Lessons on Bottom Line Basics
Corn quality, mashing, fermentation, distillation, coproducts—each year The Alcohol School provides a comprehensive course on making ethanol efficiently. In September, 90 participants flew into Montreal for the 32nd annual school, which digs into the science behind the process as well as new developments under way for both the fuel ethanol and beverage alcohol industries. The cross fertilization is deliberate, not only between the beverage and fuel alcohol producers, but also for related companies that send employees to get a broader understanding of the industries served. Ethanol Producer Magazine attended this year and gives a taste of what was learned, with a focus on areas with a big impact on the bottom line in the initial steps of the process.
A Closer Look at Corn
“If you want to save money, improve your bottom line in the biggest cost—feedstock,” said Robert Piggot, technical consultant with Lallemand Ethanol Technology, a co-sponsor of the event with the Ethanol Technology Institute. When buying corn, ethanol producers are most interested in the starch. Nonetheless, a survey of ethanol producers once asked if they would pay more for corn if they could get 3 percent more starch. “Most ethanol plants said no,” he said. For a 100 MMgy plant, that 3 percent would add up to $13 million more revenue, if ethanol were selling at $2.50 per gallon, he pointed out.
There can actually be a conflict in the goals between the purchasing department, which is looking for low cost, large quantities and flexible payment terms, and operations, which needs easy-to-process, high yielding corn to be delivered on a timely basis. Operations is looking for high starch content, low moisture, high ratios of amylopectin-to-amylose and floury-endosperm-to-horny-endosperm, and no molds or mycotoxins, which means the standard grade of No. 2 yellow dent corn isn’t all that helpful. “We are stuck with the specifications that were meant for the baking and feed industries, and they actually don’t mean anything to us,” Piggot said. Broken kernels, for instance, could be considered beneficial for the ethanol industry, while foreign material wouldn’t. Thus the maximum 3 percent BKFM is not specific enough. No. 2 specs also call for a 5 percent maximum in damaged kernels, with no more than 0.2 percent heat damaged. “If the grain is heat damaged, you lose sugars,” he explained. “Other types of damage are not a problem.” The moisture spec of 14.5 percent is well-understood, but the test weight spec of 54 or 56 pounds per bushel is meaningless, he said. “You often get better yields out of low test weight corn.”
Piggot recommends plants make their expectations clear when explaining their quality needs and setting discounts. “You’re better off to discount the grain,” he added. “Try not to get into the loop of getting a rejected load returned to you, just blended up.” Discounts need to consider hidden and indirect costs. High moisture corn, just 1 percent over the 14.5 percent spec, would translate into a direct cost of 50 gallons of ethanol lost from the load, which at $2.50 per gallon ethanol, would call for a discount of 16 cents per bushel. He recommends that number be doubled to account for other increased costs. For example, the electrical power needed to grind wet corn will be significantly higher than for properly dried corn.
Attention to details in grinding corn is the next area that can have a direct impact on yield. The ideal grind size is very much plant-dependent, Piggot said. While there are advantages to smaller grind sizes that increase the surface area exposed to enzyme action, problems can arise. Factors to consider include how well the slurry mixes and temperature parameters. Coarser grinds need slightly higher temperatures while finer grinds are needed if jet cooking is not used. Finer grinds will keep suspended longer in the fermentor, but will contribute to quicker fouling in the stripper and heat exchangers, plus impact centrifuge separation. The goal is to get the best compromise of particle size for maximum yield and good separation—too large and yields are lost, too small can increase solids in the stillage and backset.
Most ethanol plants use hammermills to grind, and many never take a close look at the configuration, Piggot added. A number of things can be adjusted to improve performance, including the speed and number of hammers as well as spacing, plus the open area on the screen, feed rate, air flow and hammer-to-screen distance.
The next step in the process, mashing, is also a key area for maximizing yields, said Garth Whiddon, technical service manager for Lallemand Ethanol Technology. In mashing, water is combined with the crushed or ground grain, adjusted for pH and temperature to match the chosen enzymes used to break the starches down into dextrins. “Having the optimal conditions for enzymatic efficiency leads to lower usage rates and higher yields,” he said. Fine tuning this step is important. If 4.5 percent residual starch is left after fermentation, it adds up, amounting to $2.6 million lost for a 50 MMgy plant.
The industry has gone through some major process changes, Whiddon added. At one time, nearly every plant used a separate saccharification tank, where the mash was cooled before adding glucoamylase. While it may have been optimal for the ideal enzyme dose, it also created a perfect environment for bacteria. Most plants have now moved to simultaneous saccharification and fermentation.
A more recent process change has been a move towards dropping the jet cooking step. In addition to decreasing enzyme use by nearly 25 percent, eliminating jet cooking prevents a possible 3 to 5 percent yield loss from a Maillard reaction—a chemical reaction that makes some sugars unfermentable and also reduces the free amino acids needed for yeast health. If jet cooking is eliminated, however, the grind requirements are more stringent, he adds, to ensure proper starch conversion.
