This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Addition of solidified gelatin to a gel solution. Lloyd states that gelation is similar to crystallization, in that it takes place from a saturated solution. There are various theories to account for gel formation. It has been suggested that nuclei, strands, rods, or micelles form in the solution and solidification takes place upon cooling, with the formation of a gel structure. If some gelatin that has solidified is added to freshly made gelatin solution solidification takes place more rapidly. Alexander suggests that this is due to the addition of nuclei, and that other particles aggregate around these nuclei. This is similar to "seeding" or adding of crystals to crystalloidal solutions to hasten crystallization. Dahlberg, Carpenter, and Hening find that this property of gelatin has a practical application in ice-cream making. The gelatin is added to ice cream to improve the texture and body of the ice cream. If the unfrozen ice-cream mix is aged at a temperature just above the freezing point a smoother ice cream is obtained. One reason attributed for this is the low viscosity of the freshly made gelatin solutions compared with the viscosity of the aged solutions, and another is the speed at which the gelatin gel previously aged reforms after whipping during freezing.
Hydrogen-ion concentration. The acidity of the gelatin affects its stiffening power. According to the kind and amount of ash and its buffer action the gelatin may have a hydrogen-ion concentration greater or less than its isoelectric point, which is pH. 4.7. The initial pH of different brands and samples tried in the laboratory has been above the isoelectric point. Probably the greater the acidity or alkalinity, unless it is beyond the maximum or minimum pH for the greatest water absorption, the less firm the resulting jelly. However, the pH of a gelatin solution does not entirely control the time for setting or the stiffness of the resulting gelatin. Two gelatin solutions may have the same pH and the same proportions of ingredients added, but one may require much longer than the other to set and never produce as firm a jelly.
Acids and alkalies. The addition of fruit juices or acid salts to the gelatin solution increases its acidity, the increase depending on the acidity of the fruit juice added, its buffer capacity, and the quantity added. Lemon juice has a lower pH than most fruit juices. Its pH varies slightly but is usually between 2.0 and 2.5. The addition of 1/4 cup of lemon juice to 1/2 ounce of gelatin with an initial pH 6 to 6.5, 3 1/2 cups of water and 1/2 cup of sugar, lowers the pH of the mixture around 3. This is lower than the isoelectric point of the gelatin. Many recipes use larger quantities of lemon juice than the proportion given above. This would increase the acidity still more. With a definite concentration of gelatin a less firm jelly is obtained as the amount of lemon juice is increased. A longer time is also required for the gelatin to set with the increased lemon juice or acidity.
Patten and Johnson found that a 3 per cent gelatin solution liquefied on standing when the pH was sufficiently low. Liquefaction also varied with the temperature. It occurred at a temperature of 18°C. when the gelatin solution was at pH 3.6 or lower.
Some of the less acid fruit juices do not increase the acidity of the gelatin mixture as much as the lemon juice. The addition of fruit or vegetable juices such as tomato juice may bring the pH of the gelatin close to its isoelectric point. The pH of tomato juice varies somewhat but is usually between 4.2 and 4.8. The juices that bring the gelatin close to the isoelectric point do not increase the time required for gelation to the extent that the lemon juice does. Gelatin made with tomato juice varies in the length of time required for setting. But in recipes lemon juice is usually added in combination with other fruit juices, and either lemon or vinegar is often added with tomato juice for tomato gelatins. Recipes containing a larger quantity of a very acid fruit juice may need a higher concentration of gelatin to produce a texture desired for serving.
Lemon jellies were made from the same sample of gelatin, using different proportions of lemon juice. When the quantity of lemon juice added was just sufficient to bring the hydrogen-ion concentration of the mixture to about pH 4.7, the isoelectric point of gelatin, the jelly formed was cloudy and more turbid than when the pH of the mixture was lower than 4.7. Jellies with a pH lower than the isoelectric point of gelatin were clear and sparkling. They also required a longer time for the gel to form than those with a higher pH. When the series was repeated but citric acid was substituted for lemon juice, similar results were obtained.
Tartaric acid added to the gelatin solution also increases the time for gelation. A teaspoon of tartaric acid (about 3.5 grams) or a teaspoon of citric acid (about 3.5 grams) added to 3.5 grams of gelatin and 236.5 cc. of water gives the mixture about pH 2.7. The jellies are clear and tart.
Patten and Johnson have reported that a 3 per cent solution of gelatin at 20° to 22°C. begins to liquefy between a pH of 8.4 and a pH of 9.2. It is not completely fluid at pH 9.6.
