METHODS FOR DETERMINING THE FERMENTABILITY AND PREDICTED SPIRIT YIELD OF

Transcription

1 J. Inst. Brew., November-December 1996, Vol. 102, pp METHODS FOR DETERMINING THE FERMENTABILITY AND PREDICTED SPIRIT YIELD OF DISTILLING MALTS By Thomas A. Bringiiurst, James M. Brosnan, Bruce McInnes and Gordon M. Steele (The Scotch Whisky Research Institute. 84 Slateford Road. Edinburgh EHll IQU) This Paper is dedicated to the memory of Eric Dcwar Received 29 March 1996 Fermentability and predicted spirit yield (PSY) are two fundamental quality parameters of distilling malt, both of which rely on empirical factors for their calculation. Experiments were carried out using modern barley varieties to assess whether these factors were still valid for use in today's distilling industry. Using the 450 g IOB mash, an average attenuation factor of from fifty-two deter minations was found which was not significantly different from the existing factor of A novel method to calculate the attenuation factor at different alcohol strengths was demonstrated which confirmed the factor to be The PSY factor was shown to be 6.11 for a 450 g IOB mash, calculated using methods based on predicted and actual alcohol produced. However, a high gravity mashing procedure which more closely simulated distillery conditions confirmed that the current standard factor of 6.06 was still acceptable. Results indicated that it was not necessary to change either the attenuation or PSY factors when changing from the 515 ml mash to the 450 g mash in the standard IOB Recommended Methods. Key Words: Malt, fennentahility, predicted spirit yield, attenuation factor Introduction The fermentability method has been the principal method of analysis used for trading distilling malt and monitoring dis tillery efficiency since It was accepted as a Recommended Method of the Institute of Brewing (IOB) in I9834 and was published as Method 2.15 in the 1986 edition of the Recom mended Methods of Analysis6. This has remained essentially unchanged in the current edition of I99l7. It has been generally considered that the fermentability method gives satisfactory results for good quality, well modified malts based on a coarse extract wort'-3-5. However, over the last few years there has been growing concern within the Scotch Whisky industry about the reliability of the method. This con cern is in light of technological changes in the malting and distilling industries and a large expansion in the availability of new barley varieties. These factors have resulted in a broaden ing of the quality spectrum of malted barley which can be processed. Similarly in 1996, the Institute of Brewing will be adopting a 450 g standard extract mash in place of the SI 5 ml mash volume which was used when the original fermentability method was being developed. The attenuation factor of 81.4 used in the calculation of distilling malt fermentability was derived empirically and thus might well be expected to change under these different modern conditions. Determination of fcrmentability and soluble extract allows the fermentable extract to be calculated. Although not part of the Recommended Method, distillers multiply the fermentable extract of a malt by another empirical factor. 6.06, to give a predicted spirit yield (PSY) figure in litres of alcohol per tonne of malt. PSY values are used to monitor distillery efficiency by comparing the predicted figure with the actual spirit yield achieved from the malt under production conditions. The PSY figure is not used when trading malt but is employed in the distilling industry as the most common indicator of the quality of a particular malt. However, doubts have been expressed about the current validity of the PSY factor similar to those outlined above for the attenuation factor. An investigation has been carried out using modern malting barley varieties in order to assess whether the empirical factors used to determine fcrmentability and PSY have changed. As part of this study, cftbrl has been made to unravel the under lying principles which determine how these factors may be calculated from experimental data. The results of this work show whether it is necessary to modify or discard what has been the single most important malt analysis within the distilling industry for the last twenty years. Methods The IOB Recommended Methods7 were used to determine hot water extract (Method 2.4), final gravity (FG) ferment ability (Method 2.15), moisture content (Method 2.3), residue gravity (RG) (Method 8.5.1) and the actual alcohol in wash (Method 8.5.3). Except where specifically noted to the contrary, all IOB mashes were made up to 450 g. All specific gravity and alcohol measurements were made using the Anton-Paar DMA 55 density meter. High Gravity Determination of Malt Predicted Spirit Yield Malt grist (57 g, 0.2 mm grind), was mashed with tap water (150 ml) at 63-64cC for I hour. The mash was then centrifuged (l200# for 15 minutes) and the supernatant decanted into a 250 ml measuring cylinder. The grains were transferred to a Buchner filter and filtered under vacuum through a cotton filter to apparent dryncss. The filtrate was combined with the supernatant and the total volume of the first worts measured (VI). The grains were then extracted with second water (75 ml) at 75 C for 30 minutes and filtered as described previously. The volume of the second worts (V2) was measured. The grains were then re-extracted twice more for 15 minutes with 75 and ml of tap water at 85 and 95 C respectively. The third and fourth waters were obtained by filtering as before and the volumes measured (V3 and V4). The volume of fourth water is calculated as [3I5 (VI+V2+V3)] ml. The worts were combined (V1+V2+V3+V4). The wort was transferred to a 500 ml round bottomed flask, pitched with fresh pressed Quest M yeast (0.4% w/v) and fermented at 30 C for 67 hours. Following fermentation the spirit yield was determined by distilling the entire wash as described in the Institute of Brewing Recommended Methods of Analysis 199I (Method 8.5.1), except that 190 ml of spirit was collected and made up to 200 ml before determining the alcoholic strength.

