Fermentation

Phenomenon

In recent years, process descriptions on coffee bags have been getting longer: "anaerobic fermentation, 72 hours", "carbonic maceration", "lactic fermentation", "yeast-inoculated", "barrel-fermented". The flavor notes have grown louder too: lychee, cinnamon, rum, liqueur chocolate. The prices are often higher.

Some people find these coffees stunning; others find them "too much — it does not taste like coffee". And some hear for the first time that coffee is "fermented" and feel uneasy: isn't fermentation just going off?

What do microbes actually do during processing? And what do these long names really tell us?

Variables

As Chapter 7 put it, microbes are active during both washed and natural processing Confidence: [Strong Evidence]. Fermentation is not a step owned by one process; it is what happens whenever the fruit's sugar is present and wet. The variables that change it:

"Controlled fermentation" simply means managing these variables on purpose.

Experiment

Thought experiment: split one batch of depulped seeds between three containers.

• A: an open tank, washed after 24 hours;
• B: a sealed barrel, washed after 72 hours;
• C: a sealed barrel, 72 hours, but somewhere much warmer than B.

Write down your prediction before reading on

Which of the three will have the most different microbial community? Which is most likely to over-ferment? If the bags say "washed", "anaerobic 72 h" and "anaerobic 72 h", will the last two taste the same?

Your prediction

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Second experiment: two farms both write "anaerobic fermentation" on the bag.

• Farm 1 packs whole cherries into a plastic barrel, closes the lid tight and leaves it in a shed;
• Farm 2 puts the cherries into a stainless-steel tank, flushes out the air with carbon dioxide, and holds it at a fixed temperature.

Write down your prediction before reading on

What differences between them does the word "anaerobic" tell you about? What does it not tell you?

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Observation

First experiment: B and C differ only in temperature, and the warmer it is, the faster microbes work Confidence: [Established Mechanism], so C is the most likely to over-ferment. A and B differ not only in time but in oxygen: when oxygen is scarce, microbes that depend on it are held back, and yeasts and lactic acid bacteria, which can live without it, gain the upper hand Confidence: [Established Mechanism]. All three may differ in microbes, products and flavor — and the last two carry the same name.

Research offers some clues:

• in washed fermentation, lactic acid bacteria and yeasts grow the most Confidence: [Strong Evidence] (Chapter 8);
• in one study where yeasts were suppressed with a drug, the seeds contained far less isoamyl alcohol, ethanol, acetaldehyde and ethyl acetate; the differences persisted after roasting, and cupping scores were lower Confidence: [Strong Evidence];
• when chosen yeasts were inoculated into a semi-dry process, the inoculated yeasts were detected throughout fermentation, and tasters found a caramel note in the inoculated coffee that was absent from the control Confidence: [Strong Evidence];
• when natural and pulped coffees underwent "self-induced anaerobic fermentation" (sealing the cherries so the microbes use up the oxygen themselves), the fruity character was more intense Confidence: [Strong Evidence];
• in a method borrowed from winemaking, carbonic maceration, both fermentation temperature and time affected the cupping score Confidence: [Strong Evidence].

On the other side, reviews repeatedly note that well-controlled fermentation can bring desirable flavors, while uncontrolled fermentation produces off-flavors Confidence: [Strong Evidence].

Second experiment: both farms say "anaerobic", but one is whole cherries in a plastic barrel at ambient temperature, and the other a CO₂-flushed, temperature-controlled steel tank. In the trade, "anaerobic" is a loose umbrella term, not a standardized set of steps Confidence: [Common Practice]. It tells you only that some step happened with limited oxygen.

Model

Fermentation is not spoilage, and not magic

Fermentation and spoilage often involve the same kinds of microbes and the same kinds of reactions: eating sugar and producing acids, alcohols, esters and other compounds. The difference lies in control and outcome:

• fermentation is steered toward what people want, and stopped at the right time (by washing, by drying);
• spoilage is what happens when nobody steers, or steering fails, and unwanted things are produced.

The acids made during fermentation make the environment more acidic, which by itself holds back some microbes that cannot tolerate acid Confidence: [Established Mechanism]. That is why humans use fermentation to preserve food, not just to let it go bad.

