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Biomedical subjects

W F Stevens

Publications and source records attributed to W F Stevens.

At least 19 recordsLinked to original sources
PubMed2005

Ionotropic alginate beads for controlled intestinal protein delivery: effect of chitosan and barium counter-ions on entrapment and release.

Alginate beads containing the model protein haemoglobin (Hb) were prepared by coagulation with various counter-ions to improve the controlled release of the protein. The effect of Ba(2+) and Ca(2+) ions and of the polycationic polysaccharide chitosan was investigated. Coagulation with Ba(2+), Ca(2+) and/or chitosan showed differences in the swelling index of the beads, in the encapsulation efficiency of Hb entrapment and in the release of the entrapped protein. Chitosan in the coagulation fluid markedly enhanced the encapsulation efficiency of the Hb. Release studies were conducted in simulated gastric fluid (SGF pH approximately or equal to 1.2) and subsequently in simulated intestinal fluid (SIF ) at 37 degrees C. Beads were stable in the gastric fluid but released their protein upon transfer to intestinal fluid. The release coincides with the burst and disintegration of beads. Rate of protein release from the beads was affected by the Ba(2+) and chitosan concentration in coagulation fluid.

Alginates↗
PubMed2004

Kinetic growth parameters of different amylolytic and non-amylolytic Lactobacillus strains under various salt and pH conditions.

Four Lactobacillus species were studied for their ability to grow at high NaCl concentrations and different initial pH values. Lactobacillus plantarum 541 and A6 could ferment glucose and produce lactic acid in the presence of 8% salt in the medium. For strain 541, the specific rate of lactate production (q(lac)) and the yield of lactic acid relative to substrate (Y(p/s)) remained constant, whereas the yield of biomass relative to substrate (Y(x/s)) decreased up to 6% salt. In contrast, for strain A6, Y(p/s) decreased up to 6% salt whereas Y(x/s) did not vary markedly. Combined effects of salt and pH studied through a factorial design did not show significant interaction between salt and pH. The pH was the dominant factor in glucose fermentation for both the strains. Considering overall performance, 4% salt and pH between 6.0 and 6.6 can be taken as appropriate conditions, for the use of both strains as starters in processes where higher salt concentrations are required.

Bioreactors↗
PubMed2001

Shrimp chitin as substrate for fungal chitin deacetylase.

The fungal chitin deacetylases (CDA) studied so far are able to perform heterogeneous enzymatic deacetylation on their solid substrate, but only to a limited extent. Kinetic data show that about 5-10% of the N-acetyl glucosamine residues are deacetylated rapidly. Thereafter enzymatic deacetylation is slow. In this study, chitin was exposed to various physical and chemical conditions such as heating, sonicating, grinding, derivatization and interaction with saccharides and presented as a substrate to the CDA of the fungus Absidia coerulea. None of these treatments of the substrate resulted in a more efficient enzymatic deacetylation. Dissolution of chitin in specific solvents followed by fast precipitation by changing the composition of the solvent was not successful either in making microparticles that would be more accessible to the enzyme. However, by treating chitin in this way, a decrystallized chitin with a very small particle size called superfine (SF) chitin could be obtained. This SF chitin, pretreated with 18% formic acid, appeared to be a good substrate for fungal deacetylase. This was confirmed both by enzyme-dependent deacetylation measured by acetate production as well as by isolation and assay for the degree of deacetylation (DD). In this way chitin (10% DD) was deacetylated by the enzyme into chitosan with DD of 90%. The formic acid treatment reduced the molecular weight of the polymeric chain from 2x10(5) in chitin to 1.2 x 10(4) in the chitosan product. It is concluded that nearly complete enzymatic deacetylation has been demonstrated for low-molecular chitin.

Absidia↗
PubMed2000

Critical factors in chitin production by fermentation of shrimp biowaste.

Factors affecting Lactobacillus fermentation of shrimp waste for chitin and protein liquor production were determined. The objective of the fermentation is medium conditioning by Lactobacillus through production of proteases and lowering of the pH. The efficiency was tested by conducting fermentation of biowaste in 1-1 beakers with or without pH adjustment using different acids. Addition of 5% glucose to the biowaste supported the growth of lactic acid bacteria and led to better fermentation. Among four acids tested to control pH at the start and during fermentation, acetic acid and citric acid proved to be the most effective. In biowaste fermented with 6.7% L. plantarum inoculum, 5% glucose, and pH 6.0 adjusted with acetic acid, 75% deproteination and 86% demineralization was achieved. Replacement of acetic acid by citric acid gave 88% deproteination and 90% demineralization. The fermentation carried out in the presence of acetic acid resulted in a protein fraction that smelled good and a clean chitin fraction.

Acetic Acid↗
PubMed1979

Effects of innervation by ciliary ganglia on developing muscle in vitro.

Electrophysiological and pharmacological properties of neuromuscular junctions formed in tissue culture between chick ciliary ganglia and chick skeletal muscle cells have been studied. Functional neuromuscular junctions are formed already within 24 h. No functional acetylcholine (ACh) esterase is present at these end-plates. The neurites conduct action potentials to the neuromuscular junctions, where EPPs are generated. Tetrodotoxin (TTX) blocks this nerve conduction but in the presence of TTX MEPP-like potentials remain whose amplitudes are lowered when the Mg2+/Ca2+ ratio in the medium is raised. It is speculated that these large TTX-resistant potentials are multiquantal. The relatively high resting membrane potential in non-innervated muscle fibers was not changed by innervation. ACh-sensitivity was determined by iontophoretical application of ACh to the myotubes. Non-innervated myotubes exhibited an evenly distributed ACh-sensitivity. Local differences in ACh-sensitivity were always gradual and never exceeded a factor of 3. Innervation did not alter the overall ACh-sensitivity, but on functionally innervated muscle cells loci hypersensitive to ACh were found. Hypersensitivity was located within sharply defined areas. Apparently the parasympathetic neurons of the ciliary ganglion are able to form functional neuromuscular junctions with skeletal muscle cells in tissue culture and to induce the formation of regions of high ACh-sensitivity.

Animals↗