Application of membrane technology to food process

Application of membrane technology to food process Application of membrane technology to food process Application of membrane technology to food process Application of membrane technology to food process Application of membrane technology to food process Application of membrane technology to food process Application of membrane technology to food process Application of membrane technology to food process Application of membrane technology to food process

Review

I

Membrane technology is still evolving, finding more and II

more applications in food processing Conventional tech-

niques such as micro- and ultrafiltration or reverse osmosis

can now be regarded as more or less standard unit operations

that are being implemented in numerous processes Newer

techniques such as pervaporation and bipolar membrane

technology offer new possibilities, but are still in the process

of development The present trend of membrane technology

in food processing is to produce more specialized membranes

that are dedicated to one process, one product or even only

to improving the quality of an existing product

Ancient papyrus rolls suggest that centuries ago the

Egyptians used a type of ceramic clay mesh for the clari-

fication of wine I These ceramic filters might be consid-

ered as the first membranes used in food processing

The commercialization of membrane preparation and

membrane processing originates from the beginning of

the 20th century, when Zsigmondy in Germany began to

manufacture microfilters At that time, the filters were

used only in laboratory-scale sterility tests rather than in

industrial filtering devices Nevertheless, the intro-

duction of the microfilter marked the initiation of one of

the largest modem membrane applications: the cold

sterilization of numerous food and dairy process streams

Membranes for use in large-scale commercial pro-

cesses were not developed until the invention of the

asymmetric membrane in the late 1950s With this type

of membrane, the high fluxes across the membrane that

are essential for commercial applications were estab-

lished The first advantages to the tbod industry

appeared quickly when reverse osmosis membranes

were developed tbr purifying (desalting) water From

that time onwards membranes were introduced into sev-

eral conventional processes (e.g concentration by ultra-

filtration instead of evaporation) Moreover, membranes

permitted the development of entirely new processes

and products (e.g bioreactors with continuous product

withdrawal, and desalted products)

The main advantages of using membranes in the food

industry are summarized below 2

• Membranes help to separate molecules and micro-

organisms

• Thermal damage of products and microorganisms is

minimized

• The use of membranes requires only moderate energy

consumption

Undoubtedly, the first two points are the most signifi-

cant In principle, almost all separations currently prac-

F Pelrus Cuperus and Herry H Nijhuis are at the Agrotechnological

Research Institute (ATO-DLO), PO Box 17, NL-6700 AA Wageningen, The

Netherlands

Applications of membrane technology to food processing

F Petrus Cuperus and Herry H Nijhuis

tised in food and process technology can be achieved using conventional unit operations For instance, steril- ization was traditionally achieved by thermal treatment

In cold sterilization, microfiltration is used to remove bacteria from products at low temperatures The intro- duction of cold sterilization initially took place mainly

in high-value markets (e.g the pharmaceuticals industry)

to prevent irreversible product damage due to thermal treatment Cold sterilization has since become standard practice in food processing, since it both preserves prod- uct quality and avoids the introduction of off-flavours

Membrane technology is still rapidly evolving and new processes are presently undergoing development 3

In 1991 the worldwide application of membrane tech- nology to food processing amounted to -US$40 million;

this is expected to increase to US$230 million by 1997 (Ref 4) These figures represent a steady 10% share of the total market for membranes

A review of the applications of membrane technology

to food processing indicates that the ultimate goal of the technology is changing, in the past, membrane technol- ogy research involved mainly the development of a membrane, its application to a process and the subse- quent manufacture of a product without much feedback

to the first development stage The present trend is towards more extensive integration of the processes of membrane preparation, membrane function and manu- facture of the desired product This means that a mem- brane tends to be developed for a certain application, being specialized to apply to one particular product or even to improving the quality of that product In recent years, a number of extensive reviews on membrane fil- tration in the food industry have been published 5"6 in this review the trend towards dedicated systems is illus- trated by describing a number of more or less estab- lished membrane applications in the food industry together with some new technical advances that appear

to have great potential in the production of high-quality food

From sieving to dedicated separation

There is an increasing tendency for membranes to be developed for one particular separation problem The selection of the appropriate membrane is one of the key steps during process development, and frequently specific membrane preparation or modification is involved 3 A rough classification of membrane types 7

