SOME ASPECTS OF THE IMPACT OF BREWING SCIENCE ON SCOTCH MALT WHISKY PRODUCTION

Transcription

1 J. lust. Brew., May-June, 1976, Vol 82, pp SOME ASPECTS OF THE IMPACT OF BREWING SCIENCE ON SCOTCH MALT WHISKY PRODUCTION By T. C. S. Dolan (The Highland Distilleries Co. Ltd, Tamdhu, Knockamh, Moray) Received 29 January 1976 Some benefits accruing from the application of brewing science to the production of Scotch Malt Whisky are described. The major function of a distillery laboratory is to provide meaningful information to the production units to allow them to function in an optimal manner. The major areas of activity are, to ensure that the raw materials used (barley, malt, yeast, water) are of the most suitable quality, that plant cleanliness is of the highest possible order and that the process is operated as efficiently as possible. The effect of lack of maintenance of these standards is described, as are the practices to be followed to ensure their maintenance. Key words: fermentation, luctobacilli, mashing, spirit, yeast. Introduction The aim of the malt whisky distiller is to manufacture, in the most economical and efficient manner, the maximum amount of organoleptically acceptable whisky, from a given quantity of his own malt or bought malt. This paper describes some areas where brewing science is currently helping the dis tiller to achieve this aim. The manufacture of Scotch Malt Whisky has been fully described by Simpson,12 who outlined distillers' requirements in terms of malt quality and mashing, fermentation and distillation performance. Recent quality control and process studies have pointed to the need for increased attention in three major areas: 1. The need to ensure that the raw materials used are of the most suitable quality. 2. The need to ensure that the process plant is kept at as high a standard of cleanliness as possible, to minimize bacterial competition in what is inevitably a nonsterile system. 3. The need to ensure that the process is operated as efficiently as possible. Laboratory methods have been adopted or devised to provide relevant information to help these needs to be met. Artificial malting aids arc not used, and as a result, the malt ing time is generally slightly longer with the cycle being around 48 h steeping, 5-7 days germination and h kilning, although in some recently-developed malting plants4 these times arc shorter. Malt. The distiller requires malt which on mashing will release into the wort <99% of the available extractable carbohydrates. As much as possible of the carbohydrate must be fermentable, an acceptable minimum being 86%. The Recommended Methods of Analysis,7 which arc used as the basis of distillers' malt analysis, do not provide all the information required. Major requirements arc to be able to predict the possible spirit yield of a malt and estimate its phenols content and its microbial population. The possible spirit yield can be estimated to within about 1 %, if the stan dard hot water extract figure and fermentability of the de rived wort are known. Various methods of estimating wort fermentability arc in use in different laboratories, and all involve a laboratory mash followed by fermentation of the wort with the spent grains in situ or of the filtered wort. After fermentation, the fcrmentability of the wort can be calculated from the final specific gravity or the residual specific gravity, using the respective formulae: (i) F(FG) = (OG - FG) x -819 x 1 OG-1 Raw materials Barley. Distiller-maltsters purchase the same cultivars of barleys as do brewers, and apply the same scientific practices during malting, which have been fully described elsewhere, e.g., by Hough, Briggs & Stevens." Healthy barleys which will yield the highest extract on malting arc bought, due attention being paid to cultivar, total nitrogen content and corn size. Using the Institute of Brewing Recommended Equation for Extract Prediction,7 the possible spirit yield eventually obtainable from the barley can be reasonably estimated if normal fermentability of the eventual wort is assumed. The protein content of barley is ca times the total nitrogen (T.N.) content, thus a small difference in T.N. has a large effect on the protein content, and in turn on the carbo hydrate content, with consequent lowering of the possible spirit yield by up to six times any rise in the T.N. content of the barley, since the alcohol is derived from the carbo hydrate of the grain. The thousand corn weight has much less effect on the possible extract as can be seen from the prediction equation, but it has been reported that larger corned barleys give malts which produce worts of higher fermentability.3 Barley handling, drying and storage are carried out by distiller-maltsters themselves, or by outside firms, and the most modern practices arc followed.8 Prior to malting, the Recommended Methods7 arc used to assess the readiness of the barley to malt, and suitable steeping and kilning pro grammes arc devised, in many cases by micromalting. (ii) F(RG) = (OG - RG) x 1 OG-1 where OG = Original Specific Gravity, FG = Final Specific