Pericarp Fiber Separation from Corn Flour Using Sieving and Air Classification Radhakrishnan Srinivasan1 and Vijay Singh2,3 ABSTRACT
Cereal Chem. 85(1):27–30
In the dry-grind process, starch in ground corn (flour) is converted to ethanol, and the remaining corn components (protein, fat, fiber, and ash) form a coproduct called distillers dried grains with solubles (DDGS). Fiber separation from corn flour would produce fiber as an additional coproduct that could be used as combustion fuel, cattle feed, and as feedstock for producing valuable products such as “cellulosic” ethanol, corn fiber gum, oligosaccharides, phytosterols, and polyols. Fiber is not fermented in the dry-grind corn process. Its separation before fermentation would increase ethanol productivity in the fermenter. Recently, we showed that the elusieve process, a combination of sieving and elutriation (air flow), was effective in fiber separation from DDGS. In this study, we evaluated the elusieve process for separating pericarp fiber from corn flour. Corn flour remaining after fiber separation was termed “enhanced corn flour”. Of the
total weight of corn flour, 3.8% was obtained as fiber and 96.2% was obtained as enhanced corn flour. Neutral detergent fiber (NDF) of corn flour, fiber, and enhanced corn flour (dry basis) were 9.0, 61.5, and 5.7%, respectively. Starch content of corn flour, fiber, and enhanced corn flour (dry basis) were 68.8, 23.5, and 71.3%, respectively. Final ethanol concentration from enhanced corn flour (14.12% v/v) was marginally higher than corn flour (13.72% v/v). No difference in ethanol yields from corn flour and enhanced corn flour was observed. The combination of sieving and air classification can be used to separate pericarp fiber from corn flour. The economics of fiber separation from corn flour using the elusieve process would be governed by the production of valuable products from fiber and the revenues generated from the valuable products.
Fuel ethanol production from starchy cereal grains like corn is increasing rapidly due to the need for alternate energy sources (Farrell et al 2006). Biorefining, the recovery of value-added coproducts by separating individual components, is crucial for increasing revenues from fuel ethanol production (Ragauskas et al 2006). Fiber separated from cereal grains would be a valuable coproduct that can be used to produce additional “cellulosic” ethanol (Dien et al 1997). Fiber can also be used as combustion fuel, cattle feed, and as feedstock for producing valuable products such as corn fiber gum, oligosaccharides, phytosterols, and polyols (Crittenden and Playne 1996; Moreau et al 1996; Doner et al l998; Buhner and Agblevor 2004). Corn wet-milling is a process used to biorefine corn to produce starch and valuable coproducts. The starch is further used to produce either ethanol or high-fructose corn syrup. The wet-milling results in valuable coproducts but the capital investment required is high. A dry-grind process is preferred over the wet-milling process for ethanol production from corn, even though it results in only one major coproduct. In the conventional dry-grind process, whole corn is processed, starch in the corn is converted to ethanol, and the remaining corn components (protein, fat, fiber, and ash) are recovered at the end of the process to form the coproduct, distillers dried grains with solubles (DDGS). Fiber does not ferment in the dry-grind process and its separation before fermentation could increase ethanol productivity in the fermenter. Processes such as the quick germ quick fiber (QGQF) process (Singh et al 1999; Wahjudi et al 2000) and the dry degerm defiber (3D) process (Murthy et al 2006) have been developed to recover pericarp fiber in dry-grind corn processing. QGQF processing involves soaking corn, wet degermination, and separation of pericarp fiber in aqueous medium. QGQF processing involves rerouting of water in a dry-grind corn plant to achieve fiber separation. In the 3D process, short tempering of corn with steam or hot water followed by dry degermination is required before pericarp fiber separation by aspiration.
Recently, we showed that the elusieve process, combining sieving and elutriation (air flow), was effective in fiber separation from DDGS (Srinivasan et al 2005; Srinivasan 2006). Unlike the QGQF or 3D processes, the use of elusieve processing to remove pericarp fiber from corn flour would not require any soaking of corn and would not require germ removal. Germ has several micronutrients that are essential during yeast fermentation of sugars (Murthy et al 2006). Capital investment required for the elusieve process is expected to cost less than the QGQF or 3D processes. The process for ethanol production from enhanced corn flour obtained after pericarp fiber separation using the elusieve process is called the “prelusieve” dry-grind process. The objectives of this study were to 1) evaluate fiber separation from corn flour using the elusieve process, and 2) determine and compare ethanol yields from conventional and prelusieve dry-grind processes. In masa processing, alkali steeping of corn separates pericarp fiber, which results in waste water (called nejayote) and creates disposal problems (Rosentrater 2003). Pericarp fiber separation from corn flour using the elusieve process could eliminate the need for debranning that uses alkali, and the remaining corn flour could be used for masa production. The combination of sieving and air classification could also apply to barley processing for separation of β-glucans (Sundberg et al 1995). Removing β-glucans from barley before fermentation would decrease viscosity of fermentation mash, and increase ethanol productivity and nutritional value of the DDGS coproduct used as animal food (Hicks et al 2005).
1 Former
graduate student, University of Illinois at Urbana-Champaign. Current assistant research professor, Department of Agricultural and Biological Engineering, Mississippi State University, Mississippi State, MS 39762. 2 Associate professor, Department of Agricultural and Biological Engineering, University of Illinois at Urbana-Champaign, 360G, AESB, 1304 West Pennsylvania Avenue, Urbana, IL 61801. 3 Corresponding author. Phone: 217-333-9510. Fax: 217-244-0323. E-mail address:
[email protected] doi:10.1094 / CCHEM-85-1-0027 © 2008 AACC International, Inc.
