Journal of the Society of Dairy Technology, Vol. 18, No. 2, 1965

T H E C H A R A C T E R I S T I C S A N D P E R F O R M A N C E

111

O F A L A B O R A T O R Y - S C A L E H T S T M I L K

P A S T E U R I Z I N G P L A N T

BY I . C . F R A N K L I N

National Institute for Research in Dairying, University of Reading

(Received 6th November, 1964)

SUMMARY An apparatus for the HTST pasteurization of milk in the laboratory is described and satisfactory methods of operation and cleaning are given. Treatment of milk at varying flow rates within the temperature range 156"-162"F, followed by deter- mination of the residual phosphatase, showed that a flow rate of c 65 ml/min resulted in phosphatase destruction curves closely agreeing with those obtained with commercial plants. The results indicate, therefore, that this apparatus yields an end-product similar in terms of residual phos- phatase to milk heated to the same temperature in commercial HTST pasteurizing plants and that milk heated to 161 O F at a flow rate of c 65 ml/min is similarly comparable to commercially pasteur- ized milk.

INTRODUCTION Most liquid milk retailed to the public in the United Kingdom is pasteurized by a high-temperature short-time (HTST) process during which, by law, it is required to be heated to a temperature of not less than 161°F and held at this temperature for at least 15 sec (The Milk (Special Designations) Regulations, 1963). A method of simulating the commercial treatment is desirable in the laboratory to facilitate studies of the effect of raw milk quality on the keeping quality of the corresponding heat-treated milk. The overall effect of commercial pasteurization results from a combination of the holding conditions (i.e. at least 161 O F for I5 sec) and the heating-up and cooling-down processes. Ideally a laboratory apparatus should provide a treatment equivalent t o this and should also be able to treat quantities of milk sufficiently small to permit tests on samples from the milk of individual producers, but sufficiently large to ensure results that are reasonably representative of the bulk.

Several methods for the laboratory treatment of milk by an HTST process have been described or referred to in the literature (Quinn & Burgwald, 1933; Yale, 1933; Plock & Walzholz. 1936; Hileman & Leber, 1941 ; Gilcreas & O'Brien, 1945; Solberg, 1946; Thome & Olsson, 1947; Pascoe,

1948; Lindqvist, 1949; ThomC & Gynning 1958; Hadland & Steinslund, 1958; McFarren, Thomas, Blake & Campbell, 1960; Aule, 1962; Lembke & Gantz, 1963) but most of these do not give a treatment equivalent to the commercial process because of vastly different heating and cooling times. Some of the plants are complex and difficult to clean and the volumes of milk treated are some- times too large to allow milk from individual producers to be tested, or too small for the samples to be representative.

Recently, MacWalter & Wright (1962) described a laboratory HTST pasteurizer capable of repro- ducing the commercial process with a throughput of c 1 gal/h. A plant based on their design was subsequently manufactured by Astell Laboratory Service Co. Ltd.* and kindly loaned to the Institute for testing and experimental purposes. The appar- atus as supplied did not satisfy all the requirements largely because of variations in milk flow due to difficulties with the milk pump, and a number of modifications were necessary before a satisfactory performance could be achieved. This communi- cation is concerned with the lay-out, operation, behaviour and characteristics of the modified Astell laboratory HTST pasteurizer.

METHODS Description of plant The apparatus (Fig. I , p. 112) is shown diagram- matically in Fig. 2 (p. 112). The inverted 2 1 conical flask K and the 500 ml stainless steel beaker B comprise the raw milk reservoir and constant-head system for the apparatus. Maintenance of a constant-head at the top of the beaker necessitates a greater quantity of milk initially to fill the system whereas a constant-head sited at the bottom of the beaker resulted in frequent blockage of the beaker outlet by air bubbles formed during discharge of milk from K, thereby interfering with the milk flow. The constant head of milk is maintained, therefore, about half way down the beaker by siting the end of the outlet tube from K in this position. The beaker

*Address: 172 Brownhill Road, London S.E.6.

112 Journal of the Society of Dairy Technology, Vol. 18, No. 2, 1965

Fig. 1. The modified Astell laboratory HTST pasteurizer. B = beaker; D = milk cooler; F = flow controller; H = milk heater; K = raw milk reservoir flask; 0 = outlet from milk cooler; T = holding tube; W = hot water set;

Y = primary diversion line. Fig. 2. Diagrammatic lay-out of the modified Astell labora-

tory HTST pasteurizer.

is placed at a convenient height above the rest of the plant to allow a gravity flow of milk through the apparatus.

