Agitation in mashing

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0960–3085/03/$23.50+0.00# Institution of Chemical Engineers

www.ingentaselect.com=titles=09603085.htm Trans IChemE, Vol 81, Part C, March 2003

ASSESSMENT OF THE EFFECTS OF AGITATION ONMASHING FOR BEER PRODUCTION IN

A SMALL SCALE VESSEL

K. L. TSE, C. D. BOSWELL, A. W. NIENOW and P. J. FRYER

Centre for Formulation Engineering, Chemical Engineering, University of Birmingham, Edgbaston, Birmingham, UK

Mashing is the brewhouse operation concerned with producing the fermentable sugarsnecessary for the successful fermentation of wort into beer. The process involvesadding grist to water and heating to promote the enzymic conversion of malt starch

into sugars. The conversion process depends on several factors including raw materials, the nalproduct specications and on the equipment used downstream in the subsequent unit operationsto recover and stabilizethe wort. Althoughthe biochemistry of the process is well understood, theinteractions between operating parameters and mash quality are less so, despite the fact that thisinformation holds the key to process improvements in the brewing industry. A study of the effectsof agitation in mashing has been carried out, to examine the impact of this parameter on mashquality and to investigate agitation as a possible process intensication route for brewing.Experiments have been carried out in small-scale agitated vessels with well characterizedgeometry. Results of the studies suggest that for the materials studied here the conversion of starch into fermentable sugars is independent of the agitation conditions. Agitation does notprovide a viableroute for process intensication.The primary effect of increasing agitation speedis to increase the number of ne particles formed, which may compromise the efciency of downstream recovery operations through reduced ltration rates. Under agitation conditionsencountered in industrial mash tuns, solubilization of b-glucan from the grist and the proposedconsequent increase in viscosity is unlikely to be the reason for the increased separation timesreported. Likewise, processing conditionsare such that the viscosity of the mash itself cannot be

considered to present a challenge to the agitation requirements of the process.

Keywords: mashing; agitation; viscosity; particle size; process intensication.

INTRODUCTION

The brewing process can be neatly divided into two sub-sections (Figure 1), brewhouse and fermentation. Brew-

house operations are those concerned with producing afermentable extract from malt, and recovering and thenstabilizing it, and comprise ve processes: milling, mashing,lautering (or mash ltration), boiling and trub separation(Lewis and Young, 1995). Mashing is the extraction step,where the milled grist (usually barley malt) is combinedwith hot water and possibly other cereal ingredients(adjuncts) and heated to produce a fermentable substrate,which contains all the nutrients and precursors necessary forthe yeast fermentation downstream.

Mashing primarily utilizes the action of two enzymespresent in the grist, a- and b-amlyase, to carry out theconversion of malt starch into sugars of differing molecularweights. These two enzymes act in concert to degrade thetwo different forms of starch present in malt; a-amylase isprimarily responsible for digesting starch into lower mole-cular weight fermentable sugars and dextrins, whereas theaction of b-amlyase produces maltose, which is the primary

sugar required by the yeast (Lewis and Young, 1995). Theprocess begins with the gelatinization of the starch, which isachieved by heating the grist in the presence of water toabout 65¯C and is followed by enzymic action to convert the

free starch polymers into sugars of varying chain lengths. Atthe same time, proteins and other materials are solubilizedboth by the physical process and enzyme action, such thatthe nal wort produced is a complex mixture containingsugars, dissolved proteins, polyphenolic species, vitaminsand minerals (Briggs et al., 1981).

Operating conditions for mashing vary depending on boththe initial conditions of the malt and also on the nal wortdesired. Conditions are designed to maximize the action of the different enzymes, which have different optimum work-ing temperatures (Briggs et al., 1982). The process typicallyinvolves raising the mash temperature with a series of holding times at temperatures conducive to the enzymeaction required. In the UK, a typical mashing ‘prole’involves combining the grist and hot water (‘liquor’) at¹65¯C, holding the mash at this temperature until iodinetesting no longer shows the presence of free starch (approxi-mately 45 min) and then heating to a nal temperature

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of ¹78¯C, to halt further enzyme activity and ‘x’ the wortcomposition. The design of operating equipment varies

widely, especially depending on geographical location, butin the UK, grist and liquor are typically pre-mixed beforeentering the mash tun and then intermittently mixed by aso-called ‘low-shear’ paddle-type agitator (example shownin Figure 2, Wilkinson and Andrews, 1996). Alternativemashing processes include decoction and infusion mashing.Decoction mashing is typically used in Germany and uses atemperature program to allow gradual enzyme action toconvert the malt. Grist and hot water are mixed in a malthydrator and transferred to a stirred vessel. The temperatureprogram is achieved by removing part of the mash, boiling itand returning it to the main mash, which is stirred to try toensure temperature homogeneity. This process may be

repeated two or three times (Lewis and Young, 1995).Infusion mashing is a traditional process and involves nofurther stirring of the mash after the grist and hot water havebeen mixed together in equipment resembling a static mixer.Temperature control of the mash in this process is achievedby ensuring the grist and water are at the correct temperatureat the point of mixing and minimizing heat losses from themash tun (Lewis and Young, 1995).

