beer clarification

Journal of Membrane Science 194 (2001) 185196

Beer clarification by microfiltration product quality control andfractionation of particles and macromolecules

Q. Gan a,, J.A. Howell b, R.W. Field b, R. England b, M.R. Bird b,C.L. OShaughnessy c, M.T. MeKechinie c

a Department of Chemical Engineering, The Queens University of Belfast, Stranmillis Road, Belfast BT9 5AG, UKb School of Chemical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, UK

c Brewing Research International, Lyttel Hall, Nutfield, Redhill, Surrey RH1 4HY, UKReceived 15 May 2000; received in revised form 21 May 2001; accepted 29 May 2001

Abstract

Beer clarification by microfiltration demands a finely balanced retention of colloidal particulates (yeast cells, chill hazeflocs, etc.) and transmission of soluble macromolecules including carbohydrates, proteins, flavour, and colour compoundswhich give the whole some quality of a beer. The required porous transmission of these macromolecular species led toan unavoidable, complex and dynamic in-pore membrane fouling in terms of fouling constituents, formation, structure andkinetics, which are the main obstacles in obtaining an economically viable flux and consistency in permeate quality.

This experimental study was carried out with the aims of understanding the dynamic inter-relation between flux, fouling andsystem selectivity during a cross-flow beer microfiltration process so that an effective operating strategy for flux optimisationcould be formulated in conjunction with the parallel objective of good product (permeate) quality control. Tubular ceramicmembranes (Ceramem) with nominal pore diameters of 0.2, 0.5, and 1.3m were used. Simultaneous measurement of fluxand permeate qualities, such as specific gravity and chill haze level enabled identification of the effect of anti-fouling tech-niques, such as backflushing on transmission of essential beer components and on the filtered beer quality. The experimentalevidence lead to an understanding that the drastic flux enhancement achieved by employing backflushing at reversed mem-brane morphology was associated with enhanced solute transmission which could, without careful control, upset a balancedtransmission of essential beer components and the retention of unwanted chill haze components. Further operating param-eters and varying system configurations were investigated over their effect on both flux performance and system selectivity.These include membrane pore size, filtration temperature, and the addition of an amorphous silica particles as coagulationagent for hydrophilic proteins. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Beer microfiltration; Particle fractionation; Macromolecular transmission; Backflushing; Reversed membrane morphology

1. Introduction

Cross-flow microfiltration (CF-MF) is attractingincreasing technical and commercial interest as an

Corresponding author. Tel.: +44-1232-274-253;fax: +44-1232-381-1753.E-mail address: [email protected] (Q. Gan).

alternative method for fluid clarification/pasteurisation/sterilisation in the beverage, brewing, and dairy indus-tries [1]. The dairy industry uses CF-MF for milkpasteurisation and ultrafiltration (UF) for separatinglipoproteins and whey protein fractions [24]. Wineand vinegar producers have developed CF-MF forsimultaneous clarification and sterilisation with majorbreakthrough [5,6]. The cider and fruit juice industry

0376-7388/01/$ see front matter 2001 Elsevier Science B.V. All rights reserved.PII: S 0376 -7388 (01 )00515 -4

186 Q. Gan et al. / Journal of Membrane Science 194 (2001) 185196

use MF for microbiological stability, clarification andspent fluid recovery with improved product quality[710]. Unfortunately low flux and unsatisfactoryseparation properties with undesirable product char-acteristics has retarded application to brewing [11,12].

Beer clarification serves the objective of obtainingbeer stability which can be divided into three broadaspects: microbiological, colloidal, and flavour sta-bility. Microbiological stability is achieved throughthe removal of active yeast cells. Colloidal stability isachieved through the removal of other large particles,especially flocs formed by the coagulation of polyphe-nolic and proteinaceous compounds which appear aschill haze in the clarified beer at low temperature.Flavour stability is achieved through retaining flavourcompounds and by minimising dissolved oxygen inthe clarified beer.