Understanding the Workhorse
More than one Alcohol School instructor spoke about the importance of yeast and pointed out that proper management would enhance yield and therefore profit. The longtime scientific director of the Ethanol Technology Institute, Mike Ingledew, and the new director, Graeme Walker, both dug into the details of keeping yeast working at peak efficiency. Ingledew is professor emeritus at the University of Saskatchewan and Walker is a professor and director of the Abertay University Yeast Research Group in Scotland.
“Saccharomyces cerevisiae is the number one industrial organism, making more money than any other microbe,” Walker said. Keeping yeast cells viable and actively growing is essential for consistent fermentations. “Growing yeast cells produce alcohol 33 times faster than nongrowing cells,” he explained, adding that the rate of budding parallels ethanol production. An expert on yeast physiology, he described the micro-organism’s needs in detail, including factors such as oxygen, pH, temperature, moisture, nutrients, physical factors such as agitation and pressure as well as stress factors that include inhibitors and contaminants. While glycerol plays a role in yeast metabolism, overproduction reduces ethanol yield and is a key indicator of stress. “The theoretical maximum yield of ethanol from 100 parts of glucose is 51.1 percent,” he adds. “You’d never get 100 percent—a living organism needs to use sugar to grow—but industrial alcohol producers should aim for greater than 90 percent of this theoretical yield.”
Very high gravity fermentations lead to higher concentrations of alcohol than what was previously considered possible, Ingledew explained. Some plants are getting 15 to 16 percent ethanol volume to volume (v/v), and many are achieving up to 20 percent, compared to the traditional expectation of 7 to 12 percent.
Paying close attention to yeast nutrition is a major part of achieving these high alcohol levels—beginning with water. Attention must be paid to water quality when recycling various process flows and even with well water, he cautioned. “They can contain significant amounts of ions, solvents and organic acids that are harmful to yeast viability.” Proper aeration is important and the timing of oxygen supply can be more critical than the amount. “Many plants overaerate,” he added, “and many poorly aerate.”
“Useable nitrogen is the most critical limiting nutrient,” he said. “It makes the most difference of all nutrients added to fermentors with regard to yeast growth.” Nitrogen in yeast cells is normally found at concentrations of 6 to 8 percent weight to weight (w/w) and phosphorus is around 1.4 to 2 percent. The amounts of useable nutrients needed, therefore, relate to the overall biomass weight that will be made in the fermentor, which can amount to over 5 tons of yeast biomass in a 250,000 gallon fermentor. “If your mash is purified, deficient, or you don’t know what’s in the mash, add nutrients,” he said. If using urea, he recommended using 0.48 grams per liter (g/L) for normal gravity mashes and doubling that for very high gravity mashes. Ammonia’s higher nitrogen content calls for 0.134 g/L for normal mashes and double that for very high gravity mashes, which will require a pH adjustment. Phytases may be useful to liberate useable phosphorus in grain mashes, he added, and most of the other minerals and vitamins are generally supplied by the mash, although yeast foods provide insurance and lead to consistency. “Nutrients will vary from crop to crop, from farm to farm and day by day,” he explained.
Author: Susanne Retka Schill
Contributions Editor, Ethanol Producer Magazine
More on Fermentation
A 2 or 3 percent yield increase would help any plant. Walker gave Alcohol School attendees a checklist of important points for optimal yeast management:
*Minimize microbial contaminants, especially lactic bacteria.
*Optimize yeast nutrition, especially nitrogen and minerals.
*Minimize yeast stress and remove inhibitors.
*Precondition yeast and use nonflocculent strains. Flocculation is appropriate for some brewers because near the end of fermentation, the single yeast cells begin clumping and drop to the bottom of the fermentor, leaving clear beer.
*Reduce glycerol and other secondary yeast metabolites.
*Use correct process parameters, including temperature, pH, pressure and oxygen.
*Use efficient distillation and correct downstream technologies to eliminate known losses of ethanol.
*Consider very high gravity fermentations.
Switching to high gravity fermentation requires both time and some re-engineering, Ingledew said. He listed key differences and adjustments required:
*Prepare mashes with increasingly high solids and less water.
*Remove solids with a rinse prior to fermentation, if possible, as is done in brewing.
*Supply sterile oxygen to fermentation when the yeast are actively growing, with a goal of 5 parts per million per liter per hour for strong yeast cell membranes.
*Supply enough useable nitrogen as it stimulates fermentation rate, increases yeast cell numbers and improves consistency.
*Supply other nutrients as needed.
*Gelatinize and liquefy but don’t saccharify starch in mash before fermentation.
*Ensure that pumps can handle the higher viscosity.
*Use a yeast strain which tolerates alcohol well and thrives in high sugar media.
*Use simultaneous saccharification and fermentation.
*Work up slowly in mash specific gravity to ensure that problems don’t occur.
*Condition yeast by preparing it in lower gravity mashes before inoculating the very high gravity mash.
*Do not reuse yeast.
*Keep the fermentor temperature down or use temperature staging.
*Keep the mash free of bacterial contamination and minimize stresses.