The brewing industry is such that the same product isoften manufactured at different sites using different equip-ment and it is obviously desirable to produce a product withminimal site-to-site variation. In order to achieve this, an

improved knowledge of the inuence of operating condi-tions on the mash quality is required. This increased

understanding should also provide insight into possibilitiesfor process intensication, allowing a reduction in the overall

cost per unit product. In the mashing process, the primaryparameter of interest which offers a route to achieving theseaims is the degree of agitation used in the mash tun. Mash isknown to be a poor conductor of heat (Hudson, 1969) andtherefore, in processes where heat input or removal isrequired, efcient stirring is also required. This stirringwill ensure even distribution of heat and maintain thetemperature proles necessary for enzyme conversionto occur, whilst also preventing local overheating in theliquid or burn-on of material to the heat transfer surface(Herrmann, 1999; Herrmann et al., 1997). However, there isvery little information available on whether improvedmixing in the process could accelerate the rate at which

conversion occurs. There are reports in the literature inwhich ne grinding of the grist is recommended (Hudson,1969) as this is thought to facilitate the penetration of waterand subsequent gelatinization of starch. If this is the case,then improving the mixing of mash, beyond that required fortemperature control, may facilitate the mass transfer of wateracross the grain boundaries.

It may be that the predominant effects of increasing agita-tion are entirely detrimental to the brewing process; almost allstudies reported in the literature have focussed on this aspect(Andrews, 1996; van Wæsberghe, 1986; Uhlig and Vasquez,1991). For example, it has been shown that the mean particle

size of the mash decreases with increasing agitation (Buhleret al., 1995), a nd which is supported by anecdotal reportsfrom brewers. The ne particles cause problems in the wortseparation step which follows mashing, particularly when thisis carried out using traditional ‘lauter’ tuns. (In lautering, thewort is ltered through the spent grain and the presence of large numbers of ne particles causes high pressure drops andsubsequent slow operation.) In addition, increased agitatorspeed has been linked to increased extraction of b-glucan fromthe malt which also impacts on the lterability of the mash(Uhlig and Vasquez, 1991).

Analysis of the process suggeststhat there are three aspectsto be considered with regards to agitation in the mashing: thechallengeof mixing a uid with complex rheology,the effectof agitation on the biochemistryof the mash and theinuenceon the particle size of the grist. The aim of this work wasthus to study the impact of mixing on all three aspects, toidentify their effects and any interaction between them.

Figure 1. A schematic diagram of the brewing process, showing the split between the brewhouse and fermentation operations. The ve brewhouse operationsare: milling, mashing, wort ltration (either lauter tun or mash lter), wort boiling and trub separation (using the whirlpool).

Figure 2. A schematic diagram of the Briggs ‘low-shear’ paddle agitator formashing, showing the off-centre positioning and low clearance of theimpeller from the bottom of the mash tun. This impeller is typical of those used in the UK (Wilkinson and Andrews, 1996).

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MATERIALS AND METHODS

The grist used in all experiments was Optic (RM205) malt,which was provided milled by BRi (using a Bobby mill,1.6 mm gap setting). Following milling, the grist was sievedinto seven different particle sized fractions (‡1400, ‡1000,‡710,‡500, ‡250, ‡106 and less than 106 mm) and storedin airtight containers at ¡20¯C until used. For each experi-

ment, the grist was brought to room temperature and knownamounts of each fraction combined to give a reconstitutedstandard grist, as determined from initial milling and sievingtrials carried out at BRi (Table 1). This method ensured aconsistent initial grist composition for all experiments andavoided sampling errors that may have arisen from segrega-tion of the contents during transport.

In large-scale brewery plant, the mash tun is oftenagitated by an offset, paddle impeller (Wilkinson andAndrews, 1996) in a fairly poorly characterized process.In the laboratory, bench-scale studies were carried out usinga much smaller but well characterized, standard agitatedvessel, with the geometry modied to approach that of plant

set-up. A glass, 2 l stirred vessel (diameterˆ 12 cm,heightˆ 24 cm was tted with four bafes (1=10 vesseldiameter, with a 1 cm clearance from the bottom of thevessel) and a standard six-blade Rushton turbine. The vesselwas equipped with a water jacket for temperature controlof the contents. To approximate the industrial process set-up, the impeller diameter ( Dˆ 6 cm) was made relativelylarge (half the vessel diameter) to enhance bulk mixing andthe impeller clearance set lower than usual (one-sixth vesselheight) to aid solid suspension. An aspect ratio of 1 gave aworking mash volume of 1.5 l. To allow for scale differencesbetween laboratory and plant-scale processes, the scale-

down parameter chosen was impeller tip speed (T s) as thisis the parameter most commonly used in the brewingindustry (Wilkinson and Andrews, 1996; Barnes, 2000). Inbreweries, mash impellers are typically operated at tipspeeds of about 3 ms

¡1. However, in the much smallerlaboratory scale vessels, such high tip speeds resulted invery high agitation speeds with consequent increased airentrainment into the mash. As this is not desirable (due toincreased oxidation of mash components; Herrmann, 1999),tip speeds were signicantly lower in the laboratory scalevessels, typically half those used in plant. For all the tipspeeds utilized, the mash solids appeared visually to be fullysuspended and in motion.

