Walczak, Malgorzata
(2025)
Evaluating the role of waxes on oil stability in frying applications.
PhD thesis, University of Nottingham.
Abstract
Deep-fat frying (DFF) is one of the most popular food processing methods. However, during this process, the oil is exposed to high temperatures, air, and water from the food, resulting in a degradation of the frying medium. Various strategies have been proposed to enhance oil stability during DFF, including blending different oils, genetic modification of the oil crops and antioxidant addition (Aladedunye et al., 2017; Erickson et al., 2023; Przybylski et al., 2013).
In recent years, edible oleogels have gained widespread use in the food industry as alternatives to solid fats. Oleogelation structures liquid vegetable oils into solid-like gels using organogelators such as waxes and fatty acids. Initially used in products like spreads and dairy products, oleogels were first explored as a frying medium by Lim et al. (2017) who used soybean oil–carnauba wax oleogels for frying instant noodles. While the study focused on fried product quality, findings suggested improved oil stability and reduced oil uptake. Since 2021, research has expanded, though most studies still emphasise the effects on fried foods rather than the stability of the frying oil.
Therefore, the overall aim of this PhD research was to evaluate for the first time the effect of rice bran wax (RBW) on physiochemical behaviour and thermal stability of conventional sunflower (SFO) and high-oleic sunflower (HOSFO) during deep-fat frying. To achieve this goal, 0.06 and 0.15% (w/w) RBW-SFO and 0.06% (w/w) RBW-HOSFO dispersions were used as a frying medium. To evaluate the changes in the oil quality, this PhD research assessed the primary and secondary oxidation products measuring peroxide value, p-anisidine value, the thiobarbituric acid reactive substances (TBARS) value, conjugated diene and triene. The changes in fatty acid composition, viscosity, total polar materials formation and colour were monitored throughout the trying processes. High-performance thin-layer chromatography and the free fatty acid value were used to assess the extent of hydrolytic reactions. In addition, a two-part thermogravimetric study was used to determine fried oils' oxidative stability and thermal decomposition.
The experimental data suggest that adding RBW to conventional SFO enhanced oxidative stability during frying. Adding RBW to conventional SFO significantly reduced the formation of secondary oxidation products, the breakdown of triacylglycerols and the release of free fatty acids. Some enhancement in HOSFO stability was observed upon the incorporation of RBW. Adding RBW to both oils reduced the darkening of the oil.
The findings indicate that RBW contributes to protecting oil against thermal degradation. This effect is likely due to the presence of gamma-oryzanol in the wax, which plays a role in mitigating lipid oxidation and delaying oil deterioration. Analysis of the frying mediums suggests that incorporating RBW into SFO during prolonged frying enhances oil stability and extends its functional lifespan.
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