Disgusting oil experiment: effect on oil degradation due to long heating during deep frying process

Backstory

Upon seeing the deep dried insects, instead of being disgusted by the insects themselves, we are more shocked by the blackish oil. As we know, degradation of oil can happen under oxygenated environment, heat and light, creating substances with unpleasant smell and taste. Which surely happens in those reused "thousand years" oil for street food. We can already imagine why would they always add excessive spices to cover the rancidity of oil. In fact, we can even see in many restaurant, the usage reused oil can be seen by the weird precipitate on the bubbling oil, which are mostly carcinogenic. Here we want to investigate the effect of long term heating on rancidity of oil in deep frying processes.

Theory

Oxidative rancidity is associated with the degradation by oxygen in the air. The double bonds of an unsaturated fatty acid can be cleaved by free-radical reactions involving molecular oxygen. This reaction causes the release of malodorous and highly volatile aldehydes and ketones, and acidic substances like butyric acid. Because of the nature of free-radical reactions, the reaction is catalyzed by heat during deep frying process, resulting in unpleasant taste and odor, and even carcinogenic substances. The amount of unwanted crosslinked or acidic substances produced can be tested using flow analysis to measure its change in viscosity and pH meter to measure the change in acidity. By measuring the difference in mass change before and after cook, we can also hypothesize the change in composition of deep fried potato cubes.

Hypothesis

1. There will be increment of viscosity due to production of crosslinked precipitates after long term heating
2. The precipitates are acidic in nature which will cause drop in pH
3. The precipitates will attach on the food and can be observed by differences in mass change

Materials

5mmx5mmx5mm potato cubes
Oil
Thermometer
pH paper/meter
Syringes
Balance

Method

1. Preheat 1L of oil to 200°C and maintain the temperature
2. Weight every potato cube and mark them individually
3. Deep fry five potato cubes for 2 minutes
4. Take out potato cubes for measurement without turning off heat
5. Measure the pH of the oil using pH meter
6. Measure the viscosity of the oil by taking small amount of oil and press it through a syringe using weight of known mass
7. Measure mass of the potato cubes
8. Wait for 30 minutes
9. Repeat step 3 to 7 until 2 hours elapsed
10. Tabulate the data and analyse

Analysis

Controlled variables:
Size of potato cubes, temperature of frying oil, deep frying duration, 

Independent variable:
"Age" of frying oil (how long has it been heated from t = 0)

Dependent variables:
pH change of oil, mass change of potato cubes, viscosity change of oil

Reference

1. T Majchrzak et al,Thermal degradation assessment of canola and olive oil using ultra-fastgas chromatography coupled with chemometrics, Monatsh Chem. 2017; 148(9): 1625–1630.

2. M Aniołowska et al, The Effect of Type of Oil and Degree of Degradation on Glycidyl Esters Content During the Frying of French Fries, J Am Oil Chem Soc. 2015; 92(11-12): 1621–1631