OIL AND FAT: RANCIDITY IN FATS
Diagram
Two ways a fat turns rancid
Hydrolytic rancidity
Water and enzymes split the fat into glycerol and free fatty acids. The free fatty acids smell soapy and rancid.
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Oxidative rancidity
Oxygen attacks the unsaturated fatty acids. Hydroperoxides form, then break into aldehydes and ketones that smell stale and painty.
Figure 1: Hydrolytic rancidity splits fat with water, while oxidative rancidity attacks fat with oxygen. Drawn in our own style from the rancidity discussion in Allen and Hamilton, Rancidity in Foods, and in Akoh and Min, Food Lipids.
Introduction
Hydrolytic rancidity and oxidative rancidity both make fats and oils smell and taste bad, and both are called rancidity, but they work in different ways. Hydrolytic rancidity is the splitting of a fat by water, helped by enzymes called lipases, which frees fatty acids from the glycerol backbone. Oxidative rancidity is the attack of oxygen on the unsaturated fatty acids in a fat, which forms hydroperoxides that break down into smelly aldehydes and ketones. A fat can suffer from either one, and the smell tells which one is at work: hydrolytic rancidity gives a soapy, sweaty smell, while oxidative rancidity gives a stale, painty smell.
Quick comparison
Table 1: Hydrolytic Rancidity compared with Oxidative Rancidity
| Aspect | Hydrolytic Rancidity | Oxidative Rancidity |
|---|---|---|
| Cause | Water plus lipase enzymes splitting the fat | Oxygen attacking unsaturated fatty acids |
| What is split | The ester bond between glycerol and fatty acids | The double bond region of unsaturated fatty acids |
| First products | Glycerol and free fatty acids | Hydroperoxides |
| Smell | Soapy, sweaty, rancid, from short chain free fatty acids | Stale, painty, fishy, from aldehydes and ketones |
| Needs water | Yes, water is a reactant | No, but water activity affects the rate |
| Needs oxygen | No | Yes, oxygen is a reactant |
| Needs enzymes | Yes, lipases speed it greatly; heat and moisture also split fat slowly without enzymes | No, but light, heat and metals speed it |
| Fats most at risk | Fats rich in short chain fatty acids, such as milk fat | Fats rich in unsaturated fatty acids, such as vegetable oils |
| How it is measured | Free fatty acid content, by titration with alkali | Peroxide value and other tests for oxidation products, described in Akoh and Min |
| Prevention | Keep the fat dry, keep lipases out, keep it cool | Keep air out, keep it dark and cool, add antioxidants, use closed packs |
| Example | Butter kept damp and warm turning soapy | Cooking oil left open in light turning stale and painty |
Scientific explanation
Definition
Hydrolytic rancidity is the rancidity caused by hydrolysis, the splitting of triglycerides by water into glycerol and free fatty acids, usually with the help of lipase enzymes. Oxidative rancidity is the rancidity caused by oxidation, the reaction of oxygen with unsaturated fatty acids to give hydroperoxides, which then break down into aldehydes, ketones and other smelly compounds.
Working principle
In hydrolytic rancidity, a water molecule breaks the ester bond that joins a fatty acid to glycerol, and a lipase enzyme makes this splitting fast. Each split frees one fatty acid, and short chain fatty acids, such as butyric acid in milk fat, smell strongly even in small amounts, so the fat smells soapy. In oxidative rancidity, oxygen adds to the carbon chain next to a double bond in an unsaturated fatty acid, forming a hydroperoxide. Hydroperoxides have no smell themselves, but they are unstable and break into aldehydes and ketones, which are the compounds that smell stale and painty. Light, heat and traces of metals speed oxidation, and once it starts it runs as a chain reaction.
Apparatus
The books used here name the methods, not a single apparatus list. Free fatty acids are measured by titration with alkali, and oxidative rancidity is measured by the peroxide value and related tests for oxidation products, both described in Akoh and Min, Food Lipids. No full apparatus list with named instruments is given in the books used here, so none is stated.
Measurement procedure
The books used here give no standard laboratory procedure with volumes and steps for either form of rancidity, so no procedure is invented here. In practice, the analyst titrates the free fatty acids for hydrolytic rancidity and measures the peroxide value for oxidative rancidity, and the smell of the fat is the first sign a maker notices.
Factors affecting readings
For hydrolytic rancidity, moisture, temperature and the presence of lipases decide the rate: wet, warm fat with active lipases turns rancid fast. For oxidative rancidity, oxygen, light, heat, the number of double bonds in the fat, and traces of metals decide the rate: a highly unsaturated oil in an open bottle in the light oxidises fastest. Antioxidants slow oxidation but do not stop hydrolysis, and drying stops hydrolysis but does not stop oxidation.
Formulas and calculations
Hydrolysis, in words. Triglyceride + water, with lipase, gives glycerol + free fatty acids. Each water molecule frees one fatty acid from the glycerol backbone.
Oxidation, in words. Unsaturated fatty acid + oxygen gives a hydroperoxide, and the hydroperoxide breaks down into aldehydes and ketones. The books used here give no numerical formula for the rate of either reaction, so none is given.
Industrial applications
- Hydrolytic rancidity: a defect to prevent in butter and milk fat, where lipases and moisture free butyric acid and give a soapy smell.
- Hydrolytic rancidity: a defect to prevent in stored oilseeds and nuts, where moisture lets lipases split the fat.
- Oxidative rancidity: a defect to prevent in cooking oils, fried foods and snacks, where oxygen and light turn the oil stale and painty.
- Oxidative rancidity: the reason oils are packed with little headspace air and kept dark and cool.
- Oxidative rancidity: the reason antioxidants are added to fats and oils in food manufacture.
Advantages and limitations
- Hydrolytic rancidity, advantage: none as a defect, but the same hydrolysis by lipases is wanted in cheese ripening, where free fatty acids give flavour.
- Hydrolytic rancidity, limitation: it makes butter and milk fat smell soapy and unfit to eat, and it needs only moisture and lipases to start.
- Oxidative rancidity, advantage: none as a defect, but measuring it by peroxide value lets a maker judge the shelf life of an oil.
- Oxidative rancidity, limitation: it is a chain reaction, so once it starts it runs on, and it spoils oils, fried foods and snacks even in the dark if air is present.
FAQ
What is the difference between hydrolytic and oxidative rancidity?
Hydrolytic rancidity splits fat with water and lipases into free fatty acids, while oxidative rancidity attacks unsaturated fatty acids with oxygen to give hydroperoxides and then aldehydes.
Which one smells soapy?
Hydrolytic rancidity. Short chain free fatty acids, such as butyric acid from milk fat, smell soapy and sweaty.
Which one smells painty?
Oxidative rancidity. Its aldehydes and ketones smell stale, painty and fishy.
How is each one measured?
Hydrolytic rancidity is measured by free fatty acid titration, and oxidative rancidity by peroxide value and related tests in Akoh and Min.
Can a fat have both at once?
Yes. A damp, warm fat open to air can hydrolyse and oxidise at the same time.
References and further reading
- FTN books on disk: Technology of Fats and Oils.
- Allen, J. C. and R. J. Hamilton. Rancidity in Foods. Springer, 1995.
- Akoh, Casimir C. and David B. Min (eds). Food Lipids: Chemistry, Nutrition, and Biotechnology. 2nd ed., CRC Press, 2002 (Google Books page read)
- Google Books pages read for this article:
- Akoh, Casimir C. and David B. Min (eds). Food Lipids: Chemistry, Nutrition, and Biotechnology. 2nd ed., CRC Press, 2002 (Google Books page read)
