Primary fermentation is where the yeast turn honey water into mead. It is the busiest phase of the batch, and the decisions you make here will shape almost everything that follows. In this sixth part of the Intro to Making Mead series I will explain what primary fermentation actually is, when to add ingredients to the must, and how to avoid the most common mistake beginners make: moving on too soon.

What Primary Fermentation Is

Primary fermentation is the first stretch after you pitch your yeast. During this phase the yeast consume sugars from the honey and produce ethanol, carbon dioxide, and a range of flavor compounds. In a typical mead, primary fermentation lasts anywhere from one to four weeks, depending on the yeast strain, temperature, starting gravity, and nutrient availability.

The most obvious sign of primary fermentation is bubbling in the airlock. That bubbling is carbon dioxide escaping from the fermenter. Early on it can be aggressive, sometimes pushing foam and liquid through the airlock. After a few days the activity usually settles into a slower, steadier rhythm.

A lot happens below the surface during this phase. Yeast do more than produce alcohol. They also generate esters, phenols, and other aromatic compounds that contribute to the final flavor. They reproduce and build a population large enough to finish the job, and they drop sediment — called lees — as they begin to tire.

Adding Ingredients to Primary

One of the first decisions you will make is whether to add fruit, spices, or other flavorings during primary fermentation or wait until secondary.

Adding additions in primary has some advantages. The yeast activity helps extract color, flavor, and aroma quickly. It can also help sanitize ingredients to a degree, since the active fermentation creates a low-oxygen, acidic environment that discourages many spoilage microbes. Some brewers add fruit in primary for a more integrated, fermented-fruit character.

There are downsides too. The vigorous CO2 production can scrub away delicate volatile aromatics, especially from lighter ingredients like flowers or subtle spices. Fruit added in primary will often lose its fresh character and take on a more cooked or wine-like quality. Yeast can also settle into fruit pieces, making racking and cleanup more difficult.

I generally keep primary simple. Honey, water, yeast, and nutrients go into the fermenter. If I want a strong integrated fruit character — for example, in a berry melomel — I might add the fruit in primary. For delicate additions like rose petals, hibiscus, or spices, I prefer secondary. That gives me more control and preserves the qualities I am after.

Temperature, Oxygen, and Nutrients

Temperature is one of the biggest factors in how a mead ferments. Each yeast strain has a preferred range, and staying inside it matters. Too cold and fermentation can stall or produce sulfur aromas. Too warm and the yeast may create fusel alcohols and harsh flavors that take months to age out. I keep my primary fermenter in a stable spot, away from direct sunlight and big temperature swings.

Yeast also need oxygen early in fermentation. For the first few days, I aerate the must by swirling the fermenter or using a sanitized whisk to introduce oxygen. This helps the yeast build a healthy population. Once fermentation is active, I stop adding oxygen and keep the airlock in place. Oxygen after that point can oxidize the mead and create off-flavors.

Nutrition is just as important. Honey is low in nitrogen and minerals, so I use Go-Ferm during rehydration and add Fermaid O or Fermaid K in stages during primary. The first addition usually goes at pitch, the second about 24 hours later, and the third around the one-third sugar depletion point. If I miss this and the ferment starts smelling like rotten eggs, I add more nutrient right away and degas gently.

When Primary Fermentation Is Actually Over

This is the part where patience pays off. The airlock is not a reliable indicator on its own. Bubbles may slow down or stop long before the yeast are finished, especially in a cooler room where fermentation happens more slowly. The yeast may still be working through the remaining sugars, just without producing visible bubbles.

Even when the airlock looks completely still, the yeast may be in a cleanup phase. After the bulk of sugar consumption, yeast continue to metabolize byproducts and improve the flavor of the mead. Moving the batch off the yeast too early can leave off-flavors behind or stall the fermentation before it is truly finished.

The best way to know primary is done is to take gravity readings with a hydrometer. When the specific gravity is stable across two or three readings taken over several days, the yeast have likely stopped consuming sugar. Only then do I consider racking to secondary.

I also pay attention to the visual cues. The foam cap, if there was one, will have collapsed. The must will look clearer as yeast begin to settle. The sediment layer at the bottom will be thickening. Those signs, combined with stable gravity, tell me the yeast are winding down.

Final Gravity and Estimating ABV

When I decide primary fermentation is over, I take one last gravity reading just before racking to secondary. This is the final gravity, or FG. Comparing it to the original gravity tells me how much sugar the yeast consumed and gives a reasonable estimate of the alcohol by volume.

A hydrometer measures the density of the must compared to water. Sugars make the must denser, so fresh honey water reads high — often around 1.080 to 1.120 for a standard mead. As yeast consume sugar, the gravity drops. A dry traditional mead may finish near 1.000 or slightly below, while a sweet or sack mead finishes higher because residual sugar remains.

The simple homebrew formula is:

Estimated ABV = (OG − FG) × 131.25

The number 131.25 comes from the chemistry of fermentation. When yeast consume sugar, each gram of sugar produces a predictable amount of alcohol and CO₂. The factor 131.25 is a rounded empirical constant that converts the gravity drop — how much denser the must was than water at the start versus at the end — into an approximate alcohol percentage. It is not exact for every must because different sugars, yeast strains, and fermentation conditions change the conversion slightly, but it is accurate enough for home mead making.

For example, if the OG was 1.100 and the FG is 1.010:

(1.100 − 1.010) × 131.25 = 11.81% ABV

If the mead finished sweeter at 1.030:

(1.100 − 1.030) × 131.25 = 9.19% ABV

I record both readings in my brewing log. This estimate is close enough to know what you are pouring and to compare batches over time.

My Process

I let primary run until gravity is stable for at least three days. I do not rush it, and I do not move the batch just because the airlock has gone quiet. Once the readings hold steady, I take a final gravity reading, estimate the ABV, and rack the mead off the lees and into secondary.

The biggest lesson I have learned is that fermentation has its own timeline. The yeast are not watching the calendar. Trust the hydrometer, trust your eyes, and give the yeast time to finish their work.