AsA is a water soluble vitamin that is widely used as an indicator
of food quality changes during fruit and vegetable processing
(Negi and Roy, 2000; Alibas et al., 2007; Chuah et al., 2008; Orikasa
et al., 2008). Fig. 6 shows the residual ratio of AsA in a kiwifruit
sample after drying. The residual ratio of AsA in the kiwifruit samples
during hot air drying and vacuum drying was 0.75–0.88 and
0.90–0.99, respectively. The residual ratio of L-ascorbic acid after
vacuum drying is higher than that after hot air drying at all tested
temperatures. The AsA in fruits and vegetables was decomposed by
an enzyme or by non-enzyme oxidation, and the decomposition of
AsA was accelerated more at higher temperatures (Miura and
Araki, 1988). The enzymatic decomposition of AsA was caused by
ascorbic oxidase, and the non-enzymatic decomposition was
caused by oxygen in the atmosphere. During the vacuum drying
process (3.00 kPa), a small amount of atmospheric oxygen might
inhibit the non-enzymatic decomposition of AsA in the sample.
Vacuum drying could be a more desirable drying method for protecting
the AsA in the kiwifruit sample. Because the AsA content
was maintained at a high level during vacuum drying, only the
decomposition of AsA during the hot air drying process is discussed
in the following theoretical analysis.