After the peak the compost is brought down at about 1.5 °C per hour and held at 45 to 48 °C. Surviving heat-loving fungi turn free ammonia into their own bodies, which the mushroom later eats. This step is what makes the compost selective. ICAR-DMR says it takes 3 to 4 days in a modern tunnel, or until the compost is free of ammonia.
Two limits protect it. Never condition below 40 °C, because mesophilic fungi set in and spoil the compost. Keep oxygen above 10 percent, since the fan is not optional.