Produce Degradation Pathways and Prevention

328 Produce Degradation: Reaction Pathways and their Prevention

Water activity in fruit or vegetable juices or purees is also reduced by concen-
tration, which is also used to preconcentrate foods prior drying or other processing
to reduce the volume. Liquid food can be concentrated by evaporation, reverse
osmosis, or freeze concentration. In spite of the development of evaporators and
optimization of the process, evaporation causes changes in nutritional and sensory
properties of concentrated food because of the severe heat treatment. Reverse osmo-
sis and freeze concentration, however, are less severe and allow achievement of a
higher degree of concentration.
The addition of solutes such as sugar or salt is the principal technological
operation in the production of fruit jams, spreads, toppings, and syrups. After
concentration, these products are classified as intermediate moisture food and their
stability must be achieved in combination with additional treatments (Kyzlink, 1990).

TABLE 10.13

Importance of Water Activity in Food

aw Phenomenon Degree of Protection Required

1.00

0.95 Pseudomonads, bacillus, Clostridium

perfringens, and some yeasts inhibited

Intermediate-moisture foods

(aw = 0.90–0.55)

Other preservation treatments/other

hurdles are necessary

0.90 Lower limit for bacterial growth (general),

salmonella, Vibrio parahemolyticus,

Clostridium botulinum, lactobacillus, and

some yeasts and fungi inhibited

0.85 Many yeasts inhibited

0.80 Lower limit for enzyme activity and growth of

most fungi; Staphlococcus aureus inhibited

0.75 Lower limit for halophilic bacteria

0.70 Lower limit for growth of most xerophilic fungi

0.65 Maximum velocity of Maillard reactions

0.60 Lower limit for growth of osmophilic or

xerophilic yeasts and fungi

Minimum protection or no packaging

required; depending the storage

conditions, if the relative humidity is

higher than the corresponding water

activity of the product the prevention of

water uptake is necessary

0.55 Lower limit for life

0.50

0.40 Minimum oxidation velocity

0.30

0.25 Maximum resistance of bacterial spores

0.20

Sources: Adapted from Bibek, R., Fundamental Food Microbiology, CRC Press, Boca Raton, FL,

1996, Section 6; Lund, B.M., in The Microbiological Safety and Quality of Food, Vol. 1, Aspen

Publishing, Gaithersburg, MD, 2000, p. 122; Archer, D.L., Int. J. Food Microbiol., 90, 127, 2004;

and Li Bing, J. Food Eng., 54, 175, 2002.