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What Are Log Kills? The Difference Between 99.9% and 99.9999%


It’s common knowledge that a detergent cleaner removes more bacteria than water alone and that a sanitizer or disinfectant is more effective than just soap and water. However, what many people don’t think about is specifically how effective a product is and how that’s measured. What percent of bacteria are killed, and how many are left behind? Although these details might not be critical in a household, when it comes to food safety and biosecurity, log kills matter. 


What Are Log Kills?

When disinfecting a surface or decontaminating a facility, the efficacy of a product is measured by its log reduction, also commonly known as log kills. The term comes from the logarithm scale, which is used to indicate the percentage of bacteria killed. The term “log reduction” indicates a 10-fold reduction, which means that with every step, the number of bacteria present is reduced by 90 percent.

As an example, if there are one million bacteria present on a surface, a 1-log reduction would reduce the number of bacteria by 90 percent, or 100,000 bacteria remaining. A 2-log reduction removes 99 percent, leaving behind 10,000 bacteria, 3-log removes 99.9 percent to leave behind 1,000 bacteria, and so on through a 6-log kill, which leaves behind only one cell in one million. With this knowledge, log kills on a sanitizer label take on new meaning. The difference between a single digit can have a major impact, which is critical for food safety professionals to understand when selecting the sanitizing products that will be used in a facility.


How Log-Kill Tests Are Performed

Log kills are measured in a laboratory using time-kill tests. To determine the log reduction, the product is brought into contact with known pathogens for a certain period of time. At specified intervals, samples are removed and neutralized, then the colonies are counted. An untreated sample is also used for comparison, and typically water is applied in lieu of a sanitizer. Specimens of the untreated sample are also neutralized at the same time, and the bacterial colonies are counted for comparison purposes. The difference between the number of bacteria in the untreated sample versus the treated sample indicates the log-reduction capability of the product being tested. 


The Significance of Enhanced Sanitation Measures

Because the logarithm scale is being used, the difference in log kills can be dramatic. As an example, a 2-log reduction leaves 100 times more bacteria behind than a 4-log reduction. This might not mean much in some scenarios, but when it comes to food safety, every pathogen left behind presents a risk. E. coli, one of the more common foodborne pathogens, can double every 20 minutes under ideal conditions. This means that one cell can produce one million cells in about seven hours. Daily sanitation protocols that use products that deliver a minimum 6-log reduction can help keep bacteria like this at safe levels. On the other hand, if a colony is allowed to grow for a longer period of time, the risk of an outbreak increases significantly.


Log Kills Matter

Although you don’t need to be a microbiologist to implement effective sanitation protocols, you do need to understand the importance of log kills. Choosing a product with a greater than 6-log reduction and using it appropriately will help reduce the risk of an outbreak in a food processing facility or farm. D7 is effective against ([Staphylococcus aureus [(ATCC 6538)]]) [and] ([Pseudomonas aeruginosa [(ATCC 15442)]]) biofilms with a 6 log reduction. In addition to daily protocols and periodic deep cleaning, using a disinfectant or sanitizer at entry points and between zones will also help reduce cross-contamination from people and equipment moving between areas. Learn more about how D7 helped a turkey farm achieve a minimum 6-log reduction in entry point bacteria by reading this case study.

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