Basically it's the time it takes the rennet to start working on the curd from the point of adding it. When you visually see that the milk began coagulating (usually less than 15 minutes, explained below), you take that time and multiply it to figure out when to cut the curd. For Tomme type cheese for example, you would want a multiplier of 3. So if the rennet began working 11 minutes after you put it in, it meant you cut the curd 11x3=33 minutes after rennet was put in (or 22 minutes after you discover it because the first 11 have already passed by then). If it's a Camembert for example, some recipes would call for a multiplier of 6, so 11x6=66 minutes (55 minutes left at the time you figure it out).
This is far superior to just counting time because your milk isn't reacting the same every time, so it is better to do it based on the milk's real reaction. Temperature, humidity, animal feed, season, time of day in which the animal was milked, pasteurization process, age of milk... these are just a few of the many variables that can change your flocculation time from one week to the next. This system produce better and more consistent cheese as well as increases your yield.
These multipliers are generic standards, each general cheese family has its own suggested multiplier, so this is usually not a surprise in a recipe:
- Grana/Parmesan, Alpine/Swiss = multiplier xof 2 to x2.5
- Cheddar, Havarti, Tomme, Gouda, Provalone Mozarella (classic Mozarekka, not the quick recipe) = multiplier of x3 to x3.5
- Roquefort, Gorgonzola and other blues, Feta/Bulgarian - multiplier of x4
- Soft or semi soft surface or smear ripened Camembert/Brie, Telaggio = multiplier of x5 to x6
Some cheese is out of the ordinary: You will find the Gorgonzola recipes with as little as x2., Tellagio with x4 and some softer washed rind cheese like Port Salut can have as little as x3. Such differences are usually made when a recipe aims to obtain some of the needed acidification at later stages, such as in the press or during drainage. (or vice versa, a recipe may extend the multiplier and shorten or eliminate drainage/pressing to make up for it).
To know when the flocculation point occurs: one of the easiest methods is to float a small round plastic bowl or cup on top of the milk immediately after adding the rennet and start counting time. Spin the bowl and it will turn quickly and freely. Do that every couple of minutes. The moment that it all of a sudden does not spin is the flocculation point. Stop the clock and multiply it. (Remember to deduct minutes that have passed since adding the rennet if you are only trying to find out how much time you have left)