2 434 SPIRIT YIELDS OF MALTS [J. Inst. Brew. TABLE I Summary of the results of 8 replicate fermcntability analyses on 3 barley malt varieties 1993 Final Gravity Method Dcrkado Fermentability (%) (%) PSY (LA/tonnc) Residue Gravity Method Fcrmcntability (%) PSY (LA/tonnc) Results and Discussion ^ Confirmation of the Attenuation Factor and Reproducibility of the Fennentability Method Three different samples of good quality commercial distilling malt were obtained. The three indirectly kilned samples were 1994, 1994 and 1993 Derkado which are all Spring barley varieties. Eight replicate sets of fermentability analyses were carried out on each sample and the averaged data shown in Table I. The standard fermcntability method was used, based on apparent fermentability and the attenuation factor of In addition, the residue gravity after distillation was determined which allowed the actual fermentability to be calculated. The attenuation factor may be calculated from the actual (RG) fcrmentability and the apparent fermentability as shown by the equations below: Apparent fermentability (%)=(OG FG) x 100 (OG-I000) IOB (FG) fermentability (%)= x attenuation factor (81.4) (OG-1000) 3 Actual (RG) fermentability (%)= -x 100 (OG-1000). 4.. _ Attenuation Factor= RG fermentability - X100 Apparent fermentability (OG RG) 5 Attenuation Factor=- -x 100 The results show that the three malts were of similar quality with good soluble extracts, fermentability and PSY. There was excellent agreement between fermentability values calculated using the existing attenuation factor and the actual (RG) fermentability determined from the residue gravity (Table I). This confirms that the attenuation factor of 81.4 is still valid. When attenuation factors are calculated for each malt using the experimentally obtained OG, FG and RG, as described in the above equations, average values of 81.44, and 81.SI were achieved for, and Dcrkado respectively. The range of values is but most values are between 81.4 and 81.5, that is, very close to the existing factor. The coefficient of variation for the eight replicate analysis shows that fermcnt ability calculated using the attenuation factor and that calcu lated from the residue gravity have the same, high degree of reproducibility. Having established the value of the fermentability method for three good quality commercial distilling malts, ten further malt samples were examined. These samples represented a greater degree of variation in malt quality, for example, direct or indirect kilned, high or low nitrogen, Winter barley and micrornaltcd, upcoming, new varieties. In addition two samples of Camarguc which had been reported as causing process prob lems in distilleries were also included. The attenuation factors resulting from these samples are shown in Table II. The fermentability range of these samples was % which would include the normal values found with distilling malt. As before, there was very little difference between the ferment ability using the existing attenuation factor and that calculated directly from the residue gravity. When the attenuation factors were computed as described above, there was a similar degree of variation as that produced by the three malt samples dis cussed previously. However, the average attenuation factor over the complete set of fifty-two fermentability determinations is 81.46, which was very close to the existing factor of The results reported in Table II also showed that wide differ ences in malt processing had no significant effect on the attenuation factor itself. For example, when identical batches of Prisma and Derkado barley were malted using direct or indirect fired kilning, the fcrmentability of the direct fired samples was about 2 per cent lower but importantly the attenuation factors were not significantly different. Similarly micromalts, high and low nitrogen malt and distillery problem malts all gave com parable attenuation factors. It should be noted that even at the extremes of the range of attenuation factor ( ) would translate to less than a 1 percent difference in PSY. Alternative Calculation of Attenuation Factor from Spirit The attenuation factor was originally determined using the experimentally derived OG, FG and RG as described by equa tion 5. However, it would be expected that the level of alcohol in the wash, as the major product of wort sugar fermentation, would be directly related to the degree of attenuation. It was thus thought possible that an alcohol derived attenuation factor could be calculated if the alcohol produced during fer mentation were distilled and collected. This proved to be the case as the following calculation shows: TABLE II Attenuation factor determined from the fermcntability of a range of malt samples Variety 1993 Derkado Derkado (Direct) 1994 Derkado (Indirect) 1994 Prisma (Direct) 1994 Prisma (Indirect) 1994 Camargue (Process problems) 1994 Camargue (Process problems) 1994 Cooper (New variety) 1994 Riviera (New variety) 1994 Halcyon (Winter, low N) 1994 Halcyon (Winter, high N) Attenuation factor Average attenuation factor (n=52)

