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Why does my preserved garlic turn blue/grey ?

The sulfur compounds in garlic (thiols of some type) can be broken up by active enzymes in the garlic allowing the sulfur to react with any copper in the solution. This results in copper sulfides which is what you see as the "blue" discoloration. The enzymes can be denatured (inactivated) by high temperature processing.

The enzymes are more abundant in immature garlic. The amount of copper required is tiny and can be present in the garlic, the salt or any other ingredient. In any case the blue discoloration presents no hazard.

The conventional wisdom for eliminating blue garlic seems to be:

1.) Use mature garlic (lower enzyme content)

2.) Process at high temperature (denature enzyme)

3.) Use "canning salt" (generally very low in copper)