Yeast Management Provides Stable Fermentation Performance

New propagation technique offers savings while increasing yields
By Peter Krasucki | May 13, 2011

Grain pricing volatility and the comparatively low margins associated with commodity products such as ethanol require diligent reevaluation of material and energy balances of every unit operation in the ethanol industry. To remain competitive, these significant market pressures demand absolute focus on optimal ethanol yield and maximum process efficiency.

There are inherent limitations due to the complexities of carbohydrate processing and yeast metabolism, although 88 to 93 percent of the theoretical maximum yield is frequently achieved in the corn ethanol process. The typically closed-loop bioprocessing configuration of ethanol plants and the dependence on the biocatalytic activity of the living microorganism, Saccharomyces cerevisiae (yeast), offers many challenges and a very limited number of economically feasible solutions.

While the self-limiting nature of the industrial scale, batch-fermentation process would appear to offer very few options, a promising new technology option—High Density Seed Cultivation—may significantly improve the performance of the standard dry grind ethanol plant. 

Ethanol manufacturing platforms dependent on batch- or continuous-yeast propagation may improve performance by incorporating advanced technologies already utilized in other biotechnology industries using fed-batch fermentation platforms. The core bioprocessing systems in the ethanol process require high throughput, and any modification of current systems may be economically unreasonable.  Improvements could be made, however, to the accessory bioprocesses such as yeast propagation that can be appropriately scaled while maintaining or improving performance. The modernization of current yeast management strategies through use of the high-density seed cultivation platform targets production of low-volume, high-density fermentation inoculums. When combined with cell recycling and fed-batch yeast cultivation, it can lower operations costs while improving fermentation performance.

Yeast Inoculum
Ethanol fermentation is first dependent on the availability of yeast biomass. In contrast to other bioprocess ingredients where concentrations are based on the effluent and influent composition and additions, yeast acts as a self-reproducing biocatalyst, properties of which are functions of plant processes. While the properties of various commercial yeast products have significant impacts, these properties alone do not ensure satisfactory performance. Instead, successful process performance depends on economical production and maintenance of the viable yeast biomass. Currently, the ethanol industry mostly uses active dry yeast products as the source of the initial yeast inoculum. These products are closely related industrial strains of typically polyploidy Saccharomyces cerevisiae.  A definitive review was written by W.M. Ingledew in 2005, covering the properties and performance of seven such products and similar reviews are available on yeast performance under different saccharification conditions, chemical balances and process conditions.

Fermentation Requirements
The overall fuel ethanol biological process can be separated into two distinct phases—yeast biomass propagation (inoculum build up) and ethanol fermentation. The biological process culminates in the beer well and downstream distillation and recovery. These two steps are common across various fermentation industries where a required biomass is first propagated and then used as a biocatalyst in the production phase.

In view of the overall process principles, propagation design targets are based on fermentation inoculum density requirements of 5 to 10 x 106 cells per milliliter. Additionally, propagation processes are designed and based on the selected yeast product properties, process cycle, temperature, medium composition, specific growth rate target and fermentative capacity of yeast biomass. The yeast cultivation process has a big impact on the specific fermentative capacity of the yeast biomass.  Beside yeast biomass considerations, significant attention needs to be devoted to controlling microbial contamination that could reduce ethanol yield.

Industrial-scale fermentations are run as self-limiting batch processes where only the final substrate and product concentrations are of consequence. That self-limiting nature provides, on one hand, wide latitude in initial conditions, yet, it also prevents reasonable control or optimization of the fermentation process itself. Such variability in initial conditions, which results in seemingly similar performance, frequently leads to the false sense of “process control.”

Yeast Propagation Performance
The inherent engineering-scale limitations dictating the use of the self-limiting, batch-fermentation platform offers many process control challenges resulting in often unpredictable performance. Review of the current industrial data and relevant publications reveals that in view of the specific attainable fermentation goals, the propagation process contributes an unreasonable amount of uncertainty—frequently resulting in performance losses. The unnecessary limitations imposed on the current propagation process lead to the very low volumetric productivity, and to final yeast biomass with less than optimal fermentative capacity. A focused propagation platform, however, should provide significant control over self-limiting batch fermentations.

Using standard-scale factors, fermentation inoculum density requires production of 484 to 726 kilograms of yeast in active dry yeast equivalent (ADY) during biomass propagation. This process is typically initiated by using approximately 40 kg (ADY equivalent) of yeast and results in the initial yeast propagation density of between 10 to 20 x 106 yeast cells per milliliter.  After eight hours, the final yeast biomass concentration of 320 to 640 x 106 yeast cells per milliliter is produced in four to five generations.  Regardless of the yeast strain, the batch propagation conditions can create significantly imbalanced metabolic fluxes in the yeast biomass, resulting in increased production of secondary metabolites such as glycerol, acetate and acetaldehyde that can inhibit fermentation.

Alternative Technologies
The current targets for inoculum of 1 to 2 percent, volume to volume, are excessive, and based on a low-density yeast cultivation process. Current processes producing biomass densities of only 8 to 12 grams per liter versus the more ideal 137 grams per liter, illustrate that there are clear optimization opportunities, as well as significant potential savings in adopting new approaches.

Advanced, yet inexpensive, features such as integrated fed-batch control loops can be easily incorporated into existing computer control systems.  Furthermore, additional advanced features such as medium enrichment, cell recycling, mixed yeast cultures and feedstock flexibility can all be easily implemented to realize additional cost savings and performance gains. Compared to existing propagation processes, single-stage or multi-stage, cell-recycling, fed-batch systems offer the following advantages:   

  • Production of high-density, low-volume fermentation seed.
  • Faster fermentations.
  • Improved fermentation process stability.
  • Improved material and energy balance.
  • Decreased risk of infection, lower sanitation costs.
  • Reduced costs of about 50 percent.
  • Reductions in commercial yeast use, regardless of strain, of up to 95 percent.
  • Elimination of commercial yeast product variability impacts on fermentation.
  • Stringent control over final fermentation inoculum properties and fermentation process kinetics.
  • Feedstock flexibility beyond C6 carbohydrates.
  • Pure culture and mix culture process flexibility.
  • Full process control automation.
  • Immediate return on investment.
  • Additional bioproducts options.

Economical technologies such as cell-recycle, fed-batch yeast cultivation promise to be the next step in development for biorefineries. Additional options in fermentation process management are also available to the fuel ethanol industry.

Author: Peter Krasucki
General Manager, Fermatrix