Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

Подождите немного. Документ загружается.

column using recycled hot water for removal of

higher alcohols (fusel oils), and a rectifying column

for final purification and concentration. For crude

industrial or fuel alcohol, only the boiling columns

and possibly a smaller side-draws purification

column are required. For anhydrous ethanol produc-

tion, additional azeotropic or extractive distillation

columns, a zeolite molecular sieve, or a pervaporation

membrane plant must be used.

Yeast Biomass

Single-cell Protein

0011 Yeast biomass has been produced commercially

by whey fermentation from the 1940s until the

present. Strains of K. marxianus var. lactis or var.

marxianus are those most commonly used, although

other strains may be produced successfully. In

some operations, ethanol is also obtained as a co-

product by lowering the aeration rate during the

fermentation.

0012 The requirements of a suitable strain for biomass

production are summarized as follows.

0013High specific growth rate and biomass yield to

insure a high productivity

0014The strain must not be affected by whey proteins if

these are present

0015The strain must be suited to continuous culture

0016The strain must be acid-resistant. (To control con-

tamination, it is necessary to operate the process at

low pH or to wash the yeast at frequent intervals

with acid to remove contaminants)

0017Large cell size and uniform morphology to aid cell

separation and concentration

0018Adequate protein content and acceptability in

feeding trials

0019The Bel process (Figure 3), developed in France in

the late 1950s, is a frequently cited example of this

fermentation. Sweet cheese whey is first deproteinated

and diluted to a lactose concentration of 20–25 kg

3

. Whey is limiting in nitrogen sources for yeast

growth, so ammonium salts are added to maintain a

high nitrogen content and growth rate, and trace

metals (Fe, Cu, Mn, Zn) may also be added. The

culture has been shown to comprise three cooper-

ating yeast species. The continuous fermentation is

Yeast biomass

Yeast autolysate

(autolysis)

β-Galactosidase

Kluyveromyces or Candida spp.

(Hydrolysis)

Lactic acid

bacteria

Ammonium lactate

Lactic acid

Bakers' yeast

Whey beverages

Acetic acid

Propionic acid

Methane

K. marxianus

Ethanol

Propionibacterium spp.

Mixed anaerobic bacteria

Whole or

deproteinated

whey

Saccharomyces

cerevisiae

Acetobacter spp.

fig0001 Figure 1 Major commercial products obtained by whey fermentation. Reproduced from Whey and Whey Powders: Fermentation of

Whey, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic

Press.

WHEY AND WHEY POWDERS/Fermentation of Whey 6159

operated at a dilution rate of 0.33 h

1

and a tempera-

ture of 38



C. For a serum flow rate of 5–6m

1

, air

is added at about 1800 m

1

and provides both the

mixing and the aeration of the culture in an air-lift

fermenter. The residual sugar level is about 1 kg m

3

the biomass yield is 0.55–0.6 kg yeast (dry basis) per

kg lactose metabolized, and the biomass productivity

is approximately 4.5 kg m

3

1

. The yeast is separ-

ated and concentrated in a two-stage washing and

centrifugation process, plasmolyzed to render the

yeast protein more accessible, and dried.

0020 The composition and uses of biomass produced

from whey are similar to those of other food yeasts.

The crude protein content is about 50 wt% on a dry

basis and the only significant limitation is of the

sulfur-containing amino acids. Overall, the protein

value is similar to that of casein. Fermenting whole

whey and recovering a product containing both the

yeast biomass and the whey proteins may increase the

food value of the yeast.

Bakers’ Yeast

0021 Two processes have been developed for the produc-

tion of bakers’ yeast to overcome the limitation of

Saccharomyces cerevisiae not being able to utilize

lactose. In the first, the lactose is hydrolyzed using

b-galactosidase, and the glucose and galactose are

consumed simultaneously by the yeast in fed-batch

or continuous culture. The second process utilizes a

two-stage fermentation system. In the initial stage,

lactic acid bacteria convert lactose to lactate and

this is consumed in the subsequent stage by the

yeast. Although the bakers’ yeast so produced

appears comparable in quality to that from conven-

tional processes, only very limited quantities are

manufactured by these methods.