MATERIALS AND METHODS Elusieve Processing for Pericarp Fiber Separation from Corn Flour Corn was milled using a hammer mill (Fitzpatrick, Chicago, IL) with a 0.5-mm round hole sieve. Corn flour was sieved using a vibratory sifter (model ZS30S6666 Vibro-Energy separator, Sweco, Florence, KY). Screens were 24T (869 µm), 34T (582 µm), and 48T (389 µm). Tensil bolt cloth is designated as “T”. Material passing through the sieve with a larger opening was collected and placed on the next smaller sieve. Only one sieve was used at a time. Four sieve categories (24T, 34T, 48T, and Pan) were obtained by sieving the corn flour using three screens (Fig. 1). The three largest sieve categories (24T, 34T, and 48T) were subjected to elutriation using an apparatus similar to the elutriation apparatus used by Srinivasan et al (2005). The elutriation column diameter used in this study was 155 mm instead of 63 mm as used by Srinivasan et al (2005). Apparatus with a 155-mm Vol. 85, No. 1, 2008
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elutriation column is described in detail in Srinivasan (2006). Material carried to the top of the column was called the lighter fraction, and material that settled in the bottom of the column was called the heavier fraction. Elutriation produced a 4–16% lighter fraction yield from each of the sieve categories (Table I). Fiber was obtained by mixing the lighter fractions from the three sieve categories. The remaining flour was called enhanced corn flour. Analytical Tests Neutral detergent fiber (NDF) content was determined using the procedure of Van Soest et al (1991). Starch content was determined using a glucoamylase procedure (Approved Method 77-11, AACC International 2000). Analyses were made at a commercial laboratory (Midwest Labs, Omaha, NE). Moisture content was determined using the two-stage convection oven method (Approved Method 44-18, AACC International 2000). Laboratory Dry-Grind Procedure Yellow dent corn from the 2005 crop season at the Agricultural and Biological Engineering Research Farm, University of Illinois at Urbana-Champaign, was used for the study. Samples were hand-cleaned. α-Amylase (Termamyl 120L, Novozymes NA, Franklinton, NC) and amyloglucosidase (AMG 300L, Novozymes) were obtained from Sigma (St. Louis, MO). α-Amylase is from Bacillus licheniformis and had activity of 930 KNU/g (where
KNU is kilo novo α-amylase units). Amyloglucosidase is from Aspergillus niger and had activity of ≥ 300 NU/mL (where NU is novo units). For the conventional dry-grind process, corn flour (1,000 g) was used as a starting material. Water (2,556–2,572 mL) was added to 25% dry solids content. The slurry was heated to 90°C and 2 mL of α-amylase was added to the slurry. Liquefaction was conducted for 2 hr. Mash was cooled to 30°C and adjusted to pH 4.5 using 10N sulfuric acid solution. Simultaneous saccharification and fermentation (SSF) was initiated by addition of 2 mL of glucoamylase, 7 g of active dry yeast (Fleischmann’s, Fenton, MO), and 4 g of urea as yeast nutrient. SSF was conducted for 72 hr. Fermentation was monitored by taking 2-mL samples in duplicate at 0, 2, 4, 6, 8, 10, 12, 24, 48, and 72 hr and analyzing samples using HPLC. Each slurry sample was centrifuged for 2.5 min at 16,110 × g (13,000 rpm) (model 5415 D, Eppendorf, Westbury, NY) to obtain clear supernatant liquid. Supernatant was passed through a 0.2-μm sterile syringe filter (Corning, Corning, NY) into 1-mL shell vials (Fisher Scientific, Pittsburgh, PA). Filtered liquid was injected into an ionexclusion column (Aminex HPX-87H, Bio-Rad, Hercules, CA) maintained at 50°C. Sugars, organic acids, and alcohols were eluted from the column with HPLC-grade water and 5 mM sulfuric acid. Elution rate was 0.6 mL/min. Separated components were detected with a refractive index detector (model 2414, Waters, Milford, MA). Data were processed using HPLC software (Waters). The HPLC instrument was calibrated with standards containing all components of interest at known concentrations at the beginning of each batch of samples, after every 10 samples, and at the end of the batch. Each sample was injected twice for analysis. Results presented are the means of multiple analyses. Ethanol yield was determined from weight of liquid after 72 hr and final ethanol concentration. The procedure for the prelusieve dry-grind process was similar to that for the conventional dry-grind corn process, except for the weight of starting material. For the prelusieve dry-grind process, 956–967 g of enhanced corn flour was used as starting material, which was the yield obtained from elusieve processing of 1,000 g of corn flour. Water (2,461–2,488 mL) was added to obtain 25% dry solids content. Liquefaction and SSF were conducted using the same procedure as for the conventional dry-grind process. Statistical Analyses Experiments were conducted in three replicates. Three batches of enhanced corn flour were obtained by elusieve processing, fermentation was conducted for three batches of enhanced corn flour, and three batches of whole corn flour. ANOVA analysis and Tukey’s test (SAS Institute, Cary, NC) were used to compare means. Statistical significance level was 5% (P < 0.05). RESULTS AND DISCUSSION
Fig. 1. Schematic of the elusieve process for pericarp fiber separation from corn flour. For industrial implementation of elusieve processing, aspirators may be used instead of elutriation columns.
Pericarp Fiber Separation from Corn Flour Using Elusieve Processing The weight of material (%) retained on sieve after sieving corn flour was 30.8, 13.1, 9.8, and 46.3% for 24T, 34T, 48T, and Pan,
TABLE I Weight % of Fractions from Elusieve Processing of Corn Flour and Elutriation Velocities for Sieve Categoriesa Sieve Category and Size 24T (>869 μm) 34T (582–869 μm) 48T (389–582 μm) Pan (