Trends in Food Science & Technology September 1993 [Vol 4] Science Publi,,hers Lld IUKI ()c124 -22441tI I/$(11,.0[I 277

can be made on the basis of the (molecular) size of the

product to be separated, as is shown in Fig 1

Nanofiltration, ultrafiltration and microfiltration involve

separation mechanisms in porous membranes, while

reverse osmosis and pervaporation make use of tight,

dense membranes Ultrafiltration and microfiltration

membranes separate on the basis of a simple sieving

me chrism., R e particle dimensions in relation to the

pore size distribution of the membrane determines

whether or not a particle can pass through the mem-

brane

Reverse osmosis and pervaporation are able to sep-

arate species that have comparable sizes, such as sodium

chloride and water, In such cases, the affinity between

the membrane and the target component is important, as

well as the velocity of the component permeating the

membrane Components that have greater affinity for

the membrane material dissolve more easily in the

membrane than other components; therefore, the mem-

brane material acts as an extraction phase Differences

in the diffusion coefficients of the components across

the membrane causes separation According to the

'solution-diffusion' theory, solubility and diffusivity

together determine the membrane selectivity 7 The way

in which nanofiltration membranes work is not en-

tirely clear Possibly both size exclusion and solution-

diffusion mechanisms play a role

In electrodialysis, charged (ion exchange) membranes

are used to separate molecules or ions in an electrical

field on the basis of differences in charge and transport

velocity through the membrane Often, these mem-

branes have very narrow pores ( I - 2 n m wide) and

charged sites " In an electrodialysis cell a number of

cation and anion exchange membranes are placed

between an anode and a cathode When a current is

(nm)

!j

I

i'i

0.5 0,2 0,1

10 000, sooo

lOOO 50o

=

1 0 0 -

5 0 -

i

lo

= 1

F

!

r != ~

._ Erythrooyte - Cancer cell

- - Saccharomyces

(beer fermentation)

" - Staphylococcus

Tetanus

ShigeUa

- - Haemoglobin

- - Japanese encephalitis virus - Polio virus

- - Haemoglobin - Pepsin _ Vitamin B12

• Sucrose

- - Na+, O H -

= Zn2*

HaO, e l -

Fig 1

Different membrane processes separate different substances according to size (given in nm) ED, electrodialysis; Nanofiltr., nanofiltration;

PV, pervaporation; Re, reverse osmosis (Taken from Ref 7.)

applied, positively charged ions migrate through the cation exchange membrane while the negatively charged ions migrate through the positively charged anion exchange membrane Some ion exchange mem- branes can even discriminate between monovalent and muitivalent ions, such as Mg 2÷ and Na ÷ The basic prin- ciple of electrodialysis is known as the 'Donnan ex- clusion principle' and can be described by equilibrium thermodynamics 9

In contrast to the processes mentioned above, in which the membrane material is typically solid (mostly

a polymer), liquid membranes consist of a thin liquid film that controls mass transfer and acts as a separation medium Mass transfer can be facilitated by adding a special component that exerts a carrier function This carrier can be very selective and, in principle, very ef- ficient membranes can be made However, because of membrane stability problems, liquid membranes have so far been applied on only a very limited scale 7.1°

