Playing with nature’s rainbow

From turmeric to paprika to chlorophyll, natural colours offer a rich range of choices — and challenges. S Roenfeldt Nielsen and S Holst explore the critical parameters to consider

The range of natural colourings goes from yellow through orange, red and green to brown and black. In many cases, different raw materials or blends of these can be used to achieve the same or similar shades, whereas others are very distinct.

Learning how to juggle these many choices isn?t brain science, but becoming acquainted with the unique properties of each major natural colour can make a formulator?s job much easier.

Below, we will explore the raw material, processing, typical end products, applications and critical parameters of the major natural colours on the market today.

The turmeric plant, Curcuma longa, is cultivated in many tropical countries, with the primary commercial production taking place in India. It has been used as a spice for thousands of years and is one of the principal ingredients in curry. The main colouring pigment is curcumin. The turmeric roots are harvested around February, ground to a powder and subsequently extracted with solvent. The resulting turmeric oleoresin has a curcumin content of 37-55 per cent and the same relative proportion of flavour compounds and colour as in the spice. The pure colour is produced by further crystallisation of the oleoresin resulting in a product with minimum 90 per cent curcumin and very little of the flavour compounds. It is insoluble in water and poorly soluble in other solvents

Typical products for use in the food industry have a curcumin content of 4-10 per cent and are achieved by dispersing pure curcumin in a mixture of food-grade solvent and emulsifier, by dissolving in vegetable oil, or by spraying onto starch.

Curcumin in aqueous media has a lemon-yellow colour with a distinct green shade at low pH. Typical applications include dairy products, sugar confectionery, ice cream, water ice, bakery and savoury products. Curcumin has good heat and acid stability, but is sensitive to light. It can, however, be used in products packed in nontransparent packaging.

Annatto is the seed of the bush Bixa orellana, which is mainly found in Central and South America, and East Africa. The seeds grow in large clusters of capsular fruits that are dried in the sun, cracked open and taken out by hand. The basic colour pigment is bixin, a carotenoid that is found in the thin resinous coating of the seed. It can be extracted in different ways to yield oil-soluble extracts, oil suspensions or water-soluble extracts.

Oil-soluble extracts are made by extraction of annatto seeds with hot vegetable oil, resulting in products with low colour content, typically 0.1-0.2 per cent bixin, as bixin has poor solubility in oil. By suspending the undissolved colour pigment in vegetable oil or fat base, the content of bixin can be increased by up to five per cent. Oil-soluble extracts and oil suspensions are yellow-orange and suitable in products with a high quantity of oil present, ie, margarine and butter, salad dressings and extruded snacks.

By extracting the annatto seeds with water and potassium hydroxide, bixin is converted into the water-soluble pigment norbixin. Water-soluble solutions typically contain 0.5-4 per cent norbixin, and powders with concentrations as high as 15 per cent can be achieved through further spray-drying. Bixin and norbixin can also be blended to produce colours applicable in both water and oil-based foods.

Typical applications for norbixin products are cheese, ice cream and water ice, bakery, sugar con-fectionery and beverages, where they give a yellow-orange colour. Both norbixin and bixin are reasonably stable to heat, whereas light stability is best if norbixin is bound to protein, such as cheese. Norbixin may precipitate at low pH and cannot be mixed with products containing calcium. However, it is possible to formulate norbixin products stable at low pH.

The source of paprika is found in the pods of the sweet red pepper, Capsicum annuum, which is grown in Spain, Central Europe, the US, Asia and Africa. It is used by the food industry for both colouring and flavouring purposes, giving an orange-red colour and a characteristic taste. Paprika consists of several different carotenoid pigments that develop during ripening. The most important are capsanthin, capsorubin and beta-carotene, accounting for about 90 per cent of the total pigments. The strength is expressed as colour units (CU) per gram. For use as a colour, paprika oleoresin is obtained by solvent extraction of the pods. The extract is standardised with vegetable oil to the desired colour strength, the most common being 40,000, 60,000, 80,000 and 100,000 CU. These extracts can be further processed to obtain water-miscible products, typically by incorporating an emulsifier during production.

Paprika is typically used in savoury products such as meat, soups, sauces and snacks. Deflavoured products are available for use in sugar confectionery and beverages. Stability to heat and light is fairly good and can be improved considerably through stabilisation with antioxidants.

Other carotenoids
Carotenoids are widespread in nature; most fruits and vegetables contain mixtures of carotenoids, and more than 600 different types have been identified. Besides annatto and paprika, commercial colouring of food and beverages primarily uses mixed carotenoids, beta-carotene and lutein. Nature?s richest source of mixed carotenoids is the fruit of the oil palm tree, Elaeis guineensis.

Predominantly grown in Malaysia and Indonesia, the oil palm tree contains a mixture of beta- and alpha-carotene with traces of gamma-carotene and lycopene. The fruits are first processed to crude palm oil and further steps include distillation, purification by HPLC, and standardisation to an oil suspension from which oil- and water-soluble products in different strengths can be made. The oil-soluble products give a fresh yellow colour shade in application whereas the water-soluble emulsions can range from yellow to yellow-orange.

Beta-carotene is the most common of the single carotenoids and is used extensively in the food and beverage industries. Most beta-carotene applied today is manufactured by synthesis resulting in a molecule equivalent to that found in nature. However, natural sources are available.

