Fats and Oils are effectively the same chemicals, with fats solid at room temperature and oils liquid. Fats are important molecular components of food in terms of taste and mouthfeel, providing a smoothness and tenderness to a food's texture. For example, the flavour of meat comes from the fat, so a piece of beef without any fat has hardly any flavour difference from a lean piece of mutton. The human diet needs only a small amount of fat each day, because it can make most of the essential fatty acids. A few grams of fat are needed to carry certain vitamins - A, D, E and K which are insoluble in water. Then the only fatty acid that is needed is probably linoleic acid, with the other two necessary fatty acids (linolenic acid and arachidonic acid) probably made by the body. In general, health practitioners believe that the amount of fat eaten in Western diets is excessive and unhealthy, and can (and should) be reduced wherever possible - in the UK, the average person gains 12.6% of their energy from fats when the recommended level is 11%.
Fats and oils are complex molecules built from a group of substances, called lipids. Complex lipids form molecules called esters, of which fatty acids are a class; these then combine with an alcohol to give us fats - usually a combination of a fatty acid and glycerol (commonly known as glycerine). There are many types of fatty acids and all fats and oils are combinations of these. The simplest fatty acid is formic acid (HCOOH) and the next acetic acid (CH3.COOH). These chains of fatty acids can get longer and longer as in most fats, varying in length and weight. The shortest molecules are volatile and have strong smells, hence the strong rancid smells when fats break down; butyric acid is a short fatty acid and is the main smell of rancid butter, while caproic, caprylic and capric acids have "goaty" smells. Longer, heavier molecules are not volatile, have no smell and tend to be solid at room temperature, whereas shorter and medium-sized fatty acids are usually liquids at room temperature.
Saturation and Rancidity
As above, fatty acid molecules include both carbon (C) and hydrogen atoms (H) in long chains. When these chains contain only carbon atoms and the fullest complement of hydrogen atoms, they are termed saturated, i.e. totally filled with hydrogen. If some fatty acid molecules include extra bonds between pairs of carbon atoms in the fatty acid chain (double bonds), there is one less hydrogen atom in the chain; these double bonds are chemically less stable than a single bond. These fatty acids are unsaturated, i.e. more hydrogen atoms could theoretically be added in. Following on from this, then if only one double bond is include then it is mono-unsaturated, but if there are many (or more than one) it becomes poly-unsaturated. Most of the longest fatty acids are saturated, while unsaturated fatty acids tend to be shorter, but the shortest fatty acids are saturated. Therefore, increasing the level of saturated acids in a liquid oil tends to turn it to a solid fat.
The more double bonds, i.e. more unsaturated, in a fatty acid chain, then the less stable the fatty acid molecule is. This double bond is an active site in the fat chain, so unsaturated fats are less stable - especially those that contain linolineic acid (e.g. soya oil) and more easily attacked by oxygen in the air. This attack results in oxidation of the fatty acid chain, breaking down the molecular chain. These shorter molecules are volatile and have strong, unpleasant smells, so unsaturated oils develop an unpleasant smell and taste - we term this rancid or rancidity. Fish oils and soya oil, both of which are highly unsaturated, are very likely to become rancid. However, where double bonds are replaced by extra hydrogen via a process called hydrogenation, then this oxidation process is prevented. Hydrogenation also raises the melting point and the unsaturated oil becomes saturated - this is what happens in margarine, i.e. a vegetable oil is hydrogenated and so hardened.
Alternatively, bakers and food manufacturers add antioxidants to fats to slow down the fats becoming rancid. Sugar is a good antioxidant - biscuits with sugar go rancid much more slowly than those without sugar. Some herbs and spices are also powerful antioxidants, because their essential oils are mildly antiseptic so restrain the microorganisms that cause rancidity. Effective herbs and spices against rancidity include: allspice, cloves, rosemary and thyme.
Oil is affected by cold temperatures, because some of the heavier molecules solidify when they get below their solidification point. The oil will cloud and a solid deposit forms at the bottom of the bottle. This does no harm and the oil will reliquefy if warmed.