Conventional beer clarification process employs fil-ter press or pressure vessel filters which are commonlypre-coated with porous particles of diatomaceousearth (DE) as the filter aids, which play an importantrole not only in acting as a second filtration barrier, butalso in absorbing the chill haze components. Recentresearch works on replacing the conventional processwith CF-MF have suggested that the new technologyholds a number of advantages, including the elim-ination of using filter aids and associated handlingand disposal problems [13,14], reduced beer losses[15], high solids handling capacity, and the substi-tution of heat pasteurisation and, therefore, a betterproduct quality and cost saving [16,17]. However,there still exist considerable technical and economicalbarriers have restrained large scale operation. Theseinclude severe membrane fouling, inconsistent prod-uct quality, uncertainty over productivity, and largeflux/quality variations among different beer brandsfiltered on one membrane system [18,19].

In theory, beer stability can be achieved in a singlemicrofiltration process by retaining unwanted materi-als and transmitting essential beer components. Micro-biological stability is readily achievable because yeastcells with a size range of 510m can be read-ily rejected by MF membranes with pore diametersin the range of 0.21.3m. Colloidal stability is adifficult task since the membrane selectivity basedon size-exclusion is unable to discriminate againsthydrophobic proteins, which give the essential foamproperties, and hydrophilic proteins which give rise

to unwanted chill haze. The size range of these twogroups of protein molecules overlaps with moleculesin the hydrophobic group only slightly bigger thanthose in the hydrophilic group [20]. With the varia-tion of membrane pore size, system configuration andoperating conditions, there is always a risk of overtransmission of haze proteins or under transmission offoam proteins, resulting in impaired product (perme-ate) quality. This delicate trade-off can be furthercomplicated by a dynamic and continuous membranefouling process which may alter selectivity and otherperformance characteristics of a carefully pre-selectedmembrane system.

Flux is largely an economical issue. The eco-nomical flux rates are reported to range between 10and 100 kg h1 m2 usually based upon recirculationexperiments [11,21]. The assumptions of these eco-nomical calculations are rather roughly defined. Inthis work, an average flux rate of 40 kg h1 m2 over24 h continuous filtration is considered as being ofcommercial interest.

Whilst the mechanisms of fouling and flux declinein beer microfiltration are now better understoodand documented [12,22,23], there has been littlereported on a systematic analysis of the dynamicand inter-dependent relationships between separationcharacteristics, permeate quality, flux and membranefouling. There is especially a lack of understandingof how flux and permeate quality are affected by themechanism of transport of macromolecule throughthe micropore phase of a membrane. The rate of thetransport determines the permeation rate and permeatequality, and depends primarily on the particle/pore sizeratio and complex particlepore interactions. A sys-tematic understanding of these dynamic inter-relationsand finding an optimal fluxpermeate quality relation-ships should lead to important process improvements.

A previous study of beer microfiltration has beenconducted through a Link project involving BRFInternational, Courage Brewing plc (now ScottishCourage) and the University of Bath. We have alreadyreported the effects of membrane fouling [23], fluxenhancement techniques [24] and membrane cleaning[25]. This paper reports on the analysis and optimi-sation of the relationship between flux and permeatequality performance. It highlights specifically thesynergetic relationship existing between the nature ofmembrane fouling and the permeate quality variations

Q. Gan et al. / Journal of Membrane Science 194 (2001) 185196 187

under different system configurations and operatingregimes, especially with high frequency backflush-ing and reversed membrane morphology with whichdramatic flux improvement has been achieved. Mostimportantly a high product quality has been achieved.

2. Experimental methods and materials

A schematic flow diagram is shown in Fig. 1. Inaddition to the provision for conventional cross-flow,special features of the experimental rig include (i)techniques for enhancing surface hydrodynamics andproducing secondary flow through two-way reversingflow pulsation; (ii) creation of helical flow patternsthrough insertion of helical baffles within the flowchannel; and (iii) an automated multi-stage backflush(BF) facility which generates backpulses of control-lable frequency and strength.

