A mesophilic culture refers to microorganisms that thrive in moderate temperatures. They can grow well at room temperature without requiring extreme heat or cold. Full definition
2 cups heavy cream, room temperature (65 - 68F) 1/8 teaspoon mesophilic culture (optional, for cultured butter. (blog.kitchenaid.com)
A small measure of thermophilic «heat loving» (108 ° -112 °F) culture is added to the milk versus mesophilic culture that is used for making room temperature yogurt (68 ° -78 °F). (whiteleycreek.com)
You could also try purchasing «mesophilic culture» or «vegan probiotics» online and following the instructions to use those. (yupitsvegan.com)