Food Lipids: Chemistry, Nutrition, and Biotechnology

lipids from oilseeds. Diethyl ether (bp 34.6�C) has a better solvation ability for lipids
compared to petroleum ether. Petroleum ether is the low boiling point fraction (bp
35–38�C) of petroleum and mainly contains hexanes and pentanes. It is more hydrophobic
than diethyl ether and therefore selective for more hydrophobic lipids
[5,7]. The main component (>95%) of dietary lipids are TAGs, while the remaining
lipids are mono- and diacylglycerols, phospho- and glycolipids, and sterols. Therefore,
nonpolar solvent extractions have been widely employed to extract and determine
lipid content of foods. However, oil-soluble flavor, vitamins, and color compounds
may also be extracted and determined as lipids when less polar solvents are
used.
In determining total lipid content, several equipment and methods have been
developed that utilize single-solvent extraction. Among them the gravimetric methods
are most commonly used for routine analysis purposes. In gravimetric methods, lipids
of the sample are extracted with a suitable solvent continuously, semicontinuously,
or discontinuously. The fat content is quantified as weight loss of the sample or by
weight of the fat removed. The continuous solvent extraction (e.g., Goldfisch and
Foss–Let) gives a continuous flow of boiling solvent to flow over the sample (held
in a ceramic thimble) for a long period. This gives a faster and more efficient extraction
than semicontinuous methods but may result in incomplete extraction due
to channeling. In the semicontinuous solvent extraction (e.g., Soxhlet, Soxtec), the
solvent accumulates in the extraction chamber (sample is held in a filter paper thimble)
for 5–10 minutes and then siphons back to the boiling flasks. This method
requires a longer time than the continuous method, provides a soaking effect for the
sample, and does not result in channeling. In the direct or discontinuous solvent
extraction, there is no continuous flow of solvent and the sample is extracted with
a fixed volume of solvent. After a certain period of time the solvent layer is recovered,
and the dissolved fat is isolated by evaporating the organic solvent. Rose-
Gottlieb, modified Mojonnier, and Schmid–Boudzynski–Ratzlaff (SBR) methods are
examples, and these always include acid or base dissolution of proteins to release
lipids [6]. Such procedures sometimes employ a combination extraction with diethyl
and petroleum ethers to obtain lipids from dairy products. Use of these solvents may
allow extraction of mono-, di-, and triacylglycerols, most of the sterols and glycolipids,
but may not remove phospholipids and free fatty acids.
3. Methods Using Organic Solvent Combination
A single nonpolar solvent may not extract the polar lipids from tissues under most
circumstances. To ensure a complete and quantitative recovery of tissue lipids, a
solvent system composed of varying proportions of polar and nonpolar components
may be used. Such a mixture extracts total lipids more exhaustively and the extract
is suitable for further lipid characterization. The methods of Folch et al. [8] and
Bligh and Dyer [9] are most widely used for total lipid extraction. Use of a polar
solvent alone may leave nonpolar lipids in the residue; when lipid-free apoproteins
are to be isolated, tissues are defatted with polar solvents only [10]. It is also accepted
that the water in tissues or water used to wash lipid extracts markedly alters the
properties of organic solvents used for lipid extraction.
Commonly the chloroform–methanol (2:1, v/v) solvent system [8] provides an
efficient medium for complete extraction of lipids from animal, plant, or bacterial
tissues. The initial solvent system is binary; during the extraction process, it becomes
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