Products from Yeast Biomass

0022Yeast cells can be a useful source of several food

additives or processing aids. Following cell growth

and recovery, the biomass can be broken down mech-

anically or biologically to release cellular components

which may be purified or transformed to yield high-

value products. The major products that can be

derived from biomass are yeast autolysate, flavour

nucleotides, enzymes, and cell wall components such

as glucan and mannan.

Autolysate and Flavor Nucleotides

0023Autolysis is a process of self-digestion mediated by

degradative enzymes present within the cell. Elevated

200 m

Vented

Fermented beer

2.3% (v/v)

ethanol

Crude

ethanol

90% (v/v)

Final product

96.5% (v/v)

ethanol

SteamSteam

Steam

Hot water

to storage

Purified

ethanol

8% (v/v)

Still bottoms

to irrigation

Yeast propagation

25 l

0.5 m

10 m

Fermentation

Pasteurized whey

permeate

Boiling column 1

Boiling column 2

Extractive distillation column

Rectifying column

fig0002 Figure 2 Typical process for production of ethanol from natural-strength whey permeate. Reproduced from Whey and Whey

Powders: Fermentation of Whey, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler

MJ (eds), 1993, Academic Press.

6160 WHEY AND WHEY POWDERS/Fermentation of Whey

temperatures and the presence of NaCl enhance the

rate of lysis during which protein is solubilized and

cell wall polymers are broken down. Optimal condi-

tions for autolysis of Klyveromyces strains are

50–55



C, pH 5.1–6.5, and 8–12 h; addition of salt

may not be necessary. Yeast nutrients are more readily

available in autolysates than in intact cells and the

autolysates find uses in foods, animal feeds, and

microbial cultures, e.g., for enhancing the growth of

dairy starter cultures.

0024 RNA can be isolated from lysed cells and used

to produce 5

-ribonucleotides which are important

flavor enhancers in food systems.

b-Galactosidase

0025 The enzyme b-galactosidase (EC 3.2.1.23), or lactase,

can be produced from selected strains of Kluyvero-

myces spp. following growth on diluted whey. The

enzyme is intracellular and is induced by lactose and

galactose. One study showed that expressed levels

may vary up to 60-fold between strains, but commer-

cial yield data are not available. The optimum condi-

tions for enzyme activity are pH 6–7and35–45



and Mn

2þ

and Mg

2þ

are strong activators. b-

Galactosidase may be used to hydrolyze lactose to

overcome the problem of lactose intolerance and

to generate syrups for food use which are sweeter

than lactose and which do not crystallize as readily

(hence reducing ‘sandy’ defects). Oligosaccharides,

which have useful nutraceutical properties, are

alternative and interesting byproducts of the hydroly-

sis process.

Organic Acid Fermentations

Lactic Acid and Ammonium Lactate

0026Whey has been a traditional feedstock for lactic acid

production by fermentation, but the process struggles

to be economic compared to the utilization of corn

syrups. Lactic fermentations have typically been con-

ducted in batch mode using homofermentative strains

of Lactobacillus such as L. delbrueckii sbp. bulgari-

cus. Complex nutritional sources such as corn steep

liquor, malt sprouts, and malt or yeast extracts may

be supplemented. The pH is controlled in the range

5.5–6.5 by addition of Ca(OH)

