Biomedical Engineering Reference
In-Depth Information
Oxidation of
Nonencapsulated
Lipids
Molecular mobility
Mechanical changes
Crystallization
Enzymatic activity
Release of encapsulated
-Flavors
-Lipids
Diffusion
controlled
reactions
Critical a w
0
1
WATER ACTIVITY
FIGURE 1.13 A food stability “map” showing the effect of water activity on various time-
dependent changes and rates of diffusion-controlled reactions as well as the growth of micro-
organisms in food materials.
Knowledge of material properties is extremely useful in the production of encapsu-
lated flavors, extruded products, confectionery, development of totally new products,
such as dehydrated enzymes or starters, and avoiding quality changes that may result
from mechanical changes, e.g., loss of crispness and recrystallization phenomena.
The temperature-, water content-, and time-dependent changes, which have been
problems in manufacturing and storage of food powders and other low-moisture
foods, can be reduced by avoiding exceeding their critical values based on the T g
determination or by compositional adjustments that provide sufficiently high values
for critical a w and T g . The most important applications of producing high quality
dehydrated foods include reduced collapse and improved flavor retention in dehy-
dration processes. The kinetics of enzyme activity are important to food quality and
applying the knowledge in food industry may allow the design of improved products
with extended shelf lives or even improved retention of activity, e.g., in enzyme
preparations. The applicability of the various kinetic models (Arrhenius and WLF)
should be tested with more data on reaction kinetics. Establishing relationships
between the physical state and kinetics of quality changes in relation to T g and other
relaxations even in the glassy state are particularly important.
Relevant research needs to be undertaken to establish clear relationships between
changes observed by various techniques and to evaluate them in terms of a w and
molecular mobility. A combined use of knowledge of both a w and water plasticization
is advantageous. The a w concept is a more traditional approach to understand quality
changes in foods. Water activity has been successfully used in setting limits for microbial
stability in foods and qualitative characterization of relative rates of deteriorative reac-
tions in low-moisture foods. The combined use of a w and glass transition theory has a
high practical applicability as it provides criteria for critical a w and, together with
sorption isotherm, for water content at a constant temperature. Water activity and glass
transition alone do not explain why various changes in texture or rates of deteriorative
 
 
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