boar_d_laze wrote:

Since the "bean mass" is not a unitary thing, BMT is an artificial construct. Nevertheless, it's the roast master's best guide as to where the roasting process stands at any given temperature.

The BMT is measured at the point(s) of greatest density in the (fluid dynamic) moving beans, and is usually a combination of beans contacting the probe and air in the roaster. The more its a function of contact, the more representative the reading.

Charge:
When the beans are introduced into the roaster.

Turning Point:
When the roaster is charged, the temperature in the drum, including at the points where Bean Mass Temperature is measured, drops. The moment when BMT begins to rise is called the turning point. It's a function of the thermal mass of the roaster, its retained heat, the mass of the charge, and the amount of agitation.

It's important because readings during the interval when the BMT readings are entirely inaccurate in the sense that the bean mass temperature is not actually dropping, and therefore not useful.

Accurate BMT readings don't begin until the TP, and most roasters give it a minute or so after TP before taking the readings too seriously.

Nevertheless, time to TP tells you something about the consistency of the preheating process.