Mashes were carried out at a standard grist to liquor ratioof 1:3, although to investigate the effect of mash composi-tion, ratios of 1:2.5 and 1:5 were also used. In each case, thegrist and liquor were manually combined before beingsubject to controlled agitation conditions at tip speeds of 0.5, 0.95, 1.5 or 2.0m s

¡1. For a specic series of experi-ments designed to investigate the effects of very high

agitation intensity, the contents of the mashing vessel weretransferred at intervals to a ‘high shear’ homogenizer(Moulinex Vitamix) and agitated at high speed (approxi-mately 4600 rpm) for short intervals during the mashingprocess. This procedure was necessary to allow the tempera-ture prole of the mash to be maintained whilst applying thehigh agitation intensity desired.

Temperature control of the vessel was provided by a

waterbath (Grant LTD6G, Grant, UK) and followed one of two proles. For the standard mashing prole (‘Standard’),grist and liquor were combined at 65¯C, and then held atthat temperature for 45 min before the temperature wasincreased (at about 1¯Cmin

¡1) to 78¯C. The total mashing

time was 66 min. A small number of experiments werecarried out using an alternative mashing prole (‘Tempera-ture Ramp’), where the grist and liquor were rst combinedat 40¯C, the temperature raised to 50¯C and held at thattemperature for 10 min and the temperature then increasedfurther to 65¯C over a period of typically 7 min, followed bya 45 min hold before increasing the temperature to 78¯C

over about 12 min. Total mashing time was then typically90 min. This procedure was followed in order to betterfollow the viscosity changes associated with the gelatiniza-tion of malt starch and subsequent conversion throughenzyme action.

Analyses

The effect of agitation on the mashing process wasfollowed by analysis of changes in

(1) viscosity measurements of both the whole mash (solids

present) and mash extract (no solid material present);

(2) determination of sugar concentrations, both by HPLCanalysis and measurements of specic gravity; and

(3) particle size and particle size distribution.

Viscosity Measurements

The viscosity of the whole mash was investigated todetermine (1) how the complex rheological behaviour of the uid affected the agitation process, (2) as an indicationof the progress of the starch conversion, and (3) to deter-mine how the mash itself was affected by the differentoperating regimes.

Two methods were used. Initially, a basic assessment of the whole mash viscosity was made using a torque meterattached to the impeller to monitor changes in torque drawnover the lifetime of the process. Whole mash viscosity wasalso measured by mashing in the cup and bob facility of aContraves Rheomat 30 (Contraves AG, Switzerland) usingthe largest gap, which allowed the changes in whole mash

Table 1. Weights of grist and water used to prepare the initial mash composition for the three mash concentrations, 1:2.5, 1:3 and 1:5.

Weight of fraction (g)

M ash ratio Water (g ) ‡1400 ‡1000 ‡710 ‡500 ‡250 ‡106 ‡106 Total grist (g)

1:2.5 1070 124.11 77.43 61.35 42.56 50.15 39.85 33.50 430.001:3 1125 108.48 67.69 53.62 37.20 43.85 34.86 29.30 375.001:5 1250 72.32 45.13 35.75 24.80 29.23 23.24 19.53 250.00


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viscosity to be monitored as a function of the time,temperature and shear prole. Finally, the viscosity of themash extract (with no solids material present) was measuredto determine the effect of different operating conditions onthe nal product. In this case, measurements were madeusing the Contraves Rheomat 30 double gap facilityimmersed in a waterbath for temperature control. Measure-ments were taken at 65 and 78¯C, the latter being approxi-

mately the temperature at which the spent grain is separatedfrom the mash liquor.

Particle Size Analysis

Changes in particle size distributions were determined ona percentage weight basis for the whole mash. At the end of each mash, the vessel contents were passed though a sievestack (the same as that used to determine the initialcomposition) and allowed to drain. Samples were taken of the mash liquor to allow measurement of viscosity, specicgravity and dry weight of solids. In additions, samples of the

smallest solid fraction (less than 106 mm) were taken forparticle size analysis in a Mastersizer S (Malvern Instru-ments Ltd, UK). The fractions were rinsed to wash off adhering nes, using 3 l of hot water (T º 80¯C) to preventsolids precipitation, which was observed to occur when themash temperature was below about 30¯C. Solids in thesmallest size fraction (less than 106 mm) were separatedfrom the liquor by centrifugation (Jouan C422, 3500 rpm,20 min). All solid fractions were then dried overnight at60¯C. After the initial drying period, the fractions weresieved further (2.5 mm amplitude, 30 min, Analysette 3Pro,Fristch, Germany) to separate the different sizes and werethen dried for 3 h at 105¯C to give the nal dry weight of solid. The grist retained in each sieve was expressed as apercentage of the total recovered, resulting in mass-basedparticle size distributions. As the solids content of the mashcontinues to evolve over the process, interval sampling of the mash was not used and comparisons were made onthe basis of the change between initial and nal fractionweights.

