Corn fiber as a raw material for hemicellulose and ethanol production

Com fiber, a byproduct of the wet milling process, has shown to be a substrate of particular interest due to its abundance, ready availability and low value. Corn fiber is primarily composed of the outer seed covering or pericarp of the kernel, along with adherent starch with typicallyl 20% xylose and 10% arabinose in the form of arabinoxylan, 18-24% cellulose and 20% starch although its apparent composition varies considerably according to its source and the method of analysis. As estimate, ethanol yields from com could be increased by approximately 10% if the constituent sugars of corn fiber could be efficiently utilized. Since the xylan in seed fiber is highly branched with arabinose and may cross-links with phenolic acids, GH 10 xylanase and ferulic acid esterases are suggested to supplement for enzymatic hydrolysis.

The first engineered trait designed for the ethanol industry

The news: A corn called Enogen,  the first crops genetically engineered to contain a hydrolytic trait that has been approved or commercial growing by the Department of Agriculture. The crop with this self-hydrolysis trait will increase ethanol output while reducing the use of water, energy and chemicals in the production process.

Heterogeneous azeotropic distillation with n-hexane

Heterogeneous azeotropic distillation is a widely used technique for separating binary azeotropic mixtures into their components. To produce pure ethanol,  heterogeneous azeotropic distillation using hexane instead of benzen as the entrainer has been developed recently because  n-hexane is a common compound found in gasoline and any trace amount of hexane in the anhydrous ethanol will not be a problem for its subsequent use as a fuel. The composition of the ternary azeotrope determined by numerical interpolation was reported as  0.105, 0.236 and 0.658 mole fraction of water, ethanol, and n-hexane, respectively, and the temperature is 329.21 K.

The heterogeneous azeotropic distillation with n-hexane can produce pure ethano with relatively low energy input.

The opportunities of enzyme expressed grain

Successful expression of enzymes in corn grains will bring the following opportunities for bioethanol production in low cost:

1. Recover suars from DDGs corn fibers by enzymatic hydrolysis using extracted proteins from exzyme expressed corn grains.

2. Mix exzyme expressed grains with pretreated biomass to reduce external hydrolytic enzymes.

Currently, the technology for such a expression is avaible; therefore it is expected such cost effective applications will integrate into bioethanol production.

Vacuum fermentation and distillation

It is known that high ethanol concentrations (10% v/v) were inhibitory to the industrial yeast strains and would reduce the yeast growth and cell density in the high solids mash. However, high slurry solids saccharification and fermentation are right direction for cost effective production of fuel ethanol. To overecome the challenge, vacuum feremtation and vacuum distillation can be applied to remove ethanol produced during SSF. As a result,the ethanol concentration was maintained as negligible during the entire fermentation process, which can lead to high ethanol productivity.Combining hydrolytic enzyme hydrolysis and SSF under a vacuum allows the integration of all four unit operations (liquefaction, saccharification, fermentation and distillation) into one single step, which eliminates both substrate (glucose) and product (ethanol) inhibition of the yeast and allow very high slurry solids (~40%) ssacharification and fermentation.

The higher slurry solids and removal of water during vacuum distillation will also result in higher percentage of stillage solids at the end of the fermentation, which can be sold directly as wet grains without any need for centrifugation and thin stillage evaporation to remove water, therefore to reduce operational cost to a great extent.

Corn Fractionation

The germ, pericarp fiber, and endosperm fiber can be recovered as coproducts in addition to ethanol and DDGS through dry-grinding ethanol process or a modified dry-grind process (E-Mill)

Removal of the suspended solids improves both the fermentation rate and ethanol productivity because such solids interfere with the enzyme kinetics, heat transfer and mixing of the mash. In addition, other benefits can be obtained as follows:

   • Recovery of germ as valuable coproducts that can increase plant capacity and protein content of DDGS

   • Recovery of coarse fiber as valuable coproducts that can be used for the production of corn fiber oil,corn fiber gum, as well as corn ethanol, resulting in an increase in capacity of plant,protein content of DDGS, and reduction in fiber content of DDGS.

  • In Enzymatic Milling Process, recovery of pericarp and endosperm fiber as valuable coproducts can increase plant capacity and protein content of DDGS as well as reduce significantly fiber content in DDGS. In addition, the use of GSH enzymes during E-milling can results in a synergy effects, i.e. improving the fractionation process and converting starch into dextrins and sugars for fermentation.