Fats and Oils in Food
Generally, vegetable oils contain a higher proportion of monounsaturated and polyunsaturated oils than animal fats. However, there are exceptions - coconut oil and palm oil are highly saturated, whereas fish oils are mostly unsaturated. Saturated fats are the "bad" fats, because the liver converts these into cholesterol that can result in fatty deposits "furring" up the arteries and so restricting blood flow. The recommended daily amounts of saturated fats are: 30 g per day for men; 20 g per day for women; and less for children. Levels of saturated fats are given below:
Ratio of saturated, monounsaturated and polyunsaturated in common oils
|Sesame seed oil||16||39||45|
|Sunflower seed oil||13||25||62|
From a food perspective, the best oils are cold-pressed, retaining most of their flavour and nutritional qualities and as used in the best olive oils and nut oils. Unfortunately, while cold pressing gives the best flavour and most of the "good" natural substances from the plant, it also contains enzymes which will attack the oil, resulting in it not keeping well and tending towards rancidity. Butter is another example of a simply made fat with good flavour, but which does not keep well.
Cold pressing does not remove all the oils from the nut or seed, so the crushed plants are then heated and solvents often used to wash the remaining oils from the plant tissues. The solvents are then evaporated and recovered to be reused, leaving the oil. Most oils that come from oilseeds taste revolting and/or are impure and inedible (e.g. cottonseed and rapeseed oils), especially if heat-treated or extracted with solvents as is the case with many mass produced oils. Therefore, these impure oils must be refined. Refining removes gums and unwanted flavours. These oils are, also, decolourised and deodourised, so stripping out much of their flavours but also destroying any beneficial vitamins.
Fats and Oils In Cooking
Fats and oils contribute to the palatability of food, by providing a smooth and tender texture, plus also carrying a decent amount of flavour. These flavours derive from chemicals that dissolve in fat rather than water, but are not the fat itself - e.g. the fats in meat provide the distinguishing flavours of beef and lamb.
Fats and oils are mainly used for their ability to aid cooking, because they are not significantly changed by moderate heating which enables them to diffuse heat well through food via deep frying and sautéing. Oils do start to break down when overheated and they begin to give off smoke - e.g. soy oil has a "smoke point" of 256oC (492oF) and does not catch fire until c. 350oC (662oF). Oils will, however, burn when food breaks off and falls into the oil, reducing the "smoke point".
Oils used in deep frying degrade from repeated heating to the "smoke point", breaking the molecules down into free fatty acids that become rancid. Other oils polymerize, clump together and make the oil "heavy and sticky".
In addition to their use as oils, fats and oils are used as ingredients. Fats have a rich flavour and provide a pleasant texture, or mouthfeel to foods - good olive oil or butter on bread tastes delicious without the need for additional flavours. Solid fats in pastry gives it a crumbly mouthfeel. The fat crystals force the starch particles in the flour apart and retard the formation of gluten, keeping the pastry "short". If the pastry becomes warm when making it, the crystals melt, gluten forms and the pastry becomes tough - that's why it is best to make pastry on cold boards, e.g. marble, and with cold hands. Fats with large crystals, e.g. lard, make better pastry, while smaller crystals, e.g. vegetable oils make better cakes. In cakes, these vegetable fats spread evenly through the flour and lubricate the surfaces of starch and gluten, so they slide together when eaten and the cake's texture is tender.
However, fats can become rancid, so to retard rancidity in baked goods, antioxidants are added - bakers add sugar to biscuits and cakes, so increasing the life of these by four times over biscuits without sugars in them. Other antioxidants that aid the shelf life of fats include: allspice, cloves, oregano, rosemary, sage and thyme.
- Public Health England (2014) New National Diet and Nutrition Survey shows UK population is eating too much sugar, saturated fat and salt, 14 May 2014, accessed 14 June 2016 
- NHS (undated) Fat: the facts, accessed 14 June 2016 
- Davidson, A. (1999) The Oxford Companion to Food, p.292, Oxford University Press. ISBN 0192115790