Complex yeast nutrient
Fermaid O, Fermaid K, organic nutrient
Nitrogen plus the amino acids, vitamins and sterols the yeast also needs.
Typical dose
Per the product sheet, staged through the first half of fermentation.
When to add it
Staged, as for DAP, and often combined with it.
In practice
Inactivated-yeast nutrients supply organic nitrogen, micronutrients and survival factors that DAP alone lacks, which matters most in the fruit, honey and grape musts short of their own. The usual practice is a blend: complex nutrient for the range of needs, plus some DAP where a large nitrogen gap remains.
Linked faults are the ones this addition either treats or, used wrongly, causes. The fault page says which, and whether it can be undone.
Others in nutrients and enzymes
Diammonium phosphate
Inorganic nitrogen for the yeast, the direct fix for a starved ferment.
Rehydration nutrient
Primes dry yeast with micronutrients before it is pitched.
Thiamine
A micronutrient that helps a ferment finish and lowers SO2 binding.
Yeast hulls
Detoxify a stuck ferment and give the yeast a surface to work from.
Pectic enzyme
Breaks down pectin to clear haze and free juice and colour.
Glucanase
Breaks down Botrytis glucans and speeds lees ageing and filtration.
Beta-glucosidase
Frees the bound aroma compounds locked up in aromatic white grapes.