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

and bakery goods, upon losing moisture below an a w of 0.5 to 0.7, become unacceptably
hard (Kochhar and Rossel, 1982). Recrystallization phenomena of dry amorphous sugars
caused by reaching an a w of 0.35 - 0.4 affect texture and quality loss reaction rates, as
already mentioned.
Besides the specific critical a w limits, water activity has a pronounced effect on
chemical reactions. This effect plays a very important role in the preservation of IMF and
dry foods. Generally, the ability of water to act as a solvent, reaction medium and as a
reactant itself increases with increasing a w . As a result, many deteriorative reactions
increase exponentially in rate with increasing a w above the value corresponding to the
monolayer moisture. This can be represented schematically in a global food stability map
(Figure 7) .
Figure 7. Global Food Stability Map (adapted from Labuza et al., 1969).
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