Cold Weather Care for Starter Cultures

Fall and winter present special challenges for culturing. As the weather cools, how we care for starter cultures must also change.

As a general rule, fermented and cultured foods appreciate a nice warm spot. Warmth promotes the growth of the beneficial bacteria and yeast that allow foods and beverages to culture properly. As temperature outside drops, the normal spot you may use to make your yogurt, sourdough, kombucha, and other cultured foods may also cool down, affecting how your cultured foods behave.

Some cultures will actually behave differently during the winter even when the amount of light and heat are constant!

Why Cool Temperatures are a Problem for Culturing and Fermenting

There is an inverse relationship between yeast and bacteria and ambient temperature. As temperatures drop, development of yeast and bacteria slows. As temperatures rise, development of yeast and bacteria quickens (although there is an upper limit based on the needs of the particular culture). What does this look like?

Yogurts and Buttermilk. The bacteria in these cultures work by consuming the lactose and producing lactic acid. The lactic acid causes the milk proteins to break down slightly and clump together. The cultures also contain bacteria that produce specific textures and flavors. When yogurt and buttermilk are too cool, they do not consume lactose, or don’t consume it in enough quantity to produce the acid necessary to affect the proteins. At cooler temperatures, the bacteria also don’t reproduce well, so there are fewer of them to do the work.

As the temperature rises, the bacteria become more active, and start pumping out those important acids.

Yogurt makers and dehydrators help keep the temperature constant while yogurt or buttermilk are culturing, but even those appliances can fluctuate in temperature if the ambient temperature is too extreme. (A yogurt maker in a 55°F kitchen will not be able to keep yogurt warm enough to culture properly!)

Milk Kefir. The grains that culture milk into kefir work similarly to yogurt and buttermilk cultures, but they are more complex. They are made up of bacteria and yeasts in a symbiotic relationship, and they work by consuming the lactose to produce lactic acid and carbon dioxide. The lactic acid causes the milk proteins to coagulate, and the carbon dioxide provides fermentation. The grains also use some of the milk protein for growth and reproduction.

If milk kefir is too cool, the grains will simply hibernate and work very, very slowly if at all. It’s a little easier to culture milk kefir slowly and for longer periods, but you will still run the risk of having the grains use up the lactose and begin to starve if you culture for more than 48 hours.

Water Kefir and Kombucha. These cultures are also made up of bacteria and yeasts, in a complex matrix. The cultures ferment the sugar water or tea and produce carbon dioxide. In warmer temperatures the kefir will be finished in 24 to 48 hours, while in cooler temperatures, it may take the full 48 hours to finish culturing, and sometimes even up to 72 hours. The kombucha normally takes one to three weeks to ferment, depending on your taste, but allow an extra week in cooler temperatures.

Vegetables. What is happening with cultured vegetables is that the natural enzymes on the plant surface, plus whatever bacteria you add to the culturing liquid, are working together to break down the tough plant fibers and make the plant nutrients more available. This will actually happen fairly well in cooler temperatures, but too cool, and the micro-organisms will just go dormant. The vegetables, being in a salty bath and basically refrigerated, will not suffer, but they won’t ferment much either.

Can You Just Culture the Food for Longer?

Sometimes a longer culturing period helps, but only to a point. With milk products, keep in mind that while cool temperatures keep the yeast and bacteria from developing effectively, competing bacteria resident in milk is often not impeded and may even be encouraged. The bacteria in the milk multiplies quickly, and without strong starter culture to keep it in check, it can turn the milk sour before the yogurt culture gets a chance to do its work. The result is a curdled milk, but without the special textures and flavors provided by the starter culture.

If the temperature is cool enough, some of the bacteria may produce changes in the milk’s flavor, but there won’t be sufficient acid production to affect the texture of the milk. With a long enough culturing time, the bacteria will consume all the lactose and die off, leaving you with a thin, bitter milk.

Water kefir grains can be left to culture at even a cool room temperature for up to 72 hours, but after that they are the risk of running out of sugar and starving. Under 60°F or so, they will begin to hibernate. Water kefir grains are especially resilient, and can actually produce a lightly carbonated beverage if left in the refrigerator in sugar water for several weeks! However, it weakens the grains to keep them dormant or under-productive for long periods of time. They will be more robust, and grow better, if they are allowed to work at optimum temperatures on a regular basis.

Since a kombucha scoby can be stored in the refrigerator for several weeks, sitting in a cool kitchen is not especially harmful to it, but the culture may go dormant and not produce much fermentation. Culturing it for longer will help if the temperature is just slightly cool, but if the scoby is dormant, a longer culturing time won’t produce any effect.

How Can You Keep the Cultures Warm Enough?

There are a variety of ways to help keep cultures warm enough to culture well during cooler weather. Here are some suggestions. You might think of more!

Appliance Boost. Many people have appliances in their homes such as DVRs that are “always on”. Wrapping a dishtowel around the culture may make it too warm, but you can simply set the jar on top of the appliance to culture. With electronic equipment, it is strongly advised that you put the jar in or on a dish or pan to protect against accidental spills. 

Seedling or Reptile Mat. A seedling mat from a nursery or greenhouse or a reptile mat from a pet store can be set to around 75°F which is perfect for a mesophilic yogurt, kefir, buttermilk, or other products that need to culture at a cozy room temperature.

Elevation. It may be sufficient to simply set your culture in a high place in your kitchen, where the air is a little warmer than at counter height.

Hot Water Bath. In a pinch, you can use a hot water bath to keep your culturing food warm enough to ferment. Set the culturing jar in a larger bowl, and fill the bowl with heated water, to anywhere from an inch or two up from the bottom of the jar, to an inch or two from the top of the jar. You can use a thermometer to measure the temperature of the water. 75°F will feel lukewarm. As the water cools, you can pour it out and replace it with more. (Lift the culturing jar out very gently, being careful not to disturb it, then pour the water out of the larger bowl and replace the culturing jar, then the heated water.) This is pretty labor- and time-intensive, and really only useful as an emergency measure when there are no other options.

Culturing Box. You may be able to maintain a “room temperature” environment by just putting your culture inside a food cooler along with a jar of hot water. Check the cooler occasionally and replace the hot water if necessary to maintain a temperature inside the cooler of at least 70°F.

Oven. Your oven may be able to maintain an ideal temperature with just the inside light on. Make sure you check this first, as the temperature can range from 85° to 110°F or so. Test the oven temperature by leaving a jar of water inside for an hour or so, then taking the temperature of the water.

Insulation. You can help a culture retain its own heat by wrapping it in a dishtowel or putting a man-size sock over the jar you’re culturing in.

Heat Source. Sometimes just setting the culture near an incandescent lamp can provide it with enough heat to culture well. Putting it right under a lamp may be too warm, though.

Related Articles & Recipes:

• Learn to Make Kefir
• How to Make Sourdough Bread (VIDEO)
• How to Make Labneh (Yogurt Cheese)

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