Chemistry of Almond Oil

The term “Almond Oil” is sometime a confusing term. Almond oil can be extracted by cold crushing of the almond kernel and contains mainly saturated and unsaturated fatty acids. Oleic, Linoleic and Palmitic acids are the main constituents. This oil has many uses but one major use is in the Cosmetic/Health industries where it is used as body oil or as solvent allowing other non polar chemicals to be mixed. “Essential Almond oil” or “Essence of Almond” is predominantly benzaldehyde and is used in food as an essence of “almond”.

Although almond oil contains natural antioxidants it does have a “shelf life”. Health regulations require chemical testing to be carried out on the “safeness of the oil”. FFA, Free Fatty Acids, and Peroxide Values are two such tests.

Free Fatty Acids

Free Fatty Acids (FFA) or Acid Value (AV) is the basic criterion for grading almond oils. A reliable estimate of free acidity is of great importance in the quality assurance and control of almond oil. The "free" fatty acid is that which has separated from the triglyceride base leaving one or two fatty acids as a mono or di glycerol molecule. FAA is expressed as %w/w oleic acid is the predominant acid measured. A value of 0. 5% w/w would be considered to be excessive.

The method relies on titration of a strong base (sodium hydroxide) against the weak acid (mostly oleic acid) in the oil which has broken off. Hot ethanol is used to dissolve the oleic (or other free acids) into a polar solution which can be titrated against with the known concentration of sodium hydroxide solution using an indicator (phenolphthalein). See practical sheet “Quality Tests on Almond Oil”

Peroxide Value

Almond oil can develop unpleasant odour when it goes “off”. The Peroxide Value of oil is used a measure of the oxidative rancidity reactions that have occurred during storage. These reactions are dependent on temperature, air (oxygen) and sunlight to which the oil is exposed. Autoxidation is a free radical reaction involving the double bonds found in the oil and unpleasant flavours and odours are produced. Peroxides are intermediates in the autoxidation reaction. The peroxide value is defined as the amount of peroxide oxygen per 1 Kg of oil. See practical sheet “Peroxide Value”

Peroxide values of “fresh” almond oils are less than 10 milliequivalents /kg. When the peroxide value is between 20 and 40 milliequivalents/kg, a rancid taste is noticeable. Milliequivalents are equivalent to a millimole.

The peroxide value is determined by measuring the amount of iodine which is formed by the reaction of peroxides with the iodide ion.

2 I- + H2O + ROOH → ROH + 20H- + I2

Note that the base produced in this reaction is taken up by the excess of acid present.

The iodine liberated is titrated with sodium thiosulphate.

2S2O32- + I2 → S4O62- + 2 I-

The indicator used in this reaction is starch which forms a blue to black solution with the iodine produced.

The odours and flavours associated with typical oxidative rancidity are mostly due to carbonyl type compounds. The shorter-chain aldehydes and ketones isolated from rancid oils are associated with advanced stages of oxidation. The carbonyl-type compounds develop in low concentrations early in the oxidative process.