Cold Weather Care for Starter Cultures
Fall and winter present special challenges for culturing. As the weather cools, the methods for keeping cultures at the proper temperature 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 temperatures outside drop, the 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 a direct relationship between ambient temperature and the activity of yeast and bacteria. 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).
Yogurt and Buttermilk. The bacteria in these cultures work by consuming the lactose in the milk 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 do not 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 there is still a risk of the grains' using up the lactose and beginning to starve if cultured 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 24-48 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 taste preference, but an extra week may be required in cooler temperatures.
Vegetables. What is happening with cultured vegetables is that the natural enzymes on the plant surface, plus any bacteria added to the culturing liquid, are working together to break down the tough plant fibers and make the plant nutrients more available. This process 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 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 may run out of sugar and starve. 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:
Appliance Boost. Many people have appliances in their homes such as DVRs that are always plugged in. 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 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 the culture in a high place in the kitchen, where the air is a little warmer than at counter height.
Hot Water Bath. In a pinch, use a hot water bath to keep the culturing food warm enough to ferment. Set the culturing jar in a larger bowl, and fill the bowl with heated water. Use a thermometer to measure the temperature of the water. As the water cools, 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 method is labor- and time-intensive, and really only useful 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. The oven may maintain an ideal temperature with just the inside light on. Make sure to check the temperature first, as it can range from 75°-110°F, depending on the oven and bulb wattage. Test the oven temperature by leaving a jar of water inside for an hour, then taking the temperature of the water.
Insulation. Help a culture retain its own heat by wrapping it in a dishtowel or putting a large sock over the jar.
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, so check the temperature.
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