Note: A version of this article appeared on Chemistry in Sri Lanka January 2011 Vol.28 No.1
Sensory properties of food such as colour, flavour and texture make food appetising and attractive to the consumer. Colour is an important attribute of food. We judge the safety or freshness of a food based on its colour. Dinner guests compliment the hostess by looking at the food laid on the table, even before tasting! Our visual perception of food therefore is very important.
Before the implementation of food laws and regulations, colours were used indiscriminately by food vendors. The results were disastrous. Lead, copper and arsenic salts were used to colour food. Meat and fruit products were coloured using azo dyes, originally used in the textile industry. Some of these dyes are now known to be carcinogenic. Today, the list of permitted food colouring is regularly updated so that consumer safety is adequately looked after.
The trend now is to adopt new colours derived from natural pigments that occur in food, such as turmeric, paprika and carotene. However, natural food colours are not easy to isolate and use as colouring material because of their instability outside the natural environment and under processing conditions.
Natural Food Colours
Chlorophyll
The structure of chlorophyll (a) and (b) is shown below.
Chlorophyll a, X = CH3
Chlorophyll b, X = CHO
The presence of magnesium in the molecule is very important in producing the bright green colour of the pigment. Cooking and processing conditions can result in the loss of magnesium which will impart a ‘dirty’ brownish colour due to the formation of pheophytin. This occurrence is highly undesirable in green peas. When peas are canned the acidity formed results in the replacement of Mg 2+ with protons, i.e. the formation of pheophytin. Frequently artificial colouring is added to make the peas look green. One must be cautious that poisonous dyes are not used for this purpose.
Synthetic dyes should always be used with caution. Food chemists are constantly encouraged to examine the possible use of natural derivatives as alternatives.
In the case of chlorophyll, it is known that in the past, cupric sulphate was used, but now a mixture of tartrazine and Green S is most frequently used. In the 18th and 19th centuries vegetables were pickled in vinegar, in copper pans. This gave a bright green colour to the vegetables due to the leaching of Cu 2+ ! Legislation now ensures that pickles are free from copper.
Sodium copper chlorophyllin is now used as an artificial colouring material. It is more soluble and heat resistant than natural chlorophyll, and the amount of copper is negligible: too small to be harmful
Carotenoids
The bright yellow and orange colours in vegetables such as carrots and pumpkin, the bright red colour of tomatoes, and the attractive pink of watermelon are due to the presence of carotenoids.
Carotenoids are generally insoluble in water, being soluble only in non polar organic solvents. Though very stable in their natural environment, heating and extraction into organic solvents make them unstable. Exposure to sunlight, and dehydration can cause fading of colour. Isomerisatin of the trans double bonds to cis causes this loss of colour.
TABLE 1 CAROTENOIDS COMMONLY USED AS FOOD COLOURANTS.
| Pigment | Commercial source |
|---|---|
| Beta carotene, mixed carotenes | Palm oil, carrots, and algae |
| Cis-bixin, norbixin | Annato |
| Capsanthin | Red peppers (Capsicum annum) |
| Lycoopene | Tomato |
| β -Apo- 8’-carotenal | Chemical synthesis |
| Ethyl ester of β -Apo- 8’-carotenal | Chemical synthesis |
| Lutein | Marigold, alfalfa |
| Canthaxanthin | Chemical synthesis |
| Crocin, crocetin | Saffron (Crocus sativus) |
Anthocyanins
A range of pink, red, mauve, purple and blue colours of fruits leaves and flowers are due to the presence of anthocyanins.
The different glycosilation patterns and acylation result in a wide range of natural anthocyanins. The aglycones are known as anthocyanidins, the common ones being cyanidin, pelargonidin, delphinidin, and malvidin. Most anthocyanidins are found in black grape, cyanidin in red fruits such as strawberries and delphinidin in black current, dark grapes and aubergines. At low pH of fruits in the natural environment anthocyanins are stable. However they are very sensitive to pH changes and processing conditions such as treatment with sulphur dioxide and sulphites resulting in bleaching. Anthocyanidins form dimers and polymers with certain flavanoids present in fruits such as grapes, especially during the fermentation of wine, where procyanidins are formed. Procyanidins are more stable than anthocyanidins and give a brown red colour to wines. Some sources of anthocyanins used as food colourants are grape skins and red cabbage.
