Ong, Law - Hygrothermal Properties of FVFssusceptibility of protein to heat denaturation (Desrosiers et al., 1987b). As the amount of
water absorbed by protein is reduced (aw < 0.5), the protein becomes more resistant to
denaturation because of electrostatic interactions that stabilize the protein. In contrary,
severe and irreversible denaturation occurs when protein is heated in between aw 0.5 to
0.97 due to the cross-linking between functional groups of amino acids side chains with
the presence of water as a solvent. Similar phenomenon occurs in proteins of fava bean
where denaturation temperatures of legumin and vicilin is in a function of water content
(Figure 2.4) and the denaturation temperature is higher at lower water content (< 0.9 g-
(^1) water g-1 protein). This could be due to insufficient water in the vicinity of protein to
bring about the thermal transition at the low water activity (Leung, 1987).
On the other hand, some studies on soybean meal show that high loss of lysine is
possible even for short drying if reducing sugar levels are high in the matrix (Labuza and
Tannenbaum, 1972). The loss of lysine is maximum when aw = 0.68 and negligible at aw =
0.4. Hence, it can be associated to non-enzymatic browning which is also optimum at the
medium aw range. Sun et al. (2002) reported that the protein solubility of freeze-dried
ground beef patties was the lowest at medium aw (0.33-0.66) at 49°C and it could be re-
lated to Maillard-type browning reactions. In both cases, protein amino groups react
with reducing sugars, leading to cross-linking and polymerization of proteins. Thus re-
sult in low protein extractability which in turn affects the availability of these amino ac-
ids biologically.
Figure 2.4. Effect of water content on denaturation temperature of legumin and vici-
lin (Arntfield et al., 1985)
2.2.4. Vitamins
Drying which involves simultaneous heat and moisture transfers can be a destruction
process for both water soluble and fat soluble vitamins. Water soluble vitamin C (ascor-
bic acid) is the most labile component among all the vitamins contained in foods. Since
ascorbic acid is soluble in water, it is readily lost via leaching from cut or bruised surfac-
es of food and also takes part in chemical degradation such as non-enzymatic browning