Cider; yeast; and sulphur smells.

The general belief is that sulphur odours (rotten eggs) are created when yeast are stressed and lack nutrients, especially nitrogen. This is a true for wine but not cider. Ciders have much lower sugar levels (about half) , which therefore requires less nutrients. This is evident because natural unadulterated cider ferments to dry at about  6-8% not 10-14% as wines can do.

Apples tend to naturally have enough nitrogen for yeasts to process all the available sugar. With cider, sulphur aromas coming from your barrel are a sign some is wrong such as the type of yeast or it’s overfed.

Although most fermentation yeasts belong to the Saccharomyces cerevisiae family, (also known as brewer’s or baker’s yeast), different strains have different genes.

These genes determine how the yeast process various compounds and the yeast’s preference for those compounds. The methods some yeast strains use to process these compounds can liberate small sulphur compounds. One of those is when the yeast is lacking nitrogen needed to process sugar and reproduce. They breakdown various amino acids or protein building blocks present that can create hydrogen sulfide (H2S) or various mercaptans and disulfides that produce the sulfur aromas. This risk is greatest with wine due to its higher sugar levels and the higher level of  nitrogen required.

For cider, the risk of creating sulphur aromas is not lack of sufficient nitrogen but having too much inorganic nitrogen!
The most common way the apple juice has too much nitrogen is because the cider maker adds "yeast nutrients". Most yeast nutrients are inorganic diammonium phosphate, commonly  called DAP.

Yeasts  love it. It is easily and quickly assimilated. It is the quick part that is the concern. Adding most yeast nutrient will generally make the yeast process sugar quickly. That leads to heat and that leads to sulphur aromas. Adding this type of yeast nutrient is usually detrimental to the cider. Not only does it create heat but fast fermentation means lots of CO2 is produced and released fast and removes the volatile aromas. This is both good and bad as most sulphur aromas are volatile. But so are positive esters and other compounds that give cider its aroma. Slow cool fermentation is therefore best.
If you feel the ferment is ‘stuck’ and want to add some nitrogen to be safe, it’s best to add an organic nitrogen sources like Fermaid-O, which is derived from inactivated yeast.

This nitrogen makes the yeast work to unlock the nitrogen and that process unlocks aroma and aroma precursor that can contribute positively to the aroma profile of your cider. At TeePee Cider I have yet to add additional nitrogen. I go for cool fermentation at 17C and over 6 months at least with wild yeasts and then EC1118 for the secondary ferment. EC1118 also known as ‘champagne’ yeast is known for its low sulphur odours. Many cider makers in cooler climes such as Britain have their cider freeze in winter. This does not kill the yeast but slows its metabolism. Fermentation restarts as the cider warms in Spring.