The outlet from B is connected via a flow- controller F (Fig. 3) to the inlet of the stainless- steel heater unit H by means of flexible silicone rubber tubing. The internal design of the heater is similar to that of MacWalter & Wright (1962), a thin annular column of milk passing up the heater and being heated from both sides by hot water pumped from the hot water set W. The top of the heater connects directly with a stainless steel holding tube T, which is lagged with an inner jacket of rubber and an outer jacket of expanded polystyrene bound with plastic-backed adhesive tape. The milk temperature in T can be measured directly by means of a thermometer M. The top of the holding tube is fitted with a two-way cock enabling milk to be directed to the cooler D, a cold water jacketed stainless steel tube, or diverted to waste.

The hot water set W consists of a vessel of c 2 1 capacity fitted with a 1 kW heating element con- nected in series with a Sunvic energy regulator and a Sunvic thermostat to control the water tempera- ture. The hot water is circulated through the milk

heater and back to the hot-water set by means of a Charles Austen C 16/300 centrifugal pump P.

Operation of the plant The hot water set heater and pump are switched on 15 min before the milk treatment commences so that the water and plant are at the required temper- ature. Flask K is filled with milk, the screw clip L closed, and the flask invet ted and placed in position over the beaker B. With the flow controller F closed, clip L is opened to allow milk to fill the system to F, taking care that the tubing and beaker outlet are completely free of trapped air. The cold water supply to the cooler D is turned on and the two-way cock at the top of the holding tube turned to the diverted position. With a screw clip on the diversion line Y open, the flow controller is care- fully opened by means of an adjusting screw (Fig. 3) until milk is seen to flow into the heater. The flow rate of the diverted milk is measured and adjusted to the required value. The temperature of the heating water is controlled by means of the thermostat and energy regulator so that the milk is heated to the desired temperature. Then, when flow rate and temperature are steady, the two-way cock is turned so that the milk flows into the cooler

Journal of the Society of Dairy Technology, Vol. 18, No. 2, 196.5 113

Fig. 3. Flow controller.

and the cooled treated milk is collected at the outlet 0.

Testing of the plant The ability of the modified Astell laboratory pasteurizer to reproduce the commercial HTST process was tested by a method similar to that used by MacWalter & Wright (1962). Milk was treated at varying flow rates over a temperature range of c 156-162"F, the residual phosphatase was measured by the method of Aschaffenburg & Mullen (1949), and the resultant phosphatase destruction curves obtained were compared with the results obtained for two commercial APV Paraflow pasteurizing plants by MacWalter & Wright (1962). The results, shown in Fig. 4, indicate that flow rates of 64-66 ml/min give phosphatase destruction curves in close agreement with those obtained with the commercial plants. The modified Astell apparatus used to heat milk at 161°F at these flow rates, therefore, yields an end-product comparable with regard to phosphatase destruction to commercially HTST pasteurized milk.

Measurements of milk temperature in the holding tube indicated the temperature drop during passage through the tube to be (0.25"F. The hot water/milk temperature differential was found to be dependent on the height of the heater above the hot-water set and the position of the heater was

Treatment temperature O F

Fig. 4. Phosphatase values of milks subjected to different treatments in an Astell laboratory-scale HTST pasteurizer. ____ Laboratory pasteurizer (figure denotes flow-rate

- - - - Commercial HTST plants A and B (from Mac-

adjusted to give the most satisfactory differential of 2-3"F, i.e. similar to that in commercial plants. The use of larger differentials resulted in more rapid deposition of milk solids in the heater whereas reducing the differential necessitated slower flow rates to heat the milk up to temperature, with the corresponding increase in treatment times. It was possible to maintain the milk temperature constant 10.25"F throughout a run provided the flow remained unchanged and the heater remained free of milk deposits.

Cleaning and disinfection of the plant The degree of soiling of the plant by milk deposits is dependent on the treatment used, the length of run and the compositional quality of the milk. Experiments showed that when milks flowing at 64-66 ml/min were heated to 161-162"F, the maxi- mum length of run possible before milk deposits in the heater interfered with the treatment varied from c 40-70 min (i.e. +-1 gal quantities). It was considered that for any cleaning process to be satisfactory it must be capable of dealing with the worst conditions which might be found at the end of the maximum treatment time. A number of materials and cleaning routines were tested before the following satisfactory method was adopted : On completion of the milk treatment, clip L is closed, the flow controller F is opened and the beaker B is allowed to empty. The tubing is then disconnected from the heater outlet and con- nected to a mains cold water supply. Water is circulated through the plant to rinse and the beaker B and flask K are removed for separate cleaning by hand. When the plant has been thoroughly rinsed, the thermometer and two-way cock are removed from the holding tube and the tube sealed with a rubber bung. The plant is then assembled for

in rnl/min).

Walter & Wright, 1962).

