Alyce Hartvigsen (1); (1) Alfa Laval Copenhagen A/S, Soborg, Denmark

Yeast, Fermentation, and Microbiology

Many fermentation issues can be related to sub-optimal conditions for yeast viability. High-gravity brewing (HGB) 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 CO2. 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 SO2, 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 rotary jet mixing 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 super-saturated CO2 in the wort, thereby reducing dissolved CO2 concentrations and 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 experience from both commercial-scale experimental trials and full cellar implementation of the system has found no evidence of an adverse impact on 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 tenth generation of the yeast. In order 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 presentation 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 yeast health and performance, general recommendations are provided regarding how the system can maximize yeast viability and lead to improved performance of the fermentation process, producing a more uniform, higher quality, and better tasting final product.

  Alyce Hartvigsen was born in Philadelphia, PA, in 1967 and completed her B.S. degree in chemical engineering at Case Western Reserve University in 1988. 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, TX, and moved to Denmark in 1997. 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 sales and technology manager in the Brewery Market unit. Here, she is responsible for the global sales and technical support for rotary jet mixers and other tank equipment in the brewery market. 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 full cellar installations and 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 fast-growing craft brewing sector.