Dynamic Fermentation with Iso-Mix Rotary Jet Mixing: Optimizing Yeast Viability and System Performance

Transcription

1 MBAA TQ vol. 53, no pp SUPPLIER PERSPECTIVE Dynamic Fermentation with Iso-Mix Rotary Jet Mixing: Optimizing Yeast Viability and System Performance Alyce Hartvigsen Alfa Laval Copenhagen A/S, Soborg, Denmark Many fermentation issues can be related to suboptimal conditions for the yeast. High-gravity brewing exposes the yeast to higher ethanol concentrations. Premature yeast settling brings the yeast to the bottom of the tank, where it is subject to higher pressures and concentrations of dissolved CO 2. Packing of the yeast in the cone limits access to the nutrients in the fermenting wort and can result in the generation of hot spots. All these conditions are suspected to contribute to increased yeast stress, adversely affecting viability and resulting in higher concentrations of stress by-products such as acetaldehyde and SO 2, off-flavors in the beer that decrease the flavor score. In some cases, the fermentation essentially ceases prior to full attenuation, leading to stuck fermentation and possibly a high degree of residual extract. The use of the Iso-Mix rotary jet mixing system during fermentation has demonstrated considerable potential in addressing such issues. The system maintains the yeast in homogeneous suspension in the beer during the course of the fermentation, provides improved heat transfer and a homogeneous temperature profile through forced convection in the fermenting beer, and nucleates supersaturated CO 2 in the wort, thereby reducing dissolved CO 2 concentrations and thereby toxicity for the yeast. While there were some initial concerns that the mixing process could impart shear stress on the yeast and cause cell death, the accumulated ABSTRACT experience from both commercial-scale experimental trials and fullcellar implementation of the system has found no evidence of adverse impact to the yeast viability from the system. On the contrary, yeast viability measurements are typically equal to or higher than those of conventional fermentation. In one instance, trials performed in adjacent tanks fermenting 16 P wort using serial repitching of lager yeast demonstrated consistent yeast viability of higher than 95% through the 10th generation of the yeast. To achieve the best performance of this technology, it is important to examine in more detail its effects on the fermentation process, particularly on the yeast itself, as well as the considerations that should be made prior to implementing the system. This article examines the results of implementation of rotary jet mixer systems in fermentation, effects on the viability of the yeast cultures, and the overall fermentation performance. Through a better understanding of the relationship of mixed fermentation to the yeast health and performance, general recommendations are provided regarding how the system can maximize the yeast viability and lead to improved performance of the fermentation process, producing a more uniform, higher quality, and better tasting final product. Keywords: Fermentation, Mixing, Yeast, Stress, High-gravity brewing Alyce Hartvigsen was born in Philadelphia, PA, in 1967 and completed her B.Sc. in chemical engineering at Case Western Reserve University in She began her career in process engineering in the petrochemical industry. In 1994, she accepted a technical sales position with a Danish technology company in Houston, and she moved to Denmark in Her arrival coincided with the beginning of the craft beer revolution in Denmark, and over the years, she developed a keen interest in beer and brewing, joining the Danish Beer Enthusiasts and a home-brewing club. In 2012, she took advantage of a unique opportunity to combine career and personal interest and joined Alfa Laval as a sales and technology manager in the brewery market unit. She is responsible for the global sales and technical support for rotary jet mixers and other tank equipment for breweries. Her work has primarily focused on developing and promoting applications for rotary jet mixing in the brewing industry. She has overseen the implementation of both commercial-scale trials and fullcellar installations, and she works closely with the end users in the commissioning and process optimization of the systems. Planned activities include expanding the applications for rotary jet mixing within the fastgrowing craft brewing sector. alyce.hartvigsen@alfalaval.com Master Brewers Association of the Americas Challenges of Conventional Fermentation Modern industrial lager fermentation typically takes place in large cylindroconical tanks. These tanks typically range in volume from several hundred to 10,000 hl, with typical diameters of 3 9 m and overall heights from 8 to over 20 m. The tanks normally have external cooling jackets to absorb the heat of fermentation and for the crash cooling (chill-back) of the beer at the end of fermentation. The tank cone functions to collect the yeast at the end of fermentation, allowing for easy and convenient harvesting. While cylindroconical tanks have been optimized over the years for production of bottom-fermented beer, they still can pose some considerable challenges to the vitality and viability of the yeast, and thereby the efficiency of the fermentation and the quality of the beer produced. Table 1 summarizes the main challenges, typical causes, and possible effects on the yeast health and fermentation performance. Observations of Premature Yeast Settling The phenomenon of premature yeast settling was well documented in a study performed by Christopher Boulton from the 200

