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f BULLETIN: Not 210.

Contribution from the Bureau of Animal! Industry A. D. MELVIN, Chief

Washington, D. C. PROFESSIONAL PAPER July 13, 1915.

PASTEURIZING MILK IN BOTTLES AND BOTTLING HOT MILK PASTEURIZED IN BULK.

By S. Henry AYERS, Bacteriologist, and W. T. JOHNSON, Jr., Scientific Assistant, Dairy Division.

CONTENTS. Page. Page. NTE ROUIICELOM ies ee ae ue Ses Le 1 | Prevention of bottle infection by Method of bacteriological analysis __ 3 bottling hot milk and by pasteur- Method of pasteurizing in bottles___ 3 IZATION IAW DOEELES =e a oe wea 14 Bacterial reductions by pasteuriza- Cooling milk which has been bottled ELOM IMA OE] CS ea nee ie 3 MCG oe RI os FPO les NaS aN 16 Advantages and disadvantages of The cream line and flavor of pas- pasteurization in bottles_________ 7 teurized milk cooled by various Machinery for pasteurizing milk in TLE TINO CLS eee ae OS tee ae 23 OTe Sees ae 7 | Bottles to be used in the process of Method of pasteurizing in bulk and . bottling hot pasteurized milk_____ 24 Houtlineswhileshoh==_s2 ieee ii | Process of bottling hot pasteurized Comparison of bacterial reductions in milk under commercial conditions_ 24 milk pasteurized in bottles and DURST Ta eh Teas aan a as ea al ec ter ane 25 milk pasteurized in bulk and bot- Citation=topliteratunes se" Swe 27 TEIUEG UA OUTS) 3) BVO) reseice ens ein me lane 12 INTRODUCTION.

_ The process of heating milk in bottles is by no means a new one, for it probably dates back to the work of Soxhlet (1),! from 1886 to 1891. In general, however, the object has been partially or completely to sterilize the milk by the use of high temperatures rather than simply to pasteurize it at low temperatures. While the practice of steriliz- ing or partially sterilizing milk in bottles has been extensively prac- ticed in several countries in Europe, the pasteurization of milk in

bottles has not been so common. 2 1 The figures in parenthesis refer to the list of citations to literature at the end of the paper. Notr.—This paper is of interest to milk dealers, health officials, and all who have to do with the milk supply of cities; it is suitable for distribution in all parts of the country. 94289°—15

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2 BULLETIN 240, U. S. DEPARTMENT OF AGRICULTURE.

It is evident from the report of Gerber and Wieske (2) that pas- teurization in bottles has been practiced in certain localities for a considerable period of time. According to these authors, pasteuri- zation in bottles by the process of Gerber, which consists of heating milk in bottles for one hour at 65° C. (149° F.), during which they are agitated, had been practiced in certain dairies for 15 years pre- vious to 1903. Met:

In this country milk has been pasteurized ee in bottles at various Strauss infant milk stations for several years, but this proc- ess has not been used on an extensive commercial scale until within the last two years. During the summer of 1910 an investigation was started of the bacteria which survived pasteurization in flasks and of the efficiency of the process. A report of this work has been pub- lished in Bulletin 161 of the Bureau of Animal Industry (8).

While this work was in progress North (4) suggested the pasteuri- zation of milk in bottles on a commercial scale by the use of machines similar to those which have been in use in breweries for several years.

The process of pasteurizing in: bottles consists in bottling the milk in specially constructed bottles of sufficient size to allow a space in the top of the bottle to take care of the expansion of the milk during heating. The bottles are capped with special water-tight caps and submerged in hct water. After the milk in the bottles has reached the pasteurizing temperature, the temperature is maintained for 30 minutes; the hot water is then replaced by cold and the milk cooled. In. sein it takes about 30 minutes to heat the bottles, 30 minutes for the holding period, and 30 minutes to cool. Miulk is also pas- teurized in the bottle -by heating and cooling with water which is sprayed over the bottles. By this method of spraying, ordinary caps with a protective covering can be used; this will be described in another place in this ballon

This precess of pasteurizing in bottles is now used on a commer- cial scale in a number of milk plants throughout this country.

Numerous advantages of this method of pasteurization over the . ordinary methods have been claimed particularly in relation to the far superior bacterial reductions obtained. The most obvious point of advantage of this process is the prevention of reinfection after pasteurizing, but it seems as though a modification of the present system of “ holder” pasteurization by bottling the pasteurized milk while hot, as suggested previously by the senior writer (5), would help to solve the problem of reinfection.

Accordingly, the general object of the work hereinafter described has been to compare on a laboratory scale pasteurization in bottles with the process cf bottling hot pasteurized milk. The special ob- jects have been to determine the bacterial reductions in each process, to study any special points which must be considered in the opera-

PASTEURIZING MILK. 3

tion of each process, and to present preliminary data on the cooling of milk in bottles by an air blast.

METHOD OF BACTERIOLOGICAL ANALYSIS.

Since bacterial counts are widely influenced by differences in media and incubation it is always essential in discussing the results of bac- teriological work to explain exactly how the counts were obtained. In this work plain inftision agar, made according to the recom- mendations of the committee cn milk analysis (6), was used. The plates were incubated for five days at 80° C. (86° F.) and counted.

METHOD OF PASTEURIZING IN BOTTLES.

Milk was placed in special bottles, similar to those supplied to the trade, and capped by machine with patented metal caps. The bottles were heated by being submerged in hot water at a temperature of from 145° to 147° F. After the temperature in the bottom of the bottles had reached 145° F. they were held at that temperature for -380 minutes and removed, plates being made while the milk was hot. The bottles were so constructed that after a full quart of milk was poured in there remained an air space of sufficient size to allow for the expansion during the heating. While heating it was noticed that the milk expanded and pressure enough was generated to lift the caps slightly so as to allow air to escape. Special care was taken to see that the temperature in the bottom of the bottle of milk was maintained for the full 30 minutes.

The method of pasteurization was the same as is used on a com- “mercial scale; hence, the results obtained are directly applicable to commercial conditions. The fact that the bacterial counts were taken directly after heating has no effect on the results, since it has been shown that cooling plays no part in the destruction of bacteria in the pasteurizing process (3).

BACTERIAL REDUCTIONS BY PASTEURIZATION IN BOTTLES.

It has been claimed that remarkable bacterial reductions have been obtained by pasteurization in bottles which were far superior to those obtained by other methods even when the same temperature and holding period were used. In order to determine what reductions could be obtained, 34 samples of milk were pasteurized in bottles.

The results are seen in Table 1. The bottles for samples Nos. 2 to 23, inclusive, were washed clean in hot water, but not steamed, before they were filled with raw milk. The bottles for the other samples were steamed two minutes and then cooled before they were filled with raw milk.

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BULLETIN 240, U. S. DEPARTMENT OF AGRICULTURE.

TABLE 1.—Buaecterial reductions during the process of pasteurization in bottles.

