This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Time of boiling pectin and acid, minutes
Time of boiling pectin, acid and sugar, minutes
Jelly strength, pressure in centimeters of water
Rooker states that the powdered pectins will eventually replace the liquid pectins. This is because the "powder is cheaper, is easier to handle, haul, cart, and store. It does not spoil and deteriorate. Powdered apple pectin retains its jellying strength indefinitely while sirup pectins deteriorate if kept any material length of time. A container of powdered pectin may be opened and used as needed, while a container of pectin sirup must be used almost at once or the balance of the opened can will spoil."
Rooker gives many uses of pectin in addition to making jelly and jam. Among these are use in emulsions for pharmaceutical purposes, in emulsions for tree spraying, in crushed fruits for soda fountains, as "candy doctors," in confections, etc.
Myers and Baker after extensive experiments with the jellying properties of pectic substances, recommend that pectin should be bought and sold on a basis of jellying strength.
Olsen states that, commercially, pectins are graded on the basis of the amount of sugar they will carry. Thus a 160-grade pectin, as supplied by a definite source, is one which, when used in a jelly containing 65 per cent of sugar by weight with a proper amount of acid in the proportion of one part of pectin to 160 parts of sugar, yields a satisfactory jelly. Olsen has also shown that the strength of a jelly will vary with the temperature to which the pectin is heated. Hence a standardized procedure for testing pectin strength is essential.
With the aid of commercial pectin, which the housekeeper can add to fruit juices that are deficient in pectin, jelly and jam can be made from many varieties of fruit.