or CaCO

and the

optimum temperature is about 43



C. The medium is

usually pasteurized, but contamination is not usually

a serious problem. For natural whey the fermentation

will be complete in less than 24 h with a yield of

90–95%. The productivity of the fermentation is

typically 1–3kgm

1

0027Recovery and purification of lactate represents a

major cost in the process and are complicated by the

highly corrosive nature of the product. This requires

all equipment to be 316 stainless steel or better and

removal of metal ions introduced by corrosion may

be required for some grades of product. In traditional

processes, the fermentation liquor is initially heated

BEL LACTIC YEAST

Filtration

Fermentation

tower

Cooler

Deproteinized

whey 70˚

Heat

exchanger

15˚C

Deproteination

Pasteurization

Whey from

cheese plant

WHEY PROTEINS

Decanting

Cloth

Filtration

Drying

Grinding

LACTALBUMIN

Air

Nitrogen salts

Yeast

suspension

Water

Separation

and washing

Packaging

BEL LACTIC YEAST

Grinding

Drying

Plasmolysis

FLOW DIAGRAM

fig0003 Figure 3 Fromageries Bel process for yeast production from deproteinated whey. Reproduced from Whey and Whey Powders:

Fermentation of Whey, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993,

Academic Press.

WHEY AND WHEY POWDERS/Fermentation of Whey 6161

to 80–100



C and the pH is raised above 10 to solu-

bilize calcium lactate, kill all microorganisms, and

precipitate calcium phosphate and proteins. The

liquor is then decanted, filtered, and treated with

carbon to increase the purity. Following evaporation

under vacuum, a technical-grade lactic acid can be

recovered by acidification with sulfuric acid, filtra-

tion of the precipitated CaSO

, and further carbon

and purification treatments. To obtain higher-grade

products, calcium lactate crystals may be recovered

following cooling of the liquor; these are then

washed, redissolved, and carbon-treated in several

cycles. Several alternative recovery processes are

available, e.g., both solvent extraction and distilla-

tion of lactate esters are practiced commercially.

0028 Lactic acid has attracted much recent interest as a

base for the manufacture of biodegradable polylac-

tate plastics. A large-scale pilot plant was commis-

sioned in Wisconsin, US, but full-scale production did

not proceed.

0029 Ammonium lactate is produced by the addition of

liquid ammonia to the fermentation broth. After

completion of the fermentation, the broth is evapor-

ated, neutralized by addition of further ammonia,

and processed to a range of liquid or solid products.

These are used as animal feeds and are especially

suitable for ruminants. Ammonium lactate is superior

to urea and similar to soybean meal in its nutritive

value and digestibility.

Acetic Acid

0030 Following the ethanol fermentation of whey, the alco-

hol can be further metabolized to acetic acid by Acet-

obacter spp. This process has been commercialized,

most notably by Kraft, and the resulting whey vinegar

may be used in salad dressings and other foods. The

manufacturing process is assumed to be similar to

that for conventional vinegar production. The pro-

duction of acetic acid from whey as a base for synthe-

sis of other chemicals has also been considered.

Recent attention has focussed on production of

acetate deicers as a possible market.

Propionic Acid

0031 Propionic acid is added as a fungistatic agent to bread

and bakery products and can be produced by whey

fermentation using strains of Propionibacterium

freudenreichii ssp. shermanii. Whole whey is typically

used and the fermentation must proceed under

sterile conditions at around 30



C and pH 6.5–7.5.

Nutritional supplements, such as yeast extract,

considerably enhance propionate production and

a typical yield of about 40% of the lactose fer-

mented is achieved after 60–70 h. The culture fluid

is spray-dried to obtain a powder containing both the

acid and whey proteins.

Methane Fermentation

0032The microbial production of methane from whey

offers the major benefits of an efficient waste treat-

ment process coupled with the production of a z-

convenient energy source that can be utilized on-site.

The methane fermentation, or anaerobic digestion,

requires a limited input of energy and nutrients and

produces very little sludge for ultimate disposal

in comparison to conventional aerobic treatment

processes. A number of bioreactor (digester) configur-

ations are available for the treatment of high-strength

effluents such as whey. All of these rely on retaining

the microorganisms mediating the fermentation in the

digester, so that the hydraulic residence time of

the waste stream can be very much less than the

biological solids (or biomass) residence time. This

separation is necessary as the cells grow only slowly,

yet large volumes of waste must be processed daily.

