JFS: Food Chemistry and Toxicology
Physiochemical Properties of Polysaccharides
Extracted from Tofu Processing Wastewater
T.S. SONDA, R.J. KAIN, AND H. YAO
ABSTRACT: Yield, gelation, viscosity, emulsifying properties, and sugar composition of polysaccharides ex-
tracted from tofu processing wastewater under acidic conditions at different temperature levels and incubation
times were determined. Optimum extraction for water-soluble polysaccharides was at pH 1.5 and 100 °C over an
incubation period of 6 hours. Extraction ratio (%) and yield of high-molecular-weight polysaccharides were
higher at pH ranges of 1.5 to 3.0 with extraction temperatures of 80 to 100 °C. Most water-soluble polysaccha-
rides extracted under strong acidic conditions and above 80 °C remained fluid, while most extracted within the
range of 45 to 80 °C gelled. Pure polysaccharides exhibited better emulsifying properties than water-soluble
polysaccharides. Apparent viscosities of water-soluble polysaccharides were dependent on sugar concentra-
tion.
Keywords: tofu processing wastewater, extraction ratio, polysaccharides, apparent viscosity, emulsifying
Introduction
OFU SERVES AS AN IMPORTANT SOY-
bean [Glycine max (L.) Merr.] product
Materials and Methods
HE WASTEWATER DISCHARGED JUST
after soybean soaking and hot-grind-
D-galactose, and D-galacturonic acid on
hydrolysis. Very few reports have been
published on the extraction of water-solu-
ble soybean polysaccharides under acidic,
neutral, and/or alkaline conditions, while
the extraction of polysaccharides from
tofu processing wastewater has yet to be
reported.
T
T
that has been widely used in a variety of
dishes by some Asians for many centuries.
It is a highly digestible and nutritive prod-
uct (Schroder and others 1973) and also
serves as an inexpensive source of protein.
As a result of the economic importance of
the oil and protein constituents of soy-
beans, most scientific investigations have
been concerned primarily with those con-
stituents rather than with the accompany-
ing carbohydrates.
Oligosaccharides, especially sucrose,
raffinose, and stachyose, have long been
recognized as constituents of soybeans
(Aspinall and others 1967; Mulimani and
others 1997). Soybean oligosaccharides
exhibit important physiological functions
in humans (Koga and others 1993). Some
reports have been published on soybean
polysaccharides (Morita 1965; Yamaguchi
and others 1996). Interesting results on
soybean polysaccharides and water-solu-
ble soybean polysaccharides and their ex-
tractions under different conditions have
also been reported (Morita 1965; Yamagu-
chi and others 1996; Yoshi and others 1996;
Furuta and others 1998). Some of these
findings have revealed that soybean
polysaccharides are composed of ara-
binogalactans and galacturonic acid, re-
gardless of their solubility and the fact
that soybean cotyledon meal contains
complex polysaccharides that yield L-ara-
binose, D-xylose, L-fucose, L-rhamnose,
ing was obtained from a small-scale tofu
processing factory located at the outskirts
of Wuxi, P. R. China. The enzyme AS1398
(origin Bacillus subtilis; type proteinase;
cellulase activity e”1 H 105 µ/g) was ob-
tained from Genecor Bio-Products Co.
Ltd., Wuxi, P. R. China. The catalyst 1-me-
thylimidazole used in the acetylation of
monosaccharides and standard dextrans
were bought from the Shanghai branch of
Sigma Chemical Co. Some of the dextrans
obtained from ICN Biochemicals were giv-
en to us as a gift. All other chemicals used
were of reagent-grade and obtained either
from other laboratories or from the Chemi-
cal Department of Southern Yangtze Uni-
versity.
Recently, the use of water-soluble
polysaccharides extracted from soybean
okara as emulsifiers or viscoelastic re-
agents has been found to be economical
in the food industry (Yoshi and others
1996). This work, therefore, was undertak-
en to investigate and establish an opti-
mum level for the extraction of water-solu-
ble polysaccharides and high-molecular-
weight polysaccharides from tofu process-
ing wastewater, and to investigate the ex-
tent to which the application of heat in the
range of 45 to 100 °C at varying pH levels
(1.5 to 6.0) can affect the yield, sugar com-
position, gelation, and apparent viscosity
of water-soluble polysaccharides extract-
ed from tofu processing wastewater under
acidic conditions. The emulsifying proper-
ties and heat stability of pure polysaccha-
rides and water-soluble polysaccharides
extracted are investigated. Also an at-
tempt is made in this study to extract
polysaccharides from tofu processing
wastewater by using isopropanol and
deionized water reaction mixture.
Sample and pre-treatment
Tofu wastewater (25 L) was centri-
fuged (5000 H g for 30 min) using a tripod
centrifuge produced by Zhang Jiagang
Centrifuge factory, Zhang Jiagang, Jiang-
su Province, P. R. China. The supernatant
was discarded and the centrifugate was
oven-dried for 18 hours at 60 °C. The
oven-dried centrifugate was milled into a
fine powder using a portable electrically
operated laboratory milling machine
(Shanghai, Shenbo Instruments Compa-
ny, Shanghai, P.R. China). The powder
(52.0 g) was put into airtight containers
and stored in desiccators.
The word “wastewater” as used in
this study refers to water oozed during
pressing after soybean soaking and hot-
grinding.
1682 JOURNAL OF FOOD SCIENCE—Vol. 67, Nr. 5, 2002
© 2002 Institute of Food Technologists