Precipitation of Grain-Curd Casein from Pasteurized Milk, Including

It was found throughout the experimental study of the separation of casein from pasteurized milks that when the identical conditions of the grain-curd...
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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY

Vol. 13, No. 6

Precipitation of Grain-Curd Casein from Pasteurized Milk, Including Sweet Cream Buttermilk'tz By Harper F. Zoller u. s. DEPARTMENT OF

DAIRY DIVISION, BUREAUOF ANIMALINDUSTRY,

The application of t h e normal grain-curd method t o the manufacture of casein from milk, which has sometime during its history been heated t o pasteurizing temperatures or higher, cannot be made without certain modifications. It was found throughout the experimental study of t h e separation of casein from pasteurized milks t h a t when t h e identical conditions of the grain-curd precipitation were observed the resulting casein was of a decidedly different texture from t h e normal type of curd from unheated milk. The curd was softer, and t h e grains were less definitely formed and much smaller in size. Changing the velocity of stirring during t h e precipitation had no appreciable effect upon this peculiar physical texture. It was found t h a t this curd could not be economically handled upon the drain rack. Clogging of the pores of t h e cloth resulted, rendering rapid draining of the whey impossible. Working of the curd upon t h e cloth caused many fine particles t o pass through t h e pores, while t h a t which remained upon t h e cloth became soggy and puddled. Furthermore, an additional phenomenon was experienced during the washing of the curd. When the curd was leached with water adjusted with hydrochloric acid t o t h e isoelectric condition of casein, i. e., pH 4.6, there was a marked retrogression of the hydrogen ion. This phenomenon throws some light on the influence of heat upon the protein andsalt equilibrium in milk. The discussion of this question is reserved for a later paper. Some of the experimental procedures aimed towards t h e restoration of t h e normal texture t o the curd will now be reviewed. I N F L U E N C E O F D I F F E R E N T PASTEURIZING T E M P E R A T U R E S U P O N NATURE OF CURD

Twenty-five-pound portions of fresh skimmed milk were separately heated t o t h e temperatures indicated in Table I. After cooling t o 34" C., the casein was precipitated ,with normal hydrochloric acid, using methyl red as indicator of the end-point as in the regular grain-curd method.3 The curd was then thrown upon a cloth in a drain rack and allowed t o drain free from whey. T a p water adjusted t o pH 4.6 was put into a vat large enough t o accommodate the drained curd. The edges of t h e drain cloth were gathered together, and the curd was lifted from the tray and immersed in the wash water for a period of 10 rnin. When possible, the curd was agitated in the water so that the washing would be as thorough as possible. The washing process was repeated in three changes of adjusted water. The retrogression of the hydrogen ion was marked in these experiments, especially in B, C, and D. Within 1 2 8

Received January 26, 1921. Published with the permission of the Secretary of Agriculture. THISJOURNAL, 12 (1920), 1163.

AGRICULTURE, WASHINGTON, D. C.

TABLEI Portion A B

C

D

Temp. of Heating ' C. 50 63 80

Time Min. 60 60

60

98

30 Unheated control

E

Per cent Ash 3.80

4.28 4.84 5.21 2.02

R dm. ______

gression ' of Nature of H + Ion Curd Yes Soft Yes Soft. featherv Yes Very soft, fine and dark Yes Very s o f t ,fine and dark No Firm and normal

a few minutes from the time the curd was suspended in the wash water (at pH 4.6) t h e pH of the supernatant water had increased t o 5.6. The change in p H could be followed quite accurately with methyl red. It must be remembered t h a t t h e curd had just been taken from its whey which registered a pH of 4.6 t o methyl red. From hydrogen-electrode measurements' i t is found t h a t when methyl red registers a pH of 4.6 in skim milk t h e actual concentration of hydrogen ion is greater (pH 4.1 t o 4.20) t h a n t h a t demanded by t h e isoelectric point of casein. Hence this retrogression of the hydrogen ion in the wash water is even more astonishing. This was further magnified when i t was found t h a t t h e retrogression occurred even after as many as ten-changes of wash water, although in these instances with protracted washings t h e curd began t o disperse rapidly in the medium. This dispersion of t h e curd in t h e wash water is, again, contrary t o the experiences with normal grain curd. INFLUENCE

