Chemistry in Cooking


By Gilly French


Out of the fire into the frying pan....if you've never thought about all those chemical reactions occurring every time you cook some food, here is a little enlightenment to whet your appetite....

Why does the human animal like its food cooked? After all, the earth's entire animal population had eaten its food raw for thousands of millions of years; it was only a mere million years ago that some early humans began to apply fire to a variety of objects that their ancestors had eaten raw for eons, and the new charred version prevailed. Now we enjoy cocoa and coffee beans and the crust on a roast, with sushi and steak tartare being somewhat of a novelty, an acquired taste.

Certainly cooking serves some very practicable purposes: it makes food easier to chew, more digestible, slower to go off and less likely to cause illness. The mechanical advantages may well have given rise to our smaller, less protruding jaws, compared to those of our primate relatives. But this does not explain why we should have come to enjoy cooked foods: humans didn't cook before the 'discovery' of fire, and cooked flavours are very different from the raw originals. Perhaps the answer might lie in a look at some of the chemical changes caused by cooking. This is not a simple task: hundreds of flavour molecules have been identified in, for example, cooked meat, but it is a starting point.

The modern day chemistry of food flavour dates from the discovery in the nineteen-teens of the browning reaction, also known as the Maillard reaction, which generates much of the characteristic colour and aroma of foods cooked over a flame, in the oven, or in oil. The 'simplest' browning reaction is the caramelisation of sugar, but it is not a simple reaction. Glucose produces at least a hundred different products, including organic acids, fragrant molecules and brown-coloured polymers. This takes place at a relatively low temperature of about 154C, which is why most foods brown on the outside during the application of dry heat. Maillard reactions proper occur between amino acids (found in proteins) and sugar molecules: when these are heated together they produce, rapidly, a whole range of highly flavoured molecules that are responsible for the brown colour and distinctive taste of cooked meat. Foods that have been boiled, and moist interiors of meat and vegetables, do not exceed 100C and will therefore look, and taste, plain. To make a rich tasting stew the meat, vegetables and flour must be browned before adding any liquid; conversely, if a cook wants to highlight natural flavours, high temperatures should be avoided. The food industry uses purified sugars and amino acids to approximate to otherwise costly flavours: for example, a well-known coffee substitute is simply a mixture of roasted wheat, bran and molasses.

High temperatures are also used, of course, to increase the rate of chemical reaction: if the rate doubles with every 10C rise in temperature, which is a reasonable approximation, then a reaction which would normally take a day can be over in a matter of seconds. Frying, for example, is possible at temperatures of up to 250C. The first stage of the cooking allows for heat transfer; the second, at a higher temperature, encourages Maillard reactions. At this temperature the fat is no longer a heat transfer agent and starts to influence the taste of the resulting Maillard compounds.

However, we have still not answered the question of why we should prefer our food cooked. To do this, we have to look at what happens during the caramelisation of sugar. Plain crystalline sugar has no odour. Heated to 160C it will melt; at 168C it begins to colour and to develop a rich aroma. At this point several hundred molecules have been formed, as the carbon, hydrogen and oxygen atoms interact with each other and with oxygen in the air at high temperatures. The volatile products include acids, aldehydes, alcohols and esters (those familiar fruity smells).

Caramelised sugar also includes ring compounds: furans (which have nutty or butterscotch aromas), and pyrones (eg maltol, which tastes of caramel) are examples.

The interesting point is that several of these flavours are contributed by chemical families of compounds that are common in nature, particularly in fruit. Alcohols, esters and the suchlike are found throughout the living world because they are all associated with the process of energy production. Cooks generate them by breaking up sugar molecules under the influence of heat; fruits generate them during the ripening process. So some of the components of the caramelised aroma would have been familiar to our ancestors in the form of fermented fruit.

The important families of aroma compounds produced in the Maillard reaction (which occurs between amino acids and sugars at a lower temperature than caramelisation) include pyrroles, thiophenes, thiazoles, pyridines and pyrazines. Several of these contribute a nutty flavour, some a 'roasted' impresion, even with hints of chocolate. And several contribute floral odours, or are reminiscent of green leaves and vegetables: flavours our ancestors would have encountered long before they had 'discovered' fire. For example the compound 2-methyl thiazole, which is reminiscent of green vegetables, is found in cooked beef. Pyrazines have been identified in molasses, coffee, green peas, Gouda cheese, red beans, asparagus and other green vegetables. The American food scientist Harold McGee, whose work has inspired this article, suggests that 'fruits probably provided our evolutionary ancestors with refreshing sensory interludes in an otherwise bland and dull diet...perhaps cooking with fire was valued in part because it transformed blandness into fruitlike richness'. Our ancestors have been encountering molecules characteristic of the roast for probably hundreds of millions of years.

Some animals, eg ants, produce their own cyclic aroma molecules by way of chemical communication - pheromones. It is suggested that, in terms of smell, there is not that much difference between us and some insects. It has been important for all animals to detect a wide variety of aroma molecules, particularly those generated by plants and other animals. McGee suggests that' our powerful response to odours may in part be a legacy of their prehistoric importance to animals, which have used them to recall and learn from experiences'.

References

Raymond Blanc: 'Blanc Mange', BBC books
Harold McGee: 'On food and cooking', Unwin
paperbacks;
'The Curious Cook', Collier Books
T.P. Coultate: "Food, the chemistry of its components' Royal Society of Chemistry

Hooke Magazine - Issue 10