Water in Dairy Products - Advanced Dairy Chemistry - page 455

þ ln ð g w X w Þ

ln a w ¼ V w
RT

P 2s
r

(8)

With food products, a w is more commonly considered to be controlled
by the solutes only, probably because capillary and pressure effects are
quantitatively small; they should not be ignored, however.

11.2.2. Relevance

It must be remembered that the definition of a w and the derived expres-
sions assume that the system is in a state of thermodynamic equilibrium. It is
sensible to question the validity of their application to food products (ISO-
POW, 2000).
Actually, solutions of small solutes, even if concentrated, can be con-
sidered to be at thermodynamic equilibrium, whereby the a w concept is valid.
Microbial cells cannot be considered to behave as true osmometers; other
parameters, such as pH, nature of the solutes in the medium and mobility of
the metabolites, must be taken into account. Water activity, however, is an
essential parameter for all aspects of their activity: germination and growth,
production of toxins and of aroma. a w is therefore an important parameter of
processing and storage to obtain the desired organoleptic properties and
safety standard of the product especially for cheeses (Figure 11.2). Some
studies have demonstrated a relationship between molecular mobility and
some aspects of microbiological activity (Kou et al., 1999). However, from an
extensive review (Chirife and Buera, 1996) of the then available knowledge, it
was concluded that, although mobility factors, in addition to a w , may be
useful for a better prediction of microbial behaviour in foods, glass transition
concepts do not provide any better alternatives than a w as a predictor for this.
The texture of cheese was found to be better correlated to water content
(coefficient of correlation with an extrusion force ¼ -0.867) and fat content,
than with a w (coefficient of correlation ¼ -0.548) (Ruegg, 1985). Water
activity, however, will control the exchanges of water between cheese and
the environment and is thus an important parameter controlling texture.
For solid low-moisture or semi-moist food products, being closer to
their glass transition, it must be recognized that what is measured is the
relative humidity of the atmosphere in contact with the product, at best in a
pseudo-equilibrium state. The term ''apparent water activity'' should be used
(ISOPOW, 2000). As will be discussed later, the current sorption models
provide no reliable information about the physical state and properties of
water. From a practical point of view, however, the a w concept is very useful,
because it allows description of the gradient that will determine the transfer of