Food Lipids: Chemistry, Nutrition, and Biotechnology

preparing nutritional labeling material, as well as for promoting and understanding
the effects of fats and oils on food functionality. At the same time, knowledge about
the structural characteristics of lipids may allow development of tailor-made products
designed for a particular function or application.
II. EXTRACTION OF LIPIDS FROM FOODS AND
BIOLOGICAL MATERIALS
Lipids in nature are associated with other molecules via (a) van der Waals interaction,
e.g., interaction of several lipid molecules with proteins; (b) electrostatic and hydrogen
bonding, mainly between lipids and proteins; and (c) covalent bonding among
lipids, carbohydrates, and proteins. Therefore, to separate and isolate lipids from a
complex cellular matrix, different chemical and physical treatments must be administered.
Water insolubility is the general property used for the separation of lipids
from other cellular components. Complete extraction may require longer extraction
time or a series or combination of solvents so that lipids can be solubilized from the
matrix.
The existing procedures of lipid extraction from animal or plant tissues usually
include several steps: (a) pretreatment of the sample, which includes drying, size
reduction, or hydrolysis; (b) homogenization of the tissue in the presence of a solvent;
(c) separation of liquid (organic and aqueous) and solid phases; (d) removal of
nonlipid contaminants; and (e) removal of solvent and drying of the extract. Standard
methods for lipid extraction have been established by the Association of Official
Analytical Chemists (AOAC) International for different types of materials/tissues.
However, when it comes to practical situations, each case might require modification
of the method.
A. Sample Preparation
As with any form of chemical analysis, proper sampling and storage of the samples
are essential for obtaining valid results. According to Pomeranz and Meloan [5], an
ideal sample should be identical in all of its intrinsic properties to the bulk of the
material from which it is taken. In practice, a sample is satisfactory if its properties
under investigation correspond to those of the bulk material within the limits set by
the nature of the test. Sample preparation for lipid analysis depends on the type of
food and the nature of its lipids. Effective analysis calls for a knowledge of the
structure, chemistry, and occurrence of principal lipid classes and their constituents.
Therefore, it is not possible to devise a single standard method for extraction of all
kinds of lipids in different foods.
Extraction of lipids should be performed as soon as possible after the removal
of tissues from the living organism so as to minimize any subsequent changes. Immediate
extraction is not always possible; however, the samples usually are stored
at very low temperatures in sealed containers, under an inert (nitrogen) atmosphere
or on dry ice. Yet the freezing process itself may permanently damage the tissues as
a result of osmotic shock, which alters the original environment of the tissue lipids
and brings them into contact with enzymes from which they are normally protected.
Thawing the sample taken from frozen storage before extraction may enhance this
deterioration. Therefore, tissue samples should be homogenized and extracted with
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