Pei Nia ee Nn 58, 000 1, 630 SUS 20 ee aes sir 80, 000 2,010 97. 48 Oe NS Gan 63, 000 1,070 S80 MED Ses epaceae 160, 000 29,500 81.56 Aint regs soe see 5, 100, 000 11, 800 SR ETMD NN 777 Gee ree cauiele 151, 000 12,500 91.72 iid ae ese 580, 000 8, 000 OSG 2a 23 ee eee 81, 000 9, 800 87. 90 Oper ee 5, 900, 000 15, 600 QO UTA 2a eee tees oe 24,900 570 97.71 (Ee tye ees 99, 000 980 QOSON Tl p2oe ee ees ae 94, 000 2, 200 97. 66 Sa Empires aa 7, 400, 000 7,100 SO A ee meade nc 305, 000 55, 800 81.70 OF ose 191, 000 7,600 QOS 022i eee eo eee 235, 000 7,600 96. 76 I Se a eee 14, 100, 000 14, 200 OOF SO S28 sae ae 176, 000 11, 400 93. 52 1 ee a 24,700 5,780 COS OOH G20) ees eras 97, 000 8,350 91.39 De Beri icr titty 75, 000 28, 000 G2H6O RI SOMeee meee 230, 000 5,500 97. 61 GS Bes ee earapee 126, 000 1,720 OSOSallte Peers eee 124, 000 1,500 98.79 AT Geran ee eee 4,100, 000 2,410 O94) Hagia es ae 450, 000 11, 400 97. 46 LORET eee eee 76, 000 3,550 VRS pears aseaseencel 3, 950, 000 3,520 99. 91 Oe Soe easenaee 8, 100, 000 1, 660 SERS bGY eee deacscnce| 985, 000 | 18, 400 98. 13 It ge 18, 900 710 96224 35 ose c eee} 190, 000 9,300 95. 10 US aa eases 24, 000 10, 900 50. 41 | LOR ue sisi ele 28, 300 23,300 17. 67 Average.| 1,570,493 9, 863 90. 86

As may be seen from the table, the bacterial reductions were high as a rule, but there were exceptions. The average total count of the samples of raw milk was 1,570,493 and after pasteurization 9,863 bac- teria per cubic centimeter. It is interesting to note that the percent- age reductions averaged 90.86 per cent and ranged from 17.67 per cent to 99.98 per cent. When the latter reduction was obtained the ‘raw milk contained 8,100,000 bacteria per cubic centimeter; when the minimum reduction was obtained the raw milk contained 28,300 bac- teria per cubic centimeter. These results further substantiate the con- clusion expressed in Bulletin 161, page 58 (3) ,that percentage bacterial reduction has no special meaning, since it is influenced by the number and kinds of bacteria in the milk when pasteurized. Considering the results as a whole, it is evident that low counts may be obtained by pasteurization in bottles. :

While carrying on these experiments the following points were noted which are worthy of attention:

TEMPERATURE OF THE MILK DURING HEATING.

In the process of pasteurization it was found that the temperature of the milk in different parts of the bottle was quite different during the time the milk was being heated. Several experiments were made, heating water in sealed bottles to determine the differences in the top, middle, and bottom of the bottles. Three thermometers were inserted through a rubber stopper into a bottle so that the stems were at the top, middle, and bottom of the bottle, respectively. The bottles were then submerged in hot water at a temperature of from 145° to 146°

F. and the temperatures of the water in the bottles were recorded.

‘PASTEURIZING MILK. 5

Four pint bottles and four quart bottles were used. The averaged temperatures in the pint bottles are shown in figure 1. It will be seen from the curves that in a pint bottle with water at 50° F. submerged in hot water at about 145° F. it took 104 minutes longer for the tem- perature in the bottom of the bottle to reach 140° F. after the top had reached that temperature and 42 minutes longer for the temperature in the middle of the bottle. When the temperature in the top of the bottle was 140° F., in the bottom it was only 118° F.

The averaged temperatures of four quart bottles are shown in figure 2. When the temperature in the top of the bottle was 140° F., that in the bottom was only 127° F., and it took 91 minutes longer for the temperature in the bottom to reach 140° F.

Olena A OES lO 12| -I4 16 [x8 20 22 | 24 201 20 NsO) UG 2nes Aes OMmms oun AO 12.4 17.6 2B)n2

Heating period in minutes.

Fie. 1.—Variations in temperature in different parts of pint bottles of water during the process of pasteurization in the bottle.

It is evident that when pasteurizing in the bottle care must be taken to record the temperature in the bottom of a bottle and to date the holding period of 30 minutes from the time the bottom temperature has reached 145° F. In recording the temperature an accurate ther- mometer should be used, and it should reach to within one-half inch of the bottom of the bottle.

COOLING THE MILK AFTER PASTEURIZING.

After the milk is heated in bottles on a commercial scale it is cooled by replacing the hot water with cold and gradually changing the temperatures so as not to break the bottles. Upon cooling, the hot milk contracts and a partial vacuum is formed in the bottle when the caps are tight. It is recommended by the manufacturers of some of the patent caps that after heating the bottles be allowed to

6 BULLETIN 240, U. S. DEPARTMENT OF AGRICULTURE.

cool for a few minutes in air until the cap becomes concave, as this is said to hold the cap on tight and helps to make it water-tight. Obviously, it is of utmost importance that the caps be water-tight, since they are submerged in water during cooling, and if not tight the milk may become infected by polluted cooling water.

When bottles are submerged the ordinary cardboard cap is of no value for pasteurization in the bottle, since water will easily pene- trate during cooling. This makes it necessary to use some form of. patented cap, of which both specially treated cardboard and metal caps are on the market. It 1s almost needless to state that if the edge of the bottle is chipped or otherwise imperfect almost any seal cap will not be water-tight during the cooling. Imperfect bottles must not be used. It is claimed by the manufacturers of patented seal

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Heating period in minutes. Fie. 2.—Variations in temperature in different parts of quart bottles of water during the process of pasteurization in the bottle.

caps that they are tight on perfect bottles. It would be advisabie, however, for the dairyman to test the tightness of his caps by the following method: Fill the milk bottle with a 0.05 per cent solution of barium chlorid (BaCl,). The barium-chlorid solution should be made up with distilled water, since the sulfates present in ordinary water will cloud the solution. Cap the bottles in the usual way with a seal cap and heat to 145° F., submerge, and cool in a 10 per cent solution of magnesium sulfate (MgSo,).

If any of the magnesium sulfate leaks into the bottle during cool- ing the barium-chlorid solution will become cloudy, owing to the formation of barium sulfate, which is insoluble. This test is very delicate and will show even a slight leak. Both these chemicals may be obtained at any drug store. Since barium chlorid is poisonous, after testing bottles in which it has been used care must be taken to

ia

PASTEURIZING MILK. ul

wash the bottles thoroughly in order to remove the barium solution. Care must also be exercised to keep the chlorid solution from all edible products about the plant.

ADVANTAGES AND DISADVANTAGES OF PASTEURIZATION IN é BOTTLES.

From a bacteriological standpoint the advantage of pasteuriza- tion in bottles lies in the fact that reinfection after pasteurization 1s usually prevented. In the ordinary methods of pasteurization there is a great opportunity for infection from coolers and in bottling. Of course the proper handling in the ordinary method of pasteurization reduces and may prevent subsequent reinfection, but the possibility ~ still remains.

Tt is the general opinion that the process of pasteurization in bot- tles also effects a great saving in milk by doing away with the loss in evaporation over the coolers and with the loss in milk which adheres to the apparatus in the process of pasteurization. Undoubt- edly this saving is quite a considerable factor. There may also be a saving in the expense of machinery and in the interest on the capital invested, but it is not the province of this paper to discuss the finan- cial aspect of this process.

On the other hand, in a plant where pasteurization is now per- formed in the ordinary way, it would be necessary to install an entirely new equipment for this system of pasteurization in the bottle. When bottles are heated and cooled by submerging in water perhaps the greatest disadvantage is the cost of water-tight caps. This item of expense is important, since it may increase the cost-of pasteurization as much as one-fifth of a cent per bottle. Whether the saving in milk losses is sufficient to overcome this added expense can be determined only by the actual operation of a milk plant. In some processes of pasteurization in the bottle ordinary caps can be used, as the bottles of milk are heated and cooled by a spray of water, and the tops of the bottles are protected by metal coverings.

MACHINERY FOR PASTEURIZING MILK IN BOTTLES.

