Fermentation of Barley by Using Saccharomyces cerevisiae: Examination of Barley as a Feedstock for Bioethanol Production and Value-Added Products

The objective of this study was to examine the ethanol yield potential of three barley varieties (Xena, Bold, and Fibar) in comparison to two benchmarks, corn and wheat. Very high gravity (VHG; 30% solids) fermentations using both conventional and Stargen 001 enzymes for starch hydrolysis were carried out as simultaneous saccharification and fermentation. The grains and their corresponding dried distiller''s grain with solubles (DDGS) were also analyzed for nutritional and value-added characteristics. A VHG traditional fermentation approach utilizing jet-cooking fermentation revealed that both dehulled Bold and Xena barley produced ethanol concentrations higher than that produced by wheat (12.3, 12.2, and 11.9%, respectively) but lower than that produced by corn (13.8%). VHG-modified Stargen-based fermentation of dehulled Bold barley demonstrated comparable performance (14.3% ethanol) relative to that of corn (14.5%) and wheat (13.3%). Several important components were found to survive fermentation and were concentrated in DDGS. The highest yield of phenolics was detected in the DDGS (modified Stargen 001, 20% solids) of Xena (14.6 mg of gallic acid/g) and Bold (15.0 mg of gallic acid/g) when the hull was not removed before fermentation. The highest concentration of sterols in DDGS from barley was found in Xena (3.9 mg/g) when the hull was included. The DDGS recovered from corn had the highest concentration of fatty acids (72.6 and 77.5 mg/g). The DDGS recovered from VHG jet-cooking fermentations of Fibar, dehulled Bold, and corn demonstrated similar levels of tocopherols and tocotrienols. Corn DDGS was highest in crude fat but was lowest in crude protein and in vitro energy digestibility. Wheat DDGS was highest in crude protein content, similar to previous studies. The barley DDGS was the highest in in vitro energy digestibility.The growing need for energy independence and proposed renewable fuels has led recently to a major expansion of fuel ethanol production. In North America, this activity primarily uses corn as a feedstock. The need to find other cost-effective and efficient grains for ethanol production has increased in significance. Cereal grains are high in starch and are currently being utilized for ethanol production (26, 41). To ensure long-term viability of the industry, fermentation strategies that focus on holistic utilization of the feedstock that maximize value addition will increase in importance. The focus of industry is slowly moving from biorefineries that anticipate subsidy and government policy to integrated biorefineries that produce multiple products. Multiple product streams and integrated by-product management are thought to ensure better financial stability and opportunities for diversified income streams.Barley is a potential candidate for industrial ethanol production (10) since its ethanol yield is comparable to that of wheat but below that of American corn, which is currently the preferred industrial feedstock. Barley contains on average 63 to 65% starch, 8 to 13% protein, 2 to 3% fat, 1 to 1.5% soluble gums, 8 to 10% hemicellulose, ca. 2.9% lignin, and 2 to 2.5% ash (15, 27). Barley also contains a hull that could be fermented using cellulolytic enzymes, providing opportunities for integrated biorefineries that utilize more feedstocks than corn. Potential coproducts of ethanol production from barley include protein, fiber, fatty acids, tocopherols, and tocotrienols (40). The nutritional value of barley, based on amino acid content, is greater than that for corn and is not significantly affected by the fermentation process (40). A range of nutraceutical and functional food products, as well as amylase, amylase inhibitors, β-amylase, and oxalate oxidase, are found in barley grains and may have potential for extraction and commercial applications (6, 22, 33). Hull-less barley lines, high in both protein (particularly lysine) and starch, and low in fiber, have recently been developed (11, 14, 32). Since starch recovery and thus ethanol yields are lower for barley than corn, coproduct recovery becomes even more essential for profitability (43).Enzymes used for the pretreatment of grains prior to fermentation have traditionally been α-amylases and glucoamylases. The α-amylase decreases the viscosity of the mash (25) and performs the liquefaction of the pretreatment process. The liquefaction step is typically done at high temperatures of 100 to 120°C (38) with direct steam injection (jet-cooking). The α-amylase action serves to break starch at α-(1,4)-glucosidic bonds, producing smaller dextrin chains. During the saccharification step of the pretreatment, the dextrins produced by α-amylase are then acted on by glucoamylase. This conventional method has a considerable economic drawback, because the mash must undergo a cooking step prior to fermentation. Many industrial ethanol producers use jet-cooking to raise the mash temperature to 100 to 120°C. Because of this temperature requirement, the conventional process uses a large amount of energy to produce ethanol.Recently, a new line of cold starch hydrolyzing enzymes was developed. An example of these enzymes is Stargen 001, which is referred to as a raw starch hydrolyzing enzyme because starch is hydrolyzed to fermentable sugars while the temperature remains at or below a temperature of 48°C (38). Stargen 001 replaces the liquefaction and saccharification steps performed by conventional digestion enzymes (i.e., α-amylase and glucoamylase) and releases free glucose and other fermentable sugars for use by yeast cells. Stargen 001 is a cocktail of modified α-amylase and glucoamylase enzymes that work together to convert starch into dextrins, followed by the hydrolysis of dextrins to fermentable sugars (37, 38). With the absence of a cooking stage in the cold hydrolysis method, the potential exists that the dried distiller''s grain plus solubles (DDGS) produced by fermentation would have less damage so that the proteins contained in the DDGS could be of more value (18).The objectives of the present study were to examine the ethanol yield potentials of three barley varieties (Xena, Bold, and Fibar) and two benchmark grains (Pioneer Hi-Bred corn and CPS wheat) using conventional (jet-cooking) and cold starch hydrolysis with Stargen 001. In addition, dehulling was tested for the potential to increase ethanol yields, because hull does not contain fermentable starch; both hulled and dehulled mashes were studied where possible. The grains and their corresponding DDGS were analyzed for nutritional value and the presence of potential value-added products such as fatty acids, tocopherols, tocotrienols, sterols, and polyphenols.