4. They precipitate gelatin from solution and form insoluble compounds
w^ith gelatin-yielding tissues, a property which enables them to
convert hide into leather.
5. They are precipitated from solution by many metallic salts such as
copper or lead acetates, or stannous chloride.
Astringent qualities due to tannins. The astringent properties give
a slightly bitter taste to some foods, the degree depending on the amount
of tannins present. Thatcher states that "Tannins are of frequent occur-
rence in green fruits imparting to them their characteristic taste. They
nearly always disappear as the fruit ripens." Two explanations are sug-
gested for the disappearance of the tannins in ripening fruit. The anthocyan
pigments may be derived from the tannins, so that as the fruit colors the
tannins disappear ; or the tannins may be changed into an insoluble form
and therefore are not so apparent to the taste.
Persimmons contain a very large amount of tannin. Green persimmons
have a bitter, astringent, puckery taste, that is not easily forgotten, once
they have been tasted. Gore's work has shown that most of the tannin of
persimmons is enclosed in cells, which he terms giant tannin cells. By arti-
ficial processing in carbon dioxide, or by ripening, the membrane containing
the tannins becomes hard and insoluble so that the astringent taste is not
evident.
Blue or purple discoloration in English walnuts. English walnuts
are more astringent than other nuts and often develop color changes that
are unattractive when combined with other foods. When they are combined
with apples in a salad they often develop a blackish blue or purple color.
This may come from having prepared the apples with an iron knife, the
acid and enzyme of the fruit acting on the iron to produce ferric salts.
When this small amount of ferric salt comes in contact with the tannin of
the nut and particularly with the skin the purplish color develops. In nut
bread, English walnuts often produce a dark color.
Tannins may produce a gray shade in sugar. Zerban has found
that the color of dark greenish cane juice, which produces a sugar with
a gray shade, may be due to tannins, oxidizing enzymes, and iron. When
the cane is crushed the acid of the juice forms some ferrous salts with the
iron of the roller. Oxidizing enzymes of the juice oxidize the ferrous salts
to ferric ones and they combine with the tannins of the juice. A similar
change occurs when fruits are pared with iron knives and especially when
the juice is left to stand on the knife and it darkens.
Tannins and discolorations in canned goods. Kohman has reported
that the discoloration, a darkening in color, often found in spots in canned
ENZYMES 123
sweet potatoes is due to tannins. The tannin combines with the iron of the
can to produce the dark color. Oxygen is necessary to oxidize the ferrous
iron to ferric, so that, unless oxygen is found in the can, either through not
exhausting all of the air in processing or to a leak in the can, the dis-
coloration does not occur.
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4. They precipitate gelatin from solution and form insoluble compounds
w^ith gelatin-yielding tissues, a property which enables them to
convert hide into leather.
5. They are precipitated from solution by many metallic salts such as
copper or lead acetates, or stannous chloride.
Astringent qualities due to tannins. The astringent properties give
a slightly bitter taste to some foods, the degree depending on the amount
of tannins present. Thatcher states that "Tannins are of frequent occur-
rence in green fruits imparting to them their characteristic taste. They
nearly always disappear as the fruit ripens." Two explanations are sug-
gested for the disappearance of the tannins in ripening fruit. The anthocyan
pigments may be derived from the tannins, so that as the fruit colors the
tannins disappear ; or the tannins may be changed into an insoluble form
and therefore are not so apparent to the taste.
Persimmons contain a very large amount of tannin. Green persimmons
have a bitter, astringent, puckery taste, that is not easily forgotten, once
they have been tasted. Gore's work has shown that most of the tannin of
persimmons is enclosed in cells, which he terms giant tannin cells. By arti-
ficial processing in carbon dioxide, or by ripening, the membrane containing
the tannins becomes hard and insoluble so that the astringent taste is not
evident.
Blue or purple discoloration in English walnuts. English walnuts
are more astringent than other nuts and often develop color changes that
are unattractive when combined with other foods. When they are combined
with apples in a salad they often develop a blackish blue or purple color.
This may come from having prepared the apples with an iron knife, the
acid and enzyme of the fruit acting on the iron to produce ferric salts.
When this small amount of ferric salt comes in contact with the tannin of
the nut and particularly with the skin the purplish color develops. In nut
bread, English walnuts often produce a dark color.
Tannins may produce a gray shade in sugar. Zerban has found
that the color of dark greenish cane juice, which produces a sugar with
a gray shade, may be due to tannins, oxidizing enzymes, and iron. When
the cane is crushed the acid of the juice forms some ferrous salts with the
iron of the roller. Oxidizing enzymes of the juice oxidize the ferrous salts
to ferric ones and they combine with the tannins of the juice. A similar
change occurs when fruits are pared with iron knives and especially when
the juice is left to stand on the knife and it darkens.
Tannins and discolorations in canned goods. Kohman has reported
that the discoloration, a darkening in color, often found in spots in canned
ENZYMES 123
sweet potatoes is due to tannins. The tannin combines with the iron of the
can to produce the dark color. Oxygen is necessary to oxidize the ferrous
iron to ferric, so that, unless oxygen is found in the can, either through not
exhausting all of the air in processing or to a leak in the can, the dis-
coloration does not occur.