It is not advisable to add additional rennet if you have a slow-setting cheese. Rennet’s first role is to set the curd, and the second role is it remains in the cheese as a protease enzyme, it develops flavour within the cheese. During aging, the proteins are broken down, and this causes a desirable change in the texture and flavour of the cheese as it ripens, e.g. it helps make your Camembert go runny and makes your harder cheeses less rubbery.

Rennet sets can be slow for several reasons; see this article, ‘Why Doesn’t My Milk Set the Same Way Each Time I Make Cheese?” in the Blog. If you add more than the required volume of rennet, you will get your firmer curd, which will solve your set problem, and you will feel good at this point of the cheesemaking. However, with an increased rennet volume, you will also get an increased rate of protein breakdown during the maturing of the cheese. Excess protein breakdown may cause bitter and ammonia flavours to develop, as well as body and texture defects. These defects may also intensify as the cheese matures. Note some cheeses, such as Halloumi, can have high dosages of rennet, only marginally, but this rennet is destroyed with the 90°C cook step.

There are 3 courses of action that you may consider, but please note that they do not always work out as well as can be expected:

1. If, for example, you added rennet by 10 times less the volume, e.g. you added 0.3 ml of rennet instead of 3 ml. If you found this out within 10 or 15 minutes of adding the 1^st rennet, add the remaining 2.7ml as quickly as possible.
2. If the cheese has set within 20 minutes after the set time is due (so 20 minutes late), then just wait for the set to occur. Make the cheese as usual. But ideally, keep the prescribed hoop time as per if the rennet set worked on time (so you still get the correct level of acid development).
3. However, if the cheese set goes greater than 20 minutes, then over-acidification occurs and with that, all sorts of issues. Over acid and bitter flavours, changes in texture, changes in ripening…..You can try one of these options:

• • Make the cheese into a halloumi
  • Make the cheese into a Fromage Blanc or Quark style, first whisk the curd, add a few pinches of salt, then drain it in a cloth for several hours or overnight;
  • Make the cheese into an acidic style such as a Greek Feta, where a slightly stronger acid style and crumbly texture are required.