( Originally Published 1939 )
Pasteurization. The word pasteurization takes its name from the great French biochemist, Louis Pasteur. He discovered that microscopically minute forms of plant life were causative agents in the spoilage of wine and also in the development of certain diseases. When any of these germs gained entrance to a food product or the tissues of a person, they found nutrient materials which enabled them to multi-ply at a very rapid rate. They attacked the constituents of the medium and broke them down into simpler products, causing spoilage in the case of food products. Their metabolic processes produced toxins in diseases. When the medium containing them was heated to about 145° F., all the active germ cells would be killed, and the processes of spoilage by these agents would be stopped. Heat treatment to this degree was called pasteurization.
Pasteurization is not sterilization. Sterilization means the complete destruction of all microbic life. This is not desired in milk. The desideratum is to destroy all disease germs. The natural flora of the milk should be left to the extent that as the milk ages the acid-forming organisms will multiply and "sour" the milk, thereby preventing the growth of the ubiquitous putrefactive organisms.
Disease germs are usually killed at lower temperatures than the non-pathogenic strains. Among the pathogens most commonly found in milk, the tubercle bacillus is the most heat-resistant. Therefore, a heat treatment that is sufficient to kill this organism is adequate to kill the other pathogenic organisms. A large amount of careful bacteriological investigation has shown that the tubercle bacillus will be killed when subjected to a temperature of 140° F. (60° C.) for 20 minutes. This is the basis for the pasteurization of milk. In order to allow a reasonable margin of safety without damage to the other properties of the milk, it is customary to specify that pasteurization shall constitute a heat treatment of 142° F. (61.1° C.) for 30 minutes. The holding time may be shortened if the temperature is raised. A temperature treatment of 160° F. (71.1° C.) for a period of 15 seconds kills all pathogenic organisms that are found in milk.
Early commercial significance. Pasteurization was early adopted by the dairy industry to retard the souring of milk. Milk would often be held for relatively long periods, and sometimes it was pasteurized several times. This abuse led to great public disfavor. Pasteurization of milk was considered by many uninformed persons to be a commercial method to hide the effects of unsanitary milk handling. Even now, many persons think that it is a substitute for cleanliness in milk production.
Types of processes. In general, milk is pasteurized by three types of processes, namely, the flash, the high temperature-short time, and the holding methods. These are characterized chiefly by the temperature to which the milk is heated and the duration of the treatment.
Flash. This method consists of heating the milk quickly to the de-sired temperature, and then rapidly cooling it. No effort is made to maintain the milk at this temperature for any definite period of time or to use any very precise heating. The temperature is usually about 180° F.
In the early days of milk pasteurization, this procedure was quite generally followed. However, the discovery that this type of equipment was not properly designed to give an adequate heat treatment brought the practice into such disrepute that it has been outlawed in most communities for treating milk for consumption. It survives in butter and cheese manufacture.
High temperature-short time (conveniently designated high-short). This procedure is the outgrowth of the discredited flash method of pasteurization. Careful tests have shown that, when a milk is heated to a temperature of 160° F. (71.1° C.) and held there for 15 seconds, all pathogenic organisms are killed, and there remains a reasonable margin of safety. Inasmuch as this treatment exerts no deleterious effect on the commercial value of the milk, it has led to the development of several types of milk-handling machinery. The heating agents are hot water and electric current, respectively. In the former, hot water at a temperature slightly above that of the milk is flowed counter-current through plates or tubes on the opposite sides of which the milk is flowing. In the electric high-short pasteurization process, the milk is heated by its resistance to the passage of an electric current as it flows between two carbon plate electrodes. The temperature attained by the milk is a function of the amount of current and the rate of flow of the milk.
The introduction of high-short pasteurization has been slow. This is probably due in the first place to the inertia of interest resultant from the condemnation of any process other than the conventional type of holding process. In the second place, it probably derives from a reluctance to introduce a process whose successful operation depends so largely on a relatively complicated system of automatic temperature-control devices and electrically operated cut-offs, flow-diversion mechanisms, and automatic starters. Added to these are the difficulty of checking its performance by parallel laboratory test-tube experiments, greater metallic contamination, and increased damage to cream line to secure bacterial counts as low as those obtained by the holding process. The advantage of this method of high-short pasteurization is the small amount of space required, the facility of cleaning, and the continuity of the operations. Great improvements have recently been made under the supervision of the Chicago Health Depart-ment,' giving promise of wider acceptance.
Holding. This procedure involves the heating of the milk to the required pasteurization temperature (usually 142° F.), and holding it there for 30 minutes. The advantage of this method is its relative mechanical simplicity, and the facility with which the effect of the heating of the milk can be followed by test-tube experiments in the laboratory. Two general types of equipment are used in this treatment, namely, the vat and the pocket types.
The vat type is the simplest. It consists of a single vat or tank which is steam or hot-water jacketed. Proper pipes and valves admit the milk, and others remove it. Agitators or stirrers of one kind or another (often in the form of rotating heating coils) keep the milk in gentle agitation to avoid overheating local portions. Covers retard the cooling of the top layers of the milk. Indicating and recording thermometers reveal the heat treatment. Many installations are equipped with automatic heat control, but usually the smaller plants rely on hand-operated valves for heating, cooling, emptying, and filling. Vat installations are limited to the small plants. The flow of milk is intermittent because the operations of the whole plant are held in abeyance while the vat is filling and during the pasteurization. Generally, an hour or more may be required for a complete cycle from the time that the vat is empty until a return to emptiness. A large plant must be kept in continuous operation with uninterrupted milk flow. This is accomplished by using a series of vats which are so arranged that one is always available to receive an incoming stream of milk, indicated as follows:
TANK 1 TANK 2 TANK 3 TANK 4
1st 15 minutes filling emptying holding holding
2nd 15 minutes holding filling emptying holding
3rd 15 minutes holding holding filling emptying
4th 15 minutes emptying holding holding filling
The cycle is then repeated. In some plants there may be an extra tank, depending on the detail as to the rates of filling and emptying, size of the plant, and other local considerations.
A modification of this system is the pocket-type of pasteurizing unit. In principle, this equipment consists of a single large vat, either circular or rectangular, divided into a series of compartments. Each compartment operates as an individual vat. In most of these multiple vat or pocket type holders, a specially designed valve is geared to a motor, and is regulated so that at given and regular intervals it opens certain ports and closes others, thereby controlling the flow of milk into the respective compartment. After the operator (and the health officer) checks on the accuracy of this valve adjustment and ascertains the constancy of the speed of the driving motors, there remains no variable except possibly the temperature and a slow change in the speed of the motor from service wear.