Gravity and RG = Residual Specific Gravity, where the SG of water at 2 C is taken as 1. The method currently used in this laboratory involves fermenting filtered wort and determining the residual specific gravity of the filtered final beer (known as 'wash'). The original gravity of a standard hot water extract wort is ca. 1-29, while distillery OGs are usually ca A large number of laboratory worts in this OG area have been tested against the 1-29 worts, and there was no significant difference between the results. As a result of collaborative work in several laboratories, a standard method for estimating fermentability of standard extract wort has been devised. It is based on estimating the residual specific gravity and has been found to be repro ducible in this laboratory to ±-5%. The prediction of spirit yield is obtained by the equation: Predicted Spirit Yield (proof gallons/tonne) = SE x F (RG) x a, where SE = the malt's standard hot water extract, F(RG) = the derived wort's fermentability determined by the RG method, and a = an experimentally derived factor. The factor a (-1784) was derived by reversing the equation and substituting known values for standard hot water extract, fermentability and spirit yield over long periods of production. The factor is slightly higher in the more modern types of

2 178 dolan: brewing science and scotch whisky production [J. Inst. Brew. distilleries where lauter mash tuns, as opposed to the tra ditional mash tuns, are used as extraction is generally more efficient. The possibility of predicting spirit yield from an in-grains method where the unfiltered laboratory mash is fermented followed by distillation of the resultant alcohol was investigated, but it tended to give too high results when the IoB Standard grind was used, as would be expected from what is in effect an immobilized enzyme system. The phenols content, the level of which is generally accepted to indicate the 'peatiness' of the malt and to affect the flavour of the whisky, is estimated by the method of McFarlane." Malts carry a large population of micro-organisms, including potentially harmful positive cocci and lactobacilli, which can survive the mashing process where relatively low temperatures are used in order to protect the carbohydrases, which are required to continue to function in the fermenting vessel. The bacteria are carried through the process and actively compete with the yeast for fermentable carbohydrate, with significant loss of spirit if there are sufficient bacteria. The bacterial population of a malt is estimated by quan titatively separating the bacteria from the malt and plating the bacterial suspension on W-L differential agar followed by identification using staining, selective media and specific tests. Of the Institute's published analytical methods, the malt moisture is of use to the distiller in that it initially guides him in being able to adjust his mill to give the required grist, this being optimized by a standardized sieving analysis and by visual inspection. It also allows the purchaser to ensure he is not paying for excess water. Diastatic power is of diminishing importance, the malt distiller only being concerned that there is enough enzymic activity present to effect conversion. The generally acceptable minimum is a D.P. of 5-6. It may give some indication whether the malt under analysis has been ovcrkilned and possibly more delicate enzyme systems, such as endo-/?- glucanase, have been damaged. For estimation of modi fication, the measurement of a-amylase is useful, as is the use of the fine-course difference of extract. Griffin* has reported that well-modified malts which gave high values for hot water extracts did not necessarily give good alcohol yields and malts with high cold water extracts regularly show poor fermentability. The reason for this effect has yet to be explained, but it indicates that the determination of cold water extract is of some use to distillers. Griffin also suggested that rises in total soluble nitrogen and soluble nitrogen ratio appear to depress fermcntability and this appears to be borne out in practice. Yeast. Malt distillers generally use two types of yeast viz pure culture yeast manufactured specifically for use in malt and grain whisky distilleries (distiller's yeast) and brewers' yeasts which are a by-product of the beer-making process. A mixture of the two types of yeasts tends to give a more efficient fermentation in terms of alcohol produced. Yeasts used in distillery fermentations require to meet an acceptable standard on species, fermentative ability, viability, moisture content, bacterial contamination and keeping quality and must be washed, handled and stored in an appropriate manner. A yeast specification and a corresponding set of analytical methods have been drawn up to enable the supplier and purchaser to ensure that yeasts received are up to standard. Pressed yeast is preferred. The moisture content of yeast must not exceed 24%, thus ensuring that the correct dry weight of yeast is supplied. The viability is required to be at least 85% (methylene