I've made fermented garlic paste many times before.
It's essentially a bunch of raw garlic cloves blended with 2% salt by weight, then left in a container to ferment at room temperature for 2-3 weeks before being stored in the fridge. The paste turns a deep amber colour and should have a deep, savoury garlic smell.
However... my latest batch had a problem. I made it exactly as before except this time I made more than usual and stored it in a large, deep, bucket-like container.
I used it for a few weeks and it looked/smelled/tasted totally fine, until one day I went to use it and I noticed that the paste beneath the surface in the middle of the container looked a lot lighter in colour (white instead of amber) and it had a really strong, acrid smell.
It wasn't a "rotten" or mouldy smell; it just smelled extremely bitter, like if you've ever put a pill in your mouth without water.
There was no sign of mould but I discarded the entire container anyway (3KG of garlic wasted!).
What might have happened, and how can I avoid it happening in future? Is it possible the garlic under the surface didn't ferment at all? That would explain the difference in colour. Or was there some other chemical reaction?