Slide 1

Ong, Law - Hygrothermal Properties of FVFs

Figure 2.5. Moisture dependence degradation rate constants of ascorbic acid and -

carotene in dehydrated sweet potato (Haralampu and Karel, 1983)

2.2.5. Microbial Growth and Enzymes Activity

Enzymatic reactions can occur in low moisture foods if the enzymes are not inacti-
vated by heating. Hence, reduction of water activity in final product is a very important
mean to ensure the stability of the dried foods. Final product with sufficient low water
activity is safe from enzymatic spoilage in general because active water is not available
for microbial growth (Chieh, 2006). Error! Reference source not found. shows the water
activity limits for the growth of some microorganisms and food examples within the
range. It can be seen that the growth of most harmful microorganisms is inhibited at wa-
ter activity below 0.60 or moisture content less than 10% in general. On the other hand,
the amount of water content and value of water activity have direct influence on the ac-
tivity of enzymes that is naturally present in the food system as well. Table 2.3shows
the minimum water activity for enzymatic reaction in selected foods and substrates
compiled by Drapron (1985). Generally, enzyme activity increases with increasing water
activity which also postulate increased subtract mobility. Water activity threshold for
enzyme activity may vary with food composition and substrates structure. Leung (1987)
pointed out that substrates of high molecular weight, such as starch and protein, are less
mobile than low molecular weight substrates, such as glucose, and thus generally have a
higher water activity threshold. In addition, enzyme activity still can be traced even at
very low water activity for non-aqueous substrates, such as the lipases in olive oil, tri-
laurin and triolein, because water is not needed to provide mobility of these oil liquid
substrates.

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

aw

Effective first order rate constant (1/hr)

β-carotene

Ascorbic acid