blue test)7 on arrival. A double staining technique,11 using mcthylene blue and safranin O, which gives guidance on the state of health of the yeast, has also been found useful. Sacch. cerevisiae is the preferred species as it is more tolerant of the fermenting temperatures in distilleries, which arc C at pitching and C at removal, since there is no attemperation. It is essential that bacterial contamination of the yeast is at a minimum (3-15 total bacteria, >5 }-ve, :H lactic-acid-producing bacteria per 1" yeast cells) to minimize bacterial competition with the yeast. Since the whisky-making process uses a non-sterile system the only effective method to minimize bacterial infection is to ensure that the incoming raw materials are contaminated as little as possible. As in brewing, microbiology is playing an increasingly important role in the malt whisky industry. Pure culture yeast leaving the factory and brewers' yeasts at the point of coming from the fermenting vessels are of very acceptable quality and the imposition of washing, handling and storage conditions are designed to maintain this stale of affairs. Cold rooms have been constructed by brewers and yeast merchants and at distilleries and yeast is transported in refrigerated vehicles, all this to ensure that the quality of the yeast is maintained as long as possible. Only when all these steps are taken is serious consideration of the intrinsic keeping properties of value. The fermentative ability of the yeast is also of the utmost importance, and we have devised a test to check this property. In this test a standard extract wort of known high fermen tability is prepared and the yeast under test is pitched along with a portion of pure culture yeast. Using a wort with a fermentability of 88%, the mixture will ferment it to the extent of 86-87% ('known wort unknown yeast'). The wort in this test must possess high fermentability to allow the yeasts to ferment to the limit of their ability, while in the wort fermcntability test ('known yeast unknown wort') the yeast must be able to ferment the wort out. It has been reported by Harrison & Graham5 that 1 million bacteria/ml must be present in the wort before there is substantial competition with the yeast. Clearly, a heavily contaminated yeast could introduce bacteria which could rapidly rise to this quantity during fermentation and the causing of even a 1 % loss at a plant aiming to produce I million proof gallons/year, means a loss of 1, proof gallons of whisky, worth at current rates ca. 17, before duty and 238,, duty paid. Water. The majority of distilleries have used the same water supply for many years; indeed it is often the quality of the water available for mashing which has determined the position of distilleries. Pure soft spring water is preferred, and the chemical materials dissolved in the waters are mea sured by modern instrumental methods. The possibility of bacterial contamination and consequent possible competition with the yeast during fermentation is routinely monitored by use of membrane filtration techniques1 and any necessary steps are taken when the bacterial levels rise above the norm. Plant hygiene Many distilleries have cast-iron infusion mash tuns, and wooden fermenting vessels, but the trend in modern plant is towards stainless-steel plant. In all plants constructed both of traditional and more modern materials, the need for maximum hygiene is imperative. Since there is no wortsterilizing step in the malt whisky process, the carbohydrases in the wort being required to continue to be active in the fermenting vessel, any organisms which enter the wort via a raw material or from the atmosphere are carried through the plant into the fermenting vessel and compete with the yeast for substrate. Clearly, if the mash tun or wort lines are not sterilized between wort runs these parts of the plant are sources of bacterial pick-up, and every batch passing through them will be infected. Lack of sterility in a particular washback will only critically affect the wort entering that vessel. Mash tuns are flushed with boiling water between each mash and are washed with a solution of proprietary alkalibased cleaning compounds every third or fourth mash or between each mash if time allows. Wort lines and yeast plant arc similarly treated. After each fermentation, wooden washbacks must be thoroughly cleaned of any debris (by