Valves. The operation of valves entails a certain amount of wear which sooner or later develops into leaks. This may be enhanced by the corrosive action of the milk itself, as well as by the "scoring" or scratching or bruising of the ground parts from careless handling by the plant operators. If an outlet valve of a pasteurizing tank leaks into the outlet line, some of the milk trickles down the pipe lines into the finished batches of pasteurized milk and thus is not held for the full period of 30 minutes. If the inlet valve leaks, the raw untreated milk may trickle into a batch which is being pasteurized, but this incoming trickle cannot possibly be given the full duration of heat treatment because it enters the batch subsequent to the beginning of the timing period. In both cases, the pasteurized milk is infected with milk that has not been adequately pasteurized. To overcome these difficulties, lead detector valves have been designed to prevent the leaking milk from contaminating the treated batches. Some types automatically steam the parts which have been wet by the leak before the valve is opened to discharge the main batch. The leaking milk itself is usually allowed to escape to the floor through grooves or by-passes in the valves.
The thickness of the insulation on pasteurization vats precludes the installation of valves without interposing several inches of pipe between the inside of the tank wall and the valve seat. When the milk in the vat is heated, the temperature of the batch as a whole may be adequately heated for the full period of 30 minutes, but the part that lies in this short length of pipe is isolated from the treatment accorded the batch of which it forms a part. This milk in the dead space, or pocket, does not receive the full heat treatment and is not properly pasteurized. When the valve is opened to discharge the batch of pasteurized milk, the first milk that emerges is this unpasteurized milk from the pocket. It infects the whole batch. To overcome this difficulty, a specially designed valve has been invented, called a flush-seating valve. The valve seat is built into the valve in such a way that it lies flush with the inner wall of the tank, thus precluding the possibility of the forming of a pocket. Valves are now provided that embody the principles of leak detection and flush-seating. Proper practice requires that pasteurization vats be provided with this type of valve.
Foam. In vat or pocket types of holding equipment, the temperature of the air above the surface of the milk may be as much as 15° F. lower than that of the batch of the milk. Moreover, there may be floating islands of foam on the milk, and this may be in the form of a thick continuous blanket over the whole surface. Unless this foam and air are heated to the pasteurization temperature, some disease organisms entrapped in it may survive to infect the rest of the batch. It is now required to heat this supernatant air and foam to a temperature at least 5° F. above the existing milk temperature, both during the heating of the milk and during its holding period. This heating is often done with jets of live steam, properly trapped to prevent the discharge of water into the milk.
Temperature control. In order to maintain proper temperature control of the pasteurization process, two types of thermometer are required, namely, an indicating thermometer to enable the operator to read the temperature of the batch of milk directly, and also a recording thermometer to provide the health officer with a continuous record of the heat treatment, including the time of holding. The indicating thermometers are usually of the mercury-bulb type, with an etched line opposite the graduation at 1431/2' F. The recording thermometer records the temperature by an inked pen on a circular graduated chart fastened to the drum of the rotating clocklike mechanism. In the pocket type or multiple vat installations in which the milk is brought to the final pasteurization temperature outside the holder and in which the holding time is automatically controlled, the indicating and recording thermometers are installed on both the inlet and outlet manifolds of the 30-minute holders. The temperature charts are dated daily, and kept on file in the dairy for perusal by the control officials.
Heat regeneration. The cost of the steam that is necessary to heat milk constitutes such a large part of the expense of pasteurization that economical operation requires that as much as practicable be re-covered. This has led to the development of heat-recovery units or regenerators. In principle, they consist of an arrangement of pipes whereby the hot milk flows on one side and the cold milk flows on the other. In such a system where cold and hot milk are exchanging temperatures against an intervening pipe wall, the procedure is known as milk-to-milk regeneration. In the vertical "washboard" and cabinet types, the milk flows down the outside of the regenerator, whereas the hot milk (which has just been pasteurized) is pumped into it at the bottom and out at the top. This places the pasteurized milk under positive pressure so that, if a leak developed in the equipment, the pasteurized milk could not be contaminated with the raw milk. Another type of regenerator employs a pipe concentrically held within a larger pipe, each one of which is connected to similar pipes horizontally arranged one over the other. Hot pasteurized milk and cold unpasteurized milk are pumped and flowed, respectively, through this equipment in counter-current principle. This type is known as an internal tubular regenerator or cooler. The plate type is built and operated similar to the plate type of heater (see page 117).
Some operators employ milk-to-water-to-milk regeneration, whereby the pasteurized milk gives up its heat to water, and this heated water gives up its heat to cold milk. Their advantage lies in simplicity of equipment and ease of cleaning; their disadvantage lies in their greater heat loss and increased amount of equipment.
For details of arrangement and operation to preclude the contamination of pasteurized milk by raw milk, the reader is referred to the Journal of Milk Technology, 1, July, 1938.
Cooling. Inasmuch as pasteurization does not sterilize the milk and continued heat treatment impairs various properties of the milk, it is necessary that pasteurization be followed by immediate cooling, usually to about 40° F. The cooler is made of horizontal pipes stacked one above the other and connected to headers so that the cooling medium (salt brine or liquid ammonia) may freely circulate inside the equipment. The milk is pumped into a perforated trough or perforated pipe at the top of the cooler, and flows by gravity down the vertical surface. The bulb of the recording thermometer on the cooler is usually placed in the trough at the bottom.
Filtering. Milk in the plant must be filtered to remove any sediment that may have escaped the farmer's straining, as well as to re-move any stray particles of dust, soot, scale, or other foreign material which may have been picked up in the plant. Some plants remove this material by centrifugation through machines called clarifiers. The filter usually is located between the regenerator and the heater in order that the milk may be filtered when hot before pasteurization. An in-creasing number of plants are now filtering the milk while it is cold to preclude the breaking up of the foreign particles or their increased suspension.
Heaters. When the milk is pasteurized in single vats, the heating is usually done in the vat itself by means of steam or hot water in the jackets or rotating coils. However, in the multiple-vat system, the milk must be heated in a continuous stream because it is flowing continuously into the series of vats or pockets. Several types of heaters are on the market, but the ones in most common use are those known as the tubular heaters and plate heaters. Tubular heaters are built very similarly to the internal tubular regenerators (see page 116). Plate heaters look very much like plate and frame filter presses, and are built of hollow rectangular or square plates connected to manifolds so that the milk and the heating medium (steam or hot water) flow on opposite sides of alternate plates.
Fittings. Cold milk makes equipment greasy, and hot milk leaves a deposit of an insoluble, adhering, porous "milk stone," consisting of milk salts impregnated with organic matter including large numbers of bacteria. The grease can be fairly readily removed with hot water and an alkaline detergent, but the milk stone must be scrubbed off with a hot detergent solution and a stiff brush. The only way to clean milk pipes thoroughly is to make them demountable, and, every day, dismantle, and thoroughly scrub, cleanse, and sterilize the entire piping. Each piece is made in a relatively short length, seldom more than 10 feet long, to facilitate handling. All elbows, tees, and fittings are constructed so that every part of the inner walls can be seen. Couplings and unions are designed so that there are no corners, cracks, holes, cavities, or open seams where milk residue can accumulate. Long-handled brushes are provided for scrubbing the insides of all the piping. Troughs long enough to hold the longest pipe are installed so that every piece can be immersed, and scrubbed inside and out.