OF

TIME,

AT

CONSTANT

TEMPERATURE,

U P O N PHYSICAL NATURE O F CURD

Like quantities of fresh skim milk were placed in shotgun milk cans, and these were set in a large vat of water heated t o 63" t o 64" C. and maintained at this temperature throughout the experiment. The milk was thoroughly stirred in each can during t h e heating. When the period of heating was over each can, in turn, was plunged immediately into running water a t 18" C. The casein was precipitated under exactly t h e same conditions as in the above experiments. The results are given in Table 11. TARLEI1 RetroMilk Temp, of Time OF gression Portion Heating Heating of C. Min. Hf Ion 63 20 ' Yes A 63 40 Yes B C 63 60 Yes D 63 90 Yes E Unheated No

..

Nature of Curd Softer than control Too soft t o wash Feathery Very soft, and disperses in w h e y Firm and normal

It is evident t h a t the duration of heating has nearly as much effect upon t h e nature of the resulting curd as t h e degree of heating. It has been shown, however, t h a t i t does not have quite the same effect upon the equilibrium of milk salts.2 RESTORATION OF F I R M N E S S TO T H E CURD BY U S E D I F F E R E N T ACIDS FOR P R E C I P I T A T I O N

OF

Before proceeding with the influence of t h e various anions of acids upon t h e firming of t h e casein curd 1

W. M. Clark, H. F. Zoller, A. 0. Dahlberg and A. W. Weimar, Tms 12 (1920), 1163. Unpublished results.

JOURNAL, 2

June, 1921

THE J O U R N A L O F I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y

lrom heated milks, attention should be called t o another possible factor i n t h e acid precipitation of t h e casein. This idea is not original, since Lacquer and Sackurl found t h a t casein which had been dried at high temperatures suffered “cleavage,” and t h e alkali-soluble portion, which they designated as “isocasein,” possessed a n increased base-binding capacity over normal casein. From their conductivity measurements they arrived at t h e conclusion t h a t this body was a much stronger acid t h a n ordinary casein and consequently possessed a greater dissociation constant. With a set of buffer mixtures covering the probable range of Hf-ion concentration in which t h e isoelectric point would be found, a n d a 0.1 per cent solution of sodium caseinate a t pH 7.2 made from t h e curd from milk heated t o 80” C. for 1 hr., t h e writer was unable t o note any marked displacement in t h e probable isoelectric point. The idea was t o note t h e pH zone in which t h e casein precipitated. Of course, this method is very rough, and a more elaborate study should be made of this question. I n t h e manufacture of casein b y t h e grain-curd process in t h e factory t h e writer frequently noticed t h a t when t h e skim milk was slightly sour from lactic acid fermentation the resulting casein curd was extremely firm a n d excellent t o handle. Repeated trials in the laboratory confirmed this experience. Solutions of t h e following acids were prepared in normal concentration with respect t o t h e hydrogen ion: lactic, citric, oxalic, tartaric, acetic, phosphoric, sulfuric, nitric, and hydrochloric. Two volumes of t h e normal acid were mixed with one volume of normal hydrochloric acid, and t h e mixtures were used as t h e precipitants in fresh skim milk pasteurized at 63’ C. for 1 hr. The casein was precipitated a t 34” C. under t h e control of methyl red. The physical nature of t h e curd was carefully noted. The results appear in Table 111. TABLEI11 Temp. of Retrogression Acid HC1 Precipitation of H + Ion Character of Curd 2 Vols.: 1 VOl. Lactic 34 No uite firm and washable but short (brittle) N O 34 Quite firm and washable but Citric short (brittle) Oxalic 34 Slight Not as firm as lactic Tartaric 34 No Quite firm and washable (brittle) Acetic 34 Very slight Curd washable but brittle Phosphoric 34 Yes Soft and disperses Sulfuric 34 Yes Soft and disperses Nitric 34 Yes Firmer but not washable Hydrochloric 34 Yes Soft and disperses

+

c.