3 Vol. 82, 1976] dolan: brewing science and scotch whisky production 179 hosing and brushing), washed with a modern quaternary ammonium compound and finally steamed for about one hour. Failure to physically scrub such vessels leads to a build-up of beer stone (calcium oxalate) which then harbours bacteria. Quaternary Ammonium Compounds are essential, as steaming alone is insufficient to ensure sterility, due to the insulating properties of the wood. Enclosed stainless steel or lined mild-steel wash-backs are generally cleaned by inserting a modern cleaning-in-place head which delivers alkaline solution as required. To check if plant is sterile, swabs are taken from various points throughout the area and are incubated in sterile allmalt wort, the degree of lack of cleanliness being judged by the cloudiness of the incubate in due course. The basic method can be semi-quantified by swabbing a particular surface area of plant. Any bacteria detected are identified by staining, plating out on W-L differential agar, followed by plating out of typical colonies on selective media.followed by definitive tests, the methods used being generally similar to the Institute's Recommended Microbiological Methods of Analysis." Qualitative evidence of the type obtained by the methods outlined above has been found to agree well with quantitative results, dirty plant leading to poor fermentation results with less alcohol produced owing to bacterial com petition. Process efficiency New plant should be laid out to minimize the chances of the spread of infection; for example the mill-room and grist cases should be separate from the mash house and tun room to reduce contamination from airborne malt dust. Pipes should always have a fall and no low points, to avoid pools of wort or washing water lying and allowing bacteria to grow. Mashing, Assuming that the mechanical operations of the plant are functioning correctly and temperature control is accurate, the extraction efficiency of mashing can be deter mined by analysis7 of the spent grains or draff, when it can be shown whether conversion is complete, extraction is complete and if sparging is efficient. A refraction saccharometcr is used to give an immediate indication of the specific gravity or the running wort. As the wort runs from the mash tun through the underback, wort cooler and lines to the washback, samples of wort are drawn off and various analyses are carried out on them. Typically, the SG, ph, % total acid content,1 optical rotation2 and bacterial content are determined. A sample can be fermented in the laboratory using a yeast of high fermentative ability, the result giving an insight into whether the temperature control during mashing has been as required. As fermentation proceeds in the washback, its progress is monitored by measuring the parameters listed above. Fermentation. Analysis of worts typically set (i.e. the washback filled and declared) at ca. 1-5 gives the figures: ph 5, % total acid 1, ad + 3 and a detected bacterial content ranging from zero to 1, bacteria/ml. The bacteria are generally negative rods, with positive lactic acid producers present in much lower numbers. The bacterial content depends on thecleanlinessofthcplant and the numbers entering the system via the raw materials. Fermentation proceeds vigorously for the first 3 h, during which time the aerobic bacteria present die, and at about this time, when alcoholic fermentation is virtually complete, the lactic acidproducing bacteria become dominant, as shown in Table F, with consequent fall in ph and optical rotation, and rise in % total acid content. A fair indication of the bacterial state of health of a fermentation can be obtained by measuring these parameters at a given time, say 4-48 h from setting. Fig. 1 illustrates the changes in SG, ph, % total acid and ad with fermentation time in a fermentation of acceptable standard, while Fig. 2 shows typical parameters found in an unacceptably infected fermentation. TABLE I. Typical bacterial contents of Malt Whisky Wash Age (h) At Mlling Minimally infected vc rods 2 ISO bacteria/ml + ve cocci 3.7 not delected. Laclobacilli 1-53 x 1* 12 x 1" 5 x 1' ve rods 32. Heavily infected bacteria/ml + ve cocci 6 Lacto* bacilli 6 x 1«2-3 x 1* 18 8 x 1* 96 x 1* 52 x 1* 1 x 4* Fn a good fermentation, the SG falls rapidly during the first 24-3 h owing to the removal of sugars and the pro duction of ethanol, and then tails off as less readily fer mentable materials only are available. The ph falls from around 5 at setting, passes through a minimum of ca. 4- at 15-2 h and rises again to around at 4 h after which it begins to fall rapidly due to the lactic acid pro duction. Fn an infected fermentation the ph at setting may be \\r V\ ^" v " so -15 I 5 Fig. 1. Changes in SG, ph, acid content and ccd of wort with lime in Scotch Malt Whisky fermentation acceptable infection level. = SG; = ph; = acid; = «D below 5- and on reaching the minimum it remains in that region or falls from it. The total acid content of the wash in a good fermentation barely rises from 1--12% at setting until about 4 h when it begins to rise sharply, as shown in Fig. 1. In an infected fermentation, the acid content rises more rapidly from about 35 h, as shown in Fig. 2. There is often up to four hours difference between detection of rise in acid content and fall in ph, this being due to buffering in the wash. The optical rotation of the wash at 4 h also em pirically indicates its bacterial state of health, since a negative value at this stage indicates a large amount of lactic com pounds. The optical rotation of an all-malt wort is large 1O n-sos S +2-2 g 15 J> ' Fig. 2. Changes in SG, ph, acid content and ad of wort with time in Scotch Malt Whisky fermentation unacceptable infection level. = SG; = ph; -acid; ad.

