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Lipids  -  Fat


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Updated 2019-11-03

 Lipids - fats - oils

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/).
 


 References


 



 News
NEVO-Online 2019

2019-11-19
New edition of Dutch food composition database NEVO-online: compositional data on more than 2150 food items - 2019. See NEVO website.
 
USDA ARS FoodData Central

2019-10-01
USDA's Beltsville Human Nutrition Research Center (BHNRC) has established FoodData Central. Available at fdc.nal.usda.gov.
 
UK Composition of Foods online

2019-05-23
Food Databanks National Capability extended dataset based on PHE’s McCance and Widdowson’s Composition of Foods Integrated Dataset.
 
Composition of foods integrated dataset (CoFID) 2019

2019-05-22
McCance and Widdowson’s 'composition of foods integrated dataset' on the nutrient content of the UK food supply. Available from Public Health England's website.