Ripening the milk
Either naturally occurring lactic bacteria or added bacterial cultures are used to acidify the milk through the digestion of lactose (a sugar) into lactic acid. The use of direct set cultures helps ensure consistency across batches. The correct temperature of the milk, as called for in the recipe, must be maintained during the ripening process.
Cheeses that are prepared at lower temperatures will tend to use mesophilic cultures. Cheeses that call for higher temperature cooking call for thermophilic cultures. Of course, there are hundreds of cultures that are used in commercial cheesemaking. The home cheesemaker can benefit from prepared direct set cultures from the sources listed on this site.