Cooking Tips

Understanding Oil Degradation: A Scientific Perspective on Cooking Oil Quality

In culinary applications, particularly deep frying, the quality of cooking oil is paramount not only for achieving optimal flavor and texture but also for maintaining nutritional value.

This blog post delves into the scientific aspects of oil degradation, utilizing findings from a detailed academic study that investigates the physicochemical changes in cooking oils over repeated uses.

Here, we present data-driven insights on how oil properties such as viscosity, density, and the formation of harmful compounds evolve with repeated heating, and the sensory impacts these changes have on food.

What is Oil Degradation?

Oil degradation in cooking involves a series of complex chemical reactions that deteriorate the oil’s quality when exposed to heat, air, and moisture. These reactions include oxidation, hydrolysis, and polymerization, each contributing to changes in the oil’s physical properties and the formation of potentially toxic compounds.

The Scientific Basis of Oil Changes

Oxidation Processes

During frying, oils undergo oxidation, a reaction with oxygen that forms peroxides and free radicals. These intermediates break down into secondary products like aldehydes, ketones, and acids. In the study, Fourier Transform Infrared (FT-IR) Spectroscopy was employed to detect these changes, noting significant shifts in spectral bands indicative of oxidation. For instance, in corn oil, an increase in peroxide value from 0.162 meq/kg at room temperature to 2.994 meq/kg after the first cycle of frying was observed, highlighting rapid oxidative degradation.

Hydrolysis and Polymerization

Hydrolysis reactions contribute to the increase in free fatty acids, impacting the oil’s taste and potentially its health properties. Polymerization, on the other hand, leads to an increase in oil viscosity and the formation of larger molecular complexes, which was evident from the increasing viscosity measurements from 90.07 to 94.36 millipoise in corn oil after repeated frying sessions.

Physicochemical Properties

Viscosity and Density:
Repeated frying results in increased viscosity and decreased density, affecting the oil’s thermal conductivity and heat transfer capabilities. The study documented that the viscosity of corn oil increased with each fry, indicating higher levels of polymerized compounds. Density measurements corroborated the formation of volatile compounds, as corn oil’s density decreased from 0.8632 g/mL initially to 0.8486 g/mL after three frying cycles.

Formation of Harmful Compounds:
The FT-IR analysis showed a buildup of secondary oxidation products, especially evident from the shifting and intensification of specific IR bands. For example, the spectral region around 1745 cm^-1, which corresponds to ester carbonyl groups in the oils, showed marked changes post-frying, indicating degradation and formation of harmful compounds.

Sensory Impacts on Food

Degraded oils can significantly affect the sensory properties of food, imparting off-flavors and altering textures. Foods fried in reused oils tend to be greasier and have a darker appearance, which can be attributed to the polymers and oxidized materials present in the oil.

Given the rapid degradation observed, particularly reflected in the increase of peroxide values and viscosity changes, it’s crucial for both chefs to monitor oil quality rigorously. Techniques like FT-IR provide a scientific method for assessing oil quality, although simpler methods such as sensory evaluation and periodic testing with kits can also be effective.

Best Practices for Oil Management

To mitigate oil degradation:

  • Maintain Optimal Frying Temperatures: Excessive temperatures accelerate degradation. Keeping temperatures between 350°F and 375°F is advisable.
  • Use Oils with High Smoke Points: Such oils are more resistant to breakdown at high temperatures.
  • Regularly Filter and Replace Oil: Removing food particles and replacing oil when degradation indicators are first noted can prolong oil life and improve food quality.


The scientific investigation into oil degradation offers profound insights into how cooking oils change with repeated use and the implications for food quality and safety. By adopting rigorous monitoring and management practices, based on scientific findings such as those presented in the academic study, culinary professionals can significantly enhance the quality of their offerings and ensure a healthier dining experience.