Managing nutrition throughout fermentation

Most sluggish or stuck fermentations do not start as a yeast problem — they start as a nutrition problem. Yeast needs assimilable nitrogen (along with oxygen and a controlled temperature) to build enough biomass to carry the fermentation through to dryness. When that nitrogen runs short, or is badly timed, the result shows up as reduction (H₂S, the “rotten egg” smell), heavy aromas and, at worst, residual sugar nobody wanted.

This article sums up a practical approach to fermentation nutrition and shows how GrapeFlow turns it into concrete alerts in the cellar — from the must going into tank to a dry wine.

Start with the number: the must’s YAN

YAN (yeast assimilable nitrogen) is the sum of the ammonium nitrogen and the alpha-amino nitrogen that yeast can actually use. It is the starting point of any nutrition plan: without measuring it, you are adding blind.

• Musts below ~150 mg N/L are typically deficient and are candidates for sluggish fermentations and reduction.
• Demand rises with sugar: a very ripe must (with higher potential alcohol) needs more nitrogen, in the range of 200–300 mg N/L.
• Excess is not free either: too much nitrogen, especially added late, leaves a residue that feeds spoilage organisms and microbial instability.

The secret is staging, not a single dose

Dumping all the nutrient in at the start is a common mistake. Yeast makes better use of nitrogen delivered in the right windows, and organic nutrients (yeast autolysates, amino acids, vitamins) are not the same as inorganic ones (DAP — diammonium phosphate, fast-acting nitrogen).

• Rehydration: a dedicated rehydration nutrient alongside the active dry yeast, to get a healthy population started.
• Onset (first 24 h): a complex organic nutrient — it supplies amino acids and survival factors, not just nitrogen.
• At ~1/3 sugar depletion: the classic window for the main addition (DAP + complex nutrient), when the population is at full strength and there is still enough activity to take it up.
• Avoid DAP after ~2/3 of the fermentation: the nitrogen is no longer well assimilated and tends to stay behind as residue.

Rule-of-thumb conversion: 1 g/hL of DAP supplies about 2.1 mg N/L (DAP is ~21% nitrogen). To raise YAN by 50 mg N/L you need ~24 g/hL of DAP — within the typical 20–40 g/hL range.

Temperature and kinetics: the other half of the equation

Nutrition and temperature go hand in hand. Whites typically ferment at 14–18 °C, reds at 25–30 °C — spikes outside the window stress the yeast as much as a nitrogen deficit does. What lets you act in time is the curve: regular density (or Brix/Baumé) and temperature readings for every tank.

• A density drop that suddenly slows down is the first warning of a fermentation in trouble.
• Cross-checking the curve against temperature and the addition history tells you whether the problem is thermal, nutritional, or both.

In GrapeFlow today

All of this is built and tied to the lot. It starts with an initial must analysis, before fermentation even begins:

• Initial must analysis (in the tank, before fermentation): records sugar and measured YAN. As you enter the numbers, the app immediately calculates potential alcohol, the target YAN (which rises with potential alcohol) and the deficit — with a suggested DAP addition in g/hL.
• Fermentation readings (density/Brix/Baumé and temperature) per tank, with the curve drawing itself day by day.
• Every nutrient, DAP or yeast addition is logged against the lot — traceable from must to bottle and ready for an audit or certification.

And the Today dashboard stops making you go looking: fermentation alerts appear on their own, configurable under Settings → Alerts:

• YAN deficit — the measured YAN is below the must’s target (flagged right after the initial analysis, while there is still time to correct it).
• High-sugar must — an early warning that the must will need more nitrogen, so you can plan staged nutrition.
• Nutrient addition window — the tank has entered the ideal window for the main addition (≈1/3 to 2/3 sugar depletion).
• Slowing fermentation — the daily sugar drop has fallen below what is expected, with the wine still sweet.
• Temperature out of range, and no recent reading — so nothing slips through.

And the reminders clear themselves: as soon as you log a nutrient addition against the lot, that lot’s addition-window and YAN-deficit alerts stop appearing — no need to mark them as done.

On the horizon

What we are still working on from here:

• Phone notifications, in addition to the dashboard, for the most critical alerts.

Note: the values above are practical guidelines. Always confirm against your own must analysis, bench trials and the product manufacturer’s recommendations before dosing a tank.