0033Several large-scale whey or whey permeate

digesters are in operation and many laboratory or

pilot-scale trials have been reported for a variety of

retained-cell systems. Mesophilic operation at

30–37



C is favored. Overall COD (chemical oxygen

demand) reductions of about 90% are possible for

loading rates of up to 20 kg m

3

day

1

, although

loading rates in large-scale operations are typically

lower, at 1.5–10 kg m

3

day

1

. Gas production is

about 35 l

1

of whey processed and has a methane

content of 55–65%. These figures are consistent with

the theoretical yield of 350 litres CH

at standard

temperature and pressure per kg COD removed.

0034Success has also been achieved with treatment of

slops from whey distilleries by anaerobic digestion.

The reduced strength of the slops from ethanol

fermentation can promote more stable operation of

the digester, although the total methane yield is

less. However, regardless of the initial substrate,

some aerobic polishing of the effluent is required

before digester effluent can be discharged to natural

waterways.

Fermented Whey Beverages

0035Whole or deproteinated whey can be fermented to

produce a range of beverage products. The major

advantages offered by whey as a substrate for bever-

age production are that whey drinks have a greater

nutritive value and are more thirst-quenching than

most soft drinks, and are less acidic than fruit juices.

The marketing of these products generally empha-

sizes the health and nutritional benefits of the

6162 WHEY AND WHEY POWDERS/Fermentation of Whey

beverage, especially if they still contain the whey

proteins. A variety of whey drinks are available in

many countries, although they are most popular in

Europe.

0036 Lactic or alcoholic fermentations can provide de-

sirable properties in the beverage. Lactic fermenta-

tions use conventional starter organisms or probiotic

strains and Kluyveromyces yeast strains are com-

monly used for the alcoholic fermentation. A com-

bined lactic–ethanol fermentation may be conducted

with a kefir-type culture, but whey health drinks usu-

ally have a low (< 1%) or negligible alcohol content.

A typical recipe for preparation of a fermented-fruit

whey beverage is outlined in Table 1. Whey ‘wines’

can be manufactured by fermentation of deprotei-

nated wheys and can serve as a base for wine coolers.

Other Fermentation Products

0037 Many alternative fermentation processes have been

investigated for whey utilization. Possible products

include mixed solvents (acetone, butanol, and etha-

nol), yeast oils and specialized lipids, glycerol, extra-

cellular polysaccharides, other organic acids (citric,

acrylic), vitamins, poly(b-hydroxybutyrate), amino

acids, flavoring agents, pigments, and lantibiotics

such as nisin. Of these, polysaccharides (xanthan

gum) and flavoring agents (based on diacetyl) have

been commercially produced in limited quantities,

and the production of lantibiotics using lactic acid

bacteria also appears feasible.

See also: Casein and Caseinates: Methods of

Manufacture; Cheeses: Chemistry of Gel Formation;

Effluents from Food Processing: On-Site Processing of

Waste; Composition and Analysis; Enzymes: Uses in

Food Processing; Fermented Milks: Other Relevant

Products; Lactic Acid Bacteria; Lactose; Vinegar

Further Reading

Belem MAF and Lee BH (1998) Production of bioingredi-

ents from Kluyveromyces marxianus grown on whey: an

alternative. Critical Reviews in Food Science and

Nutrition 38: 565–598.

Driessen FM and van den Berg MG (1990) New develop-

ments in whey drinks. Bulletin of the International

Dairy Federation 250: 11–19.

Kilara A and Patel MT (1992) Whey and lactose fermenta-

tion. In: Zadow JG (ed.) Whey and Lactose Processing,

pp. 409–448. London: Elsevier Applied Science.

Litchfield JH (1996) Microbiological production of lactic

acid. Advances in Applied Microbiology 42: 45–95.

Mawson AJ (1994) Bioconversions for whey utilisation and

waste abatement. Bioresource Technology 47: 195–203.

Meyrath J and Bayer K (1979) Biomass from whey. In: Rose

AH (ed.) Microbial Biomass. Economic Microbiology,

vol. 4, pp. 207–269. London: Academic Press.