Oxygen is a knob, not a brand

"Anaerobic" is, at heart, turning the oxygen knob: limiting oxygen changes which microbes can grow and which win. But two "anaerobic" coffees can sit far apart on all the other knobs:

• whole cherries, or depulped seeds;
• spontaneous, or inoculated;
• what temperature, how many hours;
• how it was dried afterward.

So one "anaerobic" label can cover very different coffees.

From the tank to the cup is a long way

For something microbes make in the fruit to become flavor in your cup, it has to pass at least three gates:

1. Does it get into the seed?
2. Does it survive drying and storage?
3. Does it survive roasting — or turn into something else during it?

Research has found some evidence of compounds passing the gates, such as the alcohols and esters above that were still higher after roasting. But overall, which fermentation products end up affecting cup flavor, and how, does not yet have a complete answer Confidence: [Open Question]. Most of these studies also come from a few origins and a few lots, so their conclusions cannot simply be applied to all coffee.

Why a name cannot guarantee flavor

Putting the pieces together:

• under one name, the microbes, temperature, time, oxygen and raw fruit can all differ;
• how microbial products become flavor is not fully understood;
• cup flavor also depends on origin, variety, ripeness, drying, roasting and brewing (Chapter 7).

A name tells you what type of thing was done, not what result came out.

Was anything else added?

There is another family of practices that adds fruit, spices or other ingredients during fermentation, sometimes called "co-fermentation". The trade is still debating whether such practices are appropriate and how they should be labelled Confidence: [Common Practice]. As a coffee drinker, the question you can ask is simple: apart from the coffee cherry itself, what was added?

Transfer

In the Processing Mill scene you can compare the three pathways: where fermentation happens on each, and how much fruit around the seed is available to microbes. When reading a bag:

Whether you like an intense fermented style or prefer a clean traditional one is preference Confidence: [Preference]. But "anaerobic" by itself is no proof of quality.

Misconceptions

"Fermentation means the coffee has spoiled." Fermentation and spoilage use similar microbes and reactions; the difference is whether the process is controlled and whether the result is wanted. Almost all coffee goes through some fermentation during processing.

"Anaerobic fermentation guarantees quality." "Anaerobic" only says that oxygen was limited at some step. Done well, it can bring welcome flavors; done badly, it gives over-fermented off-flavors like anything else. Nor can it make up for problems with ripeness, drying or storage.

"'Anaerobic' names one standardized process." Different producers use different vessels, temperatures, times and raw material, and all call it "anaerobic".

"The longer and stronger the fermentation, the better." Longer and stronger is a style choice with its own risks, and some people dislike it. "More is better" does not apply to fermentation.

Real Lab

There is no registered lab card for this chapter. Here is a blind tasting you can do at home to separate "I like it" from "it is better".

You need: two coffees from the same roaster — one traditional washed or natural, one labelled "anaerobic" or another special fermentation — ideally from similar origins and at similar roast levels.

1. Before brewing, read both labels and write down your predictions: which cup will smell stronger? Which will taste more "like coffee"? Which will you prefer?
2. Brew each with the same recipe, water and grind setting (Chapter 24).
3. Have someone label the cups A and B without telling you which is which.
4. Taste them together once they reach a similar temperature. Record two things separately: a description (aroma, acidity, sweetness, body, finish, any vinegary, boozy or muted-fruit notes) and how much you like it (1 to 5).
5. Reveal the answer and compare with your predictions.

If you find you prefer the traditional cup — or the special fermentation — that does not mean one is "higher quality". That question belongs to Chapter 35.

Check Your Model

Predict. A producer turns one batch of cherries into two "anaerobic" lots: one sealed for 48 hours at 18 °C, the other for 96 hours at 30 °C. Which lot do you expect to show a stronger fermentation style, and carry more risk?

Diagnose. You buy an "anaerobic natural" with notes of "strawberry, cream", but the cup has a sharp vinegar smell and a nail-polish-like note. What could be going on?

Explain. In your own words, explain why the difference between "fermentation" and "spoilage" lies not in the microbes but in control.