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YAN and yeast nutrient in cider
If a cider throws a rotten-egg or struck-match smell partway through fermentation, the most common cause is nitrogen-starved yeast. Apple juice is naturally low in yeast-assimilable nitrogen (YAN) — much lower than grape must — and stressed yeast produces hydrogen sulfide (H₂S) as a byproduct. Understanding YAN is how you prevent the problem instead of chasing it.
Why apple juice runs low
Yeast needs nitrogen to build cells and ferment cleanly. Grape must often carries 200+ ppm YAN; apple juice frequently sits well below that — sometimes under 100 ppm. When the available nitrogen runs out mid-ferment, the yeast scavenges nitrogen from sulfur-containing compounds and releases H₂S along the way. The same deficiency also raises the risk of a sluggish or fully stuck fermentation.
Dosing nutrient
If your YAN is low, a measured nutrient addition keeps the yeast fed. Two common families:
| Nutrient | Notes |
|---|---|
| DAP (diammonium phosphate) | Pure inorganic nitrogen; fast-acting, easy to overdose |
| Complex/organic blends | Nitrogen plus vitamins and micronutrients; gentler, often preferred |
Add nutrient early-to-mid fermentation rather than all at the start, and don't overshoot — excess nitrogen feeds spoilage organisms and can leave the cider tasting worse, not better.
Keeving is the deliberate exception. Traditional French/English keeving intentionally strips nitrogen to slow fermentation and retain natural sweetness. If you're keeving, low YAN is the goal — don't "fix" it with nutrient. Know which path you're on before you add anything.
Acting early helps — splash-racking to aerate and a nutrient addition can rescue a ferment before H₂S oxidizes into harder-to-remove compounds. Left too long, sulfur faults become much more stubborn.
Press flags low YAN before it bites
Enter your juice details and Press estimates whether your must needs a nutrient addition and how much — helping you head off H₂S and stuck ferments, with guidance from extension cidermaking sources. Free to download.
Sources
- Cornell University hard-cider program (G. Peck) — fermentation management and YAN
- University of Vermont Fruit Program (T. Bradshaw) — cider nutrient and H₂S guidance
Nutrient needs vary by juice and yeast strain — measure where you can and add conservatively.