Direct-Steam Injection System for Processing Fluid Milk Products1

OUR DIRECT-STEAM

INDUSTRY

TODAY

INJECTION SYSTEM FOR PROCESSING FLUID MILK PRODUCTS 1 W. ~f. ROBERTS AND C. W. DILL

Department of Food Science and Processing, North Carolina State College, Raleigh I n recent years a trend has developed towards ultrahigh-temperature processing of fluid milk products. Several commercially manufactured units are available for heating fluids to various high temperatures. These can be classed as heat exchangers (plate or tubular) or direct-steam injectors. The use of direct-steam injection heating in food processing has been discussed by Morgan (2) and the mechanics of control over the steam system have been established (1-3). A number of potential advantages to be realized, particularly from high-temperature treatments, are better keeping quality in the finished product, removal of volatile flavor compounds when vacumn treatment is incorporated, and better process control in terms of quality characteristics of the finished product. This p a p e r will describe the system developed, with some adaptations for use in a research program involving physical and chemical changes in the constituents of products subjected to the process.

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Description of heater. A drawing of the steam-injection heater is given in F i g u r e 1. Milk and purified steam enter the heater at the openings indicated and are mixed at high velocities in the heater. The constriction in the mixing chamber is provided to enhance the mixing process. Spacers may be added where indicated, to adjust the heater for proportion and velocities, to insure mixing, and to provide a wide range of processing volumes. Steam is supplied to the heater in sufficient quantity to raise the temperature of the mixture to the desired point. The equipment may also be operated with excess steanl to provide better steam distillation of volatile compounds. Description of process. A flow diagram of the entire heating system is given in F i g u r e 2. Fluid product can be preheated in a vat, or continuously through a high-temperature shorn time pasteurizer, as high as 190 F, depending on the factor(s) being studied. The preheated product is passed through a homogenizer to provide positive flow through the steam-injection heater. The product is mixed with steam instantaneously in the heater and is ejected

~i(]. 1. Schematic diagram of the steam-injection heater.

into a holding tube of the desired length. Skimmilk has been heated from 160 to 300 F for times ranging from 2 sec (calculated) to 180 sec (measured). F r o m the holding tube, the product passes into a chamber wherein sufficient Published with the approval of the Director of Research, North Carolina Agricultural Experi- vacuum is maintained to cool the product to approximately the preheat temperature by ment Station, ~aleigh, as Paper No. 1390 of the Journal Series. flash evaporation, which restores the original 937

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solids concentration. The product is removed continuously from the bottom of the vacuum chamber and pumped through a plate cooler to lower the temperature to 40 F. The desired heating temperature is automatically maintained by means of a Taylor recording controller (Series No. 122R) which operates a 3~-ineh diaphragm valve in the steam supply line. Sufficient pressure is maintained on the holding tube by means of an airoperated pressure release valve to prevent flashing and to assure a smooth liquid flow during the entire length of the holding tube. Flow rates of steam consmned may be recorded continuously by means of a Minneapolis-Honeywell recording flowmeter (Model No. 292D15) operating on the principle of a pressure differential across an orifice of known size. The recorder is connected to the steam supply line on each side of the orifice with x/2-inch tubing as shown in the diagram. The condensable vapors removed from the product by the flash evaporation treatment may be sampled from the condensate pump (Figure 2). The noncondensable vapors are sampled at the vapor trap as shown in the figure. I n addition, if it is desired to cool the product without the use of vacuum and/or removal of volatile components, an alternate holding tube may be used which passes directly to a plate cooler or other cooling equipment. Analyses made on thls::iatter product, however, must be corrected for dilution. RESULTS AND mscussmN Many devices have been developed for mixing live steam with fluid milk products for the purpose of rapid and efficient heat transfer. Since these devices have not been used for pasteurization, accurate temperature control has not been required. Therefore, very little emphasis has been placed on instrumentation and control. However, since direct steam-injection heating has become more widespread, it is desirable to have a system that is properly instrumented and controlled so that data can be obtained on the effects of a wide range of time and temperature treatments of milk and milk products. Some of the problems encountered in bringing this system under control were: 1. Uniformity of flow rates with changes in temperature. 2. Correct volume and pressure relations between product and steam. 3. Msintenance of liquid conditions in the holding tube. 4. Location of the temperature sensing controller in relation to the incoming mixture of steam and product. When a temperature of 300 F is desired, it is necessary to maintain a holding tube pressure of 60 to 70 lb (gauge). Obviously, pressures of this magnitude decrease the rate of