4 18 dolan: brewing science and scotch whisky production (J. Inst. Brew. and positive, due to the dissolved glucans and maltosaccharides, and as fermentation proceeds the rotation falls, in a good fermentation to around -f -5\ and in an infected one to zero or to a negative value. A value of > r-5 at 4 h can also be indicative of incomplete fermentation of available carbohydrates, which in turn may point to malting or mash ing problems. It is common distillery practice to judge fermentation per formance from the final specific gravity of the wash at re moval, rather than by measuring the spirit content. However this practice can lead to erroneous results since the SG of washes over 4 h old can fall, remain static, or even rise, depending on the relative contribution of numerous factors, examples of which are the utilization of bactcrially-fcrmentable material, the evaporation of alcohol, the release of yeast cell contents following autolysis or the production of lactic acid. The residual specific gravity is also affected by most of these factors and generally rises after 4 h. Fig. 3 shows typical alcohol content and SG changes in the later stages of a fermentation. Alcoholic fermentation is essentially com plete by 4 h but washes are generally held in the washback for a further 8-1 h, when the rapidly rising bacterial popu lation breaks down residual large molecular-weight com pounds, and much of the dissolved carbon dioxide comes out of solution, both factors facilitating settling of the fermented wash during heating in the wash still. Typical bacterial counts found in fermenting washes are illustrated in Table I. In a minimally infected wash, the bacterial count is low at setting and as fermentation proceeds the acrobes die out. The lactobacilli grow rapidly from about 3 h causing the changes in ph and acid content outlined earlier (Figs. I and 2). In a heavily infected fermentation the acrobes once again die out rapidly, while the lactobacilli multiply at an almost maximum rate, doubling every 4 hours. The species of lactobacilli which are active arc L. brevis and L. fermenti, the latter being a faster reproducer and causing the most problems if it is present. TABLE II. Loss of Spirit yield in Malt Whisky fcrmcntalion due to bacterial competition In fee lion rating* a * d Sec text. Raclcrta/ml in 3 hr old wash (million) > 1 Approximate loss in spirit yield % < >5 Approximate financial loss (thoi sand*) At lilling duty paid <I7 < > >II9O is not carried out, the analytical results obtained in routine wash analysis, since they are profoundly affected by the bacterial content of the wash, can in turn be used to judge the loss in spirit yield. Table HI shows typical results obtained by analysing 48-h old washes which fell into the infection groups in Table II, and the approximate reduction in spirit yield. In distilleries where quality control is effective, those with modern stainless steel plant are usually in the 'a' Group while those with more traditional plant are generally in the 'b' Group. Falling below these groups indicates the need for tighter control and cleaning. TABLE HI. Approximate analytical parameters of 48-h-old washes Infection rating a b c 4 P»l Total Acid > SG at 2'C < *o' ve or high 1 ve Approx. loss in spirit yield '. nil d >O high t ve Se* text ii Fig. 3. Alcohol content and specific gravity of Scotch Malt Whisky wash in the later stages of fermentation. = SG; %P.S. The enumeration and identification of the bacteria present is clearly of use in isolating and minimizing problems in distillery practice, and the performance of a distillery can be gauged by determining the bacterial content of a number of washes at a given time from setting. 