Sugars

The composition of sugars in the mash extract is ameasure of its suitability as a yeast substrate. An indication

of the total sugar content was determined from specicgravity measurements (DA-100M, Mettler-Toldeo Ltd, UK).Information on the specic sugar composition was acquiredby HPLC analysis [Gilson, Rezex RHM (300£ 7.8mm)column, Phenomenex, UK]. Three fermentable sugars werechosen as markers (glucose, maltose and maltotriose) andindividual sugar release proles were determined over themashing process. Mash samples (8 ml) were taken at times0, 15, 30, 45 and 66 min during the mash and dosed with2 ml of 1% mercuric chloride solution to quench enzymeactivity. Samples were then diluted 1:10 for analysis.

RESULTS AND DISCUSSION

Changes in Viscosity on Agitation and i ts Effects

Measurements of the mash viscosity were carried out todetermine the rheological behaviour of the uid. Initially,

this characterization was carried out using a torquemeterattached to the shaft of the impeller to follow the mashingprocess. Results of a typical experiment are shown inFigure 3. No change in power draw was observed over theentire mashing process, except where there was a decreasein impeller tip speed after about 8 min. Some literature hadreported that a very viscous uid was formed duringmashing (Herrmann et al., 1997). Interestingly, there was

no signicant increase in power consumption at the initialpoint of mixing together the grist and water (the mashing-inphase), as might be expected. The material had the consis-tency of a thin slurry, quite different to the similarity to‘porridge’ reported by some brewers. This was surprising asmuch of the literature concerning mash viscosity suggeststhat mash can be very viscous.

One reason for the failure to observe a signicant increasein viscosity over the duration of the mash may be ascribed tothe operating regime used. Reynolds numbers for the benchscale mashes were estimated to be of the order of 10–100and thus in the transition zone between laminar and turbu-

lent ow. In this region of the power curve, the powernumber is weak though complex function of the Reynoldsnumber, essentially remaining almost constant (Edwardset al., 1997). This weak functionality is likely to explainwhy the torquemeter failed to record an increase in powerdraw as the mash progressed. In addition, at higherReynolds numbers the power number becomes essentiallyconstant, a fact which implies that simply monitoring thepower draw in industrial mash tuns is not likely to besuccessful as a means of obtaining information aboutmash viscosity, as the Reynolds number of these systemsis likely to be much higher.

However, it was also considered that as mashing-in of the

grist and liquor was carried out at 65¯C, gelatinizationof thestarch (and the expected principal cause of any increasein mash viscosity) may have occurred too rapidly to bedetected (i.e. before mashing-in was completed and all gristand liquor combined). The Temperature Ramp mashingprole was used to examine the changes in mash viscositywhich arise from gelatinization of the starch and differentviscosity proles were obtained. Temperature and powerdraw proles are shown in Figure 4 for an impeller tip speed

Figure 3. The change in power draw over the duration of the mash, asmeasured for the Standard mash prole, using a torquemeter attached to theimpeller shaft. No change in power draw is observed that is not related to achange in impeller speed. Mash to liquor ratio 1:3; Rushton turbine.


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of 0.95ms¡1. As the temperature of the mash approaches65¯C, there is a small but obvious increase in the powerdrawn by the impeller. It is suggested that, in this case, theincrease in mash viscosity associated with starch gelatiniza-tion was sufcient to lower the Reynolds number enough tocause an detectable increase in power number in the transi-tion region of the power curve.

On the other hand, when mashing was carried out in theContraves Rheomat 30 (and therefore under laminar owconditions) using the same Temperature Ramp prole, a

much more pronounced viscosity prole, i.e. torque spike,was obtained (Figure 5). There is a very rapid and largeincrease in viscosity (by more thanan order of magnitude) fol-lowed by a similarly rapid decrease to a value approachingthe initial viscosity. The breakdown of the starch granulesand gelatinization of the starch polymers causes the largeincreasing mash viscosity; the subsequent decrease observedresults from enzymic action which breaks down the largermolecular weight polymers into smaller species. Figure 6

shows the results of changing the solids concentration on themash viscosity. As can be seen, although the solids contenthas a small effect on the initial viscosity of the mash, it doesnot inuence the value of the maximum viscosity peak observed.

Experiments were carried out to identify the effect of changing the heating rates and shear rates in the rheometerduring mashing on the magnitude of the viscosity peaksseen. The ‘Temperature Ramp’ mashing experiments wererepeated with different heating rates between the holding

times. Figure 7 shows that this has a signicant effect on themaximum viscosity obtained; as the heating rate is reducedthe value of the maximum viscosity recorded also decreases.The temperature peak results from a balance between twoconicting factors: the rate of starch gelatinization andsubsequent enzymic breakdown into smaller carbohydrates.

