Assessment of repeated frying quality of some selected cooking oils available in the local markets in Piliyandala, Sri Lanka

Abstract

Plant-based cooking oils are used in various food processing methods. Deep frying is considered one of the most popular methods among them. Since deep frying requires comparatively a large amount of oil, both industrial and domestic users tend to recycle the frying oil to minimize the expense of food preparation. However, the majority of the general public is unaware of how oil quality is changed with repeated deep frying. Therefore, this study aimed to evaluate the oil quality variations of three commonly consumed cooking oil types in Sri Lanka, with repeated deep frying. To achieve this objective, a bulk coconut, a bulk palm, and a branded sunflower oil samples were purchased from the local market in the Piliyandala area, and potato and fish were deep fried separately for three conservative frying cycles. After each frying cycle, the acid value, peroxide value, and moisture content of oil samples were determined. These physicochemical properties act as important strategies in monitoring the oil quality. Accordingly, sunflower oil had a higher initial quality than the bulk coconut and palm oil samples, with less acid value, peroxide value, and moisture content. Initial acid values of coconut, palm, and sunflower oils ranged from 0.4225, 0.1845 to 0.1877 mg KOH/g. Corresponding peroxide values varied from 3.7844, 6.9336 to 3.7259 mEq O2/kg and moisture contents varied from 0.594%, 0.491% to 0.199%. As per the results, regardless of the frying material, the acid value and the peroxide value increased significantly with the increased number of frying cycles whereas the moisture content decreased. However, the number of frying cycles that oil can be recycled varies depending on the frying material. When considering the repeated deep frying process, at a frying temperature of 180 °C for 20 minutes, all three oil samples are not suitable for repeated deep frying of fish, due to their acid and peroxide values, which were not aligned with the recommended levels of the Codex Alimentarius Commission. Bulk coconut oil exhibited an acid value of 1.0440 mg KOH/g and bulk palm oil had a 0.5536 mg KOH/g acid value, which was closer to the recommended level. Even though sunflower oil did not exceed the recommended acid value, its peroxide value was closer to the recommended level with 9.2898 mEq O2/kg. However, when potato was used as the frying material sunflower and bulk palm oil samples can be recycled up to two frying cycles with peroxide values 9.3084 and 9.5656 mEq O2/kg respectively. But the bulk coconut oil sample could not be used for repeated deep frying of potato, since it had an acid value higher than the recommended level after the first frying cycle which was 0.8478 mg KOH/g. When considering the peroxide value, coconut oil exhibited the highest oxidative stability while sunflower oil exhibited the lowest. The experiment needs to be conducted with many different oil staples available in the local markets.