Lipids and Fatty Acids

Edible oil refining stages

Crude vegetable oil contains several classes of non-triglyceride impurities, and refining is a sequence of steps in which each step removes one class. Each stage targets a specific impurity, so the stage and the impurity should be learned as a pair.

• Degumming — removes phosphatides (phospholipids), which are hydratable. Treatment with water or dilute phosphoric acid hydrates them into an insoluble gum that settles out and is separated. e.g., Adding a small amount of dilute acid to crude oil produces a gum layer that can be filtered off.
• Neutralization (alkali refining) — removes free fatty acids. Caustic soda converts them into soap stock, which is separated from the oil. e.g., A high free fatty acid content in crude oil is lowered by alkali treatment before bleaching.
• Bleaching — removes pigments such as carotenoids and chlorophyll by adsorption on activated clay or activated carbon. e.g., Adsorbent treatment makes a dark crude oil pale in colour.
• Winterization — chills the oil so that waxes and high-melting glycerides crystallize and can be filtered off. It is a physical separation. e.g., Cooling sunflower oil to a low temperature removes waxes that would otherwise cloud the oil.
• Deodorization — removes volatile odorous compounds by steam distillation under high temperature and vacuum. It is normally the final stage.

The usual order is degumming, neutralization, bleaching, and deodorization, with winterization used where clarity at low temperature is required. Degumming and neutralization must precede bleaching so that the adsorbent is not overloaded with gums and soaps.

Essential fatty acids

Essential fatty acids are polyunsaturated fatty acids that humans cannot synthesize because the body lacks the desaturases needed to introduce double bonds beyond the Δ9 position. They must therefore be supplied in the diet. The two parent essential fatty acids are linoleic acid (C_{18:2}, n-6) and α-linolenic acid (C_{18:3}, n-3).

The omega number ('n-') is counted from the methyl end of the chain. The position of the first double bond identifies the family, and the families are not interconvertible.

• Omega-6 family — first double bond at carbon 6 from the methyl end. Linoleic acid is converted to γ-linolenic acid by Δ6-desaturation, then to dihomo-γ-linolenic acid, and finally to arachidonic acid (C_{20:4}, n-6) by Δ5-desaturation.
• Omega-3 family — first double bond at carbon 3 from the methyl end, as in α-linolenic acid.

A common trap is that two fatty acids can share the same carbon number and number of double bonds yet belong to different families because the double bond positions differ. For example, γ-linolenic acid and α-linolenic acid are both C_{18:3} but are n-6 and n-3 respectively.

Lipid oxidation measurement methods

Lipid oxidation is measured by different methods that reflect different stages of the process, and each value answers a different question about the oil.

• Peroxide value (PV) — quantifies peroxides and hydroperoxides formed in the primary stage of autoxidation, so it indicates hydroperoxide content.
• Thiobarbituric acid (TBA) test — detects malondialdehyde and related secondary products that contain carbonyl groups, so it indicates carbonyl content and secondary oxidation.
• Rancimat method — heats the sample under a stream of air and records conductivity change caused by volatile acids. The time before the sharp rise is the induction time, which is a measure of oxidative stability.
• Iodine value (IV) — the grams of iodine absorbed per 100 g of fat. It is proportional to the number of double bonds and therefore indicates the degree of unsaturation, not oxidation products.

The key distinction is between primary products (hydroperoxides, measured by PV) and secondary products (carbonyls, measured by TBA), with induction time reflecting the overall resistance to oxidation. Iodine value is a structural property rather than an oxidation index.

Dietary fatty acids and LDL cholesterol

Dietary fatty acids affect blood lipids in different ways, and the effect on LDL-cholesterol is an important basis for classifying them in nutrition.