All valves and pumps are constructed so that they can be readily and completely dismantled. All filters, heaters, tanks, filling machines, and, in fact, every piece of equipment in a milk plant are made according to these principles of sanitary construction to facilitate cleansing and sterilizing.
Metals. The increasing attention which is being given to the flavor of milk and to the bacteriological cleanliness of the plant has directed attention to the materials of construction of dairy-plant equipment. Exposure of milk to copper or its alloys, or to iron, often imparts an off-flavor. When the coating of tin has worn off such metals, the milk deteriorates in quality. When the coating on glass enameled tanks cracks or chips off, it is difficult to clean the equipment satisfactorily. Conn et al.' found that 18 samples of uncontaminated raw milk contained 0.051-0.132 part per million of copper, averaging 0.077, and that 7 samples of pasteurized milk varied in copper content from 0.088 to 0.741 p.p.m. For a more complete discussion see Journal of Milk Technology, 1, September, 1938.
Bottles. The cleansing of milk bottles is now done almost exclusively by machinery. They are immersed in solutions of alkali varying in strength from about 0.5 percent to about 3 percent depending on local regulations, and soaked for several minutes at temperatures of about 150°-160° F. Sodium hydroxide is the most common base, supplemented with other chemicals according to the hardness of the water, as for example, sodium carbonate, trisodium phosphate, sodium meta-phosphate, and sodium metasilicate. The latter two chemicals are particularly effective in helping to remove deposits and make the bottle shiny. Many plants mix 1 part of sodium hydroxide and IA part of trisodium phosphate, made up to a 1.0 percent solution, and used at about 110° F. Others mix 3 parts of sodium hydroxide and 2 parts of sodium metasilicate, made up to a 1.0 percent solution of total alkali, and used at 140° F. The bottles are then rinsed with clean water and sterilized by steaming or by rinsing in chlorine solution of a strength of about 25-50 p.p.m. Inasmuch as many bottles have been utilized by the public to contain cement, paint, oil, and other sub-stances that are difficult to clean, it is necessary that every bottle be inspected for any residues of dirt as it leaves the bottle-washing machine.
The hot alkali may be in a succession of compartments whose temperatures are increased by steps up to the desired maximum in order to minimize bottle breakage. Various devices are employed to maintain effective concentration of the alkali and prevent its increasing dilution or decreasing strength during the cleansing operations.
An increasing number of dairies are using single-service milk containers. These are made from a fiber-board or cardboard base, and then dipped in hot paraffin at a temperature of about 185° F. This treatment moisture-proofs the containers, closes the seams, and imparts rigidity. Some types are assembled in the milk plants, whereas others are shipped already assembled. All the paper stock is made from clean virgin wood pulp, and the plant processes are under sanitary control. The use of these containers precludes the need of ex-pensive bottle-washing machinery in the milk plants, but the machinery to assemble the cardboard containers in the milk plant is also expensive. For full information on the technical, sanitary, and regulatory control aspects, see the papers in the Journal of Milk Technology, March and July, 1938, and January, 1939.
Capping. It is a general public-health requirement that all capping of milk bottles must be done by automatic machinery. The circular, cardboard discs are packed at the factory in cardboard tubes sealed at both ends. The operator at the capping machine tears off the closures and installs the tube on the capping machine. As each bottle of milk passes under the tube, a cap is automatically seated on the bottle. For those dairies that are too small to install an automatic capping machine, a small portable capper has been developed for cap-ping by hand. The operator passes from bottle to bottle, engages the capper over the mouth of each, and operates the release to seat the milk cap.
There is an increasing demand from the public that the lip of the milk bottle be protected from dirt and other contamination.' This has led to the development of protective caps of paper or metal, some of which cover just the rim and others extend down over the neck. Arnold has designed a rim which greatly reduces lip contamination.
Refrigeration. As soon as the milk is capped, it is rushed to the refrigerator rooms. Some plants give the empty milk bottles before filling an initial chill so that, when the cold milk is filled into them, there is no rise in temperature of the milk. Other plants spray a jet of cold water on the capped milk bottles. This serves the double purpose of washing off any milk splash or drip on the cap, and of cooling the package. Residual water which clings to the cap is blown off with an air jet. The bottles are then packed in crates, and stacked in the refrigerator with aisles left between the stacks to facilitate ready pas-sage of cold air.
Equipment cleansing and sterilizing. All the equipment in a milk-handling plant must be dismantled, and thoroughly cleansed and sterilized daily. It is first rinsed with lukewarm water to remove most of the residues of milk. The pipe is dismantled as described above. The large pieces of equipment, such as tanks, heaters, coolers, filters, and pumps, are cleaned by circulating a detergent solution through them to soften the accumulations of milk stone, and then vigorously scrubbing them with a stiff brush. A solution may be made of 2 parts of sodium carbonate and 1 part of trisodium phosphate made up to a 0.5 percent solution, and used at 120° F.; or a solution of sodium metasilicate made in 0.5 percent strength and used at 145° F. The equipment is then reassembled and rinsed with clean water. It is then sterilized by blowing steam through it for half an hour, or circulating a chlorine solution of 50-100 p.p.m. The next morning before operations begin, the assembled equipment is again sterilized, usually by circulating chlorine.
Milk cans must not only be thoroughly cleansed and sterilized, but also dried. It is important that they be designed so that all portions of the interior can be seen, thereby precluding the necessity of having to feel them with the hands to ascertain whether they are clean.
Delivery. During delivery to the consumers, milk must be protected from freezing in winter, and from warming up in summer. Freezing destroys the butterfat emulsion, so that the cream "oils off" when the milk thaws. When the milk warms, the residual bacterial flora multiplies, so that the milk becomes sour or develops other off-flavors. Also, the cream may form a "plug" or semi-plastic mass in the top of the bottle. All these changes can be prevented by proper handling of the milk.
RELATION TO THE PUBLIC HEALTH
Composition. The pasteurization of milk does not entail any appreciable change in the proximate composition. The butterfat, the carbohydrate, the protein, and the minerals will remain the same in percentage concentration. However, certain changes are brought about which are measurable by special methods. The milk, when heated, loses its content of carbon dioxide and other gases. Some of its vitamins are affected. Its minerals are changed from their original physical-chemical equilibrium to reduce their solubility slightly. Small traces of ammonia are formed. The enzymes are mostly inactivated, and the proteins assume a new form or condition. These changes are being lessened as pasteurization technology is improved.
Nutritional value. Some of these changes are nutritionally measurable, and some are not. None of them is practically significant from the standpoint of the actual use of such milk in human nutrition be-cause human beings are never isolated and fed a monotonous diet as test animals are. It is important to determine quantitatively the nutritional effects derived from different production and processing procedures, because it is by such means that improvements are made in the nutritive value of the milk and in dairy technology. All such improvements cumulatively result in a better product, but the significance of slight differences may be completely masked by dietary customs. For example, raw milk contains so small an amount of vitamin C that sound pediatric practice supplements milk with some really effective source of this vitamin, such as orange juice.