There is some overlap in the applications of different membrane types For instance, reverse osmosis and electrodialysis can both be used in the preparation of potable water In such cases, other factors besides the membrane characteristics are important in the selection

of a membrane process TM ~ Some of these considerations are discussed below

Decisive engineering parameters

In principle, all membranes can be regarded as selec- tive sieves that are more permeable for certain (target) compounds than for others This means that on a mol- ecular level, in the direct proximity of the membrane, a build-up in the concentration of the retained molecules occurs This phenomenon is called concentration polar- ization The retained, accumulated (but still dissolved) molecules are an additional resistance to solvent per- meation The direct consequence of this fully reversible concentration polarization is a resistance that arises from the osmotic pressure, which leads to a decrease in the driving forcC-' However, the solute concentration, especially of proteins, near the membrane interface c a n

reach such high values that gel layer formation occurs Gel layer formation is usually referred to as 'membrane fouling' and is irreversible or, at best, only partly reversible Membrane fouling can also be caused by other phenomena, such as the adsorption of proteins and plugging of pores (see Fig 2) Hence, a reversible and direct decline in flux across the membrane might be defined as concentration polarization, whereas irre- versible and long-term decline in flux are termed mem- brane fouling Both generally occur in every membrane process, but the effects are most dominant in micro- filtration, ultrafiitration and reverse osmosis 12,~3, and sometimes in pervaporation ~4

Although concentration polarization and membrane fouling are inevitable in membrane processes, their detrimental effects can be reduced Extensive studies describing both of these phenomena have led to improvements in membrane configurations, new modules, optimization of fluid dynamics and spacer

278

Trends in Food Science & Technology September 1993 IVol 41

development Spacers are open structures separating

stacked membranes in spiral wound and plate-and-frame

modules Materials and structures have been developed

that promote turbulence, thus reducing concentration

polarization and membrane fouling However, despite

these advances, concentration polarization and fouling

are still problematic, although some innovations to

prevent fouling are practised ~s

The first membrane filtration setups were used in the

dead-end mode (Fig 3a) This kind of classic filtration

allows liquid to pass while retaining the target com-

pounds With this technique severe fouling and concen-

tration polarization (sometimes accompanied by cake

formation) can occur, leading to an extremely large

decline in flux and inefficient processing Although

dead-end filtration is a very simple operation, in practi-

cally all processes the cross-flow filtration principle

(Fig 3b) is currently used In this technique the feed is

pumped parallel to the membrane surface, thus dimin-

ishing the thickness of the hydraulic stagnant layer and

decreasing the tendency towards concentration polariz-

ation and fouling Cross-flow filtration is often used in

combination with a backflushing procedure, whereby

the filtration flow is reversed for a short period of time

so that stationary concentration polarization profiles are

disturbed and obstinate particles are removed from the

membrane surface 16 The cross-flow velocity, trans-

membrane pressure and backflush frequency are the im-

portant process parameters that are normally tuned to

the optimum for low fouling, high flux and low energy

costs Dead-end microfiltration has been practised for

many years because, compared with normal filtration,

the high pressures used render reasonably good fluxes

However, detailed studies have indicated that low press-

ures (~100 kPa) and cross-flow velocities of 3-5 m/s can

lead to more econon,eal and reliable processing ~7

Although the driving force for filtration is low in

these cases, fouling and concentration polarization are

decreased Therefore, installing a greater membrane

area is more beneficial than increasing the feed

pressure

An interesting new approach for the control of fouling

has been developed by Van Boxtel 's A 'self-learning'

mathematical model estimates the characteristic process

dynamics of the system t2"~ring the first 15 minutes of

operation Using these characteristics, process par-

ameters are then regulated throughout the rest of the

process Van Boxtel has demonstrated that, using this

approach, the reverse osmosis of cheese whey can be

optimized to yield 10% more cash flow with 20% lower

energy costs

The evolution of membrane processes

Table 1 lists some of the membrane processes used in

food processing The dairy industry has accepted and

developed membrane processing quite extensively A

certain trend from 'technology push' to 'product pull'

can be seen Previously, membranes were simple sieves

that were bought and accepted as they were, whereas

now systems are specially designed to manufacture a

membrane

0

0

0

blocking

I

@

0

retentate

@

0

concentration polarization Fig 2

Membrane fouling may be caused by the adsorption of proteins or by pore blocking Concentration polarization may also lead to membrane fouling if the solute concentration near the membrane interface becomes too high, leading to formation of a gel layer

