h. As these cylinders revolve, the beans pass
between them rubbing against themselves and the rough sides of the
cylinders. This action serves to remove dirt and other foreign matter
that may be clinging to the beans, and also gives them an attractive
polish. An exhaust fan sucks away the dirt milled off in the process.
This type of machine will mill about forty bags of green coffee in an
hour.

_Mixing or Blending Green Coffee_

Most roasters blend the different types of coffee while green. Some
blend them after they have been roasted separately. When blended before
roasting, the coffees are mixed by a machine built especially for that
purpose. The mixing machine in general use in all countries consists of
a large metal cylinder which, in wholesale operations, is revolved by
the factory's general power plant or by a separate motor. The cylinder
is equipped on the inside with sets of reverse-screw mixing flanges that
tumble the beans around until they are thoroughly blended; and there is
usually a fan attachment to remove dust. This operation serves also to
smooth down and to polish the surfaces of the beans, which adds to the
style of the coffee when roasted. The average blending machine will mix
from ten to twenty bags of coffee at a time. The actual mixing requires
less than five minutes, but a longer period is needed for feeding and
discharging. This is the last of the so-called "green-coffee
operations". The next step is roasting.

_Roasting by Coal, Coke, Gas, and Electricity_

Coffee is roasted commercially in cylinder or ball receptacles revolving
in heated chambers, the degree of heat reaching about 420 deg. Fahr. The
cylinder type of roaster is invariably used in the United States; while
both the cylinder and the ball types are popular in England, France,
Germany, Holland, and other foreign countries.

[Illustration: AN ENGLISH FOUR-MACHINE GAS COFFEE-ROASTING PLANT

The equipment includes three Morewood indirect-flame, and one quick
direct-flame machines]

Each roasterman has his own opinion about the fuel that gives the best
result, and throughout the world the choice lies between anthracite
coal, coke, and gas; though hard wood is frequently used in countries
where other fuels are not available or not economical. Electric heat has
been tried for commercial roasting in Germany (1906), in England (1909),
and in the United States (1918); but the experimenters have always found
the cost of electric