Info

°C (°F)

Time

63 (145)a

30 min

72 (161)a

15 s

89 (191)

1.0 s

90 (194)

0.5 s

94 (201)

0.1 s

96 (204)

0.05 s

100 (212)

0.01 s

a If the fat content of the milk product is 10% or more, or if it contains added sweeteners, the specified temperature shall be increased by 3°C (5°F).

Source: U.S. Public Health Service, 1995.

a If the fat content of the milk product is 10% or more, or if it contains added sweeteners, the specified temperature shall be increased by 3°C (5°F).

Source: U.S. Public Health Service, 1995.

(Boor and Nakimbugwe, 1998); and up to 6 months for UHT (Dunkley and Stevenson, 1987). Whereas HTST and ultrapasteurized products require refrigeration at 4°C or less during storage, UHT products can be stored at 25°C.

Currently, both direct and indirect methods are used to bring raw milk to pasteurization temperatures (Bylund, 1995). Direct heating strategies, which are most commonly used for UHT and ultrapasteurization, involve injecting raw milk with hot culinary steam until the desired temperature has been achieved. Controlled pressure changes during cooling ensure that the amount of water vapor that was injected into the milk is equal to the amount of water that evaporates from the milk during cooling, thus preventing dilution or concentration of the milk. Indirect heating strategies, which are most commonly used for LTLT and HTST pasteurization, utilize a heating fluid which is separated from milk by a physical barrier; typically a stainless steel pipe, plate, or vat. The two fluids flow side-by-side and either gain or lose heat via conduction through the metal barrier and convection within the fluids.

The effectiveness of heat treatment depends on three main factors: temperature to which milk is raised, length of time milk is held at the temperature, and resistance of microorganisms in milk to thermal destruction. Two graphical representations describe the interaction between these variables. The thermal death rate curve, also known as the survivor curve, plots time versus number of surviving organisms at a given temperature. The reciprocal slope of this curve, also known as the D value, indicates the length of time required to kill 90% of the microbial population at that specific temperature (Potter and Hotchkiss, 1995; Jay, 2000). Destruction of 90% of the microbial population is known as a one-log reduction. Thermal death time curves plot time versus temperature for a given number of organisms killed. The negative slope of this curve, known as the z value, indicates the degrees Fahrenheit needed for a 1 log cycle reduction in the thermal destruction curve (Potter and Hotchkiss, 1995; Jay 2000).

Resistance of microorganisms to thermal destruction depends on several factors, including product water activity, product pH, quantities of protein and colloidal particles present, number and physiological status of organisms in the total population, and the presence of heat-stable antibiotics or inhibitory compounds in the product (Jay, 2000). Water activity, which is a measure of unbound water present in a solution, is determined primarily by concentrations of sugars, fats, and salts in milk and heavily influences microbial resistance to thermal destruction. The higher the water activity of the product, the lower the heat resistance of organisms present in the product. This is likely to be the result of the increased rate of heat-induced protein coagulation caused by the presence of water. The effect of pH on thermal destruction characteristics depends on the particular bacterium, as organisms are most resistant at their optimum growth pH. In general, the optimum growth pH of most organisms, about 7, coincides with the pH of raw milk, suggesting that pH generally does not contribute to thermal destruction of organisms in raw milk. The presence of protein and colloidal particles has a protective effect on bacteria, increasing their heat resistance by serving as a thermal buffer. Larger numbers of organisms similarly result in increased bacterial resistance to thermal destruction. The individual bacteria in a species are no more or less heat resistant; rather large numbers of bacteria present in milk act as a thermal buffer, raising the time necessary for all bacteria to reach the appropriate destructive temperature. Stationary phase cells tend to be more resistant to thermal destruction than logarithmic phase cells. The presence of heat-stable antibiotics or inhibitory compounds typically reduces resistance to thermal destruction.

C. Centrifugation

Two techniques known as clarification and Bactofugation (e.g., Westfalia Separator, Inc., Northvale, NJ) rely on the greater relative densities of bacterial cells and of other foreign particles to separate milk from contaminants. Centrifugation of milk causes denser bacteria, dirt particles, somatic cells, animal hairs, and bacterial spores to migrate outward, whereas lighter fat globules and casein micelles migrate inward. Appropriately designed outlet nozzles allow for separation of milk from contaminant sludge. Clarification is primarily designed to remove dirt particles, somatic cells, and animal hairs, whereas Bactofugation is specially designed to remove bacterial spores from milk (Spreer, 1998). Using high-force centrifugation, the spore load of raw milk can be reduced by greater than 99% (Olesen, 1989; Torres-Anjel and Hedrick, 1971).

D. Filtration

Microfiltration and ultrafiltration utilize the larger relative size of bacterial cells to separate out microbial contaminants. Filters with very small pores allow milk components to pass through while blocking bacteria, thus separating contaminants (Olesen, 1989). Typically rated in terms of pore diameter, microfiltration filters range from 0.2 to 5.0 |m. Using microfiltration, lactose, minerals, and small proteins pass through into the permeate, whereas fat, very large proteins, and bacteria are retained. Typically rated in terms of the largest molecular weight molecule that can pass through the pores, ultrafiltration filters range from 103 to 105 D. Using ultrafiltration, minerals and lactose pass through into the permeate, whereas proteins, fats, and bacteria are retained (Smith, 2000).

Although filtration can not remove all microorganisms, it can achieve a 99.99% reduction of the total bacterial count and a 99.95% reduction in the total spore count while allowing 5-6% of the solids in the bulk liquid to flow through into the permeate (Eckner and Zottola, 1991; Olesen, 1989). Effective bacterial retention appears to be determined primarily by the type and manufacturer of the

Alter and the design and configuration of the filtration unit; the morphology of contaminating microbes does not appear to affect bacterial retention (Eckner and Zottola, 1991). Although the fat level does not affect bacterial retention, milk with higher fat percentages causes membrane fouling, making this technique most useful for treating skim milk.

E. Additional Microbial Control Methods

Several less commonly utilized techniques exist for controlling microbial growth in milk. Addition of carbon dioxide to milk at 10-30 mm/L inhibits growth of the common spoilage organism P. fluorescens (Muir, 1996). This technique has been reported to extend the shelf life of refrigerated milk by several days. The use of the natural antibiotic nisin to inhibit gram-positive bacterial growth in milk has also been explored (Muir, 1996). Addition of nisin to milk intended for clotted cream and processed cheese is currently approved in the United Kingdom. Addition of lactic acid starter cultures to raw milk has been shown to inhibit growth of psychrotrophs (Muir, 1996). Although the lactic acid bacteria do not multiply at refrigeration temperatures, their metabolism results in a pH decrease to below 6 and possible organoleptic changes.

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