BEER CLARIFICATION BY CROSS-FLOW

MICROFILTRATION:

Fouling Mechanisms and Flux Enhancement

Q. GAN, R. W. FIELD (MEMBER), M. R. BIRD (GRADUATE), R. ENGLAND (MEMBER), J. A. HOWELL (FELLOW),M. T. MCKECHNIE* and C. L. O’SHAUGHNESSY*

School of Chemical Engineering, University of Bath, Bath, UK*BRF International, Nut® eld, Redhill, UK

An experimental study of beer micro® ltration has been carried out on ceramicmembranes with the eventual aim of carrying this process through to the commercialscale. Enhancement of surface hydrodynamics through ¯ ow pulsation had little impact

on ¯ ux suggesting that pore blocking by in-depth adsorption/deposition was the dominantfactor and this was indeed found to be so. The nature of the foulants has been determined bystudying the ® ltration rates of beer treated with various enzymes which degrade potentialfoulant species. Speci® c classes of carbohydrates and minerals have been identi® ed as foulants.In particular, pentosans (carbohydrates composed of 5 numbered sugar rings) make a majorcontribution. A multi-stage back¯ ush programme was developed and optimized in an attemptto achieve maximum pore clearance with minimal use of permeate and time. Moreover, whenback¯ ush (BF) was employed, staged increases in trans-membrane pressure had a morepositive impact on ¯ ux improvement. The effect of membrane pore size on product quality and¯ ux was also investigated in this work. Use of the BF programme achieved a ¯ ux improvementof 400%.

Keywords: membrane fouling; cross-¯ ow micro® ltration; pulsatile ¯ ow; back¯ ush; beerclari® cation

INTRODUCTION AND SCOPE

The processing of beers with micro® ltration membranes formicrobiological stabilization, clari® cation and tank bottomsrecovery is potentially viable1±3. Studies have been under-taken with micro® ltration membranes but limitations interms of product (permeate) quality or low permeateproductivity have often been encountered4,5. Recent work6

using a clari® ed (kieselguhr ® ltered) lager beer and areconstituted rough beer (made from a clari® ed beer andyeast) is not directly comparable to our study because such¯ uids do not contain the chill haze that needs to be removed.Nevertheless it will be seen later that an interesting contrastcan be made between this recent work and the ® ndingsreported in the current paper.

Through increased process understanding and incorpora-tion of the latest improvements in hydrodynamics, it wasthought that a new method for the processing of a naturaltraditional material could be developed. If successful, such adevelopment would enable breweries to replace theirkieselguhr ® lters with a process not requiring the disposalof spent ® lter aid to land® ll. A LINK project involving BRFInternational, Courage Brewing plc and the University ofBath started with an experimental study of the complexfouling mechanisms in ceramic membranes during beermicro® ltration with the eventual aim of carrying this processthrough to the commercial scale. This paper presents theinitial results from this interdisciplinary study.

Membrane fouling in beer ® ltration has in most casesbeen severe and complex1,3. This has caused dif® culties inobtaining an economical ¯ ux rate as well as good productquality. This paper speci® cally highlights the foulingmechanisms and optimization of rough beer clari® cationusing ceramic membranes. These membranes have theadvantage being robust. This is important, as harsh cleaningregimes are used in brewing operations. Furthermore, unlikepolymeric membranes, the membrane structure is notcompressible which allows good comparison of databetween successive experiments. The multi-channel cera-mic membranes, formed using the technology gained fromthe development of the catalytic converter, are relativelyinexpensive, especially when a minimum 5 year life can beassumed compared with a 2 year life for a polymericmembrane. The life spans given are generally acceptedvalues but the latter value can vary signi® cantly withdifferent applications, sometimes being as short as 6months.

