z-Values A z-value (yes, it is written as a lowercase “z”) is a term used to describe the number of degrees the temperature or the increase in chemical concentration that is needed to achieve a ten-fold (i.e., 1 log 10 ) reduction in the D-value. While the D-value gives the time needed at a certain temperature or chemical concentration to kill 90% of organisms, the z-value relates the resistance of an organism to di ering temperatures or disinfectant concen- trations. The z-value allows the calculation of the equivalency of two thermal or disinfection processes if the D-value is known. This value is helpful in many situations that we encounter. Here is an example of how a z-value is used. In dairy technology, pasteurization tem- peratures are based on the target organism, Mycobacterium tuberculosis , whose classic D-value in milk is given as D 55 C(131 F) = 60 s. From this value, we regulate 1) vat pasteuriza- tion at 139–144 °F (60–63 °C) for 30 min; 2) high-temperature, short-time pasteurization at 161 °F (72 °C) for 15 s; and 3) ultra-high temperature pasteurization at 194 °F (90 °C) for 0.75 s. These are z-values. We can make similar calculations with changes in chemical concentrations, pH, water activity, and radiant energy once we know the D-value of the target organism in a given medium or environment. F-Values The best description of the F-value comes from the Food and Drug Administration (2014). The F-value was developed by Nich- olas Appert, a French candy maker in 1810 for inventing a “cook, seal-with-wax, cook again” method for preserving foods for the army of Napoleon Bonaparte. The F-value is primarily used for processes such as canning. It represents the number of minutes required to kill a known population of microorganisms in a given food under specified conditions. Since food has mass, the F-value is usu- ally set at 12 D-values to give a theoretical 12-log cycle reduction of most heat-resistant species of mesophilic spores in a can of food. For example, if there were 10,000 spores of a species of spore-forming bacteria (e.g., Clos- tridium botulinum ) in a can of food and a 12 D-value process was given, the initial 10,000 spores (10 4 spores) would be reduced to a theoretical 10 -8 living spores per can—or in theory, 1 living spore per 10 8 cans of product (1 spore per 100 million cans). For example,
cific organism is the time required in a given medium (e.g., milk, water, tomato juice, chicken soup, pork), at a given temperature to achieve a one-log (ten-fold) reduction of that organism. That is to say, the time required to destroy 90% of relevant microor- ganisms. Therefore, after an exposure time of 1 D-value, only 10% of the organisms origi- nally present in a microbial colony would remain. This information is absolutely essen- tial when examining the e ectiveness of ther- mal inactivation under di erent conditions such as cooking and preserving foods, clean- ing for infection control, and providing safe drinking water. The term originated in assessments of ther- mal resistance in microbes and in thermal death time analysis, particularly when used to sterilize something. It now has, however, other uses in death rate applications, such as for disinfectants and sanitizers. When we use D-values, we assume (with relative assur- ance) that the procedure in question causes the number of living microorganisms to decay exponentially. As a rule, each organ- ism will have its specific D-value, and that D-value is unique to the conditions of the environment in which the bacteria currently exist. For our purpose, we use the organ- ism with the greatest resistance to e ectively gauge D-value exposure time for whatever situation of concern. Although D-values were created in the laboratory to best understand how to con- trol an organism of concern within a par- ticular medium, by using a specific tem- perature you can readily find most D-values through a simple web search. In the context of thermal analysis, it is typical practice to subscript D-values with an indication of temperature. For example, given a hypo- thetical organism that is reduced by 90% after exposure to temperatures of 150 °C for 20 minutes, the D-value would be written as: D 150 C = 20 min. Likewise, the D-value can be expressed for varying concentrations of disinfectants and sanitizers. D-values tell us how long a contact time is required for a chemical—at a given concentration and at ambient temperature— to reduce the number of organisms by one order of magnitude. The use of this D-value is how disinfectants and sanitizers are evalu- ated and how label instructions for their use are developed.
if the D 240 was 1 min, the F-value for the pro- cess would be 12 min or F 240 = 12 min. When F is used without a subscript indi- cating temperature, 250 °F is assumed. When the symbol F is used, a z-value (change) of 18 °F is assumed with an exposure temperature of 250 °F. The actual processing time of a can of food is given in a retort and is always greater than the F-value due to heat penetration requirements. Under conditions of industrial practice, however, the process is modified to take into consideration characteristics of heat penetration into the in-container product and to integrate these data with microbial thermal resistance data to determine the actual steril- izing value of the new process. Industry makes extensive use of F-values in maintaining processes and in develop- ing new schedules. Optimally, old and new processes are equated to acceptable F-values. Two di erent processes are considered equiv- alent when the processes are equally e ec- tive with respect to the destruction of a given microorganism. While we know this explana- tion is a bit convoluted, it all starts making sense when we assess cooking processes that fall outside our normal retail food criteria. We urge you to think in terms of F-values in your daily work, particularly when evalu- ating the cooking time of macerated meats, and even when disinfecting laundry by heat, or using a biocidal agent on porous furnish- ings and bedding. Final Thoughts Well, there you have it. D-, z-, and F-values give some degree of rationale to the policies, rules, and regulations we are asked to enforce, specifically when they appear as part of writ- ten procedures and on instructional labels. By thinking through a particular decontamina- tion problem using approaches to these val- ues, as well as estimates of bioloads, we can suggest e ective alternatives to our clients. Trust us, they will thank you for it.
Contact: powitz@sanitarian.com
Reference Food and Drug Administration. (2014). Sterilizing symbols (D, z, F) . https:// www.fda.gov/inspections-compliance- enforcement-and-criminal-investigations/ inspection-guides/sterilizing-symbols-d-z-f
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