*

There is a marked influence upon the physical structure of t h e casein curd which suggests, aside from any practical application, a relation t o Pauli’s2 and Batschek’s3 work on t h e production of a stiffer gel with gelatin or agar by t h e addition of citrate or tartrate. This effect is undoubtedly a manifestation of a change in t h e distribution of water between the two phases. Whether we consider this change t o be wrought by t h e resulting concentration of hydrogen ion or by the distribution of electrical charges, or what not, such considerations have no place in this paper. 1 2 8

Beilr. chem. physiol. pathol., 3 (1902), 210. Arch. ges. Physiol.. T8 (1899), 315. “Introduction to the Physics and Chemistry of Colloids,” 1916, p 49.

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USE O F COPRECIPITANTS W I T Y H,YDROCHLORIC ACID I N P R E C I P I T A T I N G CASEIN

The coprecipitants which would be suggested by t h e work of Freundlich,l Linder and Picton,2 Hardy,8 and others would be those possessing polyvalent cations, This is because i t has repeatedly been demonstrated t h a t casein exists in ordinary milk in t h e form of a caseinate anion possessing a charge equivalent t o a tetra-, hexa , or octabasic acid (or multiple thereof). Hence as this charge becomes neutralized by positive hydrogen ion, as it does in acid precipitation, t h e casein finally reaches a point where its electrical charges are equivalent, or zero in external effect (the isoelectric point), and in this state is extremely sensitive, as a neutral colloid, t o physical stimuli. Thus we should expect those electrolytes which affect pure suspensoids t o affect similarly this neutrally suspended casein. Investigations by t h e above-mentioned workers h a v e shown t h a t polyvalent cations produce maximum effects upon such colloids and form firm coagula o r precipitates. Further i t has been shown t h a t these electrolytes, or “coprecipitants,” as they are termed in this paper, usually contaminate the precipitate, which leads t o t h e speculation t h a t the mechanism of this phenomenon is one of adsorption. T h e polyvalent cation salts available (readily) at this time were those of aluminium and t h e alums. Solutions of 0.2 M aluminium sulfate, ammonium alum, and potassium alum were prepared. These solutions were strongly acid in themselves (pH 2.1 t o 2.4) and served t o precipitate t h e casein alone without t h e addition of further acid, but t h e addition of so much extraneous salt was inadvisable. Ash analysis of some of t h e caseins prepared with aluminium sulfate as t h e sole precipitant showed as much as 8.5 per cent ash. The precipitation mixture which yielded an average curd from heated milks consisted of one volume of M alum solution with two volumes of N HCl. When t h e regular grain-curd process was followed with t h i s precipitant on milk heated t o 80’ C. for 1 hr., the resulting curd was fairly firm, could be washed quite well, b u t was very brittle. The caseins resulting from this treatment ashed from 4.5 t o 5.5 per cent of mineral matter. EFFECT

OF

HIGHER

PRECIPITATION

TENPERATURES

U P O N T E X T U R E OB CASEIN CURD

It was evident during the early studies t h a t the temperatures of precipitation had a marked influence upon t h e cohesion of t h e curd particles. I n following t h e effect of temperature upon the precipitation of grain-curd casein from fresh, unheated skim milk, the extreme sensitiveness of t h e coagula t o slight changes in t h e temperature of the medium which bathed them was duly appreciated. A c c u r a t e colztrol of the precipit a t i o n temperature i s one of the main f a c t o r s in the success of the grain-curd method. Now when t h e milk has been subjected t o abnormally high temperatures for varying lengths of time, the proChem., 44 (1903), 129;Z . Chem. I d . Kolloide, 1 (1907), 321. J. Chem. SOL,61 (1892), 137. a Proc. Roy. Soc. London, 66 (1900). 110; J . Physzol., 38 (19Q5), 281.

