CITRIC ACID CONTENT OF MILK AND MILK PRODUCTS.

Transcription

1 CITRIC ACID CONTENT OF MILK AND MILK PRODUCTS. BY G. C. SUPPLEE AND B. BELLIS. (From the Research Laboratory of The Dry Milk Company, New York.) (Received for publication, July 23, 1921.) Citric acid has been recognized as one of the normal constituents of milk for many years, but there is still some disagreement as to the forms in which it exists in this product. Soldner (1) claims the presence of potassium, magnesium, and calcium citrates, whereas Van Slyke and Bosworth (2) state that only sodium and potassium salts of this acid are present. Regardless of the exact form in which this constituent is found in milk, quantitative determinations have shown that there is normally between 0.Z and 0.2 per cent citric acid combined in one form or another. Interest has been directed t.o the parallelism between the citric acid content of fruit juices and their antiscorbutic properties, and to the similar association of t,hese factors in milk. If only natural products, unheated and without subjection to processing for preservation, were considered, the existence of a definite relationship between the two factors would be more acceptable. Reliable experimental evidence, however, shows that the antiscorbutic properties of natural foods are destroyed by heat whereas the citric acid content of the same products is not quantitatively affected by the application of heat even in excess of that, required for the destruction of the antiscorbutic property. Sommer and Hart (3) have shown that milk may be autoclaved at 15 pounds pressure for 1 hour wit,hout causing a diminution in its citric acid content. They have also shown that the citrates of milk are not rendered insoluble by a,utoclaving for 20 minutes at 15 pounds pressure. Since comparatively recent investigations by Hess (4, 5), Hart (6, 7), and Dutcher (8) and their coworkers have made available additional data on the antiscorbutic potency of milk as affected 453

2 Citric Acid Content of Milk by the feed of the lactating animal, and by what appears to be the method of processing in the preparation of various concentrated milk products, it has been considered desirable to record the variations in citric acid content of milk from individual cows while on a normal winter ration and when on a normal summer or pasture ration; also to determine the citric acid content of some well known concentrated milk products. EXPERIMENTAL. Estimation of Citric Acid in Milk. Due to the small amount of citric acid present in milk and because of the possible variations which it was desirable to detect, it has been deemed advisable to incorporate details of the well known methods used in this series of analyses. Determination of Citric Acid in Milk.--50 cc. of milk are treated with 10 cc. of dilute sulfuric acid (1: 1) and thoroughly agitated. 2 cc. of 40 per cent potassium bromide solution and 20 cc. of a solution of phosphotungstic acid are then added. After a thorough mixing, the precipitate is separated by filtration. To the perfectly clear filtrate in an Erlenmeyer flask is added an excess of freshly prepared saturated bromine water (usually between 5 and 10 cc.). The mixture is then placed on the water bath at a temperature of from 4%50 C. for about 5 minutes. After removing from the bath, add rapidly from a burette 25 cc. of potassium permanganate solution (5 per cent) drop by drop with frequent interruptions, and with constant and vigorous shaking, avoiding a temperature during the oxidation exceeding 55 C. Set the flask aside until the hydrated peroxide of manganese begins to settle. The supernatant liquid should be dark brown showing an excess of permanganate. Add more permanganate if an excess is not indicated. When the precipitation assumes a yellow color and most of it is dissolved, add drop by drop a clear solution of ferrous sulfate until the hydrated peroxide of manganese and excess of bromine are removed. Allow the solution to cool, shaking occasionally. Allow the mixture to stand over night. Collect by means of gentle suction on a tared Gooch crucible provided with a thin pad of asbestos previously dried over sulfuric in a vacuum desiccator: wash with water slightly acidified with sulfuric acid and finally wash twice with water. Dry the precipitate to constant weight over sulfuric acid in a vacuum desiccator protecting the precipitate from strong light. The weight of the precipitate multiplied by the factor gives the equivalent weight of anhydrous citric acid in the sample. Determination of Citric Acid in Milk Powder.-Weigh 5 gm. of powder into a beaker and reconstitute with 45 cc. of warm water. Mix thoroughly and proceed as with liquid milk.