Figure 4. The change in power draw over the duration of the mash, asmeasured for the Temperature Ramp mash prole, using a torquemeterattached to the impeller shaft. Note the change in power draw at 80 min;from the temperature prole, it can be deduced that this arises from achange in the mash viscosity, associated with the gelatinization of starch(and subsequent enzyme conversion). Mash to liquor ratio 1:3; Rushtonturbine, tip speed of impeller 0.95m s¡1.

Figure 5. Change in viscosity over the duration of the mashing process, fora rheometer mash following the Temperature Ramp prole. The very rapidincrease in viscosity arises from starch gelatinization as the 65¯C stand isapproached. The subsequent decrease is due to enzyme conversion intolower molecule polymers. Mash to liquor ratio 1:3; shear rate 23.3s

¡1.

Figure 6 . The effect of mash concentration on the viscosity–time–tempera-ture prole, as determined for three rheometer mashes using the Tempera-ture Ramp prole. Changing the mash to liquor ratio has little inuence onthe maximum viscosity value recorded, although there is a small inuenceon the initial viscosity value. Shear rate 23.3 s

¡1.

Figure 7 . The effect of heating rate on the viscosity–time–temperatureprole, as determined for three rheometer mashes using the TemperatureRamp prole. Altering the rate at which the temperature is increased fromthe 50¯C to 65¯C stand affects the balance between the rate of starchgelatinization and subsequent enzyme conversion. As the heating rateincreases, the gelatinization process occurs more rapidly than the enzymeconversion, hence the very large increase in mash viscosity. For all, mash toliquor rate 1:3, shear rate 23.3s

¡1.




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Slowing the heating rate allows the enzyme processes topredominate; as starch gelatinization is strongly temperaturedependent (Lagarrigue and Alvarez, 2001) at the highestheating rate, it occurs more rapidly than the subsequentenzymic breakdown into sugars and dextrins. Consequently,there is a large degree of non-degraded starch present in themash which results in a large viscosity increase. When theheating rate is reduced, the rate of enzyme conversionpredominates, such that the enzymes have sufcient timeto degrade the starch present in the mash and the maximumviscosity recorded reduces (Muller, 1991). Decreasing the

heating rate will, however, increase the process time.The second parameter investigated in the rheometer was

the impact of shear rate on viscosity. The interactionbetween mash viscosity and agitation is demonstrated inFigure 8, which shows the results of three shear sweepscarried out during the course of a Standard mash (mash:liquor 1:3, Standard temperature prole) in the ContravesRheomat 30. As may be expected for a starch solution(Lagarrigue and Alvarez, 2001), the mash is very clearlyshear thinning with viscosity values decreasing exponen-tially with increasing shear rate. Values measured at a givenshear rate continue to decrease between the initial and nal

shear sweeps. The effect of shear on the maximum viscosityvalue was also investigated by conducting TemperatureRamp mashes at a range of different shear rates (Figure 9).Both the initial mash viscosity and the maximum valuerecorded decreased signicantly as the shear rate increased.

Finally, to assess whetherefcient mixing in industrial mashtuns is hindered by the complex rheological behaviour of themash itself, conditionsin large-scaleplant must be considered.‘Shear rates’ in industrial mash tuns are considered, almostuniversally, to be low (Andrews, 1996), although they aregenerally difcult to quantify especially in the poorly char-acterized, non-standard geometries and at the Reynoldsnumbers found in industrial breweries. Although agitationspeeds are low (15–20 rpm in a UK brewery; Barnes, 2000),thereby potentially increasing the viscosity of the mash,Reynolds numbers are higher than in small-scale tests. Inaddition, heating rates are also low (typically of the order of 1¯Cmin¡1; Barnes, 2000, personal communication). This low

heating rate will aid the enzymic breakdown of starch asgelatinization occurs, reducing the value of the maximumviscosity peak. Unless extensive temperature programming isused (which is not generallythe case in the UK), thebulk of thestarch gelatinization and initial enzyme conversion will occuras the mash andliquorare fed into themash tun at temperaturesbetween 64 and 65¯C and will not occur during agitation. Thiswork has shown that, althoughthere is some inuence of shearrate in the rheometer on mash viscosity, this is very muchsecondary to the effects of mashing prole. Correct manipula-tionof temperatures and times during mashing will lead to low

mash viscosities and thus to a free-owing material. Poorcontrol of temperature and initial mixing conditions, on theother hand, may give high-viscosity mashes.