A carefully conducted laboratory study of the influence of the commercial pasteurization of milk on its nutritive properties for rats was conducted by Henry, Kon, and their associates? The milk was fed raw and pasteurized from the same batch. It was processed commercially in a plate holder at 142° to 150° F. and held for 28 to 44 minutes. The pasteurized milk was negative to the phosphatase test, and contained a bacterial count ranging from 460 to 44,000 organisms per milliliter. The milk contained 3.9 percent fat and 12.7 percent total solids. In brief, the rats assimilated about 80 percent of the calcium and phosphorus for both the raw and pasteurized milks, and the retention of these elements was not differentially affected. There was no effect on the digestibility or biological proteins of the two milks. No vitamin A or carotene was destroyed by pasteurization. Some of the vitamin B complex was destroyed, but its extent was not measured. Vitamin C was decreased by pasteurization to the extent of about 21 percent. When judged by any or all of the standards employed, namely, gain in weight, body length, composition of carcasses, and appetite, the raw and pasteurized milks had the same nutritive value.
These results on experimental animals were in agreement with the results of large-scale feeding tests on children.
A large-scale research was conducted on 8435 school children living at home .8 They were divided into four groups: one group of 2000 for control, one group on 1/3 pint of pasteurized milk, another group on % pint of pasteurized milk, and one on % pint of raw milk. The home diets of the children were supplemented at school with biscuits (as controls) and the above portions of milk. No supervision was exercised over the home diets. After one and one-half years, the results showed that the children at each age fed with 1/3 pint of pasteurized milk gained more in weight than those fed with biscuits, and probably—but rather less consistently—gained more in height. When the groups were compared on the basis of the clinical grades of excellent, normal, and poor, the children fed the milk supplements showed higher rates of growth than those in the corresponding clinical grade with the biscuit supplement. No consistent difference was observed between those fed % pint of pasteurized milk as compared with those fed the same amount of raw milk.
Frank ° studied the results from feeding more than 3700 children. One group had received raw milk for more than half their lives, including at least the latter half, and the children in the heated-milk group had used pasteurized milk, boiled milk, evaporated milk, and milk powder. The average weight of the children on raw milk was 33.2 pounds, and on heated milk 33.6 pounds. Their average heights were 37.4 inches and 37.5 inches, respectively. Their rates of growth were substantially the same.
Studies by numerous investigators on the effect of heat treatment on the nutritive properties of milk have been critically reviewed by Stirling and Blackwood, Hannah Dairy Research Institute, Bul. 5, 1933.
McCollum has summarized the situation very well 10 by pointing out that, since the effect of pasteurization on the food value of milk is too slight to be apparent in observation on children living under ordinary American conditions, there is no valid argument which can support the marketing of raw milk instead of pasteurized milk for the general population. It seems strange indeed that, when we accept so generally the cooking of most of our foods, there should still remain in certain areas a serious objection to the mild treatment involved in pasteurization.
Relation of pasteurization to clean milk. The observation that pasteurization prolonged the commercial life of milk by delaying spoilage led to early ideas by some people that care in the production of clean milk was not necessary. It was argued that pasteurization would kill the germs which cause spoilage, and thereby preclude the need of keeping these microorganisms out of the milk. It was further said that cleanliness was expensive and this cost might be lessened. As a matter of fact, it is common experience that, where the pasteurization of milk is most skilfully and energetically practiced, the greater is the insistence on a clean raw supply. The deteriorative changes that have occurred in an unclean milk by reason of the effects of its bacterial content cannot be neutralized or volatilized or other-wise corrected merely by pasteurization. From the discussion of these changes (see page 82), it was seen that the deleterious effects of bacterial metabolism produced in a raw milk remained after this milk had been pasteurized . Moreover, it is known that, the greater the original bacterial load, the greater is the difficulty of pasteurizing, and the greater is the final bacterial count, because the destruction of micro-organisms by heat is a logarithmic function. Off-flavors may persist, and render the milk undesirable organoleptically, thereby lowering its commercial value.
Microbiology. Pasteurization should not be confused with sterilization. Pasteurized milk contains a mixed flora of numerous types of microorganisms which were not killed at the pasteurization temperatures. These surviving organisms are not pathogenic but are pre-dominantly those which produce lactic acid and, when present in enormous numbers, will curdle or "sour" the milk. In most communities, the local ordinances set a maximum limit of 50,000 colonies per milliliter, but, as a matter of fact, the bulk of the bottled pasteurized milk actually contains much less.
The bacteriological examination of pasteurized milk reveals a great deal about the history of the milk, for example: whether it came from cows with infected udders, or was handled in dirty equipment, or was inadequately refrigerated, or was improperly pasteurized, or was not handled properly after it was pasteurized. It constitutes a most useful tool for the determination of the sanitary quality of the milk.
The determination of organisms of the Escherichia-Aerobacter group (sometimes called the coliform bacteria) can be utilized to good advantage in supervising the sanitary quality of a milk supply. How-ever, great care and discretion are required in the interpretation of coliform determinations. There is a tendency to give the same significance to these organisms in milk as is given to them in water. This is erroneous. These organisms do not grow in water, and so their quantitative determination in water serves as a useful index of its pollution. On the other hand, these organisms proliferate rapidly in milk, so that a high count may mean either gross contamination (similar to pollution) or merely lack of proper refrigeration (with attendant growth). Their presence in pasteurized milk is also complicated by the fact that some strains are heat resistant to the ordinary temperatures of pasteurization. This fact alone precludes the adoption of a requirement that pasteurized milk must be free from these organisms. However, the determination is valuable if samples of the same milk, taken immediately out of the pasteurizing tank, are found to contain no coliform organisms, because, if these organisms are then found in the bottled milk, it is apparent that there is contamination of the milk between the pasteurizing and the capping stations. In other words, the presence of coliform bacteria in bottled milk has significance only when the same milk supply has none at the pasteurizer.
Sometimes a bottled pasteurized milk supply becomes contaminated with bacteria which are not killed but may actually grow at pasteurizing temperatures. These are called thermoduric and thermophilic bacteria, respectively, and have the appearance of "pin-point" colonies on standard agar plates. They have no public-health significance from the standpoint of pathogenicity, but they do concern the quality-control officer because their presence indicates that some of the raw supply is contaminated, or that some insanitary plant practice or some faulty laboratory technic is being followed. The confirmation by Ward and Meyers of Breed's discovery that bacteria killed by the heat treatment do not stain well with methylene blue 13 affords a useful method for determining the extent of thermophilic contamination of pasteurized milk. The presence of these organisms is revealed by differential staining and direct microscopic examination 14
When milk is examined under the microscope, it is observed that, in addition to the bacteria present, there is a greater or less number of leucocyte cells (white blood corpuscles). These are a normal product of the wastage from the active gland and, when present in limited numbers, have no sanitary significance. However, when the udder becomes inflamed and infected with streptococci, the number of leucocyte cells may greatly increase. Although they may be present to the extent of millions per milliliter in milk that is apparently normal from ordinary bacteriological and chemical examination, it is generally considered that a number in excess of 500,000 per milliliter indicates possibly a pathological condition that needs further investigation and correction.