Pasteurization in the bottle has been practiced on a commercial scale in many different ways since water-tight caps made it possible to heat milk in bottles by submerging in water. When this process of pasteurization was first practiced the bottles, with water-tight caps, were placed in tanks and heated, held, and cooled by changing the water. This method, while satisfactory on a small scale, was hardly practical in large plants. Several types of machines have been invented, which make the process continuous. One of these

8 BULLETIN 240, U. S. DEPARTMENT OF AGRICULTURE.

5

machines is shown in figure 3. The machine consists of a large tank divided into two compartments and two smaller tanks. These con- tain water at different temperatures. Bottle-holding frames are car- ried through these compartments on an endless chain in the manner shown in the drawing. The raw milk is bottled and capped with water-tight caps, then placed on the bottle-holding frames of the machine on the loading end. The bottles of milk are then carried through the preheating compartment into the pasteurizing compart- ment where they remain for about 30 minutes. From the pasteuriz- ing tank the bottles are carried to the cooling tank, then to the refrigerating tank, after which they are removed from the machine. The process is continuous, the bottles of milk being loaded at one

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Fic. 3.—Machine for continuous pasteurization of milk in bottles. The bottles have water-tight caps and are conveyed on an endless chain through water compartments of various temperatures.

end, heated, held, and cooled, then unloaded at the other end of the machine. The temperature of the water in this machine is auto- matically controlled.

There are other machines on the market which differ in the man- ner in which the bottles are carried through the tanks of water, but the principle is about the same.

In other types of pasteurizers the bottles are not submerged in water and consequently water-tight caps are not necessary. The bottles of milk are heated and cooled by sprays of water and ordinary caps are used and protected from water by a metal covering. One of this type of in-the-bottle pasteurizers is shown in figure 4. The

PASTEURIZING MILK. 9

crates of raw milk are placed on an endless traveling conveyor which passes through the machine and returns under it. The bottom of the machine is divided into several compartments and each compart- ment is filled with water for supplying the machine when in oper- ation. The top of the machine is a flooding pan divided into com- partments corresponding to those in the bottom of the machine. Pumps draw the water from the lower compartments and force it into the corresponding top sections, from which it returns in the form of a shower through perforated bottoms. The process is repeated with the same water. As the crates of milk pass through the machine they pass through showers of water at different temperatures and are heated to the pasteurizing temperature, then held and finally

Wie. 4.—Another type of continuous machine in which the bottles of milk have ordinary caps and are passed through showers of water at various temperatures.

cooled. ‘The tops of the bottles are protected from water by metal caps arranged as shown in figure 5. This frame of metal caps covers the top of each bottle in the crate.

The pasteurizing section of the machine is located in the center with the preheating and cooling section at each end. The preheating and cooling sections are connected by channels, because the cool milk entering the machine has a tendency to cool the water and the hot milk emerging from the pasteurizing section has a tendency to heat

94289°—15 2

10 BULLETIN 240, U. S. DEPARTMENT OF AGRICULTURE.

it. The temperature of the water in the pasteurizing section is auto-

matically maintained. | In figure 6 is shown another type of in-the-bottle pasteurizer which

is so arranged that bottles of milk may be heated with ordinary

Fig. 5.—Metal caps in frame for protection of bottles as operated in machines shown in figures 4 and 6. eee

caps. The pasteurizer is made of sheet inetal and contains racks which hold crates of bottles. The tops of the bottles are covered with metal caps of the type shown in figure 5. The crates of raw milk covered with metal caps are placed on the racks in the pasteur-

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Fic. 6.—A pasteurizing machine in which paper-capped bottles are protected by metal caps, and heating and cooling are done, respectively, by circulation of hot and cold water.

izer and heated by means of hot water which is forced against the

botties. The water is circulated by means of a pump and is used

continuously. After the milk has reached 145° F. it is held for 50

minutes and then cooled. Cooling is accomplished by replacing the

PASTEURIZING MILK. 11

hot water by cold, while for low temperatures a special set of cooling pipes is supplied. The temperature of the heating water can be automatically controlled.

METHOD OF PASTEURIZING MILK IN BULK AND BOTTLING WHILE HOT.

For the pasteurization of milk in bulk a double-walled cylindrical tin tank with a capacity of about 3} gallons was used. The con-

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Fic. 7.—Apparatus for pasteurizing milk and bottling while hot. struction of this tank is shown in figure 7. Raw milk was placed im the milk tank, where it was heated by hot water in the outer | jacket. The surrounding water was heated by a steam jet and con- stantly agitated by blowing in a small amount of air. During the heating the milk was agitated by a paddle supported by the cover of the milk tank. The water in the jacket was kept at a temperature of about 146° F. The milk was held at a temperature of 145° F.

12 BULLETIN 240, U. S. DEPARTMENT OF AGRICULTURE.

for 30 minutes and then drawn off while hot through the outlet pipe into hot milk bottles which had been steamed 2 minutes. As stated before, this method of bottling milk while hot was suggested in Cir- cular 184 of the Bureau of Animal Industry (5), but the suggestion then was to bottle hot milk in cold bottles. In this work it seemed advisable to bottle directly into hot bottles, as it makes it possible to steam the bottles and fill them before infection can take place. Also, this method eliminates the possibility of breaking bottles. While working on this process of bottling milk hot it has been found that a similar process was apparently patented several years ago, but, so far as known, it has never been used to any extent. This process as described by de Schweinitz (7) consisted in pasteurizing the milk at temperatures from 160° to 180° F. and piacimmg it while hot in a sterilized milk jar or fruit jar with a flap top. Special paper caps were used. The jars of milk were cooled by being placed in troughs of iced water. |

COMPARISON OF BACTERIAL REDUCTIONS IN MILK PASTEURIZED IN BOTTLES AND-MILK PASTEURIZED IN BULK AND BOTTLED WHILE HOT.

Since it has been shown earlier in this bulletin that excellent bac- terial reductions may be obtained by pasteurization in bottles, a question of great importance arises as to whether or not as good results can be obtained by pasteurizing milk in bulk and bottling while hot. :

A series of 22 samples of raw milk was pasteurized by both proc- esses at 145° F. for 30 minutes. Part of the milk was pasteurized in bulk in the pasteurizer shown in fig. 7 and bottled hot in hot bottles which had been previously steamed for two minutes. In all these experiments the bottles were capped with ordinary paper caps, no precautions being used in capping by hand. Another portion of the same raw milk was pasteurized in bottles. Both samples of pasteur- ized milk were examined bacteriologically while hot in the bottles.

in the first series the bottles in which: the milk was pasteurized directly were washed with hot water and washing powder imme- diately before they were filled with raw milk.

. PASTEURIZING MILK. 13

TABLE 2.—Oomparison of bacterial reductions in milk pasteurized in unsteamed bottles and in pasteurized milk bottled while hot in steamed bottles.

Milk pasteurized at 145° F. for 30 minutes. F Hot pasteurized milk | Milk pasteurized in S le N ere in het steamed. bot- washed but un- EAETIONS) NOt ‘ ae oe tles. steamed bottles.1 Bacteria | Percentage|} Bacteria | Percentage pere.c. | reduction. | pere.c. | reduction. Dye Bsn Bs et ee SAN eo aah PG aa 58, 000 1, 160 98. 00 1, 630 97.18 OTe ee eae ee ips A BROADEN bo ND al a 63, 000 220 99.65 1,070 98. 30 GTS aii LD 6 Wh cara are a el aE a A AR PEE 5, 100, 000 8, 400 99. 83 11, 800 99. 76 Cis ii ce) AN i FER Uh hg TR trac 580, 000 8, 300 98. 57 8, 000 98. 62 CTs Os eA i NAL ei We TED on TORIES et po Ae 5, 900, 000 6, 000 99. 90 15, 600 99. 74 CLS Ee er a Re RE DI RI Rh aa 99, 000 610 99. 38 980 99. 01 So) is tS Septet rcs tee a er UU ee Lee 7,400, 000 6,300 99. 91 7,100 99. 90 OF see a estan Ores UR ge ok wee NENT gee CO 191, 000 2, 000 98. 95 7,600 96. 02 TN) Nests BUS One eee os NTN eieeer Ae aan bee 14, 100, 000 7, 000 99. 95 14, 200 99. 89 TSE OR, BAR ee a et eS i ae Pan a 24, 700 4,550 81.58 5,780 75.59 TORS GREG Aer Sc gai a yet COC a or ae ea ae eke 75, 000 3, 000 96. 00 28, 000 62. 66 TSE cc Cee ea A elie Er siete i Gs A Mla cag UN 126, 000 1,440 98. 86 1,720 98. 63 SARS es Ce IRI see Se MLC LU ie 2 4,100, 000 2,470 99. 94 2,410 99. 94 Aaa ay eee Cease My Wop OS ccs SOL ie Nay Jk AS 76, 000 1,400 98. 16 3,550 95. 32 HIG penis Srna Mi eget sn bg Saye Bene Sie aN eas 8, 100, 000 1, 620 99.98 1, 660 99. 98 Weft oie BME EOE a Ne NS eR alGe 18, 900 760 95. 97 710 96. 24 TS Ge SS OS ess ot eS A Ce 24, 000 800 96. 66 10, 900 50. 41 To SSS cys ape i Ns ee a 28,300 7,050 75. 09 23, 300 17. 67 PAO) oh tates aS BES 6 RRR Us pT elec ta 80, 000 1, 360 98. 30 2,010 97. 48 ARI etre re PN ee UNG Sosy Staal et eae 160, 000 1,830 98. 86 29,500 81. 56 DNs Posts 6 eee Se Aaa pte el nea Nn Tu ee 151, 000 3, 200 97.88 12,500 91.72 DS SEN Pernt gente Srnec eee RA 81, 000 6, 800 91. 60 9, 800 87. 90 INST OTIAS Ch pee ae yes eae ye en 2,115, 268 3, 467 96. 50 9, 083 88. 34

