Agriculture Reference
In-Depth Information
Figure
2.13.
Glucose molecule, a disaccharide, and two units of a polysaccaride (not all hydro-
gen atoms are shown; unsatisfied bond positions are occupied by hydrogen).
and starch are polymers of sugars, because of differences in bonding, starch is digested
by the body while cellulose is not. Cellulose forms the roughage, one constituent of
fiber, used by the digestive tract for bulk.
Because they can be both straight chained and branched, complex carbohydrates, such
as the various starches, require a number of different enzymes to break them down into
their component sugar molecules that can then be used by the body. Thus, one enzyme
is required to break off branches so that they are single unbranched chains. A second
enzyme is used to break the chains into individual molecules of sugar. Because complex
carbohydrates must be broken down before they can be absorbed, they provide a slower
increase in blood sugar level and release of energy over a longer period of time.
The carbohydrate content of three major foods is given in Figure 2.14. There is a
significant difference in content when presented on a dry rather than a wet or fresh
basis. On a raw- or wet-weight basis, maize and rice have more carbohydrates and
less water than potato and cassava. However, on a dry-weight basis all four have
about the same carbohydrate content with cassava having the highest amount.
Unrefined carbohydrate sources contain dietary fibers that the refining process
removes and discards. This material provides bulk that the digestive tract muscles use
to maintain tone. These materials also hold water to keep the stool moist and soft.
