Food Biochemistry and Food Processing (2 edition)

BLBS102-c40 BLBS102-Simpson March 21, 2012 14:23 Trim: 276mm X 219mm Printer Name: Yet to Come

40 Separation Technology in Food Processing 773

as particle shape and size distribution, porosity and pore size dis-

tributions, surface area, and moisture content influence solubility

and mass transfer. The presence of water (moisture content) in

the sample matrix during supercritical extraction also has an

effect on the extraction outcome. In order to improve the yield

and quality of the extracted high-value food components from

raw material, a pretreatment of the raw material is an essential

process (Yang et al. 2008, Zheng et al. 2009, Nagendra et al.

2010). Cell disruption is the most important pretreatment, and

this procedure can be conducted by several processes such as

mechanical, ultrasonic, high electronic field pulse, and nonme-

chanical treatments. With improved processing conditions and

reduced cost, high-value components extracted from natural ma-

terials by supercritical CO 2 extraction process will become even

more economical at high throughput.

Membrane-Based Separation Technology

Membrane-based technology is a rising separation technology.

At present, more and more fields use membrane technology to

separate the fluids and get good results. With the principal devel-

opment of membrane technology, the applications are expand-

ing much wider and the technology brings remarkable economic

benefits. Now, many countries in the world have noticed the im-

portance of membrane technology in the food processing field,

especially since we are short of energy and resources and the

deteriorating environment all exist in our lifetime. So, industries

regard membrane-based separation technologies as important

technologies in food processing areas.

The common membrane-based separation process is usually

run under pressure. A membrane can be described as a thin

barrier between the two bulk phases, and it is either a homoge-

neous phase or a heterogeneous collection of phases. In mem-

brane separations, each membrane has the ability to transport one

component more readily than the others because of differences

in physical and chemical properties between the membrane and

the permeating components. Furthermore, some components can

freely permeate through the membrane, while others will be

retained. The stream containing the components that pass

through the membrane is called the permeate, and the stream

containing the retained components is called the retentate. The

flow of material across a membrane is kinetically driven by the

application of pressure concentration, vapor pressure, hydro-

static pressure, or electrical potential (Mulder 1996, Cheryan

1998).

Membrane separation technology adopts a selective multi-

hole membrane as the separation medium. The separated fluid,

driven by the outside pressure, goes through the surface of

the membrane with different pore sizes. The molecules will

pass the membrane but larger molecules will be rejected. There-

fore, the fluid can be separated into different molecular weights

with high efficiency (Cheryan 1998).

Compared with the traditional separation process technolo-

gies, membrane-based separation processes can run under nor-

mal temperatures, so are especially good for separating and

concentrating heat-sensitive materials such as juices, enzymes,

or phytochemicals (Vaillant et al. 2001, Enevoldsen et al.

2007, Pouliot 2008). Moreover, membrane-based processes are

energy-efficient processes that do not involve phase change or

heat input; thus, the processes do not require ancillary equipment

such as heat generator, evaporator, and condenser. They offer

ease of operation and great flexibility, and do not require the

addition of any chemical agents. The process also provides for

minimal thermal degradation, occurs at ambient temperatures,

and is used both to filter molecular-sized particulates and also to

concentrate an isolate of interest, such as lycopene. Determina-

tion of the optimum operating conditions is vital importance.

Membrane-based processes are usually more energy efficient

than distillation, adsorption, and chromatography. Furthermore,

membrane-based separation has the advantage of compatibility

with a wide range of solvents and chemical products, an ability

to process thermally sensitive compounds and easy amenabil-

ity to automation. These advantages open up several possibil-

ities of membrane application in the production of bioactive

compounds in the areas of energy-efficient pre-concentration of

dilute solutions, fractionation of diverse classes of compounds

from complex mixtures, recovery of intermediates, and recy-

cling of solvents. The challenges posed by a membrane-based

separation process are limited selectivity, fouling leading to per-

formance decline, and a necessary and occasionally difficult

periodic cleaning process. The performance of a membrane can

be distinguished by two simple factors: flux (or product rate) and

selectivity through the membrane. Flux is defined as the perme-

ation capacity that refers to the quantity of fluid permeating per

unit area of membrane per unit time. Flux depends linearly on

both the permeability and the driving force. The flux also de-

pends inversely upon the thickness of the membrane (Yang et al.

2001, Bhanusali and Bhattacharyya 2003, Kert ́esz et al. 2005).

Figure 40.5 shows a classification of various separation-based

processes that are based upon particle or molecular size. The

five major membrane separation processes, including microfil-

tration, ultrafiltration, nanofiltration, reverse osmosis, and elec-

trodialysis, cover a wide range of particle sizes according to the

Water

Reverse osmosis (RO)

Nano filtration (NF)

Ultrafiltration (UF)

Microfiltration (MF)

Membrane separation

Figure 40.5.Schematic diagram of membrane molecular cut

property.