Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 4

time-dependent phenomena affecting shelf life and quality of low- and intermediate-
moisture foods.

WATER ACTIVITY

The purest forms of water in foods are crystalline ice and gaseous vapor. Depression
of the vapor pressure of water by solutes is probably one of the most important
factors that affect the properties of water in foods. It can be shown that water activity,
a w , is the ratio of the vapor pressure in a solution or a food material, p w , and that of
pure water at the same temperature, p 0 , (Eq. 1.1). Therefore, the equilibrium or
steady state a w is related to equilibrium relative humidity (ERH) of the surrounding
atmosphere by Eq. (1.2), and a w can be considered to be a temperature-dependent
property of water which is used to characterize the equilibrium or steady state of
water within a food material.

p w
p 0

a w = ------ (1.1)

ERH = a w × 100 % (1.2)

Water activity, temperature, and pH are the most important factors that control
rates of deteriorative changes and the growth of microorganisms in foods. 21 These
parameters are often referred to as hurdles . 22 Other important hurdles include redox
potential, modified atmosphere, oxygen tension, pressure, radiation, competitive
flora, microstructure, and preservatives. 22 Reduction of a w often affects microbial
growth, the predominant microbial culture, and it increases shelf life as a result of
the reduced availability of water for the microbial growth. According to Hocking
and Christian, 23 a w and pH, alone or in combination, often determine whether foods
are subject to bacterial or fungal spoilage.

W ATER A CTIVITY OF F OODS

Most fresh foods can be considered as high-moisture foods and their shelf life is
largely controlled by the growth of microorganisms. High-moisture foods have an
a w of 0.90 to 0.999 and they usually contain more than 50% w/w water. These foods
include fresh meat and seafood, various dairy products, and fruits and vegetables as
well as beverages. Most bacteria, molds, and yeasts are likely to grow in high-
moisture foods. However, the types of spoilage microorganisms and their species
are highly dependent on both a w and pH 23 as well as other hurdles.
Intermediate moisture foods (IMF) have an a w of 0.60 to 0.90 and the water
content is 10 to 50%. These foods include many traditional low-moisture foods, such
as grains, nuts, and dehydrated fruits, but also a number of processed foods.
Brimelow 24 classified IMF products into those consumed as is , those consumed after
rehydration , and those consumed after dehydration . All of these categories had exam-
ples of traditional and novel foods. Traditional as is consumed products included
salted, cured meats, salted fish, Parma ham, dried fruits, some cheeses, and jams. Pet