Maintaining Temperatures for Culturing Yogurt
One of the most important things to consider when you are making yogurt is the temperature at which it cultures.
Thermophilic Yogurt Cultures
Thermophilic yogurt cultures require a temperature in the range of 110°F to reproduce properly. Anywhere in a range of 105°F to 112°F will be adequate, but under 105°F or over 112°F can weaken or damage the bacteria, and will cause the milk to not set up at all, or to curdle without culturing. Temperatures over 118°F will kill the culture, and just heat the milk without culturing it or providing any probiotic benefit.
There are a variety of yogurt makers on the market that are designed specifically to keep milk at a constant temperature of around 110°F. They come with plastic or glass inserts that will hold the milk+culture, and either a timer that will turn off the machine at the end of the culturing time, or a “reminder” dial that you can set to remind you to turn off the machine itself.
To verify that a yogurt maker is running at the right temperature, fill the container(s) with water that has been heated to 110°F, turn on the machine, and check the temperature of the water every hour throughout the normal culturing time. It should be fairly constant, and within the range of 105° to 112°F. This is the correct temperature for culturing any brand of thermophilic yogurt and any type of milk.
If you don’t have a yogurt maker, there are many ways you can maintain the proper temperature for culturing. In each case you can verify the culturing temperature by heating some water to 110°F, putting it in a jar such as you would use for making yogurt, then checking the temperature of the water at the end of what would be the normal culturing time.
Food Dehydrator. A cabinet-style food dehydrator, with the shelves removed, makes a perfect warm box for culturing yogurt. The dehydrator must have a setting low enough for the yogurt, as most do. The door of the dehydrator can be left open or closed, to get just the right temperature for the yogurt.
Insulated Container. You can make an insulated yogurt incubator with a small crockpot and some 1-inch foam. Cut two circles of foam to fit the bottom of the crockpot. Then cut another strip of foam about two inches narrow than the inside height of the crockpot, and as long as the inside circumference of the pot. Put one circle on the bottom of the pot, and wrap the long strip around the inside of the pot. Put the yogurt inside the foam-lined pot, put a lid on it, put the other circle on top of it, and put the crockpot lid on top of that. You can use any sort of enclosed container with foam to make a good insulated yogurt incubator.
Warm Oven. If you turn the light on inside your oven and close the door, the inside of the oven may stay at around 105° to 110°F. You will have to test this though, as some ovens run considerably cooler, and some run warmer. If your oven maintains the right temperature with just the light on, wrap your jar of yogurt in a clean dishtowel secured by a rubber band, and set it on a cookie sheet or in a shallow pan in the oven with the light on. Or, if you have a gas oven, the pilot light alone may keep the oven at a constant culturing temperature. (Again, you should test it first.)
Appliance Boost. Many people have appliances in their homes such as DVRs that are “always on”. You can wrap your covered jar of yogurt in a clean dishtowel secured by a rubber band, and set it on top of the appliance to culture. With electronic equipment, it is strongly advised that you put the yogurt in or on a dish or pan to protect against accidental spills.
Seedling Mat. Nurseries and greenhouses often sell seedling mats (or can tell you where to buy one), which are small flat heating pads that can be set to produce heat from around 68° to around 108°F. This is a little on the cool side for a thermophilic yogurt, but if your environment is quite cold, it could be a good option.
Open Insulation. If your house is warm enough, or if you have a counter that is in a warm place in your kitchen, it may be sufficient to simply wrap your covered jar of yogurt in a clean dishtowel secured by a rubber band, and set it in that warm place.
Hot Water Bath. In a pinch, you can use a hot water bath to keep your yogurt warm enough to culture. Set the covered yogurt 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. 110°F is like a very hot bath – but still cool enough to put your finger in easily. As the water cools, you can pour it out and replace it with more. (Lift the yogurt out very gently, being careful not to disturb it, then pour the water out of the larger bowl and replace the yogurt, 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.
Yogurt cultures that work at room temperature must also work within a certain range. Below 68°F, a mesophilic culture will go dormant, and may cease to reproduce. When a mesophilic culture stops working after having been activated, it is unlikely to start working again, although the bacteria are still viable, and any probiotic benefit that is already in the yogurt will still be there.
Mesophilic bacteria will be killed at 85°F, and will not have any probiotic benefit, nor will they reproduce. These yogurts work best between 70° and 80°F, with the ideal temperature being around 75°F.
It is a little easier to keep a mesophilic yogurt within culturing range, but summer temperatures can be too warm, causing overculturing and curdling, and winter or nighttime temperatures can be too cool, causing the culture to halt.
Yogurt makers, dehydrators, and ovens are generally too warm for mesophilic cultures. Some of the other methods for warming thermophilic yogurts can also be used for mesophilic varieties.
Keeping it Warm
Appliance Boost. Many people have appliances in their homes such as DVRs that are “always on”. Wrapping a dishtowel around the yogurt 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 yogurt 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.
Elevation. It may be sufficient to simply set your yogurt 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 yogurt warm enough to culture. Set the covered yogurt 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 yogurt out very gently, being careful not to disturb it, then pour the water out of the larger bowl and replace the yogurt, 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 yogurt 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.
Keeping it CoolIn the summer, it can be a challenge to keep yogurt cool enough to culture! There are a few tricks that can help you here too.
Culturing Box. Just as you can use a food cooler to keep yogurt warm in winter, you can use it to create a cool environment as well. In very hot weather, a cooler with an icepack inside will usually provide just the right temperature for mesophilic yogurt to culture successfully. Experiment with this to find the right size icepack and cooler combination.
Cooling Surface. Marble tends to stay cooler than the surrounding environment. If you have marble countertops, or a marble slab, you can place the yogurt there and it will keep the culturing temperature down.
Evaporational Cooling. Place the jar of yogurt in shallow bowl of cool water. (A pie plate or cake pan can work.) As the water in the bowl evaporates, it provides a cooling effect. You don’t need to change out the water, as it will stay fairly cool, but replace it if it evaporates away.
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