are present in all living creatures: animals, plants, algae, fungi, and bacteria. They regulate the complex chemical reactions of life. Enzymes are catalysts: that is, they take part in chemical reactions without themselves being permanently changed by the reaction. Thus one molecule of an enzyme can work successively on many molecules of other substances, so enzymes are effective in tiny amounts. And effective they are: the artificial catalysts used in industry usually need high pressures and temperatures to work, but enyzmes operate quite happily at ambient pressure and body temperature.
All enzymes are proteins. They are very diverse in nature. Some work in combination with other substances, which are termed coenzymes (several vitamins are coenzymes).
The substance on which an enzyme operates is known as its substrate. Many enzymes are highly specific in their action. They may work on any of a class of substrates; or produce any one of a limited number of effects; or be capable of producing only one effect on one substrate. Their names usually end in ‘-ase’ added to the name of the substrate. Thus pectinase is an enzyme which acts on pectin. (Some of the first enzymes to be discovered do not have names of this type, such as the digestive enzymes pepsin and trypsin; but even these belong to the larger class of proteases, enzymes that act on protein.)
The one-to-one relationship of an enzyme with a substrate is often described as ‘lock-and-key’. It is a useful way of thinking about enzymes because, like a real key, an enzyme depends on its shape to work. Its molecule is almost exactly the right shape to fit closely against a molecule of the substrate. Electrical forces on the surface of the enzyme pull the substrate towards it. But because the fit is not quite perfect, when the enzyme and substrate come together they become distorted. This sets up stresses in both molecules which can break bonds in the substrate, causing it to separate into two new and different molecules. That causes a further change in shape, so that the enyzme no longer fits against the new molecules. It comes unstuck and floats away, immediately springing back into its original shape so that it can attach itself to a new substrate molecule; only substances which can spring back in this way can work as enzymes. Other enzymes have the right shape to attract two other molecules and join them together.
Enzymes, despite their complexity and the precision of their action, are not in themselves living things. They can thus continue to work after the death of the creature they belong to, though of course no new supplies of enzyme will be produced, so the reactions will proceed more slowly.
From a cook's point of view enzymes can have useful effects. For instance, fruit continues to ripen after it is picked. Meat becomes more tender when it is hung. In these examples the enzymes break down the larger molecules of carbohydrate and proteins into smaller pieces, resulting in a softer texture, better flavour and greater digestibility. However, if such enzyme action is allowed to proceed indefinitely it will result in the food becoming too soft and finally rotten. And many enzymes have actions that are purely deleterious; they turn fruits and vegetables brown, and fats rancid (enzymes that work on fats are called lipases). It is therefore important in preserving foods to limit or exclude the action of enzymes.
Enzymes, like other proteins, are affected by heat; a few can recover if not too strongly heated, but most are permanently denatured. Cold inactivates them but does not destroy them.
Enzymes need water to be active. As food is frozen the watery fluids in it turn to pure ice, expelling all the substances dissolved in them into the remaining liquid, which eventually becomes such a concentrated solution that it cannot freeze. Thus there is always a little liquid in the food in which enzymes can continue work. Where possible, foods that are to be frozen are first blanched—briefly cooked—to destroy the enzymes.
Where blanching is not appropriate, as in the case of fruits to be eaten raw, there is another possibility. Many enzyme-regulated reactions need oxygen; so, if the foods are frozen under vacuum or in an inert gas, the enzymes will be inactivated. A strong sugar solution can also be used to fill the spaces between the cells and thus exclude oxygen.
Enzymes may also be inhibited by the presence of chemicals which interfere with the reactions they regulate. Strong solutions of salt or sugar have this effect. So do high levels of acid. Some plant enzymes stop working when acidity reaches the moderate level of pH 6.5 (pH 7 is neutral); others can hold out as far as pH 4, but that figure can easily be exceeded in a strong vinegar pickle. Animal enzymes mostly work only in the limited range from pH 6.5 to 8.
When food is fermented, many of the changes that occur in it are due to the action of enzymes. Sometimes these enzymes are naturally present in the foodstuff, as in the preparation of black tea. Sometimes they are provided by micro-organisms which take part in the fermentation; for example, the Penicillium moulds which form the white rind on Camembert cheese produce proteases which soften the texture of the interior of the cheese.
Enzymes may also be taken from foods and used by themselves: papain (another protease) is extracted from unripe papayas and used as a meat tenderizer.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
