Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 22

E FFECTS OF S TRUCTURAL T RANSFORMATIONS ON S TABILITY
Collapse
Collapse in low-moisture foods may significantly affect effective diffusion coeffi-
cients, mainly as a result of the loss of porosity and formation of a dense structure.
Karmas et al. 76 related some of the differences in observed rates of the nonenzymatic
browning to collapse of the matrix. Collapse was also observed by Nelson 72 to affect
the rate of ascorbic acid degradation in maltodextrin matrices.
The formation of glassy carbohydrate matrices is of great importance in flavor
encapsulation and in the protection of emulsified lipids in food powders from
oxidation. 87 Flink 38 pointed out that the encapsulated compounds are stable as long
as the physical structure of the encapsulating matrix remains unaltered. Encapsulated
compounds may be released due to collapse, which results in loss of flavors and
exposure of lipids to oxygen. However, Flink 38 pointed out that collapsed structures
may hold encapsulated compounds and protect them from release because of the
high viscosity of a collapsed media. Labrousse et al. 88 found that an encapsulated
lipid was partially released during collapse, but during the collapse reencapsulation
occurred and assured stability of the lipid. However, differences in the rates of
diffusion within glassy carbohydrate matrices and supercooled, liquid-like, viscous
matrices have not been reported. Therefore, it may be difficult to evaluate various
effects of collapse on the release of encapsulated compounds or effects of diffusion
on quality changes in collapsed matrices.
Crystallization of Food Components
Crystallization of amorphous sugars is known to result in serious quality losses in
food powders. The crystallization of amorphous lactose in dehydrated milk products
has been observed to result in acceleration of the nonenzymatic browning reaction
as well as other deteriorative changes and caking. King 89 reported that crystallization
of lactose coincides with an increase in free fat, which presumably facilitates lipid
oxidation in milk powders. Crystallization of amorphous lactose also occurs in other
dairy powders, such as whey powder above a critical a w . 90 Lactose crystallization
in dairy powders results in increasing rates of non-enzymatic browning and loss of
lysine. 91,92 Saltmarch et al. 92 found that the rate of browning at 45°C increased rapidly
above 0.33 a w and showed a maximum between 0.44 and 0.53 a w . The rate maximum
for browning occurred at a lower a w than was found for other foods. Crystallization
of lactose occurs within the reported a w range and the a w which allows crystallization
decreases with increasing temperature. Saltmarch et al. 92 found that the rate maxi-
mum was coincident with extensive lactose crystallization which was observed from
scanning electron micrographs.
Crystallization in low-moisture carbohydrate matrices which contain encapsu-
lated volatiles or lipids results in a complete loss of flavor and release of lipids from
the matrix. Flink and Karel 93 studied the effect of crystallization on volatile retention
in amorphous lactose. Crystallization was observed from the loss of adsorbed water
at relative humidities, which allowed sufficient water adsorption to induce crystal-
lization. At low relative humidities, the encapsulated compound was retained at water