114 Journal of the Society of Dairy Technology, Vol. 18, No. 2, 1965

Fig. 5. Diagrammatic lay-out of the cleaning assembly for the Astell laboratory HTST pasteurizer.

cleaning as shown diagrammatically in Fig. 5 . The screw clip on the diversion line Y is closed and 13 1 of nearly boiling 5 per cent ODC* detergent solution is poured into vessel B and recirculated through the plant by means of a Multifix M 80 peristaltic pump A, taking care initially that any air in the system is displaced.

A bleed of steam S is allowed to traverse the outer jacket of the cooler D, so that heat losses incurred by the circulating detergent are compen- sated. When the temperature of the recirculating detergent in B exceeds 190"F, the treatment is continued for a further 10 min. It is important that the steam supply should be cut off whenever the pump A is not working otherwise local boiling may occur with possible damage to the silicone rubber tubing. The hot-water pump P is not operated during cleaning to prevent cooling of the detergent solution passing through the heater H. After cleaning, the steam supply is cut off, the screw clip on the diversion line Y is opened and the detergent is pumped out of the system. The plant is then rinsed with hot water at c 180°F to remove all traces of the detergent before reassemb- ling in preparation for the next run.

DISCUSSION The time taken by the milk to pass through the heater is c 1 min and the holding time is c 20 sec. The overall treatment time, therefore, approxi- mates closely to that in commercial HTST plants. However, although the modified Astell laboratory HTST pasteurizer has been shown capable of simulating the commercial HTST process, it might differ from the latter in its effect on other chemical, physical or bacteriological properties of milk. In addition, its uses are limited. The volume of milk necessary to fill the system and to establish steady

'Manufactured by Reddish Detergents Ltd. and consisting of a mixture of alkalis, polyphosphates and organic sequestering agents.

operating conditions of 64-66 ml/min flow rate and 161-162°F temperature is c 1 litre; it is therefore impossible to operate the plant for samples of smaller volume than this. The total time necessary to establish operating conditions and to collect 1 I of properly pasteurized milk is c 30 min. When the time for cleaning and assembling is included, the resultant throughput of samples is c 1 per h, although this would be dependent on the quantity of milk to be treated.

It is not possible to settle the plant using water because the flow rate, and consequently the tem- perature, changes appreciably when switching from water to milk due to the difference in viscosity. A glycerine-water mixture, or some other fluid of similar viscosity to milk might be used to start up, but this would not increase the through-put of samples although it would reduce the volume of milk required. However, the extra quantity of milk necessary for starting up should not be economic- ally prohibitive because all the milk is recoverable. Nevertheless the apparatus does not appear to be suitable for the routine pasteurization of a large number of milk samples. It is also not possible to treat samples greater in volume than 3-1 gal because of the deposition of milk solids in the heater resulting in restrictions to the milk flow. However the apparatus should prove a useful research tool and demonstration instrument.

ACKNOWLEDGMENTS The author wishes to thank Astell Laboratory Service Co. Ltd. for the loan of the laboratory HTST pasteurizer and Dr. R. J. MacWalter and Dr. R. C. Wright for making available their commercial HTST plant results.

REFERENCES Aschaffenburg, R. & Mullen, J . E. C. (1949) J . Dairy Res.,

16. 58. --, .. Aule, 0. (1962) Proc. X V I l ~ t . Dairy ConEr., Vol. C .

Gilcreas, F. W. & OBrien, J. E. (1945) Rep. N . Y . SI. Ass.

Hadland, G . & Steinslund, T. (1958) MeieripoAterr, 47, No.

Hileman, J. L. & Leber, H. (1941) J . Mi/k Tech.,4, 128. Lembke, A. & G?ntz, H . (1963) Kieler Milchwirtschaft.

Lindqvist, B. (1949) Proc. XI1 Itit. Dairy Congr., 2 (2). 731. MacWalter, R. J. &Wright, R. C. (1962) J . Soc. Dairy Tech.,

McFarren, E. F., Thomas, R . C., Blake, L. A. & Campbell.

Pascoe, J . V. (1948) Aust. J . Dniry Tech., 3, 5 . Plock, K. & Walzholz, G. (1936) Molkereiztg, Hildesh.,

Quinn, J . D. & Burgwald, L. H . (1933) Mifk PI. Mon.. 22.26. Solberg, P. ( 1946) Arsskr. Alttarps Lautbr.-Mejeri-och Trarlg.-

The Milk (Special Designation) Regulations (1563) Statutory

Thorn& K . E. & Gynning, K. (1958) Milchwissenschnft, 13,4. Thome, K . E. & Olson, T. (1947) Mejeritek. Medd., 4. Yale, M. W. (1933) Tech. E d / . N . Y . SI. agric. Exp. Slu.

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