The main difference between kefir and yogurt stems from the types and amount of probiotic bacteria they contain. Kefir usually contains three times as much probiotic cultures as yogurt, depending on the brand. Kefir also has a thinner consistency and can be consumed as a beverage.
To make yogurt, milk is fermented with only a few types of probiotic yeasts and bacteria, while kefir is made with 10 to 20 types. Probiotics are believed to strengthen the immune system and stop the spread of bad bacteria. Probiotics are live organisms, such as yeast or bacteria, found in the digestive tract's intestinal flora.
While both yogurt and kefir are good sources of probiotic bacteria, the beneficial bacteria in yogurt are called transient bacteria because they pass through the digestive tract. Milk kefir contains a larger variety of bacteria, as well as yeasts, that colonize the intestinal tract.
A mesophilic yogurt starter is cultured at room temperature, while a thermophilic starter requires heat of approximately 100 degrees Fahrenheit, which is usually provided by a yogurt maker. Milk kefir is mesophilic, culturing at room temperature. Yogurt starters can be reusable or single-use. The reusable starters can be recultured weekly by mixing fresh milk with a small amount of the activated batch of yogurt. Single-use, or direct-set, yogurt starters are thermophilic and delivered in packets of powder that can be recultured only a few times until a new packet is needed.
Milk kefir is cultured with gelatinous grains that contain a large variety of yeast and bacteria capable of producing continual batches of kefir. The grains require transfer to a new batch of milk every 24 hours. Powdered kefir starter culture may be used, but requires replacement after a few uses.