Food Lipids: Chemistry, Nutrition, and Biotechnology

perature under vacuum. Acid hydrolysis also results in decomposition of phospholipids
and possibly TAGs to a certain extent [6].
Quantification of lipids from the extracts is mostly carried out as the weight
difference of an aliquot after solvent removal. Removal of the solvent from lipid
extracts should be conducted under vacuum in a rotary evaporator at or near room
temperature. When a large amount of solvent must be evaporated, the solution must
be concentrated and transferred to a small vessel so that the lipids do not dry out as
a thin film over a large area of glass. Lipids should be stored immediately in an inert
nonalcoholic solvent such as chloroform, rather than being allowed to remain in dry
state for long [4].
III. INDIRECT METHODS OF TOTAL LIPID DETERMINATION
Several techniques and instruments have been developed and applied to the indirect
and rapid determination of total lipid content of samples. These methods are really
not lipid extraction methods, but they are gaining popularity because they are rapid
and largely nondestructive. Most of these methods rely on a standard reference procedure
and must be calibrated against a methodology to be validated.
A. Density Measurement
It has been reported that the density of flaxseed is highly correlated (r = 0.96) with
its oil content [50]. Thus, measurement of seed density may be used as a means of
screening flax genetic lines for high oil content.
B. Dielectric Method
The dielectric constant of a selected solvent changes when fat is dissolved in it. After
an oilseed sample has been ground with a solvent and the dielectric constant of the
mixture measured, the lipid content is determined from standard charts that show
variation of the dielectric constants of different amounts of lipid in the same solvent
[5]. According to Hunt et al. [51], the amount of induced current and oil content
determined by solvent extraction of soybean are linearly related (r = 0.98).
C. Near-Infrared Spectroscopy
The near-infrared (NIR) reflectance is in the range of 14,300–4000 cm �1 (700–2500
nm) due to overtone and combination bands of C-H, O-H, and N-H. NIR spectrometry
can be used for determination of contents of oil, protein, and moisture and serves
as a very useful tool in the routine analysis of oilseeds.
Rodriguez-Otero et al. [52] have used NIR spectroscopy for measurement of
fat, protein, and total solids of cheese. Lee et al. [53] have used short-wavelength
(700–1100 nm) NIR with a bifurcated fiberoptic probe to estimate the crude lipid
content in the muscle of whole rainbow trout. A very good correlation was observed
between fat content determined by chemical analysis and NIR reflectance spectroscopic
values obtained for farmed Atlantic salmon fillets [54]. The use of mid-IR
spectroscopy to determine lipid content of milk and dairy products has been described
by Biggs [55]. Lipids absorb IR energy at the wavelength of 5730 nm, and
the energy absorbed depends on the lipid content of the sample. Quantification is
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.