4.3.1.2.1 Effect of Membrane Configuration
Membrane configuration has a significant effect on the retention of flavor com-pounds during apple juice concentration (Chou et al. 1991). Permeation of n-hexanal is found to be about 11.5% in the plate-and-frame configuration compared with 0.5%
in the spiral wound. The higher permeation is probably due to higher membrane packing density and membrane area of the spiral wound (a total membrane surface area of 3.90 m2) when compared with the plate and frame (a total membrane surface area of 0.36 m2). A lower concentration of n-hexanal in the permeate is observed in the spiral wound system due to increased retention and increased flavor compound absorption due to greater membrane surface.
4.3.1.2.2 Effect of Membrane Type
Types of membranes are found to have significant effect on both permeate flux and solute retention. Chua et al. (1988) studied permeate flux and flavor retention by CA and polyamide (PA) membranes. Polyamide was shown to be advantageous in terms of both permeate flux and flavor retention than cellulose acetate membrane. Sheu and Wiley (1983) studied different cellulose acetate (CA) and high resistance (HR) RO membranes to concentrate apple juice. They reported that during concentration of apple juice from 10 to 20 °Bx, at a pressure of 0.45 MPa and at a temperature of 20°C, CA-865 membrane results in a permeate flux of 26.9 L/m2 h compared with the HR-95 membrane with a permeate flux of 15.9 L/m2 h. Permeate flux about 15.0 L/m2 h seems to be economically feasible in the RO process. The HR membrane generally shows better apple flavor volatile retention than the CA membranes for a fixed operating pres-sure. About 88% and 16.9% retention of apple flavor volatiles are noticed using HR-95 and CA-865 membranes, respectively. Hence, the application of CA membranes with low retention of flavor volatiles results in significant quality loss leading to less suitabil- ity for apple juice concentration. Characteristics of the permeate collected from dif-ferent membrane processes are shown in Table 4.6. Lower retention of flavor volatiles using cellulose acetate membranes is also observed by Matsuura et al. (1974). Chou et al. (1991) investigated the retention of apple juice volatiles using polyether-urea and polyamide thin film composite RO membranes and reported that the apple juice con-centrated using polyamide membrane would result in a more intense flavor than juice concentrated using the polyether-urea membrane. Moreover, polyamide membranes have longer shelf life than cellulose acetate.
4.3.1.2.3 Effect of Feed Concentration
Permeate flux is a strong function of feed concentration during the RO process. The concentration polarization increases with feed concentration, resulting in a higher osmotic pressure near the membrane–solution interface during RO process and, thereby, a decrease in the available driving force (i.e., the TMP). This leads to a decrease in permeate flux.
4.3.1.2.4 Effect of Temperature
Temperature has a significant effect on both permeation rate and recovery of aroma
HR-95b 35 53.7 <0.1 5.91 <0.002 71.4 62.0 73.8
40 54.0 <0.1 5.92 <0.002 85.2 87.7 75.0
45 53.5 <0.1 5.89 <0.002 83.8 85.0 77.2
40 (45°C) 53.2 <0.1 5.82 <0.002
HR-98b 35 53.8 <0.1 5.45 <0.002 68.9 60.9 71.3
40 53.5 <0.1 5.47 <0.002 86.0 85.6 74.0
45 53.6 <0.1 5.3 <0.002 86.0 87.0 75.2
40 (45°C) 53.2 <0.1 5.46 <0.002
Source: Reprinted from Sheu, M.J. and Wiley, R.C., J. Food Sci., 48, 422, 1983. With permission.
a Concentrated to 25 °Bx.
b Concentrated to 20 °Bx.
They have found that with an increase in temperature from 20°C to 40°C, recoveries are decreased from 41.7% to 29.2% for hexanol and from 56.6% to 50.8% for ethyl-2-methylbutyrate while permeation rates are increased from 0.2% to 0.7% for hexanol and from 0.15% to 0.25% for ethyl-2-methylbutyrate. According to Sheu and Wiley (1983), the concentration of aroma-stripped apple juice by RO at higher temperature is feasible with the advantages of high permeate flux and high recovery of nonvolatile components. They have reported that a twofold increase in permeate flux with an increase in the retention of soluble solids and sugars is obtained with an increase in temperature of 25°C (Figure 4.11).
0290 4 8 12 16
300 310 320
T (K) 11 °Brix
15 °Brix 19.5 °Brix 22 °Brix
J (m/s) · 106
FIGURE 4.10 RO of apple juice. Influence of temperature on permeate flux at constant concentration. Operating conditions: 2.2 m/s, 550 kPa. (Reprinted from Alvarez, V. et al., J. Membr. Sci., 127, 25, 1997. With permission.)
0150 2 4 6 8 10
250 350 450 550
11 °Brix 15 °Brix 19.5 °Brix 22 °Brix
J (m/s) · 106
ΔP (kPa)
FIGURE 4.11 RO of apple juice. Influence of transmembrane pressure on permeate flux at constant concentration. Operating conditions: 298 K, 2.2 m/s. (Reprinted from Alvarez, V.
et al., J. Membr. Sci., 127, 25, 1997. With permission.)
4.3.1.2.5 Effect of Pressure
A significant enhancement in permeate flux is observed with an increase in TMP.
Figures 4.12 and 4.13 show that an increase in pressure not only increases the per-meate flux but also increases the maximum concentration that can be obtained, the limit being 19, 24.5, and 27.5 °Bx at 0.35, 0.45, and 0.55 MPa, respectively (Alvarez et al., 1997). Chou et al. (1991) have observed that with an increase in pressure, there is a significant enhancement in percent permeation of flavor compounds with higher molecular weight and more negative Taft number. However, no significant effect is observed in the case of flavor compounds with lower molecular weight and less nega-tive Taft number. Recovery of flavor compounds in the retentate is also found to be
010 2 4 6 8 10
16 22 28
350 kPa 450 kPa 550 kPa
°Brix J (m/s) · 106
FIGURE 4.12 RO of apple juice. Permeate flux variation as a function of concentration at different transmembrane pressure. Operating conditions: 298 K, 2.2 m/s. (Reprinted from Alvarez, V. et al., J. Membr. Sci., 127, 25, 1997. With permission.)
00 2 4 6 10
8 12
1 2 3
v (m/s) 11 °Brix
15 °Brix 19.5 °Brix 22 °Brix 25 °Brix
J (m/s) ·106
FIGURE 4.13 RO of apple juice. Influence of tangential velocity on permeate flux at con-stant concentration degree. Operating conditions: 298 K, 550 kPa. (Reprinted from Alvarez, V.
et al., J. Membr. Sci., 127, 25, 1997. With permission.)
pressure dependent in their studies. An increase in pressure results in an increase in the permeate flux. Therefore, operational time decreases and consequently smaller flavor compound losses from the retentate, which is attributed to volatilization and adhesion to the membrane.
4.3.1.2.6 Effect of Cross-Flow Velocity
During concentration by RO, cross-flow velocity has a positive effect on permeate flux. It is observed that permeate flux increases with an increase in cross-flow veloc-ity for a fixed feed concentration (Alvarez et al., 1997). Figure 4.13 shows that the effect of cross-flow velocity is more significant at low velocities while this effect is reduced for values higher than 1.5–2 m/s. Increasing cross-flow velocity increases the process efficiency through reduction of either filtration area or operational time but cannot change the maximum limit of juice concentration.