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Lipid Extraction and Analysis

Crude Fat Extraction

Crude fat is the classic measure of fat in food products. Measurement is based on the solubility of lipids in organic solvents. This process is greatly variable base don the experimental conditions, such as the specific solvent, temperature, how processed the food sample is, the solvent to sample ratio, etc. Ideally, the samples should be dried and ground before extraction.

This could also be accomplished as an aqueous extraction with organic washing stages, however prtitoning will be in effect, so this may not be accurate.


  1. The sample is dried, ground with sand and dissolved in an organic solvent
  2. The sample in the solvent is then dried with \(\ce{NaSO4}\) and gravity filtered
  3. The concentration of fat in the filtrate can then be measured by refractometry

Soxhlet Extraction

The Soxhlet extractor allows the analyte ot be extracted continuously into a set amount of solvent. This is accomplished in a number of steps:

  1. The sample is loaded into the upper chamber and the solvent into the lower.
  2. The solvent is slowly boiled off (distilled) and refluxed into the upper chamber where it will extract the analyte in question
  3. When the level of solvent in the upper chamber reaches a certain level, it will be siphoned back into the bulk of the solvent, where it can be re-distilled.

Since the solvent is constantly distilled before being mixed with sample, the effects of solution saturation/partitioning are negated and the sample can extracted to a much higher concentration than partitioning would allow.

Reichert-Meissl Value

Is a measure of how much volatile fatty acid can be extracted from an aqueous sample. It is equal to the amount of \(1M\:\ce{NaOH}\) solution (\(mL\)) required to neutralise the fatty acids in \(5\:g\) of fat/oil.

It is primarily used to measure the adulteration (how much the milk has been reduced in quality) of milk fat, however it is a rather crude value.

Polenske Value

Similar to the Reichert-Meissl value, however it measures the volatile fatty acid content in a non-aqueous sample

Iodine Number

Is a the grams of iodine which will be taken up across double bonds in \(100\:g\) of fat/oil. It is a crude method of the degree of unsaturation of the sample.

Since \(\ce{I2}\) is sparingly soluble in aqueous solutions, Wij’s solution (\(\ce{ICl}\) dissolved in glacial acetic acid) is used. When a known quantity of \(\ce{I2}\) or Wij’s olution is added, it can be back-titrated with \(\ce{Na2S2O3}\) to determine how much iodine wasn’t taken in by the fat.

Kreis Phloroglucinol

Is a measure of the rancidity of fats.

Rancidity is the amount of degradation that fats undergo, either by hydrolytic means (cleaving the ester bonds with water \(\ce{RCOOR' + H2O -> RCOOH + HOR'}\)), or by oxidative means (cleavage of the \(\ce{C=C}\) bonds by oxygen radicals).

The test involves equal quantities of oil and \(\ce{HCl}\) being combined with \(\ce{0.1\:\%}\) phloroglucinol (Right) in equal parts diethyl ether.

The presence of a coloured compound indicates rancidity, however this is only a semi-quantitative method.

Instrumental Analysis

Instrumentally, the easiest way to analyse the sample is to use GC-MS.

The sample must be thermostable and derivitisable, to be able to make it volatile for the GC analysis.

To extract the fats from the sample, chromatography must be performed, either though solid phase extraction, or thin layer chromatography (the silica can be scraped off the plate).

From here the fatty acids must be converted into more thermostable/volatile compounds through derivitisation, before being separated by GC and their structure determined through ToF based MS.