12,physik. 2

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512

teins are believed t o become denatured. T h a t is, during the heating the physical properties of the proteins are changed in such a manner t h a t the molecules have a n abnormal absorption affinity for water. This is .a progressive process, as a glance a t Table IV will show. TABLEIVI Temp. of Moisture Time of in Curd Heating Precipitation O c. Per cent Milk c. Min. 50 60 34 44.6 A 63 60 34 62.2 B 75 GO 34 68.5 c... D 100 30 34 79.4 15 34 88.2 E 120 34 40.3 F.. Control 1 The data in this table were determined by treating the above milks as in the grain-curd method, draining as free from whey as possible, and washing once by decantation with equivalent amounts of water. The curd was then thrown upon a drain cloth and allowed t o drain for 30 min. The moisture content of these curds was then determined by drying about 5-g. portions in an oven a t 98' to 99' C. t o constant weight. The moisture content is expressed in per cent. Temp. of Heating

............ ........... ........ ........... ........... ..........

..

The higher the temperature the greater amount of water will the curd hold, until a t excessive temperatures the phase approaches a reversal and the curd simulates a gel in appearance.' This water-holding power of the curd must be in some degree a reversible process, because when it is precipitated in a medium heated t o temperatures in the neighborhood of those used in pasteurizing, the curd becomes firm again, although its internal structure is still abnormal. Some of the following observations will serve t o emphasize the importance of higher precipitation temperatures with heated milk.

Vol. 13, No. 6

above conditions of the experiment. The temperature of 42.5" yielded by far the best curd in this series. EXPERIMENT B

A series of experiments were designed t o determine the effect of the duration of heating upon t h e optimum precipitating temperature, when the pasteurizing temperature was held constant. The pasteurizing temperature was 63 ', probably representing the one most commonly used. Time periods of 20, 30, 40, 60, 80, and 100 min. were studied. The results of these studies are shown graphically in Fig. 1. The grain-curd principle of precipitation was observed in all save the temperature. The optimum precipitating temperature was defined as t h a t temperature which produced a curd t h a t most nearly simulated grain curd in its uniformity of size and condition for washing. Another series of tests were performed upon milks pasteurized a t different temperatures, in order t o de-

EXPERIMENT A

Fresh skim milk was pasteurized a t 63' C. for 1 hr. It was then cooled down t o the temperatures indicated in Table V and the casein precipitated therefrom with normal HC1, using methyl red t o indicate the approach t o the isoelectric point. The curd was then drained from the whey and suspended in adjusted water (pH 4.8). Temp. of RetroPrecipi- gression tation of Milk C. H+Ion A 30 Yes B 35 Yes C 40 Slight D 50 No

E

60

No

F

42.5

No

TABLFV

Ash in Casein Per cent Texture of Curd Washable Feathery No 2.88 Soft No 3.14 Firmer and grained Not readily 3.92 Chunks Yes, but imper- 4.20 fectly Large clumps, Yes, but imper- 4.16 leathery fectly Very firm and Yes 3.85 grained

I t was noticed during - the washing- of the curd, precipitated a t 40' C., with cold water t h a t brittleness was increased, whereas if the wash water was warmed t o about 30" to 35' t h e brittleness was not so noticeable. The toughness of the curd, which is a result of the higher temperature of precipitation, remains unchanged in the warm wash water. All of the curds, such as C and D, which approximated normal grain curd in appearance were found t o be very "short" in texture. T h i s i s a characteristic property of all pasteurized m i l k caseins. The essential fact divulged in Table V is t h a t different precipitating temperatures influence the physical nature of the curd from milk pasteurized under the 1 It should be mentioned t h a t since the writing of this paper Mr. Leighton of these laboratories has actually obtained a curd gel by heating milk a t high temperature (about 140° C.) in a sealed bomb.

FIG. CONSTANT TEMPSRATURE,FIG.Z-CONSTANT TIME OF PAS63' C. TEURIZATION, 60 MIN.

termine the optimum temperature for obtaining a workable curd from each milk in question. With the exception of the temperature, the grain-curd method of precipitation was followed throughout. The time of pasteurization of the milks was held constant (1 hr.). The results of these tests are reproduced in the optimum temperature curve in Fig. 2. This effect of precipitation temperature is obviously of immense importance, and its practical application is a t once evident. Further discussion of it is reserved till later in this paper. EMPLOYMENT O F R E N N I N I N PRECIPITATION O F CASEIN PROM PASTEURIZED MILK

