F&B

Colour and cooking

The notion that 'the eye eats before the palate' stands true even today. Chefs therefore need to pay particular attention to presentation. Chef Kaviraj Khialani elaborates on the use of colour in cooking

The substances in food that give them their colour are known as pigments. Most of them belong to closely related groups of chemicals and minor chemical differences between them give different colours. All this applies to natural and artificial colours alike. Examples of the former are the green chlorophyll pigment in plants and the orange pigment in carrots and carotene. The latter include many pigments derived from coal tar (all 12 artificial colours permitted by EEC regulations are coal tar derivatives).

Most plants contain several pigments whose relative proportions may vary considerably, producing colours which differ noticeably from each other. This is not only true of broad categories such as, say, apples, but applies to apples of one particular variety or strain. Hereditary variations produce differences in chemical composition and even minute differences of this kind affect colour. Sometimes the results are quite dramatic, even freakish. Chelation is another phenomenon that affects colour, among other things.

The groups of pigments discussed below are chlorophyll, carotenoids, flavonoids (including anthocyanins, betazyanin and betaxanthin), and various kinds of globin. All but the last are plant pigments.

Chlorophyll

Green vegetables must stay green when cooked and not turn olive. Chlorophyll is not soluble in water so the green which is to be preserved will not leach out into the cooking water, which is a blessing. However, there are lots of other things which can affect it. Chlorophyll is a complex substance; it is quickly affected by an alkali or an acid, and by the application of heat.

The effect of an alkali, whether already present in the cooking water or added in the form of soda bicarbonate, is to prevent the replacement of the central magnesium atom in chlorophyll with hydrogen. The vegetable then acquires an unnaturally bright green colour. Some might find this attractive but the penalties to be paid are formidable. Not only is there a heavy loss of vitamin C but the cell walls of the vegetable are disrupted and becomes slimy. So it is best not add bicarbonate of soda.

Acid has a different effect. Chlorophyll reacts with acid to give an olive-coloured product called pheophytin. This is clearly not wanted, except in artichokes! But acids are naturally present in vegetables. A possible solution is to cook the vegetables in plenty of water so as to dilute their natural acids and to leave the pan uncovered to allow volatile acids to escape. But this increases the loss of vitamin C. The dilemma must be resolved according to the individual cook's priorities, or by resorting to a different method of cooking described below.

The effect of heat, in brief, is to decompose the chlorophyll, producing once more the unattractive olive colour. The effect is not immediate. If a green leaf or pea is dropped into boiling water its colour at first intensifies - for reasons which experts still debate but which are probably due to physical action, for instance the removal of the layer of air between the outer skin of a pea and the inside. It is only a little later that the green changes to olive. The extent of the change will depend on the temperature and time of cooking. Less chlorophyll will be decomposed and more of the green colour retained if the vegetables are cooked briefly. Cutting vegetables such as cabbage makes it possible to shorten the cooking time.

However, there is still one more hazard to consider. The action of enzymes in vegetables, which cause the chlorophyll in green vegetables to fade are being stored for a time. The enzyme that affects chlorophyll is called chlorophyllase. When blanching green vegetable before freezing, it is a good idea to bring them to boil as quickly as possible, thus hustling this enzyme through its temperature of maximum activity (70oC/160oF) and then quickly destroying it.

The above comments apply particularly to cooking green vegetables by boiling. Clearly, this has inescapable disadvantages. An alternative is to stir-fry the vegetables quickly after cutting them. This reduces the loss of colour and vitamin C.

Carotenoid

These constitute a large group of yellow, orange, and red pigments found in both plants and animals. The most common carotenoid is B (beta) carotene. It is, incidentally, the constituent of carrots which our bodies break up to produce vitamin A, that aids night vision.

The carotenoids, like chlorophyll, are insoluble in water but unlike chlorophyll, they are not affected by the presence of an alkali or an acid. The application of heat may speed up the gradual fading which will be induced by exposure to air and prolonged storage. But it does not produce very noticeable changes. One particular change which occurs is something of an oddity - the carotenoid pigments in swedes turn to a deeper yellow when cooked.

An example of a carotenoid pigment in an animal is the red colour of cooked lobsters and certain other crustaceans. A live lobster is purple or stale-coloured. So is a dead, uncooked lobster. In these situations the red pigment is chemically bonded to a protein and does not appear as red. Cooking breaks the bond and 'releases' the red colour. The yellow colour of egg yolk is due to the pigment zeaxanthin (which may be enhanced by including factory-made beta carotene in the hens' diet). Zeaxanthin occurs naturally in maize too.