Beta-carotene produced from the micro algae Dunaliella salina combines photosynthesis with microbial fermentation in open ponds in Australia, Israel and the US. The algae are grown in 30 per cent salt concentration with high light intensity, high temperature, and with or without mechanical agitation. Under these conditions, the algae produce large amounts of carotenoids and can accumulate beta-carotene up to 10 per cent of their dry weight.

Beta-carotene is purified from the dried algae by a series of processing steps including concentration, enzyme treatment, extraction, saponification, partial crystallisation and dispersion in oil at a concentration of 20-30 per cent. Beta-carotene can also be obtained from a fermentation process with the mold Blakeslea trispora. Two types of the fungus are mated in a specific ratio and large amounts of beta-carotene are synthesised in aerobic submerged batch fermentation. Beta-carotene is subsequently recovered by solvent extraction, purification, concentration and crystallisation to obtain either a crystalline beta-carotene with purity greater than 96 per cent or a 30 per cent micronised suspension in oil.

Both mixed carotenoids and beta-carotene are available as oil- and water-soluble products in a number of different strengths. They are technically suitable in a wide range of applications such as beverages, sugar confectionery, ice cream and water ice, dairy products, fruit preparations, bakery, and spreads, and have good stability toward heat, light and pH. Their application is most often limited by their price.

Lutein is the solvent extract of dried flower petals of marigold, Tagetes erecta, which has been further concentrated and deodorised. It can also be derived from alfalfa grass as a by-product of chlorophyll extraction. Following extraction it can either be suspended in vegetable oil or made water dispersible by the addition of an emulsifier.

Lutein gives a warm yellow colour in application, but is not widely used in food. It has advantages over turmeric in, for example, beverages and sugar confectionery when good light stability is required. Besides their use as a colour, carotenoids also have nutritive properties and are therefore also used as active ingredients in health products.

Anthocyanins are polyphenolic pigments responsible for the red to blue colour of a wide range of fruits and vegetables
Anthocyanins are a group of natural polyphenolic pigments responsible for the red to blue colour of a wide range of fruits and vegetables. Traditionally the most common sources of industrially used anthocyanins have been grapes, elderberry and blackcurrant, but the past years have also seen new sources launched in the market such as red cabbage and black carrot. Due to an intramolecular co-pigmentation of their phenolic structure, they are more stable to light, pH and heat than traditional sources. The fruits and vegetables used for anthocyanin production come from cultures with a yearly crop and are primarily grown in Europe.

The different raw materials are extracted with water, concentrated, standardised and pasteurised. Subsequently they can be spray-dried to powder formulations using maltodextrin as a carrier. Anthocyanins are available in different strengths from various sources and in both liquid and powder formulations. The main food applications are beverages, fruit preparations, sugar confectionery and water ice.

All anthocyanins are pH dependent and they change colour hue from red at low pH through blue to brownish-green at high pH. Their colour intensity is also reduced with increasing pH, and they are most commonly used in low pH applications. Stability to heat and light is generally good, but it varies with the source.

Red beet
Red beet colour is obtained from the beetroot vegetable Beta vulgaris, which has been cultivated for centuries in temperate climates. The pigments present in beetroot consist of a group of red and yellow pigments. In most varieties of beetroot, the red pigment betanin is the predominant colouring compound, representing 75-90 per cent of the total colour present.

Beetroot is extracted by physical means, often a press operation under acidic conditions. It is further concentrated by ultra filtration and pasteurised to yield a viscous liquid with 0.5 per cent betanin. The juice can further be spray-dried to a powder with a betanin content of about 0.35 per cent and using maltodextrin as a carrier.

Red beet gives a nice strawberry colour in ice cream, dairy products, fruit preps, jams and jellies, as well as sugar confectionery products. It is a very intense colour and dosage levels are usually low. However, the pigment is susceptible to heat degradation and oxidation, which limits its use but can, in part, be overcome by adding the colour after heat treatment. Stability is highest at pH 4.5, and red beet colour is not recommended for alkaline applications.

Chlorophyll is present in all plants capable of photosynthesis. The usual sources for colour production are grass, spinach or alfalfa, and chlorophyll is extracted from the dried plants by solvent extraction.

During extraction, a magnesium ion is wholly or partly lost from the pigment, making it more unstable and resulting in a product that is duller in appearance and of an olive-green colour. This chlorophyll extract is oil-soluble and can be further standardised with vegetable oil or blended with a food solvent or emulsifier to produce a water-miscible form. The products are sold as uncoppered chlorophyll.

The magnesium ion, which is lost during the extraction process, can be replaced with copper to produce a more stable complex with a higher tinctorial strength. This complex, copper chlorophyllin, is made by further processing of chlorophyll extract with water and alkali addition.

Chlorophyll is recommended where a green colour is required in bakery products, dairy products, sugar confectionery, cereals, jams and jellies. Chlorophyll is also used as a complementary colour, when decolourisation of yellowish cheese milk is needed. Chlorophylls are most stable under alkaline conditions, and in diluted acids they will lose colour rapidly and copper chlorophyllins will precipitate. Copper chlorophyllins are relatively heat and light stable.

Excerpted from Colour in Food - Improving Quality, Douglas B MacDougall, editor. ISBN 1 85573 590 3. Published by Woodhead Publishing Ltd, Cambridge, England. Respond: [email protected]

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