Present public-health significance. Many different persons handle the milk between the time when it leaves the cow and when it reaches the consumer's doorstep, and every such handling increases the possibilities of contaminating the milk. If the milk supply of a large dairy should become infected with pathogenic bacteria and they were allowed to multiply, the widespread distribution of this milk would cause a disastrous epidemic. The extent of such a catastrophe can be visualized in the light of the Montreal outbreak of milk-borne typhoid fever, involving more than 5000 cases, caused by a typhoid carrier in a milk plant. The increasing potentiality of such a public-health hazard has led the public to demand appropriate safeguards. For milk, this is effective pasteurization under official supervision.
Epidemiology. The effectiveness of pasteurization to make an infected milk supply reasonably safe for human consumption is indicated by the several instances where split portions of the infected supplies were consumed raw, and pasteurized respectively, by different groups of people. Epidemics of typhoid fever were traced to the unpasteurized part of the supply whereas that part which was pasteurized was consumed without mishap. Several such instances were recorded by Armstrong and Parran.
Some idea as to what pasteurization saves us from is furnished by reports on several milk-borne epidemics by Brooks. An outbreak of 33 cases of septic sore throat was traced to the consumption of raw milk taken by employees from a supply entering a pasteurization plant, although no such illness occurred among the consumers of the pasteurized milk of this firm. The epidemic was traced to a cow whose udder was infected with hemolytic streptococci. In another outbreak, a milk tank truck (on its way to a pasteurization plant) was wrecked, and 53 persons who salvaged the milk and used it developed gastroenteritis. Again, a can of milk, rejected because of high temperature at a shipping station, was taken home by the trucker. The family made ice cream mix with 6 quarts of pasteurized milk and other ingredients, and then boiled the mixture. Just before freezing, they added raw cream from the returned milk'. Every one who ate the ice cream developed sore throat, one with scarlet fever and several with atypical rashes. Experience in New York State for fifteen years revealed no outbreaks traceable to pasteurized milk, although the process itself has often been found to be improperly practiced. This indicates that the standard of pasteurization procedure possesses a considerable margin of safety.
In recent successive years, the Office of Milk Investigations of the U. S. Public Health Service has conducted an annual questionnaire survey to collect information on all outbreaks of disease traced to dairy products as reported by state and city health departments. These data are summarized in Table V.
In the larger series of Armstrong and Parran, the number of out-breaks from allegedly pasteurized milk constituted about 3.7 percent of the total number. In the above smaller but more recent series, this percentage is 5.4. When one considers that the large urban communities are well protected by pasteurization whereas the remaining 50 percent of the population resides in the smaller towns and rural communities which mostly do not have the protection that milk pasteurization affords, the above figures indicate the importance of pasteurization as a health measure.
In spite of the public-health safeguards of the pasteurization of milk, it is a matter of record that a number of epidemics have been traced to milk that was processed in pasteurizing plants. Armstrong and Parran 18 report that, during the period 1907 to 1926, there were 29 such outbreaks, most of which were traced to some faulty plant operation or to some subsequent infection. If the pasteurizing machinery is not designed to impart the proper temperatures and holding period to every drop of the milk, some inadequately processed milk may infect an entire batch. Minute leaks of raw milk may also contaminate the pasteurized product. In some plants where the milk-line valves are hand operated, mistakes have been made whereby the raw supply was run into the pasteurized product, and an epidemic resulted. Carriers of disease germs and dirty bottles have infected milk which had been properly pasteurized. The pasteurization process is no safer than the proper design of the equipment, its intelligent operation, and the protection of the milk from subsequent infection and deterioration. Milk, after pasteurization, is just as good a medium for the growth of microorganisms as it was before it was pasteurized. Its maintenance in a safe condition after this processing requires the utmost care to protect it from recontamination.
Standards and regulations. When a foodstuff is produced or handled under a permit system, its compliance with regulatory standards depends on the extent to which the operating conditions, as well as the composition of the product itself, comply with the official regulations and standards. Inasmuch as the pasteurization of milk is under the permit system, it is necessary that a milk business comply with the detailed specifications of plant equipment and operation, as well as that the milk itself comply with composition standards.
Standard. The U. S. Department of Agriculture uses the following definition for pasteurized milk:
PASTEURIZED MILK. Milk every particle of which has been subjected to a temperature not lower than 142° F. for not less than 30 minutes and then promptly cooled to 50° F. or lower.
The Public Health Service Milk Ordinance and code: 1935, defines pasteurization as follows:
The terms "pasteurization," "pasteurized," and similar terms shall be taken to refer to the process of heating every particle of milk or milk products to a temperature of not less than 142° F., and holding it at such temperature for not less than 30 minutes in approved pasteurization apparatus, provided that approval shall be limited to apparatus which requires a combined holder and indicating thermometer temperature tolerance of not more than 1˝° F., as shown by official tests with suitable testing equipment and provided that such apparatus shall be properly operated and that the indicating thermometers and the recording thermometer charts both indicate a temperature of not less than 143 1/2° F. continuously throughout the holding period. The terms "pasteurization" . shall also include the process of heating every particle of milk or milk products to 160° F., and holding at that temperature or above for not less than 15 seconds in apparatus of approved design, and properly operated... .
Specifications for operation and equipment. In order that all pathogenic organisms be killed, it is essential that every particle of the milk be given the full heat treatment. The overheating of some of the milk will not compensate for the underheating of another part of the milk. An average heat treatment is not adequate.
As the result of many carefully conducted laboratory, plant, and epidemiological studies on what constitutes a heat treatment which is adequate to kill all pathogenic organisms and leave a reasonable margin of safety, it has been concluded that the heating of the milk to a temperature of 142° F. and holding it there for 30 minutes is adequate to make the milk safe without impairing its food value or palatability. Inasmuch as the uniform transfer of heat from a gas (steam) to a liquid (milk) is very difficult to control within narrow limits, it has been found that, in the most carefully constructed equipment, there may be a maximum temperature difference of about 1° F. between the hottest and coldest portions of the milk. Moreover, thermometers may have a maximum error of about 1/2° F. The total error may be cumulative. Hence, in order to insure adequate heat treatment, it is assumed that there may be a difference of 1 1/2° F. between the indicated temperature and the actual temperature. This has led to the adoption of 143 1/2° F. as the minimum temperature which the plant thermometer must register to insure proper pasteurization. Automatic temperature control and recording thermometers are required on all pasteurizing and cooling equipment.
Specifications for satisfactory plant equipment and directions for its operation have been written up by the U. S. Public Health Service. Directions for equipping and operating a small pasteurizing plant have been published by the U. S. Department of Agriculture.