3 h is used since at this time there arc generally only lactobacilli present, and the bacterial fermentation should not be dominant. A rating of the efficiency of the distillery fermentation can be given based on the bacterial count. If facilities do not exist to conduct bacterial counts the ph and other parameters can give a good insight into the situation. The loss of whisky arising from bacterial competition is roughly known and Table II shows the effect particular levels of infection at 3 h from setting have on spirit yield, and indicates the associated financial losses to the distiller and in duty in a plant operating at a planned production level of 1 m proof gallons per year. If bacterial enumeration and identification 5 The maintenance of high standards of cleanliness and low levels of bacterial infection may cause some effect on flavour as it is likely that bacteria present in significant numbers will produce flavour compounds which may be organolcptically desirable or undesirable. Particular strains of yeasts (which would produce parti cular flavour compounds) could be used for fermentation. Fermentation vessels could be attemperated and worts accurately aerated and these activities could have effects on flavour, but at the present time distillers rely on the dis tillation stage of the process to produce a spirit of acceptable flavour. The study of flavour compounds in distillery wash and in the resultant spirit is still in its relative infancy. The alcohol content of the fermented wash is measured using the traditional distillation method, or by gas chromatography, these methods also being employed to measure the alcohol remaining in spent wash and lees, which arc the residues of the two sequential distillations. The spent wash and Ices arc checked to ensure that the stills are operating efficiently and that the maximum quantity of spirit is collected. To ensure that there are no leaks between the spirit and water sides of the still kettles or condensers, samples of condensate and condenser waters are checked to ensure the absence of alcohol, using a test involving oxidation of any alcohol present to aldehyde, concentration of any aldehyde by distillation, and reaction (if any) with 2:4-dinitrophcnylhydrazine. The new whisky is reduced in strength and casked for maturing, as described by Simpson,18 after which period blending and bottling arc carried out. These arc aided by

5 Vol. 82, 1976] oolan: brewing science and scotch whisky production 181 an analytically-biased, rather than a production-oriented laboratory effort, some activities of which have been out lined above in an attempt to describe where the application of brewing science is helping maintain efficiency in the pro duction of Scotch Malt Whisky. Acknowledgements. The author wishes to thank the Directors of The Highland Distilleries Company Limited for permission to publish this paper, and the staff of Pentlands Scotch Whisky Research Ltd for useful discussions, especially on optical rotation. References 1. Association of Official Analytical Chemists (of America) Methods of Analysis, 12th Edition 1975, Dadic, M., Brewers Digest, 1975, 5, Griffin, O. T., Journal of the Institute of Brewing, 1974, 8, Griffin, O. T., Process Biochemistry, 1972, 7, Harrison, J. S., & Graham, J. C. J., The Yeasts, Volume 3, Yeast Technology, London: Academic Press, 197, Hough, J. S., Briggs, D. E. & Stevens R., Malting and Brewing Science. London: Chapman and Hall, Institute of Brewing Analysis Committee, Recommended Methods of Analysis, Journal of the Institute of Brewing, 1971,77, Institute of Brewing Analysis Committee, Recommended Methods of Analysis, Journal of the Institute of Brewing, 1975, 81, McFarlane, C, Lcc, J. B., & Evans, M. B., Journal of the Institute of Brewing 1973, 79, Ministry of Housing & Local Government: The Bacterio logical Examination of Water Supplies, London: H.M.S.O Nishikawa, N., & Nomura, B., Proceedings of the 13/A Con vention, Australian & New Zealand Section, Institute of Brewing, 1974, Simpson, A. C, Process Biochemistry, , 9.