Granular Starch Hydrolysis

Granular Starch Hydrolysis with external enzymes or corn with expressed enzymes:


• Use granular starch-hydrolyzing (GSH) enzymes (e.g., Stargen from Genencor or BPX from Novozymes) to convert starch into dextrins at temperatures lower than 48 C and hydrolyze dextrins into fermentable sugars during SSF


• Not require heating of the corn slurry to high temperatures for cooking or liquefaction; therefore, GSH enzymes reduce the overall utility requirements of the dry grind process.


• With GSH enzyme, the liquefaction, saccharification, and fermentation steps can all be combined into one single step. Compared to conventional enzyme treatments,
      o No increase in viscosity of the corn slurry that occurs during gelatinization and cooking; Therefore, higher concentrations of solids can be used in corn slurries, which allows the fermentation to reach increased final ethanol concentrations
     o Glucose concentrations with GSH enzymes are typically lower but the final ethanol concentrations and ethanol yields remain similar.
     o glycerol concentrations are lower for GSH treatment so that yeast cells are thus subjected to a low osmotic stress; thus improving the overall productivity by lowering the production of glycerol


• The cost of GSH enzymes is approximately double that of conventional enzymes; but overall material, capital, and operational cost may be advantageous.


In addition, if using enzyme expressed corn seeds as feedstocks, the cost of enzyme will be signifcantly low.

High Gravity Fermentation

A high final ethanol concentration during ethanol production will improve the plant profitability by reducing water usage, increasing the plant capacity, and decreasing the downstream processing costs, thereby improving plant efficiency. High final ethanol concentrations (i.e., >14 %) can be achieved by high-gravity fermentation. At a high initial dry solids content (~33 %), it is particularly important to ensure that the nutritional needs of the yeast are appropriately met as a preventive measure to avoid stuck or sluggish fermentations.

It has been found that temperature staging (33 C during early stage and 28 C or lower during later stages) is also important to complete a high-gravity mash; The addition of proteases, urea, Mg2+, yeast extracts, and amino acids, have all been reported to provide useful nutrients to yeasts, thereby increasing fermentation rates and final ethanol concentrations in high-gravity mashes.

Dry-mill corn ethanol process

 In a conventional dry-grind process, corn is ground and mixed with water to produce slurry. The slurry is cooked; starch in the slurry is liquefied, saccharified, and fermented to produce ethanol. The remaining nonfermentables in corn (germ, fiber, and protein) are recovered together at the end of the dry-grind process as an animal food coproduct called distiller dried grains with solubles(DDGS). Due to its lower capital and production investments than wet-mills, it is still the dominant process for corn ethanol production nowadays. Conventionally, the process involves the following steps:

Milling. The feedstock (Corn) passes through a hammer mill to break down the corn kernel into a fine powder.

Cooking. The fine powder flour is mixed with water to obtain ground corn slurry with ~27-37% solids content. The pH of the ground corn slurry can be adjusted with anhydrous ammonia to stabilize within the range 5.5 to 6.5. The slurry is then heated to 85 C (185_F) for 30-45 min, and subsequently cooked with high pressure steam in a jet cooker at 104 C (220 _F). After cooling to 85 C (185 F) and a holding period of 30–45 min, the slurry is reheated and cooked to gelatinize the starch and break down its crystalline structure. The resulting mixture of amorphous starch is called the ‘mash.’

Liquefaction. A unit operation involving an enzymatic digestion of the starch molecules in the mash into imonoeric sugar and oligosaccharides. In this step, alpha-amylase (an endoenzyme ) applied randomly hydrolyzes a-1,4-glucosidic bonds to reduce the viscosity of gelatinized starch, producing soluble dextrins and oligosaccharides.

       • The total alpha-amylase dose:0.2 to 0.4 kg enzyme per metric ton ofvdry solids (0.02–0.04 %, w/w).

       • The common practice: add one-third of the alpha-amylase prior to cooking and the remainder after the jet cooker step. The resulting liquefied corn mash has a typical composition as: maltotetraose and other soluble dextrins 24–35%; maltotriose 1.7–3.3%; maltose 0.4–2.7%; and glucose 0.1–1.3 %; i.e. a starch hydrolysate that has a dextrose equivalent ranging from 10.4 to 19.1 (The dextrose equivalent is a measure of the percentages of glucosidic bonds that are hydrolyzed during liquefaction.)

