the substance which causes jams and jellies to set, is a carbohydrate which exists in the cell walls of fruits and vegetables.
Unripe fruits contain a predecessor of pectin, called protopectin or pectose. As the fruit ripens, enzymes convert this into pectin, the quantity of which reaches its maximum just before the fruit is fully ripe. Continuing enzyme action turns it into pectic acid. Neither pectose nor pectic acid has the setting power of pectin itself; hence the well-known difficulty of making jam or jelly with fruit that is overripe or markedly underripe.
It is equally well known that fruits vary in the amount of pectin they contain. Apples and citrus fruits have a lot. Cherries, figs, peaches, pears, pineapples, and rhubarb have much less. Strawberries and raspberries have pectin of inferior setting ability.
Pectin consists mainly of methyl pectate, which is a polysaccharide (a substance composed of long chains of sugar molecules). About half these sugar molecules are crosslinked to molecules of methyl alcohol. The nutritional value of these complex chains is negligible. Their value lies in the way they behave when fruit is made into jam or jelly.
The first step, boiling the fruit, causes the pectin to disperse through the mixture. The next, adding sugar and further boiling, crowds the chains of pectin together. The crowding effect is a simple consequence of the withdrawal of water, some of which is now occupied in holding the sugar in suspension and some of which evaporates. The crowded chains tangle. Then, when the mixture is allowed to cool and the molecular agitation within it diminishes, the tangled chains set into a continuous network with the remaining water trapped inside it. This is a gel.
The tangling effect, without which no gel can be achieved, will in this case only take place if the degree of acidity of the mixture is right, not much above or below pH 3.0. Mildly acid conditions are necessary to counteract small electric charges on the pectin chains which would otherwise attract water and maintain the pectin in solution. If the acidity is slightly too high, too strong a gel forms, giving a rubbery jam which shrinks and ‘weeps’, forcing out droplets of excess water. Strongly acid conditions prevent the pectin from setting at all.
Several other factors must be taken into account when making jams or jellies (see under jam and jelly). A familiar problem is that of making jam from fruits which contain little pectin, or pectin of low quality. One traditional answer is to mix fruits which have little pectin with others which have plenty. But one may not wish to add, say, apple to raspberry. Fortunately it is possible to obtain pectin as a liquid extract or in powdered form. These products are prepared from apple or citrus trimmings, preferably the former.
Special pectins used by the food industry include so-called low ester pectins which are capable of forming a gel in a sugarless solution, if a small amount of calcium salts is added to the mixture. These last pectins are used commercially to make aspic and to coat frozen foods with a protective layer of jelly.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
