Food Biochemistry and Food Processing (2 edition)

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40 Separation Technology in Food Processing 775

membrane, as long as the applied pressure is greater than the os-

motic pressure of the feed solution. Reverse osmosis membranes

have been widely applied in water treatment, such as desalting,

pollution control, pure water treatment, and wastewater treat-

ment. Reverse osmosis operates by diffusion from a solution.

Under the high differential pressure across the membrane, the

solvent from the solution actually dissolves in the material of

the membrane, diffuses across it, and transfers out into the clean

solvent on the other side. It is not a perfect separation, because

the dissolved species from the feed solution have definite abil-

ities to diffuse through it as well, but the diffusion coefficient

for the solvent is so much higher than that for the solute that the

separation is virtually complete. Reverse osmosis is often used

to remove dissolved organics and metals. Thus, reverse osmo-

sis, as a device solely required to separate sodium chloride from

water in the early days, is now being used to reject a wider range

of solutes.

Electrodialysis

Electrodialysis, an electrically driven membrane process, is

commonly used to desalt solutions. Ions in aqueous solution

can be separated using a direct current electrical driving force

on an ion-selective membrane. Electrodialysis usually uses

many thin compartments of solution separated by membranes

that permit passage of either positive ions (cations) or nega-

tive ions (anions) and block passage of the oppositely charged

ion. Cation-exchange membranes are alternatively stacked with

anion-exchange membranes placed between two electrodes. The

solution to be treated is circulated through the compartments and

a direct current power source is applied. All cations gravitate

toward the cathode (negatively charged terminal) and transfer

through one membrane, while anions move in the opposite direc-

tion, thereby concentrating in alternating compartments. Elec-

trodialysis is commonly used to recover spent acid and metal

salts from plating rinse. It obviously is not effective for nonpo-

lar solutions.

Membrane-Based Separation in Industrial Applications

Membrane-based processes offer many advantages in terms of

lower energy consumption, higher selectivity, faster separation

rate, and lower recovery costs (Cassano et al. 2008). There has

been increasing interest in the use of membrane-based technolo-

gies in the food industry. Various designs of membrane-based

separation equipment are used in the food industry (Figure 40.6).

In the dairy industry, ultrafiltration is widely used to fraction-

ate cheese whey and pre-concentrate milk for cheese making

(Pouliot 2008). In sequential membrane processes, ultrafiltra-

tion may also be used as a membrane reactor to produce pro-

tein hydrolysate from protein isolate (Feins and Sirkar 2005,

Charoenvuttitham et al. 2006, Sarkar et al. 2007, Annathur et al.

2010) and as a membrane fermenter to produce fuels and chemi-

cals from reverse osmosis concentrates (Greenlee et al. 2009, Qu

et al. 2009, Saxena et al. 2009). In sugar and corn syrup process-

Figure 40.6.The pilot scale membrane separation system.

ing, microfiltration and ultrafiltration are used for clarification to

remove impurities (Singh and Cheryan 1994, Jaeger de Carvalho

et al. 2008). In juice, wine, and beer processes, microfiltration

and ultrafiltration not only remove insoluble solids to produce

sparkling clear product, but also remove spoilage microorgan-

isms, eliminating the need for pasteurization (Massot et al. 2008,

Vaillant et al. 2008, Cassano et al. 2010). Ultrafiltration is also

used to concentrate natural colorants and flavors (Nawaz et al.

2005, Nawaz et al. 2006, Shi et al. 2007d, Galaverna et al. 2008,

Ren et al. 2008a, 2008b, 2008c).

The main operating problem of all membrane separation pro-

cesses is that the membrane material eventually plugs, called

fouling, causing the resistance to flow to increase. The plugging

process is accentuated by the concentration polarization that oc-

curs in the relatively quiescent fluid zone close to the membrane

surface, as the species separated from previously processed flu-

ids build up in this zone and interfere with fresh material trying

to get to the surface.

Membrane-based separation is still evolving and finding more

and more applications in a broad range of fields, and the