1 Bottles were washed clean in hot water, but not steamed, before they were filled with raw milk.

The results of the bacteriological examinations are shown in Table 2. It will be seen that the average count of the raw milk was 2,115,- 268 bacteria per cubic centimeter. After being pasteurized in bulk and bottled hot in hot steamed bottles the average count was 3,467 bacteria per cubic centimeter, while the average count when pas- teurized in bottles was 9,083 bacteria per cubic centimeter. Compar- ing the percentage of bacterial reductions, it will be noted that the average reduction of the milk bottled hot was 96.50 per cent and only 88.34 per cent in the milk pasteurized in bottles. In 19 of the 22 samples the bacterial count was lower in milk pasteurized in bulk and bottled hot. In many cases the count was much lower, as may be seen by comparing samples 4, 6, 7, 12, and 18. This difference is par- ticularly striking in sample 21, in which milk pasteurized in bulk and bottled hot showed a count of 1,830, and some of the same milk pasteurized in a bottle for the same time aa at the same temperature contained 29,500 bacteria per cubic centimeter.

In the bones that this marked difference might be due to the fact that the bottles were steamed in the first case and unsteamed when the milk was pasteurized directly in bottles, another series of samples was pasteurized in which both bottles were steamed for two minutes

14 BULLETIN 240, U. S. DEPARTMENT OF AGRICULTURE.

in order to eliminate this factor of possible infection. The result of these experiments are shown in Table 3.

TaBLeE 3.—Comparison of bacterial reductions in milk pasteurized in steamed bottles and in pasteurized milk bottled while hot.

Milk pasteurized at 145° F. for 30 minutes.

Raw milk. Hot pasteurized milkin| Milk pasteurized in Sample No. hot steamed bottles. steamed bottles.1

Bacteria Bacteria | Percentage} Bacteria | Percentage

per ¢.c. per ¢.c. reduction. per c.c. reduction.

1 AIP Gn RES Ree = Peon ow) Se ee cs et as Rotem 24,900 380 98. 47 570 97.71 ae ep mS ga Eee a aya ie Nea ep SER ery me a ea 94, 000 860 99.08 2, 200 97. 66 ToS a ee ee ees SI eee A 305, 000 21, 800 92. 85 55, 800 81. 70 Ue Sere eR TN ae ER ey Aa eee a Elon 235, 000 5, 400 97.70 7, 600 96. 76 OR Pears pratt nea a Po UY MELA) Ae hy 82 coat ee Rie 176, 000 2, 200 98. 7. 11, 400 93.52 DAD ts eh GPS a Na Ia ea Se ei Ih as HS A 97, 000 5, 900 93.91 8, 390 91.39 Ca J a se ree 2 ee rw eR go tr 230, 000 6,300 97.26 5,500 97.61 SU oe eae SS Ie Se Pts te eee pee NA 124, 000 920 _ 99. 26 1,500 98. 79 SP, Ge eae chs Oe el Een tt MI Sete Sth RES 450, 000 4, 200 97.47 11, 400 97.46 Se Se a a nm ees Le er = Boe 3, 950, 000 4,320 99. 89 3,520 99.91 SILI SS Nl Ue ae tae ad a al 985, 000 11, 800 98. 80 18, 400 98. 13 Een gy ere get Eons. Be Mem Sk aan 190, 000 7,500 96. 06 9, 300 95. 10 Nevorape th ewes. ioe a 571, 766 5,965 | 97. 46 | 11, 295 95. 48

1 Bottles were steamed two minutes, and cooled before they were filled with raw milk.

It will be seen that the results again were in favor of the milk pasteurized in bulk and bottled while hot. Of the 12 samples in the experiment 10 showed lower counts than when the milk was pasteur- ized in the bottles.

The average count of the raw milk was 571,766 bacteria per cubic centimeter. After pasteurization in bulk, followed by bottling hot, the count was 5,965, and a portion of the same milk pasteurized in ‘bottles averaged 11,295 bacteria per cubic centimeter. In several of the samples the count in the milk pasteurized in bottles was very much higher than in the same milk pasteurized in bulk and bottled hot. The explanation of these marked differences is not known. While minor differences are always within the limits of the errors of bacteriological methods, the great differences found in many .cases can not be explained in this manner.

PREVENTION OF BOTTLE INFECTION BY BOTTLING HOT MILK AND BY PASTEURIZATION IN BOTTLES.

Since the process of pasteurizing milk in bulk and bottling while hot enables the use of hot, steamed bottles which can be directly filled with hot milk, it should be expected that there would be no contamination added to the milk during bottling.

To determine this point eight samples of milk were pasteurized in bulk and bottled hot in hot, steamed bottles. The bacteriological results are shown in Table 4, column A. Two steamed and cooled

PASTEURIZING MILK. 15

milk bottles for each sample were inoculated with equal amounts of sour milk. One of these infected bottles was then steamed for two minutes and filled with hot pasteurized milk and the other contami- nated bottle not heated was filled with some of the same pasteurized milk, which had been previously cooled in a sterile bottle. An exami- nation of Table 4 shows, when the figures in columns A and C are compared, that the infectious material added to the bottle was en- tirely destroyed by the method of bottling, at least so far as bac- teriological methods can detect, since any marked increase in column C would show infection. Column B shows the bacterial counts obtained by putting cold pasteurized milk into infected bottles. From these results it is evident that the process of bottling hot pasteurized milk in hot, steamed (two minutes) bottles entirely eliminates the factor of bottle infection, which may often be serious in the ordinary processes of pasteurization on a commercial scale.

Taste 4.—Destruction of bottle infection during the process of bottling hot pasteurized nrilk.,

Hot pasteur- | Cold pasteur- Hot Pesta ized milk in | ized milk in conned hot steamed | cold infected Meee Saae Sample No. Raw milk, bottles. bottles.! tles.1 A B Cc Bacteria Bacteria Bacteria Bacteria ; per C.C. per C.C. per C.C. per C.C. DIN te Sa A Sire Daa abe cd AD NG aN eS dM A 24, 900 380 6, 400, 000 460 DOD 5 is Sta RS FERS peg ot Ry SPAT Nes Si Eg eV RA ee 94, 000 860 5, 600, 000 600 Fe MM, Men MEL ot ik 235, 000 5, 400 1, 330, 000 4, 800 ak i nie Ss eA ea toh ee Rk . 176, 000 2, 200 1,510, 000 2, 400 Dore aetna eC T ELS Mc) 97,000 5,900 235, 000 4,100 195 5 12 BU yee REE Ean SN See 230, 060 6, 300 355, 000 5, 800 Se ra ae i NI i ON oer eA SRO 124, C00 920 305, 000 950 PSD orcs SA on SATE, SNe Mata 190, 000 Te SOON ere aE aa 8, 800

1 Bottles had been previously infected with several cubic centimeters of sour milk. 2 Bottle infected with old, sour, pasteurized milk.

