Why Warmth Is the Secret Ingredient for Winter Fermentation

It happens every year. The first chilly nights arrive, and suddenly someone writes to us saying, “My kefir hasn’t set!” or “My sourdough starter looks sleepy.”

It’s not you, and it’s not your grains or starter. It’s the cold.

Fermentation is a living process, powered by tiny microbes—yeasts and bacteria—that love a cosy environment. When the kitchen cools down, they slow right down too. Think of it like us on a winter’s morning: we’re not exactly leaping out of bed when the room is freezing.

Why Does Fermentation Slow Down in Winter?

In summer, milk kefir grains can transform milk into a tangy, creamy drink overnight. By winter, the same grains may take two or even three days, producing thin, sweet milk that leaves you wondering if something’s gone wrong.

Sourdough starters tell a similar story—where they once doubled in a few hours, they now take half a day or more to show signs of life. Kombucha just sits there, stubbornly still, with no fizz to speak of.

The culprit is simply temperature. Most cultures are happiest around 22–28 °C. A kitchen sitting at 16 °C in winter feels like an ice bath to them.

Signs Your Kefir, Kombucha or Sourdough Is Too Cold

• Kefir grains produce runny, sweet milk instead of tangy kefir.

• Sourdough starter takes far longer to rise.

• Kombucha shows little fizz and minimal SCOBY activity.

If you notice any of these, chances are your ferment just needs more warmth—not a replacement.

How to Keep Ferments Warm in Cold Weather

The good news? You don’t need high-tech gadgets to help your cultures along—just a little creativity.

• Find warm corners: On top of the fridge, near the kettle, or inside the oven with only the light on.

• Wrap them up: A towel around your jar works like a little blanket.

• Use gentle heat mats: Seedling mats or fermentation mats give steady warmth without cooking your cultures.

• Work with the day: Let ferments sit out during the warmest hours, then tuck them away when it cools down.

The Benefits of Warmth During Fermentation

With a bit of extra care, your grains and starters will reward you with:

• Better flavour and aroma.

• Consistent texture and fizz.

• Stronger, healthier cultures that multiply faster.

There’s something deeply comforting about sipping a glass of homemade kefir on a cold day or pulling a warm, crusty loaf from the oven when the air outside bites. By keeping your cultures warm, you’re not just speeding up fermentation—you’re keeping the rhythm of your kitchen alive all winter long.

Winter Fermentation FAQs

What is the best temperature for kefir grains?
Kefir grains thrive between 22–28 °C. Below 20 °C they ferment very slowly, and below 15 °C they can go dormant.

Can I ferment without a heat mat in winter?
Yes. Many home fermenters use low-tech solutions like wrapping jars in towels, placing them in an oven with the light on, or leaving them near a warm appliance. A heat mat just makes things easier and more consistent.

My kefir tastes weak in winter. Is it bad?
Not at all. It simply means the grains didn’t have enough warmth to produce acids and fizz quickly. Once you give them a warmer spot, they’ll return to their normal tangy flavour.

Does warmth change the flavour of sourdough or kombucha?
Yes. Warmer ferments are usually more lactic and mellow, while cooler ferments can lean more acetic (sharp/vinegary). Temperature is one of the best tools for fine-tuning flavour.

Final Thoughts

So next time your kefir looks sluggish or your sourdough takes its sweet time, remember: it’s just waiting for a little warmth. Sometimes, that’s the missing ingredient.

And if you’re just starting your fermentation journey this winter, our milk kefir grains and sourdough starters are a great place to begin—they’re hardy, reliable, and backed by all the tips and support you’ll need to keep them thriving year-round.