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Expert: John Locke - 1/6/2008


Question
I work in a food safety lab and prepare many packaged products for analysis by spraying with an ethanol/methanol solution prior to sampling.  It is my understanding that a 70% dilution of ethanol (or other alcohol) is the most effective/lethal concentration to effectively kill vegetative state microorganisms.  The stock solution I use for preparing the diluted stock is 85% ethanol, denatured with 15% methanol.  To achieve peak bactericidal concentrations and maximum economic use of the stock should I be diluting the ethanol to a 70% dilution or the stock in its entirety to a 70% dilution.

I appreciate your consideration on the matter and look forward to your response.

Answer
You ask a good question, Richard, one that highlights the importance of attention to detail. Because the stock solution is only 85% ethanol, diluting the entire stock solution to 70% will result in an ethanol concentration below 70%, and thus below the preferred bactericidal concentration. Hence, you should be diluting the ethanol itself to a 70% concentration; the methanol that's present will also have a bactericidal effect, but you can't treat it as thought it's necessarily cumulative. That is, you can't assume that the bacteria surviving treatment with ethanol will necessarily be killed by methanol exposure. If ethanol is your bactericidal agent, then that's the substance you must dilute to 70% concentration.

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John Locke

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I will answer all biology-related questions through the undergraduate level. I will explain unclear concepts and suggest approaches to solving problems, but would prefer not to completely solve homework problems for you. If you are completely stumped on homework, tell me what you already know and I will help you as much as possible. Please do not ask me for ideas on school research projects; part of research is determining a suitable area of investigation, and that's not a task that should be completed by someone else. Please don't simply send me your homework for solutions. If you are having difficulty after you have started an analysis, I will be happy to direct your thinking; in particular, I would prefer to not simply solve pedigrees for students, but I will be happy to assist in solving pedigrees that you have already started. If you don't understand how to analyze a pedigree, I'd highly recommend watching this video, in which a biology professor explains the basic concepts of pedigree analysis: http://www.youtube.com/watch?v=HbIHjsn5cHo

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I have a BS in Bioengineering with a concentration in Chemical Engineering (which included a heavy focus on biology), and have taught biology, biochemistry, and related subjects for some time now.

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BS Bioengineering, Penn State University MCAT/DAT/OAT Instructor

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