Effect of Agitation on Conversion

Process intensication in mashing implies acceleratingthe rate of enzymatic conversion of malt starch to fermen-table sugars. Consequently, the rst step to identifyingagitation as a viable route for intensication is to determinewhether the either the total amount of fermentable sugar or

the sugar concentration proles are inuenced by the rate of agitation. Release proles for the three fermentable sugarsmaltose, maltotriose and glucose are shown in Figure 10, fora mash carried out at T s 0.5ms

¡1. As expected (Lewis andYoung, 1991; Briggs et al., 1981; Koljonen et al., 1995;Moll, et al., 1981), by far the greatest amount of carbohy-drate is present as maltose, although there are signicant,but much smaller amounts of maltotriose and glucosepresent. Figure 11 shows the release proles for maltosefor mashes produced at tip speeds of 0.5, 1.5 and 2.0 m s¡1;as can be seen there is virtually no inuence of agitationspeed on the rate of maltose formation or on the totalamount produced. Similar proles are observed for malto-triose and glucose. Comparison of the specic gravities of the mashes obtained under the different agitation conditionsdoes not show any signicant variation (Table 2). It can beseen that, in the uids studied here, agitation is not a viablemeans of increasing the enzymic conversion of starch.

Figure 8 . The effect of increasing shear rate on the mash viscosity, fromthree consecutive shear sweeps carried out in the Contraves Rheomat 30during a Standard mash. The legend refers to the time during the mash forwhich the shear sweep was carried out. As expected for a starch solution,the mash is clearly shear thinning. Mash to liquor ratio 1:3; temperature

65

¯

C.

Figure 9. The effect of shear rate on the viscosity–time–temperature prole,as determined for three rheometer mashes using the Temperature Rampprole. Although the general trend is unchanged, the maximum viscosityvalues vary greatly. Offset peaks reect slightly different temperature–timeproles. For all, mash to liquor ratio 1:3.


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The rate at which a- and b-amylase convert malt starch tofermentable sugars is considered to follow the Michaelis–

Menten model (Lewis and Young, 1995; Koljonen et al.,1995; Marc et al., 1983) whereby the reaction proceeds viathe formation of an enzyme–substrate intermediate. Limita-tions to the rate of reaction thus arise from either a lack of substrate or a lack of enzyme. As the sugar release prolesare seen to be independent of the degree of mixing in themash, it can be concluded that, at all times during the mash,

the enzymes are saturated with substrate and that there areno mass transfer limitations to be overcome in this system.The limiting factor in mashing, irrespective of the degree of mixing in the system, appears to be enzyme saturation. InFigure 12, the maltose release proles are shown forexperiments where the mash concentration was variedfrom 1:5, 1:3 to 1:2.5. In all cases, the tip speed used was1.5ms

¡1. Signicantly larger amounts of maltose are

present in the more concentrated mashes, reecting boththe increase in available starch and enzymes for conversion.

The reduced amounts of water in the more concentratedmashes has not reduced the amount of fermentable sugarproduced, suggesting that the reaction is not in fact limitedby diffusion of water to or sugar solution from the grist.

Inuence of Agitation on Particle Size

Agitation has been identied in the literature as a poten-tial cause for downstream ltration difculties (Andrews,1996; Uhlig and Vasquez, 1991; Buhler et al., 1995), boththrough the formation of a smaller mean particle size and anincreased extraction of b-glucans from the malt. Experi-

ments carried out at the three different tip speeds (grist:liquor ratio 1:3) showed clearly that the proportion of ‘ne’particles (dened as those below 710 mm, a nominal cut-off,below which the grist did not contain husk material or largepieces of endosperm material from the grain) in the totalspent grains increased with increasing agitation speed(Figure 13). As can be seen, almost twice as many neparticles are formed at an impeller tip speed of 2.0 m s

¡1, ascompared to those present following agitation at a tip speedof 0.5ms¡1. However, particle size analysis of the smallestfraction (


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nisms of nes formation were considered with eithermechanical or chemical origins. Increased uid dynamicgenerated forces may affect g rist particle size through attri-tion (break-up and abrasion), but inuences on dissolutionorpossible precipitation of dissolved species through occula-tion may also be viable. In an attempt to determine the originof the ne particles, mashes were carried out under the threedifferent agitationconditionsusing only coarse (>710 mm)oronly ne material (106 mm) from the coarse mash. Thecoarse material consists of the grain husks and larger frac-tions of endosperm, which in turn contain the majority of thecell wall material, a rich source of b-glucans. These are largemolecular weight polymers of glucose which are an impor-tant structural component in the cell walls of barley (Vis and

Lorenz, 1997). It is reported in the literature (Uhlig andVasquez, 1991) that there is an increased extraction of b-glucans with increasing agitation which may suggest thatthe ne material formed upon agitation is in fact aggregatedb-glucan molecules.

Inuence of Agitation on Viscosity

An increased extraction of b-glucans from the cell walls

of the grist into the mash can impact on the viscosity of thenal mash liquor, increasing the viscosity and therebycausing problems downstream in the process (Vis andLorenz, 1997). Measurements of the mash liquor viscosityfor the three mashes carried out with grist:liquor ratios of 1:3, but under different agitation conditions are shown inTable 3. Viscosities were measured at 65¯C and 78¯C as

Figure 13. The effect of impeller tip speed on nal grist particle sizedistribution, as determined for three mashes in the agitated vessel followingthe Standard mashing prole. There is a clear increase in the amount of material present in smallest sized fraction (less than 106 mm) as agitationintensity increases. Grist weights are presented as a percentage of the nal

total weight. For all mashes, the initial particle size distribution is thesame.

