Food Biochemistry and Food Processing (2 edition)

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34 Rye Constituents and Their Impact on Rye Processing 665

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 condi-

tion 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 shredding 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 addi-

tional 0.5% moisture is added (Zwingelberg and Sarkar 2001).

Milling of the Rye

In Europe, “roller” milling and “pin” milling are the most im-

portant rye milling processes. In roller milling, 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 dissimilar speeds, all can be

selected or adjusted. Typically, milling starts with prebreak or

flatting rolls, followed by six to seven break rolls, six bran fin-

ishers, and six reduction rolls. All rolls run at a differential 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 plan sifters 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 re-

constitution process, extraction rates decrease. 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 disintegration. 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

simple relative to that of a common wheat mill. A typical 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 is sticky and difficult

to sift, generous sifting surfaces are required (Zwingelberg and

Sarkar 2001).

Impact of Rye Constituents on Rye Milling

Rye contains higher levels of arabinoxylans than wheat (Aman

et al. 1997) and is accordingly more difficult to mill. Because

the arabinoxylans are hygroscopic, the flour absorbs ambient

humidity and tends to clump. This interferes with sifting. For

this reason, rye is milled at a lower moisture content and requires

more sifting surface than wheat (Zwingelberg and Sarkar 2001).

Protein content also influences the performance of rye in the

mill. When it exceeds 12%, rye becomes more difficult to grind

and has a lower extraction rate (Weipert 1993a).

Rye Bread Making

The bread-making behavior and performance of rye differ con-

siderably from those of wheat (Weipert 1997). The principal

reason for this is that rye proteins cannot form a gluten network,

which lies at the basis of wheat bread quality. In addition, rye

starch gelatinizes at a lower temperature than wheat starch and

is therefore more prone to enzymic degradation during the oven

phase than wheat starch. The main differences between rye and

wheat breads are observed in terms of volume yield, crumb tex-

ture, and shelf life. Baking volume of rye breads is normally

only about half that of wheat. However, rye bread has a longer

shelf life and is richer in taste and aroma (Weipert 1997).

A wide assortment of rye-containing breads exists. They vary

in size, shape, formulation, bread-making process, and sensory

properties. Rye flour and whole meal rye can be incorporated

into rye bread (minimum 90% rye), mixed rye/wheat bread

(minimum 50% rye), or mixed wheat/rye bread (minimum 50%

wheat). Other ingredients can be crushed rye grains, malted and

crushed rye grains, malted rye kernels, or precooked rye kernels

(Kujala 2004). Examples of rye bread types are pumpernickel

(containing whole kernels), soft sourdough bread, crisp bread,

rolls, and buns.

Rye Bread-Making Process

For rye bread production, acidification is a prerequisite to obtain

a good quality product. The traditional method of preparing rye

bread uses sourdough, although sometimes, direct acidification

by lactic acid or other organic acids is used.

Preparation of Sourdough Sourdough is classically made by

mixing rye flour or whole meal rye with water, yeast, and a

starter and allowing the mixture to ferment (Kariluoto et al.