It helps, but it's not critical. I incubate Mesos at room temp (around 70F) for 18-24 hours. I incubate Thermos at 110F for just 6 or 7. When the MC thickens up a bit, it's ready to use or refrigerate. If you start accumulating a little whey at the top off your container, you have gone too far. Still usable, but the acid kills off some of the bacteria. Fresh MC is more active and will work faster in your makes.