Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
Residual bran particles, fibers, and endosperm pieces that have not been disintegrated
are removed the same way. The clarified starch suspension is refined further in
nozzle centrifuges with a central wash–water device by removal of small-size starch,
pentosans, sugars, low-molecular-weight proteins, and minerals. In this manner, Astarch,
concentrated to a suspension of approximately 35% dry matter, leaves the
separator via nozzles. For final purification and concentration, multistage hydrocyclone
systems are applied in a countercurrent washing. The resulting starch milk of
23°Bé (equal to approximately 40% dry substance) is then dewatered over a vacuum
drum filter or peeler centrifuge. Finally, starch cakes resulting from dewatering are
dried in flash dryers similar to those for maize starch.
With wheat starch, however, optimum dewatering is hardly ever achieved when
small-size granules are present in large amounts. In such cases, dewatering can be
accomplished alternatively by pressure filtration. With rotary pressure filters, the
water content can be reduced to 33 to 35%, and the handling of the resulting starch
cakes is less problematic. Also, drying costs can be significantly reduced. 17 Pressure
filtration has been found to be effective even under practical conditions, wherein a
pressure level of 3 bars, a drum rotation speed of 1.2 to 1.6 rotations/min −1 , and a
specific throughput of 650 to 800 kg m −2 h −1 can be used. 18
8.3.4 WESTFALIA PROCESS
8.3.4.1 General Remarks
The Westfalia three-phase decanter process was first designed in the 1980s and has
undergone various improvements to achieve its present design, presented in Figure
8.1. The principal design feature of this process is the use of centrifugal forces to
separate a diluted mixture of wheat flour and water into its main components —
starch, gluten, pentosans, and process water. Prior to separation, the undiluted mixture
undergoes shearing treatment, by the energy of which flour particles become
fully hydrated and protein bodies are transformed into agglomerated gluten.
8.3.4.2 Flour Slurry Preparation and Gluten Agglomeration
The flour coming from suitable dosing systems is mixed with water (30 to 40°C)
in a 1:0.85–0.95 ratio to produce a lump-free and fully hydrated slurry, in which
flour proteins are allowed to form agglomerated gluten. Gluten development is
frequently supported by a homogenization step with subsequent maturation.
Depending on the applied pressure (up to 80 bars) and maturation time, the treatment
results in improved differentiation of gluten and pentosans when centrifuged.
Also, the relative amount of A-starch increases whereas gluten quantity reduces.
However, gluten produced is more pure and offers a more clear separation from
the superincumbent pentosans, and a distinct process water layer is developed. Prior
to decanter separation, the pretreated flour–water mixture is further diluted by
adding process or fresh water, or both. The generally used water quantity of 0.3 to
0.9 m 3 provides acceptable segregation of flour components in the decanter bowl
according to their density. Because starch has the highest density of ca. 1.5 kg l −1 ,