In the introduction, we have seen that yeast turns sugar into alcohol. Honey provides this sugar. But yeast needs other nutrients on top of sugar: nitrogen, minerals, vitamins. Honey provides some of these nutrients. Darker honeys generally provide more minerals and nitrogen. But, in general, we add additional nutrients: nitrogen in the form of diammonium phosphate, urea or amino acids, magnesium sulfate and vitamins can also be added. For ecologists, it is possible to decide not to add chemicals, but then fermentation is not reliable. One sometimes has to choose between poor diet and chemical diet. Fruits can also be added to provide nutrients (see adding fruits).
Berry and Brown describe in detail the nutritive needs of yeast [B10]. H11 describes various experiments of mead fermentation with various combinations of ammoniated compounds and vitamins.
What amount to add?
Follow the directions on the box. Are they reliable? Hard to say, they must be suitable for wine or beer where the need for nutrients is not so critical. Unless the weather is awful, it should not be necessary to add nutrients to make wine. But, as far as mead is concerned, a fair amount of nutrients come from what we add. Does the seller overestimate quantities so that yeasts receive more nutrients than they need so that fermentation is as fast as possible in order to prove how efficient their stuff is?
Commonly used are tartaric, malic and citric acids. They are not related to honey and it is not sure that they are the most suitable ones but they are the acids of grape, and so they are the ones everybody uses (copycat).
Tartaric acid is the main acid of grape. It can react with K+ ions (potassium) and form potassium bitartarate (cream of tartar) which will fall to the bottom of the bottles if they are cooled (see crystals).
Malic acid is mainly found in apples, peaches, pears, etc. [B3]. Some yeasts and bacteria can metabolize it. Malo-lactic fermentation, due to bacteria, turns malic acid into lactic acid (sensory threshold: 400 p.p.m. [B10]). Lactic acid being less acidic than malic acid, the result will taste less acidic. This phenomenon is used to improve the quality of acidic wines.
Citric acid is found in citrus, cherries, etc. It can be metabolized (consumed) by yeasts. It is therefore likely that citric acid added at the beginning of fermentation will have vanished at the end of fermentation.
See also addind acid in Syntheses.
Tartric acid is harsher, malic is greener and citric is fresher [Mowbray in V4]. Malic acid is slightly less sour and is a bit fresher tasting than tartaric acid [V9].
Vitamin C (ascorbic acid) is an antioxidant. It is not used instead of sulfite which is also an antiseptic, but it allows to reduce the amount of SO2 to be used. Vitamin C is never used without SO2 [V4, V6].
The legal limit in France is 100 mg/L but there is hardly a limit in the U.S. [V4].
It is a non-toxic non-saturated fatty acid. It prevents yeasts from multiplying and from refermenting sugar but does not kill them [Peynaud in V4]. It is used to prevent the fermentation from restarting in sweet meads. In many countries, adding of 200 to 300 mg/L of sorbic acid is permitted [V6], the maximum is 200 mg/L in France [V4] and 1000 mg/L in the US [V6]. The sensory threshold is 135 mg/L [V6].
Its action is reinforced by [V4]:
Solubility of sorbic acid in water being low, potassium sorbate is used instead. 270 mg of sorbate give 200 mg of sorbic acid. It is added slowly to the mead (which must contain only few yeast cells), with a vigorous agitation [V4]. See also geranium smell in A problem?.