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TABLZV-MOLECULARWEIGHTS OF VARIOUS GELATINS Cc. 0.2 M HCl t o Change 1 G. Jell Strength Gelatin from Molecular pH 4.2; pH 4.7 to Weight of pH 3.6 Gelatin GELATIN 21.00 c. 3.79 1319 X 69 2 430 2.85 1753 2.70 1852 2 E 565 2016 800 2.48 3 3 B 1026 2.40 2083
These results, burdened as they are by assumptions as to the monacid character of the gelatin and the absence of hydrolysis of the gelatin acid compound, plainly indicate the order of magnitude of the molecular weight and its progressive increase with the grade of gelatin. Additional evidence is also furnished as to the formation of a compound
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between the gelatin and the acid. Gelatin a t the isoelectric point has a low viscosity. Addition of acid results in the formation of a gelatin acid compound having an increased viscosity. Addition of acid alone to water lowers the viscosity tremendously, a few per cent sufficing to reduce the viscosity to about one-half that of pure water.21 Consequently, when the gelatin acid combination is complete additional acid exerts its influence and the viscosity is again lowered. Van Slyke and Van Slykef2 give an excellent bibliography of work supporting the view of the formation of compounds between proteins and acids and bases. 51 12
Wagner, 2. physik. Chem., 5 (1890),31. A m . Chem. J . , 88 (1907). 383.
A Study of the Relation of Pectin and Acidity in Jelly Making' By Lal Singh UNIVERSITY
OF
CALIFORNIA, BERKELEY, CALIFORNIA
Jelly can be prepared from a mixture of pectin, acid, sugar, and water. A definite relation exists between the amount of acid present in fruit juices and the amount of sugar necessary to form jelly. Between certain limits, the greater the acidity of the juice the lower the amount of sugar required. I t is therefore desirable to increase the acidity of the juice to the maximum limit compatible with taste, in order to save sugar. Within certain limits, the higher the percentage of pectin in a fruit juice the lower the amount of sugar necessary to form a jelly. After a certain concentration of pectin in a jelly is reached, any excess remains inactiue. B y increasing the pectin in a juice from 0.9 per cent to 1.5 per cent, a jelly mater can easily save over 15 per cent of sugar. It is also noted in this article that the lemon peels which go to waste at citric acid factories would yield pectin at the rate of ouer 90 Ibs. per ton of waste peels.
ECTIN and acid are considered the two necessary constituents of fruit juices in jelly making. It has also long been recognized that juices of high acidity yield firmer jellies than juices equally rich in pectin but deficient in acid. However, no published data are to be found upon the effect of acid concentration upon the jelling quality of citrus juices. Consequently the following series of experiments were conducted in duplicate to determine whether or not any direct relationship exists between the amount of acid present in citrus juice and the jelling point. EFFECTOF ACIDCONCENTRATION In this series of experiments pure pectin solution, pure citric acid, water, and sugar were used. To prepare the pectin solution commercial, powdered citrus fruit pectin was leached in boiling water and filtered through felt. Four extractions were made. The resulting dilute pectin solution was concentrated to about one-fourth of its original volume. Triplicate analysis by alcohol precipitation showed this solution to contain 5.5 per cent pectin. A citric acid solution containing 25 g. of acid per 100 cc. was used as a source of acid. Small (100-g.) lots of jelly were made in which the amount of pectin (1.5 g.) was kept constant. The acid concentration and sugar were varied in a definite and carefully con-
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1
Received May 18, 1922.
trolled manner in order to determine the effect of acidity upon the jelling point. For example, in preparing a sample to contain 1.5 per cent pectin, 1per cent acid, and 60 per cent sugar there were added to 27.3 cc. of pectin solution (equivalent to 1.5 g. of pectin) in a weighed aluminium stew pan, 4 CC. of citric acid solution, 60 g. of sugar, and a small amount of water. The mixture was boiled to dissolve the sugar and to concentrate the solution until it reached a weight of exactly.100 g. It was then poured into a jelly glass, covered to prevent evaporation, and stored for 12 to 24 hrs. to observe whether or not the product formed jelly. In case it jellied the same experiment was repeated with smaller amounts of sugar, e. g., 57, 55, 53, etc., g., successively. If it did not jell, larger amounts of sugar were used successively. By this process of elimination the sugar concentration was determined within 1 per cent for the jelling point at 1 per cent acidity. In a similar fashion, more than one hundred samples were prepared and the minimum concentrations of sugar necessary barely to form jelly at a number of different acid concentrations were determined. The results appear in Table I and Fig. 1. TABLEI-RELATIONOF ACIDITY AND MINIMUM SUGARCONCENTRATION TO PRODUCE JELLYPECTINCONCENTRATION CONSTANT TNROUDHOUT THE SERIES AT 1.6 PER CENT Total Acid Present in Sugar Required Barely to Cause Jelling Finished Product Grams Per cent EXPT. 1 75 2 64 3 61.5 4 56.5 5 6 53.5 7 8 52 9 10 60.5 11 50 12 13 50 14
Sugar tended to crystallize out on the surface of jellies in Expt. 1 after about three days; in Expt. 2 after about a week; in Expt. 3 to a very slight extent after about a month. In the other lots this tendency was not observed. This absence of crystallization may be partly due to the hydrolysis of the cane sugar by the high concentrations of acid used,
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but is more probably due to the lower sugar concentrations required for jelly formation a t the higher acidities. No jellies of 65 per cent sugar concentration, or higher, showed any tendency to mold, while the jellies with less than
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Various lots of jelly containing 0.5, 0.75, 0.9, 1, 1.25, 1.5, 1.75, 2, 4.2, and 5.5 per cent, respectively, of pectin, based upon the finished product, were prepared, and the minimum concentration of sugar required barely to form jelly in each case was determined. The results are given in Table I1 and Fig. 2. TABLE11-RELATION OF PECTIN CONCENTRATION T O JELLING POINT Sugar Present in Finished Pectin in Product at Lowest Sugar Finished Product Concentration for Jelling Per cent Per cent 0.6 No jelly formed a t any 0.76 concentration 0.9 65
1. 1.26 1.5 1.75
2.
