This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Sugar, if its concentration and that of acid and pectin is sufficient, precipitates the pectin. Spencer explains this precipitation as follows. When a pectin sol is formed, "an equilibrium is reached between the partial pressure of the adsorbed water" of the pectin "and that in the dispersing medium." When sugar is added to the pectin sol, this equilibrium is disturbed, for when the sugar dissolves in the dispersion medium it lowers the vapor pressure of the dispersing medium. As a result the pectin particles lose water and are less stable. Increases in sugar accelerate the setting of the jelly, probably because of increased dehydration, although rate of setting is also modified by other factors.
Jelly without added sugar. Jelly can be made by boiling down some fruit juices until they yield a "jelly test." The concentration of 2 cups of gooseberry juice yields less than 1/8 cup of gummy, tough, sticky jelly. If left with the lid off for a few days it becomes dry enough to handle. It is very dark in color, looking and feeling very much like licorice candy. The fruit flavor is present, although scarcely noticeable at first, because of the extremely acid taste.
When to add sugar in jelly making. As previously stated, Tarr has found that the jellying strength of the pectin is lessened if boiled before the addition of sugar. Yet, if the sugar is added to the pectin and acid prior to boiling, the jelly strength is not decreased even when boiled as long as 42.8 minutes. Directions for making jelly often state to boil the juice for a certain number of minutes before the sugar is added. From Tarr's results it would seem a much better practise to add the sugar to the juice and concentrate it after the sugar is added. Long boiling of the sugar and fruit juice produces a darker-colored jelly, the extent of the darkening depending upon the juice and the length of time of boiling. With dark fruit juices this is not desirable, but it may improve the appearance of light-colored ones. To prevent darkening after the sugar is added, the jelly can be made in small lots so that boiling to the desired concentration can be accomplished quickly. In extracting the fruit juice too much water should not be added, as a large quantity of water only dilutes the pectin, and then boiling is required to concentrate it.
Concentration of sugar in finished jell. Jelly can be made with a wide range or proportion of sugar to juice, from about 40 to 70 per cent in the finished jell. From the reports in the literature, the average percentage of sugar in commercial jelly, unless acidity or pectin is abnormally high, varies from 60 to 65 per cent. The concentration of sugar desirable in the finished jelly varies somewhat with the juice from which the jelly is made. Thus gooseberry juice, which usually has a pH 2.6 to 2.8 and also has a high pectin content, is usually of better texture if the final concentration of sugar is short of or about 60 per cent. Cox states that for cranberry sauce too stiff a jelly can be prevented by stopping the boiling process when the sugar concentration is somewhat below 60 per cent. Cox states that premature gelation can be brought about by too much pectin, acid, or sugar, or all three. For some fruit juices Tarr and Baker have found that 70 per cent of sugar in the finished product gives a jelly of good texture. With higher concentrations than 75 per cent the sugar may crystallize from the jelly.
Proportion of sugar and jelly yield in illustrations. In general, other factors remaining constant, the yield of jelly parallels the amount of sugar added. The gooseberry juice for the jellies in the illustrations is from the same source, so that the pectin content and viscosity of the juice were the same. The percentage of sugar in the finished jell is about 60, since all were boiled to 103°C. In Experiment 24 the yield of jelly parallels the sugar added. See Figs. 15 and 16. The firmness of the different jellies varies. The 2 cups of juice boiled down yield less than 1/8 cup of jelly. It is very firm, hard, and difficult to pull apart. The percentage of sugar in it is not known, but both the pectin and acid have been enormously concentrated. With 1/4. cup of sugar to a cup of juice the yield is a little more than 1/4 cup of jelly. This jelly is very firm, solid, and hard. With 1/2 cup of sugar to a cup of juice the yield is a little more than 1/2 cup of jelly. This jelly is a little more tender than the one with 1/4 cup of sugar but produces a jelly that is still too firm. It should be remembered that whereas each cup of juice had the same concentration of acid and pectin before the sugar was added, the addition of varying amounts of sugar and boiling to 103°C. gives a concentration of practically 60 per cent of sugar in all the jellies. But the boiling to produce a concentration of 60 per cent of sugar thereby concentrates the pectin and acid, the concentration being greater per unit of weight in the jelly to which the smallest proportion of sugar was added. That the acid is concentrated is shown by the flavor, for the
Fig. 15. - The yield of jelly obtained when the proportion of sugar to juice is varied. Experiment 24.
