Colours and their influences on sensory perception of products

Consumers have clear (although unconscious) expectations of the optical appearance of their food. The appearance of a product is generally the first feature that can be perceived by the senses. Alongside the colour, other appearance factors include the shape, surface condition, transparency or visible texture properties. However, it is the colour that stands out most, that triggers likes or dislikes, and that supplies the first information about the product quality (for instance the ripeness) or about product properties (such as degree of roasting).

1. Colour and sensory analysis

1.1. Colour vision & colour vision deficiencies

Colours influence the way we perceive a food – provided that we are in a position to see colours. The retina of the eye contains two kinds of photoreceptors – cones and rods. These convert incident light – a physical signal – into an electrical signal that is passed on to the brain. Three different types of cone are responsible for short, medium and long-wave rays, i.e. are sensitive for colour vision.

Rods are responsible for black and white vision. People with normal colour vision can differentiate colours well, but it is much more difficult to remember colours. Colour vision deficiencies are certainly frequent, but they are divided unequally between the genders. For instance one out of twelve men but roughly only about one out of two hundred and fifty women have limited colour vision.

• Protanopia = red colour blindness

• Deuteranopia = green colour blindness

• Tritanopia = blue colour blindness

Just like the other sensory organs, the eye is affected by aging processes. That is why the ability to see colours also changes in old age, as the eye lens becomes cloudier.

Colour vision can be important in the food industry. When panellists, persons trained in sensory analysis, describe products with their properties, they usually mention the colour too. In quality control, typicity of the colour can be examined instrumentally (see DLG Expert report 4/2015) or visually by staff. References such as colour charts or colour fans can be used for this purpose. Comparing the product being analysed with a reference sample or colour standard is known as the “similarity method”.

1.2. Influence of the food colour on the sensory perception of a product

The colour of a food generates a certain product expectation. From childhood onwards yellow desserts are linked with vanilla. We learn this from the colour of vanilla custard and vanilla pudding, even though vanilla pods are black. Red is associated with fruity and ripe, while green is associated with a lack of ripeness, for fruits that are both green and ripe (Granny Smith apples) or fruit vegetables (green zebra tomatoes) are relatively rare.

The eye can deceive other senses by the product expectation generated. In a study conducted with untrained testers, Chardonnay that had been coloured pink was assessed as the fruitiest sample but with the least body, least maturity and least complexity. When the wine was coloured red, it was assessed as having the most body, the most maturity and most complexity.

1.3. Influence of the crockery on the sensory product perception

The influence of the crockery on the perception of foods and beverages has only recently been a subject of more detailed investigation. The colour can act in two ways – it can intensify or weaken the perceived intensity of foods, and it can generate expectations that influence the tasting.

According to a study, strawberry mouse tastes more intensive, sweeter and better on a white plate than it does on a black plate. Hot chocolate drinks are enjoyed to different extents depending on the colour of the beaker. The hot chocolate was rated best in orange beakers. An orange beaker also boosted the sensation of a chocolate taste by comparison with a red or a white beaker.

The colour surroundings or the lighting can also influence the perception of a food. This starts during shopping in the retail environment and ends with consumption. Initial studies have produced interesting results. British test persons characterised the same whiskey differently depending on the room – in a green room they found it was grassiest, in a red room sweetest, and in a wooden room it tasted woodiest. The willingness to eat apples was greatest in yellow light, followed by white light. The willingness was lower in cases of red or green illumination, and lowest of all in blue light. Yellow and white light also triggered the greatest willingness to eat red peppers. The crunchiness of the apples and peppers was not affected by the colour of the light. Butter was felt to be more spreadable when it appeared yellower. Under red light, on the other hand, the yellower butter samples in a study were not rated as being more spreadable.

1.5. Consequences for sensory analysis practice

As all colour environments can influence the perception of the product to be tested, sensory analysis laboratories are fitted out in white, beige or light grey. The light and crockery are kept constant. If the colour of the product to be tested is to be masked, appropriate glasses (e.g. black wine glasses, blue olive oil tasting glasses) are used, or coloured light is switched on.

Panellists who are required to assess product colours are subjected to colour vision and colour differentiation tests. This can be done with the aid of an Ishihara test, in which numbers have to be identified on colour plates. Another Fig. 1: The colour of the cup influences the taste of the beverage 4 DLG Expert report 3/2017 method is to produce coloured solutions that the test persons have to arrange in the right sequence. Details of how to produce the coloured solutions can be found in the standard DIN EN ISO 8586.

The colour is also a quality criterion for many products in DLG tests, for instance in fruit juices. In the case of bread, the degree of browning is considered, in the case of cheese any unnatural colour, and in the case of scalded sausages whether these are too pale, too dark, uneven in colour or off-colour.

2. Colouring substances in foods

Many foods contain colouring substances in a high concentration by nature. This is conspicuous and unusual for instance in the case of blue potatoes, violet cauliflower or black maize. These are rarities or old varieties which are, however, seldom ever used in industrial processing, for instance violet maize crisps.

