Thermal Processing of Oils in Deep-Fat Frying

Deep-fat frying subjects culinary oils to sequential high-temperature treatments that initiate hydrolytic, oxidative and polymerisation reactions. At frying temperatures (typically 160–190 °C), triglycerides undergo hydrolysis to free fatty acids and mono- or diacylglycerols, while molecular oxygen drives lipid peroxidation to form primary hydroperoxides. These intermediates fragment into a complex mixture of secondary products, including aldehydes, ketones and polymeric compounds, which degrade oil performance and may transfer into fried foods. The extent of degradation depends on oil composition—degree of unsaturation, minor antioxidants and natural tocopherols—together with frying duration, food moisture and turnover rate. Degraded oils exhibit increased viscosity, darkening colour and elevated acidity, impairing heat transfer and sensory quality. Beyond culinary considerations, thermal breakdown products have been linked to oxidative stress and chronic disease risks in consumers, prompting studies into oil replacement intervals, antioxidant fortification and alternative frying media. Advances in analytical methods have enabled molecular-level characterisation of oxidation pathways, guiding the development of stabilised or low-polyunsaturate oils that maintain functional integrity under repetitive frying.

Research from Nature Portfolio

Recent studies have demonstrated that monounsaturate-rich algal oils exhibit markedly greater resistance to peroxidative breakdown during both laboratory-simulated shallow frying and repetitive deep-fat frying episodes. Quantitative analysis by proton nuclear magnetic resonance revealed substantially lower formation of cytotoxic and genotoxic aldehydes in these oils compared with polyunsaturated-rich sunflower and soybean frying media. Moreover, examination of fast-food restaurant samples confirmed that aldehyde levels in potato chips correlate directly with oil composition and frying episode number. These findings underscore the potential of tailored fatty-acid profiles in mitigating the generation of harmful lipid oxidation products during commercial and domestic frying operations.

Thermal Processing of Oils in Deep-Fat Frying publication trend

The graph below shows the total number of articles in thermal processing of oils in deep-fat frying across all publications each year (not limited to Nature Index journals).

Technical terms

Triglyceride (TG): The principal lipid in edible oils, composed of three fatty acids esterified to a glycerol backbone.

Lipid peroxidation: Oxidative degradation of unsaturated lipids initiated by free radicals, leading to hydroperoxides and secondary decomposition products.

Lipid oxidation products (LOPs): Secondary compounds, such as aldehydes and ketones, formed from hydroperoxide breakdown during thermal or photo-oxidation.

Peroxide value (PV): A measure of primary oxidation, quantifying hydroperoxide concentration in an oil sample (meq O₂/kg).

Total polar compounds (TPC): The aggregate proportion of oxidised, polymerised and hydrolysed species in frying oil, expressed as a percentage of total oil.

Free fatty acids (FFA): Fatty acids released from triglycerides by hydrolysis, often used as an indicator of hydrolytic rancidity.

References

1. Application of cellulose‐ and chitosan‐based edible coatings for quality and safety of deep‐fried foods. Comprehensive Reviews in Food Science and Food Safety (2023).
2. Potential Adverse Public Health Effects Afforded by the Ingestion of Dietary Lipid Oxidation Product Toxins: Significance of Fried Food Sources. Nutrients (2020).
3. Evaluating the rancidity and quality of discarded oils in fast food restaurants. Food Science & Nutrition (2019).
4. Toxic aldehyde generation in and food uptake from culinary oils during frying practices: peroxidative resistance of a monounsaturate-rich algae oil. Scientific Reports (2019).