AQA C1b (ii Plant Oils)

Plant oils

Some plant materials (eg nuts, fruit and seeds) are rich in oils. These can be removed by crushing the material and then filtering or distilling the oil. As well as providing a source of energy, these oils can supply other nutrients.

We all know that water and oil don’t mix well. When shaken together they form an emulsion. On standing, the emulsion usually separates out into its components unless an emulsifier is added. Mixing oil and vinegar (which water-based) together to make mayonnaise is a good example. If no egg is added, the mayonnaise soon breaks down to give the separate oil and vinegar but if a little egg is used in the mix, it makes the emulsion much more stable. The egg is said to be an emulsifier.

The texture and appearance of emulsions make them suitable for many uses such as salad dressing and ice-creams. An interesting example is at http://www.rsc.org/chemistryworld/News/2008/February/19020801.asp

In general, vegetable oils are unsaturated (whereas animal fats are generally saturated). The unsaturated molecules have kinks in their chain so they cannot lie together snugly. This means they have a lower melting point so they are usually liquids at room temperature where most animal fats are solid.

We can measure the amount of unsaturation in a molecule by testing with bromine water. Unsaturated molecules change bromine water from orange to colourless. (Iodine can also be used and it goes from purple to colourless or brown to colourless depending on the conditions.).

Although it is generally accepted that vegetable oils are better for you health-wise, most people prefer to spread a soft solid (like butter) on their bread. By partial hydrogenation of the oil, we can remove some of the double bonds and make it more like an animal fat.

Hydrogenation involves treating the oil with hydrogen gas in the presence of a nickel catalyst at moderately high temperature. This process adds hydrogen across some of the double bonds. It can be controlled to put in just the right amount of extra hydrogen so that the product has the desired qualities of texture.

Unfortunately, sometimes this hydrogenation can lead to an increase in the trans-fat content and this is considered to be bad healthwise.

The health problems associated with saturated fats are to do with cholesterol. Unsaturated oils increase the amount of “good” cholesterol (HDLs) while saturated ones increase the amount of LDLs, the “bad cholesterol”.

Food additives are not all based on plant oils but this is a convenient place to consider them. “E-numbers” are given to chemicals for which there has been a series of safety tests and a judgement made that the chemicals are safe to use in food.

Preservatives, colourings, antioxidants, emulsifiers, sweeteners and flavourings can help us to have food that is longer-lasting, better tasting and more attractive. Not all “E-numbers” are artificial substances. Many are natural substances. Even so, there are some disadvantages to their use. In particular, there is some concern about the link between some colourings and hyperactivity in children.