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Smoke Point When cooking oil reaches its smoke point it will degrade and oxidize. The smoke point is the temperature at which: triglycerides in the oil begin to separate into fatty acids and glycerol free glycerol is converted into a toxic compound called acrolein The more refined the oil, the higher the smoke point. Virgin oils will have lower smoking points. For example: Olive Oil Unrefined: 320°F (160°C) Extra Virgin: 375°F (190°C) Virgin: 390°F (199°C) Pomace 460°F (242°C) Extra Light: 468°F (242°C) Avocodo oil Virgin 375-400°F (190-205°C) Refined: 520°F (271°C) See Comparison of Dietary and Cooking Oils for list of smoking points. Heating Polyunsaturated Fats Grootveld et al (2001) warns that heat degrades polyunsaturated fatty acids to toxic compounds Possible risks of consumption or exposure of air-borne breakdown products of heated oils (Grootveld 2001) atherosclerosis (heart diseases) inflammatory joint disease including rheumatoid arthritis pathogenic conditions of the digestive tract mutagenicity and genotoxicity (carcinogenesis) teratogenicity (possible birth defects) Halvorsen & Blomhoff (2011) observed 2.9–11.2 fold increases in alkenal concentrations (a measure of oxidation) when various vegetable oils were heated for to 225°C for 25 minutes. Despite the biological toxicity of several lipid oxidation by-products (Halvorsen 2011) safe doses of these compounds has not been established little is known about what doses may constitute a health hazard for humans. Guillén & Uriarte (2012) found that cooking oils produces aldehydes when heated to frying temperature 190°C for extended periods (20 hours). aldehydes were found in both oil and fried foods aldehydes may be associated with some neurodegenerative diseases Przybylski, et al (2012) did not detect partially hydrogenated oils, or trans fats when the oil was heated to 275°C in a stir fry pan and smoking heavily before the food was placed in it. It only decreased the amount of polyunsaturated fatty acids because of oxidative degradation However he noted that linolenic acid appears to be the most prone to isomerization with the highest amount of trans isomers formation irrespective of the cooking procedure. See Comparison of Dietary and Cooking Oils for amounts of polyunsaturated fat in various oils.

Storing Oils Rancidity Unsaturated Fats including polyunsaturated and monounsaturated fats are susceptible to rancidity There may be possible negative health effects from ingestion of oxidized lipids from fish oils and cooking oils (Halvorsen 2011) Storage Containers Auto-oxidation and photo-oxidation during processing and storage (Halvorsen 2011) Undesirable taste, decomposing nutritional quality, production of toxic compounds Depends on fatty acid composition (degree of unsaturation), processing, exposed heat, light, metals, and antioxidants. Plastic containers Placicides can contaminate food with Bisphenol A (BPA) BPA from plastics links to lipid molecules BPA is found in: certain plastic containers plastic coating in some metal cans and other plastics See Hormone Disrupters: Bisphenal A (BPA) Shelf Life Shelf life for cooking oils vary from 2 to 12 months 3-6 weeks Flax and hemp oils May be kept up to a year in the freezer 2-4 Months Walnut, Sesame Oils 3 Months Sunflower 3-6 Months Grapeseed oil 6 Months Safflower, Soybean, and Peanut oils 6-12 months Olive, Macadamia, and Almond Oils 9-12 months Corn Oil 12 months Coconut, Palm, Avocado, and Canola Oil Refined oils will keep refrigerated for 6 months to a year. Unrefined oils are less stable and will only keep for 4 or 5 months purchase them in small quantities References Grootveld M, Silwood CJL, Addis P, Claxson A, Serra BB, Viana M (2001). "Health effects of oxidized heated oils". Foodservice Research International 13: 41–55. Guillén M, Uriarte P (2012). Aldehydes contained in edible oils of a very different nature after prolonged heating at frying temperature: Presence of toxic oxygenated a,ß unsaturated aldehydes. Food Chemistry 131 (3), 915–926. Halvorsen BL, Blomhoff R (2011). Determination of lipid oxidation products in vegetable oils and marine omega-3 supplements. Food Nutr Res. 2011;55. doi: 10.3402/fnr.v55i0.5792. Przybylski O, Aladedunye FA (2012). Formation of trans fats during food preparation. Can J Diet Pract Res. 2012 Summer. 73(2), 98-101.