Permits. Before anyone may operate a milk business, he must apply for a permit from the proper official agency, usually the health department. If an official inspection of the plant reveals that it complies with the local requirements, the application is approved, and the dealer is given a permit number which is usually required to be displayed on his vehicles. All the farmers who ship milk to him must either be licensed directly by the local health department or must regularly be inspected by the quality-control officers of the regularly licensed plants to which they ship. (See page 71 for more extended discussion.)
Grading provisions. An increasing number of communities are adopting the principle of milk grading. This necessarily requires that the several grades be kept separate in the plant. In some communities, a whole plant may be required to handle only one grade. If a plant is degraded from a Grade A level to a Grade B level, all the milk handled in that plant must be labeled Grade B milk. (See page 91 for more extended discussion.)
Protection from contamination. A milk-pasteurizing plant may be sanitary in its construction and equipped with the most modern designs of milk-handling machinery, but, unless the plant is operated with a high degree of public-health intelligence, there is a possibility that all the effort and expense may be nullified.
The dripping of condensed moisture from damp ceilings, sweating pipes, and leaky overhead drains, and the inevitable leaks from milk and water lines through bruised pipe connections and valves, may seriously contaminate a milk supply. When a drop falls, it splashes droplets in all directions, thereby infecting all equipment and products over a relatively large area. Floor splash may infect the milk trough of the pasteurizer or cooler, or the open bottles between the bottle-washer and the milk-capper. Flies, coughing of operators, and handling of equipment by the workmen are common sources of infection unless all the milk is kept covered.
Water. The water supply must be clean and pure. This means that the water must be free from dirt, sediment, scale, rust, muck, ash, cinder, coal dust, and all such physical impurity. Of course it must be potable and free from all pollution. Particularly hazardous are cross connections to sewerage systems 22 or to polluted wells (see page 59). A hard water should be treated to soften it in order to reduce scale on the plant and to facilitate the effectiveness of the cleaning operations. Scale is a source of contamination because it prevents equipment from being properly cleansed and sterilized, and accordingly harbors many resistant microorganisms. Furthermore, it acts as insulation to reduce the proper heat exchange, and therefore may seriously affect proper pasteurization and cooling. Dug wells on the premises may be potential hazards, and the water should be examined frequently.
Operators. There is no agreement among health officers as to just what items should be included in the supervision of the health of employees (see page 130). In some communities it is required that a milk-plant operator must be in good health as determined by a physician. The extent of the examination is generally left to the discretion of the examining physician and may range from a perfunctory inquiry as to how the applicant feels, all the way up to a careful and detailed clinical and laboratory examination.
The clinical examination should include a search for signs of tuberculosis, primary lesions of syphilis, and a history of typhoid fever, together with any other ailment or deformity which the examining physician considers to constitute a health hazard. Laboratory examination always requires the submission of stools and sometimes urines for examination for typhoid and paratyphoid fever organisms. Sometimes throat and nasal swabs are required for the detection of diphtheria germs and occasionally for the beta hemolytic streptococci of septic sore throat. There is a growing tendency to limit the laboratory examination to the stools to detect typhoid carriers. All these tests have frequently been required once a year or oftener for each employee, but they are now usually required only at the beginning of employment. Moreover, any worker who has a sore throat or diarrhea should be excluded from the milk handling, bottling and capping operations. It is desirable in the interest of safety that the operators at these stations be given as adequate a medical supervision as that afforded the workers in a certified-milk dairy (see page 103).
Freedom from infection. Human operators. When the milk-pasteurization plant is well laid out and properly equipped, all the milk is required to be in sanitary piping, covered tanks, protected coolers, and automatic filling and capping machines. However, it is necessary for a group of workers to handle the clean bottles, and to adjust the fillers and cappers. These operators have to work in restricted quarters in close proximity to the clean bottles as they are conveyed to the filling machines, and also to the milk-filled bottles as they travel to the capping machines. These people may contaminate the clean empty bottles or infect the open bottled milk. To reduce this possibility, shields are placed over the bottle conveyor belts, and tight covers over the filling machines.
Bottles. The milk bottle has long been considered a potential health hazard. Formerly, no bottles were allowed to be removed from quarantined premises until a health officer had disinfected them. It is now recognized that this precaution is not necessary. The development of automatic machinery for cleaning bottles and treating them with germicidal agents such as hot water, steam, and chlorine removes any hazard that may exist. However, it is very important that the bottle-washing equipment be adjusted to operate effectively. All these machines use a caustic solution as a detergent, and follow this up with a scalding treatment which may be superseded or followed by a chlorine treatment. Some health officers require that a milk bottle shall contain no more than one organism for each milliliter of bottle capacity (about 500 for a pint bottle and about 1000 bacteria for a quart bottle), but many dairies consistently operate with counts ranging from 0 to 50 for the entire bottle.
No appreciable public-health end is served by substituting the cardboard container for the glass bottle. Neither one is sterile. The relatively few microorganisms that are found on both kinds of containers are non-pathogenic. See also discussion on page 119.
Toilets. Every milk plant should be required to provide adequate toilet facilities for its help. This should consist not only of the requisite number of urinals and seats but also provision for washing the hands with soap. No toilet should open directly into the plant but should be separated by the wash room and a vestibule. Signs should be prominently displayed to remind the employee to wash his hands before returning to work.
Screens must be installed to keep out flies. Doors must be self-closing. Ventilation must be adequate. Floors of toilet rooms must not drain to the floor of the milk plant but be directly connected to a sanitary sewer. Light must be adequate to facilitate cleaning. In addition to these requirements, the toilets must, of course, comply with the local sanitary code.
Loose milk. This term is applied to milk that is dispensed directly from bulk containers. In practice, the milk is sold over the retailer's counter by dipping it out of a milk can or by drawing it from an urn. Several factors operate to render this practice unsanitary or even hazardous. The cream soon rises to the top with consequent deprivation of this valuable constituent from purchasers who happen to be served from the bottom of the container. Drippings from the dispenser's hand contaminate the milk. Its sale is gradually being prohibited. For full discussion of this subject the reader is referred to the admirable report of the New York Milk Commission.
Types of violation and adulteration encountered. A proper super-vision of the milk industry is a far more involved procedure than that of any other food product because, in addition to the chemical, physical, and microbiological control of the quality of the products them-selves, there is a vast and complicated regulatory structure of permit provisions, sanitary requirements, equipment specifications, production and operating demands, and medical examinations. The control officers and the quality-control staffs of the operating companies re-quire compliance with all these regulations and specifications. These are so multitudinous, often contradictory in neighboring communities, and sometimes meticulous in their detail that continual compliance with all of them is a huge task for the operators to meet or the inspectors to enforce. Violation of any of them constitutes the basis for regulatory action. Milk-control officers probably spend more time in enforcing the laws, rules, and regulations which cover the production and handling of milk than they do in the examination of the product itself.