Flow pulsation has been successfully employed inreducing surface particle accumulation in micro® ltrationof yeast suspensions7,8. In this work, however, turbulenceenhancement through ¯ ow pulsation had little effect on ¯ uximprovement, at best doubling a very low long-term ¯ ux.This level of ¯ ux was less than half that required for thecommercial implementation of the process. High transmis-sion of beer components is required to preserve productquality but extensive adsorption/deposition of proteins,

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carbohydrates, and minerals will always occur inside themembrane pores. Fouling of this nature would be insensitiveto the change of hydrodynamics on the membrane surface.Yeast cells, cell debris and the larger proteinaceouscomponents will contribute to surface fouling but the ¯ uxstudies indicated that the predominant fouling factor wasin-pore fouling. Therefore the in¯ uence of hydrodynamicsconcentrated on a study of the ef® cacy of back¯ ush (BF) intackling fouling caused by in-depth pore plugging. Thenature of the foulants was also determined.

EXPERIMENTAL METHOD AND MATERIALS

Rig Set-up

A schematic ¯ ow diagram is presented in Figure A. Inaddition to provision for conventional cross-¯ ow, specialfeatures of the experimental rig include (i) techniques forenhancing surface hydrodynamics and producing secondary¯ ow through two way reversing ¯ ow with and withoutinserts; and (ii) an automated multi-stage back¯ ush facilitywhich generates backpulses of controllable frequency andstrength.

Methods and Materials

Tubular ceramic membranes basically composed ofalumina from Ceramem (Waltham, USA) with 0.2, 0.5and 1.3 l m nominal pore diameters were used in cross-¯ ow® ltration. The membranes consisted of twelve channels ofsquare cross-section with dimensions of 4 ´4 ´300 mm.Two way reversing pulsatile ¯ ow was introduced byoperating an air driven double diaphragm pump (WildenM1, Cheshire, UK). Rough beer types A, and C werecommercially supplied. Unless speci® ed otherwise the beerused was beer A. The beer has a typical suspended solidsconcentration of 0.17-0.21g l-1. All ® ltration was carriedout at a temperature of < 38 C to ensure that the beer wasrepresentative of that being transferred from `cold con-ditioning’ to the ® lter line. The brewery term `coldconditioning’ refers to the conditioning of beer byrefrigerated storage for typically 8 days. This is currentlyconsidered necessary in order to ensure successful ® ltration.Beer quality was assessed according to recommendedanalytical methods9,10.

Small scale laboratory studies for foulant speciesidenti® cation were undertaken using an Amicon stirredcell with 47mm diameter Anotec 0.2 l m pore sizemembranes. The enzyme treatment of the beer was carriedout with either pure enzymes (Megazyme and Aldrich) orcommercial grades (ABM, Biocatalyst or Novo Nordisk).50 l l of the enzyme preparation was added to 50 ml of beer,69 kPa (10 psi) of CO2 pressure applied, the stirrer set at250 rpm and the volume of ® ltrate collected in a set timenoted. Percentage increases or decreases in ® ltrate collectedconsequent to enzyme addition (as compared to runswithout enzyme) were calculated.

To detect the concentration of the minerals in the foulantlayer the deposited minerals were washed out with 0.1 NHCl. Atomic Absorption Spectroscopy was then used

Back¯ ush Programme

The principal aim in designing a good back¯ ush regime isto minimize permeate usage as back¯ ush media whilstachieving the maximum pore clearance within the shortestpossible time. In this work, back¯ ush was governed by aprogrammed multi-stage backpulse routine with timevarying frequency and pulse strength. The variables aremany and include the CO2 pressure, the duration of thepulse, duration between end of the pulse and opening of thepermeate valve and cycle frequency. These variablesin¯ uence the membrane cleaning ef® ciency of the backpulse, dead-time and the `loss’ of permeate. The `lost’permeate is permeate recycled from the product to the feed-side and so this portion does not contribute to the net ¯ ux.

RESULTS AND DISCUSSION

Identi® cation of Key Membrane Foulants

Initial studies focused on treating the beer withcommercial enzymes and measuring the ® ltration rates ofthe resultant solutions in a stirred cell apparatus. As theenzymes are known to digest various particulate andmacromolecular species, the nature of foulants could bedetermined by studying these rates. The Anotec membranewas chosen as it is one of the few ¯ at sheet aluminamembranes available. Table 1 shows the effect of thecommercial enzymes.