certain product ~9.2° The production of dried egg-white protein is an example of how membrane technology has evolved (Fig 4) In the conventional process, the pro- tein was concentrated using a vacuum oven, which is rather time consuming and expensive Although for a completely dry product a final heating stage is still necess- ary, the incorporation of a membrane unit for concen- trating the protein (to a purity of 50-60%) before heat- ing diminishes costs and increases throughput With the membrane it is impossible to concentrate the protein by more than 60% because viscosity, and hence pressure drop, becomes too high Another example from the dairy industry is the production of 'Quark' In the orig- inal process the curds that formed in the reactor were concentrated by means of a centrifuge, which resulted in

(a)

feed

permeate

(b}

permeate

time - - ~

I " r i p I

tim~

Fig 3

Dead-end filtration (a) compared with cross-flow filtration (b) The detrimental effect of concentration polarization and fouling is greater in dead-end filtration than in cross-flow filtration, which breaks up accumulating retentate The diagrams on the right-hand side show that by using backflushing to restore flux, the cross-flow technique is made even more efficient

Table 1 Examples of membrane operations in food processing

Pl1~e~ or

application Typical product

Cold sterilization Beer, wine, milk

Clarification Wine, beer,

fruit juices Drying, thickening Proteins (whey),

fruit juices

Membrane process

MF

MF, UF

UF, RO

(often combined)

Fractionation Proteins (egg, OF

whey, blood), carbohydrates Product recovery Lactic acid, UF, ED

citric acid Product improvement Aromas, flavours PV, RO

potable water, NF desalted cheese

Industry

Dairy, beverage Beverage

Potato, dairy

Beverage Dairy, meat, egg, sugar

Biotechnology

Beverage Beverage, dairy

ED, electrodialysis; MF, microfiltration; NF, nanofiltration; PV, pervaporation;

RO, reverse osmosis; UF, ultrafiltration

egg yolk

egg white

ultrafiltration

vacuum dryer permeate Jr flakes protein

Fig 4

A membrane process for producing protein flakes from dried egg

white Ultrafiltration concentrates the protein before it is dried

ultrafiltration

pasteurization 1

I juice

~oncentrate J -

war

reverse osmosis

Fig 5

Outline of a process for the production of clarified fruit juice The

recovery of aroma compounds is established using reverse osmosis

membranes

a considerable loss of protein The application of an ultrafiltration membrane prevents this loss and yields a more tasty product

The improvement of the organoleptic properties of foods is also an important motive for using membrane processes in sterilization and clarification However, selective extraction is difficult due to the complexity of the mixtures usually involved Therefore, highly selec- tive membranes are essential For instance, during the dealcoholization of beer and wine, aroma compounds may be removed along with the alcohol Recovery of the flavour compounds from the alcohol by cooling and subsequent recycling of the aromas may be successful, but makes the process more costly 2t-23

Recently, membranes have been developed for the recovery of taste and odour compounds Reverse os- mosis is used to extract the vital components from the aqueous (or vapour) stream that is produced by the con- centrater unit (e.g a reverse osmosis or ultrafiltration unit)2L A typical example of such a process is shown in Fig 5 The concentrated flavour extract may be either added to the concentrated fruit juice or used in bever- ages or other food products Pervaporation or vapour permeation is a good alternative to reverse osmosis in recovering aroma compounds (see Refs 22, 23) The feasibility of using pervaporation to recover the mushroom flavour I-octen-3-ol from biotechnological processes has been assessed by Voilley et ai 24 A high- boiling lactone (6-pentyl-(z-pyrone) produced by the fungus Tric'hoderma virMe was extracted from a fer- menter 25, and a product enrichment of 10-20% was established using homogeneous organophilic polyether- polyamide copolymer membranes

