Jo u r n a l o f M ilk T e c h n o l o g y 11

Engineering of Pasteurization * Λ · , C. A. Holmquist and W. D. Tiedeman

Director, Bureau of Sanitation; Chief, Bureau of M ilk Sanitation; State Department of Health, Albany, N. Y. Downloaded from http://meridian.allenpress.com/jfp/article-pdf/1/1/11/2392536/0022-2747_1_1_11.pdf by guest on 02 October 2021

The principle of pasteurization was de­ pasteurizing equipment requires a knowl­ veloped by Pasteur to meet a . practical edge of physics, hydraulics, mechanics, need in the beverage industry, namely, sanitation, ‘electricity, metal processing, to check the growth of organisms causing strengths and qualities of materials, in undersirable fermentation in wines. The addition to an understanding of the effect commercial application of the theory of of heat on bacteria in milk and on the heat treatment to wine was fairly simple. physical and chemical properties of milk. With the application of this principle to Perhaps it is to be expected that the milk and milk products came new prob­ first pasteurizers, like the first efforts along lems. other lines, should fall far short of accom­ The bacteriologists soon established plishing what we recognize as proper points on the thermal death curves of pasteurization today. There were two various pathogenic organisms which may general faults: first and most important, be found in milk. The effects of various faulty design and construction leaving ave­ degrees of heat on the Creaming of milk, nues for viable pathogenic bacteria to get on albumin, fat, sugar, casein, enzymes, through and second, faulty design and and taste were also determined. It soon construction causing scorching of portions appeared that there was a range within of the milk, damaging cream line, encour­ which the temperature of pasteurization aging the development of thermophylic must fall in order to secure enough heat bacteria, and other undesirable effects. treatment to destroy any pathogenic bac­ The last mentioned faults are of greater teria which might chance to be present health significance than might first appear and yet not enough to damage the physi­ because when both raw and pasteur­ cal or chemical properties of milk. This ized milk are available to the public, range or zone is shown on North’s consumers have turned to the use of raw Curves, with which you are undoubtedly milk when the flavor of the pasteurized familiar. milk was objectionable for one reason or The application of the principle of another. The interesting point is that the pasteurization to milk and milk products application of sound engineering princi­ on a commercial scale was first done by ples to pasteurizing equipment has resulted operators who apparently did not grasp in replacing the defective equipment so the fundamental technological principles commonly used during the first and second involved, if such principles were given decades of the commercial pasteurization any thought. After participating in one of milk with equipment that will of the first engineering studies of pasteur­ "pasteurize” milk without damaging izing equipment, Phelps said ". . . . the flavor, creaming or any other important lack of sound engineering in a distinctly property. By the term "pasteurize” as engineering field has been conspicuous just used is meant subjecting every particle and rather deplorably so.” of milk to a temperature of 143 °F. or The proper design and construction of more for not less than. 30 minutes, or to a temperature of 160°F. or more for not * Delivered at the Food Technology Conference of the Department of Biology & Public Health, Massa­ less than 15 seconds in pasteurizing equip­ chusetts Institute of Technology, Boston, September 14, 1937. ment of an approved type. The approval 12 Jo u r n a l o f M il k T e c h n o l o g y of types of pasteurizers has become an mi;tal scale had slipped and no etched important function of health departments. mpk on the tube for use in resetting the The first comprehensive engineering sqile, (9) leaks in heat exchangers per- tests of commercial pasteurizers were car­ mjtting unpasteurized milk to· enter the ried on .at Endicott, New York, in con­ pasteurized supply, (10) continuous flow junction with bacteriological tests by pasteurizers designed upon the fallacious Phelps, North, et al, during a period of tbpory that actual holding time would about a year and a half, beginning in equal the theoretical and (11) inaccurate December 1921. The results were re­ temperature control and lack ·οί protec­ Downloaded from http://meridian.allenpress.com/jfp/article-pdf/1/1/11/2392536/0022-2747_1_1_11.pdf by guest on 02 October 2021 ported in Public Health Bulletin No. 147 tion against low temperature. of the United States Public Health Service The remedies for these defects which entitled "Commercial Pasteurization” with engineers have developed are (a) valve- which most of you are familiar. Many less outlets or outlet valves installed flush serious defects we're found in the com­ wjth the inner wall of the pasteurizer mercial equipment of that period. This and so built that milk tending to leak served to awaken the industry as well as past the valve will run to waste instead o f, the health departments to the need for entering the outlet line, (b) valveless in- better engineering in the field of pasteur­ lefs or inlet valves protected against leak­ ization. Later Whittaker in Minnesota, age and with air relief to permit milk Putnam and Frank in Chicago, and our which has passed the valve to drain into own department did considerable work thp pasteurizer, (c) foamless pumps, on engineering tests of commercial foamless inlet lines or automatic foam pasteurizers using thermocouples to de­ heaters, (d) recording and indicating termine, temperatures of the milk at sig­ thermometers with 1/16 inch scale divi­ nificant 'points throughout the pasteuriz­ sion per degree, (e)' indicating ther­ ing operation. mometers so installed in pasteurizers as You will remember that among the to be used for operation and for checking serious defects found in these and similar the recording thermometer, (f) indicat­ studies were: (1) dead-ends caused by ing thermometers with marks etched in such things as outlet valves located some the glass at pasteurizing temperature to distance from the holder and in which aid in detecting and resetting scales which some milk was either not heated to have slipped, (g) safeguards against leak­ pasteurizing temperature or was not main­ age of unpasteurized milk into pasteurized tained at that temperature during the in1 enclosed heat exchangers and similar holding period, (2) leaky outlet valves protection of surface type regenerative which permitted unpasteurized milk to heat exchangers by the use of leak protec­ accumulate in the outlet line during tor grooves and the extension of the ends pasteurization, (3) leaky inlet valves of tubes beyond the collecting troughs which permitted unpasteurized milk to using diverting fins at the end of each drip into the pasteurizer during the tube, (h) designing continuous flow hold­ holding period, (4) unpasteurized milk ers upon actual holding time determined held in the· inlet line by atmospheric pres­ by color or similar test, (i) the develop­ sure when the inlet was submerged and ment of automatically controlled precision discharged with the' pasteurized milk heating for continuous flow pasteurizers, when t ie holder was. emptied, (5 ) foam aqd (j) the provision of automatic de­ on the surface of pasteurized milk, the vices for flow diversion of underheated temperature of which was likely to be as milk and automatic low temperature milk much as 15 degrees below that of the pump stops. , body of milk in the holder, (6) recording Perhaps a few concrete examples of and indicating thermometers with 5 ° scale cfefects in equipment w ill serve to bring divisions closely spaced which could not out the engineering features involved in be read accurately. (7) recording ther­ tests better than an abstract discussion of mometers badly out of adjustment and no the subject j although it is getting to be means provided for checking them, (8) an old story. You are familiar with the indicating thermometers on which the so-called "flash” . type consisting of a Journal of Milk T echnology 13