Saccharification. After the corn mash cools to 32 C, a second enzyme, glucoamylase (,an exoenzyme) is added (0.05–0.08 %, w/w) to catalyze the release of successive glucose units from the non reducing ends of soluble dextrins by hydrolyzing both linear a-1,4-glucosidic and branched a-1,6-glucosidic linkages, resulting in the fermentable sugars, mainly glucose.

Fermentation. The glucose generated by glucoamylase during saccharification step will be fermented into ethanol by yeasts (typically Saccharomyces cerevisiae) and carbondioxide as a by-product. Saccharomyces cerevisiae is the yeast species commonly selected because of its quick, efficient production of alcohol and its ability to withstand heat, osmotic stress and high alcohol concentrations. The fermentation process generally takes about 50 to 60 hours with a final ethanol concentrations of14%- 20% (v/v) depending on the corn slurry solids contents from improved fermentation process. Batch or continuous fermentation systems may be used, although batch processing is more common due to its simple, easily cleaning, and flexibility. Some new fermentation systems are designed to minimize dilution water, which reduces the evaporation requirements in the feed processing stages after fermentation.

Distillation. Distillation is the process of separating the ethanol from the solids and water in the mash. The fermented mash, now called “beer”, is pumped to the continuous flow, multi-column distillation system where the ethanol is removed from the solids and the water. Conventional distillation/rectification methods can produce 95% pure (190 proof) ethanol because the ethanol and water form an azeotrope so that further separation by heat cannot occur. Therefore, the ethanol leaves the top of the final column in the distillation syetem at about 95% concentration, and the residue mash (stillage), is transferred from the base of the column to the co-product processing area.

Dehydration. The ethanol from the top of the column passes through a dehydration system to remove remaining water to obtain anhydrous ethanol (pure, without water, and approximately 200 proof). Most modern dry-grind ethanol plants use a molecular sieve system to produce pure ethanol.

Stillage Processing. The solid and liquid fraction remaining after distillation is referred to as “whole stillage”, which includes fiber, oil and protein components of the grain, and the non-fermented starch.

      The “thin stillage” is first separated from the insoluble solid fraction using centrifuges or presses/extruders and then sent to evaporator units to remove excess water to obtain the thick, viscous syrup, which is mixed back with the solids to create a feed product known as Wet Distillers Grains with Solubles (WDGS) with about 65% moisture.

     Due to its low shelf life and high transportation cost, WDGS is usually dried to 10 to 12% moisture to produce a product known as Dried Distillers Grain with Solubles (DDGS). Although drying distillers grains is energy-intensive, consuming about one-third of the energy requirements of the entire dry-grind plant, , it is essential to produce a uniform, stable, high-quality feed co-product to the profitability of the plant, resulting in most plants producing DDGS rather than WDGS.

Typical Yield from a Bushel of Corn from a Dry-Mill Ethanol Plant

• 2.7 gallons of ethanol
• 17.5 pounds of distillers dried grains
• 17 pounds of carbon dioxide

One step liquefaction,saccharification and fermentation in dry-mill corn ethanol process

    In the typical dry-grind process corn ethanol production, high sugar concentration often occurs at SHF process as well as the initial phase of SSF process, which cause the mash to have a high osmotic pressure resulted in the production during the fermentation process of higher concentrations of glycerol as glycerol aids yeast with osmoadaptation. As a result, the osmotic stress negatively affects yeast performance. One of the technological improvements is to use granular starch-hydrolyzing enzymes during corn liquefaction and saccharification.

Granular starch-hydrolyzing (GSH) enzymes can convert starch into dextrins at temperatures lower than 48 C and hydrolyze dextrins into fermentable sugars during SSF. Therefore, there is no need to heat corn slurry to a higher temperature for liquefaction in the dry-grind process, which allows liquefaction, saccharification, and fermentation steps to be combined into one single step. The advantages are found as follows:

• avoid the increase in viscosity of the corn slurry that occurs during gelatinization and cooking in the conventional process because the rate of glucose production by GSH enzymes parallels the rate of glucose fermentation by yeast
•  yeast cells are subjected to a low osmotic stress due to the low production of glycerol 
•  Low glucose concentration but similar final ethanol concentrations and ethanol yields