Monosaccharides generally find broad use in commerce, and of the monosaccharides glucose in particular finds varied usage. Glucose is used as the chief substrate in fermentation media for production of ethanol, is isomerized to afford fructose, which is broadly used as a sweetener, and is itself used as a sweetener, especially in confectionary products, although glucose is only perhaps three-fourths as sweet as sugar.
A major source of glucose is cellulose, a polysaccharide of cellobiose, which is the (1.fwdarw.4)-linked disaccharide of beta-D-glucose. In the United States corn cobs are a major source of cellulose, for cobs are an abundant waste material resulting from one of the largest agricultural crops. The use of corn cobs is not without disadvantages which arise chiefly from the presence of lignin. Corn cobs are within the class known as lignocellulosics, where cellulose is embedded in a matrix of amorphous lignin and hemicellulose. Pretreatment of lignocellulosics is necessary to disrupt the lignin matrix so that cellulose becomes more available to subsequent hydrolytic agents, and a whole class of processes are directed toward removal of lignin from lignocellulosics. The disadvantages of delignifying pretreatments are amplified by the relatively low value of lignin and the need to dispose of the chemical waste arising from the delignification process.
In the context of glucose production it would be highly advantageous either to develop a more efficient, less costly delignification process, or to find an essentially lignin-free source of cellulose. Such a source would be desirably available in abundance and byproducts accompanying glucose production would be of distinct commercial value so that the source could be efficiently used with a minimum of waste disposal problems. We have found such a source of cellulose in corn kernel hulls, a waste product of corn milling operations, which contain little or no lignin. Consequently, corn kernel hulls can be hydrolyzed in high yield without any delignifying pretreatment to afford a mixture which is mainly D-glucose, D-xylose, and L-arabinose. The latter two monosaccharides, which are pentoses, have independent utility as components of culture and fermentation media for some microorganisms, with D-xylose also being used in dyeing and tanning, so that virtually all the monosaccharides arising from hydrolysis have a commercial niche, a circumstance with important economic advantages.
Recognizing the advantages accruing from an abundant source of cellulose which requires no delignification pretreatment to make cellulose available to hydrolytic agents, we have developed several variants on a theme of hydrolyzing corn kernel hulls to a mixture of monosaccharides. In one variant acid hydrolysis at elevated temperature followed by enzymatic hydrolysis affords a maximum yield of glucose and total monosaccharides. A second variation employs acid hydrolysis at lower temperature to afford a solution whose monosaccharides are chiefly those from hemicellulose, with subsequent enzymatic hydrolysis cleaving cellulose to liberate glucose. A third variation features a very mild base pretreatment followed by enzymatic hydrolysis of cellulose to afford a solution whose monosaccharide is virtually exclusively glucose, and further acid treatment then hydrolyzes the hemicellulose component. Each variant exhibits particular advantages recommending its use, depending upon the marketplace and the needs of the processor, making the theme especially harmonious. Each variant also has unique features discovered during its development which may be indigenous to the use of corn kernel hulls as a feedstock. The remainder of this specification will be devoted to the exposition of the underlying theme and the development of its different variations.