"barrel” heater in which milk flowing type of holder was discredited and was through tubes was rapidly heated by sur­ soon taken off the market. Before many rounding hot water which was in turn years, old installations of this equipment heated by direct steam under manual coil· were replaced with So-called "positive trol without recirculation of the water. holders.” Milk pumps used in conjunction with Among the early types of pasteurizers these were of variable speed and no at­ were various ones designed to heat, hold tempt was made to determine holding and cool the milk in the bottle in which time at pasteurizing temperature. Ther­ it was to be delivered to, the customer. mometers used were small, slow to react This intrigued health officials because it Downloaded from http://meridian.allenpress.com/jfp/article-pdf/1/1/11/2392536/0022-2747_1_1_11.pdf by guest on 02 October 2021 and difficult to read. The temperatures eliminated the hazards of recontamination used ranged from 158°F. to 165°F. and involved in cooling and bottling the milk the time of exposure from a few seconds after pasteurization and insured that the to two minutes. Engineering tests showed bottles themselves would be at least a great variation in temperature during pasteurized if not sterilized. Bacteri­ operation at any predetermined temper­ ological tests generally showed a good re­ ature and similarly great Variations in duction in plate count with an occasional holding time. With the. exposure of unexplained increase. Engineering tests these defects and the advent of the hold­ made by reading temperatures by means ing type of pasteurization, "flash” pasteur­ of an especially constructed system of. ization was discredited and legislated out thermocouples installed at different levels of existence. Good engineering might in bottles of . milk at various locations in have saved this process as has since been the pasteurizer while in commercial opera­ demonstrated, although the equipment tion gave a different story. They brought then in use was entirely unsuitable. out the fact that milk cannot be heated One of the outstanding examples of in­ uniformly in an upright milk bottle with­ efficient pasteurization was the continuous out resorting to agitation. The greater flow cylindrical holder or so-called "short heating surface in the narrow neck of flow system.” When flash pasteurizers the bottle ,per unit volume of milk to be were banned this was the next most heated resulted under the most favorable economical equipment for large plants. conditions in a differential of about 7°F. Installation usually consisted of two cylin­ in the temperature of the milk'in the top drical tanks, one at a slightly higher ele­ and bottom of the same bottle. The re­ vation than the other and so piped as to sults of a typical test of one of these introduce milk in a downward stream at types of pasteurizers as made by our own the center of the upper one. The over­ engineers showed that at the be­ flow from the upper tank was similarly ginning of the 30 minute holding introduced into the lower tank from period when every particle of milk was whence the overflow went to the cooler.' supposed to be at not less than 142 °F. Because it took thirty minutes to. fill the (the legal requirement at that time), the two tanks it was assumed that it took the temperature of milk at 12 different points same time for all particles of milk to flow in various bottles varied from 85.5 °F. to through. The Endicott experiments, 136°F. while at only one of the points made on an improved type of short flow selected was the temperature found to be holder provided with perforated inlet adequate, that is 147°F. At the end of spreader troughs to break the flow showed the holding period the temperatures of that "some milk passed through in from milk at three points was ' still below 10 to 12 minutes and that 25 per cent of 143°F., i.e. 125.5°, 127°, and 141°. At the flow passed through in .about 20 min­ the other ten points temperatures ranged utes or less.” The reasons for this should from 142°F. to 153.5°F. This type has have been perfectly obvious to the de­ also been taken out of plants and taken signer and are apparent to the engineer off the market. Although with good who has studied currents in streams or engineering equipment could be designed flow through sewage sedimentation tanks. to pasteurize milk in the bottle, the opera­ As a result of these and other tests this tion is not economical and has the com­ 14 Jo u r n a l o f M il k T e c h n o l o g y mercial disadvantage of leaving a partially purpose. Tests have shown that the re­ filled bottle when the milk contracts on sults are equivalent to that obtained on cooling. electrically heated milk. Apparently the More recently pasteurizers have been decree of heat and the holding time developed to subject milk effectively to rattier than the m ethod o f applying the a temperature of not less than 160°F. for heat are the important factors. 