3.7. Ascorbic acid content
Table 4 shows the effects of pretreatment and drying temperature
on the ascorbic acid content of sweet potato flours. The
ascorbic acid content of sweet potato flours ranged from 14.84
to 24.41 mg/100 g, which was similar to previous reports (Huang
et al., 2006). USF and UF showed higher retention of ascorbic
acid content than PSF and PF. The peel and NaHSO3 solution
may act as a shield against heat and oxidation. It is well known
that ascorbic acid is relatively unstable to heat, oxygen and light.
Drying temperature had a detrimental effect on the retention of
ascorbic acid since heated air inherently exposes the products to
oxidation, reducing their ascorbic acid content. Ascorbic acid decreased
with increasing drying temperature for all samples. However,
PSF and USF had higher ascorbic acid content than PF and
UF. This investigation demonstrates that sweet potato slices immersed
in NaHSO3 solution, before drying, retain more ascorbic
acid. The results may indicate that the immersion solution prevents
chemical deterioration (oxidation) by turning ascorbic acid
into dehydroascorbic acid.
3.8. Microstructure
Figs. 1 and 2 show the scanning electron micrographs of peeled
and unpeeled sweet potato slices and flour prepared after pretreatment
and at different drying temperatures. The granules from the
dried slices of PF and UF (Fig. 1a–c and g–i) were more pronounced
than those of PSF and USF slices (Fig. 1d–f and j–l). On the other
hand, PSF and USF granules (Fig. 2d–f and j–l) were more aggregated
and disrupted than those of PF and UF (Fig. 2a–c and g–i).
This variation might be attributed to the internal modification of
starch granules through the action of NaHSO3 during processing.
These results are in agreement with Hoover and Perera (1999)
who reported that potato starch granules are affected by treatments
such as NaOH and Na2SO4 Wootton and Manatsathit
(1984) reported that, under alkali conditions, the inter- and intra-
molecular hydrogen bonds of the starch chain can be destroyed,
thereby weakening the granular structure. The fusion of
granules were higher in PSF and USF (Fig. 2d–f and j–l) than PF
and UF (Fig. 2a–c and g–i). This could be attributed to the introduction
of hydrophilic groups to the starch molecules, which resulted
in increase of hydrogen bonding (Singh, Chawla, & Singh, 2004).
The PSF and USF starch granules swelled more than PF and UF
due to interaction between amylase chains. Svihus, Uhlen, and
Harstad (2005) reported that the swelling is accompanied by a loss
of polysaccharide, due to the amylase, from the granule structure.
It was seen that at higher temperature PSF and USF granules
(Fig. 2e–f and k–l) could be increased by the hydrogen bonding
than PF and UF (Fig. 2b–c and h–i) which leads starch to fusion
granules.
4. Conclusions
The effects of pretreatment and drying temperature on the
physicochemical and nutritional characteristics of sweet potato
flours were investigated. These results showed the effect of
NaHSO3 treatment on the quality characteristics of sweet potato
flour as compared to untreated samples. Sweet potato flour could
be used to enhance the quality of food products such as colour,
flavour, natural sweetness, and supplemented nutrients.
Therefore, treated flour could be used to make a higher quality
product that is more attractive to product developers and
consumers.