The question naturally arose as to whether or not pasteurization in bottles would destroy infection in bottles specially infected before being filled with raw milk. To determine this point nine samples of milk were pasteurized which had been previously steamed and cooled. The results are shown in Table 5. One bottle for each sample was steamed, cooled, infected with several cubic centimeters of sour milk, and filled with some of the original raw milk. Samples were then plated from this bottle to show the extent of the infection, the results of which may be found in column B of the table. The bottle of infected raw milk was capped with a seal capsand the milk pasteurized directly in the bottle. Plates were made directly after the heating and the bacteriological results are shown in column C. Any increase in the counts in column C over those in column A shows

16 BULLETIN 240, U. S. DEPARTMENT OF AGRICULTURE.

~

the amount of infection introduced by placing milk in an infected bottle. It is evident that in only two samples, Nos. 28 and 35, was the infection entirely destroyed.

TaBLeE 5.—Destruction of bottle infection during the process of pasteurization in

bottles. Milk pasteur-| Bottles in- Seah bet | ized in clean | fected with | ME Dee previously |sourmilkand qnfected 2 : steamed filled with b idl Sample No. | Raw milk. | $oitles. raw milk. | ottles. ae | | | | A | B Cc | Bacteria Bacteria | Bacteria Bacteria per c.c. per Cc. Cc. DERE C- DEE CAC ss A eae Ae le eM ras Re ars Mend See AS Sie 24, 900 570 3, 700, 000 | 2, 090 Deere ee ep Oe Ae ee a ee ae ee ee 94, 000 2, 200 3, 300, 600 | 6, 200 Poe a aI ais Sa ee PN yep ae pega ger 235, 000 7, 600 760, 000 | 9, 500 PASS a ss eae ee re A ee MR EL Gs a ar ess BE eee 176, 000 | 11, 400 650, 000 | 11, 090 pe Re ne esas MUNG eS Cnr Mees OR es a 97, 000 } $, 350 530, 000 20, 000 TUTE =e are aban eg eget eed ee oe tan Toe ine te Sele 230, 5.500 645, 000 | 20, 900 SA” pe eg So ee Seer te ee ee pe ee ee 124, 000 1, 500 400, 000 | 28, 600 SS Sea ee age oe 4 fg ee ee eee eS 190, 000 9,300 | 230, 000 9, 600 ae ee I Ca ya agen a oe ee 38, 000 5, 600 } 92, 000 17, 700

1 Bottle infected with old, sour, pasteurized milk.

It is quite possible that infection from unclean bottles might be- come a serious factor in bottle pasteurization. When one considers that in pasteurization in the bottle the bacteria which are left are either heat-resistant vegetative cells or spores, 1t is easy to see that if a large number are left in a bottle and it is again filled with milk and pasteurization again performed in the bottle these same bacteria will again survive and increase the number left. It is advisable to steam the bottles at least two minutes before filling with milk for ‘pasteurization in the bottle

COOLING MILK WHICH HAS BEEN BOTTLED HOT.

When a water-tight cap is used it is, of course, possible to bottle the milk while hot and ccol by submerging in cold water, but experi- ments have been made with a process by which the milk may be cooled in bottles capped with ordinary cardboard caps. Briefly — stated, the process consists in exposing the hot bottled milk to an air blast. The air-blast system is used at present in the hardening rooms in ice-cream plants, but, so far as known, this system has never been apphed to the cooling of milk.

Several experiments were tried on a laboratory scale which gave promising results. When a bottle of hot milk is allowed to cool in still air a film ef warm air forms about it which can move away only by convection, and. naturally, the cooling process is slow. If some means were provided for moving the film of warm air and forcing

‘PASTEURIZING MILK. my

cool air against the bottle, heat would constantly be given up with more rapidity by the milk and the cooling process hastened. In figure 8 are shown the temperatures in three bottles of milk cooled for 30 minutes in air. One bottle was cooled in still air at 77° F., one was cooled in an air blast from an electric fan at a temperature of 77° F

and one was cooled in still air at 35° F. At the beginning of the cooling the temperature of the milk was about 145° F. As will be seen from the curves, after 30 minutes’ cooling the temperature of the milk in the bottle cooled in still air at. 77° F. was about 127.5° F., while that of the milk cooled in an air blast at 77° F. was about 102° F. It is noted that ae cooling in an air blast for 80 minutes

150° F.

140° ~

130°E, ae ot alin a 120'‘F. Sag Wap | SHEE ee - (SRS R BRR ae begs See. ee i eee Pees | ee a anes

ane iii

fo) 2 14 16 ES 20 30 Cooling period in minutes.

Fic. 8.—Effect of cooling a quart bottle of milk in still air and in an air blast.

there was a reduction in temperature of about 25.5° F. in excess of that obtained under the same conditions in still air. The temperature curve of the milk in the bottle cooled in still air at 35° F. follows closely that of the milk cooled in still air at 77° F. It is also inter- esting to note that after cooling for 30 minutes in still air at 35° F. the temperature was 122° F., while that of the milk cooled in an air blast at 77° F. was about 102° F., a difference of 20° F.

Since these experiments indicated that hot bottled milk might be cooled more rapidly by using a blast of cold air, another experiment was conducted in which one quart and one pint bottle were cooled in still air which averaged 39.4° F. and another set in an air blast the temperature of which averaged 44.3° F. The blast of cold air was

cbtained by placing an electric fan in a refrigerator. The fan de-

18 BULLETIN 240, U. S. DEPARTMENT OF AGRICULTURE.

livered air at a velocity of about 1,250 feet per minute. The tem- perature curves in figure 9 show the results of this experiment. The temperatures of the hot milk at the beginning of the cooling ranged from 140° to about 143.5° F. in the different bottles. It will be seen from the curves that five and one-half hours were required for the temperature of the quart bottle of milk in still air to reach 50° F., while the milk in a quart bottle in an air blast was cooled to 50° F. in a little over two hours. The milk in the pint bottle cooled in still air reached a temperature of 50° F. after about three and one-half hours, while only one and one-half hours were required to cool the milk in the pint bottle which was in a blast of cold air.

From these results there can be no doubt as to the value of an air blast for cooling bottles of hot milk, at least as compared with still air as a cooling medium. As these experiments were made on single

150° F.

CERUURGDBEDSsueces >” INGE ae ANS eee ctor arate Paste |

ioe se |

140° F.

130° F. 120°F.

r10° F.

roo° F. 90° F. 80° F. 70° F. 60° F.

KIRA i Ss eS eS a ye eo

Lio aS

25 5 Cooling period in ine

50°F.

40°F.

bole

Fic. 9.—The cooling of pint and quart bottles of hot milk in still air and in an air blast at refrigerator temperature.

bottles it was thought advisable to try cooling several crates of bottled hot milk by an air biast. Specially constructed skeleton- frame steel crates were used, so as to allow a free circulation of air.t Milk was pasteurized at 145° F. for 30 minutes and bottled hot in ordinary milk bottles by the aid of a hand bottle filler. The bottles were then capped with the ordinary cardboard caps and placed in crates. Four crates were used in these experiments, two filled with quart and two with pint bottles. The two crates which contained quart bottles were placed in a refrigerator room one above the other, and directly back of them were placed the two crates of pint bottles one above the other. The air blast was generated by a 16-inch desk

i Mr. John T. Bowen, of this division, assisted in this work.

