If you've never cooked with bacterial cultures before, making homemade yoghurt is a wondrous experience. With proper care and feeding, live probiotics such as Lactobacillus acidophilus or Bifidobacterium in a few spoonfuls of yoghurt will grow, thrive and turn fresh milk into a huge batch of thick, creamy homemade yoghurt. Electric yoghurt makers are convenient, but not necessary -- you can make yoghurt at home without any special kitchen equipment.
Warm milk in large saucepan over gentle heat, stirring occasionally, until it reaches 82.2 to 85 degrees Celsius, and hold it at this temperature for a few minutes. If you don't have a thermometer, bring the milk just to a boil, then immediately turn down the heat until it's barely simmering. Allow it to simmer for a few minutes. Remove the milk from the heat and cover it.
Cool the milk to 46.7 degrees Celsius, if using a thermometer. It's OK if the milk cools to as low as 41.1 degrees C, but don't allow it to become any cooler than that. If you don't have a thermometer, award-winning food writer Claudia Roden advises cooling the milk to the point where you can put a clean finger in the milk for a count of 10 before it becomes uncomfortably hot.
Whisk one-half cup plain yoghurt into the hot milk until completely combined. Choose a commercial yoghurt with live cultures and a taste you enjoy. Transfer milk to a large glass bowl and cover it. Cling film works well if your bowl doesn't have its own cover.
Place your inoculated milk in a warm environment of approximately 46.1 degrees Celsius. If you have a food dehydrator, place the bowl in the dehydrator at 46.1 degrees C. If not, you have three options. You can place your bowl of milk in a sink full of 115-degree water, adding warm water as needed to maintain 46.1 degrees C, using your thermometer or the finger test to gauge heat. You can also turn on your oven briefly to the lowest heat setting, turn it off, and let it sit with the oven door open for a few minutes. If you have an oven thermometer, wait until the oven reaches 46.1 degrees C. If not, use your best judgment. Place the bowl of milk, wrapped in a towel or blanket, into the oven and close the door. Or, Roden suggests wrapping the bowl of milk in a wool blanket or shawl and leaving it in a warm place.
Let the milk mixture sit undisturbed for four to eight hours. After four hours, check to see if the mixture is thick, creamy and slightly sour. If not, check again every hour until it reaches the desired consistency. The yoghurt will grow more acidic over time; let it culture longer for a tangier end result.
Chill your freshly made yoghurt in the refrigerator for several hours. It will continue to firm up as it gets cold. Your finished yoghurt may produce some whey, a thin yellowish liquid. Simply drain it off or drink it; it's full of beneficial yoghurt cultures and milk protein.
Use antibiotic-free milk and be sure your saucepan and bowl are clean and completely free of detergent residue, as these factors can prevent yoghurt from thickening, according to Cheese Queen Ricki Carroll, author of "Home Cheese Making." Whole milk makes the thickest yoghurt, but you can use low-fat milk instead. To bolster its thickness, add some powdered milk to the low-fat milk before scalding.
If your yoghurt doesn't thicken properly and becomes fizzy, bubbly or strange smelling, discard it. Cultures of unhealthy bacteria may have colonised your milk rather than healthy yoghurt cultures.