3 Vol. 102,1996] SPIRIT YIELDS OF MALTS Spirit Indication (Sl)= 1000(1-SG) (SG=Specific Gravity of distillate in IOB) From standard tables (IOB Recommended Method Tables 1 A-l D)7 the SI can be used to give the degrees of gravity lost ( GL). As shown in the IOB Recommended method 8.5.1, SI, GL and RG can then be used to produce calculated values for OG and FG as follows: 7 Calculated OG= GL+RG 8 Calculated FG=RG-SI (Both of these equations arc taken from IOB Recommended Method 8.5.1)7 As has already been shown foft R CT\ 5 Attenuation Factor=- -x 100 Substituting for calculated OG and FG into this equation ( GL+RG)-RG Attenuation Factor=- -xloo ( GL+RG)-(RG-S1) Resolving the brackets gives: 10 Attenuation Factor GL -X100 GL+SI Thus it is possible to calculate the attenuation factor based on one single experimental parameter, that is the amount of alcohol produced during fermentation. Values for the attenua tion factor were calculated from the alcohol produced during the fermentability analysis from the malts discussed above. The results were remarkably uniform being from as compared with the wide range of values for the conventionally derived factor ( ). The average attenuation factor calculated from the alcohol level is 81.46, exactly the same as that predicted from the fifty two conventionally derived attenuation factors. Using the standard tables for degrees of gravity lost, as shown above, it is possible to calculate how the attenuation factor will alter as a function of alcohol in wash. This is shown graphically in Figure I. The relationship indicates that at the level of alcohol associated with the fcrmcntability method (4-5 percent vol/vol) the existing 81.4 factor is still valid. A high gravity of distillery wort (OG approximately 1055 ) producing percent vol/vol alcohol would be expected to require an attenuation factor of Similar high attenuation factors have been derived empirically in a number of "in-house" high gravity fermentability methods used by some companies within the distilling industry. Calculation of the PSY Factorfrom Unboiled IOB Fermentability Worts Based on Predicted Alcohol Levels From the information generated during the calculation of FG and RG fcrmentability it is possible to calculate a PSY factor which will link the amount of fermentable extract with the alcohol produced. This can be achieved from the calculated spirit yield derived from the degrees of gravity lost in fermentation. This method requires the weight of wort in the fermenter to be estimated. In the IOB method for Hot Water Extract (450 g mash), 50 g malt gives g of filtered wort and the grains are con sidered to weigh g2. In the fermentability method 250 ml of wort is fermented and the weight of this wort (Wf) can be determined as follows: II Wort Density (kg/m3) Wf(g)=250x 1000 This allows the equivalent weight of malt in the fermenter (Wm) to be calculated. 12 Wm=50x- Wf Experimental determination of OG and RG allows the degrees of gravity lost to be calculated (Equation 7) which can in turn be used to obtain the spirit indication from standard tables (IOB Recommended Method 8.5.1)7. From the spirit indica tion, the alcohol density is derived (Equation 6) and thus the amount of alcohol from 250 ml of wort may be estimated using standard laboratory alcohol tables. This can be directly related to the weight of malt (Wm) to give the predicted spirit yield in units of litres of alcohol per tonne of malt. 13 Predicted alcohol Spirit yield (LA/tonnc)= xlooo Wm The PSY factor can then be determined using this predicted spirit yield and the malt fermentable extract. Spirit Yield 14 PSY Factor=- Alcohol strength (% vol/vol) Fig. I. The relationship between the alcohol strength and the attenuation factor calculated from the expression ( OL/( GL+SI)). 10 The above derivation of the PSY factor assumes that the weight of grains taken into account during extraction remains con stant as described by Buckee2. Also any effects of yeast growth, CO2 evolution, filtration of wash and volume contraction due to alcohol production are assumed to be negligible. The PSY factors and predicted spirit yields produced from Derkado, and using the above calculations are shown in Table III alongside the PSY values resulting from using the existing factor of The three malts all produced PSY factors of approximately 6.11 which is 0.8% greater than These higher PSY factors gave rise to a similar increase in PSY. As these calculations were made from predicted alcohol levels, it could be argued that some of the assumptions made above were not valid. However, it is also possible to calculate a PSY factor using the actual alcohol produced during the fermentation of fermentability worts as the following section shows.

