EPM: Briefly, what is the history behind "active dry yeasts" for fuel ethanol (or beverage alcohol) production?

Ingledew: The production of active dry yeasts (ADY) for alcoholic beverage manufacture began in the 1940s but came into its own in the 1960s as an attempt to produce a dry product with high viability and a long shelf life that could be used to inoculate grape juices during the short six to seven week period of intense wine fermentations in California. Knowledge of yeast and perfection of the drying technologies for active dried yeast now allow commercialization of specific yeast strains with attributes appealing to specific industries. Use of active dried yeasts has now spread from the baking and wine industries to the distilling industry, the fuel ethanol industry and even into brewing (and not just for beer kits).

Today, yeasts are available to ethanol producers that have characteristics well suited to producing high levels of alcohol while withstanding the particular stresses associated with fuel ethanol fermentations, such as acidity and temperature. Ethanol plants today can operate with a high quality yeast and save on the costly investments in microbiology and capital equipment needed to culture, prepare, scale up, process and harvest a suitable yeast.

EPM: How is active dried yeast processed, packaged and stored?

Ingledew: Processing of active dried yeast includes harvesting, compression of the paste through extruders, and drying to about five percent moisture under the controlled conditions needed to maintain viability. Emulsifiers are added to provide best dispersal of yeast in mash after inoculation. The pellets are packaged - often under inert gas or in a vacuum. It has a shelf life over one year and losses of activity of one percent per month are common. Refrigeration under inert gas is recommended for best storage, and a regime for rehydration (38°C for 20 minutes) is recommended for best recovery. Purchases large enough for only one or two months at a time are recommended.

EPM: What advantages would a producer have from purchasing active dry yeasts?

Ingledew: Use of active dry yeasts or compressed fresh yeasts (if located near a yeast plant) is now the norm in distilleries. In the recent article "Propagation of Yeast in a Distillery: Is it Worth the Risk? Why I Don't Believe So," printed in the January, 2003, issue of Ethanol Producer Magazine, I stressed the advantages in their use over methods of propagating yeast in-house.


EPM: In terms of effectiveness, what should ethanol producers be trying to achieve for with active dry yeasts?

Bellissimi: A goal of approximately 22 billion viable yeasts per gram was the standard in the industry until 2001-2002. Today, some ADY preparations contain as many as 30 billion viable cells per gram and one high-temperature yeast is so small that the numbers are even higher. Then, it was quite typical to inoculate fermentors at 2.2 pounds per 1,000 (U.S.) gallons to obtain a reproducible inoculation rate of about six million viable yeasts per millimeter of mash or must. Active dried yeast are grown under strong aerobic conditions with the computer-controlled addition of sugar (molasses) such that they grow aerobically where the yield of yeast from substrate is 0.5 (0.5 grams of yeast per gram of substrate used). Because they are grown aerobically, they can often be used for a single batch fermentation (one cycle) with little need or regard for further aeration.

EPM: What about contamination?

Bellissimi: All active dried yeast contain small (when compared to the numbers of viable yeast) numbers of bacterial and wild yeasts. In fact it is impossible for yeast companies to produce such large batches of propagated yeasts without contamination (infection). Fear of increasing the numbers of such contaminants is one of the strongest arguments against continuous (re)propagation of yeasts in distilleries.

EPM: What can producers expect from these so-called "new yeasts?"

Ingledew: For many years, the yeasts available to the fuel ethanol industry have included bakers' strains, some distillery strains and even brewing strains; and all of these yeasts will produce alcohol from molasses or enzyme-modified, starch-based crops in a reproducible fashion. However, today, the industry is not satisfied with alcohol concentrations of 8-10 percent, with low ethanol tolerance or with slow fermentations at less than 35°C (95°F). Yeast suppliers have responded with what we have termed the "new yeasts." How are they different? How do they perform? How do they compare? What should you know about them? Here at the University of Saskatchewan, we have an ongoing study that will, with time, answer many of these questions.

Here's a brief preview:
Many new yeasts are specially selected strains for fuel ethanol production. Some claim to be more temperature tolerant, with higher yields and higher viable cell counts during fermentation than was seen with the former strains. Some may be more tolerant to ethanol or to carbohydrates. At least one of them we know has been used in plants that produce alcohol at much higher concentrations than what the industry average has been and one company claims 17 -19 percent ethanol in 40 to 48 hours. Some may produce less foam, or reduced glycerol. Stuck fermentations may be reduced. We know that some of these strains are yeasts that produce smaller cells. As a result, it is not uncommon to see active dried yeast which are well above the previous norm of 22 billion per gram - in fact some are over 50 billion viable cells per gram. This may not mean that the amount of viable biomass per gram is any different than the yeasts provided in the past, because the new yeast cells are smaller than cells found in normal active dried yeast. The proof of the claims made by the suppliers will surely be provided with time, as it becomes known whether a new yeast produces alcohol at faster rates, to lower gravities, with higher yields or higher alcohol concentrations.

It appears that one or more of these new yeasts may be unusually thermo-tolerant; experiences in the industry with these strains are building. If so, savings in energy will accrue in cooling fermentors. Does this allow higher alcohol concentrations when strains are thermo-tolerant? Our laboratory is conducting studies to determine whether claims on the benefits of the new yeasts can be substantiated, and hopefully to provide new information on how to get the best performance from the new strains available.

EPM: Will it be difficult for producers to stay abreast of new yeasts developments - and purchasing decisions - as the science evolves?

Ingledew: For now, the choice in purchasing these yeasts is up to each individual producer. Early data indicates some advantages in their use. When research is complete, the differences between some of these varieties will become known. Eventually, you will have to determine if the differences are so great that you will have to analyze incoming yeasts as part of your company's quality control program in order to understand their use. Remember also that yeasts available prior to the introduction of the new strains of ADY have already been shown to produce up to 23.8 percent alcohol v/v under lab conditions using very high gravity (VHG) conditions in batch fermentation. These yeasts were the standard in an industry where alcohol concentrations over 16 percent are becoming common. What should producers expect form these new yeasts? Time will tell. But the future will be exciting! EP