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939

flow of steam. Thus, it was necessary to use a 3~-inch steam-regulating valve, rather than a 1/2-inch valve, as previously calculated, to provide an adequate volume of steam. Positive pumps were found to allow some by-passing of product resulting in a reduction in rate of flow at the higher temperatures and pressures. This was corrected by using a homogenizer as the product pump. Cheek-valves were placed in the incoming product and steam lines to prevent backing-up in either of the lines. The injection heater was designed with spacers so that the opening for product could be adjusted for changes in volume of product processed. Since the product flow was standardized at approximately 2,500 lb/hr, there was no need for changing the spacers once the optinmm operating conditions were dete~nined. This was found to vary some between fluid milk and ice cream mix. A glass holding tube was used to deternfine the state of the mixture at the operating temperature. A hand-operated air-pressure control valve was located on the discharge end of the holding tube. Sufficient pressure was applied on the valve to maintain a liquid state in the holding tube. This pressure varied from about 10 lb at 1 6 0 F to 60 lb at 300F. There was some vibration of the discharge valve as a result of the large pressure drop from 60 ]b to a 25-inch vacuum. This vibration was eliminated by placing an orifice in the holding tube a short distance before the valve. Times in the holding tube were accurately measured with a Solu-Bridge automatic timer (Industrial Instruments Company) and were found to vary less than 0.05 sec under a given set of operating conditions. I t was necessary to place the temperature controller near the heater, to get an immediate response to temperature changes. This was especially true when long holding times were used. I t was found that an orifice placed at the discharge of the heater reduced fluctuations that resulted from incomplete mixing and thus gave more uniform temperature control. The degree of temperature control with the equipment as described is shown in Figure 3. I t is noted that very accurate temperature control was mMntained on the system over a range of temperature varying from 160 to 300 F. The major fluctuations in the chart, e.g., at 270 and 300F, were due to adjustments in holding tube pressure. Also, the system was brought under control with large changes in temperature in ~ relatively short period of time. Since the discharge valve on the holding tube is manually operated, some experience and skill are necessary to make the adjustments quickly. As more operating data on pressure relationships are obtained, it should be possible to preset the conditions for stable uniform operation.

940

ffOURNAL OF D A I R Y SCIENCE

FIG. 3. Section of recorder chart showing temperature control at various operating temperatures.

SUIE:~¢[ARY A steam-injection system for heating fluid milk products is described in which temperatures are automatically and accurately maintained as high as 300 F. H o l d i n g times are varied by adjustments in the length of the holding tube. Thus, accurate heat treatments of fluid products can be provided over a wide range of total treatments. The system is very adaptable f o r research and commercial use because of the wide range of treatments and volumes possible with the equipment. The system has been used f o r the commercial processing of fluid milk and ice cream nfixes in conjunction with a high-temperature shorttime pasteurizer, t t has also proved v e r y useful in research on fluid milk, ice cream mix, and skimmilk f o r cultured milk products. The vacuunl system is being used to provide quantities of volatile materials f r o m milk for identification purposes. The entire system is cleaned easily in-place and should be v e r y adaptable to an automated process.

ACKNOWLEDGMENTS The steam injection heater and bisie vacuum equipment were manufactured by the CherryBurrell Corp., Cedar Rapids, Iowa. The authors acknowledge the assistance of D. J. Fitzmaurice and associates of Cherry-Burrell Corporation for their assistance on this project. They also express appreciation to Dr. K. A. Jordan, Agricultural Engineering Department, and J. I. Middleton for assistance in developing various aspects of the process control system. REFERENCES (I)BRowN. A. H., LAZAR, M. E., WASSERgAI,r, T., S M I ~ , G. S., AND COLa, M. W. Rapid Heat Processing of FIuid Foods by Steam Injection. Ind. Eng. Chem., 43: 2949. 1951. (2) ~/IOF~GAN ~, A. I., Ja. Use of Direct Steam Injection in Food Processing. J. Dairy Sci., 43: ]693. 1960. (3) M o r ~ r , A. I., Jm, ~ D CArLSOn, R. A. Steam Injection Heating. Ind. Eng. Chem., 52: 2'19. 1960.