THE C H m r I c u AND PHYSICAL CHANGES ATTENDAST UPOX THE STERILIZATION O F MILK.
When milk is sterilized by heating in loosely plugged flasks immersed in a steani bath surface evaporation does not occur,' a n d there is little or no pellicle formed on the surface, t h e sterilized milk presenting the same appearance, if t h e heat is not continued longer than one or two hours, as t h e original. Under the microscope the only visible chaiiges effected in the boiled and sterilized milk is the coalescence of some of the fat globules into others of larger size, and the appearance of more shreds or coagula of separated caseinous matter . 'I'lie differences in the behavior of raw, boiledand sterilized milk when treated with dilute acid were not by any means as striking as anticipated. I t is frequently stated that t h e precipitate formed with the two latter, requires less acid for its formation, is formed more quickly, and is in softer, finer, more flocculent masses t h a n the clot or cnrd of ram milk. B a t on treatment with equal quantities of dilute acid a6ded in successive portio!is, and waiting for equal intervals, I have not beeii able to satisfy myself t h a t much difference between t h e three milks, exists. T h i s is especially true when tlie separated casein, which is present in raw milk, has illready been partly removed with tile skin formed on heating, and which itself tends t o iiicrcLise the size and density of t h e clots of acidtilated raw milk. It is indeed w r y d:fficalt to form a judgment when tho undiluted milks %re used,. since the clots remain suspended in the remaining cloudy liquid, and their size and appearaiice ciiii readily be modified by manipulation and shaking. 13etter to see wli;tt takes place, each milk was diluted to twenty times its original volamc with water, ten cubic centimetres of the
35
STERILIZATION O F MILK, ETC.
milk being taken, and then a cubic ccntimetre of aceticacid which itself had been diluted ten times was added. At first the flocks in t h e raw milk formed a precipitate more dense and odcupying a t t h e ,bottom of the vessel a notably less volume, than that precipitating out of the sterilized milk. In the former case t h e supernatant liquid was also more opaque from unprecipitated white particles, b u t after staiidiiig 12 hours, these differences in the bulk of t h e precipitate had disappeared, while the supernatant liquid from t h e sterilized liquid had a white turbidity and t h a t from t h e raw milk was perfectly clear. The boiled milk failed to precipithte a t all, and an additional cubic centimetre of acid had to be added to before its casein was thrown down. It is necessary t o have recourse to more accurate methods to discover the nature of the changes effected by heating. And the more important and striking is the following. When ordinary raw milk is diluted with 20 times its volume of water, and the precipitate is filtered off, the dilute acid filtrate gives on boiling a n additional precipitate. Milk which has been heated for an hour (sterilized), or boiled for half an hour, behaves altogether differently. It gives a larger amount of precipitate with dilute acid, but yields no further precipitate on boiling the filtrate. The precipitate obtained with dilute acid must certainly be different in character, therefore, when raw milk is used and when cooked milk is tested, even though the differences are difficult to recognize by the eye alone. If we call the albuminoid matter first precipitated C ; t h a t separated on boiling A, and t h a t remaining i n solution even after boiling the acid liquid, P, the result obtained by experiment on a sample of milk, raw, boiled for half an hour, and sterilized for one hour were as follows : Raw. Boiled. Sterilized. Total Albuminoids 3,303 per cent, 3.451 per cent. 3.353 per cent. c ("Caseine") 2.514 3.129 " 2.928 " " A ((I Albumen") 0.144 '( none '( none " P (Soluble) 0,335 (' 0.352 0.253 " " Of course the total albuminoids must have been the sanic k amount in each case, but the formation of t h e skin on the o w k d '9
'(
'(
*(
((
((
('
milk by making it difficult to obtain ;t homogeneous sample, introduced slight discrepancies i n the analyses. I have put tlie name albumen i n quotation marks, liecanse while simillrr to the albumen of sotne authors, t h e precipitate obtained in this niatiner would not ezactl>- correspond. Raising t h e tcmperature merely to tlie boiling point is ! ! < I t sufficient. Some d b u men is still o1,ttiiiied on boiling tilt! xcid filtrate from the caseinthe heating must be continued f o r sonic time. Inasmuch as gastric digestion is performed in it11 ; i c d medium, and precipitation would a t once occur, t,lie more complete precipitation of the :dburninoid matter in cwokecl :is conipitreil with raw milk n-ould exert a correspoiidiiig influence. Tlie ratio of C to A and P appears to be very variable. In another sample of milk obtaincci from the same dairy C= 2 . i 1 3 ; X=0.4134 ; P=0.33?: T o t n L 3 . 5 1 9 . This gives a ratio of total albuminoids tocaseiii of 100 : 7 7 . ‘I’lie above sample of sterilized milk gives for the same ratio 100 : $9. It is important to note tliRt :t like cliange is produced i i i the coiiilensatioii of milk for canning. Though the heating is carried on i n va(:iio, yet (it is sufficient to mise the percentage of the albuminoids precipitable by diiute x i t l i n t h e cold very notably. If the heating is continued long enougli :rnd is sufficiently raised to render the canned milk entirely sterile, the cJffect abovc noted is very evident. Thus in it sample of ‘‘ sterilized condensed Swiss milk,” the total nlbnminoids were 10.>!)!)$and the casein 9.Sl$ giving the ratio of 100 : 93.
