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 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 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 CO2 in the wort, thereby reducing dissolved CO2 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 experience from both commercial-scale experimental trials and full-cellar 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