2 Dynamic Fermentation with Iso-Mix Rotary Jet Mixing MBAA TQ vol. 53, no University of Nottingham, in a major brewery in the United Kingdom. A series of encapsulated Aber biomass probes were installed in a cylindroconical fermentation tank at various depths in the tank, from the top of the liquid down to the base of the cone. Using these instruments, the concentrations of live yeast cells at the different levels in the tank were monitored throughout the course of a normal fermentation cycle. The results of this test are depicted in Figure 1. During the test, some minor settling of the yeast into the cone was observed early in the progress of the cycle. After about 12 h, the yeast concentrations became homogenized in the tank, most likely as a result of the natural convection stimulated by the rising bubbles of CO 2 that evolved during the fermentation. About two days into the fermentation cycle, an abrupt increase in the measured concentrations of yeast in the cone of the vessel was observed, while the yeast concentration in the upper half of the tank decreased. These results suggest a massive settling of the yeast into the cone well before the completion of primary fermentation. Iso-Mix with Rotary Jet Mixer Experience has demonstrated that the above-listed challenges of modern lager fermentations can be addressed by the implementation of an Iso-Mix system based on a rotary jet mixer in the fermentation tanks. The system functions by circulating the wort from the bottom of the fermentation vessel to the rotary jet mixer nozzles using a pump. Figures 2 and 3 illustrate a rotary jet mixer and a typical configuration of the Iso-Mix system based on this technology. Mechanisms of Iso-Mix and Expected Benefits The benefits provided by rotary jet mixing during fermentation are the result of three primary mechanisms: 1. Maintenance of the yeast in homogeneous suspension in the entire tank throughout the primary fermentation (prevention of premature settling into the cone) 2. Provision of uniform temperature distribution throughout the tank and improved heat transfer with the tank jackets through forced convection (prevention of hot spots and temperature stratification in the tanks) 3. Nucleation of supersaturated CO 2 (reduction of dissolved CO 2 levels in the wort and provision of gradual CO 2 release from the beer, thereby minimizing foaming) Reexamining the challenges of conventional fermentation introduced in the first section, the potential benefits provided by Iso-Mix are clearly deduced from the mechanisms described above. These effects are summarized in Table 2. Process Considerations with Iso-Mix Rotary Jet Mixing The implementation of the Iso-Mix system in fermentation is a significant change to the fermentation process, and it requires as a result some considerations of its potential effects on the process itself and on the yeast. Several of the primary considerations are reviewed below. 1. Mixing shear stress on the yeast. A common concern of brewers considering the Iso-Mix process is the potential Figure 1. Actual measurements of viable yeast concentrations throughout a cylindroconical tank during the course of a conventional (unmixed) fermentation, clearly illustrating the premature settling of yeast in the cone of the tank. Figure 2. Iso-Mix rotary jet mixer. Table 1. Challenges of fermentation in modern cylindroconical tanks Challenge Typical root causes Possible effects Hot spots High outdoor temperatures and sun load Insufficient heat transfer from tank jackets, inadequate mixing, large tank dimensions Yeast stress, variable fermentation conditions, off-flavors CO 2 toxicity Premature yeast settling, inadequate mixing, supersaturation, tall tanks/high hydrostatic pressures, high fermenter pressures, high-gravity fermentations Yeast stress, stuck fermentation, variable fermentation conditions, off-flavors, yeast autolysis, deterioration of foam quality Alcohol toxicity High-gravity brewing Yeast stress, premature yeast settling, stuck fermentation, yeast autolysis, off-flavors, deterioration of foam quality Premature yeast settling Flocculant yeast, inadequate mixing, high-gravity brewing CO 2 toxicity, hot spots, poor access to beer nutrients, yeast stress, stuck fermentation, off-flavors, yeast autolysis, deterioration of foam quality CO 2 supersaturation Accumulation of high levels of dissolved CO 2 in the beer, gas releases, excess foaming Yeast stress, off-flavors, yeast autolysis, product loss, bitterness loss

