Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 19

content at a constant temperature. The change in the physical state due to glass
transition or other phase transitions has been considered in only a few studies. These
studies have revealed that reaction rates are increased above glass transition, but
some reactions have occurred also at temperatures below T g . 72,74
The relative rate of deteriorative changes is traditionally related to water content
and a w with the assumption that stability of low-moisture foods can be maintained
at water contents below the BET monolayer value. 4,30,77 Duckworth 78 used wide-line
nuclear magnetic resonance spectroscopy to determine “mobilization points” for
solutes at a constant temperature in low-moisture food matrices. The mobilization
point was found to be peculiar to the system and the level of hydration needed to
achieve mobility was solute-dependent. Duckworth 78 also found that no solute mobi-
lization occurred below the BET monolayer value, and results for a system that
contained reactants of the nonenzymatic browning reaction suggested that browning
initiated at the mobilization point. An increase in the reaction rate was apparent with
increasing a w and the rate maximum occurred at the a w corresponding to the hydration
level allowing complete mobilization. Extensive water contents resulted in dilution
and reaction rates were reduced. Molecular mobility in low-moisture foods is obvi-
ously important in defining rates at which reactants may diffuse within the solid
matrix. According to Duckworth, 78 mobilization of solutes required sufficient
amounts of hydrated reactants. The theory assumed that food materials with water
contents less than the BET monolayer value were composed of a hydrated matrix
with undissolved reactants and reactions did not occur. Above the monolayer value,
some of the reactants were dissolved, which allowed mobility in a saturated solution
and an increasing rate with increasing water content. However, Duckworth 78 did not
consider that the systems studied were amorphous and the solutes were in the
dissolved, but solid, state. It may be assumed that the mobility of the reactants at
low water contents was restricted by the high viscosity of the glassy state. Reaction
rates at low water contents but at a constant temperature may be considered to be
restricted by diffusional limitations, 18 as the transport of the reactants as well as
transport of the products become the rate controlling factors.
It is likely that most of the nonfat solids in low-moisture foods are amorphous
and therefore mobility may be achieved by plasticization. 79 Such plasticization may
result from an increase in temperature (thermal plasticization) or addition of plasti-
cizers such as glycerol or water (water plasticization). Roozen et al. 80 used ESR to
study molecular motions of dissolved probes in malto-oligosaccharides and malto-
dextrins with various water contents as a function of temperature. The rotational
motions were detected from rotational correlation time,

τ c which was related to the
rotational diffusion coefficient. Roozen et al. 80 found that τ c , decreased linearly with

increasing temperature at temperatures below T g , suggesting that temperature-depen-
dent molecular motions occurred in the glassy state. However, a dramatic decrease
of the rotational correlation time occurred over the T g temperature range, which
indicated the dramatic effect of T g on molecular mobility. Roozen et al. 80 also noticed
the decrease of the temperature at which the change in molecular mobility occurred
with increasing water content as a result of water plasticization. The dramatic
increase of molecular mobility above, but in the vicinity of T g , obviously has an
effect on rates of quality changes in low-moisture foods.