Ceramic membranes (Ceramem Corporation,Waltham, USA) with nominal pore diameters of 0.2,0.5, 1.3m and 12 4 mm 4 mm square-shaped flowchannels inside a ceramic monolith were used in thiswork. Modules were 500 mm long and 50 mm dia-meter with an internal total surface of 0.096 m2 and

Fig. 1. Schematic flow diagram.

an external surface of 0.0785 m2. In all experiments,0.5m pore diameter was selected except in theexperiment where the effect of pore sizes was in-vestigated. Baffles were made of 0.1 mm stainlesssteel wire helically wound and soldered to a corewire of 0.25 mm diameter at a pitch of 21 mm. Whenthe baffles were inserted in the monolith, the centralcore wires were individually tensioned and fixed atboth ends of the channel to control vibration. Fullyreversing pulsatile flow was provided by a modified airdriven double diaphragm pump (Wilden M1, Cheshire,UK). This could be superimposed on the feed flow.

Fresh cold-conditioned rough beer, Beer A, wassupplied daily by Courage Brewing plc at its on-sitedevelopment brewery where this study was carriedout. It is a slightly different beer as that used in a pre-vious study [23] as it was brewed in a small scale fer-menter (10 m3) with a shorter cold conditioning timein a smaller storage tank (10 m3). The beer has a typ-ical suspended solids loading of 0.190.23 g l1 (dryweight) and total dry solids content of 3.533.67 wt.%.Mean yeast cell diameter measured by laser Mas-tersizer (Malvern Instrument, Malvern, UK) is6.40.2m. The feed was stored and filtered at 2.00.9C except in the experiment where the effect of


Table 1Measured beer qualities of the cold-conditioned rough beer (feed)and the conventionally filtered beer according to the EBC standards

EBC standards Feed Product ofDE filtration

HRV (s) >100 128 112Chill haze (EBC)


Table 2The 24 h average fluxes at steady cross-flow filtration conditionsa

TMP (bar) 24 h average flux (kg h1 m2)0.4 3.80.8 4.91.2 4.6

a Re = 1552, T = 2.0 0.9C.

causing rapid flux reduction (>95%) in the first hourof filtration. Steady flux, defined as the flux value atwhich further flux reduction due to fouling is lessthan 2% per hour, was less than 5 kg h1 m2 underconventional cross-flow filtration conditions. It tookmore than 5 h to reach the steady-state. It is alsonoted that the performance of the cold-conditionedbeer used in this study behaved very differently tothe beers used in some other studies [11,22,26] as therough beer was undiluted and has a higher specificgravity and total solids concentration.

Extended 24 h filtration operations were carried outto assess the long run flux performance. The averagefluxes under three different trans-membrane pressuresare shown in Table 2.

Enzymatic analysis in a stirred cell apparatus hadbeen carried out to identify specific classes of mem-brane foulants [23]. Carbohydrates were important,in particular pentosans, glucans and hydrophilic hazeproteins. Trace minerals (Ca2+, Cu2+) also played animportant bridging role in the surface complexion ofthe key membrane foulants.

The formation of the fouling structure can besubdivided into three major types: (i) the cakelayer consisting of a compact deposit of yeast cells,

Table 3Measured permeate beer qualities against filtration time at steady cross-flow filtration conditionsa

Filtration time (h) Colour (EBC) Present gravity (S) HRV (s) Haze (EBC) Bitterness (BU)0.05 14.8 12.5 128 9.53 17.40.3 11.9 12.0 112 0.49 16.72.5 12 11.8 111 0.49 16.6

10.0 10.9 9.9 97 0.48 16.817.0 10.1 9.2 84 0.44 16.224.0 9.5 8.6 63 0.39 16.1

Average 10.4 0.4 9.8 0.3 74 2 0.46 0.02 16.8 0.4EBC standard 11.5 12.0 >100


3.2. Effect of flow hydrodynamics onflux and permeate quality

Flux improvement through employing hydrody-namic techniques, such as superimposed flow pulsa-tion and a helical flow has been effective in a numberof microfiltration applications for solids concentra-tion and separation where particle fractionation andtransmission of macromolecular species through thepore phase are not demanding [2729].

