Effect of Different Bran, Germ and Endosperm Composition of White and Brown Sorghum Varieties on Iron-Binding Polyphenols and Phytate: Implications to Iron Bioavailability During Fermentation
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Date
2013-06-06
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Addis Ababa University
Abstract
The improvement of sorghum varieties for beneficial agricultural traits might affect their
nutritional and processing properties. The nutritional profile of agriculturally improved varieties
(low tannin and high tannin) and the effect of traditional decortication and injera fermentation on
nutritional characteristics and iron bioavailability were investigated. Low- (Meko) and hightannin
(Seredo) sorghum varieties were collected and characterized for their grain characteristics
and nutritional profile. Household decortication was characterized and its effect on nutrient
retention and mineral chelating agents was evaluated. Further experiments were conducted by
extending and shortening of the decortication time and its effect on nutritional profile was
investigated. The effect of fermentation on mineral chelating compounds was evaluated. Iron
bioavailability was estimated using phytate to iron molar ratio and Hallberg and Hulthén’s
algorithm. Seredo has slightly higher pericarp thickness than Meko, but had lower seed size and
nearly the same germ proportion. The iron-binding polyphenols content of Seredo was six to nine
times higher than that of Meko (P< 0.05). Decortication had numerous effects on nutrient
retention. Significant losses in iron-binding polyphenols were observed for both varieties (P<
0.05). However, the losses of iron-binding polyphenols were affected by extraction rates. Seredo
had higher loss of phytate and tannin than Meko. Decortication led to iron losses of 27.3% in
Meko and 23.7% in Seredo. Despite the loss of chelating compounds; the phytate to iron molar
ratio was above 1. Further reduction in chelating compounds was observed during fermentation.
Fermentation led to significant losses of iron-binding polyphenols for Seredo (P< 0.05) but not
for Meko. Losses of tannin and phytate were also observed. However, the observed phytate
degradation was not enough to improve the phytate to iron molar ratio to values that are likely to
improve iron bioavailability (phytate/Fe< 1). Nevertheless, iron bioavailability estimated using
Hallberg and Hulthén’s algorithm predicted two times more fractional iron absorption in Meko
than in Seredo. The present study has shown that retention of nutrients during decortication was
dependent on variety. Furthermore, the degradation of phytate achieved during fermentation did
not result in significant improvements in iron bioavailability estimates. However, combining
decortication and fermentation showed the possibility of reducing the iron-binding phenolics
content of the high tannin variety to the extent of the low tannin one. This suggests the need for
improving the efficiency of phytate and polyphenols degradation during fermentation of dough
for injera preparation.
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Keywords
Sorghum, Traditional Decortication, Fermentation, Iron, Phytate, Iron-Binding, Polyphenols, Bioavailability