The very first ingredient in your cheese making is rennet. As you may already know, there are recipes using lemon juice, tartaric acid, citric acid but those recipes are not for real cheeses. They are usually made fresh and consumed straight away.
Rennet is an enzyme or actually couple of enzymes. It is secreted from the 4. stomach called abomasum of grazing animals. When a young grazing animal drinks milk from its mother, the milk needs to get into a solid form so that it can be digested. Otherwise it would go quickly in the digestive track and the animal can not get the beneficial vitamins, minerals, fat and proteins out of milk. The enzymes named as chymosin, bovine and pepsin and a real rennet would have all 3 in it in different ratios. These days vegetarian rennet is mostly chymosin created by injecting the DNA of chymosin into a bacteria and let the bacteria create the enzyme.
Chymosin used to be called rennin thinking that it is secreted from kidney glands but it is renamed to give a better name and to prevent confusion for the science minded people.
Rennet is available at cheese making supply stores. In Australia, you can get it from web shops like http://cheeselinks.com.au, http://www.thecheesemaker.com.au and http://greenlivingaustralia.com.au. Only vegetarian rennet is available through these shops. Although I looked for real calf or goat rennet but could not find it. There are some in 10 litres containers which is not practical for home cheese makers. I here thank you again to Barry Lillywhite for sending me some real calf rennet.
There are also two shops in Canberra selling rennet. These are Buts and Brew in Kaleen shopping plaza and Cooking Coordinates in Belconnen Fresh Fruit Market where I have given workshops before. These two shops are selling Mad Millie cheese making kits and they supply rennet and other things.
Rennet comes in a 50 or 100 ml bottles and the usage is written on it. It is usually added to at 2 ml or 1 ml per 10 litres of milk depending on the strength of rennet.
You can not take rennet through customs, so don’t try doing this. It is an animal product and not allowed.
Rennet can be kept in the fridge (not in freezer) and will service you till it expires. Even when they are expired, you can still use it by increasing the dosage as its strength is diminishing.
Also get couple of plastic syringes from chemists as you will need these to measure the rennet correctly. Using more rennet than specified in hard cheeses results in bitter taste and defects. In fresh cheeses it is not so much important but still obey the instructions that came with it.