The effect of sugar on the firmness of fruit. The effect of sugar
of course varies somewhat with the amount used in proportion to the
quantity of fruit. Osmosis occurs when a sirup has greater sugar concentra-
tion than that of the fruit to which it is added. The vapor pressure of
the sirup solution is lower than that of the fruit so that water passes
from the fruit to the sirup. This transfer from a region of high concen-
tration to a low one is called osmosis. If a membrane separates the regions
of high and low concentrations it is dependent upon the membrane whether
all the dissolved constituents of a solution can pass through the membrane.
With a semi-permeable membrane only the water can pass through the
membrane. Without discussing the possible role of the membrane in osmosis
or the osmotic pressure that may be produced, it is sufficient for the purpose
here to state that as the permeability of the membrane increases larger
and larger ions or molecules dissolved in the water may pass through the
membrane. The skin and cell walls of the fruit serve as membranes. These
132 FRUITS AND VEGETABLES
membranes in the different fruits seem to have a greater or a lesser permea-
bility to sugar.
When cooked in a sirup some fruits tend to keep their shape, some be-
come mushy, some shrivel, and others tend to collapse or flatten out. For
instance, sweet apples tend to stay whole whether cooked in sirup or in
water, whereas sour apples have a greater tendency to break up during
cooking. In general, the fruits tend to stay whole better if cooked in a
sirup. For purees the fruit is cooked in water and the sugar added later.
Cooking seems in most instances to increase the permeability of the fruit
membranes to sugar. Some apples, some plums, and most berries can be
cooked directly in the sirup without noticeable toughening of the fruit.
Peaches, apricots, most apples, and most plums absorb sugar satisfactorily.
Lathrop states that the firm texture of the cherry skin and of the peach
and blackberry flesh retards penetration of the sugar during cooking. On the
other hand, a fruit like Keifer pears is dehydrated if cooked directly in a
sirup and becomes shriveled, tough, hard, and rubbery. It is necessary to
soften the fruit by cooking and increase its permeability to sugar before
the sugar is added. It is of course possible that many changes other than
increasing the permeability of the fruit to sugar occur when the fruit is
cooked in sirup. Later work has shown that fruit like Keifer pears and
quinces can be cooked by adding the sugar directly to them without pre-
liminary cooking. But it is necessary to start with an excess of thin sirup
and cook slowly to evaporate the excess liquid, as the sugar penetrates the
fruit. So long a time is required that the method is not practical. The fruit
becomes more transparent in appearance. It may be that the sugar has
some effect on the cellulose.
In fruits like strawberries and seeded sour cherries, and particularly if
they are heated rapidly in a sugar sirup, flattening occurs. If a large number
of cell walls are broken the fruit becomes mushy. Time is required for
sugar to pass through the cell walls of fruits. If osmosis has not progressed
far enough, i.e., if the concentration of the sugar within the berry is not as
great as in the sirup surrounding it, the berry floats when canned.
Page 106
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The effect of sugar on the firmness of fruit. The effect of sugar
of course varies somewhat with the amount used in proportion to the
quantity of fruit. Osmosis occurs when a sirup has greater sugar concentra-
tion than that of the fruit to which it is added. The vapor pressure of
the sirup solution is lower than that of the fruit so that water passes
from the fruit to the sirup. This transfer from a region of high concen-
tration to a low one is called osmosis. If a membrane separates the regions
of high and low concentrations it is dependent upon the membrane whether
all the dissolved constituents of a solution can pass through the membrane.
With a semi-permeable membrane only the water can pass through the
membrane. Without discussing the possible role of the membrane in osmosis
or the osmotic pressure that may be produced, it is sufficient for the purpose
here to state that as the permeability of the membrane increases larger
and larger ions or molecules dissolved in the water may pass through the
membrane. The skin and cell walls of the fruit serve as membranes. These
132 FRUITS AND VEGETABLES
membranes in the different fruits seem to have a greater or a lesser permea-
bility to sugar.
When cooked in a sirup some fruits tend to keep their shape, some be-
come mushy, some shrivel, and others tend to collapse or flatten out. For
instance, sweet apples tend to stay whole whether cooked in sirup or in
water, whereas sour apples have a greater tendency to break up during
cooking. In general, the fruits tend to stay whole better if cooked in a
sirup. For purees the fruit is cooked in water and the sugar added later.
Cooking seems in most instances to increase the permeability of the fruit
membranes to sugar. Some apples, some plums, and most berries can be
cooked directly in the sirup without noticeable toughening of the fruit.
Peaches, apricots, most apples, and most plums absorb sugar satisfactorily.
Lathrop states that the firm texture of the cherry skin and of the peach
and blackberry flesh retards penetration of the sugar during cooking. On the
other hand, a fruit like Keifer pears is dehydrated if cooked directly in a
sirup and becomes shriveled, tough, hard, and rubbery. It is necessary to
soften the fruit by cooking and increase its permeability to sugar before
the sugar is added. It is of course possible that many changes other than
increasing the permeability of the fruit to sugar occur when the fruit is
cooked in sirup. Later work has shown that fruit like Keifer pears and
quinces can be cooked by adding the sugar directly to them without pre-
liminary cooking. But it is necessary to start with an excess of thin sirup
and cook slowly to evaporate the excess liquid, as the sugar penetrates the
fruit. So long a time is required that the method is not practical. The fruit
becomes more transparent in appearance. It may be that the sugar has
some effect on the cellulose.
In fruits like strawberries and seeded sour cherries, and particularly if
they are heated rapidly in a sugar sirup, flattening occurs. If a large number
of cell walls are broken the fruit becomes mushy. Time is required for
sugar to pass through the cell walls of fruits. If osmosis has not progressed
far enough, i.e., if the concentration of the sugar within the berry is not as
great as in the sirup surrounding it, the berry floats when canned.