Care must be taken when interpreting data fromcommercial enzymes, as many of them are impure andmay contain more than one active component. Synergistic

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Trans IChemE, Vol 75, Part A, January 1997

Figure A. Schematic ¯ ow diagram (PT - pressure transmitter, TT -temperature transmitter).

Table 1. Stirred cell studies (commercial enzymes). Percentage increase ordecrease in ¯ ux.

Type of Enzymes Source Beer A Beer C

Bact. a -amylase Bacillus amyloliquefaciens +0.25 +32.80Bact. b -glucanase Bacillus subtilis -3.06 +25.27Bact. a -amylase Bacillus stearothermophilus -20.08 -7.0Fung. a -amylase Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð -5.8 -3.53Pullulanase Klebsiella planticola -10.77 -8.1Bact. proteinase Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð -12.55 -7.97Fung. a -amylase Aspergillus oryzae +21.33 +1.76Fung. b -glucanase Penicillium funiculosum +23.7 +63.62Trypsin Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð +7.2 +8.1Papain Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð +3.7 +7.8Fung. cellulase Penicillium funiculosum +12.3 +31.75

and antagonistic effects between enzymes are not uncom-mon. The proteases trypsin and papain are the exception tothis rule, being speci® c to proteins. The precise activities ofthe enzymes vary from type to type and also betweenbatches, making direct quantitative foulant analyses dif® cultwithout time consuming assays. All enzymes were added asa standard volume of preparation (25 units of activity per50 ml of beer). These data can only be used to identify thenature of the foulants. Some of the enzyme data in Table 1indicate a reduction in ¯ ux on adding certain enzymepreparations. In these cases there is no or little bene® cialenzyme activity and the enzyme preparation itself fouls themembrane. However, the increases in ¯ ux of over 20% dueto the activity of certain other enzymes clearly indicates thatcarbohydrate species such as b -glucans and starch mole-cules/particulates affect performance. Interestingly, pro-teases had only a small effect indicating that proteins are notthe major foulant. This result was slightly surprising as it isknown that micro® ltration of beer can reduce its foamstability (Head Retention Value, HRV). This reduction inHRV is attributed to protein loss and therefore it follows thatproteins are retained by small pore membranes. This factorin¯ uences the choice of membrane pore size. As our resultsshow, it does not follow that fouling is mainly a matter ofprotein adsorption.

Following the stirred cell studies, commercial gradeenzymes were used with both the stirred and cross-¯ ow® lters. Two commercial grade enzymes (Table 2) wereevaluated in the laboratory stirred cell with two beer typesand then the experiments repeated by treating 50 litres of thebeers with enzymes from the same batches and carrying outa cross-¯ ow experiment with the 0.5 l m Ceramem module.It was found that the small-scale studies had been successfulin indicating enzymes for use on the larger scale. Some datais shown in Figure 1. Subsequent work, still ongoing, withpure enzymes has quanti® ed the trends reported here andalso identi® ed pentosans (carbohydrates composed of 5numbered sugar rings) as a speci® c foulant. The AtomicAbsorption Spectroscopy results, giving the mineral deposi-tion on the membrane surface, are shown in Table 3.

Single layer ion adsorption will only cause a very limiteddegree of fouling because of its very small physical size,even allowing for hydration. However, membrane foulingcould be aggravated by the problem that divalent ions areliable to act as sequestering agents to form large molecularcomplexes of carbohydrates and proteins when suchmaterials are concentrated on the membrane surface.Formation of such macromolecular aggregates may causedramatic reduction of the effective hydraulic pore radius. Asindicated, the ions detected were calcium and copper.

Flux Under Steady Cross-¯ ow

The effect of pressure upon ¯ ux is shown in Figure 2.

Below a transmembrane pressure (TMP) of 0.8 bar the ¯ uxrates attained a similar steady-state level after an initial andrapid decline in ¯ ux. When TMP was over 0.8 bar, thesteady ¯ ux level was lower. For the conditions employedthis indicated a TMP limit of 0.8 bar for conventional cross-¯ ow micro® ltration.

