of foods is a familiar occurrence in the kitchen, and often a welcome one. People prefer loaves to be brown outside, and like a slice of bread to be browned when toasted. Grilled or roasted meat should have a brown exterior. On the other hand, no one likes to see cut fruits or vegetables turning brown, as many will if remedial action is not taken.
Some compounds formed during browning have distinctive flavours, often ones which are liked, such as that of caramel, but sometimes stale and even repulsive. An understanding of the different ways in which browning occurs will help to illuminate these contrasts.
There are four main causes of browning, which may act separately or in combination at various temperatures.
The simplest is caramelization, which happens only at high temperatures. It is caused by the breakdown of sugars or, indirectly, of starches which first decompose into sugars.
Another kind of browning directly affects starch when it is heated in dry conditions; this is known as dextrinization.
A very common cause of browning is a more complicated reaction known as sugar-amine browning. Sugars and amino acids (or proteins, which are composed of amino acids) react together, usually at cooking temperatures but sometimes at room temperature or below.
Enzyme browning is also widespread. It occurs at low or moderate temperatures only, since the enzymes which cause it are quickly destroyed by heat.
Sugar molecules begin to disintegrate at temperatures above 170 °C (340 °F). They break up in various ways, and the number of different compounds which can thus be yielded is over a hundred. Some of them are brown in colour and bitter in taste, producing the characteristic colour and flavour of caramelization. If heating is continued, caramelized sugars break down further into pure black carbon.
The various types of sugar differ noticeably in the extent to which they caramelize. Fructose and sucrose caramelize readily, but dextrose (glucose) hardly does so at all. The pentose sugars, whose molecules contain only five carbon atoms instead of six, caramelize very well. Since small amounts of these are present in wheat bran and in rye, wholemeal and rye breads tend to colour quickly when toasted.
Caramelization can take place both in air and away from it, as at the bottom of a saucepan. The sticky black coating on the bottom of an overheated pan is mostly caramel and carbon.
Caramelized sugar can be used as a brown colouring and is the basis of ‘gravy browning’, which is made from glucose. Such products are popular in Britain and northern countries, though little seen elsewhere.
An example of pure caramelization is the well-known dessert crème caramel. Sugar and water are boiled until the sugar is caramelized, and this is then used to line a small mould. A vanilla-flavoured custard is poured in, and the mould is placed in a bain-marie in the oven. See also crème brûlée.
Curiously, the sweets called ‘caramels’ have not undergone caramelization. They acquire their flavour and pale brown colour largely from sugar-amine reactions caused by heating the milk with which they are made.
Dextrins are the remains of starch molecules which have been broken down by heating, or by enzymes in the course of digestion. When starch is heated in dry conditions these products include pyrodextrins, which are brown in colour and have a characteristic flavour recognizable in bread crust and toast.
Again, excessive heating can yield black carbon.
This occurs in a wide variety of foods which contain both sugars and proteins. It is also called the ‘Maillard effect’, after the Frenchman who first identified it. It is strange that there is no more common term for it, since such reactions are a fundamental part of cooking. The products are not only coloured but also have flavours which give much of the taste of roasted and grilled foods.
The chemistry is complex and many different compounds are formed at various stages. The reactions can take place both in air and without it. The coloured end products are known as melanoidins. Proteins, whether whole or in the form of isolated amino acids, are not all equally prone to engage in these reactions. Sugars, too, vary in their behaviour. Ordinary sugar (sucrose) does not react at all. But in most foods there are other sugars which do; and some sucrose will anyway be split into simple sugars during cooking, or by the action of yeast in bread dough.
Sugar-amine reactions are usually desirable, as in cooked meat, roasted coffee beans, dried prunes, and maple syrup; but they may also be unwelcome, as in fruit juices. It is therefore useful to know both how to encourage and how to inhibit them.
Among the encouraging factors are a high temperature and alkalinity. Thus, in making maple syrup, the desired colour and flavour are deliberately developed by concentrating the syrup at a high temperature and taking advantage of its slight natural alkalinity. Bicarbonate of soda, the mild alkali which is added to some cakes and biscuits and peanut brittle, promotes browning. Certain organic acids, such as fruit acids, also help, as do phosphates, iron, and copper. The provision of additional sugar and protein, as when milk is brushed on to bread to help the crust turn brown, is an obvious technique.
Inhibiting factors are fewer. Moderate, but not extreme, drying of a product slows down browning. Sulphur dioxide blocks it effectively, in a manner not yet fully understood. This chemical is therefore added to various food products some of which, including fruit squashes, would otherwise discolour quickly even at room temperature.
This typically occurs in fruits and vegetables that have been cut or bruised, breaking open their cells and allowing the natural enzymes in them to decompose other substances into compounds with a dark colour and, often, an ‘off’ flavour. This is seldom desirable; an exception is the preparation of ‘black’ tea, where the leaves are deliberately bruised to allow enzymes to work.
We normally try to frustrate the enzymes responsible for this kind of browning. The simplest way is to cook the food, which destroys the enzymes. Freezing, on the other hand, does not, so vegetables to be frozen must be thoroughly blanched beforehand to prevent them from slowly browning after they are frozen. Salt blocks the action of the enzymes, but such large quantities are needed that it is not generally used for the purpose, although sliced apples may be left in brine for a short time before they are used. Acids have the same effect and are more practical. Lemon juice, vinegar, and ascorbic acid (vitamin C) are all used. Sulphur dioxide (which forms sulphurous acid in water) is used for commercial dried fruit. Finally, enzyme browning can be prevented by the exclusion of air, since, unlike the other kinds of browning, it needs oxygen to work. Putting foods in water is not effective by itself, because of the air dissolved in the water. Syrups afford better protection. Vacuum packing, although the vacuum is never total, is quite effective. The use of an inert gas, as when apples are stored in nitrogen, is best of all.
The browning of bread crust and of toast involves the first three mechanisms acting simultaneously. Sugars released into the dough by the action of yeast, or painted onto the loaf, undergo caramelization. On the dry outside of the loaf starch breaks down into pyrodextrins. In the moister conditions just below the surface, sugar-amine browning also takes place. The same three, and especially the last two, occur when breadcrumbs are sprinkled on top of a dish and heated to produce a brown crust over it.
There is an element of caramelization in the browning of foods which are deep fried; but this is subordinate to the more important sugar-amine browning.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
