Cream sours when lactose meets Lactobacilli, the beneficial bacteria found in cultured dairy products. This reaction produces lactic acid -- not unlike the fermentation process of sauerkraut and pickles. You only need to introduce Lactobacilli to cream to make sour cream -- lacto-fermentation takes care of the rest. Use a cream with 18 to 20 percent fat content for a rich, creamy consistency that works well with food.
Cultured buttermilk, yogurt and powdered sour-cream starter culture all kickstart fermentation in pasteurized cream. You can also work with store-bought sour cream to start your own batch of sour cream.
Pour room-temperature pasteurized cream in a sanitized food-grade container -- glass canning jars work well -- and stir in 1 tablespoon of room-temperature cultured buttermilk, yogurt or sour cream per cup. If you're using starter culture, add 1 gram to every 1 to 4 quarts of cream. Cover the container with cheesecloth and secure it with a rubber band. Let the cream sit at room temperature for 24 to 48 hours and stir. Store the sour cream in the refrigerator until the expiration date of the cream used.
To make reduced-fat sour cream, substitute half the cream with non-fat milk.
Raw milk produces sour cream in its truest form -- thickened and cultured with the naturally occurring bacteria already present in the milk -- but consuming raw-milk products pose a health risk.
The first thing you notice about sour cream made from raw milk is its yellow-beige tinge, which signifies the presence of beta carotene. You don't see this color in pasteurized milk -- small-farm organic being the exception -- because commercial dairy cows typically eat about 50 percent foraged grass and hay and 50 percent grains. Quality raw milk comes from cows raised only on pastured produce, which naturally colors the milk they provide.
Pour room-temperature raw milk in a sanitized food-grade container -- glass canning jars work well -- leaving about 1/2 inch of space at the top. Cover the container with cheesecloth and secure it with a rubber band. Let the milk set for 4 to 5 hours, then spoon the fat off the top and discard it. Cover the container again with cheesecloth and let it sit for 24 to 48 hours. Store the cream in the refrigerator and use within three to four days.
Sour Cream Shortcuts
You don't have to wait 24 to 48 hours for passable sour-cream substitute -- you can create a shortcut by using cottage cheese, milk and lemon juice or lemon extract. Cottage cheese, when blended, supplies the texture consistent with sour cream, while the lemon adds the tang. Milk helps blend everything together.
Blend 1 cup of cottage cheese with 2 tablespoons of milk and 2 tablespoons of lemon juice until smooth and creamy. If using lemon extract, blend the milk and cottage cheese, then add the extract to taste. Blend again to combine.
Sanitize the food-grade container or jar you will use to culture the sour cream and the spoon before you begin. Mix a standard sanitizing solution consisting of 1 tablespoon of bleach and 1 gallon of water. Submerge the container and spoon in the solution and let them sit for 15 minutes. Air dry the container and spoon.
The U.S. Department of Agriculture advises against consuming raw milk or using raw milk to make sour cream.