Flux Under Pulsatile Flow

The increase of ¯ ow turbulence through ¯ ow pulsationcan be described in terms of a peak oscillatory Reynoldsnumber Rep and a ratio of the size of the amplitude to thesize of the channel. In this study the size of the channel wasnot varied. The de® nition of Rep is:

Rep = q m pD / l

where m p = peak velocity of oscillatory flow = 2p fx

x = amplitude (m) q = density

f = frequency (s-1) l = viscosity

Two way reversing pulsatile ¯ ow was generated byoperating the air driven diaphragm pump so as to oscillatethe cross-¯ ow. The steady ¯ ow was oscillated (with the netvolumetric ¯ owrate the same as in Figure 2) to give peakReynolds number from 1550 to 4950. Such an increase in¯ ow turbulence, however, failed to produce a signi® cant¯ ux improvement. The results in Figure 3 show a de® niteimprovement in the ¯ ux compared with Figure 2 but theabsolute values are less than half that required for thecommercial implementation of the process on a green ® eldsite. (For implementation on existing sites that already usekieselguhr ® lters, a higher ¯ ux is required.) Differentfrequency and amplitude parameters controlling the dia-phragm pump were used and similar results were obtained.

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Trans IChemE, Vol 75, Part A, January 1997

Table 2. Stirred cell studies (commercial enzymes). Treated beers to beused in cross-¯ ow ® lter experiments.

% increase in ¯ uxEnzyme Source beer A beer C

Cellulase Penicillum funiculosum - 120Amyloglucosidase Aspergillus niger 34 -

Figure 1. Effect of amyloglucosidase addition on cross-¯ ow ¯ ux rates,TMP = 1.3bar, Re = 6700.

Table 3. Mineral deposition on ceramem membrane surface.

Membrane Surface ConcentrationType of Ions (mg m- 2)

Ca+ + 0.87Cu+ + 0.49Fe+ + 0Mg+ + 0

This result suggests that either increases in bulk ¯ owturbulence did not produce a suf® cient enhancement ofsurface shear stress (which is critical in removing surfacedeposit) or that the in-pore plugging mechanism (whichis insensitive to the change of surface hydrodynamics)is dominant. The work with membranes of different poresizes (see next section) suggested that the latter was thecase.

Effect of Membrane Pore Size

Figure 4 shows the effects of altering pore size on ® ltrate¯ ux. Clearly the smaller pore size gives a higher ¯ ux. Thiscon® rms that one of the major mechanisms is in-depth porepluggingÐ as smaller pores will exclude much of the ® nematerial from the membrane matrix.

One important factor is the impact of pore size onresultant beer quality. Figure 5 shows the head retentionvalues (HRV) of the ® ltered beer as a function of time for a0.2 and a 0.5 l m membrane. The HRV value is a qualitycontrol measure indicating `foam stability’ and is related tothe presence of head forming protein. Clearly the 0.5 l mpore membrane will consistently transmit such componentsbut the 0.2 l m pore size membrane tends to remove them.Indeed, the rate of removal increases as the membranebecomes fouled. The implication is that with the 0.2 l m

pore size membrane, some of the ® ner particles andmacromolecules are being incorporated in to the foulinglayer, and that this layer is ultra-® ltering or adsorbingproteinaceous material. A similar trend was noted withgravity (density) measurements; for the 0.2 l m membranethere was a general loss of carbohydrate molecules. It is thusobvious that product quality issues dictate the use of the0.5 l m pore size membrane.

