Biomedical Engineering Reference
In-Depth Information
It should be noticed that proteins in foods are also plasticized by water. Protein
denaturation occurs in the presence of water, but interestingly, although addition of
polyhydroxy compounds tends to decrease the denaturation temperature, an increase
in the denaturation temperature of globular proteins has been found to occur with
increasing T g of the polyhydroxy additive. 86
Microbial growth requires a minimum a w , in addition to optimal pH, temperature,
and other factors that may influence the growth of microorganisms. 21 Water activity
is considered as one of various hurdles that can be varied to provide stability and
safety in foods. 22,100 The minimum requirement for microbial growth is a w 0.62 which
allows growth of xerophilic yeasts. 1,101 An increasing a w allows the growth of molds,
other yeast, and finally bacteria at high water activities. The most important a w value
for the safety of food materials is probably 0.86 which allows the growth of Sta-
phylococcusaureus , 1,2,102 a well-known pathogen. Minimum a w values for the growth
of microorganisms are given in Table 1.1 . It should be noticed that microorganisms
may also have maximum a w values above which their growth is declined.
Microbial stability is often the most important criterion in food preservation.
The a w limits for growth of various microorganisms ( Figure 1.12 ), although being
slightly dependent on growth media, are well established and successfully used in
food development and manufacturing. 1,104 Gould and Christian 104 recognized the
possible secondary influence of high viscosity and diffusional factors on the growth
of microorganisms. Slade and Levine 6 have emphasized the effects of water dynam-
ics, which are based on the theories of food polymer science, on the growth of
microorganisms and criticized the use of the a w concept in predicting microbial
stability. Slade and Levine 6 suggested that germination of mold spores is a mechan-
ical relaxation process that is governed by the translational mobility of water. The
effect of glass transition on the heat resistance of bacterial spores was studied by
Sapru and Labuza. 103 They found that the inactivation of spores followed the WLF
relationship, which fitted to the data above T g better than the Arrhenius relationship.
Sapru and Labuza 103 also found that the heat resistance of bacterial spores increased
with increasing T g of the spores. These findings emphasize the importance of water
plasticization for microbial growth and heat inactivation. However, the growth of
microorganisms has been observed to occur within glassy food materials 101 and other
factors in food formulation, including a w and pH, should be considered in addition
to the physical state for the increase of microbial stability.
Chirife and Buera 3,101 showed that dehydrated fruits and vegetables have large
T- T g values over the a w range of microbial growth and the physical state has little
influence on observed microbial growth. Using wheat flour as an example, they
demonstrated that growth of microorganisms may occur even at conditions which
support the glassy state. However, it should be remembered that a w is a property of
a water-solute system while glass transition of the same system is usually measured
as the behavior of the water-plasticized solids. Obviously, the possible effect of glass
transition on the growth of microorganisms in foods remains questionable. Rigorous
studies are needed to establish possible relationships between a w , physical state, and
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