Food | Refractometric Determination of Oleic and Linoleic Acid Content in Peanut Oil

Traditional peanut seeds consist of approximately 50 % oil, of which approximately 50 % is oleic acid and 30 % linoleic acid. The shelf life of peanut products, such as peanut butter, roasted snack nuts, confections, and peanut oil, is limited by the oxidative stability of the oils.
Oleic acid (formula CH₃(CH₂)₇CH=CH(CH₂)₇COOH) is a monounsaturated fatty acid. In contrast to that, linoleic acid (formula CH₃(CH₂)₇CH=CH(CH₂)₂CH=CH(CH₂)₃COOH)) is polyunsaturated.
Because of the different degree of unsaturation, the oxidative state of peanut oil is strongly influenced by the oleic/linoleic (O/L) ratio. The higher the O/L ratio the lower the total degree of unsaturation within the oil. This results in a more stable product, and extends the shelf life of peanut products.
The normal O/L ratio in non-high O/L peanuts is usually smaller than 9, with an average of 1.5 to 2.0 commonly observed in the US. Peanut farmers put a lot of effort into the growing of peanut seeds with high O/L ratios of 9 and even larger.
The definitive method to determine the fatty acid profile of an oil would be gas chromatography. This method is time consuming, expensive, and requires a skilled and dedicated operator. Therefore, gas chromatography is not the method of first choice for the peanut industry.
A rapid, simple and inexpensive method is required for the determination of the oleic/linoleic ratio in peanut oil. The measurements of oil refractive indices have been investigated for decades to characterize seed oil chemistry. The recent advances in Anton Paar‘s Abbemat refractometers allow the fast determination of the O/L ratio in peanut oils with refractive index measurements. This rapid and cost-effective application to determine the O/L ratio in peanut oil has recently been published by Davis et al. (2012).