A whole stillage sieving process to recover fiber for cellulosic ethanol production
Introduction
In a conventional dry grind ethanol process, corn is milled, liquefied, and simultaneously saccharified and fermented to produce ethanol. Ethanol is distilled, and underflow from the distillation column is called whole stillage (non-fermentable components). Whole stillage consists primarily of water, fiber, protein, oil, unconverted starch and dead yeast cells. Whole stillage is centrifuged to produce thin stillage (supernatant) and wet cake (suspended solid). Thin stillage is further evaporated and concentrated to syrup. The wet cake and the syrup are blended and dried to produce distillers dried grains with solubles (DDGS).
The conventional dry grind process converts storage carbohydrate (starch) in corn kernels into ethanol. However, to increase ethanol yields, much research has been focused on utilization of structural carbohydrates from pericarp (coarse) fiber and endosperm (fine) fiber (Dien et al., 2005a). Corn fiber is composed of 11–23% starch, 12–18% cellulose, 18–28% xylan, 11–19% arabinan, 11–12% protein and 2% oil on a dry basis (Leathers, 1998). About 2.04 kg (4.5 lb) of corn fiber is obtained from 25.4 kg of corn (56 lb) with theoretical ethanol yield of 1.14 L (0.3 gal) (Saha et al., 1998). Another advantage of recovering fiber in the dry grind process is that it improves the nutritional value of DDGS by increasing protein, amino acids and fat contents (Martinez-Amezcua et al., 2007). Due to high fiber contents, DDGS produced from conventional dry grind plants is mainly used as an ingredient in ruminant animal diets. However, DDGS with decreased fiber content would be suitable for nonruminant animals (Rausch and Belyea, 2006).
Several processes have been developed to recover fiber in the dry grind ethanol process. Fiber can be separated by wet fractionation (enzymatic milling (E-Mill) corn process) (Singh et al., 2005), dry fractionation (quick fiber process) (Singh et al., 1999) and elusieve process (Srinivasan et al., 2005). In the E-Mill process, germ, pericarp fiber and endosperm fiber are recovered before liquefaction. Corn kernels are soaked in water for 6–12 h followed by coarse grinding. Ground samples are incubated with protease and starch degrading enzymes for 2–4 h. Then, germ and coarse fiber are recovered by flotation. The remaining slurry is screened using a 200-mesh screen (74 μm openings), and endosperm fiber is recovered on the screen (Singh et al., 2005). In the quick fiber process, pericarp fiber is recovered before liquefaction. By increasing the density of corn slurry, pericarp fiber floats at a specific gravity of 1.090–1.098 (12–13 Bé) (Singh et al., 1999). Unlike the E-Mill and quick fiber processes, the elusieve process entails separation of fiber in DDGS by sieving and elutriation (Srinivasan et al., 2005). This process requires a low capital cost for a dry grind ethanol plant to recover fiber from DDGS, of which the payback period is less than 2 years for 287.69 million liter (76 million gallon) ethanol production per year (Srinivasan et al., 2006).
Fractionation technologies prior to fermentation, such as the E-Mill process or quick fiber process, require substantial retrofitting of a conventional dry grind ethanol plant and significant capital investment. Fractionation technologies after fermentation, such as the elusieve process, require drying of DDGS or DDG prior to separation of fiber. In this study, fiber was recovered from whole stillage by sieving prior to centrifugation. This strategy to recover fiber requires minimal change in the conventional dry grind process compared to other strategies and no drying step is required prior to separation of fiber. The objectives of this study were: (1) to investigate the effects of sieving on fiber recovery in whole stillage, (2) to determine the nutrient value of fiber-removed DDGS (enhanced DDGS), and (3) to determine sugar yields from enzymatic hydrolysis of recovered corn fiber.
Section snippets
Whole stillage, enzymes and chemicals
Whole stillage sample was obtained from One Earth Energy, LLC in Gibson City, IL and stored at 4 °C prior to testing. Protease (Fermgen) was obtained from Genencor International (Palo Alto, CA). Fermgen is an acid proteolytic enzyme obtained by controlled fermentation of a selected strain of Trichoderma reesei. Protease activity is 1000 SAPU/g (SAPU = Spectrophotometer Acid Protease Units). The surfactant, polyethylene sorbitol ester (TWEEN 80), was purchased from Fisher Scientific (Fair Lawn, NJ).
Whole stillage treatments
Effects of treatment and sieve size on fiber recovery
Retentate recoveries, NDF recoveries and compositions of samples are summarized in Table 1. To remove oil and protein from fiber, whole stillage was ground, incubated with protease, or incubated with surfactant. Grinding is used to separate starch attached to fiber and soften the protein matrix (Singh and Eckhoff, 1996). Protease is a proteolytic enzyme that breaks down the protein matrix surrounding starch particles (Johnston and Singh, 2001), and effectively reduces residual starch in the
Conclusion
A sieving process that can be directly integrated in the current dry grind process was effective to recover fiber from whole stillage, increasing both nutrient value of DDGS and ethanol production. eDDGS showed lower fiber, higher oil, and similar or higher protein contents compared to non-sieved whole stillage samples. Moreover, high sugar yields after pretreatment and hydrolysis demonstrate that samples recovered by sieving have potential to be a bioethanol feedstock. Ground whole stillage
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