All three chemical tests indicate that the reactivity of sorghum
tannins increases through the milk and early-dough stage, peaks during
the mid- to late-dough stage and falls sharply at maturation. The
increases are attributed to tannin synthesis during seed development.
The decrease at maturation is similar to the behavior of tannins in
fruit. This has been attributed to the loss of alcohol solubility and
reactivity of large complex tannin molecules that develop during the
final ripening process in fruit. It is reflected by the decreases in
V/FD and V/LA ratios. This same pattern in sorghum helps us better
understand the numerous references in sorghum literature to "loss of
tannins during maturation." Tannins are not lost; they become inac-
tive with increased polymerization, the degree of which varies con-
siderably among varieties.

Biochemical tests: The biochemical reactivity of tannins depends on
the size and shape of the molecule. Astringency, tanning ability,
enzyme inhibition, and nutritional quality are all dependent on the
ability of the tannin molecule to form bonds with protein molecules.
Tannin molecules with molecular weights below 500 or above 3,000 are
too small or too large to bind with proteins. Four tests (enzyme
inhibition, protein binding, hemanalysis, and astringency) are good
indicators of this binding process and were used to evaluate the
biochemical reactivity of sorghums at different stages of maturity
(Table 15).

Table 15. Mean biochemical reactivity values for extracts from eight
 BR sorghum varieties in four stages of maturity.


 Enzyme Protein
 Stage of inhibition binding Hemanalysis Astringency
 maturity (%)a (TAE) (TAE)b (% preference)c

 Milk 19.88 0.81 0.83 38.24

 Light dough 23.78 1.05 0.58 34.17

 Firm dough 23.53 1.14 0.79 30.81

 Mature 11.98 1.21 0.24 37.47

 a Percent loss in enzyme activity caused by respective sorghum
 extract.
 b Tannic acid equivalent.

 c Percent consumption of treated (0.1-percent sorghum extract) hulled
 proso millet by five birds (Quelea quelea) during 6 days of exposure.