Description
The pectic enzymes are those which take part in the degradation of pectic substances. These pectic substances are found widely in plant tissues, particularly in the fruits. The pectic substances include a multitude of compounds of complex and variable nature. The pectins are polymers made up of chains of galacturonic acid units joined by α-(l-4) glycosidic linkages. In native pectins, approximately two-thirds, more or less depending upon the source, of the carboxylic acid groups are esterified with methanol. These highly esterified pectins yield semi-solid gels with sugar and acid of the kind familiar in jellies and jams. Since crude natural pectins on complete hydrolysis give small amounts of several different sugars and acetic acid along with galacturonic acid, it was once thought that other carbohydrates such as arabans and galactans were present as impurities. It now seems more probable that rham- nose, arabinose, galactose and traces of other sugars are integral parts of the pectin molecules, and some of the free hydroxyl groups of the galacturonic acid are acetylated.
The pectic enzymes may be classified into pectin methylesterases which hydrolyze the methyl ester linkages, the polygalacturonases which split the glycosidic bonds between galacturonic acid molecules, and the pectin transeliminases, or lyases. The latter enzymes bring about non-hydrolytic cleavage of α-(l-4) linkages forming unsaturated derivatives of galacturonic acid. Further divisions exist with the enzymes depending upon whether they act on the methylated or on the free polygalacturoic acid substrates, and whether they attack internal linkages.
Pectic enzymes occur rather widely in many plants, including fruits, and in many microorganisms. The commercial pectinases are derived from fungi, most commonly from strains of the Aspergillus niger group. These commercial pectinases, widelyemployed in processing fruit products, are mixtures of several of the pectic enzymes, particularly pectin methylesterases and polygalacturonases. Differences in the effectiveness of different commercial preparations with different fruits and under different conditions are undoubtedly due to variation in the kinds and amounts of particular pectic enzymes present.
Freshly pressed fruit juices contain pectin, the hydrophilic colloidal nature of which makes them viscous and holds dispersed solids in suspension. This is highly desirable in some cases, as in tomato, orange and apricot juices, and the natural pectic enzymes must be destroyed by heat in order to maintain the desired stable cloud and viscosity. In other fruit juices, such as apple and grape juices, a clear product is usually desired. To obtain such clear juices a commercial pectinase is used, and the major application for pectic enzymes is in the production of brilliantly clear apple juice, grape juice, other fruit juices, and wines.
The appropriate commercial pectinase added to crushed apples results in higher yields of the juice. The enzyme added to the juice reduces the viscosity, and degrades the soluble pectin-protective colloid, permitting finely divided insoluble particles to flocculate. Considerable variation exists in practical use of pectic enzymes in clarifying apple juice depending upon season and varieties of apples, and the factors of time, temperature and enzyme concentration employed. The floe formed may be allowed to settle or may be removed by centrifugation or filtration to yield crystal clear brilliant juice. When this juice is pasteurized it is free from the boiled taste characteristic of untreated pasteurized juices, and the pasteurized juice can be marketed the year around. Vinegar and jelly produced from depectinized juice are superior in brilliance, color, and aroma. For Concord grapes, pectic enzymes are indispensable in the hot press process in order to increase the yield of grape juice and improve color extraction. The de-stemmed and crushed grapes are mixed with the pectIn the United States citrus, apple, grape and prune juices make up the major part of the juice production. However, juices from cranberries, blueberries, strawberries, black currants, cherries, plums and many other fruits are prepared for beverage use or for jelly manufacture, and pectic enzymes are commonly depectinized completely in order to obtain clear juices and make uniform jelly possible by adding back a standard amount of apple or citrus pectin when the jelly is made. The variable quality and quantity of the natural pectins in the original juices do not then interfere.
