Lipids - Fat |
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| | Updated 2019-11-03 |
The three terms fat, oil and lipid are often confused. The strict
definitions in chemistry/physics are that the overall term lipids covers fats, which are
solid at room temperature (20° C), and oils, which are liquid at
room temperature.
However, in daily language the
terms fat/fats and lipid/lipids are often used synonymously.
Lipids in foods are a very diverse group of components and
includes a long range of constituents; triglycerides/fatty acids,
phospholipids, wax esters and sterols being the most common.
Lipids are substances that are insoluble in water and soluble in
organic solvents such as ether, chloroform and benzene. Due to the
lipid fractions difference in solubility in different solvents, the
values for total lipids/total fat are very dependent on the chosen
solvent/analytical method. One of the reasons for this is that the
socalled 'free' lipid constituents can be extracted by less polar
solvents (light petroleum fractions/diethyl ether), whereas 'bound'
lipids require more polar solvents such as alcohol for their
extraction. The specific term 'crude' fat is used for lipids
extracted with diethyl ether.
Due to the wide variety and complexity of food matrices combined
with the large ranges in content, the determination of lipids in
foods present substantial analytical challenges. This is also
reflected in the amount of official standard methods for
determination of total lipid/total fat.
Srigley and Mossoba, CFSAN, US Food and Drug Administration, have
published a good overview of these methods. The wide range of
analytical procedures include not only solvent extraction procedures
as mentioned above, but also the two-step hydrolytic procedure
involving (enzymatic) hydrolysis before the solvent extraction.
Traditionally these procedures are both gravimetric procedures.
Newer procedures for determination of total fat (and fatty acids)
include gas chromatographic and near-infrared spectroscopy
procedures.
It is important here to point out that there are several 'modes
of expressions' for expressing total lipid/total fat. The conventional
way is as the gravimetric measure of the extracted fat. With the gas
chromatographic procedures fatty acids are (usually) measured as
fatty acid methyl esters (FAME). Under the Nutrition Labeling and
Education Act (NLEA) in USA, total fat is defined as the sum of fatty acids
expressed as triacylglycerol (TAG) equivalents, whereas those for
saturated fat are expressed as free fatty acid equivalents, and
mono-unsaturated/poly-unsaturated fatty acids as the sum of cis-forms
of the free fatty acids (see also
Fatty Acids).
It is also important to note that FAO Food and Nutrition Paper
No. 77 mentions that "the preferred method for determining lipid/fat
for energy purposes (FAO, 1998) is by summation of fatty acids and
expression as triglycerides as this approach excludes wax esters and
the phosphate content of phospholipids". Accepted method in the
report is "although less desiable, a gravimetric method (AOAC, 2000)
according to FAO.
However, the reference "FAO, 1998", which is Carbohydrates in
human nutrition (Report of a Joint FAO/WHO/UNU Expert
Consultation, FAO Food and Nutrition Paper No. 66, Rome 1998) does
not have any mention of a preferred method for determination of
total fat.
For more information about determination of total lipid/total fat
in foods, see the EuroFIR e-learning module: Nutrient-Analysis for
non-chemists (membership required; demo on analysis of fat is available at the EuroFIR
site:
http://www.eurofir.org/e-learning/EuroFIR%20e-learning%20demo/).
- Greenfield H. and Southgate D.A.T.:
Food Composition Data: Production, Management and Use.
2nd Edition, Food and Agriculture Organisation of the United Nations, Rome, 2003
English:
- as Google book
Particularly pages 106-110.
- Kirk, R.S. and Sawyer, R.:
Pearson's Composition and Analysis of Foods, 9th ed.
Longman Scientific and Technical, Essex, England, 1991.
Particularly pages 22-26 + specific explanations for each food
group.
- Srigley, C.T. and Mossoba, M.M.:
Current Analytical Techniques for Food Lipids.
United States Food and Drug Administration, 2017.
- Public Law 101-535 - Nutrition Labeling and Education Act of
1990.
- CFR - Code of Federal Regulations, Title 21:
Sec. 101.9 Nutrition labeling of food.
- Food energy - methods of analysis and conversion factors.
Report of a technical workshop, Rome, 3-6 December 2002.
FAO Food and Nutrition Paper 77.
Food and Agriculture Organization of the United Nations, Rome,
2003.
- Busstra, M.C.; Hulshof, P.J.M., Hollman, P.C.H.:
E-learning module: Nutrient Analysis for Non-Chemists.
EuroFIR Network of Excellence 2013.
Freely available with EuroFIR AISBL membership. Demo version
available at the EuroFIR AISBL website
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