Fig 2. Transitions in the anthocyanin structure
Fig. 3 Structure of malvidin (3-coumaryl gluscoside)
Betalains
Betalains are divided into two groups, the purple red betacyanins and the yellow beta xanthins. Pigments of significance in food are those present in beet rood (red beet) Beta vulgaris. Ninety percent of beet root betalaines are betacyanins. These are quite different from anthocyanins chemically and in their stability, though earlier they were mistakenly named as ‘nitrogenous anthocyanins’. Their colour is not affected by pH normally found in food and are more resistant to processing. However, they are unstable to heat at neutral pH.
Fig 4. Betanidin
Curcumins
These are derived from turmeric (Curcuma longa). The powdered dried root of the plant is used as a spice and a food colourant especially in oriental cookery. Curcumin is obtained by solvent extraction of the powdered turmeric and is a bright yellow pigment. Due to increasing concern about the potential toxicity of synthetic food dyes, curcumin has become useful in colouring desserts, pickles, icecreams etc. Turmeric also has many medicinal, antioxidant and antiseptic properties.
Fig. 5 Curcumin
Colours formed during processing and manufacturing
In addition to natural colours of plants and synthetic dyes added to food, colours are formed during food processing and manufacture. Browning due to enzyme activity, sugar caremalisation and Maillard browning (initiated by the reaction between reducing sugars and amino acids) are such examples.
The most important attribute of black tea, for which Sri Lanka is famous internationally, is its charatersitic colour and flavour. The orange-red compounds in tea, the theaflavins, are formed during the fermentation stage of tea manufacture. This is in fact an enzymic oxidation and not a microbial fermentation. The tea catechins form dimmers which have a benzotropolone ring system responsible for the colour. Thearubigins are the brown pigments in tea. The ratio of theaflavins to thearubigins determine the final colour of the tea brew. Tea polyphenols cause the astringency of the beverage and are known to have antioxidant properties as well as many beneficial therapeutic effects.
Fig.6 Theaflavin
OR = OH or gallate
Artificial food colourants
As mentioned previously, artificial colouring of food has given opportunities to the unscrupulous vendor to cheat the consumer. In the past, as far back as 1857, a survey of adulterants detected in food revealed that substances such as lead chromate, mercuric sulphide, red lead, and even copper arsenite were used to colour sweets. Aniline dyes which were subsequently developed eliminated the use of metallic compounds.
Synthetic dyes are cheaper, more stable and brighter with a wider range of shades than natural colours. The use of synthetic dye stuffs to colour food therefore steadily increased but at the same time strict legistlation removed protentially toxic chemicals from the list of food dyes, limiting the permitted dyes to only eight in Sri Lanka. These are listed below
- Carmoisine
- Ponceau 4R
- Sunset Yellow FCF
- Tartazine
- Erythrosine
- Fast Green FCF
- Indigo Carmine
- Brilliant Blue
Food colours are mostly used in fruit containing products such as desserts, confectionery, icecreams etc, and in colouring canned peas. Erythrosine is used to colour canned meats. Addition of colour to raw or unprocessed food (meat, poultry, fish, vegetables, tea, coffee..) is prohibited.
Testing food colourants is often confined to looking at evidence for carcinogenicity . However, food colours such as tartrazine are known to cause intolerance in some individuals, manifesting as allergic reactions such as asthma or eczema.
Despite the growing concern about synthetic dyes, colour remains an important attribute by which the consumer is guided when selecting food. Food chemists and technologists therefore play a vital role in ensuring the safety of consumers, while looking after their culinary preferences.
References:
Tom Coultate , Food The chemistry of its components (5th Edition) RSC Publication (2009)
S. Carmen Food colorants: Chemical and Functional Properties CRC Press Boca Raton, Florida (2007)
