What is rennet?
Rennet is an enzyme used in the cheese making process to coagulate or set milk and allow cheese curds to form.Traditional Animal Rennet is an enzyme derived from the stomachs of calves, lambs, or goats before they consume anything but milk. Rennet used in cheese making is about 90% pure chymosin. Vegetable Rennet is an equivalent chymosin product which works equally well but is not animal derived. Vegetable Rennet can be found in tablet form.
Rennet thrives at temperatures in the 85-105F range, but it won't be deactivated completely until it reaches the 140F's. Rennet continues working to set the milk as long as it has the right conditions. So, when a recipe calls for cutting the curds after a certain time period, it is important to follow the directions. Otherwise, your curds may be too firm for the cheese you are trying to make.
Which rennet should I use?
Rennet is standardized, so all the different kinds of rennet (liquid, tablet or powder) work the same to set milk. Liquid is the easiest to work with because you can measure it very precisely. However, the powders and tablets will keep better under more adverse conditions. Calf rennet is considered to be the best choice for longer aged cheeses because some of its residual components help to complete the breakdown of proteins. Some of the more complex proteins in the vegetable rennet can have a slightly bitter taste after 6 months of aging. The liquid vegetable rennet is Kosher, but it has been re-packaged without Kosher supervision.
I am not getting curds. Is it the milk or the rennet?
The odds are very good that it is a milk problem. Check the milk or curd below the surface. Bring a spoonful up from the bottom and if it is different, you have a problem milk. Try your recipe again with a good local cow's milk. Whenever possible, look for a local brand. This is because long, cold storage and transportation is problematic for milk. The closer the milk is to the source (cows, goats, etc.) the better for cheese making. Another factor is the heat at which the milk has been pasteurized.If the milk clearly states that it is ultra-pasteurized (UP), do not purchase it. It has basically been stripped of friendly bacteria. More importantly, the alteration of the proteins is so severe that the milk will not form a proper curd. Unfortunately, even if your milk is not labeled UP, it may have been heated to just short of ultra-pasteurization. When milk has been pasteurized at a high temperature (anything over 172F), it is problematic for cheese making. The only way to know how high a processor has heated their milk is to call them. We encourage you to do this because we believe it will let the large processing plants know where we stand.