). Exclusion or limitation of O 2 by nitrogen flushing or
vacuum packaging reduces redox potential and slows down undesirable reactions.
Further, the presence and relative amount of other gases, especially carbon dioxide, and
secondly ethylene and CO, strongly affects biological and microbial reactions in fresh
meat, fruit and vegetables. The mode of action of CO 2 is partly connected to surface
acidification (Parkin and Brown, 1982) but additional mechanisms, not clearly established,
are in action . Quantitative modeling of the combined effect on microbial growth of
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