“PASTEURIZING MILK. 19

fan, which gave an air velocity of about 1,250 feet per minute. The fan was placed about 24 feet in front of the pile of four crates directly facing the crates with quart bottles. ‘Temperatures were taken in two quart bottles, one in the front and the other in the back row. In this experiment the crates were cooled in a refrigerator room, the temperature of which varied from 40° to 44° F. The results of this experiment are shown in figure 10, together with the results of a similar experiment in which the crates were cooled in an air blast at a temperature of about 76° IF. for a period of 24 hours. The crates were then placed in a refrigerator and the cooling con- tinued, a blast of air with a temperature of about 41° IF. being used. The curves in figure 10 show the averaged temperatures of two quart

es SO SC See am BeBe a

Ea 1M

>

90° F. rs 80° F. en Oe pera d 27 2773 Rah ee | Z eae a eee 40° F |_ 6

Cooling period.

Fic. 10.—Effect of cooling crates of bottled hot milk in an air blast at different temperatures.

bottles. It will be seen from curve A that about 3 hours and 7 min- utes were required to cool the milk in quart bottles from 140° to 50° F. when cooled in a blast of cold air during the entire period. A comparison of curves A and B shows that it took only about 45 minutes longer to cool to 50° F. the milk in bottles exposed to an air blast at room temperature for the first 24 hours. It is interesting to note that curves A and B follow each other fairly closely during the first 30 minutes of cooling. These results suggest that the cooling of hot pasteurized bottled milk may be accomplished by cooling with an air blast at ordinary room temperature and completed by cooling in a blast of cold air in a refrigerator room. The greater the number of

20 BULLETIN 240, U. S. DEPARTMENT OF AGRICULTURE.

heat units which can be removed from the milk by an air blast at room temperature the cheaper the cost of cooling, since refrigeration would be saved and about the only cost would be the operation of a blower.

These experiments, although by no means conclusive as to the value of this method of cooling by an air blast on a practical scale, since many complications may arise in the practical application, indicate great possibilities for such a system.

THE EFFECT OF QUICK AND SLOW COOLING ON THE BACTERIAL FLORA OF THE MILK.

It is believed that any system of pasteurization in which the milk is not cooled immediately after heating will be looked upon with suspicion and will excite comment. It has always been supposed that immediate cooling was an indispensable part of the process of pasteurization, first, because sudden changes in temperature were believed to have a destructive effect on the bacterial cells, and second, because it has been supposed that bacteria left after pasteurization would immediately begin to grow unless the milk was cooled at once.

As stated earlier in this bulletin, it was shown in Bulletin 161 (3) that sudden cooling played no part in the destruction of bacteria. ‘There remains, therefore, one question to be answered, How quickly must pasteurized milk be cooled in order to check bacterial growth?

From the writers’ former studies of pasteurization it seemed apparent that the bacteria which survived heating were somewhat weakened or at least did not begin to grow as might theoretically be expected. These observations naturally gave rise to the idea that pasteurized milk might be cooled directly in bottles by a cold air blast, provided the cooling period did not extend over a few hours.

In order to obtain data on this question 10 samples of milk were pasteurized and bottled hot in steamed bottles. Two bottles for each sample were cooled as follows: One bottle was cooled within half an hour in ice water and placed in a refrigerator at 45° F. for 174 hours; the other bottle was cooled slowly at room temperature for 4 hours and placed in a refrigerator at 45° F. for 14 hours. At the end of that time each bottle of milk was 18 hours old; one was cooled quickly and had been at 45° for 174 hours; the other had been cooled slowly and had been at 45° for probably a very short time, because, although it had been in the refrigerator for 14 hours, - the milk was warm when placed there, and cooling in still air is a slow process. Both bottles after the 18-hour cooling period were allowed to stand at temperatures of from 75° to 86° F. for a period of 6 hours. The bacterial results are shown in Table 6.

PASTEURIZING MILK. 21

TABLE 6.—Number of bacteria per cubic centimeter in pasteurized milk. bottled hot, cooled quickly and slowly, and subsequently held at room temperature.

Sample No. Aver- | age of Method of cooling. 10 x | sam- 1 2 3 4 5 6 vi 8 9 10 ples. Bacteria in the raw milk. .. .|95, 000)176, 000/176, 000|97, 500/97, 500]...... 450, 000|.....- 985, 000} 38, 000|264, 375

Bottle No.1, cooled quickly: Directly after pasteur- WAIROM 65h coe 4eBeee 600} 1,870] 1,570) 5,900) 5, 900)22, $00 890} 4, 800} 8,300} 5,500] 5, 823 After one-half hour in i ice water and 17% hoursiat 45> Bs 22222: 1, 000} 4 2,050) 1 2,370).....-|.--.-- 16,600} 1,700) 2,500} 8,900} 5,200) 5,040 After 6hoursat86° F....)...... 5, 750] 8, 400] 6,600) 5, 900)......|..-.---].-.--- 29,600] 25, 200} 6, 908 Bottle No.2, cooledslowly: Directly after pasteur- IWAN ENON 0 Leap al ae ate ea 860] 1,320] 1, 220) 5,900} 5, 900)21, 800 890} 5,400] 7,500} 6,500} 5, 729 After 4 hours at room temperature and 14

houmnsrai4 om he eeee ee 00) PaO: ei syY-O BS aaa lladaaes 12,300} 2,200) 715} 9,800] 5,200) 4,678 After 6hoursat 86° F....|...... HRSOO NOOO SeOO(S. 700) sole sees |eeseee 28 900) 25,300] 5, 583 1 Held at 45° F. for 21 hours in place of 18 hours. 2 Held at 75° F. instead of 86° F.

As may be seen from Table 6, bacterial counts were made of the raw milk on each bottle directly after pasteurization, at the end of the 18-hour cooling period, and again after the milk had been at -room temperature for six hours. The bacterial results obtained showed that there was no more increase in the pasteurized milk cooled slowly than in similar milk cooled within half an hour and held at low temperatures for 18 hours. Neither was there any difference in the bacterial numbers even after milk cooled by both processes had been removed, after 18 hours’:cooling, and allowed to stand for six hours. ‘The various counts from 10 samples have been averaged and are given in the last column in order to show more plainly the effect of the two systems of cooling on the bacterial numbers in milk. It will be seen that the average bacterial counts of the milk cooled slowly are even lower than those of milk cooled quickly. While this difference is probably an experimental error, it is evident that bac- terial growth in the pasteurized milk was not increased by the slow- cooling process.

The writers do not wish to convey the idea that pasteurized milk need not be cooled at all. The cooling of any milk is absolutely essen- tial in order to restrain bacterial growth, and the fact should be emphasized that the process of cooling pasteurized milk slowly does not dismiss the cooling process but simply makes use of a slower cooling process than is in use at present.

In order to show, respectively, the effect on the bacterial content of cooling quickly, cooling slowly, and not cooling to low tempera- tures at all, three experiments were made. Milk was pasteurized in bulk and three steamed and hot quart bottles were filled with the hot milk. One bottle was cooled in iced water in half an hour to 50° F. and refrigerated at 45° F. Another bottle was cooled in a blast of air at room temperature for half an hour during which time the

22 BULLETIN 240, U. S. DEPARTMENT OF AGRICULTURE.

temperature dropped from 145° to about 100° F. The milk wasthen allowed to stand at a temperature of from 100° to 80° F. for five hours, after which it was placed in a refrigerator at 45° F., where it cooled slowly in still air. The remaining bottle was cooled for half an hour in an air blast at room temperature and allowed to remain at a temperature of about 75° F. through the entire experiment. The results of these experiments are given in Table 7.

TABLE 7.—Hffect of different methods of cooling on the bacterial content of pasteurized milk.

Sample No.. Method of cooling. Bacteria per c.c. | Bacteria per c.c. | Bacteria per c.c.

