Effect of frying oil degradation on surface tension and wettability

Abstract

Frying oil degrades via exposure to heat, oxygen and water resulting in the formation of volatile and non-volatile products, which act as surface active substances and change heat and mass transfer rates. Effects of oil degradation during frying were quantified by measuring viscosity, surface tension, and static and dynamic contact angles of fresh oil (Total polar materials, TPM 3–4%) and used oils (TPM 10–20%). Oil viscosity decreased exponentially with increasing temperature (40–200 °C). Used oil viscosity was higher than fresh oil at room temperature; no significant difference was recorded above 60 °C. Pendant drop technique was used to measure air-oil (24–200 °C) and steam-oil (100–200 °C) surface tension of all oil samples. Surface tension decreased linearly as temperature increased. There was no effect of surrounding medium (air or steam) or oil quality on surface tension. Surface tension was time independent for both oils, as observed with a 5 h measurement using rising-bubble technique. Static contact angles of all used oils were lower than fresh oil, indicating increased wettability of used oil, which can affect amount of oil absorbed during frying and post-fry cooling. Hysteresis of used oil (13°–15°) was lower than fresh oil (18°), which can impact drainage during post-fry cooling.