Sienkiewicz T and Riedel C-L (1990) Whey and Whey

Utilization, 2nd edn. Gelsenkirchen-Buer: Verlag Th.

Mann.

Yang ST and Silva EM (1995) Novel products and new

technologies for use of a familiar carbohydrate, milk

lactose. Journal of Dairy Science 78: 2541–2562.

Principles of Dialysis

J J Roberts, University of Pretoria, Pretoria, South

Africa

J F Mostert, ARC-Animal Nutrition and Animal

Products Institute, Irene, South Africa

This article is reproduced from Encyclopaedia of Food Science,

Background

0001Dialysis is the oldest, most natural, and flexible

technique for separating components from a solu-

tion. Electrodialysis (ED), however, has been known

and studied since the beginning of the twentieth

century, but it was not until around the end of the

1940s that stable membranes of low electrical resist-

ance were developed, allowing the commercial appli-

cation of ED using membrane stacks with several

membranes in series. The modern process of ED has

been made possible by the development of highly

selective membranes for anions and cations. Both

processes may be used in conjunction with other

tbl0001 Table 1 Production of a fermented-fruit whey drink from whey

Recipe

Stabilizer 030%

Fruit concentrate 2–7%

Sour whey 857%

Sugar 6–10%

Flavor and color As required, to 100%

Possible technical pH range 36–42

Method

1. Coarse filtration of sour whey

2. Disperse stabilizer in the sour

whey, avoiding lumps

3. Maintain gentle agitation for 30 min

to allow the stabilizer to swell

4. Heat to 85



C and cool to incubation

temperature (c.42



5. Inoculate with Lactobacillus helveticus

6. Incubate to pH 40

7. Cool

8. Add fruit and sugar

9. Check pH and adjust, if necessary,

with 50% citric acid solution to

pH36–42

10. Thermal treatment

11. Place in chill

WHEY AND WHEY POWDERS/Principles of Dialysis 6163

membrane-separation processes (osmosis, reverse

osmosis, ultrafiltration, and microfiltration) in ex-

perimental and large-scale industrial applications. A

major disadvantage is that an equilibrium is quickly

reached. However, specific molecules may be com-

pletely separated, provided a high concentration gra-

dient is maintained, and the permeate is constantly

removed. (See Membrane Techniques: Principles of

Reverse Osmosis; Applications of Reverse Osmosis;

Principles of Ultrafiltration.)

Theory of Dialysis and Electrodialysis

Dialysis

0002 Dialysis is a separation process whereby smaller

molecules are segregated from larger molecules

(macromolecules) in a solution by virtue of their dif-

ferent rates of diffusion through a specific membrane

(or by nondiffusion, which is the criterion of the

colloidal state and forms the basis of separation of

colloidal materials from crystalloids). Alternatively,

dialysis may be regarded as a membrane transport

process in which solute molecules are exchanged be-

tween two liquids separated by a membrane. Hence,

the primary driving force is the difference in concen-

tration of the permeable species between the solution

in the dialysis membrane system and that on the

outside. The membrane might be natural, such as a

pig bladder, or artificial (i.e., consisting of materials

such as cellulose derivatives or collodion).

0003 To illustrate the principles of dialysis, a test solu-

tion can be placed in a sack or bag that may consist of

a membrane with, for example, a 24-nm pore diam-

eter. The bag is closed/knotted and soaked in distilled

water at a given temperature and for a given time; the

bags are usually agitated. The small molecules diffuse

out into the water, while the larger molecules remain

in the solution in the bag. This method is commonly

used to separate proteins from salt solutions. In add-

ition, it is an indispensable technique in the recovery

and purification of materials in food, chemical,

biological, and pharmaceutical preparations.

Electrodialysis

0004 ED is a unit operation that uses semipermeable mem-

branes for the separation or concentration of electric-

ally charged particles (ions) from nonionic particles

or species in a solution. The key to the process is the

use of ion-selective membranes. These membranes are

ion-exchange resins cast in sheet form, which allow

the passage of positively charged cations (e.g., sodium

or potassium) or anions (e.g., chloride or phosphate).

