Gelation is the formation of a gel or solidification of a gelatin solution. In food preparation it is referred to as the setting of the gelatin. The setting point is no clearly defined point. The gelatin solution becomes sirupy, like a thick sugar solution, then gradually thickens to finally form a firm gel. Thus there is no distinct point at which solidification occurs. If samples of solutions are placed in test tubes, the time at which they cease to flow may be taken as the setting point. In larger containers this is not so easy to ascertain, and it is necessary for members of the class to compare results and all try to select a definite stiffness as the setting point. Sheppard and Sweet devised an apparatus for determining the setting point. They take the setting point as the point at which a stream of air bubbles ceases to flow through the gelatin solution.

The stiffening power and the viscosity of gelatin are affected by several factors. Usually stiffening power and viscosity run parallel, but this is not always true, for gelatins giving the highest viscosity do not always yield the stiffest jellies. Viscosity of gelatin is measured at a temperature of 35°C. by some workers and at 60°C. by others. At these temperatures gelatin is a liquid and will flow. Viscosity is measured by the rate of flow under standardized conditions; stiffening power or jelly strength is measured when the gelatin is set.

Different brands of gelatin show decided differences in stiffening power when made into gelatin to serve. If two gelatins cost the same per pound, one may be more economical than the other, if equal quantities of the gelatins when combined with the same amounts of sugar, water, and flavoring show different stiffening power.

Time, concentration, and temperature all affect the gelation process and the stiffness of the resulting jelly.

Time of gelation. When concentration and temperature are constant a definite length of time is necessary for gelation to occur. This is characteristic of many colloidal changes. With lessened concentration a longer time is required for gelation. At higher temperatures a longer time is required for the solidification to take place.

The firmness of the jelly is also affected by time. Some time may be required after the gelatin solution has cooled to the refrigerator temperature before gelation occurs. The jelly becomes stiffer with longer standing. Gelatins allowed to stand in the refrigerator over night at a temperature of 8° to 10°C. are stiffer than those standing only a short time after gelation has occurred. Clark and Du Bois state that after gelatin has cooled 3 hours at 0°C. little or no further increase in jelly strength occurs. Dahlberg, Carpenter, and Hening found that, for the weak gelatin solutions used in ice cream, gel formation requires a long period of time. They allowed 18 hours for gel formation and aging in most of their experiments. However, they found that the gumminess in ice cream due to too much gelatin increased during 4 to 5 weeks' storage of the ice cream. They attributed this to the "slow rate at which gelatin gels in weak concentrations come to a final equilibrium."

Concentration. With a definite temperature and time, solidification occurs only with a definite concentration. With increasing concentration of gelatin the time for setting is shortened. The amount of liquid that a gram of dry gelatin can bind and hold in the form of a gel is high, 1 to 2 grams forming a gel with 99 to 98 grams of water.

Ostwald states, that the solidification of gelatin in 1/2 hour depends upon the mode of preparation, but usually requires about 2 per cent at 15°C and 1 per cent at 0°C. To serve as food the concentration should not be too high, or the jelly is too stiff to be desirable. Some food gelatins produce a desirable texture for serving with a 1.5 per cent concentration, some require 2 per cent, and some may require as much as 3 or 4 per cent.

Manufacturers often place a quantity of gelatin in a packet or envelope and state the amount of water to be used with it. This amount of water is an average amount required under all conditions for cooling. In some instances it might be desirable to change it, on account of variation in proportion of ingredients or variation in temperatures at which it solidifies, or for other reasons. Recipes containing sauces thickened with egg or starch do not need so high a concentration of gelatin.

It is preferable to use bulk or sheet gelatin by weight, rather than by measure. On account of differences in the size of the gelatin particles, a definite measure does not yield the same weight, hence a different concentration of the gelatin would be used.

Temperature. With a definite concentration and time for setting, solidification occurs only with a definite temperature. For a high solidifying temperature a high concentration of gelatin is required. The slower a gelatin solution is cooled, the higher the temperature at which it solidifies. A gelatin solution may be cooled down, by packing ice around its container, to a temperature below that at which setting would occur if a longer time is allowed. On the other hand, gelatin solutions may be mixed and left standing for 2 or 3 hours at room temperature. If they are then put in ice or the refrigerator they set quite rapidly and as if gel nuclei might have started to form while standing at room temperature. At low temperatures all gelatins become firmer. At high temperatures they are liquid, for no gel occurs at temperatures above 35°C. with any concentration of gelatin. With the same concentration one gel may set at 10°C, another at 12°, still another at 14° to 16°C, and others require lower temperatures for setting. The gelatins that set at higher temperatures show jellies of greater firmness than those that set at low temperatures. The ones requiring lower temperatures than 10°C. to set do not serve very well, for they also soften at a low temperature.