High isostatic pressures up to 600 MPa were applied to samples of skim milk before addition of rennet and preparation of cheese curds. Electron microscopy revealed the structure of rennet gels produced from pressure-treated milks. These contained dense networks of fine strands, which were continuous over much bigger distances than in gels produced from untreated milk, where the strands were coarser with large interstitial spaces. Alterations in gel network structure gave rise to differences in rheology with much higher values for the storage moduli in the pressure-treated milk gels. The rate of gel formation and the water retention within the gel matrix were also affected by the processing of the milk. Casein micelles were disrupted by pressure and disruption appeared to be complete at treatments of 400 MPa and above. Whey proteins, particularly β-lactoglobulin, were progressively denatured as increasing pressure was applied, and the denatured β-lactoglobulin was incorporated into the rennet gels. Pressure-treated micelles were coagulated rapidly by rennet, but the presence of denatured β-lactoglobulin interfered with the secondary aggregation phase and reduced the overall rate of coagulation. Syneresis from the curds was significantly reduced following treatment of the milk at 600 MPa, probably owing to the effects of a finer gel network and increased inclusion of whey protein. Levels of syneresis were more similar to control samples when the milk was treated at 400 MPa or less.
The ripening characteristics of modified tomatoes (Lycopersicon esculentum Mill cv Ailsa Craig), which express antisense RNA to polygalacturonase (PG) and thus have very low activity of this enzyme, were compared with control fruit. Previous studies of these fruits showed that although PG activity was reduced to approximately 1% of that in untransformed tomatoes, this reduction had no effect on softening. Further detailed mechanical assessments have now been performed which revealed small, but significant differences in the fruit ripening characteristics between control and antisense fruits. Compression along the polar axis of the PG antisense fruit was significantly reduced relative to the control. Deformation at failure (h), 'modulus' (M), coefficient of compression ( K J , and coefficient of shear (K,) values from probe tests along the equatorial axis also indicated that the antisense fruit was firmer. Although these significant differences in texture properties were observed between the antisense fruit and the control, they were quite limited in extent, as compared to the normal evolution observed during ripening. Scanning electron microscope studies on the subexocarpic region of the pericarp showed that cell wall separation was reduced in the ripe antisense tomatoes, and this may explain the change in mechanical properties. Optical measurements made with both Hunter Color Difference (Hunter Lab Ltd, Fairfax, VA, USA) and Micromatch 2000 (ICS Texicon Ltd, Altrincham, UK) spectrophotometer systems showed that the 'a' (green/red) component of colour of the antisense tomatoes was increased relative to that of the control samples. The effect of PG on softening and of cell wall breakdown on mechanical and optical properties is also considered.
Reading RG6 6AP, UK Concentrated yogurts were produced by traditional (control), direct reconstitution, ultrajiltration and reverse osmosis techniques. The membrane techniques were applied either before or soon after incubation. The physical properties of the samples were monitored using a penetrometer (set yogurt) and viscometer (stirred yogurt), and the results indicated that different manufacturing techniques led to differences in the rheology of the concentrated yogurts. As expected, samples with high protein contents had greater gel strengths. Also, the concentration techniques caused large differences between the samples, even at the same protein level. The rheological properties correlated well with the microstructure as monitored by confocal laser scanning microscopy. In general, larger compartments in the network were associated with a weaker structure.
Clostridiwn thennocellwn, a thermophilic anaerobe, produces an extracellular cellulase system capable of extensively degrading crystalline cellulose, in the presence of Ca2+ and D l T [l]. However, the cellulase system of this bacterium is present in the form of a complex, termed cellulosome with a molecular mass of -2x106 12).Using three strains (YS, LQRI and NCIB 10682) of C.thennocellwn, we have demonstrated that this bacterium produces extracellular cellulosomes when grown on cellobiose and Avicel [3]. In the present paper, cellulosomes produced on cellobiose and Avicel by the above three strains were compared on the basis of in vitro Avicel solubilization, dissociation pattern and the cellulase and xylanase activities of various subunits.All three strains were cultured on cellobiose and Avicel and the cellulase system produced was isolated as described [3]. The cellulase system produced on both carbon sources by these strains solubilized Avicel linearly up to 48 h ( Fig.1). However, the rate and extent of Avicel solubilization by the cellulase system of all three strains of C.thennocellwn obtained on cellobiose (1.6 -1.8% h-1 and 70 -80%) was marginally lower than that produced on Avicel (2.1 -2.8% h-1 and 95 -98%). Furthermore, the cellulase system produced by strain NCIB 10682 on Avicel showed the highest rate and extent of Avicel solubilization, followed by YS and LORI while the rate and extent of Avicel solubilization by the enzyme system produced on cellobiose by all three strains were comparable. Electron microscopic observation of the above mentioned strains, during the growth on cellobiose and Avicel also revealed the presence of cell surface cellulosomes. Thus, biochemical and morphological studies confirmed the production of fully active cellulosomes on cellobiose and Avicel by these strains of C.rhennocellwn.Purified cellulosomes of all three strains from cellobiose and Avicel cultures were dissociated at pH 5.0 and at 25°C for 30 min and separated by SDS-PAGE. The pattern of dissociation showed overall similarity between the cellulosomes from all three strains produced on both carbon sources. However, the S, subunit of cellulosomes from cellobiose culture showed a diffused protein pattern while that from Avicel culture migrated as a sharp protein band. Also, based on the intensity of protein bands, S, and S, were found to be the major subunits in cellulosomes from cellobiose culture whereas S, and S,, appeared to be the most prominent subunits in cellulosomes from Avicel culture.In order to identify whether the subunits of cellulosomes from cellobiose and Avicel cultures show the same type of enzyme activity, the cellulosomes were dissociated as described above and separated by SDS-PAGE. Proteins eluted from the SDS-gel slices were assayed for endoglucanase, exoglucanase and xylanase activities. Cellulosomes produced on both carbon sources contained one major PNPCase and one major CMCase activity peaks corresponding to S, and S,, subunits, respectively. In addition, two minor PNPCase and several ...
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