15 seconds or more which tests have Some of the earlier devices, particularly shown is equivalent to treatment at not outlet valves designed to correct defective equipment, were far from being satis­ less than 143 °F. for 30 minutes or more. Downloaded from http://meridian.allenpress.com/jfp/article-pdf/1/1/11/2392536/0022-2747_1_1_11.pdf by guest on 02 October 2021 This may be called a revival of "flash” factory. It was necessary to permit the pasteurization but has been more correctly use of some of these that were theoretic­ called high temperature short time ally sound until defects developed in pasteurization to indicate the definite actual practice. The end result has been holding period. This is simply another equipment that overcame the fault with­ point on the pasteurization curve selected out developing new ones. Notwithstand­ as a matter of convenience and utility. ing all this development it is disconcerting This was first attempted by use of equip­ to |ind that the modern vat pasteurizer so ment designed to heat milk quickly to commonly used is not as yet equipped 160°F. by electrical resistance during its with devices for showing positively the passage between two energized, water- holding time at pasteurizing temperature. cooled, flat carbons spaced about 2l/2 Along with the perfection ofjhe effici­ inches apart. The older equipment had a ency of heat treatment of milk have gone fixed electrical input and the temperature other important .engineering develop­ of the milk was automatically regulated ments. Many metals have been tested by varying the rate of flow through the ancj new . alloys developed in the effort to medium of a variable speed pump. Pro­ ■secpre better milk contact surfaces. Glass vision of a pump stop which automatic­ surfaces fused on steel are used ex­ ally stopped and reversed the flow of milk tensively. Most modern pasteurizers have was required before the use of the process milk contact surfaces of such material or was permitted by health officials. Dr. of stainless steel. Pulleys, shafts and belts Prescott and associates at this institution have given way to concealed motors for were among those to subject this equip­ driving agitators. Welded joints have ment to Critical tests. Our own depart­ taken the place of soldered' joints. Sur­ ment in cooperation with the United faces that need cleaning have been made States Public Health Service also ran tests smpother and more easily accessible. Cov­ on a commercial unit to pasteurize quanti­ ers have been constructed so that anything ties of milk to which human and bovine falling or dripping on them will fall pathogenic oganisms had been added. The on the floor instead of into the milk vats. bacteriologists collected continuous flow Methods of applying heat have been so samples while the engineers subjected the improved as to provide low temperature unit to stress conditions· such as failure differentials and to avoid. surfaces with of electric current, interruption of cur­ hot spots that scorched some of the milk. rent in certain circuits, reduction in volt­ Milk pumps have been redesigned to age and sudden increase in flow of milk. minimize foaming,, to facilitate cleaning As a result the use of the equipment was anil prevent damage to creaming. permitted in New York State. Similar While safety is the primary considera­ action has been taken in some other sec­ tion of public health engineers it has been tions of the country. Many improvements necessary to accomplish this without de­ have been made in this equipment since stroying flavor or any of the good prop- these tests were run. , erties of milk. Early crude attempts at Other equipment built to similarly commercial pasteurization resulting in heat milk under precise automatic control scorching or in producing an oxidized or by hot water has been developed and metallic flavor in milk turned consumers tested. Of particular interest is the plate against pasteurization and led them to type heat exchanger developed for this seek raw milk that tasted better. As you Journal of Milk T echnology 15

