1 Food Biochemistry—An Introduction 5BIOCHEMICAL CHANGES IN
CARBOHYDRATES IN FOOD
CHANGES INCARBOHYDRATES INFOOD
SYSTEMS
Carbohydrates are abundant in foods of plant origin,
but are fairly limited in quantity in foods of animal
origin. However, some of the biochemical changes
and their effect(s) on food quality are common to all
foods regardless of animal or plant origin, while oth-
ers are specific to an individual food. Figure 1.
shows the relationship between the enzymatic de-
gradation of glycogen and starch (glycolysis) and
lactic acid and alcohol formation, as well as the citric
acid cycle. Even though glycogen and starch are glu-
cose polymers of different origin, after they are con-
verted to glucose by the appropriate respective en-
zymes, the glycolysis pathway is common to all
foods. The conversions of glycogen in fish and mam-
malian muscles are now known to utilize different
pathways, but they end up with the same glucose-6-
phosphate. Lactic acid formation is an important
phenomenon in rigor mortis and souring and cur-
dling of milk, as well as in the manufacturing of
sauerkraut and other fermented vegetables. Ethanol
is an important end product in the production of
alcoholic beverages, bread making, and to a much
smaller extent in some overripe fruits. The citric acid
cycle is also important in alcoholic fermentation,
cheese maturation, and fruit ripening. In bread mak-
ing,-amylase, either added or from the flour itself,
partially hydrolyzes the starch in flour to release glu-
cose units as an energy source for yeast to grow and
develop so that the dough can rise during the fermen-
tation period before punching, proofing, and baking.
CHANGES INCARBOHYDRATES DURINGSEED
GERMINATION
Table 1.1 lists some of the biochemical reactions
related to germination of cereal grains and seeds,
with their appropriate enzymes, in the production of
glucose and glucose or fructose phosphates from
their major carbohydrate reserve, starch. They are
then converted to pyruvate through glycolysis, as
outlined in Figure 1.1. From then on, the pyruvate is
utilized in various biochemical reactions. The glu-
cose and glucose/fructose phosphates are also used
in the building of various plant structures. The latter
two groups of reactions are beyond the scope of this
chapter.
METABOLISM OFCOMPLEXCARBOHYDRATESBesides starch, plants also possess other subgroups
of carbohydrates, such as cellulose, -glucans, and
pectins. Both cellulose and -glucans are composed
of glucose units but with different -glycosidic link-
ages. They cannot be metabolized in the human
body, but are important carbohydrate reserves in
plants and can be metabolized into smaller mole-
cules for utilization during seed germination. Pectic
substances (pectins) are always considered as the
“gluing compounds” in plants. They also are not
metabolized in the human body. Together with cel-
lulose and -glucans, they are now classified in the
dietary fiber or complex carbohydrate category.
Interest in pectin stems from the fact that in
unripe (green) fruits, pectins exist in the propectin
form, giving the fruit a firm/hard structure. Upon
ripening, propectins are metabolized into smaller
molecules, giving ripe fruits a soft texture. Proper
control of the enzymatic changes in propectin is
commercially important in fruits, such as tomatoes,
apples, and persimmons. Tomato fruits usually don’t
ripen at the same time on the vines, but this can be
achieved by genetically modifying their pectic
enzymes (see below). Genetically modified toma-
toes can now reach a similar stage of ripeness before
consumption and processing without going through
extensive manual sorting. Fuji apples can be kept in
the refrigerator for a much longer time than other
varieties of apples before getting to the soft grainy
texture stage because the Fuji apple has lower pectic
enzyme activity. Persimmons are hard in the unripe
stage, but can be ripened to a very soft texture due to
pectic enzyme activity as well as the degradation of
its starches. Table 1.2 lists some of the enzymes and
their reactions related to these complex carbohy-
drates.METABOLISM OFLACTOSE ANDORGANIC
ACIDS INCHEESEPRODUCTIONMilk does not contain high molecular weight carbo-
hydrates; instead its main carbohydrate is lactose.
Lactose can be enzymatically degraded to glucose
and galactose-6-phosphate by phospho--galactosi-
dase (lactase) by lactic acid bacteria. Glucose and
galactose-6-phosphate are then further metabolized
to various smaller molecules through various bio-
chemical reactions that are important in the flavor
development of various cheeses. Table 1.3 lists some