However, the hydrodynamic techniques had onlylimited flux improving effect in the beer microfiltra-tion in which pore phase transport of large amount ofmacromolecules is required. By creating a two-wayreversing flow oscillation, the presence of a highly en-hanced flow turbulence and unsteady flow conditionsfailed to produce a meaningful flux improvement(Table 4). It suggests that the increase in bulk flowturbulence did not produce a sufficient shear stressclose to the membrane surface which is critical inremoving particle deposit.

Further attempts to improve flux by modifyingsurface hydrodynamic conditions were made throughcreating a helical flow pattern accompanied by a super-imposed secondary flow. This was done by installinghelically wound baffle inserts inside the tubular flowchannel. The combined effect of the two-way reversingflow pulsation and baffle inserts was also investigated.The 24 h average flux improved by 48% by using thebaffle inserts at steady cross-flow. By using the baffleinserts under two-way reversing pulsatile flow con-ditions with a peak Reynolds number Rep = 4950, amaximum 57% flux improvement was obtained.

Table 4Steady-state flux at different feed flow hydrodynamic conditionsa

Filtration time (h) Steady-state flux (kg h1 m2)CF, Re = 1552 CF + PUL, Re = 1552,

Rep = 3400CF + BFL, Re = 1552,Rep = 4950

CF + PUL + BFL,Re = 1552, Rep = 4950

5 7.6 7.7 10.7 11.110 5.8 5.7 8.1 8.815 4.6 4.8 7.3 7.620 4.3 4.6 6.9 7.3

a CF: conventional cross-flow filtration; CF + PUL: cross-flow filtration with two-way reversing flow pulsation; CF + BFL: cross-flowfiltration with superimposed helical flow pattern; CF + PUL + BFL: cross-flow filtration with combined flow pulsation and a superimposedhelical flow pattern; Re: bulk fluid Reynolds number; Rep: peak fluid Reynolds number under flow pulsation and at helical flow pattern;TMP = 0.8 bar, T = 2.0 0.9C.

Permeate quality had very minor change as a resultof changing flow hydrodynamics. Quality analysisof permeate collected over 24 h filtration is givenin Table 5. This indicates that in-pore fouling wasdominant and insensitive to the conditions of sur-face hydrodynamics. It also suggests that bulk fluidhydrodynamic conditions had little influence on thetransmission of macromolecular species through themembrane pores. The pore flow of a permeate streamcontaining large solutes may have to overcome amuch greater frictional force than flow of pure liquidas the steric drag and electrostatic force on the solutescan be highly significant. The transport mechanism ofsolutes through cylindrical pores has been reviewedby Deen [30]. More recently, the coupled liquid/soluteflow through micropores was extensively studied byBowen and co-workers [3133] which shows that thecritical velocity of the solute and the overall perme-ation rate depends heavily on electrostatic interactionas well as steric frictional forces.

3.3. Effect of backflush

The improved flux after adopting the hydro-dynamic techniques is still too low to be consideredof commercial significance. Backflush has shown tobe a more effective way in unravelling the foulingstructure of steric pore entrance blocking and pathwayplugging since the formation of this type of foul-ing is largely reversible [34,35]. It is also noted thatbackflushing was not so effective in reducing foul-ing caused by surface adsorption of macromolecularspecies.


Table 5Quality analysis for permeate collected over 24 h filtration at dif-ferent flow hydrodynamic conditionsa

Quality parameters Operational modeCF CF + BF CF + PUL

+ BFColour (EBC) 10.4 0.3 10.5 0.4 10.5 0.5Present gravity (S) 9.8 0.2 10.0 0.2 10.1 0.2HRV (s) 74 1 78 3 81 3Haze (EBC) 0.46 0.03 0.49 0.04 0.47 0.04Bitterness (BU) 16.8 4 16.7 0.4 17.1 0.4

a TMP = 0.8 bar, T = 2.0 0.9C.