In addition to these items which can be controlled only through inspections of the premises and records, the milk itself may have been imperfectly pasteurized, or it may have been inadequately refrigerated; it may contain added water; it may be low in butterfat; it may exceed the prescribed limit in numbers of bacteria; it may carry much sediment, or possess an off-flavor, or exhibit abnormalities in composition or appearance from some pathological condition of the cow.
Physical tests. Temperature. Pasteurized milk in the bottles should be examined for temperature while it is in the plant, during delivery on the wagons, and in the retail stores. Temperatures are taken with ordinary mercury-bulb thermometers, usually of the small pocket type.
Organoleptic examination. All milk should be examined for taste, flavor, and appearance. The bottle itself should be clean, properly capped, and free from sediment. Dirty milk can be detected by means of the sediment test (see page 93).
Chemical tests. Butterfat. Samples of bottled pasteurized milk should be regularly analyzed to check compliance with claims. This is determined by the official Roese-Gottlieb method or the Babcock method (see page 93).
Added water. Occasionally water may contaminate bottled milk, especially in the milk from the early part of the run. The cryoscope method is commonly used for this test (see page 94).
Heat treatment. Several tests to ascertain whether a given sample has been properly pasteurized depend on color reactions caused by the action of selective enzymes which are thermosensitive near the pasteurization temperature. The most widely and effectively used of these is the phosphatase test. This technic is based on the reaction of the enzyme phosphatase in milk to liberate phenol from phosphoric phenyl esters. Substantial absence of this enzyme from pasteurized milk indicates that the milk had been properly processed, whereas the amounts of enzyme remaining in a heated sample are proportional to the degree of underheating (in time or temperature) or the admixture of raw milk. Any such residual amount of enzyme liberates a corresponding amount of phenol which is measured colorimetrically, Over all ranges of temperatures and times, Mycobacterium tuberculosis is destroyed more quickly than phosphatase, so that a heat treatment adequate to inactivate the enzyme likewise kills this organism and all the other common pathogenic bacteria.
Since the first publication of this test by Kay and Graham, several modifications of the technic have been devised. All of them are being used by regulatory and industrial quality control officials, but information as to their relative accuracy, dependability, and utility is only beginning to make its appearance. According to the Kay and Graham technic, developed in England, a sample of milk that gives a blue color less than 2.3 units on the Lovibond scale is properly pasteurized by heating at a temperature of 145° F. for 30 minutes 25 The official method of the Association of Official Agricultural Chemists (devised by Gilcreas) is based on the number of milligrams of phenol liberated by the sample of milk and is measured colorimetrically. A phenol value of 0.05 milligram per 0.5 milliliter of milk generally indicates that the milk had been heated to 143° F. for 30 minutes. The New York Health Department test (devised by Scharer) 27 defines a unit of blue as the amount of color produced by 0.001 milligram (1 gamma) of phenol under the conditions of the test (corresponding to about 0.45 milliliter of milk) or 0.2 part per million (ppm) of phenol. A phenol value of 2 units or less generally indicates that the milk had been heated to 143° F. for 30 minutes. Leahy's procedure combines elements of the above technics, using a standard of 0.01 milligram of phenol. Values in excess of these several standards respectively indicate that the milk was either not properly pasteurized or was contaminated with raw milk subsequent to pasteurization. For detailed and conveniently arranged information concerning the different technics employed in making this test, see the discussion in Standard Methods for the Examination of Dairy Products, 7th edition, American Public Health Association, New York, 1939. The method most widely used in this country is the Scharer technic. All the above procedures are sensitive enough to detect variations of 5 minutes in the heating time, a drop of 1° in the holding time, and the admixture of 0.1 percent of raw milk to properly pasteurized milk.
Microbiological tests. Total colony counts and also the presence of coliform organisms are determined by the standard methods (see page 94). The presence of thermophilic and thermoduric organisms in a bottled-milk supply can be demonstrated by using special bacteriological culture media and incubation temperatures (see page 97). The direct microscopic examination by the Breed technic is very useful to show bacteriological conditions which the agar plate method does not indicate 30 By a differential staining technic, it is possible to distinguish between living and dead bacteria.
Control procedure. The control procedure consists of three general practices, namely, the administration of the permit or grading system, the inspection of the plant operations, and the examination of samples of the milk.
Administration of the permit and grading systems. When inspection of the plant and its operations reveals that the regulations have been violated, a notice in writing is served on the management citing the specific offense and setting a time within which compliance must be effected. If a re-inspection discloses that the violation still exists, the management may be cited to show cause why the permit to operate should not be revoked.
Under the grading system, the finding of persistent violation may result in the degrading of the plant, whereby the milk must be labeled to designate the next lower grade. Correction of the infraction leads to restoration of the plant and likewise the milk to their original grades.
Inspection of the operations. The only way to ascertain whether a milk business is complying with the law is to visit the plant and examine it from top to bottom, inside and outside; to inspect its milk supply coming from the farmers and creameries (receiving stations), and during delivery on the wagons; and then to take samples of the milk as it is being delivered. By checking performance against the regulations, the infractions are revealed. Some milk may be received at the plant from non-inspected or non-approved sources. Any of the regulations for equipment may be violated, as, for example: fly screens may be broken with an attendant large fly infestation; pipe lines may not have been daily dismantled and cleansed; equipment may not be clean and sterile: cleanliness in locker rooms, toilets, stair-ways, and yard may have been neglected; pasteurization may have been improperly practiced by maladjustment of valves or temperature control; insufficient cooling may have resulted from inadequate refrigeration facilities; temperature-recording charts may have been incorrectly dated or not kept available for inspection or they may show insufficient heat treatment; grading distinctions may have been neglected; bottle-washing equipment may have been improperly operating through insufficient alkali strength, or plugged steam or water jets; bottles and cans may not be cleansed thoroughly; employees may not have been medically examined; and the plant may show signs of neglect in regular cleaning.
The most important inspection of all is the examination of the pasteurization process itself. The correct temperature reading on an indicating thermometer whose bulb is immersed in the milk in a pasteurizing vat does not necessarily mean that all parts of the batch of milk are really at that temperature. Differences in temperature may exist in different parts of the same vat. The first means to insure proper temperature treatment is to require specifications of equipment design, installation, and operations such as those written up in the U. S. Health Public Health Service Milk Ordinance and Code. The second method is to lower a standard maximum self-registering test thermometer, or insert electric thermocouples or resistance thermometers in the milk while it is being pasteurized, taking readings in different parts of the compartment. Thermocouples can be inserted inside a covered milk compartment and read outside. The check-up on the holding time of pasteurization equipment which holds successive batches is made by simply noting with a watch the time that the last milk enters and the first milk leaves. In types of pasteurization equipment where the milk is continuously flowing through the machine and the holding time is dependent on the lapse of time between the entry and exit, the measurement is made by injecting certain dyes (such as uranine) or a starch solution or even a suspension of harmless bacteria (such as coliform organisms) at the inlet of the holder, and noting the exact time that the indicator reaches the outlet. The sensitivity of thermostatic control of automatic flow-diversion valves of the milk stream in high-short pasteurization systems must be determined in operation. Milk-flow stops on continuous heaters must be checked. Indicating and recording thermometers must be dismantled and tested for accuracy and response.
