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chapter 9
Simple Carbohydrates
cells contain microvilli (thin hair-like projections on the
surface of the cells). The microvilli protrude through the
pores of the taste buds to provide a receptor surface for
the perception of taste. Highly soluble and diffusible sub-
stances, such as salt (NaCl) and sugars, enter the taste
pores and produce taste sensations. The chemical stimuli
received by the sensory cells are transduced into electrical
impulses. These impulses, in turn, are passed on to the
nerve fibers through neurotransmitters. Less soluble and
diffusible compounds, such as starch and protein, produce
correspondingly less taste sensation.
The four primary taste sensations are sweet, salty, bitter,
and sour. Each taste bud possesses different degrees of sen-
sitivity for all four qualities, but it usually has greatest sen-
sitivity to one or two. The integration of taste perception
occurs in the cerebral cortex, which receives nerve signals
arising from the taste buds that pass through the medulla
and the thalamus. An important function of taste percep-
tion is to provide reflex stimuli that regulate the output
of saliva. A pleasant taste perception increases saliva pro-
duction, whereas an unpleasant taste reduces output. Taste
also affects the overall digestive process by affecting gas-
tric contractions, pancreatic flow, and intestinal motility.
Choice of food and dietary habits are influenced by taste
and smell, which are interrelated. The sense of smell re-
sides in receptors of specialized bipolar neuronal cells (ol-
factory cells) located on each side of the upper region of
the nasal cavity. Like taste sensory cells, the receptors of
the olfactory cells with cilia protruding into the mucus
covering the epithelium, also undergo continuous renewal
but with a longer turnover rate of about 30 days. The re-
ceptors of olfactory cells are stimulated by volatile air-
borne compounds. Since perceptions of taste and smell
are triggered by chemicals, they are called
chemosensory
perceptions.
At the molecular level, they are mediated
by ionotropic channels and G protein coupled receptors
(Chapter 30). Chemosensory perceptions are affected by
a number of factors. Normal aging leads to perceptual as
well as anatomical losses in chemosensory processes. In-
creased thresholds for both taste and smell accompany ag-
ing. For example, aged persons need two to three times as
much sugar or salt as young persons to produce the same
degree of taste perception. The reduction in chemosen-
sory acuity may contribute to weight loss and malnutrition
in elderly persons. Other causes of chemosensory disor-
ders include aberrations of nutrition and hormones, infec-
tious diseases, treatment with drugs, radiation, or surgery.
Sugars exhibit different degrees of sweetness (Table 9-1).
Sucrose is sweeter than the other common disaccharides,
maltose and lactose. D-Fructose is sweeter than either
D-glucose or sucrose. D-Fructose is manufactured com-
mercially starting with hydrolysis of cornstarch to yield
TABLE 9-1
The Relative Sweetness o f Sugars, Sugar Alcohols, and
Noncarbohydrate Sweeteners
Type of Compounds
Percent Sweetness
Relative to Sucrose
Disaccharides
Sucrose
1 0 0
Lactose
2 0
Maltose
30
Monosaccharides
Glucose
50-70
Fructose
130-170
Galactose
30
Sugar alcohols
Sorbitol
35-60
Mannitol
45-60
Xylitol
200-250
Noncarbohydrate sweeteners
Saccharin
40,000
Aspartame
16,000
D-glucose, which is subsequently converted to D-fructose
by the plant enzyme glucose isomerase.
Synthetic noncarbohydrate compounds can also pro-
duce a sweet taste. Saccharin, a synthetic compound,
tastes 400 times as sweet as sucrose and has the following
structure:
O
Another synthetic sweetener is aspartame (L-aspartyl-
L-phenylalanine methyl ester), a dipeptide. Aspartame
is 160 times as sweet as sucrose and, unlike saccharin,
is said to have no aftertaste. Artificial sweeteners, be-
cause of their high degree of sweetness on a weight-for-
weight basis compared to sucrose, contribute very little
energy in human nutrition. They are useful in the man-
agement of obesity and diabetes mellitus. However, use
of aspartame during pregnancy, particularly by individ-
uals heterozygous or homozygous for phenylketonuria
(Chapter 17), may be hazardous to the fetus.
The perception of sweet taste can be elicited by a
wide range of chemical compounds. Two naturally occur-
ring sweet proteins,
thaumatin
and
monellin,
are derived
from the fruits of two African plants called katemfe and