4 436 SPIRIT YIELDS OF MALTS [J. Inst. Brew. TABLE III Calculation of the PSY Factor Based on Predicted Alcohol Levels TABLE V Calculation of the PSY Factor from a High Gravity Mash and Fermentation (Distillery Simulation) Derkado Conventional PSY Based on FG Spirit Yield Based on Predicted Alcohol Levels 6.11 Results are the mean of 8 replicate analyses RG (%) Based on Fine Extract Conventional PSY Bused on RG High Gravity Spirit Yield Corrected for Sparge Loss (litre alcohol/tonne) Calculation of the PSY Factor from Unboiled IOB Fermentability Worts Based on Actual Alcohol Levels Replicate sets of fermentability worts were prepared as previously described for and malts and fer mented normally. After fermentation each set of rcplicates was split into two sets. The first set of washes were analysed in the normal way. However, in the second set, the whole wash (250 ml) was distilled and the alcohol content determined. The actual spirit yield was calculated from the amount of malt used and a PSY factor derived using the fermentable extract data from the first set as described by Equation 14. The averaged results for the PSY factor and actual spirit yield of this experiment arc shown in Table IV along with the conventionally produced PSY figure. As previously shown, the PSY factors were larger than 6.06 by 0.5% () or 1.2% () confirming that the true PSY factor for fermentability worts from a 450 g mash is approximately Determination ofa PSY Factorfrom a High Gravity Mash and Fermentation (Distillery Simulation) It must be remembered that the purpose of the PSY is to provide distilleries with a realistic and uscable prediction of the performance to be expected from a particular malt. Although it has been demonstrated above that a PSY factor of approxi mately 6.11 is the correct value for fermentability worts, it does not mean that this value provides the most useful factor under production conditions. Distillery mashing and fermentation use worts of much higher gravity than in the fermentability analysis. It is possible to simulate these distillery conditions in the laboratory using the procedure outlined in the Methods section. This high gravity mashing technique was used to process samples of and malt. The high gravity spirit yield was determined after fermentation and distillation by relating the alcohol content of the distillate to the original weight of the malt. The results were corrected for sparge loss, since these are typically higher in the laboratory than those encountered in a modern distillery lauter tun. Previous ex perience has shown that spirit yield values obtained using this method were in close agreement with the actual distillery spirit yield. A value for the PSY factor was calculated by dividing the corrected spirit yield by the fermentable fine extract figure obtained from the fermentability method (Equation 14). The fine (0.2 mm grind) fermentable extract rather than the stand ard coarse (0.7 mm grind) extract, was chosen as a better TABLE IV Calculation of PSY Factor Based on Actual Alcohol Levels FG (%) Conventional PSY Based on FG Spirit Yield Based on Actual Alcohol Results are the means of 4 replicate analyses representation of distillery lauter, or semi-lauter, mash tuns which have been largely introduced to the distilling industry after the original PSY method had been developed. The PSY factors produced from the high gravity experiment are shown in Table V along with the spirit yield. Also included for comparison arc the standard PSY figures based on the fine fermentable extract. There was some variation in the results of the four individual high gravity spirit yields for both malt varieties but the average values gave good agreement with the PSY from the fine extract fermentability data. The PSY factors derived from the high gravity spirit yields showed a similar degree of variation but on average produced factors very close to the standard The high gravity procedure has a greater similarity to distillery process conditions than standard fermentability analysis. Thus these results indicate that the relationship between distillery' yield and PSY defined by a PSY factor of 6.06 is still valid for good quality distilling malt. There were no differences between the results for the FG and RG determinations of fcrmcntability made using the fine extract. As expected the fine extract PSY figures were % higher than the coarse extract figures shown previously. Comparison of 450 g and 515 ml IOB Fermentability Mashes The 450 g mash method for preparing fcrmcntability