Oil spoils: saturated fatty acids improve the frying quality of olive oil

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  • Published: Jul 14, 2016
  • Author: Ryan De Vooght-Johnson
  • Channels: Laboratory Informatics / Chemometrics & Informatics
thumbnail image: Oil spoils: saturated fatty acids improve the frying quality of olive oil

Vive la revolution

Toxic products are created from the repeated use of cooking oils. In response, scientists have worked to modify the fatty acid component of refined palm oil, with the aim of improving its thermostability.

No corner is left untouched by the science revolution, not even millennium-old cooking practices. Forever enshrined within the fourth-and-fifth century cookbook Apicius, the Romans describe the frying of chicken in olive oil as their pièce de résistance. The use of deep-frying to give food a crispy, golden coat may date back as far as the 5th century BC in ancient Egypt. However, the cornerstone of an evidence-based culture—and what separates us from antiquity—is the ability to adapt in light of new knowledge, which may signal the end of olive oil, or at least pure olive oil as we currently know it.

A deep fryer is a cauldron of chemical reactions. Whilst oil spits and hisses at high temperatures, it reacts with moisture contained within food—the starchy juice of potatoes, for example—and with atmospheric oxygen as its temperature and temperament cools. At the molecular level, oxidation, hydrolysis, polymerisation, isomerisation and cyclisation all occur, producing volatile and non-volatile entities. Multiply this by the repeated deep-fry cycles oils go through and you can imagine the industrial quantities of these compounds that are made.

This is where the problems lie: ‘…food fried in the deteriorated oil acquires a significant amount of decomposition products that may have adverse effects on the safety, flavour, nutritional value and the stability of fried food,’ Zribi and colleagues warn in a paper published recently in the International Journal of Food Science and Technology. What’s more, a number of these unwanted volatile compounds are inherently toxic to our genetic blueprint, potentially causing cancer within both consumers and professional fryers.

Thermostability

Like bygone alchemists concocting their master elixir, scientists tinker with the fatty acid components of their oils, adding oil blends here and there. Their aim is to improve the thermostability of cooking oils. This, they envisage, will minimise these unwanted chemical reactions and their toxic products.

Evidence shows, for example, that oils with a linoleic acid component of less than 3% of the overall fatty acids tend to have better thermostability than other oils. Others argue that oils with added saturated fatty acids (SFAs) and monounsaturated fatty acids (MUFAs) resist oxidation much better than oils high in polyunsaturated fatty acids (PUFAs). However, whilst the hydrogenation of oils improves their thermostability, manufacturers tend to shun unsaturated fats in favour of saturated fats.

In response, Italian and Tunisian researchers worked together to determine whether adding either 80% refined soya bean oil (high in PUFAs) or refined olive oil (high in MUFAs) would improve the thermostability of 20% refined palm oil (high in SFAs). They studied the formation thermostability and underlying chemical changes within these oils blends as they were put through up to 50 consecutive 180°C deep-fry cycles.

Oil change

In their paper, the authors list the vast array of changes that occur in the oil blends after 0, 25 and 50 deep-fry cycles. These include changes in the number of polar and volatile compounds, and in the compositions of fatty acids and sterols.

First of all, the accumulation of toxic products with repeated fry-and-cool cycles is clear. The number of polar compounds within both oil blends increased by 32 to 37% after 50 cycles. What’s more, the total number of mutagenic volatile compounds—an important indicator of oil deterioration—increased by 57% after 50 batches of potatoes were fried with the refined soy bean and palm oil blend. Most prominent of these is the total aldehydes which, after 25 cycles, increased ~13-fold in the refined olive oil/refined palm oil blend, and by up to almost 50-fold within the refined soya oil/refined palm oil blend.

Through a comprehensive analysis of all the data, the authors concluded that blending refined olive with 20% refined palm led to low deterioration of the oil during repeated deep-frying sessions. Now, it is down to cooks and consumers to act.

Related Links

Int. J. Food Sci. Technol. 2016, 51, 1594-1603,. Zribi et al. Quality assessment of refined oil blends during repeated deep frying monitored by SPME–GC–EIMS, GC and chemometrics.

The solution to drug prices

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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