Salts. It has been reported by several investigators that the gelation temperature is affected by electrolytes. The effect is usually given as follows for anions at equal concentrations. The first of the series elevates the gelation temperature, and it is slightly lower for each following anion, until the iodide may lower the gelation temperature below 0°C. SO4 > citrate > tartrate > acetate > CL > CLO3 > NO3 > Br > I. This is the order above the isoelectric point. When enough lemon juice or acid has been added so that the reaction is below the isoelectric point the cations may have more effect than the anions. The order of anions given above indicates that some salts would increase the tenderness of gelatin gels whereas others would increase the gel strength.
Dahlberg, Carpenter, and Hening found that the gel strength was greater in milk than in water, even when the proportion of gelatin added to the milk was based on the water content of the milk and not on its total volume or weight. They found that the change in hydrogen-ion concentration did not account for this increase in jelly strength in all the samples. The other factors that might influence the jelly strength were the salt and protein content of the milk. They state that the "influence of salts on gel strength has been observed by other investigators, so it may be one of the means by which skim milk altered gel strength."
Sugar. Small amounts of non-electrolytes do not appreciably affect the viscosity of gelatin solutions, but in large quantities sugar increases the viscosity, as a thick sugar sirup is more viscous than a thin one. Loeb has reported that cane sugar does not diminish the viscosity of gelatin solutions, but at concentrations of M/8 or over may increase it slightly. The sugar has a similar effect upon the stiffening power. Some concentrations of sugar do not seem to affect the stiffening power, but others do. Ostwald states that 1 gram of sugar added to 9 cc. of a 6 per cent gelatin solution accelerates gelation. This would give about 10 per cent of sugar in the solution. Larger quantities of sugar may retard gelation.
Opacity in gelatin. Edwards states that two factors of prime importance in gelatin are its strength and clarity of aqueous solution. In regard to clarity he says that many perplexing problems occur in this connection, for a gelatin may be clear at one concentration but, if further diluted, may appear turbid. He says that a complete explanation of the problem of turbidity has yet to be made, but some causes are as follows: (1) actual dirt, particles of animal tissue and fibers, (2) mold, (3) emulsified grease and calcium salts of fatty acids, (4) protein salts and proteins other than gelatin which precipitate and remain in suspension when the pH of the solution is varied, (5) a calcium sulfate-phosphate complex which is retained in solution by the presence of sulfurous acid, and (6) colloidal sulfur.
Edwards states that the first two enumerated difficulties should not be present and when detected the gelatin should be condemned. Emulsified grease is more difficult to remove. It results from the employment of greasy material, extraction of gelatin at the wrong pH, the agitation of liquid in the boiling vat, and improper filtration. The presence of proteins other than gelatin is more likely to occur in a skin gelatin. Mucin and chondrin are readily soluble in weak alkalies and are thrown out of solution by acids. Hence they are less likely to be found in acid gelatins. Cloudiness due to the presence of insoluble calcium sulfate-phosphate complex is more likely to be found in ossein gelatins. Colloidal sulfur is more likely to be found in glue than in gelatin stock.
Black mentions that unless the copper content of the gelatin is low a purplish discoloration may appear with meats and particularly with chicken and tongues.
Another cause for turbidity is discussed by Clayton. At the isoelectric point the gelatin solutions always show more turbidity, a 400 per cent increase in turbidity occurring with a pH variation of 0.03 (pH 4.87 to 4.90). Gelatin stock treated with lime shows greatest turbidity on the acid side of the isoelectric point and, vice versa, those stocks treated with acid possess isoelectric points in the region of pH 7 to 8.
Previous history. The temperature to which a gelatin has been previously heated will cause a variation in its viscosity. It may also change its stiffening power. This heating may be due to heating during manufacture or to heating for dissolving in the home. The latter is seldom long enough to destroy the jelly strength to any appreciable extent. A gelatin solution that has solidified and then is melted will form a gel in a shorter time for the second or third gelation.
Agitation and foam formation. Bogue and Alexander both state that agitation or stirring lessens the viscosity of gelatin solutions. This might also affect the jelly strength. Gelatins are often beaten when they have become thick but not firmly set. The beating incorporates air, forming a foam, and the gelatin mixture increases in volume. Bogue states that the ability for gelatin to form a foam is greatest at the isoelectric point. At this point the gelatin particles have a strong tendency to adhere to each other, and this favors foam formation. If the beating is done at the time when the gelatin has set enough to be quite viscid, but has not become brittle, so that the edges break apart, the volume may be increased two or three times that of the original unbeaten gelatin. The lessening of firmness may be partially due to agitation of the gelatin, but part is due to the incorporation of air. The gelatin at this stage is elastic and stretches to surround the air particles. The gelatin that is to be beaten should have the flavoring in larger quantity than for an unbeaten gelatin as the increase in volume causes the flavor to seem less concentrated. If gelatin becomes too firm before the beating is started the gelatin only breaks and air is not incorporated. Whipped cream and beaten egg white are often folded into unbeaten gelatin or into beaten gelatin.