Ong, Law - Hygrothermal Properties of FVFs
(Tannenbaum et al., 1985). It is rapidly destroyed by heat when in certain pH range and
by oxidation. Compilation of ascorbic acid decay rate as a function of aw for various food
systems done by Labuza and Tannenbaum (1972) shows that ascorbic acid destruction
rate increases with water activity and temperature. The loss of ascorbic acid significant-
ly increases at higher water content in general but the destruction rates of the organic
acid, somehow, do not fall at the same values for different foods at similar water activity
value. The authors suggested that it could be due to various interactions with other
components or diffusion limitation in each unique food system. Leung (1987) pointed
out that reaction rates of vitamin A, B1 and B2 increase with increasing aw (0.24-0.65) as
well, however, the B vitamins are more stable than vitamins A and C at various aw values.
Fat soluble vitamin such as β-carotene (pro-vitamin A) exhibits the highest stability at
intermediate water activity (Haralampu and Karel, 1983). Notable loss of fat soluble vi-
tamins in drying is probably due to interaction of peroxides of free radicals with the vi-
tamins where the peroxides and radicals are produced in the oxidation of lipids (Labuza
and Tannenbaum, 1972). Hence, retention of vitamin A and tocopherol can be increased
through minimizing lipid oxidation. The milder the heating process, the greater the re-
tention of the fat soluble vitamins. Furthermore, destruction rate of carotenoids can be
reduced when increasing the moisture content, similar trend occurs to lipid oxidation
(Chou and Breene, 1972; Labuza and Tannenbaum, 1972). However, increasing the
moisture level increases the destruction rate for other vitamins. The followings are
some examples adopted from selected literature to depict the phenomena abovemen-
tioned. It can be seen from Figure 2.5 that the degradation rate constants of ascorbic
acid and carotene in dehydrated sweet potato depend very much on the water activity of
the sample. While the degradation rate of ascorbic acid is decreasing with water activity,
the degradation rate of carotene increases at low water activity. On the other hand, Fig-
ure 2.6 and Figure 2.7 show the typical temperature dependence degradation kinetics
of ascorbic acid and β-carotene, respectively, where both vitamins degrade faster at in-
creased temperature. Nevertheless,
Figure 2. 8 illustrates that the moisture content may influence the degradation kinetics
of the ascorbic acid and lycopene as well other the temperature factor and the degrada-
tion trends are dissimilar between two different phytochemicals. Therefore, it is impor-
tant to know the destruction rates of interested vitamins in sample as a function of tem-
perature, aw and the food composition, in order to predict the losses more accurately
during drying.