Capturing the perfect colour

For many years, manufacturers avoided natural colours because they were more difficult to manage than synthetics. This is no longer true. S Roenfeldt Nielsen and S Holst explore the dozens of factors to consider — from food matrices to pH to microencapsulation — when picking a shade for your product

From a technological perspective, the use of natural colours is more complex than that of synthetics. However, by being aware of the different critical parameters, it is possible to apply natural colours with good results in most food and beverage products. Some parameters should be considered before choosing a colour in order to find the most suitable solution in a given application.

Initially, the shade and intensity of the colour solution has to match the target group as well as the consumer?s perception of the flavour. Children tend to be attracted to bright and vivid colours, whereas adults prefer more subdued shades. There are also large geographical variations in the perception of colours related to a specific flavour, and thus a strawberry colour may differ from orange-red to bright pink. The chosen colour should also match the colour of the relevant packaging and any other products within the same brand.

Food matrix
The food matrix will indicate whether to use oil- or water-soluble colours. In emulsions that have both an oil and water phase, it will generally be most economical to colour the continuous phase, ie, to use oil-soluble and suspendable colours in margarine and cream fillings, and water-soluble colours in milk drinks, dressings, etc. However, in products with a high content of the dispersed phase, such as mayonnaise and low fat spreads, a deviation from the general rule of thumb can be more affordable.

Some ingredients such as sugar and protein have a tendency to stabilise most natural colours, whereas others — such as salts, some colloids, ethanol and sometimes flavours — may have a negative influence on the stability of the added colour. High amounts of ascorbic acid, sulphur dioxide and other antioxidants may result in degradation of colours like anthocyanins.

In emulsions and suspensions, the intensity of a colour will vary with the amount of dispersed phase, the particle size and the content of dry nonfat solids. For example, a water-soluble pigment in a low-fat milk drink will look brighter and more intense compared to the same dosage in a milk product with a higher fat content.

Adapting to pH
Acid stability of the colour is required in low pH applications like soft drinks and many confectionery products. Alkaline formulations of chlorophyll and annatto colours are the most sensitive, and acid-proof formulations should be preferred. However, increased acid stability may be achieved if the annatto pigment norbixin is able to bind to protein, such as fruit pulp in fruit-based beverages.

Alkaline carmine formulations can be used in acid conditions if the colour is entrapped in the viscous food matrix. Hence water-soluble carmine has excellent stability in icelollies (Popsicles), hard-boiled candy, gums, jellies, and the like. If used in a nonviscous product like soft drinks, the aluminium bond in the carmine may break and the colour will precipitate, thereby changing hue toward a more orange shade.

Some colours, like all the different anthocyanins, change shade with varying pH. At pH around 3, where they are most commonly used, they will display bright pink and reddish hues. Moving toward neutral pH, the colour will change through purplish-red to mauve and bluish shades, and, depending on the anthocyanin source, can even appear dull and dark.

Impact of background
Background colour can influence the colour of the final food or beverage. This can come either from other ingredients, such as fruit juices, spices and dairy fat, or be generated during production, such as Maillard reaction products. It may be necessary to mask an off-colour, and it can be quite a challenge to colour a greyish or brownish base bright and clear compared to starting from a pure white background.

The choice of colour can also be influenced by the clarity of the background. Colours that look very similar and bright in a clear matrix may appear totally different in a white base — where one can appear dull and another may look vivid and appealing.

Heat stability and aeration
Processing parameters are also essential to consider, the most critical one normally being heating. Not all natural colours are equally heat stable. Colours like carmine and turmeric are generally considered to have excellent heat stability and can be used in, for example, UHT products. Red beet shows poor heat stability and tends to turn brownish during heating. The actual food matrix may also influence the heat stability.

Aeration, as seen in ice cream, desserts, some confectionery products and expanded crisps, will dilute colours, thereby lowering their intensity and increasing their lightness, so a higher dosage of colour might be needed. For some applications it is also important that the colour does not contain emulsifier, as this can reduce the expansion and influence stability of the incorporated air cells.

When recommending dosages it is also important to consider the actual size of transparent and semi-transparent food products. The distance the light is transmitted influences the intensity of the colour, and therefore a bigger jelly will look darker and more intense compared to a small one, even with the same recipe. Likewise a soft drink in a small bottle will appear lighter than the same drink in a larger bottle.