worts has now been adopted as the official procedure by the IOB. The factors used to determine fermentability and PSY in distilling malt were originally based on a 515 ml mash volume. In order to establish whether this change in methodology would have a significant effect on fcrmcntability and PSY, a direct com parison of the two methods was carried out using samples of and malt. As shown in Table VI, both methods gave similar fermentabilities, although the values from 450 g mashes were con sistently slightly lower. Consequently, the resultant PSY (using the standard 6.06 factor) was also slightly lower for the 450 g mash, but this was not significant since the accepted experi mental error of the method is about I percent. Using the 450 g mash would thus not alter the quality status of a distilling malt. For both mashing methods the FG and RG fermentabilities were in very close agreement showing that the existing attenuation factor of 81.4 was equally valid for 450 g as well as for 515 ml mashes. The only significant difference between the two methods was that 450 g mashes gave higher OG values than the 515 ml method. TABLE VI Original Gravity ( ) FG Fermentability (%) RG Fermcmability (%) Attenuation Factor Comparison of Fcrmcntabilily Dala from 450 g and SIS ml Mashes 515 ml g ml g Results are the means of 4 () or 2 () replicate analyses Results are the means of duplicate analyses

5 Vol. 102, 1996] SPIRIT YIELDS OF MALTS 437 TABLE VII Conventional PSY Based on FG Spirit Yield Based on Predicted Alcohol Level Comparison of PSY Data from 450 g and 515 ml Mashes 515ml 450g 515 ml 450g Results are the means of duplicate analyses A similar comparison between the two mashing methods was made to see if the PSY factor would be affected. When the PSY factors were determined using the predicted alcohol method (given above 3.3), the results showed that the 450 g mash generated a slightly higher factor than the 515 ml mash (Table VII). If these calculated PSY factors were to be used the end result would be to negate the slight shortfall in PSY from the 450 g mash due to its lower fermentability value. However, as stated previously, these differences are small in terms of practical use of the method. Conclusions The above results demonstrate that despite the changes which have occurred in the malting and distilling industries, the empirical factor used to determine malt fermentability is still valid. From these studies it is clear that the switch from 515 ml mashing to 450 g mashing will similarly not necessitate changing the factor. This should be confirmed with the wider use of the 450 g mash method by the industry. For the first time the direct relationship between the alcohol concentration in fermented wash and the attenuation factor has been established. Whether using this method, or the con ventional FG and RG comparison, it is clear that 81.4 should be retained as the correct factor to use for a variety of distilling malts such as those used in this study. The validity of the 6.06 PSY factor is more open to debate. The evidence shows that a factor of approximately 6.11 is prob ably closer to the "true" value for a 450 g IOB fermentability mash. However, the original factor of 6.06 is still relevant to the realistic capabilities of distilleries and this was confirmed by laboratory high gravity studies. It would be recommended that the factor 6.06 be maintained for use in PSY analysis. As advances are made in how modern analytical techniques can be applied to malt analysis (e.g. HPLC separation of fermentable sugars) the fermcntability method and consequent determination of PSY, may seem old fashioned. However, as the work discussed here has shown, the fermentability method has proved exceedingly robust and still provides valid, useful information to the distiller and maltster alike. Although the method may not infallibly predict the distillery performance for every malt it still represents the most reliable and easily interpreted test available for good quality distilling malts. References 1. Bathgatc, G. N., Martinez-Frias, J. & Stark, J. R., Journal of the Institute of Brewing, 1978,84, Buckce, G. K., Journal of the Institute of Brewing, 1985,91, Dolan, T. C. S., Dewar, E. T. & Gray, J. D., Journal of the Institute of Brewing, 1981,87, Dolan, T. C. S., Journal of the Institute of Brewing, 1983, 89, Dolan, T. C. S., Journal of the Institute of Brewing, 1991, 97, Institute of Brewing, Recommended Methods ofanalysis, London, Institute of Brewing, Recommended Methods ofanalysis, London, 1991.