It is also noted that the performance of the rough beerused in this study behaved very differently to the beers usedby Blanpain and Lalande6. Their beers were a clari® ed(kieselguhr ® ltered) lager beer and a reconstituted roughbeer (made from a clari® ed beer and yeast), whilst in ourstudy unclari® ed beers were obtained direct from thebrewery. For both of their beers, results indicated in-porefouling followed by the build-up of a cake layer with theoverall membrane resistance being very dependent uponthe latter. In contrast, our results with previously un® lteredbeer strongly suggest that in-pore fouling is dominant.Clearly the in¯ uence of the insoluble, non-yeast compo-nents, present in our feeds (but absent from theirs)dominates the fouling behaviour of rough beer.This suggests that the number of days in cold conditioning,and the ef® ciency of this process, will in¯ uence ® ltrationperformance.

Flux Improvement Through Back¯ ush

Filtration with a back¯ ush(BF) programme using thecombination of CO2 and permeate as back¯ ush media wasadopted. When only liquid permeate was used, the amount

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Figure 2. Flux under conventional cross-¯ ow ® ltration, Re = 1552.

Figure 3. Flux pro® le under two way reversing ¯ ow pulsation, Re = 1552,Rep = 3400.

Figure 4. Effect of pore size on ¯ ux, TMP = 1.3 bar, Re = 6700.

Figure 5. Effect of pore size on head retention value.

of beer recycled from one side of the membrane to the otherwas excessive and the net ¯ ux was low. Figure 6 shows arepresentative part of the ¯ ux pro® le from a typical run. Thebackpressure is the pressure in the CO2 line at the initiationof the BF routine.

In Figure 6 the following information is given:

A1 ,A2 ..An : Instant Peak Flux (IPF) immediately after aback¯ ush routine.

B1 ,B2 ..Bn : Base Flux Level (BFL) towards the end of acycle.

C1 ,C2 ..Cn : Average Flux with back¯ ush.(tC 1 -tA 1 ),.. (tC n -tA n ) : Duration when ¯ ux is above the

moving average.

The following are important characteristics and implica-tions drawn from the ¯ ux pro® les:

1. BFL is over 100% higher with back¯ ush than without. Itis fully expected that the BFL can be further enhanced byoptimizing the back¯ ush programme.2. The instant peak ¯ ux (IPF) obtained after committing aback¯ ush routine is short-lived. The longer the ® ltrationprocess, the faster the IPF declines after initiating aback¯ ush routine.3. IPF and BFL are variables whose values decline withtime. This suggests that the back¯ ush operation is notcapable of resolving continuous irreversible membranefouling caused by particle adsorption/deposition.

The duration of the period when the ¯ ux is above themoving average ranged from 60s at the beginning of the rundown to 10-20s after 5 hours of ® ltration. The implication isthat a back¯ ush programme should be designed to takeaccount of this. Excessive back¯ ush will result in high costthrough the loss of permeate and effective ® ltration time.Another consideration is that when BFL is high, a strongerback¯ ush may be used to obtain the best back¯ ush effect butwhen BFL is low, back¯ ush strength should be reduced inorder to reduce the back¯ ush cost.

A multi-stage back¯ ush programme (Table 4) wasdesigned to deliver a suitable back¯ ush frequency andstrength with regard to the base ¯ ux level at different® ltration stages. Application of the programme improvedthe ten hour average ¯ ux by 400%, to 22 kg m-2 h-1,compared to the initial baseline case of standard cross-¯ ow® ltration.

Effect of TMP and Cross-¯ ow Rate

As noted above, a limiting TMP of 0.8 bar was observedfor steady cross-¯ ow ® ltration without back¯ ush (Figure 2).When the multi-stage back¯ ush programme was used, ¯ uximproved signi® cantly with the increases in TMP (Figure 7).The suggestion is that a TMP slightly in excess of 1 barmay be optimal but an exact optimum has yet to bedetermined.

This study also revealed that cross-¯ ow velocities at orbelow 0.75 m s-1 had little in¯ uence on ¯ ux performance.

CONCLUSIONS

Membrane fouling in beer micro® ltration is severe andcomplicated. For rough beer, in-depth pore fouling byparticle adsorption/deposition was found to be the pre-dominant fouling factor. The decline in ¯ ux was insensitiveto changes in surface hydrodynamics. Carbohydrate speciessuch as b -glucans and starch molecules/particulates wereidenti® ed as key membrane foulants. In particular, pento-sans make a major contribution. Deposition of Ca+ + andCu+ + on the membrane surface may also promote furtheradsorption/deposition of protein and carbohydrate mole-cules. Overall, for the membrane used, product quality issuesdictate the use of the 0.5 l m pore size membrane.

