RA 00048.pdf - OAR@ICRISAT

45
100
4 0
N o l y s i n e a d d e d
L y s i n e a d d e d
90
3 5
80
3 0
70
2 5
60
2 0
50
15
10
5
40
30
0
W h o l e
D e h u l l e d
20
IS 8 2 6 0
IS 8 2 6 0
10
Figure 2. Biological value of whole and dehulled
grain in high-tannin sorghum line IS
8260 when fed to weanling rats with
and without lysine supplementation in
a 14-day feeding experiment.
lysine. Nutritionally this binding leads to interesting
effects. Thus in rat trials Eggum and Christensen
(1975) found that the protein digestibility was
reduced but the biological value (BV), defined as
%N retained in the body of absorbed N , increased
due to moderate levels of tannins. This is due to
the selective binding of tannins to the low lysine
prolamines which then become unavailable thus
improving the amino acid composition of the
proteins which are left for the rat to digest and
absorb, finally resulting in an increase in BV.
It is very important to recognize the tannin
interaction with protein quality when assessing
the nutritional value of any grain sorghum variety,
since failure to consider the biological effects of
tannins has led to substantial confusion in past
studies of sorghum nutritional quality.
Future progress in research on tannins will
depend on a better understanding of the biochemistry
of sorghum tannins and their interaction
with seed proteins. Detoxification of sorghum
0
0 1 2 3 4
Tannin content
Figure 3. Effect of tannin content (expressed as
P r o t e i n Q u a l i t y
catechin equivalents) on the biological
value of sorghum grain in a 28-day
weanling rat feeding experiment.
tannins by alkali treatment seems clearly beneficial,
which will be especially useful in animal feed
utilization. Research on removal of tannins from
the outer layers of the sorghum grain by decortication
and milling will also provide an important
future solution to the problem for grains to be
utilized for human consumption (Munck et al.
1982).
Two genetic mutants, one naturally occurring and
one induced, have been identified that increase
the lysine content of the sorghum endosperm and
improve protein quality of the grain. A brief review
of the origin of these mutants, and recent results
of experiments on the relationship between im-
591