Examination of milk samples. Milk samples may be taken on the receiving platform, in the various parts of the plant, or during delivery. Analysis reveals whether the milk itself is up to the legal standard in composition and in bacterial content. It may also re-veal whether the pasteurization operations have been properly practiced. The phosphatase test is very useful to determine whether a sample has been properly pasteurized (see page 132).
High bacteria counts indicate faulty operation, contamination after pasteurization, or inadequate refrigeration of the pasteurized milk. In general, experience has taught that the average milk supply carries a microbic flora which can be decreased about 98-99.5 percent as determined by the standard plate technic. Therefore, if a pasteurized-milk plate count is more than 1 or 2 per cent of the plate count of the original milk before it was pasteurized, there is likelihood that a badly contaminated milk supply is coming into the plant, or that plant operation or subsequent handling is faulty. It is possible that there are one or more producers whose milk supplies have become infected with a heat-resistant strain of organisms. The way to remedy such a situation is to locate this undesirable supply by pasteurizing each producer's milk in test tubes in the laboratory, and plating samples before and after this treatment. The amount of work can be decreased by grouping the producers in lots of 10, and then examining the milk of each producer in a given lot which is not reduced in count.
It is occasionally found that milk has a satisfactory count when it is sampled immediately after it has been pasteurized but that the bottled milk has a high count. This indicates that some contamination has occurred along the line' between these stations, or that the milk has not been adequately refrigerated. Samples are taken between the successive stations such as the filter (if used here), the regenerator, the cooler, the bottler, and the various pumps. This is called a line test because samples are taken all along the line of processing.
The conditions under which milk is held in the plant after bottling, during transportation and delivery, and also in the retail stores should be inspected, especially for adequacy of refrigeration and cleanliness.
The determination of coliform organisms (more or less loosely called B. coli) in pasteurized milk is useful. An occasional positive tube when inoculated from a 1-milliliter sample may simply have been one which survived pasteurization, but if more than 10 to 20 percent of 1-milliliter samples of freshly bottled pasteurized milk are positive, possible contamination of the milk after its pasteurization or its incubation is indicated.
1. S. H. AYRF.S, U. S. Dept. Agr. Bul. 342, rev. June, 1932.
2. W. H. PARK, Am. J. Pub. Health, 17, 36 (1927).
3. P. F. KRUEGER, J. Milk Technol., 1 (7) 29 (1938).
4. L. W. CoNN et al., Ind. Eng. Chem., Anal. Ed., 7, 15 (1935).
5. J. M. SANSBY, Arch. Pediat., 55, 22 (1938).
6. L. ARNOLD, J. Milk Technol., 1 (6) 5 (1938).
7. K. M. HENRY, S. K. KoN, and associates, Natl. Inst. Research in Dairying, Aberdeen, 1937. Quoted from J. Dairy Sci., 20, 171, Abs. (1937).
8. Editorial, Am. J. Pub. Health, 28, 1424 (1938).
9. L. C. FRANK et al., Pub. Health Repts., 47, 1951 (1932).
10. E. V. MCCoLLUM, Am. J. Pub. Health, 24, 956 (1934).
11. C. N. STARK, 9th Ann. Rept. N. Y. State Assoc. Dairy & Milk Inspectors, 1935, p. 147; J. Dairy Sci., 19, 495 (1936).
12. P. A. HANSEN, N. Y. State Agr. Exp. Sta. Tech. Bul. 196, 1932.
13. A. R. WARD and C. E. MYERS, Am. J. Pub. Health, 27, 899 (1937).
14. GEORGE KNAYSI and M. FORD, J. Dairy Sci., 21, 129 (1938).
15. R. S. BREED, N. Y. State Agr. Exp. Sta. Bul. 568, 1929.
16. H. D. PEASE, Am. J. Pub. Health, 22, 654 (1932).
17. The Production and Handling of Milk as It Affects Public Health, N. Y. State Dept. Health, Albany, N. Y.; see also: Am. J. Pub. Health, 14, 1069 (1924), and Proc. 44th Ann. Meeting Conference State and Provincial Health Officers of N. America, 69, 1934.
18. C. ARMSTRONG and T. PARRAN, JR., Supplement to Pub. Health Repts. 62, 1927.
18a. P. B. BROOKS, J. Milk Technol., 2, 168 (1939).
19. "Definitions and Standards for Food Products for Use in Enforcing the Food and Drugs Act," S. R. A. Food and Drug 2, 5th rev., November, 1936.
20. "Public Health Service Milk Ordinance and Code: 1935," U. S. Public Health Service Bul. 220, 1935.
21. F. M. GRANT and C. E. CLEMENT, U. S. Dept. Agr. Circular 214, 1932.
22. W. S. JOHNSON, Am. J. Pub. Health, 26, 229 (1936) ; see also J. Milk Technol., 1 (3) 3 (1938).
23. Is Loose Milk a Health Hazard? Milk Commission, Department of Health, New York City, 1931.
24. H. D. KAY and W. R. GRAHAM, JR., J. Dairy Research, 5, 54-62, 63-74 (1933); 6, 191-203 (1935).
25. H. D. KAY and F. K. NEAVE, Dairy Industries, January, 1937.
26. F. W. GrLCREAS, J. Assoc. Official Agr. Chem., 21, 372-378 (1938). See also F. W. GILCREAS and W. S. DAVIS, Proc. Internat. Assoc. Milk Sanitarians, 1936, pp. 15-19.
27. H. SCHARER, J. Dairy Sci., 21, 21-34 (1938); J. Milk Technol., 1, No. 5, 35-38 (1938); ibid., 2, 16-20 (1939).
28. H. W. LEAHY, J. Bact., 36, 670 (1938).
29. L. H. BuRGWALD and E. M. GIRERSON, J. Milk Technol., 1, No. 7, 11-24 (1938); W. A. HoY and F. K. NEAVE, Lancet, Sept 4, 1937, p. 595; W. D. TIEDEMAN, Amer. J. Pub. Health, 28, 316-319 (1938) ; F. W. GrLCREAS, Reference No. 26; D. M. ROGER, J. Milk Technol., 2, 21-25 (1939) ; A. J. HAHN and P. H. TRACY, J. Dairy Sci., 22, 191-200 (1939) ; H. SCHARER, Reference No. 27; H. W. LEAHY, J. Milk Technol., 2, 147-151 (1939).
30. Standard Methods of Milk Analysis, 6th ed., Am. Pub. Health Assoc., New York, 1934, p. 36; Standard Methods for the Examination of Dairy Products, 7th ed., Am. Pub. Health Assoc., New York, 1939; W. H. CHILSON, M. W. YALE, and R. EGLINTON, J. Dairy Sci., 19, 337 (1936).