Solubility and solution thermodynamic properties of quercetin and quercetin
dihydrate in subcritical water
, Jerry W. King
, Luke R. Howard
, Jeana K. Monrad
Ralph E. Martin Department of Chemical Engineering, University of Arkansas, 3202 Bell Engineering Center, Fayetteville, AR 72701, United States
Department of Food Science, University of Arkansas, 2650 North Young Avenue, Fayetteville, AR 72704, United States
Received 30 December 2009
Received in revised form 31 March 2010
Accepted 4 April 2010
Available online 9 April 2010
Scanning electron microscopy
Fundamental physicochemical data is required for the design and optimization of food engineering pro-
cesses, such as extraction. Flavonoids are present in natural products such as grapes and have numerous
health beneﬁts particularly with respect to their reported antioxidant properties. Such ﬂavonoid com-
pounds can be extracted from these natural products using a variety of solvents, among them water.
In this study, the aqueous solubilities of 3,3
,5,7-pentahydroxyﬂavone (quercetin) and its dihydrate
were measured at temperatures between 25 and 140 °C using a continuous ﬂow type apparatus. The ﬂow
rate of subcritical water was studied at 0.1, 0.2 and 0.5 mL/min to study its effect on quercetin solubility
and thermal degradation at temperatures greater than 100 °C. The aqueous solubility of anhydrous quer-
cetin varied from 0.00215 g/L at 25 °C to 0.665 g/L at 140 °C and that of quercetin dihydrate varied from
0.00263 g/L at 25 °C to 1.49 g/L at 140 °C. The aqueous solubility of quercetin dihydrate was similar to
that of anhydrous quercetin until 80 °C. At temperatures above or equal to 100 °C, the aqueous solubility
of quercetin dihydrate was 1.5–2.5 times higher than that of anhydrous quercetin. The aqueous solubility
of quercetin anhydrate and dihydrate at different temperatures was correlated using a modiﬁed Apelblat
equation. The thermodynamic properties of the solution of quercetin and its dihydrate in water were
than estimated from their solubility values. A ﬂow rate effect on the aqueous solubility of quercetin
and its dihydrate was not observed until above 100 °C where higher solvent (water) ﬂow rates
(>0.1 mL/min) were required to maintain a constant solubility in the saturation cell and with minimal
thermal degradation of the solute (quercetin dihydrate). The study of its particle morphology under
SEM indicated an aggregation of the crystals of quercetin dihydrate at subcritical water temperatures
and at lower ﬂow rates (<0.5 mL/min), thereby inhibiting stable solubility measurements and solvent
ﬂow through the saturation cell.
Ó 2010 Elsevier Ltd. All rights reserved.
For food engineering design applications, it is important to have
fundamental physicochemical data, such as solute solubilities in
extraction solvents, diffusivities of the solutes in like solvents,
and mass transfer parameters in order to optimize the process.
Our laboratory has embarked on an extensive program to experi-
mentally determine such data and to correlate it for predictive pur-
poses. As noted below, such fundamental physicochemical data
can have applications in related ﬁelds, such as pharmaceutical
technology and to the application of nutraceuticals. The molecular
complexity and sensitivity of many ﬂavonoids to environmental
factors such as light, heat, and oxygen make such measurements
challenging. However, in this study we have determined the solu-
bility of a model ﬂavonoid, quercetin, in subcritical water using a
novel experimental technique.
Flavonoids, are a diverse group of polyphenolic compounds
present in plants, that provide a wide range of health beneﬁts
due to their antioxidant, anti-bacterial, anti-viral and anti-inﬂam-
matory properties (Cook and Samman, 1996). Quercetin
-7-pentahydroxy ﬂavone) (Fig. 1) belongs to a sub-class
of ﬂavonoids known as ﬂavonols, which ﬁnd use in nutraceuticals
or food supplements (Boots et al., 2008). Studies have shown that
quercetin has antioxidant (Laughton et al., 1989), anti-inﬂamma-
tory (Orsolic et al., 2004), anti-bacterial (Cushnie and Lamb,
2005), anti-coagulative (Bucki et al., 2003), and anti-hypertensive
(Duarte et al., 2001) properties. Quercetin has also been used in
gene expression modulation (Moon et al., 2006) and in the inhibi-
tion of the growth of human cancer cell lines (Larocca et al., 1990).
Quercetin, existing mainly in the form of glycosides, can be found
in vegetables such as onions, tomatoes, lettuce & celery (Crozier
et al., 1997), fruits such as apples and berries (Bajpai et al., 2005)
and tea, fruit and vegetable juices (Karakaya and El, 1999).
Quercetin is commonly extracted from the afore-mentioned
sources using organic solvents (Wach et al., 2007) and
0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: +1 479 575 5979; fax: +1 479 575 7926.
E-mail address: firstname.lastname@example.org (J.W. King).
Journal of Food Engineering 100 (2010) 208–218
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Journal of Food Engineering
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