1. Jelly made from two cups of juice without added sugar.
2. A cup of juice and 1/4 cup of sugar.
3. A cup of juice and 1/2 cup of sugar.
4. A cup of juice and 1 1/2 cup of sugar.
5. A cup of juice and 1 cup of sugar.
6. A cup of juice and 1 1/2 cups of sugar.
7. A cup of juice and 1 1/2 cups of sugar.
Jelly from 1/4 cup of sugar and 1 cup of juice is very tart, the tartness progressively decreasing as larger proportions of sugar are added. This is true of the first extraction of most fruit juices. As the proportion of sugar is increased, the yield of jelly is increased. There is a proportion of sugar that produces the best jelly for the particular juice used. With increasing amounts of sugar beyond the proportion that yields an optimum jelly, the jelly becomes softer until a proportion of sugar is finally reached that results in a fluid sirup instead of a jell.
Fig. 16. - Showing consistency and yield of jelly from Fig. 15. Jelly made from first extraction of gooseberry juice.
Proportion of sugar for home-made jell. The proportion of sugar to add to the juice cannot be given definitely for the best texture of jelly, since it varies with the pectin, acid, and salt content of the juice. The amount of pectin varies with the ripeness of the fruit, the quantity of water added for extracting the juice, and with the kind of fruit. Gold-thwaite states that it is better to use too little sugar than too much with fruit juices. She has found with all kinds of fruit juices and average methods of extraction that 3/4 cup of sugar to a cup of fruit juice usually gives better results than a cup of sugar to a cup of juice. The author's laboratory and home results agree with those of Goldthwaite that 3/4 cup of sugar to a cup of juice gives a better jelly for most fruit juices. Currants if not very ripe need a cup of sugar to a cup of juice, but if a larger quantity of water is added for extraction a smaller proportion of sugar is needed. The pectin content of gooseberry juice is usually quite high, one reason being that they are nearly always picked while green. If the rule of adding water to barely cover is used, the gooseberry juice will often require 1 1/4 cups of sugar to a cup of juice to give jelly of the best texture.
Fig. 17. - The jelly from the second extraction of gooseberry juice is softer than the jelly from the first extraction with corresponding amounts of sugar.
1. A cup of juice and 1/4. cup of sugar.
2. A cup of juice and 1/2 cup of sugar.
3. A cup of juice and 3/4 cup of sugar.
4. A cup of juice and 1 cup of sugar.
5. A cup of juice and 1 1/4 cups of sugar.
Some idea of the amount of sugar to use can be obtained from the alcohol test. With gooseberry juice and using the larger quantity of alcohol for the test, the approximate proportion of sugar required for a cup of juice can be estimated from the character of the precipitated pectin. After mixing, turn the pectin and alcohol out in a dish. If the pectin precipitate can be picked up in one piece without breaking and feels firm, about 1 1/4 to 1 1/3 cups of sugar will be required; if the pectin precipitate can be picked up in one piece without breaking but does not feel firm, about 1 to 1 1/8 cups of sugar will be necessary; but if the precipitate breaks into two pieces when picked up, about 3/4 to 7/8 cup of sugar is required. If the precipitate is flocculent the juice needs concentrating or only 1/2 cup or less of sugar can be used. A still easier way to determine the best proportion of sugar to add is to determine the viscosity with a jellmeter.
Sugar, pectin, acid, and salt. As can be ascertained from the previous discussion there is a relation between proportion of pectin, sugar, acid, and salts, temperature to which the pectin is heated, and time of pouring. Increases and decreases of constituents may be made within certain limits. Salts in the fruit juice or added to it may aid jelly formation or hinder and delay it, according to whether they tend to precipitate or stabilize the pectin.
Inversion of sugar in jelly making. The saturation point of a sucrose solution at room temperature or 20°C. is 67 per cent. Thus jellies containing more than 67 per cent of sucrose at 20°C. are super-saturated, unless something has occurred to the sugar. The pectin tends to keep the sugar from crystallizing from solution, acting as a protective colloid. But even this protective action would not always be great enough to keep the sugar from crystallizing with a concentration of sugar that is increased to 70 per cent or over. When sucrose is heated with acid, some of the sucrose is inverted to dextrose and levulose. The amount of inverted sugar formed during jelly making depends upon the acidity of the fruit juice and the length of time of boiling with the sugar. Since there are three sugars in the finished jelly the percentage or total sugar that can be held in the finished jelly without crystallization occurring is increased. Tarr and Baker report that in a solution containing a mixture of sucrose, dextrose, and levulose, the total maximum solubility is obtained with the proportions of each sugar shown in Table 23 (obtained from Bureau of Standards).
At 20°C. if inversion has occurred to obtain maximum solubility, about 75.7 per cent of sugar is soluble. Thus jelly containing about 70 per cent of sugar is not necessarily supersaturated. The salts and organic constituents of the fruit juice may also increase the solubility of the sugar in a given solution.
Rate of inversion in different fruit juices. Tarr and Baker have determined the rate of inversion of sucrose during jelly making with equivalent solutions of sulfuric, tartaric, and citric acids. As would be expected, they found the inversion was greater with the stronger or more highly ionized acid. Longer heating with a given hydrogen-ion concentration also increased the amount of inversion.