![]() Note that the Z-value has a significant effect on F value, refer to the free download of lethality tables formulated using Z-values of 6°,8°,10° and 12☌. ![]() The default Z-value has been set at 10° C and can be varied by the user in this application. If F 70 or another F value is required, then Tr can be set at 70° C or other temperature. A Tr value of 121.1° C is used in the determination of F 0. Tr will vary depending on whether F 0 is being calculated or whether a pasteurisation process or other heat treatment e.g. Note the original work on thermobacteriology used a reference temperature of 250° F. F and P are used interchangeably throughout this website. P or PU values are often used to designate pasteurization-type heat treatments that typically give 6-log reductions of target organisms, whereas F values are often used to designate much higher heat treatment processes giving higher log reductions. The F or P value can be defined as the time or equivalent time taken to reduce initial microbial numbers, at a specified temperature, by a particular value, normally a multiple of the D-value for the target organism. ![]() Lethal rates when plotted against process time can be used to calculate the F or P value of a thermal process. Calculate the lethal rate at 110° C compared to that at 121.11° C (Tr), given that the most heat-resistant organism present has a Z value of 10° C. Use of this equation can be illustrated using the following example. The z-value measured in ☌ is the reciprocal of the slope of the thermal death curve for the target microorganism or spore 10° C is the value frequently used in F o calculations performed on low-acid foods. The lethal rate is a dimensionless number and can be calculated using equation 1 (Stobo, 1973) and is a relative term that compares the microbial killing effect at a measured temperature to one minute at the reference temperature.Įquation 1, Lethal rate = 10 (T-Tr)/z where T is the temperature, in Celsius, at which the lethal rate is calculated and Tr is the reference temperature at which the equivalent lethal effect is compared. This can be expressed by calculating lethal rate. The logarithmic reduction in time required to kill the same number of microorganisms as the temperature is increased has been well described. salmonella can survive several hours at high temperatures in molten chocolate.īecause microorganisms in foods are exposed to lethal temperatures as they reach the target processing temperature, during holding and during cooling, it is necessary to calculate the cumulative effect of heat on microbial destruction during both heating and cooling as well at the holding time at the target temperature to obtain a true estimate of the lethal effects of a heat treatment process. Note fat, sugar, salt and chocolate content have a major effect on the sensitivity of microorganisms to heat in general they markedly increase their resistance to heat e.g. The lethal effect of high temperatures on microorganisms is dependent on several factors, including temperature, holding time, pH and water activity. One of the calculators will also upload CSV files of a thermal process and provides a facility for free, independent validation of a thermal process. ![]() In general, the more values, the more accurate the value for F or P will be. The area under the time-lethality curve is determined by numerical integration using the industry standard method, the trapezoid rule or the more accurate Simpson's rules or both for comparative purposes. The lethality calculators convert temperature readings to lethal rates, plot the lethal rates against time, and determine appropriate lethality values or chemical indicators for a heat process whether using hot water, saturated steam or dry heat.
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