The strength trap
Liquid rennets are sold at different strengths and the bottles look the same. A rate of 2.2 ml per hundred pounds written against a 200 IMCU product is 18.9 ml on a 100 gallon vat at 8.6 lb a gallon. Pour that same 18.9 ml out of a 400 IMCU bottle and the vat has had twice the enzyme it was supposed to get; the page would have said 9.46 ml.
The correction is a single ratio — the strength the rate was written for, divided by the strength you have — and it is on the page rather than in your head because the two bottles sit next to each other in the fridge and the mistake is silent. It only works when both numbers are on the same scale. IMCU against IMCU is a comparison. IMCU against a label that says double strength is not, because double what is not written anywhere, and in that case the honest thing is to leave the two fields equal and ask the supplier.
Dilution is volume, not dose
Twenty parts of water to one gives 378.4 ml of water around 18.9 ml of rennet, and 397.3 ml of liquid going into the vat. The dose has not changed. What has changed is that the rennet can be distributed through the milk in the seconds you have to stir it, rather than going in as a slug. What water to use and how far to dilute is on the supplier datasheet, and this page only does the arithmetic of it.
The packet problem nobody warns you about
At 1.5 g per hundred pounds, an 860 lb vat wants 12.9 grams of culture. A 25 g sachet covers 1.94 vats of that size. That is the worst possible ratio: too much for one make, not enough for two, and it means either weighing 12.9 g off a scale that can actually read it or opening the sachet and storing the rest.
Both routes have consequences the calculator cannot see, and the supplier is the one who knows what an opened packet does over a week in your freezer. What the page can do is show you the number, because plenty of people size a vat without ever noticing it lands awkwardly against the packet they buy.
Why the cut moves and the multiplier does not
Flocculation at 12 minutes with a multiplier of 3.5 puts the cut 42 minutes after the rennet went in — 9:30 in, called at 9:42, cut at 10:12 on the default form. Now let the milk run half again as slow, which happens without warning as the season turns: flocculation lands at 18 minutes and the cut goes to 63 minutes. Cutting at 42 anyway means cutting a curd that has had two thirds of the set it was meant to have.
The reverse calculation is the interesting one for anybody who has always cut on a fixed clock. Enter the minutes you use and the page backs out what multiplier that has been on this vat. Do it for six makes. A steady number means your clock and a multiplier were the same thing all along. A number that wanders between 2.8 and 4.2 means the milk has been setting at different speeds for years and the clock has been quietly cutting different curds.
Questions people ask
How much rennet do I use for 5 gallons of milk?
That depends entirely on the rennet, and this page will not answer it with a number of its own. Put the rate your rennet supplier states into the form along with the strength it was written for and the strength you actually have, and the page scales it to five gallons. The reason it works that way is that rennets are sold at strengths that differ by a factor of two or more, so a single published figure would be wrong for half the bottles on the shelf.
What does the flocculation multiplier do?
It ties the cut to the milk instead of to the clock. You time how long the milk takes to flocculate, multiply by the number your make sheet gives for the style, and cut then. On the default figures that is 12 minutes times 3.5, so 42 minutes after renneting. When the milk sets slowly the cut moves out with it automatically, which is the whole point. The multiplier itself belongs to your recipe and is not something this page supplies.
My rennet is double strength — what do I change?
Put the strength the rate was written for in one field and the strength you have in the other, and the page divides. A 2.2 ml per hundredweight rate written for 200 IMCU, used with 400 IMCU rennet, becomes half the volume. If your bottle only says double strength without a number, there is nothing to divide by, because it does not say double what. Leave both fields equal and ask whoever sold it to you what the actual strength is.
Why does the culture packet not divide evenly into my vat?
Because packet sizes are set by the culture house and vat sizes are set by whatever tank you could fit through the door. At 1.5 g per hundred pounds an 860 lb vat takes 12.9 g out of a 25 g sachet, which covers not quite two makes. The page prints the ratio so you can see it. Whether an opened sachet is still worth using next week is a question for your supplier, not for a calculator.
Can I use this to decide what dose is safe?
No. This page scales a rate you supply and corrects it for a strength you supply, and it has no view about whether any dose, any culture, any temperature or any timing is appropriate or safe for what you are making. Dose rates come from the supplier of the enzyme or the culture; process comes from a make sheet somebody qualified stands behind; and what is permitted in a dairy comes from your regulator.