2.1. Colouring foods vs. food colours (additives)

Beetroot, spinach, hibiscus, curcuma, tomatoes, grapes, peppers or carrots are among the colouring foods. They are considered to be food ingredients. They display the respective aromatising physiological effects alongside their colouring qualities, and by contrast with additives they are not subject to any maximum limitations, obligation to make a declaration or to label with E-numbers. They are used to colour beverages, confectionery, dairy products, fruit preparations and ice-creams. Whether they are deemed to be colouring foods or additives of natural origin depends on the treatment and technology methods. Without the use of physico-chemical separation processes such as reverse osmosis or extraction methods, the product is also a colouring food, just as it is when concentrated by water loss under the influence of temperature. If colouring ingredients are not selectively enriched during extraction, the product is also generally a colouring food. However, if selective enrichment is carried out, if the colouring properties dominate, and if other characteristic attributes such as odour or taste are lost, the products are food colours, e.g. as in the case of betanin (E 162), the food colour of beetroot, or chlorophyll (E 140), which is obtained from green plants such as spinach. In 2013 the European Commission adopted a Guideline (Guidance Notes on the classification of food extracts with colouring properties) to define the boundary between colouring foods and food colours. Food colours from natural raw materials that are not foods, and those from source materials of non-natural origin, are always additives. The advantage of purely synthetic colouring substances by comparison with the natural colouring substances lies in the fact that they are heat-stable and light-stable.

Food colours are counted as additives (E-numbers). E-numbers are regulated in the EU Regulation 1333/2008 (FIAP — food improvement agents package). Food colours range between E 100 and E 180 (see table).

2.2. Purpose and history

The addition of food colours can be expedient because on the one hand the food can lose or change its original colour during production or storage, and on the other hand light, air, heat and moisture all affect the colour. For example, strawberry jam looks less red if it is exposed to light. Moreover food colours serve to match the colour shade of foods from different batches. Adding colouring substances or food colours accordingly helps to retain the original colour, or is used to reinforce the natural colours.

Food colours have been used in food production already since 400 BC. In those days the early Egyptians and Romans coloured wine with herbs and spices in order to make it more attractive. Synthetic colours were developed in the late 19th century, also for purposes of decoration and to lend a higher-quality look to foods. During this period colour additives were still toxic and there were no legal specifications. As of the early 20th century colour additives were used widely in the USA and Europe. Up to the time that synthetic colouring substances were developed, dyestuffs obtained from animal, mineral and vegetable sources were used.

Although “clean” foods are fundamentally safe and of high quality and do not constitute “misleading”, in consumer assessments the use of ingredients similar to additives is frequently considered to be “felt” as misleading. In a survey conducted by the German Federal Ministry of Food, Agriculture and Consumer Protection in May 2013, 26 % of the interviewees responded to the statement “without food colours, but coloured with carrot juice” by saying “I feel deceived” and 13 % with “I definitely feel deceived”. This is extremely surprising – carrot juice and carrot soup are appreciated because of their attractive colouring, but a fruit yoghurt coloured with carrot juice is rejected.

3. Implications for practice

3.1. Producers

Colour design affects not only packaging and formulations of the general product range, but also limited seasonal and event-related batches such as products for Halloween, St. Nicholas’ Day, major sporting events, weddings etc.

In the ready-to-eat segment intensive colour experiences can occasionally lead to extra consumption. A combination of monochrome varieties can attract more attention than multi-coloured offerings. In the same way it is possible to combine products in monochrome fashion in the frozen food segment. Furthermore, producers can provide consumers with new ideas and possibilities of use, for instance in the form of monochrome recipe ideas.

Target groups that respond particularly to monochrome or assorted coloured foods are children and senior citizens. It has been shown that children and adults eat more if jelly balls are offered in 24 colours instead of 6 colours. However, this only applies if they are offered sorted by colour. Vegetable mixtures often do not attract children, not even if they like the individual components. That is why products for children are often “colourful monochrome” and designed in colours that are sometimes not very attractive for adults, for example blue ice-cream.

Why not sort the fresh food department by colours, instead of by varieties? In the same way it would be possible to arrange fruit yoghurts not by brand, but by variety and colour.

Monochrome promotions are more conspicuous in a product programme that is in any case coloured. Many retail chains publish customer magazines with recipes. Depending on the seasonally available colours and the region, monochrome meal ideas are conceivable here too. In this way it would be possible to sell goods on a “seasonal and regional” basis without over-working these catchwords.

4. Black & white food

What foods are black or white? We have compiled the natural diversity of non-coloured foods in Table 2. This feature can also be found in old and special fruit and vegetable varieties, such as white strawberries, white aubergines or black chilli. To retain the white colour, attention must be given to low temperatures during preparation. It is primarily anthocyanins in highly concentrated form that are responsible for black foods. In sepia, this role is played by grey-black melanin. Foods are coloured black with the aid of Brilliant Black, caramel or vegetable carbon. White foods do not contain any colour pigments. Foods can be coloured white with calcium carbonate or titanium dioxide.