Our backflush programme was optimised in termsof backpulse frequency, back pressure, and the du-ration of each pulse. By applying the multi-stage,high frequency backflush programme using CO2 asthe complimentary flush media, the 24 h average fluxwas increased to 21.6 kg h1 m2, a 442% increasecompared to normal cross-flow filtration [23].

Analysis of the permeate quality showed that thelarge flux increase was accompanied by a higher trans-mission of total soluble beer components reflectedby increased specific gravity, HRV, chill haze andcolour of the permeate (Table 6). This synergetic re-lationship between flux improvement and increasedtransmission of macromolecules suggests that thepore clearing effect of backflush is beneficiary toboth flux and solutes transmission. The results alsoshow that whilst the permeate quality was improvedon the measures of increased HRV value, gravity, andcolour, it also had the side effect of a large increase inchill haze level. This again demonstrates the inabilityof the size-exclusion mechanism in discriminatinghaze-forming hydrophilic proteins and foam-forminghydrophobic proteins.

In general, it has been shown that backflushing wasa more effective flux and quality improving techniquethan enhanced surface hydrodynamics.

Table 6Quality analysis for permeate collected over 24 h filtration with backflushinga

Quality parameters Present gravity (S) Colour (EBC) Chill haze (EBC) HRV (s)24 h average value 10.2 0.3 10.8 0.4 0.53 0.04 83 2Percentage increase over conventional

cross-flow filtration without backflushing+4.3 +3.8 +15.1 +12.0

a Re = 1552, TMP = 0.8 bar, T = 2.0 0.9C.

3.4. Effect of temperature

Fermented beer has a high concentration of suspen-ded solids which may quickly saturate the filtermedium in the conventional dead-end DE filtration.A pre-filtration cold conditioning process is neces-sary to reduce the suspended solids concentrationby storing the beer at a temperature below 4C toallow the coagulation and subsequent sedimentationof cells, flocs of chill haze and other large particles.The cold conditioning process requires large storagevessels placed in a temperature controlled space andnormally takes place for more than 72 h.

Unlike the traditional filtration operating in adead-end mode, CF-MF is capable of handling a highloading of suspended solids. There is a growing inter-ests in eliminating the cold conditioning process bydirect cross-flow filtration of fermented rough beer.Provided the small compromise in flavour is accep-table, great cost saving could be achieved by takingout the entire cold conditioning process. This hasbeen considered feasible with certain types of lightlager beer.

Since the formation of flocs of chill haze is tempe-rature-dependent, beer filtration normally operates ata low temperature similar to that of the cold condi-tioning process so that chill haze can be filtered out aslarge aggregate particles. The temperature effect wasstudied in this work by filtering the cold-conditionedbeer at two different temperatures of 2 and 10C.Backflush was also applied. The data of steady-stateflux are given in Table 7, and the time profile of the twoquality parameters in Fig. 2. The 24 h average flux im-proved by 34 and 41%, respectively, at the two differ-ent operational mode when the filtration temperatureincreased from 2 to 10C. The permeate HRV valuebenefited from the increase in filtration temperature,but still gradually declined to a level below the EBCstandards because of continuous fouling. The effect of


Table 7Steady-state flux at different temperature with or without backflusha

Temperature (C) 2.0 10.0Operational mode CF CF + BF CF CF + BFFlux at 10th hour (kg h1 m2) 5.8 27.2 7.7 37.4Flux at 20th hour (kg h1 m2) 4.3 17.5 5.9 26.724 h average flux (kg h1 m2) 4.9 21.6 6.6 30.5

a Re = 1552, TMP = 0.8 bar.

Fig. 2. Effect of temperature on the transmission of HRV and chillhaze; Re = 1552, TMP = 0.8 bar.

temperature on chill haze is more pronounced leavinga chill haze value above the EBC standard at the endof the 24 h operation.

