Colour in food

The first thing a person does is to look at the food about to be eaten. An assessment of the colour gives some information about how acceptable and how fresh the food is. For instance fresh beef must be bright red in colour (look for any red lights near the meat counter). Meat that has become a little brown because it has been exposed to the air for too long would not be acceptable (when cooked it would not taste any different). Green meat is not acceptable.

The acceptable colour of a food depends very much on cultural background. Some foods must be certain colours. In NZ, for instance, the flesh of a boiled potato should be white while in Holland a cream coloured flesh is preferred. Dark red and purple-black fleshed potatoes are accepted by Maori.

The colour of a food appears to affect our perception of taste. In a simple experiment where apple juice is coloured yellow to look like lemon juice a number of people will be convinced that it is really lemon juice.

Food manufacturers take great care to stabilise and enhance the colour of food to meet public expectations. They attempt to process the food as quickly as possible to retain the original colour and they take care to stabilise the pH of the food as many food colours change colour at different pH values.

The biological significance of food colour has until recently been dismissed as nutritionally irrelevant. More recently it has been realised that the colours in food may themselves have biological effects apart from creating interest in the food. Lycopene, which accounts for most of the colour of tomatoes, has been largely ignored from a nutritional point of view because it was not a vitamin A precursor, although it is a carotenoid. However, recent evidence indicates that it is a powerful agent to trap singlet oxygen, which is potentially damaging to tissues. The flavonoids in various fruits and vegetables are responsible for some of their colour but they also have an oestrogen like activity. Anthocyanins, which account for colour in berry fruits, have been shown to have LDL-cholesterol lowering properties.

Vegetables and fruits provide the widest range of colours found in human foods. These pigments have quite different chemical and physical properties that become apparent during food preparation. Some food colours are soluble in water and can be lost on cooking (a good example is the red pigment - betaine of beet root) while others can be affected by changes in pH during cooking and preservation of a food. A clever cook tries to preserve as much colour as possible to make the meal have visual impact. People eat with their eyes first!

How colour is measured

Light is radiant energy in the range 400-800 nm. Apart from people who are colour blind the eye is very good at detecting colour and colour differences. The main problem is that we lack a range of descriptive words to cover all the range of colours we see. As far as mixing paint, printing and food are concerned there are three primary colours called the subtractive set (Red, Blue and Yellow). Technically these are called magenta (purple), cyan (sky blue) and yellow - these are used as the basic colours in a colour photocopier (along with black). The colour that is seen from a piece of paper or a food is the light that is reflected (some is absorbed). Painters start with a white piece of paper and add paint which serves to remove colours from the reflected light- the same is true for food. In theory painters and food technologists need only the three colours red, blue and yellow in fact they use many different colours to save the bother of mixing the colours.

Colour is a matter of perception, of subjective interpretation. A range of words are used to describe the colour red but do we mean the same colour? Vermilion, cinnabar, crimson, scarlet could be used to describe the colour red. These are all called common colour names. General colour names are, bright-, dull-, deep-red etc.

There are three components to colour.

Hue (colour) Lightness (brightness) Saturation (chroma)

Apples are red colours can be the yellow of lemon is vivid
Lemons are yellow separated into while the yellow of a
The sky is blue light and dark banana is dull

A number of different systems have been devised to describe colour. Munsell devised a series of colour charts, which gave a letter and numbers to each different colour. This system was widely used to record the colour of soils. The colour of a food or plant material is compared to pages of different hues and chromas, very similar to a paint chart.

When measuring the colour of food, companies depend on instrumental colour measurement as the eye can be easily fooled. Food colour can be measured in two ways either by a spectrophotometer or a tristimulus colorimeter. A spectrophotometer measures reflected light one wavelength at a time over the whole visible spectrum covering the wavelengths 380 to 700 nm.