To achieve separation by means of ED, cation and

anion membranes are alternated with plastic spacers

in a stack configuration with the anode at the one end

and the cathode at the other. Spacers are usually made

of low-density polyethylene and arranged in the

membrane stack so that all the mineralized streams

are manifolded together, and all the concentrated

streams are manifolded together. The spaces between

the membranes represent the flow paths of the demin-

eralized and concentrate streams. Hence, a repeating

section, called a cell pair, consists of a cation transfer

membrane, demineralized water flow spacer, anion

transfer membrane, and concentrate water flow

spacer. A typical membrane stack may have from

300 to 500 cell pairs.

0005A direct electric current (d.c.) (or an electrostatic

potential or gradient) applied across the electrodes

creates a driving force. This induces anions to migrate

in the direction of the anode (þ) and cations to

move in the direction of the cathode (). The ion-

selective membranes form barriers to ions of opposite

charge. The net result is that the anions attempting to

migrate to the anode pass through the anion barrier

or membrane but are stopped by the cation mem-

brane. Likewise, cations trying to migrate to the cath-

ode pass through the cation membrane but are

stopped by the anion membrane. The overall effect

is that the barriers form alternate compartments of

ion-diluting cells and ion-concentrating cells. There-

fore, ED relies primarily on voltage or electromotive

force and the use of ion-selective membranes to effect

separation between charged particles. Hence, on cir-

culation of a specific liquid product or test solution

through the diluting cells and a brine solution through

the concentrating cells, free mineral ions leave the test

solution and collect in the brine stream. The level of

demineralization achieved depends upon the initial

ash content, current density, and the duration of

time the test solution is within the membrane cells.

0006Figure 1 shows a schematic diagram of a typical ED

process for the demineralization of whey in an ED

membrane stack. One stream enters the membrane

stack and flows in parallel only through the deminer-

alizing compartments, while a brine stream enters the

membrane stack and flows in parallel only through

the concentrating compartments. Note that the solu-

tions flow across, not through, the membranes. When

a d.c. voltage is applied across the electrodes, the

electrical potential gradient created causes anions to

move in the direction of the anode and cations

to move in the direction of the cathode. The ion-

selective membranes form barriers to ions of opposite

charge. The result is that anions attempting to

migrate to the anode pass through the anion mem-

brane but are stopped by the cation membrane;

cations trying to migrate to the cathode pass through

the cation membrane but are stopped by the

6164 WHEY AND WHEY POWDERS/Principles of Dialysis

anion membrane. Hence, the membranes form

alternate compartments of ion-diluting cells and

ion-concentrating cells. By circulating whey through

the diluting cells and a brine solution through the

concentrating cells, free mineral ions leave the whey

and collect in the brine stream. The level of deminer-

alization achieved depends on the initial ash content,

current density, and duration of time the whey solu-

tion is within the membrane cells.

000 7 For a better comprehension of the process of ED

dialysis (especially the efficiency of the mass-transfer

reaction), the functioning of an electrolytic cell will be

discussed briefly. A pair of electrodes as well as an

electrolytic solution (i.e., a solution containing ions)

are placed in such a cell. When a voltage is applied to

the electrodes, a migration of cations and anions

takes place. According to Faraday’s law, the number

of mass equivalents transported by this mechanism

is directly proportional to the amount of electricity

which passes through the cell. In mathematical terms,

this can be expressed as follows:

Current  time ¼ F  E ðequivalent mass transported Þ,

ð1 Þ

where E ¼ number of moles  valency, and Faraday’s

constant F ¼ 96490 A s per equivalent or 26.8 A h per

equivalent.

0008Faraday’s law states that if, for example, a current

of 26.8 A is applied in an electrolytic cell for 1h, it

will transport one equivalent of matter from the solu-

tion to the electrodes. Hence,

i ¼ FE ðJ

Þð2Þ

where i ¼ current density (A m

2

), J

¼ molar flux of

ion j (mol s

1

2

), and Z

¼ valency of ion j.