The chill haze at 0C is fairly constant which is incontrast to its time dependence at 10C, suggestinga consistent cut-off of a proportion of hydrophilicproteinaceouspolyphenolic species existing in aphysical form of large aggregates at a temperature be-low 4C. The colloidal particles may have re-dissolvedat higher temperatures with a consequence of a highertransmission of the haze components in the form ofindividual molecules.

Table 8Permeate quality analysis (24 h average) at reversed membrane installation with backflushinga

Present gravity (S) Colour (EBC) HRV (s) Chill haze (EBC) Bitterness (BU) pH11.31 0.4 12.9 0.4 116 2 0.86 0.02 17.0 0.5 4.1 0.01

a Re = 1552, TMP = 0.8 bar, T = 2.0 0.9C.

3.5. Filtration at reversed membranemorphology

Flux increase was dramatic and more sustainablewhen the membrane was mounted in a reversed con-figuration (RV mode) to feed the process fluid to theopen substructure of the membrane and withdraw thepermeate from the former feed side. In the combinedcross-flow + backflush + reversed pore morphology(CF+ BF+ RV) operational mode, 24 h average fluxwas in excess of 42.7 kg h1 m2 which is consideredeconomically significant. The flux increase and foul-ing mechanism at reversed membrane morphology isto be reported in a separate paper [24].

Analysis of permeate quality revealed that the fluxincrease was accompanied by a corresponding largeincrease of HRV and chill haze level in the perme-ate (Table 8). The 24 h average chill haze value atthe RV mode almost doubled that at normal mem-brane installation, and just exceeded the stipulatedEBC standard (


3.6. Effect of membrane pore size

Membrane properties including surface charge,pore size and pore structure strongly influence flux,fouling and separation characteristics in most micro-filtration processes. However, it is not well establishedhow these properties influence the rate at which finecolloids and single macromolecular species are trans-ported through the micropores. In the case of beermicrofiltration, it is difficult to predict the fractiona-tion and transport of particles/macromolecules basedsolely on the relative particle/pore size ratio sincea pre-selected size ratio may immediately start tochange with the onset of fouling, and continue tochange because of the reduction of effective convec-tive pore diameter caused by dynamic and continuousfouling. Most reported data so far on the effect ofmembrane pore size in beer microfiltration are systemspecific and lack in consistency across the spectrumof different membranes, mode of operation and beerbrands [16,19,21].

Nonetheless, membrane pore size is still recognisedas the single most important membrane parameter indesigning and selecting a MF membrane system. Theeffects of pore size were investigated in our experi-ments using three different pore diameters of 0.2, 0.5,1.3m. Fig. 3 shows that at conventional cross-flowconditions, 24 h average flux decreased with increas-ing pore diameter. Flux at 1.3m pore diameter was26% lower than that obtained at 0.2m pore diam-

Fig. 3. Effect of pore diameter on 24 h average fluxes; Re = 1552,TMP = 0.8 bar, T = 2.0 0.9C.

Fig. 4. Effect of pore diameter on transmission of HRV; Re = 1552,TMP = 0.8 bar, T = 2.0 0.9C.

eter. This is contrary to expectation and the cause isattributed to a greater degree of pore entrance block-age and entrapping of large particles at 1.3m porediameter. Flux reduction by these mechanisms wasreversible as Fig. 3 also shows that flux respondedmore favourably to the anti-fouling techniques (back-flush and reversed pore morphology) at the largerpore sizes.

Analysis also showed that transmission of bothhydrophilic and hydrophobic proteins increased withincreasing pore diameter, especially under the com-bined CF + BF + RV operating mode, resulting inhigher permeate HRV and chill haze level at largerpore diameters (Figs. 4 and 5). Chill haze at 1.3mpore diameter exceeded the EBC standard, a potential

Fig. 5. Effect of pore diameter on transmission of chill haze;Re = 1552, TMP = 0.8 bar, T = 2.0 0.9C.


