Maret

Two other things worth having on the shelf before batch two. Nutrients added in stages over the first few days rather than all at the start, which most mead makers work from a schedule for. And gentle degassing in the early days, which a lot of people find takes some pressure off the yeast.

On the safety question, since people ask: a hot harsh mead is a quality fault rather than spoilage. It is unpleasant, not unsafe. Back-sweetening after stabilising or blending it into a smoother batch are the other two routes if aging only gets it halfway.

You have diagnosed it yourself in the second paragraph. That taste is fusel alcohols, which yeast makes more of when it is stressed, and you stressed it in three ways at once.

The airing cupboard is the biggest. Fermentation generates its own heat, so the liquid runs several degrees above the room, and a warm room plus that means you were well above where the yeast wanted to be. No nutrients is the second, because honey is poor in what yeast needs. And three kilos in five litres is a strong must, which is strain on its own.