Without permitting himself t o become bewildered with the diverse considerations upon the mechanism of rennin action in normal and heated milk, the writer decided t o determine the practicability of this method for the separation of casein from pasteurized milk, including buttermilk. Fresh skim milk, which had been pasteurized a t 65' C. for 1 hr.,was carefully adjusted t o the zone of the optimum activity of rennin in heated milk (about pH 6.2) 1 with hydrochloric acid, using bromocresol purple to determine the pH. The milk was then cooled t o 37' C. and the usual amount of rennin added. The clotted curd, after cutting, was digested in t h e whey for half an hour a t 60' C. t o expel moisture and salts. It was then drained upon a cloth in a drain rack and washed several times with water. After pressing, i t 1

Biochem. J., 9 (1915), 215.

June, 1921

T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY

was ground and dried like ordinary casein, a n d finally analyzed. It was found t o contain 9.5 per cent moist u r e a n d 5.8 per cent of ash. An explanation of t h e high digestion temperature used above is necessary a t this point. It was found after cutting t h e curd t h a t i t was very soft a n d mushy, a n d remained in this condition until t h e temperature was raised above that o r d i n a r i l y used in j i r m i n g r e n n i n curd from unheated milk. It was uniformly found t h a t pasteurized milk required a higher temperature for the firming of t h e curd, at t h e same time-interval a n d acidity, t h a n is necessary for unpasteurized milk. This corroborates t h e temperature effect upon t h e casein curd discovered in connection with acid precipitation. Members of t h e dairy manufacturing section inform t h e writer t h a t in their work with pasteurized cheese greater heat has always been found necessary t o firm t h e curd from pasteurized milk. Obviously, t h e time required for this method and t h e v a t space necessary would alone mitigate against its practical use. T h e character of t h e final casein is not greatly different from t h e acid-precipitated casein from heated milk, except t h a t it contains higher ash. This high ash content naturally results from t h e fact t h a t i t enmeshes large quantities of insoluble calcium and magnesium phosphates which remain insoluble a t t h e reaction in t h e pH zone in which rennin coagulation takes place. The writer finds t h a t both calcium a n d magnesium phosphates ( C a H P 0 4 and M g H P 0 4 ) are practically insoluble a t this reaction, namely, p H 6.2. Another disadvantage of this rennin casein is its slow rate of dissolving in alkalies. I n this respect i t is not unlike cooked curd casein which it also resembles in ash content. If t h e engulfed salts could be removed f r o m both of these caseins i t would increase their rate of solubility. This was actually found t o be t h e case. T h e caseins were redissolved in dilute ammonia, t h e undissolved residue separated by centrifuging a t great speed, a n d t h e resulting solution precipitated with dilute acetic acid, and thoroughly washed. T h e resulting casein curd was still characteristic of high temperature caseins in “shortness,” b u t t h e dried product dissolved more readily. T h e nonvolatile ash amounted t o less t h a n 1.6 per cent in both cases, DISCUSSION O F E X P E R I M E N T A L RESULTS

It is evident from t h e temperature a n d time studies t h a t t h e condition of t h e casein in pasteurized milks varies with t h e conditions of pasteurization and, there.. fore, it is necessary t o take these factors into consideration when attempting t o prepare casein from such milks. While t h e organic acids are found t o yield a good working curd, they would be impracticable industrially because of t h e cost. It would be possible t o consider lactic fermentation (natural-sour process), b u t this is a very unsanitary method t o apply in factory practice. I n respect t o t h e use of coprecipitants with hydrochloric: acid i t may be said t h a t , wcile t h e alums were found t o increase t h e firmness of t h e curd precipitated a t grain-curd temperatures (34 ’ C.), t h u s facilitating t h e draining and washing, they are not advocated

