The Cleaning Process
Second in a Series of Articles
by
Thom Leonard *
| Thom
Leonard is a baker and writer living in Lawrence, Kansas and working
with Farm to Market Bread Company in Kansas City, Missouri. |
In
the last issue, there was a discussion of the milling process as it is practiced
in virtually all large flour mills in this country. The cleaning of wheat was
given one all-too-short paragraph, basically saying that an array of machinery
is used to exploit various physical properties to remove foreign material from
sound wheat. This article will put the horse after the cart and describe this
cleaning process in a little more detail. It is amazing how simple mechanical
machines can be given a combination of wheat, weed seeds, chaff, dust, stones
and oats, and give back clean, uniformly sized wheat. And all the machinery in a
cleaning mill is simple; it’s the effective use and combination of it that is
complicated.
This
is not to give the impression that wheat coming from a farmer’s field is a
dirty assortment of matter that contains a little wheat that the miller must
painstakingly pick out from the trash. The percentage of contaminants is very
small, and some of what must be removed before milling gets inadvertently mixed
into the wheat during storage and transport.
The
most basic piece of cleaning equipment is the fanning mill, which uses a
combination of screens to separate wheat from large and smaller material and a
fan to remove contaminants which can be blown out by a blast of air. The first
separation is performed by running the grain over (and through) carefully
selected screens or perforated metal sheets. Depending upon opening size and
shape, wheat either falls through or doesn’t. Given two screens, if the top
one allows passage of objects the size of a wheat kernel or smaller, then larger
trash will be “overs” (remember this term from the purifiers in a flour
mill?), and everything that falls through will be “thrus.” When these thins
are run over the bottom screen, which allows passage of material smaller than
wheat, they are separated into another set of overs and thrus: the overs being
the properly sized wheat and the thrus being discarded.
The
last function of the fanning mill is the mechanical analog of the age-old method
of separating “the wheat from the chaff’: winnowing, or tossing harvested
grain in the air on a windy day. The overs from the bottom screen are exposed to
a stream of air to remove material the same size and shape as wheat, but light
enough to be blown out.
While
a few small mills or growers who sell directly in them may use fanning mills as
their primary cleaning apparatus, these are more common tools of the seed
companies. Larger flour mills will use a combination of machines to perform
these functions, as well as other machines to remove impurities that don’t get
separated so easily
Like
a fanning mill, the milling separator (“Millerator”) uses screens with
different size and shape openings to separate sound wheat kernels from material
that is smaller, larger, narrower or wider than the ideal range of wheat, but
does not winnow the grain. Gravity tables and “stoners” exploit the
different density of wheat and foreign material of the same size - hence not
removed by screens - stones, dirt clods, rodent droppings, light seeds. The
grain is run over a shaking inclined table with air blowing through it. The
lightest material will flow to one end, heaviest to the opposite, with a range
of separation possible between the two extremes. A gravity table, unlike the
winnowing process, can effectively remove material that is heavier than wheat.
Cylinder
and disc separators utilize indented surfaces to separate seeds of similar size
and density, but with different shapes. Rotating cylinders or discs have tiny
pockets of a shape that will pick up only items of a particular size and shape.
For example, oats can be removed from wheat because the kernels are longer;
grain sorghum (milo) can be removed because the grains are more spheroid than
wheat.
In
an “Entolator,” the wheat is thrown by centrifugal force against a hard
surface, the impact disintegrating unsound kernels and breaking dirt clods. Many
of these unsound kernels are such because of being hollowed by insects, and may
contain eggs or larvae. The impact will shatter the kernel and the intruder. An
aspirator then separates the particles from the remaining sound wheat. In an
aspirator, a stream of moving air removes light grain and dust from wheat, as
well as the grain and insect fragments produced in the Entolator.
The
necessity of some of the aspects of cleaning wheat is more obvious than of
others. Obviously, if we want wheat flour we don’t want to be grinding oats,
rye, or strongly flavored weed seeds with it. Nor is it difficult to understand
why we should remove insects in the various stages of their life cycle. Or
rodent droppings. Stones wreak havoc on milling surfaces, be that surface
another stone or a steel roll. And if they somehow make it into the flour, they
are extremely unkind to teeth.
At
the end of the cleaning process, all the remaining kernels of wheat are
remarkably similar in size. Uniformly sized kernels mill more uniformly because
they break into similarly sized fragments when they pass through the break
rolls. Small kernels of wheat have a higher bran-to-germ ratio and will yield
less flour per bushel of wheat. Whole wheat flour milled from small wheat will
generally have more flavor than from large kernels, but the higher percentage of
bran can make for some pretty heavy high-fiber bread. The sequence of cleaning
is important, and it is important that at each step as complete a separation as
possible is achieved at each machine before the wheat is passed onto the next
piece of cleaning machinery. A funning mill or milling separator can handle a
large volume of material, while a gravity table has considerable less capacity
(there are usually several in a cleaning house). It is much more efficient to
remove material with screens than with gravity tables. Sets of discs for disc
separators are designed for very specific purposes. By the time the wheat gets
to this step in the cleaning process, what’s left in it simply cannot be
removed with screens or on a gravity table. All that should still be in the
wheat when it reaches the Entolator are sound wheat, unsound wheat, a little
dust, and a few dirt clods the same size, shape and density as wheat. The
Entolator and its accompanying aspirator remove these remaining impurities,
delivering virtually pure wheat to the conditioning tanks.
Note:
The first article in this series, The Milling
Process, may be accessed here.
*The Artisan thanks Greg Mistell,
Executive Director of the Bread Bakers Guild of America for permission to reproduce this
series of article by Mr. Leonard.
Last Updated on09/24/1999 12:05:46 AM