Heat is lethal to microorganisms, yet each species has its own certain heat tolerance. Throughout a thermal destruction process, such together pasteurization, the rate of devastation is logarithmic, as is their price of growth. Hence bacteria subjected to warm are eliminated at a price that is proportional to the number of organisms present. The procedure is dependent both top top the temperature that exposure and also the time compelled at this temperature to accomplish to preferred rate that destruction. Thermal calculations thus involve the require for knowledge of the concentration of microbe to be destroyed, the acceptable concentration of microbe that can remain behind (spoilage organisms, because that example, but not pathogens), the thermal resistance the the target microorganisms (the most warm tolerant ones), and the temperature time relationship forced for destruction of the target organisms.

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The degree of the pasteurization treatment compelled is identified by the warmth resistance the the most heat-resistant enzyme or pathogen in the food. Because that example, milk pasteurization historically to be based on Mycobacterium tuberculosis and Coxiella burnetti, but with the recognition of each new pathogen, the forced time temperature relationship are consistently being examined.

A thermal fatality curve because that this procedure is presented below. The is a logarithmic process, an interpretation that in a offered time interval and at a offered temperature, the same percentage of the bacterial population will be ruined regardless of the populace present. For example, if the time compelled to destroy one log cycle or 90% is known, and the wanted thermal reduction has been determined (for example, 12 log in cycles), climate the time forced can be calculated. If the variety of microorganisms in the food increases, the heater time compelled to process the product will likewise be raised to carry the population down come an agree level. The heat procedure for pasteurization is usually based upon a 12 D concept, or a 12 log cycle palliation in the numbers of this organism.

Several parameters aid us to do thermal calculations and also define the rate of heat lethality. The D worth is a measure of the heat resistance that a microorganism. That is the time in minute at a offered temperature forced to damage 1 log cycle (90%) of the target microorganism. (Of course, in an actual process, every others that room less warmth tolerant are damaged to a better extent). For example, a D value at 72°C of 1 minute method that for each minute of processing at 72°C the bacteria populace of the target microorganism will be reduced by 90%. In the illustration below, the D value is 14 minutes (40-26) and also would it is in representative the a process at 72°C.

The Z value mirrors the temperature dependency of the reaction. The is characterized as the temperature change required to change the D value by a variable of 10. In the illustration below the Z worth is 10°C.

Reactions the have tiny Z values are highly temperature dependent, vice versa, those with huge Z values call for larger changes in temperature to reduce the time. A Z worth of 10°C is common for a spore creating bacterium. Warmth induced chemical transforms have much larger Z values that microorganisms, as displayed below.

Bacteria Z (°C) 5-10 D121 (min) 1-5

enzymes Z (°C) 30-40 D121 (min) 1-5

vitamins Z (°C) 20-25 D121 (min) 150-200

pigments Z (°C) 40-70 D121 (min) 15-50

The figure listed below (which is schematic and also not come scale) illustrates the relative changes in time temperature profiles because that the devastation of microorganisms. Over and to the appropriate of every line the microorganisms or quality factors would it is in destroyed, whereas below and also to the left of each line, the microorganisms or quality factors would no be destroyed. Due to the distinctions in Z values, the is evident that at greater temperatures for much shorter times, a region exists (shaded area) whereby pathogens can be ruined while vitamins have the right to be maintained. The exact same holds true for various other quality determinants such together colour and also flavour components. Hence in UHT milk processing, very high temperatures for really short time (e.g., 140oC for 1-2 s) are favoured compared to a lower temperature much longer time processes due to the fact that it outcomes in bacterial spore elimination v a reduced loss of vitamins and far better sensory quality.


Alkaline phosphatase is a naturally-occurring enzyme in raw milk which has actually a similar Z worth to heat-resistant pathogens. Due to the fact that the direct estimation of pathogen numbers by microbial methods is expensive and also time consuming, a basic test because that phosphatase activity is regularly used. If activity is found, it is assumed that either the warmth treatment was insufficient or the unpasteurized milk has actually contaminated the pasteurized product.

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A working instance of exactly how to use D and Z values in pasteurization calculations:

Pooled life milk at the processing plant has bacterial population of 4x10exp5/mL. That is to be processed at 79°C for 21 seconds. The typical D worth at 65°C because that the mixed populace is 7 min. The Z worth is 7°C. How plenty of organisms will be left after pasteurization? What time would certainly be compelled at 65°C to attain the same degree of lethality?


At 79°C, the D value has actually been decreased by two log cycles from that at 65°C because the Z value is 7°C. For this reason it is currently 0.07 min. The milk is processed because that 21/60=0.35 min, so the would attain 5 log cycle reductions to 4 organisms/mL. At 65°C, friend would need 35 minute to attain a 5D reduction.