3 G. C. Supplee and B. Bellis 455 Determination of Citric Acid in Sweetened Condensed Mill<.-Weigh out 25 gm. of the sample and add 200 cc. of 95 per cent, alcohol. Mix thoroughly and filter. To the filtrate add enough 0.25 N barium hydroxide to almost neutralize the solution and then 5 cc. of 50 per cent barium acetate in order to insure an excess of barium. Add about 150 cc. of 95 per cent, alcohol and reflux until the precipitate settles readily after being shaken. Filter and thoroughly wash the precipitate in the flask and on the paper with 95 per cent alcohol. Transfer the precipitate from the filter to the flask with a jet of hot water. Boil until alcohol can no longer be detected by odor and add enough sulfuric acid (1:5) to precipitate all of the barium originally present and to allow 2 cc. in excess. Evaporate to a volume of 60 or 70 cc.; cool and add an excess of bromine water. Filter and add 10 cc. of potassium bromide, then place on the watter bath at a temperature of C. and proceed as with liquid milk. TABLE I. Percentage of Citric Acid Recovered from Milk Products. No.1 No.2 No.1 No.2 No.l No.2 No.1 No.2. ~ ~-- - Original After adding 0.02 per cent I I I ~ The relative accuracy of these methods is shown in Table I in which is given the results of duplicate determinations on liquid milk with and without sugar, on evaporated milk, and on sweetened condensed milk; also duplicate results from each of t,hese products after known amounts of citric acid in the form of sodium citrate had been added. It will be noted that the maximum variation in duplicate results does not exceed per cent; it is believed therefore, that any significant variations occurring in the products examined were easily detected by the methods used. Citric Acid in Milk as Afected by Feed. In view of the work reported by Hess, Unger, and Supplee (4) in which it was shown that the milk produced from a highly concentrated ration contained less citric acid than that produced during pasture feeding, it has seemed desirable to obtain further

4 456 Citric Acid Content of Milk data on the amount of this constituent found in the milk of the same herd while receiving a normal winter ration and again during pasture feeding. Accordingly, samples were analyzed late in February and again late in June; the results are shown in Table II. From the results in Table II it is evident that there is a wide variation shown in the milk from individual animals receiving the same feed as represented by the difference between and per cent (Herd I). While the evidence pointing toward a variation TABLE Citric Acid Content of Milk from Winter Ration and from Pasture Feeding. Herd: III II I :ow Citric acid. II. Winter ration. Summer pasture Feed. >er cent Hay, distiller s grains, en silage, corn stover, mo lasses Hay straw, cottonseed, meal Hay, oil meal, corn-meal, bran Citric acid Feed.,er cent Fresh grass only I I in citric acid content as affected by the different feeds is not conclusive, there is, nevertheless, a tendency toward a higher percentage of this constituent in the winter milk of cows receiving ensilage and corn stover than in the milk of those herds receiving only hay as roughage. When summer and winter milks from each herd are compared there is a significant difference only in the case of Herd III in which the milk from pastured cows contains a uniformly higher citric acid content. The average citric acid content of the milk from all cows on a winter ration was per cent and from all cows when on pasture was per cent.

5 G. C. Supplee and B. Bellis 457 Citric Acid Content of Concentrated Milk Products. Since one of the purposes of this paper is to furnish analytical dat.a showing the citric acid content of concentrated milk products, it is desirable to briefly mention that in the manufacture of condensed, evaporated, and desiccated milks heat is applied in amounts varying from C. for a few seconds in the manufacture of powdered milk by the Just process, to sterilization under steam pressure in the case of evaporated milk. Therefore, the results from the products analyzed will adequately cover the temperature range to which concentrat.ed milk products are subjected during process of manufacture. Sample TABLE Citric Acid Content of Condensed and Evaporated Milks. Milk. Evaporated Sweetened condensed..... I... III ( Xric acid i n product. Xric acid :alculated to liquid nilk basis. per cent per cent The citric acid content of six different brands of evaporated milk and two brands of sweetened condensed milk are shown in Table III; also included in this table is the citric acid content calculated to the original liquid milk basis assuming a concentration ration of 2 to 1. The citric acid content of milk powder made by the spray process is shown in Table IV. The concentration ratio used for calculating to the liquid milk basis is 1 to 8.5 and 1 to 12 respectively for whole milk and skimmed milk. It has been possible to check up very closely on the citric acid content of powder made by the Just process by determining the citric acid and total solids before drying, for comparison with the dried product and with the reconstituted milk correctly diluted

6 458 Citric Acid Content of Milk on the basis of the data obtained from the total solids determinations. The results of these determinations are shown in Table V. The results from the different concentrated and desiccated milks do not show any variation in citric acid content which could TABLE Citric Acid Content of Milk Powder (Spray Process). IV. Milk. Sample Whole milk powder I I L I I I Skimmed milk powder I I I I TABLE V. Citric Acid Content of Milk Powder (Just Process). Milk. Skimmed milk I I Part skimmed milk I I X;i$x,ld before drying. per cent per cent ( 3tric acid i n powder ( Xric acid in accurately rec :onstituted powder. -_ per cent Whole milk I < not possibly be explained by variations in the liquid milk prior to manufacture. In the case of the Just process powders where complete data were available for reconstituting powder to the original milk basis, the citric acid content before and aft,er drying is in full agreement. These results are therefore in full accord