Figure 14. The effect of impeller tip speed on the particle size distributionsfor the material sampled from the smallest sized sieve fraction (less than106 mm), obtained from the Malvern Mastersizer. An initial particle sizedistribution (taken at the start of the mash) is shown for comparison. Theshape of the distribution shows little inuence of impeller tip speed.

Figure 15. The effect of impeller tip speed on the nal particle sizedistribution, as determined for thee mashes in the agitated vessel, followingthe Standard mashing prole but using only husk material (>710 mm) in theinitial grist. There is a clear increase in the amount of material present insmallest sieve fraction (less than 106 mm) with increasing agitation intensity.

(Grist weights are presented as a percentage of the nal total weight. For allmashes, the initial particle size distribution is the same.)

Figure 16 . The effect of impeller tip speed on the nal particle sizedistribution, as determined for three mashes in the agitated vessel, followingthe Standard mashing prole but using only nes material (less than710 mm) in the initial grist. Within experimental error (§5%) there islittle difference between the three distributions. (Grist weights are presentedas a percentage of the nal total weight. For all mashes, the same initialparticle size d istribution was used.)


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these are the temperatures most of interest for brewers (thelatter being the temperature at which separation of spentgrains and liquid is carried out). As can be seen there is asmall increase in viscosities measured at 78¯C between themash agitated at the lowest tip speed and the reaming two.The situation is reversed, however, for viscosities measuredat 65¯C, where the viscosities measured at the two lowest tipspeeds are similar and slightly lower than that measured fortip speed 2.0m s¡1. Notwithstanding the small differencesin viscosity and in spite of the sensitivity of ltration rate to

this parameter, it is difcult to see that such small increasesin viscosity (at such low levels overall) would signicantlyaffect any downstream separation.

To examine further the link between increased agitationintensityand solubilizationof b-glucansinto the mash, experi-ments were carried out where the mash was exposed to veryhigh values (in a blender) for short periods of time over thetime course of the process. Additionally, the experiments wererepeated using a malt known to be high in b-glucan content(866P), as the liquor obtained from this mash should show anincreaseviscosity, even at the lowest agitation conditions. Theresults of these experimentsare shown in Table 4. As expected,under both processing conditions and temperatures, there is asignicant difference between the viscosities of the mashliquors obtained with the two different malts, with thatobtained from the mash with 866P being consistently moreviscous.For the mash with Optic malt, increasing the agitationintensityduring the mash has resultedin a small increase in theviscosity measured at 78¯C. Surprisingly, this value iscomparative to those obtained at the much lower intensitiesshown in Table 3. However, the most signicant increases inviscosity at both temperatures are seen for the mashes with thehigh b-glucan malt. The results conrm reports (Uhlig andVasquez, 1991; Vis and Lorenz, 1997) of increased mashviscosity with increasing agitation intensity, although

obviously the experimental conditions in the blender aremuch more extreme than in the processing conditions in abrewery. These results suggest that the viscosity of mash liquoris much more inuenced by the type of malt used in themashing and to a much smaller extent by the processingconditions under which the mash occurs.

CONCLUSIONS

Experiments have been carried out to investigate theeffects of process parameters on the mashing process andto examine whether process intensication is possible usingagitation. The results suggest that signicant increases inviscosity occur on gelatinization, but that the enzymeinduced decrease in viscosity is aided by good mixing and

slow heating rates. In contrast to previous literature reports,the mash itself does not appear to provide a challenge to theagitation processes used in the mash tun. The scale of thesystem used here is of course signicantly smaller thanindustrial scale, but the rates of the gelatinization andstructure breakdown processes will be similar; the resultshere suggest that high viscosities can be avoided in mashingby correct manipulation of temperature and time prolesduring the process. No acceleration in enzyme action, asmeasured from the sugar release proles, was found,suggesting that the in-process mass transfer is not limiting.Some impacts of agitation on the process are negative froma brewer’s viewpoint as the increased formation of ne

particles and potential extraction of b-glucans into themash liquor both impact negatively on processes down-stream.

The ndings of this study follow quite closely thoseobtained with other solid–liquid processing operations inagitated vessels (Nienow, 1997). For example, in crystal-lization or when using ion exchange resins or catalysts,increased agitation intensity has little or no impact on therate of crystal growth or reaction respectively. This weak interaction is similar to that found here for gelatinization,enzymic conversion and b-glucans extraction. On the otherhand, particle attrition or secondary nucleation for crystals

is very sensitive to agitation due to particle–impeller orparticle–particle impacts, especially at large particle sizes(Nienow, 1997). It is this latter mechanism which is seen tobe important here, leading to nes production and thepotential for a concomitant reduction of ltration rate.

