2009_Book_FoodChemistry

4 Carbohydrates
4.1 Foreword
Carbohydrates are the most widely distributed
and abundant organic compounds on earth. They
have a central role in the metabolism of animals
and plants. Carbohydrate biosynthesis in plants
starting from carbon dioxide and water with the
help of light energy, i. e., photosynthesis, is the
basis for the existence of all other organisms
which depend on the intake of organic substances
with food.
Carbohydrates represent one of the basic nutrients
and are quantitatively the most important source
of energy. Their nutritional energy value amounts
to 17 kJ/g or kcal/g. Even the nondigestible
carbohydrates, acting as bulk material, are of importance
in a balanced daily nutrition. Other important
functions in food are fulfilled by carbohydrates.
They act for instance as sweetening, gelor
paste-forming and thickening agents, stabilizers
and are also precursors for aroma and coloring
substances, especially in thermal processing.
The term carbohydrates goes back to times
when it was thought that all compounds of this
class were hydrates of carbon, on the basis of
their empirical formula, e. g. glucose, C 6 H 12 O 6
(6C+6H 2 O). Later, many compounds were identified
which deviated from this general formula,
but retained common reactions and, hence, were
also classed as carbohydrates. These are exemplified
by deoxysugars, amino sugars and sugar
carboxylic acids. Carbohydrates are commonly
divided into monosaccharides, oligosaccharides
and polysaccharides. Monosaccharides are polyhydroxy-aldehydes
or -ketones, generally with an
unbranched C-chain. Well known representatives
are glucose, fructose and galactose. Oligosaccharides
are carbohydrates which are obtained from
<10 carbohydrate units, which formally polymerize
from monosaccharides with the elimination
of water to give full acetals, e. g. by the reaction:
(4.1)
Well known representatives are the disaccharides
saccharose (sucrose), maltose and lactose, and the
trisaccharide raffinose, and the tetrasaccharide
stachyose.
In polysaccharides, consisting of n monosaccharides,
the number n is as a rule >10. Hence,
the properties of these high molecular weight
polymers differ greatly from other carbohydrates.
Thus, polysaccharides are often considerably
less soluble in water than mono- and oligosaccharides.
They do not have a sweet taste and are
essentially inert. Well known representatives are
starch, cellulose and pectin.
4.2 Monosaccharides
4.2.1 Structure and Nomenclature
4.2.1.1 Nomenclature
Monosaccharides are polyhydroxy-aldehydes
(aldoses), formally considered to be derived
from glyceraldehyde, or polyhydroxyketones
(ketoses), derived from dihydroxyacetone by
inserting CHOH units into the carbon chains. The
resultant compounds in the series of aldoses are
denoted by the total number of carbons as trioses,
for the starting glyceraldehyde, and tetroses, pentoses,
hexoses, etc. The ketose series begins with
the simplest ketose, dihydroxyacetone, a triulose,
followed by tetruloses, pentuloses, hexuloses, etc.
The position of the keto group is designated by
a numerical prefix, e. g. 2-pentulose, 3-hexulose.
When a monosaccharide carries a second
carbonyl group, it is denoted as a -dialdose
(2 aldehyde groups), -osulose (aldehyde and keto
groups) or -diulose (2 keto groups). Substitution
of an HO-group by an H-atom gives rise to a deoxy
sugar, and by an H 2 N-group to aminodeoxy
compounds (cf. Formula 4.2). Analogous to 4-
or 5-hydroxypentanal, aldoses (starting from
H.-D. Belitz · W. Grosch · P. Schieberle, Food Chemistry 248
© Springer 2009