Light transparent packaging material will normally require light stable colours. Conventional turmeric formulations and red beet are the most light-sensitive natural colours, but low water activity tends to stabilise especially turmeric and thereby delay fading. However, due to technological developments, light-stable natural-colour products including turmeric are now commercially available.

Compared to other expenses, the price of colours is small in most food and beverage products. However, the cost in use of different natural colours is still a very important factor. Generally, yellow colours like turmeric and annatto are economical to use, whereas synthetic beta-carotene, natural beta-carotene and mixed carotenoids are rather expensive in application. Within the natural red shades, red beet is normally inexpensive but also has limitations with regards to functionality. The bright and stable colours originating from carmine and different anthocyanins are more expensive.

Ensuring quality
When receiving natural colour products from the supplier, it is normal to carry out a quality control inspection. The extent of this control will typically depend on the type of colour product, the information available on the certificate of analysis, the expected processing conditions, the finished product and the standard quality control procedures of the food or beverage manufacturer.

In some cases the colour strength or colour hue and intensity may be the most relevant parameter. In others, the microbiological standard and absence of pathogens is also important. For powder formulations to be used in dry blends, the particle size is relevant, whereas density can be central if the colour is volumetrically measured by flow meter during the production process.

Most colour products are recommended cool storage (4-8?C) to minimise both the risk of microbiological growth and the degradation of colour pigments. Colour formulations containing sugar, protein or relatively high amounts of water as, for instance, many emulsions and liquid extracts of beetroot, elderberry, etc, will be sensitive to microbiological hazards. Other products that have low colour strengths in general have shorter shelf lives due to degradation of pigments. Paprika, however, should be stored at ambient temperature to avoid separation due to coagulation of different components at low temperature. Many colours in powder form are hydroscopic and must be protected from humid air to avoid clotting.

In general it is important to follow the storage guidelines from the supplier to ensure stability during the shelf life period and an optimal performance of the natural colour during later use. Besides the many factors to be aware of when selecting natural colour products, the handling during production is also important. Good guidance on how to handle colours will normally be provided by the supplier in the safety data sheet specifying the directions of use as well as any relevant hazards.

When handling colours during processing, the risk of contamination of especially the more sensitive colours can be minimised if it takes place in clean environments. The primary hazard related to natural colours is the possible irritation of skin and eyes when using alkaline formulations.

Improving functionality
Many food and beverage manufacturers are still reluctant to use natural colours due to former issues such as dull colour shades, poor stability with rapid fading and difficulties in handling. However, over the past 10 years, natural colour manufacturers have discovered new techniques within formulation and processing that are able to resolve these issues. Combined with the use of high-quality raw materials, it is now possible to develop and manufacture products that have better functional properties both during handling and in the final food products.

Most efforts have been put into developing natural colours with improved stability toward low pH, light and heat, and with increased brightness of different pigments. The technologies that have been applied are microencapsulation, and the addition of antioxidants, emulsions and oil suspensions.

Microencapsulation of different substances is a popular technique that is being increasingly applied in a variety of industries. For flavours, microencapsulation can be used to improve their overall stability as well as delaying their release until the optimal time during cooking or eating. In the foregoing examples it is imperative that the microencapsulation is broken down at a given time or under a given circumstance. For colours, however, the challenge is to maintain the protection given by the coating, despite the influence of different chemical and/or physical factors in the final food and beverage products or during their manufacture.

As the majority of food and beverage products are water-based, microencapsulation of colours is especially beneficial for pigments, which are not soluble in water. Examples of these are various carotenoids such as beta-carotene, lutein, annatto and paprika, as well as turmeric, carmine, chlorophyll and carbo vegetabilis.

A number of different microencapsulated colours are available that vary with regard to coating, production method and final formulation. The coating can be created with diverse food ingredients such as pectins, gums and carbohydrates, but also proteins and lipids may be used. Several different production methods for microencapsulated colours have been published, which include processing steps such as blending, comminuting, standardisation and drying. A high and specified quality of raw material must be assured, and it is also important to monitor colour shade and functionality at critical control points, both during manufacture of the microencapsulated colour and in the final product. The microencapsulated colour may be available in both liquid and powder form.

There are a number of advantages that can be achieved with microencapsulation: the colours are easier to handle, weigh out and add into food systems; and the higher water dispersibility allows fast and efficient cleaning of processing equipment. Control of particle size is essential to ensure optimal use of the colour pigments. The colouring effect is a result of light reflection from the particle surface, and thus there is a close correlation between the total particle surface and the colour intensity. The particle size distribution is also of great importance for the perceived colour stability in liquid products such as beverages, as larger colour particles have a tendency to sediment.