Seas pveetiaulll pepe geereren ce loth ania crs AEN aA SUR dra O50 OUOs Sassducsooosocce 11,900, 000 Bottle No. 1, cooled quickly:;

Directly after OAS UAINVANH OL, G29 Scososodacadsosoue- 6, 450 2,110 8,500

Eel dia tia be eh eitOn 22 NOUnSsses seca ern ee eee 5,050 - 1,720 28, 400

ISON ENG GHA 18 5 HOP ONO UMS sos oa sceaceceoadosanessc 4,800 2,340 76,500

ISIC Ag 7 IDS WOR OF OI Caccascsdcaassoooosemocee 1,370,000 | . S8b5000; |Peeeeeeeeseeee Bottle No. 2, cooled slowly:

Directly ‘after PAStounizabiON sees eee see see eee eee 7,150 23580 11,900

Eeldeat/osnt eOn OmOULSse sence ence oc eas eee ae 6, 100 1,600 29,000

Meldiatdoouky for U7 noutss: 2.5 ses ee ee 6, 200 2, 400 192, 000

iGhGl eis 72 I sdioie @ INOwlsssoes s4oecasocsoosucesouad= 9, 600 2,740 : 348) 000

ISG aN 7? 1B Ole PAN OWNS 5 Geass cacogacobocuEcces 2,760, 000 850, UO Baaeoacsearasscc Bottle No. 3, cooled at room temperature:

Directly CITE OMS DICVAM HOM oo sdeaooacoasacccsococe 4,950 2,180 8, 500

TENG! hs GO> 1D OE MOWIRS e cCosuoddeauansacccocsSco5s 6, 850 2. 890 25, 000

IEIGICL eit ABS IB Soe WH) VOLES se cahoscooeeueosseooese. 700, 000 Dp 420, 000 83, 400, 000

HELO Aa Gomes LODO Sh OUTS sera eee ee eae +. 2,750,000 13, 400, 000 269, 000, 000 -

HeldtatvosH. tor total OLGGsnouTS 52sec = 32 s55- AGO; 800; 0008 Be 2= see | Jeske Se eaaseee

A study of the table shows that there was no increased bacterial growth with samples 1 and 2 caused by holding the pasteurized milk for five hours after bottling hot, even though the temperature dur- ing that period ranged from 100° to 80° F., which is the most favor- able temperature for bacterial development. With sample 3 there was an increased growth over that in the milk cooled quickly. . It must be remembered that these experiments represent extreme con- ditions in slow cooling, but the fact is apparent that the cooling process should not extend over five hours. The effect of not cooling milk to low temperatures is plainly shown in the table by a compari- son of the bacterial counts with those of milk cooled both quickly and slowly. It is believed from these experiments that it is possible to cool hot bottled pasteurized milk by a blast at room temperature followed by a blast of cold air without any more bacterial develop- ment than would take place if the milk were immediately cooled, provided the milk is cooled to 50° F. gradually within five hours. This is not made as a definite statement, because different results may, of course, be obtained when milk is thus cooled on a commercial scale.

Again let the fact be emphasized that pasteurized milk or raw ; milk must be kept at low temperatures after cooling in order to check bac- terial development.

PASTEURIZING MILK. 23

THE CREAM LINE AND FLAVOR OF PASTEURIZED MILK COOLED BY VARIOUS METHODS.

In the consideration of the process of bottling hot pasteurized milk followed by slow cooling it is of practical importance to know what effect such a process will have on the cream line and flavor of the milk. Several experiments were made to determine the effect on these points. Milk was pasteurized, and hot 500-cubic-centimeter graduated cylinders were filled with hot milk up to the 500-cubic- centimeter mark. Together with the cylinder of hot pasteurized milk one cylinder was filled with raw milk and one with pasteurized milk which had been cooled to 50° F. in 15 seconds’ time by running through a coil immersed in brine. The method of cooling the hot cylinders of pasteurized milk was varied considerably, as may be seen from Table 8. After holding the milk for 24 hours at 45° F. the numbers of cubic centimeters of cream were read off directly from the graduations on the cylinder. This method, of course, gave a very accurate means of determining the effect of heating and cooling on the cream line; in fact it was too accurate, since considerable dif- ferences in the cream line by this method of measurement were not apparent in bottled milk.

TABLE 8.—Cream-line experiments with raw milk and milk pasteurized at 145° F. for 30 minutes.

Cubic centimeters of Experi- cream in 500 e. e. ment Process. cylinder after 24 No. hours’ refrigeration at 45° F., AER vy ge TT cpapene cee at ey rune oe abe sears ofa Ve A, i MEM NE ALG NACE Rae eR ious 64.5 Pasteurized milk: Cooled quickly in 15 seconds to 50° F. and held in refrigerator at 45° F.. 64.5 1 Cooled slowly in air blast for 45 minutes and placed in refrigerator at A oe an Oi UE es a OD MEAL ee =p (Pa Ghee ee Claas aaa Ce Me WIRE Su oy 65.0 Held above 105° F. for 3 hours, cooled in ice water, and placed in re- ALS OTALOM LAOH EEA eee u Nene eee inet nal m menuiatt Sees av SA Rie 64.5 ESB ag es ON gO a USS SO DPT SG EIN 65.0 Pasteurized milk: 2 Cooled in 15 seconds to 50° F. and placed in refrigerator at 45° F_....... 62.5 Cooled slowly in air blast for 14 hours and placed in refrigerator at.45° F. 52n5 Hold above 100° F. for 14 hours and placed in refrigerator at 45° F..._.. 52.5 EVID] OS th ae 05, He Sires HOA ay Wi oe ht each aye peal Saving A ie ae aCe al PLR AEROS ALTER Ua) UPR Re eed Pasteurized milk: Cooled in 15 seconds to 50° F. and placed in refrigerator at 45° F......:. 83 3 Cooled slowly for 30 minutes in air blast, cooled quickly in brine, and placedinimeiniseravon atiAabge herpes) yey ee ee my lees) Cue 85 Held above 100° F. for 3 hours, cooled quickly in brine, and placed in LOI Senator apa cana e oui ee eek cp era Milas emia ul oat Bane 90 EVO Wrst NAL OS Caeser ee see al occa peacelnn, Sua. ena ce year eran aa cn Uris Ny ce ML Ral ase yA ALTE Dc OO ae Ae TG Pasteurized milk: Cooled in 15 seconds to 50° F. and placed in refrigerator at 45° F........ 75 4 Cooled slowly in air blast for 24 hours, cooled in ice water, and placed in REMAP CLA LOMA SO we byas aso sere eee toe en een MeN Wa. 69 Held above 100° F. for 24 hours, cooled in ice water, and placed in refrig- CONN aa) Seek eS eas oy Bier i HS eyes Sie ale eee ee a ae va Oe 75 FRENGT TOOUN UE eo ae kat he ie A ase aN Na el NL ee UL 80 Pasteurized milk: Cooled in 15 seconds to 50° F. and placed in refrigerator at 45° F........ 68 Cooled slowly in air blast for 2 hours and placed in refrigerator at 45° F.. 55 5 After cooling in air blast for 2 hours the milk was cooled quickly in brine) to 502 beand placediniretriceratonay4be We a 62 Held above 100° F. for 5 hours and placed in refrigerator at 45° F...... 55 After holding above 100° F. for 5 hours the milk was cooled quickly in brine to 50° F. and placed in refrigerator at 45° F.............-.--- 62

24 BULLETIN 240, U. S. DEPARTMENT OF AGRICULTURE.

A study of the results in Table 8 shows that the cream-line forma- tion is a variable factor. Sometimes it was reduced by pasteuriza- tion even when the milk was cooled to low temperatures within 15 seconds, and at other times there was no difference. In some ex- periments the cream line was slightly less on milk cooled slowly and again it was slightly higher. Throughout the experiments on pas- teurized milk bottled hot in ordinary milk bottles a good clear cream line was obtained. When milk stocd at temperatures above 80° F. for several hours without agitation some of the melted butter fat rose to the top of the bottle and cn cooling formed a small lump of butter. This was not observed, however, when the cooling process was begun immediately after bottling, even though the cooling was gradual. :

As to the effect on the flavor of the milk, it may be said that there was no more effect than that produced by milk pasteurized and cooled rapidly, except in instances where the milk was held above 100° F. for several hours, as was the case in some of the experiments, in which a shghtly more pronounced cooked taste was noticeable in the milk. .