7 G. C. Supplee and B. Bellis 459 with those of Sommer and Hart (3) and others in that the heating of milk does not cause a decrease in its citric acid content. The results also serve as further evidence to the lack of a definite relationship between citric acid content and antiscorbutic potency, as there is experimental evidence showing that some of these concentrated milk products apparently lack this vitamine, whereas in others it is retained to a high degree. Citric Acid Content of Milk as Afected by Age and Bacterial Quality. From the preceding data it has appeared that the variations in citric acid content of concentrated milk products could be explained TABLE Citric Acid Content of Milk as Afected by Aging. Sample. I VI. Age. Raw. Fresh days. I I Pasteurized Fresh days. I Acidity. Citric acid. per cent per cent by the variations in the milk from individual animals rather than by the method of manufacture. Another possible cause of variation is suggested, however, by the observations of Kunz (9) to the effect that citric acid gradually decreases with the aging of milk, particularly when soured. The utilization of citrates by certain microorganisms is an established fact and it would seem. quite possible that under certain conditions and in the presence of certain species of bacteria, the citric acid content of milk might be appreciably lowered. In order to secure information on this point, a sample of milk held raw and after pasteurization for a period of 28 days was analyzed for developed acidity and citric acid content at regular intervals. The samples were held at low

8 460 Citric Acid Content of Milk temperatures after the first 3 days. The results of these determinations are shown in Table VI. Even though the results given in Table VI show a marked decrease in citric acid content due to aging, there is considerable doubt as to just how far these results can be used as an explanation of the variations recorded in the preceding tables, particularly in view of the knowledge that milk for ma,nufacturing purposes must of necessity be fresher and of better quality than that in which the diminution in citric acid was detected. The results, however, are significant as indicating the general tendency toward reduction in citric acid content in milk of poor quality. SUMMARY. The conclusions which may be drawn from these investigations follow. There is a marked variation in citric acid content of the milk from individual animals, which may be explained on the basis of the data shown herein as due to the individuality of the particular animal. Certain data, however, indicate that the ration may have a slight effect upon this constituent. There is apparently no effect upon the citric acid content of milk caused by heating during the manufacture of evaporated, condensed, and dried milks. The results indicate that the amount found in each of these products, if subject to variation, must be attributed to causes other than heat. The parallelism bet,ween citric acid content and antiscorbutic properties does not hold true in the case of concentrated milk products; the potency of this factor has been shown to be absent in some of the heated products and present in others. 1 he citric acid content however, seems to be present in all of them apparently to the same degree as found in natural raw milk. The citric acid content of milk decreases during aging in the presence of high developed acidity, and is more rapid in raw milk than in pasteurized milk. It is quite probable that the effect of acidity and age is not applicable in causing as marked a diminution of citric acid in milk used for manufactured products as is shown by the data presented herein.

9 G. C. Supplee and B. Bellis BIBLIOGRAPHY. 1. Soldner, in Sommerfeld, P., Handbuch der Milchkunde, Wiesbaden, Van Slyke, L. L., and Bosworth, A. W., The condition of casein and salts in milk, New York Agric. Exp. Station, Techn. Bull. 39, 1914, Sommer, H. H., and Hart, E. B., Effect of heat on the citric acid content of milk. Isolation of citric acid from milk, J. Biol. Chem., 1918, xxxv, Hess, A. F., Unger, L. J., and Supplee, G. C., Relation of fodder to the antiscorbutic potency and salt content of milk, J. Biol. Chem., , xlv, Hess, A. F., and Unger, L. J., The scurvy of guinea pigs. III. The effect of age, heat, and reaction on antiscorbutic foods, J. Biol. Chem., 1919, xxxviii, Hart, E. B., Steenbock, H., and Smith, D. W., Studies of experimental scurvy. Effect of heat on the antiscorbutic properties of some milk products, J. Biol. Chem., 1919, xxxviii, Hart, E. B., Steenbock, H., and Ellis, N. R., Antiscorbutic potency of milk powders, J. Biol. Chem., 1921, xlvi, Dutcher, R. A., Eckles, C. H., Dahle, C. D., Mead, S. W., and Schaefer, 0. G., J. Biol. Chem., , xlv, Kunz, R., The determination of citric acid in milk, Arch. Chem. u. Mikros., 1915, viii, 129, abstracted in Exp. Station Rec., 1917, xxxvi.

10 CITRIC ACID CONTENT OF MILK AND MILK PRODUCTS G. C. Supplee and B. Bellis J. Biol. Chem. 1921, 48: Access the most updated version of this article at Alerts: When this article is cited When a correction for this article is posted Click here to choose from all of JBC's alerts This article cites 0 references, 0 of which can be accessed free at ml#ref-list-1