Stability and storage of vegetable oils

Stability of oils

The stability of an oil depends on its composition and storage conditions.

Since oils contain practically no water, there is normally no risk of microbial spoilage during storage. Oxygen can attack the oil and cause it to oxidize. The process runs from a slight increase in primary and secondary oxidation products, through a rancidity which can also be perceived by the senses, to polymerization and resinification or complete drying of the oil. It is very complex and not reversible.

Oxidation can be accelerated by light, heat and chemical catalysts (e.g. heavy metals), and slowed down by the addition of stabilizers (natural antioxidants such as tocopherols or synthetic antioxidants such as BHA). Since oxidation normally starts at the double bonds of the fatty acid molecules, polyunsaturated oils become rancid much faster than saturated fats.

Storage of oils

During storage, the following points contribute to a longer durability:

• Protection against oxygen: Closed containers, as full as possible. Use nitrogen or other inert gases. Avoid decanting or other contact with air. Packaging materials such as metal or dark glass or plastics with oxygen barrier can extend durability

• Protection from light: opaque packaging, storage not in the sun or under direct light

• Protection from heat: The lower the temperature, the slower the chemical processes. In cold storage, the oil becomes more or less cloudy or even completely solid, depending on its triglyceride composition. This process is reversible, but the whole package must be heated (if necessary also above room temperature) and homogenized to avoid separation (fractionation) in high and low melting components
   
• Protection from catalysts: stabilizers with chelating agents such as citric acid can bind heavy metals

When assigning a best-before or retest date, the composition of the oil and the packaging must be taken into account. The extension of the retest date is possible, for this purpose a sensory evaluation is recommended as well as the analysis of the peroxide value, by which primary oxidation products (hydroperoxides) are detected.

Esterification

Esterification alters the properties of fats and oils by redistributing fatty acids on the glycerol framework. During the esterification process, fats and oils are broken down briefly into their component parts, before being re-arranged and re-assembled to produce custom fats/oils for specific purposes.

Examples of this include special baking fats derived from palm seed oil and coconut oil. Esterification reactions are very slow. That is why strongly ionic catalysts, such as sodium, sodium hydroxide and sodium alcoholate, are added. Because of the large numbers of different fatty acids present in natural fats and oils, there are countless possible combinations, which is why fats and oils are used that feature certain glyceride classes in particular (e.g., oils with C8-C14 chain lengths ).