With pulsatile ¯ ow, increases of peak oscillatory ¯ owReynolds number from 1550 to 4950 failed to produce acommercially signi® cant ¯ ux improvement. However,back¯ ush was more effective. To achieve optimum back-¯ ush effect the frequency and strength ought to vary withtime to suit the continuous baseline ¯ ux decline. A multi-stage back¯ ush programme was developed and optimized toachieve a ¯ ux improvement of 400%. The absolute level of¯ ux still needs to be improved if cross-¯ ow micro® ltration isto be an economic replacement for existing kieselguhr® lters.

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Trans IChemE, Vol 75, Part A, January 1997

Figure 6. Effect of backlash (BF), TMP = 1.3, Re = 1552 BFfrequency = 0.2/minute, BF length = 0.5 s, Backpressure = 2.0 bar.

Table 4. An optimized multi stage back¯ ush programme.

Stage Frequency Length TMP Back Pressure(hr) (l/minute) (s) (bar) (bar)

1 1 0.5 0.8 2.52 2 0.3 1.3 2.53 4 0.2 1.7 4.04 8 0.1 2.0 5.0

Figure 7. Effect of TMP on ¯ ux, Re = 1552.

REFERENCES

1. Freeman, G. J. and McKechnie, M. T., 1995, Filtration and stabilisationof beers, in Fermented Beverage Production A. G. H Lea and J. R.Piggott (Eds.), 334±359, (Blackie Academic & Professional, London).

2. Reed, R. J. R., 1989 Advance in ® ltration, The Brewer, September:965±970.

3. Czech, B., 1995, Cross-¯ ow ® ltration of beer-experience within thebrewery, The Brewer, March: 103±110.

4. Ryder, D. S., Davis, C. R., Anderson, D., Glancy, F. M. and Power.G. N., 1988, Brewing experience with cross-¯ ow ® ltration, MBAA TechQuart, 25 (2): 67±79.

5. Burrell, K. J. and Reed, R. J. R., 1994, Cross-¯ ow micro® ltration ofbeer: Laboratory scale studies on the effect of pore size, Filtrat andSeparat, 31 (4): 399±405.

6. Blanpain, P. and Lalande, M., 1996, Investigations of foulingmechanisms governing permeate ¯ ux in the cross-¯ ow micro® ltrationof beer, Proc 7th World Filtration Congress, Budapest, II: 561±565.

7. Howell, J. A., Field, R. W., and Wu, D., 1993, Yeast cellmicro® ltration: Flux enhancement in baf¯ ed and pulsatile ¯ owsystems, J Membrane Sci, 80: 59±71.

8. Gupta, B. B., Field, R. W., Howell, J. A., and Wu, D., 1994, Helicallybaf¯ ed cross-¯ ow micro® ltration, Proc Engineering of MembraneProcess II: Environmental Applications, Tuscany, Italy.

9. Analytica-EBC, European Brewery Convention, (Brauerei undGetraÈ nke- Rundschau, ZuÈ rich, Switzerland).

10. Methods of Analysis of the American Society of Brewing Chemists,(American Society of Brewing Chemists, St. Paul, Minnesota, USA).

ACKNOWLEDGEMENTS

The helpful advice and encouragement of Tim O’ Rouke (CourageBrewing plc) is grateful acknowledged together with the ® nancial supportof the EPSRC and the DTI. The Director-General of BRF International isthanked for granting permission to publish this paper.

ADDRESS

Correspondence concerning this paper should be addressed to Dr R. W.Field, School of Chemical Engineering, University of Bath, ClavertonDown, Bath BA2 7AY, UK.

The manuscript was received 2 July 1996 and accepted for publicationafter revision 15 November 1996.

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