In this connection attention is called to the fact that these results hold only for milk pasteurized at 145° F. and can not be applied where higher temperatures might be used, as it is possible that with higher temperatures different results might be obtained.

BOTTLES TO BE USED IN THE PROCESS OF BOTTLING HOT PASTEURIZED MILK.

It is obvious that a quart bottle filled with milk at 145° F. will not contain a full quart when the milk has cooled to 50° F., owing to the contraction during cooling. Several experiments which were made to determine the loss in volume during cooling showed a shrinkage in a quart bottle which averaged about 18.40 cubic centimeters. As- suming a quart of milk to be 946.35 cubic centimeters, that volume at 145° F. would therefore contract to about 927.95 cubic centimeters when cooled to 50° F. If a quart bottle is filled with milk at 145° F., it will be 18.40 cubic centimeters, or 0.62 of an ounce, short of 1 quart when cooled to 50° F. To overcome this shortage bottles of a shghtly larger capacity should be used when filled with milk at 145° F. A bottle should be of sufficient size to hold 1 quart of milk measured at 50° F. which has been heated to 145° F.

~ PROCESS OF BOTTLING HOT PASTEURIZED MILK UNDER COMMER- CIAL CONDITIONS. Having discussed the various steps in the process of bottling hot pasteurized milk, the possible application of this process of commer- cial conditions may be outlined.

~~

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Milk can be pasteurized by the ordinary holder system at 145° F. for 30 minutes. It can then be bottled hot in special oversized milk bottles of the ordinary type and capped with ordinary sterile caps. Before being filled the bottles can be steamed for two minutes by running the crates inverted on a conveyer over steam jets. The bottles would then go through the bottlng machine in a hot condi- tion and would be practically sterile. The crates of hot bottled pasteurized milk can then be cooled by stacking in a refrigerator room and blowing cold air through the crates. In the cold season outside air can be used for cooling, and in the warm season re- frigerated air can be circulated through the crates.

This process can be modified. The hot milk can be held in the bottles at 145° F. instead of in a tank, and the crates of hot pasteur- ized milk can be cooled by spraying with cold water instead of air.

From the results of experiments with air cooling on a small prac- tical scale started in 1913, it is believed to be entirely practical to cool hot bottled milk by means of forced-air draft. The results of this work are being prepared for publication in the near future.

Since the process of bottling hot pasteurized milk has not as yet been worked out for practical use, it is impossible to state definitely all-its advantages and disadvantages. However, from laboratory experiments alone certain advantages are plainly shown. From a sanitary standpoint one great advantage of the process of bottling hot pasteurized milk in hot bottles lies in the fact that bottle infec- tion is eliminated. From a commercial standpoint there is also an advantage, because of the reduction of milk losses on the cooler caused by adherence of milk and by evaporation. Ordinary card- board caps may be used in this system, since they do not have to be water-tight, which is obviously a point of great advantage so far as cost is concerned. rs

At the present stage of this work it is impossible to state how the cost of air cooling will compare with the ordinary methods in prac- tice, but it 1s believed that there will be no more expense involved.

The length of time required for cooling is perhaps the greatest disadvantage of this process, and yet this would be of no consequence except in plants where the milk is delivered immediately after pasteurization. In the majority of milk plants the milk is pasteur- ized in the morning or afternoon, placed in refrigerators, and deliv- ered early the next morning. Consequently in most plants it would make little difference whether the cooling process was performed quickly or slowly.

SUMMARY.

1. The process of pasteurization in the bottle, using a temperature of 145° F. for 30 minutes, causes satisfactory bacterial reductions.

26 BULLETIN 240, U. S. DEPARTMENT OF “AGRICULTURE.

2. Bottles should be steamed at least two minutes before being filled with milk in order to destroy heat-resistant types of organisms which might survive the pasteurizing temperature and thereby in- crease the bacterial count.

3. Care must be taken to record the temperature in the bottom of the bottle during the heating process. When milk at an initial temperature of 50° F. is heated in bottles without agitation in water at about 146° F. the temperature of the milk in the top of the bottle will reach 140° F. about nine minutes before that in the bot- tom. The temperature of the milk during the process of pasteuriz- ing in the bottle should be recorded by placing a thermometer in a control bottle with the bulb of the thermometer about one-half inch from the bottom. The milk should be heated for 30 minutes at 145° F.

4, When bottles are heated and cooled under water care should be taken not to use bottles with chipped or otherwise imperfect tops, since the seal caps may allow leaks during the process of pasteuriz- ing. It is advisable for the users of patented seal caps to-assure themselves that the caps are water-tight, since leaking caps may cause dangerous infections, particularly if the cooling water is pol- luted. 3

5. The process of bottling pasteurized milk while hot in hot steamed bottles causes equally good bacterial reductions as does pasteurization in bottles. Even with the same length of exposure of 30 minutes and the same temperature of 145° F. the bacterial reductions are often much greater than those produced by pas- teurization in bottles.

6. In the process of bottling hot, bottle infection is eliminated, even when several cubic centimeters of old, sour milk are added to bottles before filling. The two-minute steaming period to which the bottles are subjected before filling with hot milk is sufficient to destroy the contamination, at least so far as bacteriological methods can detect.

7. Laboratory experiments indicate that milk may be pasteurized. bottled hot, capped with ordinary cardboard caps, and cooled by a blast of cold air.

8. It is probable that if milk is cooled from 145° to 50° F. within five hours no more bacterial increase will take place during the slow cooling than would take place if the milk were cooled immediately to 50° F. Whether or not this will be true under commercial con- ditions can be determined only by future experiments.

9. So far as the laboratory experiments indicate, when milk is heated to 145° F. for 30 minutes, the bottling of the hot pasteurized milk followed by slow, gradual cooling has no more appreciable

PASTEURIZING MILK. WA f

effect on the cream line or flavor of milk than the ordinary process of pasteurization. This is true of cooling periods of less than five hours’ duration.

10. Since milk contracts on cooling, a quart bottle filled with milk at 145° F. does not hold a full quart when the milk is cooled to 50° F. It is about 0.62 of an ounce short. Therefore slightly oversized bottles should be used.

11. The advantage of the process from the commercial stand- point are: (1) That bottle infection can be eliminated; (2) that milk losses are saved, owing to evaporation over the cooler; and (3) that ordinary cardboard caps can be used. The principal disadvantage is that the air-cooling process requires several hours. This, however, would be a disadvantage only in the few plants where milk is de- livered directly after pasteurization.

CITATIONS TO LITERATURE.

1. SoxuuetT, F. Ein verbessertes Verfahren der Milch-Sterilisirung. Mtin- chener Medicinische Wochenschrift, vol. 38, No. 19, p. 3385-838. Munich, May 12, 1891.

2. GERBER, N., and WIESKE, P. Pasteurisation des fiacons dans la grande in- dustrie (pasteurisation avec agitation). Revue Générale du Lait, vol. 2, No. 8, p. 169-177. Lierre, Jan. 30, 1903.

3. AYERS, S. HENRY, and JOHNSON, WILLIAM T,, Jr. A study of the bacteria which survive pasteurizetion. U.S. Department of Agriculture, Bureau of Animal Industry Bulletin 161. Washington, 1913.

4, NorkTH, CHARLES E. Pasteurization of milk in the bottle on a commercial scale. Medical Record, vol. 80, No. 3, whole No. 2128, p. 111-115. New York, July 15, 1911.

5. AYERS, S. Henry. The pasteurization of milk. U. 8S. Department of Agri- culture, Bureau of Animal Industry Circular 184. Washington, 1912.

6. COMMITTEE on Standard Methods of Bacterial Milk Analysis, Report of. American Journal of Public Hygiene, vol. 20, new ser. vol. 6, No. 2, p. 315-845. Columbus, Ohio, June, 1910.

7. DE SCHWEINITz, E. A. The pasteurization and sterilization of milk. U. S. Department of Agriculture, Yearbook 1894, p. 331-356. Washington, 1895.

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