Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
protein) and the concentration of residual fruit water or diluted process water
streams. 21 Alternative processes, however, operate either by partial removal or do
not have separation of fruit water. 22
Process water coming from internal water circulation (after initial fruit water
removal) has a specific BOD of 6.3–8.2 kg t −1 and a specific COD of 7.4–9.6 kg
t −1 . The specific wastewater volume is set to 1 to 1.3 m 3 t −1 . Although tuber
transportation and wash water must be considered additionally (BOD 1.3–2.6 kg
t −1 and COD 1.5–2.9 kg t −1 ), the presented figures, even when multiplied by a
factor of 5 to achieve comparable dry substance levels, are far from those representative
of wheat starch production. 2 The presented figures do not include residual
fruit water and fruit water.
8.4.3.2 Tuber Disintegration — Rasping
Because potato starch yield depends greatly on disintegration of potato tissue, rupture
as complete as possible is significant when working with specific rasping or disintegration
systems. If modern rasping machines operating at 2100 rotations/min −1
operate efficiently, starch extraction rates can reach up to 96 to 98%. Even in the
yields given, starch bound to rasps/gratings can reach 25 to 30%. The rasping systems
(such as Ultra Rasper) used allow throughputs between 20 and 30 t h −1 .
Depending on age of potatoes, oxidation of polyphenolic compounds of rasps
plays a critical role following tuber disintegration, particularly when concentrated
potato juice is separated and used for protein recovery. To minimize enzymatic
discoloration, SO 2 is added either as sodium hydrogen sulfite (500–600 g t −1 =
120–144 g SO 2) or as aqueous sodium disulfite solution (52–34 g SO 2) during
rasping. Encapsulated systems allow grinding under vacuum.
8.4.3.3 Fruit Water Separation
As stated previously, early and full separation of fruit water produces a concentrated
protein-containing liquid favorably used for isoelectric precipitation and heat denaturation.
A suitable protein concentration is required for optimum process design. 23
For separation of fruit water, horizontally oriented centrifuges, called decanters, are
used. In the one-stage decanting system, undiluted fruit water is recovered at separation
rates of 62 to 65%. 21,22 The rate can be significantly increased (up to 95%)
when process water coming from starch refining is used to dilute rasps prior to
decanting.
Alternatively, two-stage decanting systems can be used. Here, the rasps are
diluted with overflow water from the second decanter before entering the first in the
sequence. The concentrate from the first decanter is diluted, however, with process
water coming from starch refinement. The quantity of process water used affects
fruit water separation rate, which can reach over 92%. Because of early fruit water
removal, total fresh water supply (including water supply for downstream starch
extraction and refining) — water quality should preferably fulfil requirements for
soft-water quality — can be reduced to 0.4 to 0.5 m 3 per tonne of potatoes.