25 Rye Constituents and Their Impact on Rye Processing 581RYE PROCESSING
While rye processing has gained interest because of
its perceived positive health effects (Poutanen 1997),
the functional properties of the rye major consti-
tuents limit the processing performance of rye in in-
dustrial processes (Weipert 1997). Milling and bread
making are the most common processes in rye grain
processing for human consumption.
RYEMILLING
The objective of milling is to separate the bran and
the germ from the starchy endosperm and to reduce
the starchy endosperm in size, in order to obtain a
refined product, that is, flour. Rye milling differs
some what from wheat milling. Rye kernels are
smaller than wheat kernels, and the separation of the
endosperm from the bran is poorer in rye than in
wheat (Nyman et al. 1984). The extraction rate of
rye flour is lower than that of wheat flour, and at any
given extraction rate, the ash content of rye flour is
always higher than that of wheat flour (Weipert
1997).
Cleaning and Tempering of Rye
Prior to milling, rye grain is cleaned to remove
stones, magnetic objects, broken rye kernels, cockle,
oats, and ergot. Cleaning is based on a separation
between rye and the impurities on the basis of size,
density, and shape.
The second step involves tempering of the rye
grain by adding a required quantity of moisture to
bring it to an optimum condition for milling. Rye is,
in most cases, milled at a moisture level of 15%,
(which is 1% less than the preferred moisture level
for wheat) and, in general, is tempered for a shorter
time (2–4 hours with an absolute maximum of 6
hours) than wheat. This is due to the fact that water
penetrates the rye kernel faster than the wheat kernel
due to rye’s weaker cell structure. The tempering
process can be eliminated to reduce bacterial counts.
However, if rye is milled too dry, the risk of shred-
ding the bran into smaller particle sizes and thus
contaminating the rye flour exists.
After tempering, a second cleaning step removes
surface dirt, loose hairs, and outer bran layers. Just
before milling, an additional 0.5% moisture is added
(Zwingelberg and Sarkar 2001).
Milling of the RyeIn Europe, “roller” milling and “pin” milling are the
most important rye milling processes. In roller mill-
ing, each passage through rolls involves particle size
reduction by pressure and shear forces, followed by
flour separation by particle size using sieves. Rolls
are matched to the product needed. Their size, sur-
face flutes, rotation velocity, and the gap between
pairs of rolls rotating in opposite directions at dis-
similar speeds, all can be selected or adjusted. Ty-
pically, milling starts with prebreak or flatting rolls,
followed by six to seven break rolls, six bran finish-
ers, and six reduction rolls. All rolls run at a differ-
ential speed of 3:1, except the flatting rolls (1:1
ratio). The first four break and reduction rolls are
run dull to dull and the following breaks, sharp to
sharp. A complex sieving system with plansifters
and centrifugal sifters is used. In this process, rye is
fractionated into different milling streams, a number
of which are combined to give the final flour. When
practically all streams are used, an extraction rate of
approximately 100% is reached and whole meal rye
flour is obtained. When an increasing number of the
fractions containing outer layer materials are left out
of the reconstitution process, extraction rates de-
crease. For straight-run flour, a typical extraction
rate is 58–68%.
In pin milling, rye is milled in impact mills,
which work by the principle of impact disintegra-
tion. As this process requires a moisture content of
17–18%, rye is tempered a second time. In impact
milling, five to seven milling steps are used. The
extraction rate, ash content, and flour quality are
similar for impact mills and roller mills. The starch
damage, however, is lower in rye flour from impact
mills than in that from roller mills because of the
number of milling steps required.
In North America, the main objective in rye
milling is to produce as much flour of the desired
granulation as possible. The ash content and color of
rye flours are not as critical as for common wheat
flours. Therefore, the flow sheet of a rye mill is sim-
ple relative to that of a common wheat mill. A typi-
cal rye mill contains three to five break rolls, up to
two sizings, and four to six reduction rolls. All
grinding rolls are corrugated and run at a differential
rate of 3:1. To properly remove the bran, bran
dusters precede the last two or three break passages
rather than coming just at the end. Because rye flour