Coffee Fermentation: Time, Temperature and pH

Picture two lots from the same block. Same variety, same week of picking, same drying yard. One cups clean and sweet. The other tastes like someone left it in a wine glass.

Nine times out of ten, the difference happened during fermentation.

It's the least visible step in processing and the one that decides the most. So let's open the tank and look inside.

So what's actually happening in there?

After the cherry is pulped, the bean comes out wearing a sticky coat called mucilage — a sugar-rich gel clinging to the parchment. Fermentation is how that comes off.

Yeasts and bacteria arrive, eat the sugars, and produce acids, alcohols and enzymes. The enzymes break down the pectin holding the mucilage in place. Think of it like proving bread dough: you're not just waiting, you're letting microbes do a job on a clock you only partly control.

Two things come out of it, and both matter to you:

1. The mucilage lets go, so the coffee can be washed clean. That's the mechanical job.
2. Flavour compounds move into the seed, and some survive drying and roasting. That's the part you taste.

Stop too early and the mucilage won't wash off. You get patches that dry unevenly and roast unevenly. Go too long and you get sour, vinegary, onion-ish notes that no roast profile will rescue.

The four things that change everything

Time. Anywhere from a few hours to well over a day. On its own it tells you almost nothing, because the same twelve hours behave differently depending on everything below.

Temperature. This is the big one. Fermentation speeds up sharply as it gets warmer, so twelve hours at 28°C and twelve hours at 18°C are not the same process at all. Cooler and slower usually gives you a cleaner, more layered cup.

Altitude helps here, and so does shade — both pull the temperature down at the point where it counts.

pH. The most useful number anyone can hand you. As fermentation runs, acids build up and pH drops. Track it and you know exactly where you are, whatever the weather is doing.

A pH meter costs very little next to the value of a container. If a producer is using one, they're controlling the process. If they aren't, they're relying on experience — which works beautifully right up until the season turns odd.

Water. Underwater fermentation buffers the temperature and keeps some oxygen out. Dry fermentation runs faster and swings more. Water quality matters too, because dirty water brings its own microbes to the party.

How farmers have always judged it

The classic test is your hands. Rub the parchment between your palms — when the mucilage has gone, it stops slipping and starts to squeak. Push a stick into the tank and pull it out; if the hole holds its shape, you're done.

These work, and people who have done it for thirty years are very good at it.

The catch? Nothing gets written down. Nobody else can repeat it, and it doesn't travel well when the weather does something unfamiliar.

What "controlled" actually means

Plenty of people will tell you their fermentation is controlled. Here's how to tell whether it is.

Ask for these five, per lot:

• Vessel and volume
• Temperature through the ferment, and whether anyone managed it
• Duration
• pH at the start and end, or better, the curve
• Underwater or dry, and whether anything was inoculated

If those numbers exist, the process can be repeated and a good result wasn't luck. If they don't, a good lot is still a good lot — but next year is a separate conversation.

The same test applies, even harder, to anaerobic and experimental lots, where there is more to get wrong.

Where the faults actually come from

Two causes explain most of what lands on your cupping table as a defect.

Cherry sitting around before pulping. Once it's picked, cherry starts fermenting inside the fruit — unevenly, with nobody steering. Same-day pulping is the single biggest free quality win available on most farms.

Losing the temperature. A tank left through a hot afternoon races ahead of the same tank overnight. That's where the sour and phenolic notes come from.

Both are about how a farm is run, not what equipment it owns. Which is why fermentation quality tracks the operation far more closely than the budget.

What to ask before you buy

Five questions, and you can send them in one email:

1. How long between picking and pulping?
2. What do you ferment in, and for how long?
3. Do you measure temperature? Do you track pH?
4. Underwater or dry?
5. How do you decide it's finished — by measurement, or by hand?

You will learn as much from how quickly the answers come back as from the answers themselves.

How we do it

At Hoysala, fermentation is a 72-hour anaerobic ferment.

The blocks run from 900 m to 1,450 m, and about 65% of the estate sits under shade — which keeps temperatures down at exactly the point in the day when a tank is most likely to run away from you.

Want to taste what that produces? The offer list carries grade, screen, moisture and cup notes for every lot, and samples are a short email away.