More than meets the eye

Colour influences our daily lives, whether we are aware of it or not. Sight is our main sense, a large portion of our brain is primed for visual stimuli. Finding and determining potential food is one of the uses for survival. In addition we don't want to eat poisonous plants or animals. Colours help us to find food "at first sight" since red berries between the green branches are easily spotted. It seems we perceive three primary colours, red, green and blue. The theory about the way we perceive colour is not completely elucidated, though. Since the eye and brain are almost inextricably linked it is clear that neurological and even psychological factors are not to be ruled out in the perception of colour.

Not until the late seventeenth century was the nature of colour enlightened by Isaac Newton. However knowing how white light is composed of several colours did not mean there was a theory on colour vision. In 1810 Johan von Goethe published a paper "Zur Farbenlehre" (On colour theory) in which he describes colours in every possible way, physiologically, physically, chemically et cetera. He describes the phenomenon that when looking at a specific colour and then looking to a white wall, an after image remains in the eye which has the opposite colour. This gives us a hint on how our eyes perceive colours. It seems that opposite colours will be seen stronger placed next to each other then when in combination with others.

Colour opposites

The eye and light

Most humans are trichromats, meaning they have three kinds of colour sensory cells (cones). (Though strictly speaking we are all tetrachromats because the rod cells, which are much more sensitive to low light, cover a different region somewhere between green and blue (550-450 nm). This coverage is the reason for the Purkinje effect.) However it doesn't explain how we see the mixture of colours as a pure colour. A different colour can be obtained by taking the basis colours and mixing them like C=rR+gG+bB, where R G B stand for the different independent basis colours and the small letters r,g,b are the intensities with which we mix them. One might be inclined to think that this basis is the only one. This is not true, we can take any combination of three independent basis colours. This is shown in printing inks, where CMYK (cyan, magenta, yellow, and black) are used. Black is only a modifier for the intensity of the colour.

Cone (S,M and L) and rod (R) frequency response. Courtesy Wikipedia Commons

The CMYK in printing are called "subtractive" (opposed to the additive in projection) but only because the white light that is reflected off a surface painted, "loses" colours that are absorbed by the paint. This is obviously not possible with projected light in a rainbow.  Hence it seems that certain colours in the subtractive scheme might never be possible in projected light, like the colour brown. It is the mixture of colours which is left after all but blue or cyan is missing. But the way we perceive brown is in its absorptive nature, and it is darker. So it is a difference in intensity which is telling us that it is brown. It is actually quite strange that we perceive pure colours of the rainbow the same as we do mixed (secondary and tertiary) colours. This tells us something about the way the eye works. Looking at mixed colours, like white light, one cannot, as Feynman puts it in his Lecture series, look harder to disentangle the separate colours, like we can with flavour or even sound.

Food colour

The food we eat has a certain colour, but does the flavour depend on it? It seems that sometimes colour can enhance our experience of tastes, specially sweet. Sweet food has been linked to the colour red. A lot of berries and fruits are red in nature. And it is probably the experience that links the colour to a certain taste.

Beetroot, herring and 'Amsterdam onions' in
mixed UV and normal light

We expect darker coloured foods to be more intense in flavour. This may be due to a lack of water rendering a darker colour and more enhanced flavour. However, generally, the taste of certain colours is just an acquired perception and not innate. Nevertheless it can be fun to mix up perception with expectation by colouring foods in a non-natural way.

Almost all the colours of the rainbow can be found to exist in natural foods, except the colour blue. It is a strange thing that blue foods are not considered natural. For pure colours Red: strawberries, Dark-orange: tomatoes, Orange: carrots, Yellow: corn, Green: All leaf plants, Purple: Blueberries, Violet: violet flowers.

Anthocyanins - the pluriform colouring

The colour that ranges from yellow to deep purple is caused by anthocyanins. You may notice that when the fruit ripens its colour is much more weaker or even white or green. The anthocyanins are made from precursors called anthocyanidines which are colourless. Combining these with sugar the glucosides anthocyanins are produced, which is why when the fruit ripens and is more tasty due to the sugar it is also colourful. Such a nice indicator nature has given us! When the anthocyanins are combined with the already existing chlorophyll a brownish colour is often the result.

Thanks to the anthocyanins in for example red cabbage one can produce a plethora of colours, ranging from red through blue to yellow by changing the pH. Obviously changing the pH will affect the taste of the food, lower pH are acidic and higher ones taste bitter and have a soapy feel. The anthocyanins in grape juice or wine are already in an acidic environment, hence the purple colour. Notice when you wash the wine glasses with soap, the wine turns green or blue.

Anthocyanins in grapes appear purple-red

Egg whites are slightly alkaline, so adding beetroot juice renders them green or blue-ish. So indeed blue as a natural food colouring is possible, although in a certain pH climate. Anthocyanins are water soluble, which restricts their use.

Marbled Eggs

By placing foods in an immersive bath one can let the colour diffuse into the product, and acquire a colour gradient. As diffusion depends on the temperature and the concentration results may vary. Nice examples are chinese tea eggs. The egg is boiled until it is hard. Next the shell is cracked (quite firmly) and placed in a bath and let steep over night. Of course the tea can be changed for different liquids, such as, soy sauce, beetroot juice, spinach juice, curcuma alcohol solution. The original chinese tea recipe is with black tea, soy sauce and 5 spice powder containing a.o. star anise, cinnamon and fennel. 

Chinese eggs coloured with soy sauce, green tea, riboflavin and curcuma

Fluorescence

The curcuma has another interesting property, it displays fluorescence. Hence if we place the eggs under a black light, we see the eggs light up. Pretty spectacular. Fluorescence occurs when a molecule or atom receives light at a high energy (UV) and radiates it back at a lower frequency (usually visible). The quantum yield is the ratio of emitted photons and absorbed UV photons. The quantum yield of curcumin is highly dependent on environmental factors such as the medium, polar solvents producing higher yield, but is very low in water. There are substances which have an even higher quantum yield. Quinine in tonic water lights up blue. Riboflavin or vitamin b12, if you can get it pure, is a wonderful green fluorescent substance and it is safe to eat. The 'amsterdam onions' in the picture above are coloured with riboflavin. Also chlorophyll is fluorescent, however, the colour emitted is a deep red, and not very visible.

Under UV light the marbled eggs look like this.

Piccalilli containing the fluorescent curcuma

Tonic water containing quinine which fluoresces blue

Using the full palate of colours is nice to liven up a plate of food, although, obviously, it should not be made more important than the taste of the food itself. There are plenty of natural food colorants which have a neutral flavour and can be used without ruining the inherent flavour of the product. That doesn't mean that when we combine certain products on the plate we should be completely oblivious to their colour and throw them in there haphazardly. It is a joy to eat something that has been presented to you with care, and with similar tasting foods, sometimes a different coloured one might be better, or a natural colouring can be added. Happy colouring!