Halal cheese is a cheese that has been created using ingredients that are in accordance with the Islamic laws pertaining to food. There is one specific ingredient used in the production of cheese that has the potential to make it non-halal, also called haram, meaning forbidden. This ingredient is rennet and it is largely gathered from animals. The type of animal and the method by which it was slaughtered might be unknown, so the rennet could be haram and make the cheese haram. There are commercial brands of cheese that have been certified as halal, and nearly all vegetarian cheeses are halal, because they do not use any products from slaughtered animals.
Rennet is an enzyme that is essential in most cheese-making processes. It is largely acquired from the stomachs of adolescent cows, although it also is present in other animals. The problem with creating a halal cheese that uses rennet is that the source of the enzyme might be unknown. The animal might not have been slaughtered by a Muslim in a way that follows halal guidelines, and the animal from which the rennet is drawn might not even be a cow. This means the ingredient could be forbidden and, through its inclusion in milk to separate the curds and whey, would render the cheese haram.
There is actually a difference in opinion among various Islamic scholars and sects as to whether all cheeses made with animal rennet are halal or haram. It is clear that rennet from the stomach of a pig would be haram, no matter what, but the issue becomes more complex outside that clear situation. One argument that is made for halal cheese that uses animal rennet is that, because the rennet is essentially gone from the milk after it has separated into curds and whey, the final cheese is halal. Another argument points to a verse in the Quran in which the Prophet Muhammad asks for and is served cheese, which some scholars interpret as meaning it is halal. Some Muslims eat any cheese that is available, while others avoid it completely.
There is halal cheese available commercially that has been certified as using only permitted ingredients. These types of halal cheese use microbial enzymes or plant-based enzymes to separate the milk. Vegetarian cheese also is halal, because it will not contain any animal rennet. In some situations, depending on the ingredients listed on the label, kosher cheese also may be halal.