Fig. 6. Effect of the addition of silica gel on flux and transmission of chill haze; Re = 1552, TMP = 0.8 bar, T = 2.0 0.9C.

problem of colloidal instability. On the other hand,the 0.2m pore diameter seems too small to allowthe passage of sufficient proteinaceous componentsto give an acceptable HRV value.

3.7. Effect of silica gel as a processing aid

Amorphous silica particles are sometimes used as aprocessing aid for beer stabilisation in circumstanceswhere the beer bas a high content of proteinaceousand polyphenolic compounds. The silica particulatesused in this work have equivalent spherical diame-ters ranging from 5 to 15m. The stabilising effectstems from the silicas ability in selective adsorptionof hydrophilic haze forming proteins. This mechanisminvolves hydrogen bonding between protein carbonylgroups and silanol hydroxyl groups.

The effect of using silica in beer microfiltration wasinvestigated by mixing amorphous silica particles toa concentration of 500 ppm in cold-conditioned beerwhich was left at 2C for 12 h for the adsorption andsedimentation (of the silica particles) to take place.The treated beer, with much reduced concentrationof hydrophilic proteins, was then filtered using the0.5m membrane at different operating modes. Fluxshowed a general increase at all operating conditions(Fig. 6) and chill haze level was substantially reduced.Even with the combined CF + RV + BF operationalmode which normally promotes high solutes trans-

mission, a 0.53 EBC chill haze value was recordedwhich is significantly below the stipulated EBC limit.Whilst there was a great reduction in the chill hazelevel, HRV values at all three operating modes onlymarginally decreased in comparison to the values ob-tained at the same operational conditions without useof the silica particles.

The results show that selective use of processingaids before microfiltration had a very positive ef-fect on subsequent flux performance and resultantpermeate quality. It is suggested that similar exper-imental assessment should be performed for someadvanced synthetic processing aids (Nylon 66, PVPP,etc.) which have selective adsorption properties to-wards polyphenols. This is beyond the scope of thisstudy. The potential advantage of on-line addition ofthe processing aid during microfiltration may be alsoworthy of investigation.

4. Conclusions

The consistency of permeate beer quality dependscritically on a carefully controlled retention of col-loidal micro particles and consistent transmission ofmacromolecular solutes. A finely balanced particlefractionation and solute transmission are susceptible toalteration by the dynamic membrane fouling processand the techniques employed for flux improvement


during a continuous beer microfiltration process. Thisinter-relationship between fouling, flux and permeatequality flux has important implications in optimisingoperating strategies for flux enhancement and prod-uct quality control. Given the evidence that in-poremembrane fouling is the predominant cause of fluxdecline and selectivity change, backflush at reversedmembrane morphology was shown to be a much moreeffective technique for promoting flux and preservingsystem selectivity. The experimental results have alsoshown that although it was difficult to obtain a con-sistent permeate HRV and specific gravity throughoutthe 24 h beer filtration, it was possible to obtain anaverage permeate quality better the EBC standards.

Enhanced surface hydrodynamics had very lim-ited effect on improving flux and maintaining systemselectivity as the surface flow conditions had little in-fluence over the dominant in-pore fouling and on thetransmission of macromolecular solutes. The successof backflush at reversed membrane morphology wasattributed to the pore clearing and particle-holdingeffect of the open membrane substructure. At thisoperating mode transmission of both hydrophilic andhydrophobic proteins was enhanced as the microfil-tration membrane was unable to discriminate againstthese two groups of proteins of similar size range.This predicament can be largely rectified by usinghydrophilic silica particles as the processing aids toabsorb specifically the hydrophilic or charged proteinmolecules.

High temperature had a beneficial effect on fluxand HRV, but was detrimental to the permeate qualitywith respect to the increased chill haze level. Poresize was a major flux and quality control parameter.The optimised nominal pore diameter is 0.5m whichgave a more balanced flux and selectivity performancein comparison to the 0.2 and 1.3m pore diameters.

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