Biomedical Engineering Reference
In-Depth Information
Most bacteria, some yeasts,
pathogenic and spoilage organisms
Fresh Meat, Fish, Vegetables etc.
Foods with <40% (w/w) sucrose or
<7% (w/w) NaCl
Bread, Cooked Sausages,
Medium Aged Cheese
a
w
>0.95
Most cocci, lactobacilli, some molds,
acid bacteria is major spoilage flora
Salmonella,
lactic
0.91 - 0.95
Most yeasts, mycotoxin-producing molds,
spoilage often by molds and yeasts
Salami, Old Cheese,
Foods with 65% (w/w) sucrose or
15% (w/w) NaCl
0.87 - 0.90
Staphylococcus aureus
may grow
>0.86
Dried Beef, Sweet Condensed Milk,
Cereals with 15% Water
Most molds. No growth of pathogenic
bacteria
0.80 - 0.87
Jam, Marmelade,
Old Salami,
Foods with 26% (w/w) NaCl
Flour, Cereals, Nuts
Most halophilic bacteria
0.75 - 0.80
Xerophilic molds
0.65 - 0.75
Caramels, Honey, Toffee
Osmophilic yeasts
0.60 - 0.65
No
growth
Breakfast Cereals, Snack Foods,
Food Powders
<0.65
0.4
0.5
0.6
0.7
0.8
1.0
0.9
WATER ACTIVITY
FIGURE 1.12
Minimum water activity, a
w
, ranges for the growth of microorganisms in
foods and examples of various food materials having a
w
within the minimum range.
microbial growth in food materials which may also be affected by phase separation
and heterogeneity of the system.
C
ONTROL OF
S
TABILITY BY
W
ATER
A
CTIVITY AND
C
OMPOSITION
Knowledge of the physical state and physicochemical properties of food components
is advantageous in food design and formulation. The information on factors affecting
rates of various kinetic processes can be used to manipulate and control rates of
changes that occur during food processing and to develop food products that are
less sensitive to detrimental changes during storage. It is obvious that the main
factors that control stability of low-moisture foods are a
w
and composition. Low-
moisture foods are considered to be stable when they are stored in cool and dry
conditions. An increase in temperature or water content may result in a significant
change in the rates of deteriorative changes. Knowledge of the effects of temperature
and water on the physical state and diffusion in amorphous food matrices may be
used to establish relationships between food composition and storage conditions.
Traditional shelf life predictions of low-moisture foods have been based on the
information on rates of deteriorative changes and loss of nutrients at various tem-
peratures and water contents. The main assumption for stability is often that water
contents close to the BET monolayer value allow maximum stability. An increase
in water content at a constant storage temperature results in rapid deterioration as
reaction rates increase at intermediate water contents. It may be assumed that the
rates of a number of deteriorative changes in low-moisture foods are affected by
diffusional limitations. Reaction rates in the solid, glassy state at low water contents
approach zero, but an increase in temperature or in water content probably increases
diffusion and therefore the reaction rate. It is probable that at temperatures above
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