Plate cultures of this condensed milk showed i t to be entirely sterile, n o bacteria colonies appearing when the plates were kept many davs. And it is not surprising t h a t this condeiised milk is rendered absolutely sterile, when another evidence of prolonged heating is considered, which is the brownish color. Ihboratory experimeiits upon the sterilization of milk in a steam bath showed that 110 cliange of color took place a t t h e end of one hour, a n hour and a half or two hours. At the eiid of three hours the change was perceptible and in six hours the sterilized milk had become strongly brownish-yellow resembling an infusion of coffee to which a lnrge amount of milk has been added. Polariscopic determina-
STERILIZATION OP MILK, ETC.
37
tions were made of the amount of milk-sugar present after each heating, the raw milk containing 4.18 per cent. No change could certainly be estimated until t h e end of the six hours period when the percentage had fallen to 3.94, and i t then steadily diminished until a t the end of 48 hours the milk-sugar had entirely disappeared. There was a corresponding change of taste and color, the former acquiring more and more of that peculiar sweet and bitter flavor percepitable in boiled milk, and which I am inclined to attribute to t h e change of the milk-sugar, and the color a t the same time becoming of a dark brownish-yellow, T h e decomposition product of t h e milk-sugar is the chief substance concerned in the development of the brown color, but riot entirely since the m i x t u x of casein and albumen septcrated from such heated milk arid treated with ether to remove 811 the fat, has on drying, a strong yellow color. T h e casein obtained in a similar manner from raw milk dried perfectly white, and the albumen is white also. T h e fact t h a t the albumen by heating becomes precipitable by dilute acid is one evidence of its change of character, and t h a t on drying this changed albumen beomes yellow is another evidence. The decomposition of milk-sugar is attended with the development of an acid, probably glucic acid. Filtered off from the insoluble snbstauces left after heating for 48 houps, the solution which no longer contained any milk-sugar, had a brown color and acidity thus developed from 100 cubic centimetres of milk was equivalent t o 13.S cubic centimetres of dccinormal soda. It may justly be objected that such great changes as the entire destruction of the milk-sugar are never met with in the condensed sterilizcd milk of commerce, or in commercially 01- privately sterilized milk, T h e objection while true, does n6t apply to the object wliich 1 had in view which was to carry the cxperiments fur enough to find out what was the natureof the substances formed. T h a t the process of heating to prepare the sterilized condensed milks of commerce is not carried far enough to perceptibly lower their percentage of milk sugar may been seen from the following analyses of an American and B Swiss preparation. The American
had a slight tint, t h e forcigri was distinctly brownish. T h e condensation i i i both cases Iiitd been carried to such npoint thattlie crystals of milk-s!igitr liad beg1111 to separate, tile admixture of water bringing out :Lclistinctly granular appearitrice due, :is t h e microscolw also showed, to minute crystals of milk-sugar. I give t h e composition of t h e American variety when diluted with 23 times its voiunic ot water : wlieii dilut,ed to such a point t h a t its total solids amounted to 13.4:; (tliis being a b o u t t h e average total solids in 0111’ ordinary ~ r h o l ecommercid milk); a n d t h e composition of t h e original condensed milk. ‘I’lie Swiss variety ,is similarly tabulated except tlint it was diluted for purposes of analysis with only twicf its volume of water. Tile :dvertisements accompanying t h e samples stated t h a t when diliitell :ts :tl)ovc. e:icli would liiivc t h e strength of rich t:ible milk. B u t siicli is not t h e case, and :L commercial milk if prelmred from them by diluting to point reconimc i i d t ’ d , mould be condenined ly an inspector as adulterated milk. d JI 1c 111 c .\ N ( ‘0?j I ) 2+ times diluted. Xormal. Original. Jlilk-sugar- - - - - - - .3.295 4.s42 12.37 Fats. - - - .- - - - - - .- -2.625 3.Y.jD 9.1s Totitl hlbnminoids 2.64; 3 . SI)O 8..! G (C:tseiiie. - - - - - . - --1.‘JO(j 2 , SO:! 1 As11 -.- - - .- -
- - ..- -0.546
0.80“
Total Solids 9.113
13.394
h\\ I \ J “ CONDENSED,
STERILIZED CHEAM llIT,K.”
