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

Inhibitors

Inhibitors based on their chemical functional groups can be grouped as aldehydes, ketones, phenols, and organic acids.

  • Aldehyde inhibitors: compounds with one or more functional aldehyde groups, regardless of the base structure of a furan ring, a benzene ring or a phenol-related structure, including
      o furfural and HMF
           Cell walls and membranes of yeast cells grown under furfural and HMF-challenged conditions appear damaged when compared to those of controls grown in the absence of any inhibitor, resulting in the delay of cell growth and reduction of ethanol productivity
           Furfural and HMF inhibit cell growth and ethanol production rates at lower concentrations. Individual strains have been isolated that retain their ability to produce ethanol in the presence of 10 to 79 mM of either furfural or HMF, including strains of these species: Saccharomyces cerevisiae, Pichia stipitis, Candida shehatae, Corynebacterium glutamicum, Zymomonas mobilis, and Escherichia coli.
      o 4-hydroxybenzaldehyde, vanillin, syringaldehyde, isovanillin,ortho-vanillin, Cinnamaldehyde
         more inhibitory than those derived from sugar dehydration

• Ketone inhibitors:4-hydroxyacetopheone, acetovanillone and acetocsyringone
         exert a greater inhibitory effect on bacteria such as Thermoanaerobacter mathranii than on yeasts, in terms of reduced growth and ethanol yield

• Organic acid inhibitors: sharing a common carboxylic acid functional group, all contain a carboxyl functional group such as
       o acetic acid,formic acid, levulinic acid, caproic acid,furoic acid, 4-hydroxybenzoic acid, 3-hydroxybenzoic acid, 2-hydroxybenzoic acid, 2,5-dihydroxybenzoic acid, protocatechic acid, vanillic acid, gallic acid, syringic acid, 4-hydroxycinnamic acid, homovanillic acid, guaiaclyglycolic acid, and sinapic acid. These inhibitors are thought to be exert their inhibitory actions via their carboxyl functional groups.
             more toxic to isolates of bacteria than yeasts at low concentration
             The toxicity has been correlated with their degree of hydrophobicity, suggesting the involvement of a hydrophobic target such as the cell membrane

• phenol-based inhibitors: phenol, benzene-1,2-diol (catechol) etc.
      o cause increased membrane fluidity and affect membrane permeability, which may enhance synergistic inhibition when combined.
      o Phenols such as cathecol, hydroquinone, and coniferyl alcohol almost completely inhibit E. coli , but are relatively less toxic to yeast. However, eugenol and isoeugenol are inhibitory to yeasts at low concentrations.
     o The three main phenol structure building blocks in lignin show the order of inhibitory effects, ranked from strong to weak, as (1) hydroxyphenol, (2) guaiacyl, and (3) syringyl

In general, aldehydes and phenols are more toxic than organic acids.

In addition, some inhibitions will be amplified through process. A typical example is that fermentative microorganisms will be inhibited by concentrated non-volatiles such as lignin derivatives and extractives after concentrating although low initial inhibitor concentration in hydrolyzate.