I have been asked repeatedly to post why I will not endorse active fermentation in a Fido and what I have found about the issues of pressure, carbon dioxide build-up and such in ferments. Do they produce ferments of the same quality at a cheaper price? If they hurt it, how much and what do I loose? Is a separate airlock really necessary or will just the seal that makes it airtight do? These questions have been swirling for weeks.
I finally had the time this week to sit down and begin pulling my notes from the last few months together. I will link to and discuss just a handful of studies in this first part. This is the first post in a series on carbon dioxide and pressure. There’s more to come. Many thanks to those of you who have waited patiently for this post, appreciating that real life and my family has to come first for even the busiest of bloggers. If you’d like to see more fermentation posts I have written, check out the Related Posts section below for plenty of links and great info. We also have a fermentation class where you can learn more about fermentation, how to use and care for the vessels and how to make a wide variety of ferments and you receive free access to our Real Food Cooking School as a bonus. We also have a facebook group if you’d like to chat with others.
Pressure and Fermentation
First, let’s discuss the pressures involved in fermentation. From the information I could find, a separate, three-piece airlock burps itself as low as .1 psi, depending on the type. That’s pretty low. I couldn’t find any information about water moat fermentation vessels such as the Harsch Crock, however I must assume that they are similarly low, because they do not produce the characteristic pop/hiss that pressurized vessels give when opened (not to be confused with the hiss that can occur when gas is escaping through the airlock on a very active ferment) and you don’t see the mad rush of dissolved carbon dioxide returning to being a gas and rushing to escape. With this low of a pressure, carbon dioxide isn’t forced to dissolve into the brine and instead remains a gas, working its way up through the ferment and out of the airlock- that’s the little bubbles you see in the veggies and bubbling through the airlock. If the CO2 dissolves, you can’t see it. All of the gas in a ferment being visible and having an easy route of escape while preventing any outside air from getting in is a good thing.
I found varying quotes for the psi of different ferments- for example, pickles average around 15 psi. Psi is pound-force per square inch. A hermetic container with an off-gassing ferment in it that is very active (think water kefir) could easily build up pressures in excess of 15 psi, even if it does release small amounts of gas on occasion as some people have reported a Fido does. I strongly suspect from personal experience and in talking with others that water kefir can go higher than that. A realistic pressure test would have to take into account both the psi, the varying pressures in the different stages of fermentation and the length of time involved in a complete ferment.