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15 s

LTLT, low temperature, long time, or batch (vat) method; HTST, high temperature, short time, or continuous method.

LTLT, low temperature, long time, or batch (vat) method; HTST, high temperature, short time, or continuous method.

considered to be dangerous, because they are unable to germinate and grow under conditions of storage of either the mix or frozen product.

Pathogens introduced into ice cream mixes by ingredients, equipment, personnel, or the environment are killed by pasteurization, but recontamination may occur in subsequent operations. The potential for amplification of the effects of pathogens increases as sizes of dairy processing facilities increase. This is true because large plants serve large numbers of consumers over a wide trade territory.

Controls are provided on continuous pasteurizers to ensure that minimal temperatures are maintained until mix reaches the end of the holding tube. Also, pasteurizers are required to be designed and operated to provide minimal times of holding mixes at the minimal temperature. However, research by Goff and Davidson (1992) revealed that mix viscosity is a major variable that can affect time of holding a mix in a pasteurizer. They found that laminar flow characteristics are likely to exist in holding tubes of high-temperature, short time (HTST) pasteurizers when ingredients cause viscosities to become unusually high. Generalized Reynolds numbers, which are measures of turbulence in flowing liquids, ranged from 100 to 1700 in holding tubes of sizes common to the dairy industry. Laminar flow is likely to exist when Reynolds numbers are less than 2100 (Denn, 1980). In true laminar flow, mix that is at the tube wall flows one-half as fast as that in the center of the tube, whereas, in true turbulent flow, mixing is so thorough that particles travel at the same average rate in any cross section of the pipe. Because of the high potential for laminar flow of ice cream mixes in pasteurizer holding tubes, special considerations should be given to their design.

The method approved in 3A Sanitary Standard No. 603-06 (3A Sanitary Standards Committee, 1992) provides that, for most pasteurizers, the pumping rate is experimentally determined by timing the filling of a can of known volume and referencing this to a table of tube diameters and holding times of 15 and 25 s. Furthermore, holding time is confirmed by pumping water through the tube and detecting the time taken for an injected salt solution to pass conductivity sensors at each end of the holding tube. Whereas this method of testing provides reliable times for passage of products with the viscosity of milk, it is unlikely to be satisfactory for ice cream mixes that vary widely in viscosity. To overcome this problem, one approach is to design holding tubes to provide twice the holding time that would be applicable during turbulent flow. The 3A accepted practices provide that fully developed laminar flow is assumed when the desired holding tube length is calculated. This may result in more heated flavor than is desirable in the product. An alternative approach is to design pasteurizers with characteristics that ensure turbulent flow.

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