The Z-value provides information on how this changes with temperature. Note it is the decimal reduction value D that is a measure of the resistance of a microorganism to heat. An alternative explanation and perhaps one that is easier to understand, and apply, is that Z is a value expressing the increase in temperature necessary to obtain the same lethal effect in 1/10 of the time. The formal way is to describe it as the reciprocal of the slope of the thermal death curve for the target microorganism or spore 10° C is the value frequently used in Fo calculations performed on low acid foods. The Z-value, measured in ☌ or ☏ can be defined in several ways. If F 90☌ or other F value is required then Tref must be set to this temperature on the spreadsheet or On Line calculator. A Tref of 121.1☌ or 250☏ is used in the determination of Fo. Tref will vary depending on whether Fo is being calculated or whether a pasteurisation process or other heat treatment is being assessed. Note lethal rate is a relative term that compares the microbial killing effect at a measured temperature to one minute at the reference temperature. Where L is the lethal rate or lethality, T is the temperature, in Celsius or in Fahrenheit in the US, at which the lethal rate is required and Tref is the reference temperature. Lethal rate is calculated using equation 1. To determine the F value we first calculate the lethal effects of heat at the temperatures used during the heating process. B* and C* are discussed further in the article on UHT-processing. F 0 must be calculated in some jurisdictions, it is increasing common to use B* and C* as indicators of the bacteriological effects of heat against thermophilic spore formers and the potential adverse chemical effects respectively. In UHT-processing, temperatures up to 150 ℃ may be used and while F values e.g. This can be confusing and F value has been used throughout this website to denote the cumulative effects of heat on lethality.į values tend to be used up to 121.1 ℃. Note when temperatures lower than 100☌ are used (pasteurising temperatures) P or PU may be used instead of F value. The F value obtained using a Z-value of 10 ℃, and a Tref of 121.1 ℃ is known as F 0. This takes account of the lethal effects during heating, holding at the target processing temperature (this can be lower or higher than Tref) and during cooling. The calculated cumulative lethality at the Tref is known as the F value. How do the spreadsheets and the free On Line thermal process calculators work on the Dairy Science and Food Technology website? Some DSFT spreadsheets also calculate the concentration of chemical compounds produced as a result of high temperature processing known as thermal process indicators or TTI. Spores of this variant have a Z-value of 10 ℃, and a Tref of 121.1 ℃. The non-proteolytic variant that grows at low temperatures produces spores of relatively low resistance to heat whereas the variant concerned with canning low acid food such as meats and vegetables, produces much more heat resistant spores. Clostridium botulinum has two variants that must be considered differently in heat treatment processes. The organism is designated by its Z-value and its reference temperature, Tref. This is usually a bacterium or spore, selected because it is a pathogen or spoilage organism. Note I am not providing a guide to using spreadsheets but basic information that a competent Excel user should be able to use to make their own thermal processing spreadsheet.ĭownload Excel Thermal Processing Spreadsheets.Īll thermal process calculations start with a target microorganism. Here we provide an overview of the background, including a summary of the underlying mathematics, required to produce an Excel spreadsheet for performing basic thermal processing calculations. This section provides the context to using Excel to calculate the cumulative lethal effects (at all stages during processing) of heat on microorganisms and provides an explanation of how the Excel spreadsheets and On Line calculators available for download from the Dairy Science and Food Technology (DSFT) work. MICROSOFT EXCEL LETHAL RATE CALCULATORS AND TEMPERATURE TIME INTEGRATORS FOR THERMAL PROCESSES Introduction
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