68 M.J. Lewis Osmotic pressures are highest for low molecular weight solutes, so the highest osmotic pressures arise for salt and sugar solutions. Concentration of such solutions results in a large increase in their osmotic pressure. On the other hand, proteins and other macromol ecules do not produce high osmotic pressures. There will only be small increases during their concentration as well as small differences in osmotic pressure between the feed and permeate in ultrafiltration. Values for osmotic pressures are not easy to find in the literature and a selection of values is given in Table 3. 1. A further complication wi foods and other biological systems is their complexity, with not just one but many components. In reverse osmosis the applied pressure must exceed the osmotic pressure, and the driving force term in reverse osmosis is normally the difference between the applied pressure and the osmotic pressure. It could be that osmotic re is one of the factors that limits the extent of concentration. One suggested experimental method for measuring osmotic pressure is to determine the pressure that would give zero flux, by extrapolation. In ultrafiltration and microfiltration, there is little osmotic pressure differ ence over the membrane as the low molecular weight components are almost freely permeating(see equation(3. 8) Table 3. 1. Osmotic pressures of some solutions Solution Osmotic pressure Sugar beet 20°Brix 34.1 Tomato paste 33°Brix 69.0 15°Brix Citrus juice 10°BriX 34°Brix 690 Sucrose Br Coffee extract 28%TS 34.0 Sea-wate 3.5%o salt 15.0%o sal 138.0 Milk Lactose 1%w/v 3.7 Sor piled from data in Cheryan(1986)and Lewis(1982) Some equations for osmotic pressure are given by Cheryan(1986) As the membrane pore size increases, the membrane becomes permeable to low molecular weight solutes in the feed; even the transport mechanisms are likely to change Lower pressure driving forces are required as osmotic pressure differences between the feed and permeate are reduced. However, molecules of a larger molecular weight are still rejected by the membrane. Therefore some separation of the solids present in the feed takes place; the permeate contains low molecular weight components at approximately the same concentration as they are in the feed, and the concentrate contains large68 M. J.Lewis Osmotic pressures are highest for low molecular weight solutes, so the highest osmotic pressures arise for salt and sugar solutions. Concentration of such solutions results in a large increase in their osmotic pressure. On the other hand, proteins and other macromol￾ecules do not produce high osmotic pressures. There will only be small increases during their concentration as well as small differences in osmotic pressure between the feed and permeate in ultrafiltration. Values for osmotic pressures are not easy to find in the literature and a selection of values is given in Table 3.1. A further complication with foods and other biological systems is their complexity, with not just one but many components. In reverse osmosis the applied pressure must exceed the osmotic pressure, and the driving-force term in reverse osmosis is normally the difference between the applied pressure and the osmotic pressure. It could be that osmotic pressure is one of the factors that limits the extent of concentration. One suggested experimental method for measuring osmotic pressure is to determine the pressure that would give zero flux, by extrapolation. In ultrafiltration and microfiltration, there is little osmotic pressure differ￾ence over the membrane as the low molecular weight components are almost freely permeating (see equation (3.8)). Table 3.1. Osmotic pressures of some solutions So I LI ti on Osmotic pressure (bar) Sugar beet 20" Brix 34.1 Tomato paste 33" Brix 69.0 Apple juice 15" Brix 20.4 Citrus juice 10" Brix 14.8 34" Brix 69.0 Sucrose 44" Brix 69.0 Coffee extract 28% TS 34.0 Sea-water 3.5% salt 23.2 15.0% salt 138.0 Milk 6.9 Whey 6.9 Lactose 1% w/v 3.7 Compiled from data in Cheryan (1986) and Lewis (1982). Some equations for osmotic pressure are given by Cheryan (1986). As the membrane pore size increases, the membrane becomes permeable to low molecular weight solutes in the feed; even the transport mechanisms are likely to change. Lower pressure driving forces are required as osmotic pressure differences between the feed and permeate are reduced. However, molecules of a larger molecular weight are still rejected by the membrane. Therefore some separation of the solids present in the feed takes place; the permeate contains low molecular weight components at approximately the same concentration as they are in the feed, and the concentrate contains large