0009Eqn (2), therefore, states that the total current in an

electrolytic cell is equivalent to the sum of the current

conveyed by each species in the cell.

0010Similarly, the amount of electricity necessary to

produce separation by means of electrodialysis can

be calculated by means of eqn (1). First, we must

consider the cell efficiency coefficient (N

), which

accounts for the nonideality of the cell stack. This is

due mainly to the presence of the selective mem-

branes, but also to the loss of current through pipes

and manifolds as well as the diffusion of electrolyte

through the membranes. Thus, for a stack with n cell

pairs:

Brine solution

CACACACACA

Whey feed

Electrode rinse

solution feed

Anode

(+)

To positive pole

of rectifier

Electrode rinse

effluent

Partially

demineralized

product

Concentrated

brine effluent

Electrode rinse

effluent

To negative pole

of rectifier

Cathode

(−)

Electrode rinse

solution feed

1234567891011

−

fig0001 Figure 1 Schematic diagram of the electrodialysis process showing the demineralization of whey. From Batchelder BT (1987)

Electrodialysis applications in whey processing. In: Trends in Whey Processing, Bulletin of the International Dairy Federation No. 212,

pp. 84–90. Brussels: International Dairy Federation, with permission.

WHEY AND WHEY POWDERS/Principles of Dialysis 6165

n  current  time ¼ FE ðequivalents removed Þ=N

ð3Þ

can be expressed as the product of three efficien-

cies.

¼ N

 N

ð4Þ

accounts for the imperfection of the permselective

membrane and is also called the Coulomb efficiency;

corresponds to the inefficiency due to current

leakage through pipes and manifolds; N

accounts

for water transport, which always follows the

migration of ions. The latter is also referred to as

electroosmosis. For commercial equipment N

about 50%, and for laboratory equipment 90%. By

manipulating eqn (3):

¼ F ðQ

 Q

out

Þ=ðIn Þ:ð5Þ

where Q

¼ input flow (m

1

); Q

out

¼ output

flow (m

1

); C

¼ input concentration (equivalents

3

); C

out

¼ output concentration (equivalents m

3

0011 Owing to electroosmosis, Q

and Q

out

are gener-

ally different.

0012 The energy consumed by an electrodialysis system

may be computed from

E ¼ RI

t :ð6Þ

where E ¼ electric energy (W s ¼ J), R ¼ total resist-

ance of the system (ohm), I ¼ electric current (A), and

t ¼ time (s). From eqn (3), the number of equivalents

removed ¼ItN

n/F. Hence from eqn (6) the energy

per unit of equivalent removed is

e ¼ RIF=N

ðW s per equivalentÞ:

Types of Electrodialysis Membranes and

Fouling Problems

0013 The following types of membranes are used in ED:

0014 anion-permeable membranes (—NH

) (A), which

contain fixed cationic groups and repel cations;

0015 cation-permeable membranes (C), which contain

fixed anionic groups (—SO



) and repel anions; and

0016 nonselective membranes (N), which are permeable

to both anions and cations.

Commercially available ED membranes are manufac-

tured as heterogeneous and homogeneous membranes.

The first type is made by mixing ion-exchange resins

with a solution of polymeric materials, which is used

as a thin coating material on a fine cloth or mesh as a

mechanical reinforcement. Anionic or cationic mem-

branes are obtained by using the corresponding ion-

exchange resins. In homogenous membranes, the

active cationic or anionic groups are induced or

created in the polymeric material. Nonselective mem-

branes are usually manufactured from cellulose. Pore

sizes of the membranes are in the range 1–2 nm.

0017The desired characteristics of a good-quality mem-

brane for electrodialysis include:

0018good electrical conductivity;

0019good mechanical strength;

0020high ionic permselectivity;

0021chemical stability;

0022resistance to fouling by organic molecules;

0023insolubility in aqueous solutions;

0024resistance to chemical change in pH from 1 to 10;

0025long life expectancy;

0026impermeability to water under pressure;

0027operation at temperatures in excess of 46



0028A major problem with ED is that of membrane

polarization, also known as ‘limiting current density.’

