What is rancidity

Experiment: Detection of butter acid in butter
Experiment: Detection of C-C double bonds in fats with bromine water
Experiment: Detection of free fatty acids in used fat

When food gets old or goes bad, it starts to go moldy (bread), becomes sour (milk) or it goes rancid like fats and oils. Light, oxygen, moisture as well as microorganisms and enzymes are responsible for the development of rancid fat. Under the influence of these factors, the fats are broken down and breakdown products are formed that not only smell and taste bad, but are also partially harmful to health. You are probably familiar with the smell of the more than foul-smelling butyric acid that z. B. arises from the spoilage of butter.

Rancid butter
(Photo: Dani)

Fats have two weak points in their molecule that can be particularly easily attacked by oxygen, water or enzymes: The Ester bonds and the Double bonds. A distinction is therefore made between two ways of fat spoilage, hydrolytic cleavage and autoxidation.

Hydrolytic cleavage
If the bond between glycerine and the fatty acids is split with the addition of water, one speaks of hydrolytic splitting or lipolysis:

The products created in this way are usually physiologically harmless, but such fats are perceived as spoiled, since mostly low to medium-chain fatty acids are formed that have an unpleasant smell and taste. Just 1 µg caprylic acid and 10 µg capric acid per gram of fat are sufficient to make a fat inedible (by the way: 1 µg = 0.000001 g!). Long-chain fatty acids, on the other hand, can hardly be detected by the senses.
Hydrolytic cleavage can also be brought about by enzymes derived from vegetable and animal fatty tissue or from microorganisms. These enzymes are called lipases.

Lipolysis cannot always be equated with fat spoilage. There are also foods in which hydrolytic cleavage is more than desired, i.e. actually indispensable. These foods include cheese, whose ripening process requires the formation of free fatty acids for good aroma development.

In autoxidation, a multi-stage and quite complicated radical reaction, the oxygen in the air attacks the unsaturated fatty acids at their most sensitive point, the double bonds. The more double bonds there are, the higher the rate of oxidation and the resulting fat spoilage. A characteristic of this type of reaction is that it only gets going slowly and the first oxidation products can only be detected after a certain storage time - the induction period. However, as the process progresses, the rate of reaction increases exponentially.

Elementary steps of autoxidation:
1. Start reaction:
First, peroxy radicals (ROO·), Alkoxy radicals (RO·) or alkyl radicals (R·).

2. Chain growth:
In the second step, the radicals generated in the initial reaction react with additional oxygen or fatty acid molecules to form hydroperoxides (ROOH), which are very unstable and therefore continue to react immediately.

3. Chain branching:
The hydroperoxides break down again into peroxy radicals (ROO·), Alkoxy radicals (RO·) or alkyl radicals (R·).

4. Chain termination:
The radicals react with each other to form stable products such as B. ketones, alcohols, epoxides and aldehydes. The latter are largely responsible for the rancid taste and smell.

The autoxidation can produce so many radicals and peroxides that deep fryers can literally explode - a trigger for deep fryer fires, which are then intensified by incorrectly extinguishing them (namely with water).

Fatty and fatty foods always contain traces of heavy metals. You can get in there in a variety of ways, e.g. B. by heavy metal-containing enzymes, the equipment used in fat extraction or the packaging material. Heavy metals such as iron, copper and cobalt can start the autoxidation by decomposing existing hydroperoxides with the formation of radicals.

Men + + ROOH ———> Me(n + 1) + + RO· + OH-
Me(n + 1) + + ROOH ———> RO2· + H+ + Men +

(Me = heavy metal)

Furthermore, the autoxidation of the fats depends on the water content; the drier a food, the greater the risk of spoilage.

Enzymes can also transfer oxygen to fats. These are lipoxygenases that are widespread in the plant kingdom. They specialize in the essential fatty acids linoleic, linolenic and arachidonic acid.

There are many ways to prevent fat spoilage. It is important to use an oil or fat correctly in the kitchen, e.g. B. one should not use oil, which contains a lot of unsaturated fatty acids, for deep-frying french fries. But storage also plays an important role. To protect the fat from heat and light, it should be stored in a cool and dark place.
In the food industry, various additives can be added to fats or fatty foods, depending on the legal situation (additive approval regulation). These include substances that bind fatty acid radicals, as well as ascorbic acid, which can "defuse" oxygen or radicals from the outset, or citric acid, which complexes with the catalytically active heavy metal ions forms. All of these substances are summarized under the term antioxidants.

We have a tip of the month for fat spoilage.

Further texts on the subject of `` milk ''