Flavonoids

This general term embraces some groups of pigments whose names invite confusion: flavones, flavanols, etc. Flavones occur in what are thought of as 'white' vegetables like white onions, potatoes, cauliflower, and white cabbage that in certain circumstances produce a yellowish or brown colour. If this occurs because of cooking the vegetables in hard water, a little lemon juice will help.

Tea contains a common flavone - quercetin - and this is why it becomes slightly paler when a slice of lemon is put into it. Flavones can also produce trouble when vegetables containing them are cooked in aluminum vessels, which may produce yellow or brown chelates; or from contact with iron pans, steel utensils, and cans, which may cause green, blue, red, or brown chelates. Again, adding lemon juice will forestall this kind of change.

The anthocyanin group

Pigments in this group are related to the flavones but give red, purple and blue colours. They are present, for example, in strawberries and other berries, cherries, red apples, and pomegranates, and also in red cabbage, aubergines, and radishes. They are soluble in water and show a marked sensitivity to the pH factor. In an acid solution they are usually red, whereas in an alkaline one they are violet or blue. If flavones are also present, a green colour will be produced by the mixture of yellow and blue.

Since these pigments occur in so many fruits, their behavior is of interest to those making fruit punches. To achieve a pleasing red colour, for example use red or blue fruit juices and ensure acidity by adding lemon juice.

The most mutable of this group of pigments is the one found in red cabbage. In acidic conditions, it is red. When the pH factor is around neutral it is violet. And once the balance has swung over to the alkaline side it becomes blue-grey. When cooked in alkaline conditions (over pH 8) it turns green. So, to keep it red, it has to be cooked in acidic conditions with the addition of a little lemon juice if necessary.

Related pigments known as proanthocyanins are responsible for the pink colour of cooked pears and quinces. However, they will only turn pink in acidic conditions. Cooks tend to like the colour but food processors, in curious contrast, take pains to avoid it when canning pears by ensuring non-acid conditions. (Perhaps they are concerned to give the impression that canned pears are not 'cooked'.)

Betacyanin and betaxanthin

These two pigments, present in beetroot, fall outside the main groups. Betacyanin is red-violet, betaxanthin yellow. When a beetroot is cooked, its cell walls are made porous and these colours leak out, creating the familiar deep red effect. Betacyanin is an unusually stable red pigment. However, if a highly alkaline cooking medium is used, it may turn yellow. Acid conditions, on the contrary, will make the red colour more purple and the addition of an acid to beetroots which have turned out yellow will make them red again.

Tannins

The term has a more general application and extends beyond something present in tea. It refers to a whole group of substances which are related to the flavonoids but distinguished by the fact that they combine an astringent quality with a tendency to produce dark hues such as blackish-blue and blackish-green. They are generally what give unripe fruits an astringent taste. Green persimmons contain a large amount of tannin, which is why they have a markedly astringent, puckery taste. When the persimmons ripen, the membranes enveloping the cells in which the tannin lurks become hard and insoluble and the taste ceases to be evident.

Globins: The colour in meat

Globins, which provide the red colour in meat, are examples of chelation. They are numerous and each has a name which consists of globin with an attached prefix. Some explanation of how one globin changes into another, and so on, promotes understanding of the colour changes in meat, whether fresh, cured or cooked.

Some of the colour in the meat comes from the red haemoglobin in the blood, but mostly from myoglobin in the muscle tissue. Fresh myoglobin combines with oxygen from the air to give bright red oxymyoglobin, which is what the surface of recently killed meat exhibits. Inside such meat the unoxygenated myoglobin remains purple.

Prolonged exposure to the air, as when meat is hung, causes a further oxidisation of the myoglobin, which now becomes metmyoglobin. This, left to its own devices, will become brown. But there are bacteria at work on the surface of the meat and these appropriate some of the available oxygen with the result that a purplish shade is produced. So the surface of properly hung beef should be a depressing purple-brown, not an attractive deep red.

Two qualifications must be added: first, animals which are thoroughly bled after being killed lose their hemoglobin. Also, they are not usually hung so their flesh is a light, clear pink colour. Secondly, meat sold in shrink-wrapped trays is to some extent protected from oxidation, so its surface remains purplish red with less of a brown tinge. When raw meat is cooked, the outside turns brown and this colour gradually spreads inwards. The interior lightens from purple to pink. Outside, where oxygen is present, brown haemichrome is formed and inside, where it is not present, the result is pink haemochrome.

The author is the head of department for food production, Kohinoor College of Hotel Management, Mumbai. He has also written the book Arabic Cookery. He can be reached at kaviraj21@hotmail.com