65 per cent of sugar molded in most cases. All of the jellies which tended to mold were those which remained exposed to the atmosphere for about a week. Those which were tightly covered with sterile caps as soon as poured into sterilized jelly glasses gave little evidence of molding even when the concentration was about 55" Balling. It would appear justifiable from this experiment to draw the following conclusions:
2.75 4.2 5.5
62 54 52 51 49.5 48 45 43
The pectin solution employed in these experiments was prepared by dissolving a commercially prepared powdered pectin in water. Its pectin content was determined by alcohol precipitation and although it contained some impurities the results are a t least comparative. The above experiments show that, within certain limits, the higher the percentage of pectin in the juice the lower the amount of sugar required to form jelly. This fact is most 1-Contrary to the claims made in the literature, it is possible to prepare jelly from a mixture of pectin, acid, sugar, and water noticeable in jellies containing 0.5 to 1.5 per cent pectin. The curve is very sharp between these two limits, becomeven if it is quite deficient in acid. 2-There is a definite relation between the amount of acid ing almost horizontal after a concentration of 2.5 per cent present in fruit juice and the amount of sugar necessary to form pectin is reached. jelly. Between certain limits, the greater the acidity of the juice It also appears that after a certain concentration of pectin the lower the amount of sugar necessary. This relation is very is reached in a jelly any excess added remains inactive or marked, especially when the acidity of the juice is comparatively undissolved. For instance, it was noticed that all the jellies low. For instance, the sample of 0.1 per cent acidity required 66 per cent sugar to jell, but when the acidity was in2reased to, containing 2 per cent or less pectin were normal and showed 0.2, 0.5, and 1 per cent, jelly was formed at 62", 57 ,and 54 no distinct taste of pectin. But when the pectin concentraBalling, respectively. Thus, by increasing the acidity of the tion was raised to 2.75 per cent in the jelly, a crust-like formajuice from 0.1 to 1.7 almost 20 per cent of sugar was saved. Increase of acidity beyond 2 per cent did not materially decrease tion appeared on the jelly which was not only stiffer but less the sugar percentage necessary for jelling. sweet than the lower portion of the jelly. This became more From the viewpoint of the commercial jelly manufacturer, as and more evident as the concentration of pectin was increased. well as the home jelly maker, it is desirable to increase the acidity This experiment shows how essential it is for a jelly maker of the juice to the maximum limit compatible with the taste of the consumer, in order to economize on sugar. Cull lemons to see to it that his juice is rich in pectin, because it will mean a offer a cheap and excellent source of acid either in the fresh juice great saving in sugar. For instance, by increasing the pectin or €or citric acid. in the juice from 0.9 to 1.5 per cent (based on the finished 3-If jelly is prepared at sugar concentrations lower than 65 product) a jelly maker can easily save over 15 per cent sugar. per cent, the container and contents must be pasteurized. Even a saving of 10 per cent of sugar in big commercial factories amounts to thousands of dollars per month. EFFECT OF PECTIN CONCENTRATION PECTINCONTENT OF WASTELEMON PEEL An experiment similar to the one just described was conducted to determine the relation of pectin concentration to I n conjunction with this experiment the writer undertook the jelling point. to determine quantitatively the amount of pectin in waste The procedure for this experiment was similar to that lemon peels which a t present largely go to waste at citric adopted for the previous experiments on the effect of aoidity acid factories. It was found that waste lemon peels are extremely rich in pectin, 800 g. of peels yielding 37 g. of pure powdered pectin, or a t the rate of over 90 lbs. of pectin per ton of waste lemon peels. Consequently the waste lemon peels constitute a commercial source of pectin. The peels can be boiled separately and the extract or purified pectin added to orange juice to supply pectin, or it can simply be ' added to sliced oranges and boiled with the latter for extraction of juice to make jelly or marmalade. Fro. 2
upon the jelling point of juice. The concentration of acid was kept constant throughout the experiments while the amount of pectin was varied. The h a 1 product in each lot was 100 g. The amount of acid used throughout was 6 cc. equivalent to 1.5 g. of citric acid. The pectin solution contained 5.5 per cent pectin.
Carnegie Institute of Technology a t Pittsburgh has been selected by the United States Naval Academy to give advanced courses in metallurgy to its graduate officers. Beginning next September, the Academy will send two ordnance officers, graduates of the Academy, for a year's study, which will include graduate work in advanced metallurgy with some studies in electricity and physical chemistry. Other groups of graduate officers are assigned each year to various colleges or universities to study along specialized lines, but Carnegie Institute is the only school chosen for metallurgical instruction.
