Mixed Linkage (1→3),(1→4)‐β‐d‐Glucans of Grasses

The mixed‐linkage (1→3),(1→4)‐β‐d ‐glucans are unique to the Poales, the taxonomic order that includes the cereal grasses. (1→3), (1→4)‐β‐Glucans are the principal molecules associated with cellulose microfibrils during cell growth, and they are enzymatically hydrolyzed to a large extent once growth has ceased. They appear again during the developmental of the endosperm cell wall and maternal tissues surrounding them. The roles of (1→3),(1→4)‐β‐glucans in cell wall architecture and in cell growth are beginning to be understood. From biochemical experiments with active synthases in isolated Golgi membranes, the biochemical features and topology of synthesis are found to more closely parallel those of cellulose than those of all other noncellulosic β‐linked polysaccharides. The genes that encode part of the (1→3),(1→4)‐β‐glucan synthases are likely to be among those of the CESA/CSL gene superfamily, but a distinct glycosyl transferase also appears to be integral in the synthetic machinery. Several genes involved in the hydrolysis of (1→3),(1→4)‐β‐glucan have been cloned and sequenced, and the pattern of expression is starting to unveil their function in mobilization of β‐glucan reserve material and in cell growth.     

Modification of Standard Enzymatic Protocol to a Cost‐Efficient Format for Mixed‐Linkage (1→3,1→4)‐β‐d‐Glucan Measurement

The current enzymatic assay approach (AACC International Approved Method 32‐23) for the measurement of mixed‐linkage β‐glucan in small grains was modified to a cost‐efficient and high‐throughput format without compromising the accuracy of the results. Ten barley (Hordeum vulgare L.) genotypes used in the study represented a wide range of β‐glucan content levels. A reduced reaction volume is used in the new protocol to adapt to a 96‐well plate format. The volume of key components lichenase and β‐glucosidase were reduced to 25% of the volume required in the original protocol and the cost per sample was reduced to 22% of that in the original protocol. Labor cost was also decreased to 25% of the original protocol as a result of format changes. The accuracy of the measurement from the modified protocol was comparable to the current standard enzymatic procedure. β‐Glucan measurement accuracy of the modified and original protocols were also compared using 21 oat (Avena sativa L.) samples. The results indicated that the new protocol consistently produced accurate measurements in both barley and oat.     

Structure of (1→3)(1→4)‐β‐d‐Glucan in Waxy and Nonwaxy Barley

The endosperm cell walls of barley are composed largely of a (1→3)(1→4)‐β‐d ‐glucan commonly known simply as β‐d ‐glucan (Wood 2001). There has been much research into the characteristics of barley β‐glucan because of the influence of this polysaccharide on performance of barley in malting and subsequent brewing of beer, and in feed value, especially for young chicks (MacGregor and Fincher 1993). The potential for β‐glucan to develop high viscosity is a problem in these uses, but from the perspective of human nutrition, this characteristic may be an advantage. The glycemic response to oat β‐glucan is inversely related to (log)viscosity (Wood et al 1994a) and there is evidence to suggest that the lowering of serum cholesterol levels associated with oat and barley products (Lupton et al 1994; Wood and Beer 1998) is at least in part due to the β‐glucan (Braaten et al 1994) and probably also its capacity to develop viscosity in the gastrointestinal tract (Haskell et al 1992).     

Rapid Gas Chromatographic Technique for Quantifying 2‐Acetyl‐1‐Pyrroline and Hexanal in Rice (Oryza sativa,L.)

The aroma of rice plays a role in its consumer acceptability. The popcorn‐like smell of aromatic rice stemming primarily from its 2‐ acetyl‐1‐pyrroline (2‐AP) content is considered desirable by many consumers. Conversely, hexanal has been correlated with off odors in rice that develop from lipid oxidation. A rapid method for 2‐AP and hexanal quantification suitable for use in breeding programs, large‐scale research efforts, and quality assurance programs is needed. While developing such a method, sample preparation (degree of milling, particle size), solvent extraction time and temperature, and gas chromatographic parameters were studied. Particle size had no influence on 2‐AP or hexanal recovered. One extraction solubilized ≈80% of the 2‐AP and 56% of the hexanal present in milled rice. The optimum extraction method was assessed to require 0.3 g of ground brown or milled rice in methylene chloride held at 85°C for 2.5 hr. The complete gas chromatographic run requires ≈25 min, and 50 samples can be analyzed per day. The optimized method's linear response (R² = 0.99) and reproducibility was demonstrated. The stability of 2‐AP and hexanal in frozen milled rice and in refrigerated methylene chloride extracts was excellent for at least six months. Milled and unmilled commercial and breeders' aromatic rice samples contained 10–1,104 ng/g of 2‐AP and 148–2,541 ng/g of hexanal. Genotype had the greatest effect on the 2‐AP and hexanal content of two lines grown over four years and in four states.