Lipid oxidation and rancidity
Why fats go rancid: autoxidation, photo-oxidation, enzymatic and hydrolytic rancidity, what speeds them, measuring oxidation, antioxidants, and a PV sum.
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Cooking oils, nuts, whole-grain flours, fish, milk powder, crisps and biscuits all contain fats that can turn rancid, developing stale, cardboard, fishy, soapy or paint-like flavours long before any microbe spoils them. Rancidity limits the shelf life of many foods, destroys essential fatty acids and fat-soluble vitamins, and some oxidation products may be harmful. Understanding how fats deteriorate, and how to slow it, is a core part of food chemistry.
Lipid oxidation and its control are standard topics in food chemistry, oils and fats technology, and food packaging exams.
Types of rancidity
- Oxidative rancidity: reaction of unsaturated fatty acids with oxygen, the most important type.
- Hydrolytic rancidity: lipase enzymes or heat and water split triglycerides into free fatty acids; short-chain acids such as butyric acid give the sharp, soapy or cheesy taste of rancid butter and milk.
- Enzymatic oxidation: lipoxygenases in plant foods such as soybeans and peas oxidise fatty acids, giving "beany" and grassy flavours; blanching inactivates them.
Autoxidation: a chain reaction
- Initiation: heat, light, metal ions such as iron and copper, or existing hydroperoxides remove a hydrogen atom from a fatty acid next to a double bond, forming a lipid radical (L•).
- Propagation: the radical reacts with oxygen to give a peroxyl radical (LOO•), which takes a hydrogen from another fatty acid, forming a hydroperoxide (LOOH) and a new lipid radical. The chain repeats.
- Termination: radicals combine into stable products, or antioxidants donate hydrogen and stop the chain.
Hydroperoxides themselves have no smell, but they break down into secondary products, aldehydes (such as hexanal), ketones and alcohols, which cause rancid odours even at parts per million.
Oxidation typically shows an induction period, during which little change is noticed, followed by rapid deterioration.
Photo-oxidation
Light, with photosensitisers such as chlorophyll in olive oil or riboflavin in milk, produces highly reactive singlet oxygen, which attacks double bonds directly and much faster than normal oxygen. This causes "light-struck" flavours in milk and oils stored in clear containers.
Factors that speed oxidation
- Degree of unsaturation: linolenic acid (three double bonds) oxidises far faster than linoleic (two), which oxidises far faster than oleic (one). Fish oils, rich in long-chain omega-3 fatty acids, are especially vulnerable.
- Oxygen availability and surface area, as in powders and ground nuts.
- Temperature: rates roughly double for each 10 to 15 °C rise.
- Light, especially ultraviolet and blue light.
- Metal ions (iron, copper) as catalysts.
- Water activity: oxidation is slowest around the monolayer water content, about aw 0.2 to 0.3, and faster in very dry foods and at intermediate moisture.
Frying oils
During deep frying, oils are heated to about 160 to 190 °C in contact with air and water from food, so oxidation, hydrolysis and polymerisation all occur. Oil darkens, foams, smokes more and develops off-flavours. Quality is monitored by total polar compounds, which many countries limit to about 24 to 27 per cent before oil must be discarded.
Measuring oxidation
- Peroxide value (PV): milliequivalents of active oxygen per kg of fat; measures primary oxidation products; rises then falls as hydroperoxides decompose.
- p-Anisidine value (AV): measures aldehydes, secondary products.
- TOTOX value = 2 × PV + AV, combining both.
- TBARS (thiobarbituric acid reactive substances) for malondialdehyde, common in meat.
- Free fatty acids for hydrolytic rancidity.
- Accelerated tests such as the Rancimat, measuring the induction time at high temperature with air flow.
- Sensory evaluation and gas chromatography of volatiles such as hexanal.
Controlling rancidity
- Antioxidants:
- primary (chain-breaking): natural tocopherols (vitamin E), rosemary extracts, and synthetic BHA, BHT, TBHQ and propyl gallate at legal limits;
- secondary: chelators such as citric acid and EDTA that bind metals, oxygen scavengers and ascorbic acid.
- Exclude oxygen: vacuum, nitrogen flushing, oxygen absorbers, low-permeability packaging.
- Exclude light: opaque or tinted packaging.
- Cool storage and inactivating enzymes by blanching or heat.
- Avoid metal contamination and use stainless steel equipment.
- Refining oils to remove free fatty acids and pro-oxidants, and hydrogenation or blending to reduce unsaturation, although partial hydrogenation creates trans fats and is now restricted in many countries.
Measuring peroxide value
A 5.0 g sample of oil is dissolved, potassium iodide is added, and the iodine released by hydroperoxides is titrated with 0.01 N sodium thiosulfate. The sample needs 3.2 mL; a blank needs 0.2 mL.
PV (meq O2/kg) = (sample − blank) × normality × 1,000 ÷ sample weight (g)
- PV = (3.2 − 0.2) × 0.01 × 1,000 ÷ 5.0 = 3.0 × 10 ÷ 5.0 = 6 meq/kg.
Codex standards for named vegetable oils allow a PV of up to 10 meq/kg for refined oils and 15 for cold-pressed and virgin oils. With an anisidine value of 8, the TOTOX value = 2 × 6 + 8 = 20; refiners often aim for a TOTOX value below about 26 for good-quality oils.
Key points
- Rancidity is oxidative (autoxidation, photo-oxidation, lipoxygenase) or hydrolytic (lipases releasing free fatty acids).
- Autoxidation: initiation, propagation (hydroperoxides), termination; secondary aldehydes cause off-flavours.
- Faster with more unsaturation, oxygen, heat, light, metals; slowest near the monolayer water activity (about 0.2–0.3).
- Measures: peroxide value, anisidine value, TOTOX = 2PV + AV, TBARS, free fatty acids, Rancimat.
- Control with antioxidants (tocopherols, BHA, BHT, TBHQ), chelators, oxygen and light exclusion, cool storage and enzyme inactivation.
What the exams ask
- School-leaving and entrance papers: what rancidity is and how to prevent it.
- Degree and research-fellowship entrances: mechanisms of autoxidation and photo-oxidation, factors affecting oxidation, measurement methods, and antioxidant types.
- Food technology examinations: shelf-life testing of fatty foods, oil quality standards, packaging and antioxidant regulations.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do fish oils go rancid so easily?
They are rich in long-chain omega-3 fatty acids with five or six double bonds, which oxidise very quickly. They need antioxidants, nitrogen-flushed packaging and cool, dark storage.
Is rancid oil harmful?
Rancid oil tastes unpleasant and has lost vitamins and essential fatty acids; some oxidation products may be harmful in large amounts. It is best discarded.
Why does whole-wheat flour have a shorter shelf life than white flour?
It contains the germ and bran, which hold oils and lipase and lipoxygenase enzymes, so it develops rancidity faster.
Sources
- Frankel, E. N. Lipid Oxidation, 2nd edition. Oily Press, 2005.
- Damodaran, S. and Parkin, K. L. (eds) Fennema's Food Chemistry, 5th edition. CRC Press, 2017. Chapter on lipids.
- Codex Alimentarius Commission. Standard for Named Vegetable Oils (CXS 210-1999).
- Shahidi, F. and Zhong, Y. "Lipid oxidation and improving the oxidative stability." Chemical Society Reviews 39, 4067–4079 (2010).
- Choe, E. and Min, D. B. "Mechanisms and factors for edible oil oxidation." Comprehensive Reviews in Food Science and Food Safety 5, 169–186 (2006).
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