Processing of Cereal Crops

The main stages prior to fermentation for processing of starch-based materials are Cereal cooking; Starch liquefaction, and Amylolysis. In North America, ethanol is produced from corn by using one of two standard processes: wet milling or dry milling (Yacobucci and Womach 2003; Ferguson 2003) (Figs. 3.8, 3.9). The main dif­ference between the two is in the initial treatment of the grain. Dry milling plants cost less to build and produce higher yields of ethanol (2.7 gallons per bushel of corn),



Подпись: CORN
Подпись: Grinding Serening
Подпись: Starch Gluten Separation
Подпись: Starch
Подпись: ■MB
Подпись: Separation
Подпись: FBM
Подпись: Wet Gluten
Подпись: Drying
Подпись: Fermentation
Подпись: Syrup

image028Fig. 3.9 The ethanol production process—dry milling. http://www. ethanolrfa. org/pages/how- ethanol-is-made. Reproduced with permission

but the value of the co-products is less. The value of corn as a feedstock for ethanol production is due to the large amount of carbohydrates specifically starch present in corn. In wet milling, maize kernels are soaked in water (or dilute acid) to separate the cereal into starch, gluten, protein, oil, and fiber prior to starch conversion to ethanol.

The wet-milling operation is more elaborate because the grain must be sepa­rated into its components. After milling, the corn is heated in a solution of water

and sulfur dioxide for 24-48 h to loosen the germ and the hull fiber. The germ is then removed from the kernel, and corn oil is extracted from the germ. The remain­ing germ meal is added to the hulls and fiber to form corn gluten feed. A high — protein portion of the kernel called gluten is separated and becomes corn gluten meal, which is used for animal feed. In wet milling, only the starch is fermented, unlike dry milling, when the entire mash is fermented. In dry milling, from which most US bioethanol is made, maize kernels are finely ground and processed with­out fractionation into component parts (O’Brien and Woolverton 2009).

Starch-bioethanol (from US maize) currently dominates global fuel alcohol production, but the projected use of maize for ethanol production is expected to level-off (at around 6 billion bushels) unless “idle” land can be used to grow more cereal for production of biofuels (Abbas 2010). The principal stages in dry mill bioethanol processes are:

1. Milling (maize kernels ground to a fine powder or meal)

2. Liquefaction (water is added to the maize meal and temperature increased in the mash to solubilize starch)

3. Saccharification (enzymatic hydrolysis of starch liberates simple sugars, mainly glucose)

4. Fermentation (starch hydrolyzate is fermented by yeast to ethanol, CO2, and secondary metabolites)

5. Distillation (the fermented wash, or beer, at around 10 % v/v ethanol is distilled to ~96 % v/v ethanol with the solid residues processed into animal feed)

6. Dehydration (water remaining in the ethanolic distillate is removed by molecu­lar sieves to produce anhydrous ethanol).

After the corn (or other grain or biomass) is cleaned, it passes first through hammer mills which grind it into a fine powder. The meal is then mixed with water and an enzyme (alpha amylase), and passes through cookers where the starch is liquefied. A pH of 7 is maintained by adding sulfuric acid or sodium hydroxide. Heat is applied to enable liquefaction. Cookers with a high temper­ature stage (120-150 °С) and a lower temperature holding period (95 °С) are used. The high temperatures reduce bacteria levels in the mash. The mash from the cookers is cooled, and the enzyme glucoamylase is added to convert starch molecules to fermentable sugars (dextrose). Yeast is added to the mash to fer­ment the sugars to ethanol and carbon dioxide. Using a continuous process, the fermenting mash flows through several fermenters until the mash is fully fer­mented and leaves the tank. In a batch fermentation process, the mash stays in one fermenter for about 48 h. The fermented mash, now called “beer,” contains about 10 % alcohol, as well as all the non-fermentable solids from the corn and the yeast cells. The mash is then pumped to the continuous flow, multi-column distillation system where the alcohol is removed from the solids and water. The alcohol leaves the top of the final column at about 96 % strength, and the resi­due mash, called stillage, is transferred from the base of the column to the co­product processing area. The stillage is sent through a centrifuge that separates the coarse grain from the solubles. The solubles are then concentrated to about 30 % solids by evaporation, resulting in Condensed Distillers Solubles (CDS) or “syrup.” The coarse grain and the syrup are then dried together to produce dried distillers grains with solubles (DDGS), a high-quality, nutritious livestock feed. The CO2 released during fermentation is captured and sold for use in car­bonating soft drinks and beverages and the manufacture of dry ice. Drying the distillers grain accounts for about 1/3 of the plants energy usage. The alcohol then passes through a dehydration system where the remaining water is removed. Most plants use a molecular sieve to capture the last bit of water in the ethanol. The alcohol at this stage is called anhydrous (pure, without water) ethanol and is approximately 200 proof. Ethanol that is used for fuel is then denatured with a small amount (2-5 %) of some product, like gasoline, to make it unfit for human consumption.

