Control, not luck
The reference we wanted on the cellar wall: sulphite by pH, feeding the yeast, managing acid, stabilising, and working out what went wrong.
SO₂ and pH
Sulphite protects wine only in its molecular form, and the share of free SO₂ that is molecular falls steeply as pH rises. A dose that protects a wine at pH 3.2 does very little at pH 3.8. That is why sulphite should be dosed by pH and not by habit.
The usual targets are about 0.8 mg/L molecular SO₂ for whites and rosés and 0.5 mg/L for reds. The free SO₂ needed to reach them:
| Wine pH | For 0.5 molecular (reds) | For 0.8 molecular (whites) |
|---|---|---|
| 3.0 | 8 | 13 |
| 3.1 | 10 | 16 |
| 3.2 | 13 | 20 |
| 3.3 | 16 | 26 |
| 3.4 | 20 | 32 |
| 3.5 | 25 | 40 |
| 3.6 | 31 | 50 |
| 3.7 | 39 | 63 |
| 3.8 | 49 | 79 |
Potassium metabisulphite is about 57.6% SO₂ by weight, so:
grams of KMeta = (mg/L to add × litres) ÷ 576
To raise 23 L by 30 mg/L, that is 30 × 23 ÷ 576, or about 1.2 g. Some of each addition becomes bound and stops protecting the wine, so measure free SO₂ a day or two afterwards if you can, and again before bottling.
The free SO₂ needed climbs quickly and can become noticeable in the glass. It is usually better to lower the pH with a small tartaric acid addition than to keep adding sulphite.
The SO₂ Calculator takes your pH, volume and current level and gives the dose in grams of KMeta or in Campden tablets.
Feeding the yeast
Yeast needs nitrogen to build cells. Grape must often has enough; honey, most fruit and sugar-heavy musts do not. A hungry fermentation is slow, prone to sticking, and the usual source of rotten-egg smells.
- Think in YAN. Yeast-assimilable nitrogen needs run from about 150 mg/L for a modest must and an undemanding strain, to 250 mg/L or more for high sugar or a demanding strain.
- Stagger the additions. Split the nutrient into three or four doses: the first once fermentation is visibly active, the rest over the following days, with the last by the time about a third of the sugar is gone.
- Stop inorganic nitrogen early. Yeast takes up DAP poorly once alcohol builds. Late in a ferment, use an organic (yeast-derived) nutrient or none.
- Keep DAP out of rehydration water. Use a rehydration nutrient made for the purpose, or plain water at the temperature on the packet.
The Nutrient Calculator builds a step-by-step feeding plan for wine, mead or cider from your volume and starting gravity.
Step feeding for high alcohol
Putting all the sugar in at the start of a strong mead or dessert wine stresses the yeast before it has begun. Step feeding starts at a comfortable gravity and adds the rest in stages.
- Start at a moderate gravity, around 1.090 to 1.100.
- When gravity falls to about 1.010 to 1.020, add enough sugar or honey to bring it back up by 0.010 to 0.020.
- Repeat until the yeast stops taking the additions, which means it has reached its alcohol tolerance.
- Record every addition. Total alcohol comes from the sum of all the gravity drops, not from the first and last readings.
Dissolve each addition in a little of the wine first, and add it gently: stirring dry sugar into an active ferment releases a lot of gas at once and can foam over.
Temperature
| Style | Fermentation range | Why |
|---|---|---|
| Whites, rosés, aromatic fruit | 13–18 °C (55–65 °F) | Cool, slow ferments keep delicate aromas. |
| Reds | 21–29 °C (70–85 °F) | Warmth extracts colour and tannin from the skins. |
| Mead and cider | 15–20 °C (60–68 °F) | Cooler ferments give fewer harsh, hot alcohols. |
A steady temperature matters as much as the number. A swing of several degrees overnight can stall a ferment that was otherwise healthy, and fermentation makes its own heat, so a large batch can run a few degrees above the room.
Acid balance
pH governs stability and how well sulphite works; titratable acidity (TA) governs how the wine tastes. They move together but are not the same measurement, so check both when you can.
| Style | Typical pH | Typical TA |
|---|---|---|
| Whites | 3.1–3.4 | 6–9 g/L |
| Reds | 3.4–3.65 | 5.5–7 g/L |
- To raise acidity, add tartaric acid: about 1 g/L raises TA by roughly 1 g/L. Add part, mix, measure, and taste before adding the rest.
- To lower acidity, potassium bicarbonate removes about 1 g/L of TA for roughly every 1 g/L added. Blending with a softer wine is gentler on flavour.
- Adjust early. Changes made to the must integrate better than changes made to the finished wine.
- Malolactic fermentation turns sharp malic acid into softer lactic acid and raises pH slightly. Keep sulphite low until it has finished.
Stabilising and back-sweetening
Any sugar you add to a wine that still contains live yeast will ferment. To sweeten a wine safely:
- Wait until fermentation is finished and the wine is clear. Stabilisers work best when there is very little yeast left in suspension.
- Add sulphite to your pH-based target, and potassium sorbate at the rate on the packet (typically around 0.2 g/L).
- Wait a day or two, then sweeten to taste. Do trials in a measured glass first and scale up.
- Hold the wine for a couple of weeks before bottling, and check the gravity hasn't moved.
It doesn't kill yeast; it stops it multiplying, so it needs sulphite alongside it. And it should not be used in a wine that has been through malolactic fermentation, where it can produce a geranium-like off-odour.
Troubleshooting
| Symptom | Likely cause | What to try |
|---|---|---|
| Gravity stops falling above 1.000 | Temperature too low or too high, not enough nutrient, or alcohol beyond the yeast's tolerance | Bring it to about 20–22 °C and stir gently. If nothing moves in a few days, build a starter of a strong strain such as EC-1118 and add the stuck wine to it in stages. |
| Rotten-egg smell | Hydrogen sulphide from stressed, underfed yeast | Early in the ferment, add nutrient and stir to let the gas off. Later, rack with a little splashing. If it persists, treat promptly; it gets harder to remove with time. |
| Haze that won't clear | Pectin (fruit wines), protein, or yeast still in suspension | Give it time and a cool spot. Pectic enzyme helps fruit hazes; bentonite helps protein haze. |
| Browning, flat or sherry-like taste | Oxidation from too much headspace or too little sulphite | Top up vessels, check free SO₂ against pH, and limit splashing after fermentation. |
| Vinegar or nail-varnish smell | Acetic bacteria, which need oxygen to work | Mild cases can be halted with sulphite and a full, sealed vessel. A strong vinegar smell cannot be reversed. |
| Fizz in a wine meant to be still | Dissolved CO₂, or fermentation that restarted | Check the gravity over a week. If it is stable, degas by stirring. If it is falling, let it finish before bottling. |
Every answer above starts with "what were the readings?". A dated record of gravity, temperature, pH and additions turns a mystery into a diagnosis.