Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 7, 2004
Publication Date: August 25, 2004
Citation: Tomasula, P.M., Kozempel, M.F. 2004. Flow characteristics of a pilot-scale high temperature, short time pasteurizer. (abstract). J. of Dairy Science. 87:2761-2768. Interpretive Summary: Fluid milk is pasteurized for commercial sale using the high temperature short time (HTST) pasteurization process. Milk is pasteurized at a temperature of at least 72 degrees C and is held at this temperature in a holding tube for at least 15 seconds to reduce any microorganisms in milk to a level at which they do not constitute a health hazard. It is difficult for researchers to study the effectiveness of the milk pasteurization process against certain microorganisms of interest using commercial-scale units because large quantities of milk to which the microorganisms are added are needed for each study. Small HTST pasteurizer units are preferred that require small quantities of milk and fit easily into a laboratory. However, steps must be taken to ensure that the small unit pasteurizes milk to the same extent as the larger unit or else the experimental results may be misinterpreted and will not apply to pasteurization on the commercial scale. This study presents a method for extrapolation of experimental data obtained from small pasteurization units to larger units. The method uses a tracer method in which a salt solution is added to milk and its concentration followed as it passes through the pasteurizer. The amount of time each particle of milk spends in various parts of the pasteurizer, especially in the holding tube, is determined. The results can then be applied to determine the effectiveness of pasteurization in commercial units. This will allow researchers to study the effectiveness of HTST pasteurization against pathogens in milk to assure the safety and security of milk and milk products.
Technical Abstract: It has been suggested in the literature that testing of the effectiveness of pasteurization on pathogens in milk should be conducted using a commercial high temperature short time (HTST) pasteurizer, rather than laboratory or pilot scale apparatus, because the pathogens in milk would be subjected to the effects of turbulent flow conditions in the holding tube that are encountered in actual operation. However, this approach is problematic because of the large quantities of naturally or artificially infected milk that would be required for experiments as well as other logistical problems. In this study, we present a method for determining the fastest moving particle time (FMP) and residence time distribution (RTD) in a pilot scale HTST pasteurizer equipped with nominal 21.5 s and 52.2 s hold tubes to ensure that laboratory or pilot scale HTST apparatus meets the Pasteurized Milk Ordinance standards for pasteurization of milk and can be used for obtaining thermal inactivation data. Tracer studies were used to determine FMP times and RTD data to establish flow characteristics. Using brine milk as tracer, the FMP time for the short holding section was 18.6 s and for the long holding section was 36 s at 72 degrees C, compared to the nominal times of 21.5 s and 52.2 s, respectively. The RTD study indicates that the short hold section was 45% back mixed and 55% plug flow for whole milk at 72 degrees C. The long hold section was 91% plug and 9% back mixed for whole milk at 72 degrees C. This study demonstrates that continuous laboratory and pilot scale pasteurizers may be used to study inactivation of microorganisms only if the flow conditions in the holding tube are established for comparison to commercial HTST systems.