Across Regions, Africa, Health/Nutrition/Food Safety, Processing, fries, chips, Research, Studies/Reports

The changing face of oils under heat: A study on the effects of repeated frying in home and industrial cooking

This article was written by Jorge Luis Alonso G., an information consultant specializing
in the potato crop.

A research team at the Department of Food Science and Technology, Rivers State University in Port Harcourt, Nigeria conducted a study to investigate the effects of frying cycles on the physical, chemical, and antioxidant properties of four different vegetable oils during the deep-fat frying of potato chips. The study was published as a scientific paper in the journal Food Chemistry Advances earlier this year.

This article is the summary of the study and its findings.

Frying, an ancient and globally used method of food preparation involves immersing food in hot oil, typically between 150°C and 190°C. This process results in rapid dehydration and heat transfer. It significantly enhances the flavor, aroma and visual appeal of food. Because of its efficiency and versatility, this method has found widespread use in both home and industrial cooking.

However, it’s important to note that the frying process can degrade the quality of the food due to oil absorption. Repeated use of frying oil can lead to several adverse reactions. Specifically, the oil undergoes physical and chemical degradation, loses its nutritional value, and forms harmful toxins. This issue is particularly problematic given the growing consumption of fried and convenience foods in developed countries.

While commonly used oils such as soybean, sunflower and coconut are standard choices for frying, we now face new challenges. Rising oil prices and growing health concerns are driving the use of underutilized alternatives such as almonds and sesame oils. It’s critical to understand how these oils behave during frying and the potential degradation they may undergo.

In response to these concerns, the purpose of this study was to investigate the effects of repeated frying on the physical, chemical, and antioxidant properties of selected vegetable oils. The aim was to gain insights that would help maintain food quality even in the face of these challenges.

As the study unfolded, it revealed a strong degradation in the physicochemical and antioxidant properties of these vegetable oils over different frying cycles. With each successive frying cycle, the free fatty acid, peroxide and moisture levels in the oils increased. In contrast, the iodine value and smoke point began to decrease.

The researchers noted interesting color shifts in the oils due to repeated use: the a* values (using the Hunter Lab Colorimeter) of soybean and avocado oils plummeted, while sesame oil showed an increase. The b* value of soybean oil was unaffected by repeated frying, but avocado and sesame seed oils experienced significant decreases. Almond oil, on the other hand, showed a dramatic escalation from the first frying.

Among the oils, sesame oil bore the brunt of the impact with the steepest drop in total phenolic content (33.81%) and total flavonoid content (27.44%). Almond oil was also hit hard, with the greatest decrease in total antioxidant capacity (43.82%). In contrast, soybean oil was more resilient, showing the least decline in total phenolics (7.06%) and total flavonoids (10.18%).

Avocado oil came under scrutiny due to its low smoke point and high free fatty acid (FFA) and moisture content, making it unsuitable for repeated frying. The results highlight the profound negative effects of frying cycles on the physicochemical and antioxidant properties of oils, potentially reducing their health benefits. Therefore, understanding these transformations is a critical step in determining the suitability of these oils for frying processes.

Source: Ujong, A., Emelike, N., Owuno, F., & Okiyi, P. (2023). Effect of frying cycles on the physical, chemical and antioxidant properties of selected plant oils during deep-fat frying of potato chips. Food Chemistry Advances, 100338.
Photo: Pixabay

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