Why Does Everyone Say Not to Use Metal with Kombucha?

[Chris Edward]

I have heard this repeated over and over since the early 1990's and I have always wondered why you shouldn't use metal and whether people are talking about specific types of metal like iron, instead of stainless steel.
The reason I say this is because I have seen several videos of companies that produce Kombucha commercially in huge stainless steel vats and their SCOBY’s look amazing!

Is it less about SCOBY health and more about the acidic nature of the tea causing metal to leach into the tea?
I am curious.

Thank you for your help.

 

[Silver_Is_Money]

The acidic vinegar in Kombucha is highly corrosive. Components of steel such as iron, nickel, chromium, molybdenum, vanadium, boron, copper, zinc, etc... are not very good for you in excessive amounts.

 

[Chris Edward]

The acidic vinegar in Kombucha is highly corrosive. Components of steel such as iron, nickel, chromium, molybdenum, vanadium, boron, copper, zinc, etc... are not very good for you in excessive amounts.

Thank you for your reply...
I figured it had to do with the acidity.

So this means the kombucha produced by commercial brewers is tainted with metals oxides, yuk!!!

I have heard that some brewers will do an acid wash on their stainless steel tanks prior to brewing a batch of kombucha. This acid wash (usually a strong citric acid solution) will cause the stainless to create a protective oxide layer that is more difficult for weaker acids to penetrate.
But I am sure this would need to be fully tested to ensure it's safety.
A quick Google search shows that citric acid has a pH of 2.2 and kombucha has a range of pH from 3.5-2.5, so the "oxide protective layer" theory barely holds.
There isn't enough of a margin for me to trust it.

 

[RPh_Guy]

The acidic vinegar in Kombucha is highly corrosive.

Does this apply to passivated 304 stainless?
I don't think so.
Do you have any kind of scientific reference I could read for more info?

This acid wash (usually a strong citric acid solution) will cause the stainless to create a protective oxide layer that is more difficult for weaker acids to penetrate.
But I am sure this would need to be fully tested to ensure it's safety.
A quick Google search shows that citric acid has a pH of 2.2 and kombucha has a range of pH from 3.5-2.5, so the "oxide protective layer" theory barely holds.
There isn't enough of a margin for me to trust it.

FYI Citric acid, acetic acid, and lactic acid are all "weak" acids and are safe to use in passivated 304 stainless.

Stainless steel isn't just the standard for kombucha brewing, but it's standard across the entire food, beverage, and medicine industries. You think Heinz is making ketchup using glass or plastic vessels? Are there special kegs for traditional sour beer with acetic acid? Nope.

FYI, pH is a logarithmic scale, so there is actually a fairly significant margin between 2.2 and 2.5, but that's focusing on the wrong thing. What matters is the type of stainless, proper passivation, and whether the particular acid is reactive enough to corrode it.

 

[Silver_Is_Money]

I was merely parroting what I had overheard at a local LHBS while they were equipping my wife to make Kombucha roughly two years or more ago. A quick check of this issue on the web indicates that indeed many LHBS are now offering SS vessels for Kombucha, so it appears that it should be fine for this application.