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Acifying beer wort for sterilisation at normal boiling temperatures

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Posted 19 May 2018 - 08:49 PM

I hope you don't mind questions from laypeople but I've tried to fid an answer to this elsewhere without success.

I am a home beer brewer and I keep a number of brewing yeast strains on agar slants for medium term storage. The normal process for creating the agar slants is to boil water, dry malt extract and agar under 15PSI of pressure. This sterilises the slants from all pathogens including botulinum spores. On the homebrew scale most people don't have an autoclave so this is usually done with a pressure cooker capable of holding 15 PSI. Personally, I don't have a pressure cooker so when. I have done this my self I have just boilled for 30 minutes and so far everything seems to be fine.

I understand the requirement for pressure and increased temperature is for foods with higher pH, whilst foods with a pH under 4.6 can simply be boiled to sterilise.

My question is whether it would be possible to adjust the pH of the mixture before boiling, using phosphoric acid, to allow sterilisation at boiling temperature. During fermentation of beer, yeast naturally bring the pH down to about 4.3 so I don't think it would cause any problem on that front. If the pH was lowered to around 4.3 would boiling for half an hour be sufficient to sterilise?



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Posted 21 May 2018 - 03:18 PM

Boiling alone should be sufficient to kill vegetative cells; also, the low pH would prevent cells from germinating and growing.


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Posted 21 May 2018 - 07:23 PM

Props on being a thorough homebrewer.


C. bot typically isn't considered a risk in fermented or carbonated beverages, beer has the benefit of being both and ultimately naturally acidic as well, so I wouldn't worry about a C bot spore kill. The autoclave/pressure cooing spec for your agar slants is less for food safety and more for preventing spoilage of your pure cultures with surviving bacterial spores or cells.


Generally your yeast should out-compete any surviving spores if you inoculate shortly after boiling, not using the pressure method you may see some contaminated slants if you store them for some time before inoculating. More acidic foods do tend to need lower temperatures for pasteurization since the acid enhances the effect of the heat, but for complete "sterilization" (read: destruction of spores) either a much longer boil or pressure+heat would be required regardless of pH. The final low pH is just a C bot limit so you don't end up with those suriving spores germinating like they might in your media. However if you keep the agar slants exposed to oxygen or below refrigeration temperatures C. bot wouldn't grow anyway (maybe at the bottom of the slant, refrigeration is best bet).


TL;DR, Sterilization=destruction of spores, and for this standard pH is irrelevant for all but the most optimized industrial setups. Where pH becomes relevant is if you are pasteurizing/heat treating without sterilizing, then you're using pH as a control to keep those spores from growing since they didn't get obliterated. Your goal with your slants is to prevent contamination of your pure culture, which can be accomplished by cooking the heck out of them for a longer time, using heat+pressure, or possibly acidifying your slants in combination with boiling, but for your setup I would think a long boil followed by rapid inoculation of your yeast should be sufficient, monitor your cultures for contamination regularly.

Austin Bouck
Owner/Consultant at Fur, Farm, and Fork.
Consulting for companies needing effective, lean food safety systems and solutions.

Subscribe to the blog at furfarmandfork.com for food safety research, insights, and analysis.


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Posted 28 May 2018 - 04:16 AM

FFF has covered it all regarding the food safety aspects of controlling C botulinum in your home brewing process.  I have one more thought to add: 


Don't forget that your slants are for growing and protecting your yeast.  If you acidify the agar for the slants you might upset your yeast; it might grow differently or you might end up subtly altering its genetic characteristics over time.  

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