2 time, diluted. Milk-sugar- . - - - - - - -4.52’; Total Albuminoids -3.506 (Cnseiiie- - - - - .- - .- - 3 . 2 7 E”1ts - - - - _ _ - - - .- - - -3.714 A Z s l ~...- - - -.- - - - - - -0.688
‘rotdl SrJlitls
1?.43>
Normal. 4.866 3.79s 3.515 3.992 0.73s 13.394
-_
1.!11
3l.X
Original. 13.581 10.599
!).QL) 11.142 2.064 37.3YG
STERILIZATIOK OF MILK, ETC.
39
It is surprising that the composition of two milks, as may be seen from an inspection of t h e column marked c c Normal,” should be so strikingly similar when we consider t h a t the cattlefed 011 the Swim alps, and in the Western State, from which this American sample came, were of different breed, and t h a t their feeding, care, etc., were also widely diverse. By carrying the condensation to a farther point and by more prolonged heating, the Swiss variety was made of a slightly brown color. But both were sterile and when, thecans having been opened, diluted milk was allowed to staiid in flasks loosely stoppered with cottoq wool, the contents of both cans and flasks soured and spoiled very slowly. Condensed milk properly prepared is, in fact, sterilized milk in a concentrated, convenient and portable form. It is i m p w t a n t to compare i t with sterilized milk, prepared and sold in sterilized flasks, but without condensation. One of these preparations is stated t o have been prepared from the milk of registered Jersey cattle, aild to be especially adapted for iise by infants, children, invalids and travelers. T h e advertisements state that no :dterations have been in the composition of the milk, and that the only manipulation which i t has undergone is that of immediate transfer to glass bottles snitably stoppered, followed by sterilization with the heat of a water bath. The temperature of the bath is not stated, nor the length of time in which the bottles are immersed, but the heat is said to be adequate to destroy all the germs of I‘ertnentation, and t h a t samples opened after six months have shown no change. The :dvant:t,aeq claimed for this sterilized milk are its preparation from the fresh milk before any opportunity of absorption of any considerable number of microbes ; t h a t in t h e brief interval between the milking and the sterilizing there is no time for the development of spores ; t h a t it easily made such a close copy of nature’s nourishment t h a t only really good breast milk is its equal ; t h a t i t does not curd in solid masses, b u t like breast milk in small soft flakes. I n addition, the statement is made t h a t the Jersey milk is richer in the elements needed for bone and tissue building t h a n t h e milk of any other breed ; t h a t it yields an adequate and gunr-
40
hTEKII,IZ.\TIOS
O F X I L K , ETC.
anteed proportion of cream, averaging 2 5 $, m d t h a t when diluted with a n equal volume ( ~ fboiled water the ‘(cheesy element” is reduced to a proper proportion. Samples of this commercial sterilized milk obtained in midsummer exhibited it separation of tlie fats in masses of considerable size. I3y moderate warming aiid s1i:ikirig this fat could be partially diffused through tlie milk, b u t not i n such a manner as to bring the sterilized milk back to the ordinary appearance of r k h milk, o r milk on whicli the cream has risen on stsnding and then has bceii sliaken u p again with the milk. Samplcs which wore obtained later i i i tlie year, Deccmber, did not present tliis a,ppearance, b u t resembled rich milk: and the sep:iration of the fat in t h e former case was probablx due to \lie samples having been kept in my laboratory for seyeral weeks during very hot weather. ‘ I h reaction was neutral. Cultures were made witli gelatine peptone in the ordinary manlier, but n o growths were obtained, sliowiiig t h a t sufficiciit lieat and time liad btwi giren to ~ i i c l e rtjlie milk entirely sterile. The composiisioii of one of tlie midsummer samples mas as foll0TVS : Total Solids determined directly . - - - - - - - - .- - 14.05;; W;itei. - .- - ..- . - .. - .- .- - ..- - ...- . - - - - - - S5.!)5
Sum of constituents so determined
- - _,....-14.166
...- .
A midwinter sample (Dec. 12th) exhibited the following composition : Total Solids determined directly- - _-.- _ _ _ _ - -14. ;0$ Water.. - - - - - ...- - - ...- - - - - - - ...- - .- - . - - - - - 85.30
41
STERILIZAiTION OF JXILK, ETC.
