For example, if a bacterium is heat-treated at 110 ℃ and 90% of the original surviving bacteria in its population are killed within 5 minutes, then the heat resistance of the bacterium at 110 ℃ can be expressed by D110 ℃=5 minutes. D is the reciprocal of bacterial mortality. The larger D is, the slower the death rate is, the stronger the heat resistance of the bacterium is, and D is not affected by the total number of original bacteria.But subject to heat treatment temperature, strain, bacteria orSporeThe nature of the suspension has an impact, so the D value means that it will not change under certain conditions and certain thermal lethal temperature, and does not represent the whole sterilization time.
The higher the D value, the slower the death rate of the bacteria, that is, the stronger the heat resistance of the bacteria.
Therefore, the value of D is in direct proportion to the strength of bacterial heat resistance.
Please note that the D value is not affected by the original number of bacteria.
The D value varies with heat treatment temperature, strain, environment of live spores and other factors.
calculation
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Calculation of D value: D=t/(㏒ a - ㏒ b) t is the heat treatment time a is the original bacterial count b is the bacterial count after t heat treatment time
Related concepts
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F -- Heat treatment time (min) required to kill a certain number of target bacteria at the reference temperature, that is, the sterilization value of the bacteria.The reference temperature of low acid food is 121.1 ℃ or 2500F commonly used abroad.Usually, the upper and lower corners on the right side of the F value are marked with the Z value and the temperature it is based on, while F10121 can usually be represented by F0.F value can be used to compare the heat resistance of bacteria with the same Z value.
Relationship between F and D: F=nD, n is not fixed. With the change of factory conditions and the type and degree of food contaminating microorganisms, 6D is generally used to kill thermophilic Bacillus and 12D is used to kill Clostridium botulinum to ensure food safety.
Z -- Target bacteria during thermal sterilizationThermal lethal timeThe slope of the curve (min), that is, the estimation of the corresponding change of the thermal lethal time or the lethal rate when the temperature changes. Z is the change value of the heating temperature, and the corresponding change of the heating temperature when the thermal lethal time or the lethal rate (D) changes by 1/10 or 10 times.The larger Z is, the smaller the germicidal effect will be due to temperature rise. For example, the test bacteria with Z=10.0 ℃ will die after being heated at 121 ℃ for 5 minutes, which can be expressed as F10121=5 minutes, for example, Z=10 ℃,sterilizationThe temperature of 121 ℃ can usually be directly expressed by F value, and other values should be marked.Low acid food Z=10 ℃Botulinumcalculation;When acid food is sterilized below 100 ℃, it can be calculated as Z=8 ℃.
Research
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(1) D value is an important parameter in the application of disinfection technology. As a new sterilization technology, microwave sterilization D value is studied through experiments.
The results showed that the D value of microwave sterilization changed with different factors, and the output power had the most obvious effect;Different loading and packaging methods also affect the sterilization D value;The bactericidal D value of bacterial spores is about three times that of bacterial propagules.The change rule of microwave sterilization D value is not only of theoretical significance, but also of value in guiding microwave sterilization practice.[1]
(2) By changing the heating mode of microwave oven through program control, the heating mode of microwave oven is similar to that of water bath.Using this method, the D values of three food borne pathogens, Escherichia coli and Salmonella paratyphi B, were determined at 50,55,60 ℃ water bath and microwave heating sterilization, and the D values of Staphylococcus aureus at 55,60,65 ℃ water bath and microwave heating sterilization.By comparingEscherichia coli、SalmonellaandStaphylococcus aureusD value and Z value of microwave and water bath sterilization can find that the effect of microwave sterilization is similar to that of water bath sterilization, and no significant non thermal effect of microwave is found.[2]
