Evolving Corn Coproducts: Utilizing Fiber Fraction

Utilization of the fiber portion of a corn kernel may be the next big thing, writes Kurt A. Rosentrater of Distillers Grains Technology Council.
By Kurt A. Rosentrater | February 10, 2015

The biofuels story is not yet completely written. Many exciting developments are on the horizon and some are actually within our grasp. Indeed, cellulosic ethanol is here and is being commercialized. But, first-generation, dry-grind plants are continuing to operate, increasing their operational efficiencies and evolving over time. Indeed, the industry isn’t standing still. Engineers and scientists at new startups, as well as long-established companies, continue to find ways to extract more uses and value from both the upstream kernels of corn and the downstream coproduct. These include fiber, proteins and other nutrients. Which of these new technologies will become commercial reality?  Or, if they are already commercialized, which will be successful in the long term? It is difficult to predict the future.

Utilization of fiber may indeed be the next big thing. Fiber is an effective dietary component in ruminant diets as they are nature’s ready-made cellulosic processing factories. For monogastrics, however, corn fiber has historically not been a key source of energy, as it is relatively indigestible. Recent swine and poultry research, however, has found that corn fiber in dried distillers grains with soluables (DDGS) plays a greater role as a dietary energy source than previously thought. From a downstream processing point of view, fiber is not terribly useful either. It absorbs water, is difficult to grind and it does not bind well with other constituents during processing operations such as pelleting or extrusion. But, corn fiber (or DDGS fiber) is a ready source of cellulose at first-gen ethanol plants. Thus, utilizing this fiber may be the next stepping-stone on the road to cellulosic ethanol. And, if converted into ethanol, corn fiber qualifies for cellulosic RINs (renewable identification numbers used to show compliance with the renewable fuels standard).

So, how do we separate fiber for further use, cellulosic ethanol or otherwise? Fiber separation can be a tricky business. There has been a long history of researchers who have attempted to fractionate corn fiber, including from the corn itself in pre-fermentation processes, from the DDGS, postfermentation, or from other coproducts upstream from the DDGS. Which is the best approach for separation? It all boils down to costs, returns and payback.

In terms of fiber fractionation from the DDGS, there are a few key mechanisms that are commonly used. These include by particle size and shape through screening or sieving or by particle weight or density using air aspiration, for example. Combinations of these can also be used, gravity tables being a common example. All of these approaches are relatively inexpensive, and have been extensively tested since the early 1980s, with varying degrees of success. Major differences in these approaches include the sequence of operations, geometries used in the equipment, shaking speeds, air speeds and orientations or configurations of the equipment. Fractionation efficiencies have been shown to increase when the DDGS was milled prior to the separation process, because this makes particle densities more consistent, which will thus allow protein-rich particles to be better separated from fiber-rich materials based on density differences. But this approach does increase the cost of fractionation.

Some have examined wet fractionation of fiber from the whole and thin stillage wet streams prior to the drier. These wet fractionation techniques have not gained popularity in the dry grind industry, though, because they are more expensive than working directly with the dry products.
Unfortunately, it appears that highly efficient fractionation of fiber from the wet or dry coproducts can often be difficult to accomplish. This is predominantly the case because most ethanol plants use hammer mills to grind their corn, which leads to a fairly wide dispersion in ground corn particle sizes going into fermentation. This variability is compounded during drying, when the condensed distillers solubles bind various corn kernel components together into agglomerations as shown in Figure 2. Postfermentation-dissolved constituents, such as proteins, lipids, minerals, etc., solidify on the solid particles during drying, adding a layer of complexity to an already variable material, as shown in Figure 1.

In recent years, a promising work-around solution has been the treatment and fermentation of the fiber into ethanol without separation from either the DDGS or the wet coproduct streams. Keys to this approach are the enzymes that are used to break the fiber into fermentable substrates, and the microbes are then used to ferment the substrates. In this approach, after fermentation the nonfermentable components are then converted into a DDGS product, much like traditional dry grind plants. Initial research on these new coproducts indicates performance similar to that achieved with traditional DDGS.

Upstream fiber fractionation from the corn kernels themselves will be a topic for a future article.

If you have any questions or need any assistance with your DDGS or other coproducts, please feel free to contact us. We look forward to helping you as the industry continues to evolve.
Author: Kurt A. Rosentrater
Executive Director, Distillers Grains Technology Council
Iowa State University