Crude Fat Analysis
The term “fat” is a generic term that is also referred to as “lipid” in scientific terms. Fats
or lipids are a very diverse group of chemicals but they have one similarity in that they
dissolve in non-polar solvents and are not soluble in polar solvents such as water.
There are a variety of test methods for fats, but generally the method can be broken
down into “crude” or total fats. “Crude” methods involve dissolving the material in a
solvent such as ether or hexane and then evaporate the solvent. The material that
remains is called “crude fat.” The total fat test involves analytical instruments that
separate the fat components into various fatty acids. A more detailed explanation of
fatty acids and their determination can be found in another Midwest Memo titled
“Analysis of Fatty Acids.”
Crude fat is more of a screening method because the test procedures do not identify a
specific fat; instead, the methods measure the materials that can be dissolved in some
solvent. There are various types of crude fat procedures and the variability is for
different types of products on which “crude fat” is to be measured. For example, to
determine “crude fat” on dairy products, an alkaline hydrolysis method is used. For
products that have high levels of starchy materials, acid hydrolysis crude fat is used.
Both alkaline (base) hydrolysis and acid hydrolysis are steps carried out prior to
treatment with a solvent. The acid or base hydrolysis helps release the fats from its
matrix, otherwise the solvent cannot remove all the “fat” and the level of fat is lower than
what is actually in the sample.
The results obtained from acid hydrolysis fat, alkaline (base) hydrolysis, or no acid or
alkaline will give different results. Different solvents will also give different results.
Thus, use of petroleum ether (pet ether) or diethyl ether or hexane, will all give different
results. The differences are due to the ability of a solvent to dissolve a “fat” or other
material. As indicated previously, when the product to be tested is combined with a
solvent that solvent will dissolve certain chemicals in the sample to be tested. Some
solvents will dissolve glycerol while others will not. Some will dissolve small sugars or
amino acids while others will not. The different results will be seen when the solvents
are evaporated off. Since the amount of “crude fat” is determined by what is dissolved,
the solvent that dissolves the most material will results in a sample with the highest
“crude fat” level. When companies’ tests results from different laboratories or different
samples, the only way to be able to compare results is to make sure the same amount
of sample and the same solvent is used, otherwise, the final results will be different.
Many Midwest Laboratories Clients are requesting crude fat by acid hydrolysis and the
reason for the request is due to the complex matrix of the sample. To run an acid
hydrolysis fat, the sample is blended and ground to make sure it is homogenous so
there are not pockets of “fat” in one area and not another. Fats do tend to cling together
and are not equally distributed throughout the sample.
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A small amount of sample is removed and placed in a tube along with hydrochloric acid.
The hydrochloric acid helps break apart some of the starch materials or other chemicals
that “hide” the fat. Once the acid is added and mixed, petroleum ether (pet ether) is
added and the sample stirred to obtain uniform exposure of the solvent to the sample.
Materials that are soluble in the pet ether are dissolved so when the pet ether is poured
off, the dissolved materials (fats) are in the pet ether portion. The sample is treated
several times with pet ether and normal ether to make sure any material capable of
dissolving in the ether do dissolve. All of the ether that comes in contact with the
sample is combined into a pre-weighed container and the ether evaporated; leaving
behind the material that was dissolved in the ether. The container is weighed again and
any increase in weight is defined as crude fat. Since the sample was originally treated
with acid, the test procedure is called acid hydrolysis fat and is a type of crude fat
determination.
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