3 202 MBAA TQ vol. 53, no shear stress on the yeast imparted by its circulation through a centrifugal pump and the rotary jet mixer. However, the accumulated customer experiences with the system have not observed any deleterious effect to the yeast resulting from this circulation process. The flow rates through the mixer that are used for the fermentation process are considerably lower than those used for other mixing applications, in order to minimize the shear stress on the yeast. Moreover, the reduction of yeast stress associated with the maintenance of the yeast in homogeneous suspension and prevention of premature settling in the cone and resulting CO2 toxicity appear to more than offset any minimal increase in stress associated with the circulation. As a result, the yeast health and viability have been observed to be similar or improved compared with an unmixed fermentation. In one brewery, tests with serial repitching of yeast between two Iso-Mix fermenters succeeded in utilizing the yeast through the 10th generation while still maintaining a yeast viability in excess of 95% throughout all 10 generations. 2. Effect of the Iso-Mix system on the yeast harvesting process. The harvesting of the yeast in an Iso-Mix fermentation vessel is generally performed in one of two ways, either by warm harvesting (cropping) following settling at the end of primary fermentation or by harvesting Dynamic Fermentation with Iso-Mix Rotary Jet Mixing through a centrifuge. The Iso-Mix system functions effectively in conjunction with either of these methods. For warm harvesting by settling, the Iso-Mix mixer is turned off at a designated time in advance of the desired cropping time to allow the yeast to settle for cropping. A typical recommended period is h prior to the desired cropping time, but the optimal advance time depends on the yeast flocculence and is generally determined by trial and error during the optimization period following commissioning. For harvesting through a centrifuge, the mixer is located low in the tank and is operated for as long as possible during the emptying of the tank to the centrifuge. In this manner, a uniform load to the centrifuge is maintained, thereby optimizing centrifuge performance. 3. Yeast pitching and oxygenation rates. All other parameters being equal, a change from unmixed to mixed fermentation can lead to increased yeast growth rates, because the maintenance of the yeast in homogeneous suspension with better access to the wort nutrients can give improved yeast health and thereby growth. To prevent overgrowth and resulting extract loss, it may therefore be necessary to adjust yeast pitching rates and/or oxygenation rates following the implementation of Iso-Mix in order to maintain the desired growth rate and prevent overgrowth and high Figure 3. Typical configuration of Iso-Mix systems based on rotary jet mixer technology. Table 2. Expected effects of Iso-Mix on the fermentation challenges introduced in Table 1 Challenge Iso-Mix effects Expected benefits Hot spots Prevented by forced convection, giving uniform temperature profile Improved yeast health, uniform fermentation conditions CO2 toxicity Mixing nucleates CO2 and leads to lower levels of dissolved CO2 Yeast also kept in suspension at higher levels in the tank residual extract, lower off-flavor concentrations, possibly faster fermentation Alcohol toxicity Yeast kept in suspension throughout fermentation, gaining access to nutrients Uniform temperature profile and fermentation conditions residual extract levels, possibly faster fermentation Premature yeast settling Yeast kept in suspension throughout fermentation, only settling once mixer is turned off residual extract levels, possibly faster fermentation, lower off-flavor concentrations CO2 supersaturation Mixing nucleates CO2 and leads to lower levels of dissolved CO2 Start of mixer prior to CO2 saturation allows gradual release and prevents excess foaming Lower levels of foaming, decreased loss in bitterness

4 Dynamic Fermentation with Iso-Mix Rotary Jet Mixing MBAA TQ vol. 53, no extract loss. This optimization procedure is generally performed by the brewery with assistance from the system supplier over a period of time until the optimal system parameters for the individual site are determined. Results from Commercial Installations of Rotary Jet Mixing Systems The benefits of Iso-Mix have been clearly documented both in commercial-scale trials and in full-cellar installations of the rotary jet mixing systems in a variety of breweries around the world. In the following sections, data and measurements from commercial-scale installations are presented to illustrate the effects of Iso-Mix on the fermentation parameters. Fermentation Process Time Figure 4 illustrates the percentage reduction in overall fermentation process time (from fermenter full to start of chillback) observed with dynamic fermentation systems compared with conventional unmixed fermentation, for 12 different lager brands at 11 brewery sites. For each brand, the original extract (OE) is included when known. As observed from the results, Figure 4. Process