Repeatable performance depends on accurate measurement. For most varieties the quantity of rennet is selected to set the milk to a firm coagulum in 30 - 40 min. Measure the rennet accurately and monitor to ensure that coagulation rate is uniform from day to day.
Rennet must be diluted (about 20 times) in water and well mixed when added to ensure uniform distribution.
Use nearly the same dilution each time to improve the consistency when adding the diluted rennet to the vat.
Watch out for chlorine. It is imperative that the dilution water contains no chlorine. Only 2 ppm of chlorine will destroy 40% of rennet activity in 3 minutes. Similarly, do not sanitize the container used for the rennet with chlorine.
Another water quality issue is pH. Typically hard water also has pH greater than 7.0 which also decreases rennet activity.
Finally, dilute immediately before adding the rennet to the vat. After the brined rennet is diluted in water, its activity declines quickly.
Pasteurization temperature: higher temperatures increase yield by increased recovery of whey proteins, but a suggested maximum with respect to curd quality is 75C, 16 s.
Temperature history: if the milk is pasteurized and immediately sent to the setting vat, it will be necessary to adjust the mineral balance by adding calcium chloride.
The jury on selection of coagulant always seems to be out. I tentatively suggest that microbial coagulants are not advisable for high temperature varieties for reasons of heat stability, and not advisable for other varieties unless other setting and conditions are under tight control. The preferred choices, then, are rennet and recombinant rennet.
The amount of rennet must be carefully determined. Because rennet is costly, it is desirable to minimize its use, but this can be false economy if curd properties are compromised. Poor setting means increased losses of both fat and protein as fines.
Temperature control must be accurate and uniform through out the vat, because both the enzyme activity and the subsequent process of micelle aggregation are extremely temperature sensitive. Inaccurate or nonuniform temperature during setting will result in local areas of under or over set curd which in turn causes loss of fines during cutting.