know, equipment is now available that tic sore throat in a cheddar cheese and will pasteurize milk with a full factor of reports growth of these organisms safety without damaging the flavor or throughout a three month ripening materially diminshing any beneficial period. He points out that there is a properties. This has been largely due to tendency to place cheese on the market the work of engineers in testing, finding much more quickly than heretofore and faults and redesigning pasteurizers. sometimes within three months. It is Pasteurization of cream and other milk desirable and possible to adapt processes products is of considerable public health of manufacturing milk products to the

importance. Raw cream and ice cream use of milk Or cream pasteurized in the Downloaded from http://meridian.allenpress.com/jfp/article-pdf/1/1/11/2392536/0022-2747_1_1_11.pdf by guest on 02 October 2021 made from raw mix have been, found to usual manner. The result is not only a be responsible for outbreaks of disease on safer but a more uniform product. numerous occasions. Most butter manu­ The more general consumption of facturers have adopted pasteurization of pasteurized milk and milk products has the cream in order to prevent foreign resulted in a definite improvement in the organisms from overgrowing added cul­ health of the public. One of the factors tures and thus secure butter of uniformly making this possible has been the im­ high score. Cheese also has been impli­ provement in pasteurizing equipment. cated in disease outbreaks. Our recent use of the phosphatase test for Hucker of the New York State Agri­ pasteurization indicates that further' im­ cultural Experiment Station incorporated provement is necessary and may be ex­ hemolytic streptococci from a case of sep­ pected.

New Jersey Accredited for Bovine Tuberculosis

Q N September 1, 1937, New Jersey be­ dustry and under whose capable and un­ came the forty-fifth state to attain tiring supervision the work of tubercu­ a "modified accredited area” status under losis eradication in cattle was carried on for nineteen years, was presented to the the Federal-State cooperative program for Department of Agriculture by the State the eradication of bovine tuberculosis. Veterinary Medical Association. Tuberculin testing of -the cattle in Salem Only three states—New York, South and Middlesex counties completed the list Dakota and California—now remain to of New Jersey counties in which there re­ complete accreditation of the entire main less than one-half of one percent country, a task started cooperatively in reactors. 1917 and the greatest animal disease The accomplishment was celebrated at eradication program ever undertaken. a luncheon, under the auspices of the New York State will be accredited with­ New Jersey Department of Agriculture, in a few weeks and the work in South at Trenton on September 9th. Prominent Dakota and California is progressing State and Federal officials, past and pres­ quite rapidly. ent members of the State Board of Agri­ An important aspect of the bovine culture, and representatives of the State tuberculosis eradication program should ■Veterinary Medical Association, o f the be emphasized: accreditation of the na­ State and municipal Departments of tion’s cattle is a remarkable achievement Health, of State agricultural organizations but constant vigilance and continued effort and of other interested agencies which by continual retesting must be exerted to have supported the campaign during the prevent any spread of re-infection from past twenty years, were present. the small amount of disease that remains. As a part of the program, a portrait of All organizations should continue their the late Dr. John H. McNeil, first chief interest and support in order to protect of the New Jersey Bureau of Animal In- the ground gained.