14.69
Both of these samples were sterile. The most striking eature is their high percentage of solids, and especially of fat, as might be anticipated from t h e statement as to the source of the milk. Milk containing a large percentage of flit is valuable for its buttermaking qualities, b u t its advantages as a nutrient are open to question. T h u s Uffelman (Archiv. fw die gesnnziitte Physiologie, Pol. 29, p. 330), stateu, as the result of his experiments, that the miik-sugar is the constituent most completely used u p in the process of digestion, the albumenoids following next, then the fat and finally the salts. As to t h e apparatus used in sterilization I have employed t h e ordinary steam bath, which is a part of the usual Koch biological outfit, but the Arnold steamer made and sold for milk-sterilization expressly, is more convenient and Satisfactory. T h e flasks were closed by the rubber stoppers recommended by Soxhlet, with B glass rod passing down through a hole in the center. But in the infant ward of tlie Philadelphia Hospital, tlie sterilized milk is heated in flasks loosely plugged with cotton wool and placed in a bath of boiliug water, and this arrangement is entirely satisfactory when the milk is to be used 011 the spot. As to t h e number and character of thc bacteria present in commercial milk, it is not necessary to my present purpose to speakat great length. Five samples purchased for me a t the most untidy shops, in the poorest quarters of Jersey City, yielded after 36 liours culture 33G00, 44000, 24720, 5832, and 154 colonies. T h e smallest numbeis were obtained i n t h e last two samples which were thd richer milks laving a yellowish-white color, while t h e other three were poor whitc milk. Very interesting in this connection are the results obtained by t h e examination of t h e milk obtained from cows fed on brewers’
48
S T E R I L I Z I T I O S O F MILK. E".
grains, and a mixture of grains and swill. An analysis of t h e brewers' grains showed that they were rich in albuminoids, and in 80 far contained a large amount of material available for nutrition. Two samples were analyzed : I. 11. vr * Water- - - - - _ _ - - - _ _ - -74.81 per cent. I J . ( 5 per cent. Ash _ _ _ _ _ _ _ _ 1.25 " 0.851 " " Albuminoids _ _ _ _ _ _ _ _ 6.26 '' 5.807 " " Or, calculated on t h e grains after drying : 3.51 per cent. '(
23.95
"
"
T h e first were fed to cattle affording t h e milk analysed under column I n ; the second, together with some swill, affording milk I1 b. Eleven cows were fed in t h e first case in a small stable, very dirty, without bedding. T h e udders were very dirty, b u t apparently without sores. T h e cows had beeii driven into the sialls a n d secured by causing their heads to pass through it vertical slit, allowing of a n u p and down motion, and were to be kept their until their udders should dry up. Their food appeared to be passing through them with a constant fluid diarrhct.al discharge. T h e milk had a n alkaline reaction, and :I peculiar taste. I n the second case the milk had a n exceedingly disagreeable, repulsive taste, a alight butyric odor and a strongly alkaline reaction. I n fact, the taste gave rise to ike impression t h a t soda had been surreptitiously introduced into t h e milk, without attracting t h e notice of the inspector who obtained these samples for me. T h e composition was as follows : Ill. I1 6. Fats_ _ _ _ _ . - - _ _ _ _ ._ _3.92 . 23 9 Total Albuminoids _ _ 4.29 4.26 (Csseine _ _ _ _ _ _ _ _ _ _ _ _ 2 . 7 4 2.32) [Albiimen - - - - - - - - 1 . 5 5 1.94) 3.04 4.18 Milk-sugar- - - - - - - Ash _ _ _ _ _ _ _ 0.748 0.839
__
____.___ --
Total Solids. - - - . . . . 13.138
--
11.029
STERILIZATION OF MILK, ETC.
43
T h e ratio of the albuminous to the caseinous matters is sbnormally high, as if due to the feeding with so large an amount of soluble nitrogenous matter. I n tlie second sample the percentage of fats are abnormally low. T h e first milk after 60 hours culture yielded 874 colonies per cubic centimetre ; the latter 6500 colonies. After being heated for 15 minutes in the sterilizer, counting from the time when its inside temperature had reached 100" C., the latter milk had become sterile, yielding no colonies on culture. As to the length of time to which milk should in general be kept at the temperature of boiling water in order to sterilize it, I ani inclined to believe t h a t the period of one hour usually recommended, is excessive. The experiments previously detailed show that as soon 11s the temperature of milk reticlied t h e boiling point, most of t h e '' albumen " is so changed t h a t it becomes precipitable in the cold by dilute acid, and even heating less than half-hour to looo C., Is sufficient to change tlic whole of it. Theremaining albuminoids in milk are similar in their nature to peptones and a r e not precipitate by dilute acid or by boiling. But the bacteria Itre made up largely of alburninoids similar to the first two classes of bodies referred to and not to the last, and tlie probability of their escaping devitaliztition nfter half an hour's boiling is primarily not great. I n fact cxpcrimen ts have shown that in the great majority of instances the sterilization is complete in a half hour or less time, and for this reason the prolongation of heating to a n hour, with its attendant inconvenienres, is in general an unnecessary precau tion. My thanks arc due to Mr. Geo. IV. McGuire, the State Dairy 0ommissiol;er of Kew Jersey, and to my assistant, Mi-. W m . G. Johnston, for their aid in the performance of the experiments detailed in this rtrticle.
