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When pretreating woody biomass for the production of cellulosic ethanol, a mechanical downsizing step is commonly included to ensure an appropriate particle size for enzyme hydrolysis. Different methods of mechanical downsizing will result in wood particles with markedly different physical structures. Dry grinding methods, such as knife-milling, will produce a powder-like substrate, which consists of cut or truncated fiber bundles. The substrate will also have a reduced pore volume because of the required drying. Using a disc-refiner, wet wood chips are separated into single wood fibers and loosened fiber bundles, increasing available surface area and avoiding pore collapse because of drying. The following study compared knifemilled and disc-refined substrates produced from native and dilute-acid-pretreated wood chips to determine the impact of the mechanical-downsizing method on the enzyme digestibility and physical characteristics of a hardwood substrate. For dilute-acidpretreated aspen, disc-refining produced a substrate that was 58−80% digestible, while knife-milling produced a substrate that was 24−36% digestible. The difference in substrate digestibility was partially attributed to hornification during the drying step and also attributed to differences in physical structure because of the downsizing method. Analysis via microscopy indicated that disc-refined substrates had a greater length, smaller width, and greater fibrillation then the knife-milled substrates. The discrefined substrates also had a more exposed cellulose surface and a greater volume of accessible pores.

/?-Lactoglobulin (/?-lg) is the most abundant whey protein in bovine milk, comprising up to 50% of the total whey protein present. It consists of 162 amino acids with a calculated total mass of 18277 Da and contains two disulphide (-SS-) bonds, at Cys 66-Cys 160 and Cys 106-Cys 119 , and one free thiol (-SH) at Cys 121 , which is buried and unavailable for reaction in the native protein (Papiz et al. 1986). Above 55 °C progressive unfolding of the globular structure occurs which exposes the free-SH group and hydrophobic surfaces, allowing subsequent protein aggregation. Under suitable conditions of temperature, pH, salts and protein concentration, bovine /?-lg can form gels as a result of intermolecular protein-protein interactions such as hydrophobic, dipolar and electrostatic attractions, and covalent bond formation via-SH and-SS-interchange (Mulvihill & Kinsella, 1987). Porcine /?-lg contains 160 amino acids with a calculated total molecular mass of 17 8.11 Da. It has 79-6 % sequence homology at the cDNA level, and 66-9 % homology at the amino acid sequence level to bovine /?-lg (Alexander & Beattie, 1992). Porcine /?-lg contains two disulphide bonds but, in contrast to bovine /?-lg, it has no free-SH group (Kessler & Brew, 1970; Alexander & Beattie, 1992). In this study porcine /?-lg was used as a model protein to study the gelation of /?-lg devoid of a free-SH group.