Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 5

1.0

Amylopectin

Staphylococcus aureus Staphylococcus aureus

Lactose hydrolyzed skim milk

0.8

0.6

Microbial Growth
Enzyme Activity Loss of Crispness
Browning
Sugar Crystallization
Caking

Microbial Growth
Enzyme Activity Loss of Crispness
Textural Changes

Browning
Sugar Crystallization
Caking

0.4

Hardening

0.2

BET Monolayer
Oxidation

BET Monolayer
Oxidation

0

0 10 20 30

WATER CONTENT (g/100g of Solids)

FIGURE 1.1 Critical water activity and water content ranges for various changes and
microbial growth occurring in food materials. The sorption isotherms of lactose hydrolyzed
skim milk and amylopectin are shown as examples of extreme values for water activities and
water contents.

foods and some novel fruit products were classified as consumed as is novel foods.
Examples of traditional and novel IMF products consumed after rehydration were
jellies, meat-filled pasta, and condensed milk and soup, sauce, and meal concentrates,
respectively. The traditional and novel IMF products consumed after dehydration
included some fruit cakes/pies/puddings and pop-tarts, respectively.
Although microbial spoilage is prevented at a w below 0.60, 2 low-moisture foods
may exhibit deleterious changes, such as structural transformations, enzymatic
changes, browning, and oxidation, depending on a w , temperature, and, therefore,
extent of water plasticization. As shown in Figure 1.1 , critical a w values can be
defined for various changes and microbial growth resulting in loss of stability. 25
However, the critical values are specific for each food material and they may be
dependent on food composition and plasticization behavior.

W ATER S ORPTION

Water activity of high-moisture foods and several IMF products is relatively constant
and dependent on composition, especially solids content, and the type of water
soluble components. However, the a w of low-moisture foods and many IMF products
is dependent on storage relative humidity and temperature. Steady state relationships
between a w and water content at a constant temperature are described by sorption
isotherms. Typical sorption isotherms of food materials are sigmoid curves, which
exhibit hysteresis between the adsorption and desorption isotherm, as shown in
Figure 1.2 . Determination of sorption isotherms is necessary for the determination
of stability at various storage conditions and requirements for packaging materials
to ensure product shelf life. 25
The most common method to obtain sorption isotherms is the determination of
steady state water contents for food materials at constant relative humidity and
temperature conditions, e.g., equilibration of samples over saturated salt solutions
in vacuum desiccators. 5 Prediction of water sorption isotherms is then based on the