Carmine and curcumin are two of the colours where major advantages can be achieved by microencapsulation. For carmine, stability under acid conditions is greatly improved, and it is possible to use this colour in beverages to achieve a pink, cloudy appearance. In standard formulations, curcumin normally shows rather poor stability when exposed to light, but by entrapping it in appropriate encapsulation material, it is possible to achieve excellent light stability and a bright lemon colour (see graph on page 52). This is especially appreciated in sugar confectionery products, soft drinks and dressings when they are packed in transparent packaging. An increased light stability is also achieved with encapsulation of other colours, such as annatto and paprika.

Migration of water-soluble colours can be a problem in many different food products such as layered desserts, edible ice, processed fish and decorations, where a defined line between different colours is key. The advantage of using microencapsulated products is that their coating stays intact and therefore they do not migrate as opposed to many conventional colours.

Microencapsulation also increases the brightness of many natural colours, a property that is beneficial when blending. Where many conventional colours may turn dull and grey when blended, microencapsulated products stay vivid and attractive. In this way, food and beverage manufacturers can reduce the number of colours they need — an advantage both in relation to storage space and for label declarations of multicoloured products, like sugar confectionery.

Addition of antioxidants
The structure of carotenoids generally consists of a chain of conjugated carbon- carbon double bonds. This is the case with the commercially available colours-mixed carotenoids: beta-carotene, lutein, annatto and paprika. As the double bonds can be subject to oxidation, these colours will often be formulated with different antioxidants, such as various tocopherols, ascorbic acid and rosemary extract. The antioxidants can be used singly, but are more often combined specifically to give optimal protection of a given colour. The protection applies both in the colour itself during storage, but primarily during processing and shelf life of the final products.

With the use of antioxidants, it is consequently possible to increase the stability of paprika, carotenes and lutein colours considerably, and thereby match the shelf life requirements in products such as soft drinks, soups, sauces and dressings, meat products and sugar confectionery. Besides the increased stability during storage, the colours are better able to withstand conditions such as high heat treatment, severe light exposure and high salt concentrations, where conventional formulations may fail.

As previously mentioned, a number of natural colour pigments, like carotenoids, are not soluble in water. To match the technical requirements of the primarily water-based food products, emulsions of these colours have been commercially available for a number of years. Their limitations however, can be seen in liquid products such as beverages, where they might result in settling of colour in the bottleneck, the so-called ring formation.

The challenge for a natural colour manufacturer is to formulate an easy-to-disperse emulsion, which is stable in many different applications. This is done through a careful procedure of optimising processing conditions, choice of emulsifier and other ingredients to achieve a colour product with droplets of optimal size and surface properties. The emulsifying agent will often be different esters or mono- and diglycerides, but also different types of gums and modified starches can be used. By controlling the homogenisation process of the emulsified colour product, the right droplet size is achieved. Small emulsion particles (below 1lm) will give a transparent appearance in clear applications, whereas larger particles will result in a cloudy appearance. As differences in particle size may also give rise to variations in colour shade, a good process control is also essential during the manufacture of emulsion colours.

At the same time, this technology also gives good opportunities to create various products with different colour hues in the final application. Further requirements to the appropriate emulsifying system can also be stability at low pH, as well as lack of reaction with other ingredients in the final application, such as alcohol in flavour formulations.

Oil suspension
In the same way that emulsifying or microencapsulation can be used to create a broader spectrum of natural colours for use in water-based media, it is also possible to extend the range of colours for fat-based applications with pigments that are not naturally soluble in fats and oils. This is done by milling, for example, carmine, red beet, carbo vegetabilis and other natural colours in oil to add pink, red, purple and black shades to the already existing possibilities within green, yellow and orange. By optimising the shape and size of the suspended particles, it is possible to create colours with a very bright and vivid appearance, a high colouring ability and excellent stability.

Furthermore, appropriate milling will help avoid settling during colour storage and thereby make the colour products easier to handle. Oil suspensions produced by milling are suitable for fat coatings to enrobe ice cream, nuts and fruit, as well as for cream fillings and flavoured spreads.

Excerpted from Colour in Food — Improving Quality, Douglas B MacDougall, editor. ISBN 1 85573 590 3. Published by Woodhead Publishing Ltd, Cambridge, England.
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Impact of encapsulation on turmeric colour

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