Starch is an alpha-polysaccharide comprising D-glucose monomers arranged in two basic formats: amylose and amylopectin (Fig. 3.10a, b), and plant starches generally contain 10-25 % amylose and 75-90 % amylopectin (depending on the biomass source). Industrial enzymes used as processing aids in starch-to-eth — anol bioconversions are produced by microorganisms (bacteria such as Bacillus


Fig. 3.10 a Structure of amylase. b Structure of amylopectin. Based on http://www. gtconsult. com. br/ingles/artigos/What_is_Starch. pdf, http://www. scientificpsychic. com/fitness/carbohydrates1.html

spp. and fungi such as Aspergillus spp.) grown in closed fermentation tanks by specialist companies (e. g., Novozymes, Genencor). The industrial production and purification of amylolytic enzymes for bioethanol production have been dis­cussed by Nair et al. (2008). In order for starch to be converted to ethanol by yeast (S. cerevisiae), it has to be de-polymerized to constituent saccharides such as glucose and maltose. In traditional beverage fermentation industries such as brewing, this is partially accomplished using endogenous enzymes, mainly alpha and beta amylases, present in malted barley. However, for bioethanol production, more complete starch hydrolysis is required and this is conducted using exogenous (microbially derived) amylolytic enzymes including de-branching enzymes such as amyloglucosidase (or glucoamylase).

The production of ethanol is an example of how science, technology, agricul­ture, and allied industries must work in harmony to change a farm product into a fuel. Ethanol plants receive the large quantities of corn they need by truck, rail, or barge. The corn is cleaned, ground, and blown into large tanks where it is mixed into a slurry of corn meal and water. Enzymes are added, and exact acidity levels and temperatures are maintained, causing the starch in the corn to break down— first into complex sugars and then into simple sugars.

New technologies have changed the fermentation process. In the beginning, it took several days for the yeast to work in each batch. A new, faster, and less costly method of continuous fermentation has been developed. Plant scientists and genet­icists are also involved. They have been successful in developing strains of yeast that can convert greater percentages of starch to ethanol. Scientists are also devel­oping enzymes that will convert the complex sugars in biomass materials to etha­nol. Cornstalks, wheat and rice straw, forestry wastes, and switchgrass all show promise as future sources of ethanol.

In modern ethanol production, for every bushel of corn that is processed, one — third is returned to the livestock feed market. That is because ethanol production requires only the starch portion of a corn kernel. The remaining protein, fat, fiber, and other nutrients are returned to the global livestock and poultry feed markets (RFA 2007a). Thus, every bushel of corn processed by an ethanol plant produces 2.8 gallons of ethanol—and approximately 17 pounds of animal feed. This high- quality feed for cattle, poultry, and pigs is not a by-product of ethanol production; it is a co-product. During 2012 alone, the U. S. ethanol industry used 4.5 billion bushels of corn to produce an estimated 13.3 billion gallons of ethanol and 34.4 million metric tons of high-quality livestock feed. This includes 31.6 million metric tons of distillers grains and 2.8 million tons of corn gluten feed and meal (Fig. 3.11) (RFA 2013).

This level of output will make it necessary to find new markets and uses for co­products. New uses being considered include food, fertilizer, and cat litter. While the majority of feed is dried and sold as Distillers Dried Grains with Solubles (DDGS), approximately 20-25 % is fed wet locally, reducing energy costs asso­ciated with drying as well as transportation costs. Ethanol wet mills produced approximately 430,000 metric tons of corn gluten meal, 2.4 million metric tons of corn gluten feed and germ meal, and 565 million pounds of corn oil. Figure 3.12 shows production of US ethanol feed co-products (RFA 2013).



Fig. 3.11 Distillers grain consumption by species* (estimated). RFA (2013). Reproduced with permission



Fig. 3.12 Production of US ethanol feed co-products. RFA (2013). Reproduced with permission


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