The interest in electrodialysis as a membrane process has been triggered by recent developments in membrane materials2'"~' 'L These new materials offer better stability and performance, thus giving scope for new appli- cations Large-scale applications of electrodialysis can

be found in the desalination of whey used in ice cream, bread, cake, sauces and baby food 2'J Electrodialysis is preferable to reverse osmosis since it does not affect taste, colour or flavour Also, the desalting of species of high molecular mass is very effective and the process can be very precisely controlled Major disadvantages of electrodialysis are the high operation costs and the susceptibility to fouling 3° Protein separations have been achieved by adjusting the pH of the protein of interest to its isoelectric point during electrodialysis of the treated solution The uncharged protein does not move in the electric field, whereas the salts migrate according to their electrical charge Using this method, the solution is desalted with minimal loss of the solute3~-~.k

Another recent breakthrough is the so-called bipolar membrane 2'-'~'-u Bipolar membranes can be used in water-splitting processes, and offer possibilities for the recovery of organic and amino acids Bipolar mem- branes allow a salt to be split into the corresponding alkaline and acidic solutions An example of applying a bipolar membrane in combination with electrodialysis is

cathode

recycle

n

_e

=

i ,

m

o

I lactic 1 acid

anion exchange membrane

water

i

( l a c t a t e ) ] ~

T I -

NaOH

-'61.1"

!

cation exchange membrane

/

m

Na +

I.o,,um) m.m.r.n (.o,,um 1 lactate lactate

recycle

m

• i

¶

anode

Fig 6

Outline of the use of electrodialysis and a bipolar membrane to produce lactic acid from a sodium lactate solution produced by a fermenter

The negatively charged lactate migrates through the anion exchange membrane and recombines with the H* ion generated by the bipolar

membrane Sodium hydroxide is the by-product formed by combination of the residual ions

shown in Fig, 6 A sodium lactate solution from a fer-

menter is fed into an electrodialysis - bipolar membrane

unit The negatively charged lactate migrates through

the anion exchange membrane and recombines with the

H ÷ ion generated by the bipolar membrane 34 The prin-

ciple illustrated in Fig 6 can be adapted to fit different

applications, which all have their own positive and nega-

tive features This technology has good prospects for the

near future

The integration of membranes and reactors has led to

very effective production systems 34"3-s An example is the

enzyme-mediated esterification process that is used for

the production of esters, such as flavour molecules or

glycolipids Water plays a very important role in this

type of conversion It influences the reactivity of the bio-

catalyst and interferes with the thermodynamic equilib-

rium of the reaction Normally, conversions occur with

an efficiency of 40-50%; to achieve higher conversions

the water produced during the reaction must be r(~movad

from the reactor A pervaporation membrane has been

shown to be very effective in achieving this, and

enhances the conversion rate to 95% or higher -~6

Studies on the recovery of organic acids (e.g formic,

acetic and butyric acids) from fermentation beers using

liquid membranes have been reported in Japan 37 Others

studied the removal of citric acid or sodium citrate from

fermentation broths using liquid membranes -as Economic

analysis of the process showed that it could recover

citrate at half its market price A problem with the extremely selective liquid membranes is that they are not stable enough for long-term use Therefore, despite their potential, they have so far been used rarely in industrial processes

Conclusions

Membrane technology is increasingly used in food processing Early applications such as the cold steriliz- ation and filtering of fermentation broths were reason- ably simple and used standard membranes Currently, however, membranes and membrane processes are developed in conjunction with the ultimate product

Essentially this means that membrane technology is focusing more and more on specialty products It also suggests that membrane developers and membrane users should communicate with each other even more than they have done in the past

References

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2 Strathman, H (1992) Recent Prog Genie Proc~d~s 21(6), 17

3 Lonsdale, H.K (1982) J Membrane Sci 10, 81

4 Crull, A (1993) Membrane Sep Technol News 11(6}, 12

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1264

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155

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1399

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49

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282 Trends in Food Science & Technology September 1993 IVol 4]