While they both have a sour, tart taste, kefir and sour cream can be very different in terms of texture and taste. Sour cream always is made from dairy cream that has been fermented, while kefir can be made with dairy milk or cream as well as non-dairy liquid such as coconut milk and nut milks or in some cases, water. Depending on its base, kefir may be used in place of sour cream on some occasions, such as for smoothies or desserts, and is naturally lower in fat.
Kefir is a fermented drink most commonly drunk as a beverage and native to the Middle East. It can be made from any type of liquid, while sour cream is made only from dairy cream that contains at least 20-percent milk fat. While kefir often is lower in fat content, the nutritional differences between sour cream and kefir can vary greatly, as they change depending on what kefir is made from.
Changes to Taste
Kefir generally is much more sour than sour cream because of the different cultures used to make it. The taste of kefir also varies depending on the type of liquid from which it is made. Dairy kefir is similar in taste to sour cream, but kefir made with coconut milk or almond milk will retain the flavor of these liquids. Because kefir generally is much lower in fat than sour cream, it has less rich of a taste.
Differences in Texture
Because of the difference in fermentation processes, kefir sometimes has a slight carbonation, while sour cream does not. Sour cream, because of the higher fat content, is much thicker than kefir, although sour cream can be diluted to be of similar consistency. Sour cream made from low-fat cream or from a blend of milk and cream generally has a runnier consistency than full-fat sour cream, although manufacturers often add thickeners to low-fat sour cream.
Using Kefir in Place of Sour Cream
If you are using kefir in place of sour cream for drinks such as smoothies or for dressings such as a sour cream-based ranch salad dressing, you add less water to thin it out. However, for baked goods where sour cream adds minimal liquid, you may need to reduce the amount of other liquids used. If the consistency and lower fat content is a problem for your recipe — for example, in buttermilk biscuits — consider making kefir cream. Add kefir grains or kefir milk directly to full-fat cream and let it rest until the cream thickens. Use a 1-to-8 ratio of kefir milk to cream or a 1-to-16 ratio of kefir grains to cream.
References and ResourcesKQED: Soured Milks, What's the Difference?
the kitchn: How to Make Milk Kefir
Cultures for Health: How to Make Sour Cream
the kitchn: Water Kefir From "Mastering Fermentation"