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Esters | Fats and oils

Esters

Esters are compunds formed from the reaction between alcohols and acids. The word 'ester' alone now signifies by common usage that the acid is an organic acid, but inorganic acids can also form esters - ATP is an example well-known to biology students, being a phosphate ester. Even halogenoalkanes can be regarded as inorganic esters, of alcohols and hydrochloric acid. This section is, however, confined to esters of organic acids RCOOR' where R can be hydrogen or an organic group:

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Esters in the food industry

Esters are widely used for flavourings; many are 'nature-identical', that is synthetic versions of the esters present in the fruit. Fruit flavours are very complex, though, often arising from many different compounds, some of which are present in small quantities.

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Ethyl methanoate (ethyl formate): rum flavouring

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Propyl pentanoate (n-propyl n-valerate): pineapple flavouring

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Ethyl butanoate (ethyl butyrate): apple odour

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Octyl ethanoate (n-octyl acetate): orange odour

 

Fats and oils

            Fats and oils are tri-esters of glycerol, propan-1,2,3-triol, with carboxylic acids. These are sometimes called fatty acids owing to their presence in fats. The difference between oils and fats lies in their melting temperatures rather than in any fundamental structural difference. The constituent compounds include: tetradecanoic acid  CH3(CH2)12COOH; hexadecanoic acid,  CH3(CH2)14COOH;  octadeca-9,12-dienoic acid (linoleic acid),
CH3(CH2)4(CH=CHCH2)2(CH2)6COOH; octadeca-9-enoic acid (oleic acid),  CH3(CH2)7CH=CH(CH2)7COOH;  propan-1,2,3-triol (glycerol), HOCH2CH(OH)CH2OH.

 

Margarine:

Margarine contains:

 

*   oils and fats, up to 10% of which can be milk fat;

*   an aqueous phase, often skimmed milk;

*   salt and flavouring;

*   vitamins – the amounts of vitamin A and D regulated by law to be similar to those in butter

*   colour, usually natural such as b-carotene;

*   emulsifiers.

 

            Soft margarines contain a higher proportion of oils and therefore a higher proportion of unsaturated fatty acids than hard margarines. Spreads contain less oil and fat and more water. The composition of a margarine will depend on the climate where it is to be sold.

Margarine manufacture:

 

*      The oils are refined. Rapeseed, palm, sunflower and soya bean oils are used; hydrogenation and fractionation removes some impurities; fat-insoluble substances are removed with water; free fatty acids are neutralised with sodium hydroxide; colour is removed by a suitable adsorbent.

*      Unsaturated oils are hydrogenated to increase their melting temperature and improve their resistance to oxidation which causes rancidity. Hydrogen reduces C=C bonds in the presence of a nickel catalyst at about 150oC.

*      Interesterification allows the substituents on the carboxyl group to be altered, which modifies the melting temperature of the fat.

*      Further impurities are removed using aqueous sodium hydroxide to neutralise free acids and steam which removes volatile compounds.

 

Further reading:

The Essential Chemical Industry: Chemical Industry Information Centre, 4th ed., 1999.

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