long coagulation times

[wortmann]

I’ve bought this rennet over the net, it states 150IMCU. Which i take is quite low. I usually use 17-20ltrs batches of milk to make semi-hard cheeses. However i have found that the set time to get a clean break are twice the amount of time set in the recipe. Any thoughts. For 40 ltr i used 15ml. then the next time i used 7.5 for 17 ltr and still both have the same problem with a much longer time to set.

[mikekchar]

When this happened to me it turned out to be chlorine in my water interfering with the rennet.  I boiled the water and let it sit and added a drop of milk, but to no avail. Switching to bottled water fixed the problem.

Anyway, for 40 liters you will want at least 2000 IMCU, which is just over 13 ml, so I think your renneting rate it perfectly fine.  The other potential problem is a lack of dissolved calcium.  If the milk is heat treated at all, you will need to add calcium (usually calcium chloride).  Even if it is not (raw milk), sometimes there is not enough calcium dissolved in the milk for various reasons.  Also, check the pH of the milk.  If it is very high, it might cause a problem.  This might happen if there is mastitis in the herd, apparently.