What is the realation between thermal death time and the z-value???? and what exactly the z-value is?? in case of microbial growth! (related to effect of heat a

Nidula m Best Answer

Thermal death time(TDT) is how long it takes to kill a specific bacterium at a specific temperature. It can also be defined as number of minutes of heating required to destroy a specified number of organisms at a given temperature. This value is obtained by keeping temperature constant and measuring the time necessary to kill the number of cells specified. To understand what is Z value and its relation between TDT and Z value, we should understand what is D value. Decimal reduction time( D value) is the one which denotes the decimal reduction time is the time required at a specific temperature and under specified conditions to reduce a microbial population by one decimal. The D value is dependent on temperature, type of microorganism and the compositionn of medium containing microorganism . Thus after an organism is reduced by 1 decimal, only 10 % of the original organisms remain. D value can be obtained from graph or by equation. D= t/log No- log Nt where, No is the bioburden of choosen bacterium and Nt is the surviving population after an exposure time. The Z-value is the increase or decrease in temperature required to reduce or increase the decimal reduction time by one decimal. It is a measure of the change in death rate with a change in temperature.  The z-value is obtained by plotting the logarithms of at least 2 D-values against temperature or by the formula: Z = (T2-T1)/(logD1-logD2) Where T = temperature and D = D-value.