BLBS102-c40 BLBS102-Simpson March 21, 2012 14:23 Trim: 276mm X 219mm Printer Name: Yet to Come
776 Part 7: Food Processingdevelopment of new membrane materials will strongly influ-
ence separation processes in the future. Because of the great ad-
vantages of membrane separation over conventional separation
practices, the availability of the choice of membrane size (mi-
crofiltration, ultrafiltration, nanofiltration, reverse osmosis, pre-
evaporation membrane, and distillation membrane), materials
(polymeric and ceramic, hydrophilic and hydrophobic, symmet-
ric and asymmetric), configurations (spiral wound, hollow fiber,
and plate and frame), operation modes (dead-end and cross-flow,
batch, semi-batch, and continuous), and membrane technology
offers more selective, flexible, and efficient separations over a
wide range of compounds. This technology will continue to gain
recognition and acceptance in the food industry.Pressurized Low-Polarity Water ExtractionPressurized low-polarity water extraction, also known as subcrit-
ical water extraction (or hot water extraction, pressurized hot wa-
ter extraction, superheated water extraction, or high-temperature
water extraction), that is, extraction using hot water under pres-
sure, has recently become a popular green processing technol-
ogy and emerges as a promising extraction and fractionation
technique for replacing the traditional extraction methods. The
pressurized low-polarity water extraction is also used in sam-
ple preparation to extract organic contaminants from foodstuff
for food safety analysis and solids/sediments for environmental
monitoring purpose.
The pressurized low-polarity water extraction process is an
environmentally friendly technique that can provide higher ex-
traction yields from solid plant materials (Luque de Castro and
Jimenez-Carmona 1998). Pressurized low-polarity water extrac- ́
tion is based on the use of water as an extractant in a dynamic
mode, at temperatures between 100◦C and 374◦C (critical point
of water, 221 bar and 374◦C) and under pressure high enough to
maintain the liquid state. The critical temperature and pressure
of water are shown as a phase diagram in Figure 40.5 (Tc=
374 ◦C,Pc=221 bar or 22 MPa). The pressurized low-polarity
water extraction process can maintain the water in the liquid
form up to a temperature of 374◦C and a pressure of 22.1 MPa
(221 bar) (Haar et al. 1984, Hawthorne et al. 2000). A pressure
of 5 MPa would be high enough to prevent the water from va-
porizing at temperatures from 100◦C to 250◦C. Once pressure
is high enough to keep water in a liquid state, additional pres-
sure is not necessary as it has limited influence on the solvent
characteristics of water. Increasing the water temperature from
25 ◦C to 250◦C causes similar changes in dielectric constant,
surface tension, and viscosity (Kronholm et al. 2007, Brunner
2009). Pressurized low-polarity water extraction can easily sol-
ublize organic compounds such as phytochemicals, which are
normally insoluble in ambient water.
Pressurized low-polarity water extraction has the ability to
selectively extract different classes of compounds, depending
on the temperature used. The selectivity of subcritical water
extraction allows for manipulation of the composition of the
extracts by changing the operating parameters, with the more
polar ones extracted at lower temperatures and the less polar
compounds extracted at higher temperatures (Basile et al. 1998,Ammann et al. 1999, Clifford et al. 1999, Miki et al. 1999,
Kubatova et al. 2001, Soto Ayala and Luque de Castro 2001).Process SystemAs shown in Figure 40.7, the instrumentation consists of a water
reservoir coupled to a high-pressure pump to introduce the pres-
surized low-polarity water into the system, an oven, where the
extraction cell is placed and extraction takes place, and a restric-
tor or valve to maintain the pressure. Extracts are collected in a
vial placed at the end of the extraction system. In addition, the
system can be equipped with a coolant device for rapid cooling
of the resultant extract. As the unique properties of pressurized
low-polarity water, the pressurized low-polarity water extrac-
tion has a disproportionately high boiling point for its mass, a
high dielectric constant, and high polarity. As the temperature
rises, there is a marked and systematic decrease in permittivity,
an increase in the diffusion rate, and a decrease in the viscosity
and surface tension. In consequence, more polar target mate-
rials with high solubilities in water at ambient conditions are
extracted most efficiently at lower temperatures, whereas mod-
erately polar and nonpolar targets require a less-polar medium,
induced by elevated temperature.
Water changes dramatically when its temperature rises, be-
cause of the breakdown in its hydrogen-bonded structure with
temperature. The high degree of association in the liquid causes
its relative permittivity (more commonly called its dielectric
constant) to be very high at ca. 80 under ambient conditions.
But as the temperature rises, the hydrogen bonding breaks down
and the dielectric constant falls, as shown in Figure 40.5. The
most outstanding feature of this leaching agent is the easy ma-
nipulation of its dielectric constant (ε). In fact, this parameter
can be changed within a wide range just by changing the tem-
perature under moderate pressure. Thus, at ambient temperature
and pressure, water has a dielectric constant of ca. 80, making
it an extremely polar solvent. This parameter is drastically low-
ered by raising the temperature under moderate pressure. For
example, subcritical water at 250◦C and a pressure over 40 bar
hasε=37, which is similar to that of ethanol and allows for the
leaching of low-polarity compounds. By 250◦C, its dielectric
constant has fallen so that it is equal to that for methanol (i.e.,
33) at ambient temperature. Thus, between 100◦C and 200◦C,
superheated water is behaving like a water–methanol mixture.
Partly because of its fall in polarity with temperature, super-
heated water can dissolve organic compounds to some extent,
especially if they are slightly polar or polarizable like aromatic
compounds. Therefore, water can be used as extraction solvent
to extract the polar, the moderately polar, and the nonpolar com-
pounds by adjusting the extraction temperature from range of
50 ◦C to 275◦C.
The solubility of an organic compound is often many or-
ders of magnitude higher than its solubility in water at ambient
temperature for two reasons. One is the polarity change and
the other is that of a compound with low solubility at ambi-
ent temperature. Pressurized low-polarity water will have a high
positive enthalpy of solution and thus a large increase in solubil-
ity with temperature. Because of the greater solubility of some