Overall, the results suggest that agitation does not providea viable route for process intensication of mashing: it doesnot increase throughput and may reduce the quality of thewort produced. Rather, minimal agitation (to maintaintemperature homogeneity) and optimization of temperatureproles, both low-investment options, can be used toimprove the quality of the mash liquor and ensure efcientdownstream operations

REFERENCES

Andrews, J.M.H., 1996, Shear forces in brewhouse operations, Brewer’sGuardian, June: 33–36.

Briggs, D.E., Hough, J.S., Stevens, R. and Young,T.W., 1981,The chemistryand biochemistry of mashing, in Malting and Brewing Science Volume 1

Malt and Sweet Wort , 2nd edition (Chapman & Hall, London),pp 254–303.

Buhler, T.M., Matzner, G. and McKechnie, M.T., 1995, Agitation inmashing, European Brewing Convention, Proceedings of 25th Congress,Brussels, pp 293–300.

Edwards, M.F., Baker, M.R. and Godfrey, J.C., 1997, Mixing of liquids instirred tanks, in Mixing in the Process Industries, 2nd paperback edition,Harnby, N., Edwards, M.F. and Nienow, A.W. (eds) (Butterworth-Heinemann, Oxford), pp 137–157.

Herrmann, H., 19 99, Flavour stability with respect to milling and mashingprocedures, MBAA Tech Q, 36(1): 49–54.

Herrmann, H., Kantelberg, B., Wiesner, R. and John, L., 1997, Aspects of the perfect agitator for mash tuns, Brauwelt Int , 160–162.

Table 3. Effect of impeller tip speed on the viscosities of thenal mash liquor (no solid material present), as measuredfor three mashes in the agitated vessel, following theStandard mashing prole. For all, mash to liquor ratio 1:3.

T s (m s¡1) m65(mPa s) m78(mPa s)

0.5 1.06 0.881.5 1.09 0.982.0 1.16 0.96

Table 4. The inuence of very high agitation intensity in the blender on thevalue of the nal mash viscosity (mash liquor only, no solid materialpresent) as determined for mashes with Optic and for 866P (high b-glucan)malt, following the Standard mash prole. All mash to liquor ratios 1:3.

Grist

Agitation

condition

m65

(mPa s)

m78

(mPa s)

Optic 150 rpm 1.06 0.88866P (high b-glucan) 150 rpm 1.40 1.13Optic Homogenizer 1.08 0.95866P (high b-glucan) Homogenizer 1.79 1.47




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Hudson, J.R., 1969, Effect of mashing on the character of beer, Brewers Dig,Nov: 96–100.

Koljonen, T., Hämäläinen, J.J., Sjöh olm, K. and Pieti lä, K., 1995, A modelfor the prediction of fermentable sugar concentrations during mashing,

J Food Eng, 26: 329–350.Lagarrigue, S. and Alvarez, G., 2001, The rheology of starch dispersions

at high temperatures and high shear rates: a review, J Food Eng, 50(4):189–202.

Lewis, M.J. and Young, T.W., 1995, Mashing technology, in Brewing(Chapman & Hall, London), pp 84–105.

Marc, A., Engasser, J.M., Moll, M. and Flayeux, R., 1983, A k inetic modelof starch hydrolysisby a- and b-amylase during mashing, Biotech Bioeng,28: 481–496.

Moll, M., Flayeux, R., Lipus, G. and Marc, A., 1981, Biochemistry of mashing, MBAA Tech Q, 18(4): 166–173.

Muller, R., 1991, The effects of mashing temperature and mash thickness onwort carbohydrate composition, J Inst Brew, 97: 85–92.

Nienow, A.W., 1997, The mixer as a reactor: liquid=solid systems, in Mixingin the Process Industries, 2nd paperback edition, Harnby, N.,Edwards, M.F. and Nienow, A.W. (eds) (Butterworth-Heinemann,Oxford), p p 394–411.

Uhlig, K. and Vasquez, S., 1991, Zur Messung des Schereffektes wahrenddes Maischens, Brauwelt , 10: 326–328.

van Wæsberghe, J.M.W., 1986, Scherkrafte und Luftaufnahme beimMaischen, Brauwelt , 31(July): 1255–1256.

Vis, R.B. and Lorenz, K., 1997, b-Glucans: importance in brewing andmethods of analysis, Leben sm-Wiss Technol, 30: 331–336.

Wilkinson, N.R. and Andrews, J.M.H., 1996, Mashing, cooking andconversion, Ferment , 9(4): 215–221.

ACKNOWLEDGEMENT

The authors gratefully acknowledge the nancial support of the

EPSRC. This work was carried out as part of the IMI project numberGR=M15446-16146-16139 entitled ‘Process Engineering Modelling forBrewing and Fermentation’ managed by Brewing Research International.

ADDRESS

Correspondence concerning this paper should be addressed to ProfessorP.J. Fryer, Centre for Formulation Engineering, Chemical Engineering,University of Birmingham, Edgbaston, Birmingham BIS 2TT, UK.E-mail: [email protected]

The manuscript was received 16 October 2002 and accepted for

publication after revision 6 March 2 003.


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