Food Lipids: Chemistry, Nutrition, and Biotechnology

carried out by the standard curve of the IR absorption and lipid content determined
by standard analytical methods [56]. Details about the use of IR spectroscopy for
lipid analysis is provided in a later section of this chapter.
D. Low-Resolution Nuclear Magnetic Resonance Spectroscopy
Time domain low-resolution nuclear magnetic resonance (NMR) (referred to as wideline
NMR) and frequency domain NMR could be used to determine the total lipid
content of foods. In time domain NMR, signals from the hydrogen nuclei ( 1 Hor
protons) of different food components are distinguished by their different rates of
decay or nuclear relaxation. Protons of solid phases relax (signal disappear) quickly,
while protons in the liquid phase relax very slowly. Protons of water in the sample
relax faster than protons of the lipid. The intensity of the signal is proportional to
the number of protons and, therefore, to the hydrogen content. Thus, the intensity of
the NMR signal can be converted to oil content of the sample using calibration
curves or tables [57–60]. This method can be used to determine the contents of
water, oil, and solid–fat and solid-to-liquid ratio of the sample. Time domain NMR
has been used to analyze the fat content of foods, including butter, margarine, shortening,
chocolate, oilseed, meat, milk and milk powder, and cheese [61–63].
Frequency domain NMR distinguishes food components by resonance frequency
(chemical shift) of the peaks in the spectrum. The pattern of oil resonances
reflects the degree of unsaturation and other chemical properties [56,61].
E. Turbidimetric/Colorimetric Methods
Haugaard and Pettinati [64] have described a turbidimetric method for rapid determination
of lipid in milk. The milk fat is homogenized to obtain uniform globules
and the milk proteins are retained with chelating agents such as EDTA. Light transmission
of the sample is measured and then converted to the lipid content with the
use of a conversion chart.
The lipid content of milk can also be determined by using a colorimetric
method [65]. The lipids of milk are allowed to react with an alkaline solution of
hydroxamic acid for a specified period. Upon acidification and addition of ferric
chloride, a relatively stable chromophore with a maximal absorbance at 540 nm is
formed [66].
F. Ultrasonic Method
Fitzgerald et al. [67] have described an ultrasonic method to determine the amount
of fat and nonfat solids of liquid milk. The velocity of sound increases or decreases
directly with the lipid content above or below a certain critical temperature. This
method of fat determination is based on the speed of sound passing through the milk
at various temperatures.
G. X-Ray Absorption
It is known that lean meat absorbs more X-ray than high-fat meat [68]. This fact has
been used to determine lipid content in meat and meat products using a standard
curve of the relationship between X-ray absorption and the lipid content determined
by usual solvent extraction methods [5].
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