time reductions for Iso-Mix for 12 brands in 11 different breweries, including original extract (OE) of each brand when known. the implementation of Iso-Mix at these sites has typically resulted in about 15 20% reduction in fermentation process time. In some cases, particularly where moderate to high-gravity brewing is practiced, a greater degree of time savings may be realized. Chill-back Time Figure 5 illustrates the effect of rotary jet mixing on the chill-back time of an 1,800 hl cylindroconical fermenter. The cooling from 16 to 4 C took approximately 26 h for the tank when unmixed but was reduced to approximately 16 h when the rotary jet mixer was operated during the chill-back period. This time savings is evidence of the more effective heat transfer between the beer and the cooling jackets that occurs as a result of the forced convection effected by the mixer. Effect of Iso-Mix on Beer Quality Parameters Table 3 summarizes the typical effects of Iso-Mix on some key beer quality parameters, resulting from the reduction of yeast stress and improved temperature uniformity of the mixed fermentations, based on compilation of data and feedback from a number of full-scale trial and commercial installations worldwide. Effect of Iso-Mix on Super-High-Gravity Beer Some breweries are considering the implementation of increasingly high-gravity brewing to increase brewing capacity, to save costs, and for the production of special high-alcohol beers. Whereas an OE of 15 P was once considered high-gravity brewing, gravities of P or even higher are now observed. The use of super-high-gravity brewing presents particular challenges to the brewer, because many of the yeast stress factors such as alcohol and CO 2 toxicity are magnified with the increasing beer strength. For these applications, Iso-Mix is not only particularly suitable, it is virtually required in order to achieve satisfactory beer quality and uniform and optimized fermentation times. Figures 6 and 7 illustrate the effects of Iso-Mix on the fermentation profiles and quality parameters of a super-high-gravity beer (OE = 24.5 P). Compared with conventional, unmixed fermentation, the brewery experienced considerable improvements of the key quality parameters and a significant reduction in fermentation process time following the implementation of rotary jet mixing in the fermentation tank. Conclusion The accumulated experience from commercial-scale trials and full-cellar installations of Iso-Mix systems using rotary Figure 5. Effect of mixing on chill-back time in an 1,800 hl cylindroconical fermenter. Table 3. Typically observed effects of Iso-Mix on beer quality parameters Beer quality parameter Typical effect of Iso-Mix Acetaldehyde (ppm) 40 60% decrease SO 2 (ppm) 15 25% decrease %RDF/final alcohol content 1 3% increase, particularly for high-gravity brewing Residual extract Decrease, particularly for high-gravity brewing Volatiles, particularly isoamyl acetate Increase Foam duration Unchanged or increase Flavor True to type Flavor stability Same or improved Yeast viability Same or improved

5 204 MBAA TQ vol. 53, no Dynamic Fermentation with Iso-Mix Rotary Jet Mixing Figure 6. Fermentation profiles for dynamic versus standard (unmixed) fermentation of super-high-gravity beer (original extract = 24.5 P). ABV = alcohol by volume; AE = apparent extract; and LE = limit extract. jet mixer technology has demonstrated considerable benefits for the fermentation process, beer quality, and yeast viability. Reductions in fermentation process times have enabled capacity increase without the installation of additional tanks, and high-gravity and super-high-gravity brewing can be implemented while maintaining the highest beer quality, consistent fermentations, and a high level of yeast viability. With the current trends toward minimizing environmental impact and cost reduction, Iso-Mix is expected to become even more widespread in the years to come. REFERENCES Figure 7. Summary: improvements with Iso-Mix for super-high-gravity beer. Process and quality improvements for dynamic versus standard (unmixed) fermentation of super-high-gravity beer (original extract = 24.5 P). BBT = bright beer tank; EBC = European Brewery Convention; NFS = Nibem foam stability; COG = cost of goods; RDF = real degree of fermentation; and SV = storage vessel. 1. Heggart, H. M., Margaritis, A., Pilkington, H., Stewart, R. J., Dowbanick, T. M., and Russell, I. (1999). Factors affecting yeast viability and vitality characteristics: A review. Tech. Q. Master Brew Assoc. Am. 36: Kunze, W. (2010). Effects of different factors on the yeast. In: Technology of Brewing and Malting, 4th Ed., pp VLB: Berlin, Germany. 3. Speers, A. (2016). Brewing fundamentals, Part 3: Yeast settling Flocculation. Tech. Q. Master Brew Assoc. Am. 53: Stewart, G. G. (2001). Yeast management The balance between fermentation efficiency and beer quality. Tech. Q. Master Brew Assoc. Am. 38: Stewart, G. G. (2016). Brewing fundamentals, Part 1: Yeast An introduction to fermentation. Tech. Q. Master Brew Assoc. Am. 53:3-9. PFL00140EN 1611