513

because of t h e effect of t h e absorbed precipitant upon t h e ash content of t h e resulting casein. T h e hiqh and insoluble ash reduces its solubility in alkalies. Certainly t h e simplest way t o render t h e casein from pasteurized milk obtainable under factory working conditions is t o increase t h e temperature of precipitation, as t h e results of t h e studies on this factor indicate. I t is t h e easiest factor t o control in factory practice. I n t h e time-worn commercial methods of precipitating casein, high temperatures were universally employed, viz., 45’ C. a n d up. I t i s i m m e d i a t e l y evident w h y little troztble w a s m e t in p r e c i p i t a t i n g c a s e i n f r o m heated m i l k s in t h e p a s t . W i t h t h e g r a i n - c u r d method t h i s q u e s t i o n i s of the u t m o s t i m p o r t a n c e . As previously mentioned, t h e curd a t t h e isoelectric point is in a n extremely sensitive condition and responds in a maximum degree t o physical stimuli. T h e temperature of 34 ’t o 35 ’C. is t h e narrow zone for optimum working condition for grain curd from normal milk. This temperature is much too low for t h e optimum working curd from pasteurized milk. T h e modified scheme of t h e grain-curd process t o be applied t o heated milks in factory practice is as follows. OUTLINE

OF

METHOD

FOR

MANUFACTURE

OF

CASEIN

F R O M P A S T E U R I Z E D MILK

Essentially this is a modification of t h e grain-curd method described b y Clark, Zoller, Dahlberg a n d Weimar.’ (1) T h e milk should be heated t o a temperature indicated upon t h e published curves t h a t correspond t o t h e pasteurizing conditions t o which t h e milk was subjected. If the history of t h e milk is not known t h e optimum temperature may best be determined b y trial. ( 2 ) Dilute hydrochloric acid (100 lbs. of 20’ Bi.. t o 800 lbs. of water) should be added slowly t o t h e heated milk, bringing i t into contact with all portions of t h e milk as quickly as possible. A hardwood v a t and spigot (hardwood) prove t o be t h e best containers for t h e dilute acid in factory practice. When t h e milk ‘(breaks,” i. e., when t h e curd first separates from t h e whey, t h e flow of acid should be checked and t h e pH of t h e whey determined with methyl red indicator ( 5 drops of a 0.04 per cent solution of methyl red in 10 cc. of milk or whey). I t is frequently noticed t h a t in pasteurized milks t h e “break” is considerably delayed. This makes i t very easy t o overstep t h e endpoint. T h e addition of acid should be ceased when t h e indicator first shows a bright red. (3) T h e whey is then drained from t h e curd. Because of t h e delayed “break” i t is frequently impossible t o draw off a portion of t h e whey before adding t h e remainder of t h e acid necessary t o reach t h e endpoint. Wherever this is possible t h e reader is referred t o t h e regular method cited above for full details. (4) T h e curd is then washed with water a t a temperature of about 30” t o 35’ C. and adjusted with hydrochloric acid t o pH 4.8. The washing may be done in t h e v a t b y decantation before placing in t h e drain rack, or afterwards, as desired, although t h e former 1 LOC.

Cil.

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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y

method is somewhat more effective. Drain racks and screw presses are probably best adapted for very small casein plants. Where large quantities of milk are handled, t h e factory would be repaid b y installing a centrifugal which handles both operations in one. The application of the centrifugal is discussed below. ( 5 ) For t h e drying of th8 casein and other details the reader is referred t o t h e publication on grain-curd casein mentioned above. SEPARATION

OF

CASEIN

FROM

PASTEURIZED

SWEET

CREAM BUTTERMILK

The method just elaborated has been found t o work well with this type of buttermilk. Small-scale factory trials with buttermilk from cream t h a t has been pasteurized at 63’ C. for 30 min. seemed t o show t h a t 40° C. was t h e desirable precipitating temperature for satisfactorily handling t h e resulting curd.1 Cream pasteurized at t h e same temperature for 1 hr. demanded 45’ for precipitation of t h e casein from t h e buttermilk, It is remarked t h a t t h e f a t content of t h e buttermilk alters t h e “feel” of t h e curd, as well as its working conditions. But t h e amount of f a t which should contaminate sweet cream buttermilk is so small as not t o handicap seriously t h e use of this method. USE

