Hurdle Technology in Food Preservation - Minimally Processed Foods - page 22

2.4.2 Water Activity

W.J. Scott, an Australian scientist, proposed that the active water could be much
more important to the stability of a food than the total amount of water present.
The legacy of Scott allowed scientist to develop generalized rules or limits for the
stability of foods using water activity (Scott 1953 ). In general the rules of water
activity concept are (1) food products are most stable at their “BET-monolayer”
content or “BET-monolayer water activity” and unstable above or below BET-
monolayer; (2) there are a critical water activity limit for a specific micro-organism
or a class of micro-organism for their growth or toxin production, and biochemical
reactions (Scott 1953 ; Labuza et al. 1970 ). For example, there is a critical water
activity level below which no microorganisms can grow. Pathogenic bacteria cannot
grow below a water activity of 0.85, whereas yeasts and molds are more tolerant to
reduced water activity, but usually no growth occurs below a water activity of about
0.6. Labuza et al. ( 1972 ) proposed the food stability map based on the water activity
concept containing growth of micro-organisms and different types of bio-chemical
reactions. In the recent food stability map, Rahman ( 2009 ) showed the trends of
microbial growth, bio-chemical reactions and mechanical characteristics in the
three zones of water activity (zone 1: BET-monolayer, zone 2: adsorbed multi-layer,
zone 3: matrix or solvent water) (Fig. 6.5 ) . In fact, the BET-monolayer could be
only achieved in the cases of dried foods. The limitations of water activity concept
are reviewed by Rahman ( 2010 ). However, food industries are now widely used this
concept for determining the stability of their dried products.

2.4.3 Glass Transition

Considering the limitations of water activity concept, the hypothesis of glass transi-
tion concept was put forward. White and Cakebread ( 1966 ) first highlighted the
importance of the glassy state of foods in determining its structural stability. The
significant applications of the glass transition concept emerged in food processing
in the 1980s, when Levine and Slade ( 1986 ) and Slade and Levine ( 1988 ) identified
its major merits and wide applications. The rules of the glass transition concept are
(1) the food is most stable at and below its glass transition (i.e. T g or T g ′), and (2) the
higher the T-T g or T/T g (i.e. above glass transition), the higher the deterioration or
reaction rates (Levine and Slade 1986 ). Detailed Reviews on the food stability in
relation to glass transition, molecular relaxation and mobility are available in the
literature (Champion et al. 2000 ; Le Meste et al. 2002 ; Rahman 2006 , 2010 ). It is
clear from the literature that all experimental results could not be explained by the
above rules (Rahman 2009 ; Levine and Slade 1986 ), thus further developments are
necessary. The limitations of water activity and glass transition concepts would not
invalidate the concepts completely rather make it difficult to apply universally. The
water activity concept is based on the binding nature of water molecules in the matrix.