2009_Book_FoodChemistry

3.2 Fatty Acids 159
taste substances and numerous odor substances.
On the whole, fats enrich the nutritional quality
and are of importance in food to achieve the desired
texture, specific mouthfeel and aroma, and
a satisfactory aroma retention. In addition, foods
can be prepared by deep frying, i. e. by dipping
the food into fat or oil heated to a relatively high
temperature.
The lipid class of compounds also includes some
important food aroma substances or precursors
which are degraded to aroma compounds. Some
lipid compounds are indespensable as food emulsifiers,
while others are important as fat- or oilsoluble
pigments or food colorants.
3.2 Fatty Acids
3.2.1 Nomenclature and Classification
Acyl lipid hydrolysis releases aliphatic carboxylic
acids which differ in chemical structure.
They can be divided into groups according to
chain length, number, position and configuration
of their double bonds, and the occurrence of
additional functional groups along the chains.
The fatty acid distribution pattern in food is
another criterion for differentiation.
Table 3.2 compiles the major fatty acids which
occur in food. Palmitic, oleic and linoleic acids
frequently occur in higher amounts, while the
other acids listed, though widely distributed, as
a rule occur only in small amounts (major vs minor
fatty acids). Percentage data of acid distribution
make it obvious that unsaturted fatty acids
are the predominant form in nature.
Fatty acids are usually denoted in the literature
by a “shorthand description”, e. g. 18:2 (9, 12)
for linoleic acid. Such an abbreviation shows the
number of carbon atoms in the acid chain and
the number, positions and configurations of the
double bonds. All bonds are considered to be cis;
whenever trans-bonds are present, an additional
“tr” is shown. As will be outlined later in a detailed
survey of lipid structure, the carbon skeleton
of lipids should be shown as a zigzag line
(Table 3.2).
3.2.1.1 Saturated Fatty Acids
Unbranched, straight-chain molecules with an
even number of carbon atoms are dominant
among the saturated fatty acids (Table 3.6).
The short-chain, low molecular weight fatty
acids (< 14:0) are triglyceride constituents
only in fat and oil of milk, coconut and palmseed.
In the free form or esterified with low
molecular weight alcohols, they occur in nature
only in small amounts, particularly in plant
foods and in foods processed with the aid
of microorganisms, in which they are aroma
substances.
Odor and taste threshold values of fatty acids are
compiled in Table 3.3 for cream, butter and cocoa
fat. The data for cream and coconut fat indicate
lower odor than taste threshold values of C 4 -and
C 6 -fatty acids, while it is the reverse for C 8 -upto
C 14 -fatty acids.
The aroma threshold increases remarkably with
higher pH-values (Table 3.4) since only the
undissociated fatty acid molecule is aroma active.
Table 3.2. Structures of the major fatty acids
Abbreviated
designation Structure a Common name Proportion (%) b
14:0 Myristic acid 2
16:0 Palmitic acid 11
18:0 Stearic acid 4
18:1(9) Oleic acid 34
18:2(9,12) Linoleic acid 34
18:3(9, 12, 15) Linolenic acid 5
a Numbering of carbon atoms starts with carboxyl group-C as number 1.
b A percentage estimate based on world production of edible oils.