OF

CENTRIFUGAL

IN

MANUFACTURE

OF

CASEIN

The use of centrifugals for washing casein has been practiced for many years in those countries where t h e price of commercial casein is considerably less t h a n i t is in normal times in t h e United Statesa2 During t h e war the’ casein campaign permitted t h e writer t o t r y out in practice centrifugals of fairly large capacity, It was at once clearly demonstrated t h a t there was available in grain curd a type of product especially suited t o t h e centrifuging process. The commercial casein curds in the past were either so bulky and tough or else so soft t h a t even loading of t h e bowl of t h e centrifugal was impossible. But with grain curd t h e particles are so uniform in size t h a t a case of a n overbalanced bowl was never experienced in t h e number of trials conducted. Large sugar centrifugals in three different milk product factories were placed a t t h e author’s use for study. Through t h e courteous cooperation of t h e employees in t h e factories mentioned, test runs were made with grain-curd casein. I n one test with a machine possessing a 54-in. bowl and bottom discharge, t h e total curd from 5500 lbs. of milk was accommodated in one load. The time required for t h e precipitation of t h e curd, loading i t mechanically from t h e vat into t h e revolving bowl, whizzing free from excess whey, washing with 2000 lbs. of water, and pressing free from water for grinding within the bowl by increased speed of revolution, was only 40 min. I n another half hour i t was ground and placed upon trays in the tunnel dryer. T h e advantage of t h e centrifugal over t h e drainrack, cloth, and press method may be enumerated as follows: 1 The writer desires t o thank Mr. A. 0. Dahlberg of this Division for trying this method on sweet cream buttermilk a t the experimental creamery located a t Grove City, Pa. 2 “Casein,” 1911.

Vol. 13, No. B

(1) The operation is entirely mechanical from t h e precipitating vat through t o t h e final washing in t h e centrifugal bowl with adjusted water. (2) The draining, washing, and pressing of t h e curd are done in one operation. The curd may be pressed t o any degree desired by merely varying t h e speed of t h e rotating bowl. It is ready to be ground when taken from t h e bowl without further pressing, and is ready for t h e dryer. (3) The saving of considerable time by completing in one day an operation which now generally demands two by the rack, cloth, and press method. (4) The improvement of t h e sanitary conditions around t h e factory by doing away with wooden trays, press divider-boards, drain cloths, and press cloths, which now become t h e eyesore and olfactory pressagent of every casein plant using these accoutrements. (5) The main equipment necessary in a large factory would be t h e precipitating vat, or vats, centrifugal, curd mill, casein drying tunnel, and grinder for t h e dried casein. (6) The centrifugal would be especially well suited t o t h e washing and pressing of t h e casein prepared from pasteurized milk b y t h e modified grain-curd method, because of t h e short character and brittleness of t h e curd. It would receive less handling in the centrifugal and t h e loss therefore would be less. SUMMARY

I-The grain-curd method can be successfully applied t o t h e separation of casein from pasteurized milks only when higher precipitating temperatures are used. The optimum temperatures are exhibited in t h e form of curves for t h e different observed conditions of pasteurization. 11-The marked differences in t h e physical nature of t h e curd from pasteurized and unpasteurized milks are strikingly revealed by t h e grain-curd method of precipitation. Attempts t o overcome some of these physical effects by t h e use of organic acids as precipitants and with coprecipitants are described. 111-The advisability of using rennin t o precipitate casein from pasteurized milk is dismissed because of t h e time required and the large quahtity of mineral matter entrained in t h e curd. IV-Large centrifugals are recommended for washing and pressing t h e casein precipitated b y t h e graincurd method from pasteurized and normal milk. V-The phenomenon of t h e retrogression of t h e hydrogen ion was discovered in t h e whey and wash water from t h e curd precipitated from pasteurized milk by t h e grain-curd process a t 34’ C. This rapid decrease in acidity is attributed t o t h e excessive precipitation of alkaline earth phosphates during pasteurization, and their subsequent re-solution a t t h e expense of t h e hydrogen ion as they are brought into ready contact by t h e soft dispersing curd. VI-The great check in t h e rate of this retrogression wrought by using higher temperatures for precipitation is believed t o be due t o the engulfing of these precipitated phosphates by t h e firming of t h e c u r d , thus reducing t h e intimate contact between t h e solution and t h e phosphates.