Limiting current density is usually expressed as (CD/

)

lim

, where CD ¼current density (the amount of

current carried by a unit area of membrane surface),

and N

¼normality of the demineralized outlet

stream. This limit is a function of the fluid velocity

in the flow path, the stream temperature, and the

types of ions present. This condition may arise when

too large a driving potential is applied to a system.

The latter results in a rapid depletion of both anions

and cations from the region immediately in contact

with the membranes of the feed cell. Hence, ions are

removed from the boundary layers through the mem-

branes more rapidly than they can be replaced by

diffusion from the bulk feed fluid, so that nothing

remains to conduct the current. This polarization

potential is a combination of applied voltage, diffu-

sion characteristics of the ions, flow characteristics of

the feed, and the ionic concentration of the feed. Note

that polarization does not usually become significant

at both membranes at the same time. Other typical

problems with ED systems include current leakage,

back diffusion, and membrane poisoning and fouling.

Membrane poisoning is the result of ions passing into

the membrane, attaching irreversibly, thereby ob-

structing the passage of following ions. In contrast,

fouling is a surface phenomenon and is the result of

the deposition on the surface of a fouling layer that

forms a barrier to diffusion. These effects are largely

eliminated in an electrodialysis reversal system.

Electrodialysis Reversal

0029The electrodialysis reversal (EDR) system is the first

commercially available membrane process that is

symmetrically reversible. This means that the direc-

tion of ion movement through the membranes and the

6166 WHEY AND WHEY POWDERS/Principles of Dialysis

identity of the concentrating and demineralizing com-

partments are reversed at periodic intervals (e.g.,

every 20 min). To bring this about, the d.c. polarity

is reversed. The mechanism of EDR differs from that

of unidirectional ED in a very simple but profound

way, and the advantages of EDR system are many.

The process is more tolerant than other commercially

available membrane processes of a wide range of

troublesome organic, inorganic, colloidal, and bio-

logical contaminants. The single most important

advantage of EDR is that any scale or film build-up,

common in unidirectional processes such as electro-

dialysis and reverse osmosis, is limited. There is a

tendency for constituents that are capable of forming

slimes and precipitates to be deposited at the mem-

brane/solution interface, and over a period of time,

these deposits affect the membrane desalting/filtra-

tion performance. The self-cleaning characteristic of

EDR systems is analogous to that of reversing

heat exchangers used in the cryogenic industry for

removal of carbon dioxide and water scale from

low-temperature surfaces.

Transport Depletion

0030 The fouling of anion exchange membranes by nega-

tively charged organic collodial matter is a serious

problem in conventional ED systems. This problem

can be avoided by replacing anion membranes with

nonselective (neutral) membranes, with the order of

the membranes being –C–N–C–N–C–, so that con-

centration polarization is avoided.

Ion Substitution

0031 Ion substitution is another variant of ED in which one

species of ion is removed and substituted by another.

Instead of the simple, alternating –anionic–cationic–

anionic–(–A–C–A–)– membrane arrangement, the

sequence –A–C–C–A– solution to be demineralized

is fed into the C–C compartment.

Ion Replacement

0032 Another modification of classical electrodialysis is

ion replacement, where the membranes used are

anionic or cationic only. In this application, the

solutions are neither enriched nor depleted in

ions; only an exchange in specific types of ions is

accomplished.

Advantages of Electrodialysis

0033 The many advantages of electrodialysis, e.g., in pro-

tein separations, include fast and controlled removal

of salts, no product dilution, low membrane area

requirements, negligible product adsorption, easy

salt recovery in the same unit, and the use of lower

concentrations of salting-out agents than are required

by conventional direct addition processes.

See also: Membrane Techniques: Principles of Reverse

Osmosis; Applications of Reverse Osmosis; Principles of

Ultrafiltration