In¯uence of feed ¯ow velocity on pervaporative aroma recovery from
a model solution of apple juice aroma compounds
Jenny Olsson
*
, Gun Tr
ag
#
ardh
Department of Food Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden
Received 5 March 1998; received in revised form 2 November 1998; accepted 2 November 1998
Abstract
Pervaporation is a membrane separation technique being developed for aroma recovery from various aqueous solutions. This
study concerns pervaporative aroma recovery from apple juice. The objective was to study the concentration polarisation phe-
nomenon in this application and to ®nd a possible relationship between the relative composition of the permeate, i.e. the aroma
concentrate, and the feed ¯ow conditions (Re. 50±1000). The study was based on a model solution of apple juice aroma compounds.
The relative content of esters and aldehyde, trans-2-hexenal, increased as the hydrodynamic conditions were improved at the expense
of the alcohols. For the alcohols, the membrane resistance dominated over the feed boundary layer resistance to mass transfer. For
most of the esters, the opposite relation was found. For the aldehyde and ethyl acetate, the resistances were of the same order. The
Sherwood correlation was found to overestimate the mass transfer coecient of the feed boundary layer in cases of severe con-
centration polarisation. Ó 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Pervaporation; Hydrophobic membranes; Apple juice; Aroma recovery; Concentration polarisation
1. Introduction
The aroma complex of apple juice is a highly volatile
fraction of principally esters, aldehydes and alcohols,
but also includes ethers, fatty acids, lactones, terpenes
and ketones. The total concentration of aroma com-
pounds is approximately 200 ppm. Each group of aroma
compounds gives a typical character to the apple juice
¯avour. The esters give the juice a sweet, fruity ¯avour
aording a sensation of ripeness. The aldehydes give a
fresh, grassy ¯avour that gives the juice a ¯avour of
immature apple. The alcohols contribute to both the
Journal of Food Engineering 39 (1999) 107±115
Nomenclature
CPM concentration polarisation modulus (A)
D diusivity coecient (m
2
/s)
J mole ¯ux (mole/m
2
s)
L length of the ¯ow channel (m)
P pressure (mbar)
P
0
equilibrium vapour pressure (mbar)
Pe Pecl
et number (A)
R resistance to mass transfer (m
2
s mbar/mole)
Re Reynolds number (A)
Sc Schmidt number (A)
Sh Sherwood number (A)
V
p
velocity of the ¯uid perpendicular to the
membrane surface (m/s)
X mole fraction (A)
d
h
hydraulic diameter (m)
k mass transfer coecient (mole/m
2
s mbar)
k
mass transfer coecient (m/s)
v mean velocity of the ¯uid (m/s)
y length co-ordinate (m)
z relation between the volume fraction and the
mole fraction (A)
Greek letters
b enrichment factor (A)
c activity coecient (A)
d thickness of boundary layer (m)
m kinematic viscosity (A)
q mole density (mole/m
3
)
Subscripts
app apparent
bl feed boundary layer
f feed or feed side
i component
int intrinsic
m membrane
ov overall
p permeate or permeate side
tot total
Superscripts
b bulk
exp experimentally obtained
m membrane
theor theoretically obtained
*
Corresponding author. E-mail: Jenny.Olsson@livstek.lth.se.
0260-8774/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 6 0 - 8 7 7 4 ( 9 8 ) 0 0 1 5 4 - X