This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
According to Spencer's theory, the precipitation of pectin forms the jelly. Sugar, glycerin, or alcohol may be used as precipitating agents. Spencer states "The successful making of alcohol jellies requires rapidity of motion and practice, otherwise one obtains a precipitate and not a homogeneous jelly." The qualitative aspects of the boundary line between fields of the jelly and the sol shown in Fig. 14 are by Spencer. She states that the quantitative aspects of these boundary lines are meaningless, because the pectin used contained some ash. From this figure it is deduced that jellies with sugar as the precipitating agent are not possible in the neutral region owing to the solubility limit of sugar. Although alkaline jellies are interesting theoretically, practically they are of less importance, since they cannot be used as food on account of their flavor.
Concentration of pectin required for jelly. The strength of the jelly is in proportion to the concentration of the pectin, if other factors remain the same. Too small a proportion of pectin yields a jelly that is too soft and sirupy; too great a concentration gives a product that is too firm and hard. However, after a certain amount of pectin has been added any excess remains inactive. The concentration of pectin required for a jelly of good consistency varies with the proportion of sugar used, the acidity of the juice or solution, the salt content of the particular juice being used, the time, and temperature factors. Good home-made jelly may contain 0.75 to 1.0 per cent of pectin. Commercial jelly, on account of shipping, is somewhat stiffer, averaging about 1.25 per cent of pectin.
Pectins from different sources yield characteristic jellies. Olsen states that jellies made from citrus pectins are comparatively friable and have little elasticity; whereas jellies made from apple pectins are highly elastic, but require and tolerate less acid than citrus pectin jellies similarly prepared. Cranberry pectin tends to yield firm jellies which will not spread readily. Cox states that a better-textured cranberry jelly for spreading purposes is obtained if the pectin occurring in the juice is hydrolyzed by adding the enzyme pectinase, available in commercial form under the name Pec-tinol. A slower-setting pectin is then added to the juice. With a rapid-setting pectin gelation may start before the sirup is poured, and the pouring breaks the structure so that a mass of small lumps is formed.
Fig. 14. - From Spencer. "A qualitative view of the relations of the boundary curves of pectin jelly fields for sugar, glycerine, and alcohol jellies. The alcohol jelly field is the largest, and the sugar jelly field the smallest." J. Physical Chem. 33: 1993.
Three fairly easy methods of determining the amount of pectin present in fruit juice are available for the housekeeper. One is to cook a small portion of the juice with sugar to see if it will form jelly. Another is to test the juice with alcohol. The third is to test the viscosity of the juice with a jelmeter.
Pectin test by alcohol. Alcohol precipitates pectin in a jelly-like mass. By the character of the precipitated pectin an approximate estimate can be made of the amount of pectin in the juice. For the test, 1 tablespoon of juice is poured into 3 to 5 tablespoons of alcohol. Sometimes equal quantities of juice and alcohol are used, but the larger amount of alcohol gives a better test. Turn the container gently from side to side. If a test tube or graduated cylinder is used it can be turned slowly upside down and back to bring all the juice in contact with the alcohol. If the pectin comes down in a solid mass there is a sufficient quantity to make jelly. If the pectin is flocculent or in small flakes the juice needs boiling down to concentrate the pectin.
Johnston and Denton have reported that the amount of alcohol precipitate in citrus pectin extracts does not indicate the jellying power. It may also be possible that the amount precipitated in different juices will show different jellying strengths. Of course, alcohol precipitates such substances as gums and starch in addition to the pectin. The precipitated mass becomes firmer after standing in the alcohol a few minutes, so the estimate should be made just after mixing.
For the pectin test, straight ethyl alcohol, denatured, or wood alcohol can be used. The denatured alcohol is better to use than the wood alcohol and usually gives a satisfactory test. When wood alcohol is used, the precipitated pectin may be redissolved after standing a few minutes. This may also be true of some denatured alcohol.
Pectin test by viscosity. Based on the observed correlation of the viscosity of pectin solutions with the jellying power of the pectin, Baker has developed a jelmeter. The viscosity of the fruit juice is determined by the rate of flow of the juice through a given orifice compared with the time of flow of an equal quantity of water at the same temperature. The viscosity or thickness of the fruit juice is dependent principally upon its pectin concentration, although sugars, starches, and proteins influence it slightly. If the approximate viscosity is known, then the amount of sugar to use and the weight to which the jelly should be cooked can be determined. Baker has prepared such tables.
The jelmeter is similar to a graduated pipette. The test is easy to make and practical. It works satisfactorily with the fruit juices that have been tested.
Effect of high temperatures and a long period of heating upon pectin. Numerous investigators, among them Johnston and Denton, Sucharipa, and Tarr, have found that high temperatures decrease the jellying power of pectin. Myers and Baker state that "Prolonged heating of pectin solution has a deleterious effect on the jellying power of the solution. The higher the temperature the greater the decrease in jellying power of the pectin." They find that heat has the same effect on the powdered pectin as upon the pectin in solution.
The boiling of a pectin solution with acid decreases the amount of pectin in the solution because of the formation of pectic acid by hydrolysis. Some of Tarr's data seem to indicate that, if the sugar is in the juice during the boiling period, hydrolysis is delayed. The following table is from Tarr's results and shows that the decrease in jelly strength was considerable when the pectin and acid were boiled before the sugar was added. Commercial pectin. Commercial pectin is on the market in two forms: (1) the liquid form and (2) the powdered dry pectin. The first type of commercial pectin is a concentrated liquid pectin. It usually contains 4 to 4.5 per cent of pectin. The second form is a mixture of dry powdered pectin and sugar. It is similar to dry gelatin and sugar mixtures on the market in that it contains the sugar, and only the juice needs to be added for making jelly. Wilson states that there is no uniformity in jelly strength of these pectin preparations but that a dry pectin that will jell about 40 times its weight of sugar is a practical one. The dry powdered pectin is sold in large quantities to commercial jelly and jam makers. The dry pectin without the added sugar is now available for the housekeeper.