The aim of this paper is to define optimal conditions for the extraction of chitin from shrimp shells. The kinetics of both demineralization and deproteinization with, in the latter case, the role of temperature are studied. The characterization of the residual calcium and protein contents, the molecular weights, and degrees of acetylation (DA) allows us to propose the optimal conditions as follows. The demineralization is completely achieved within 15 min at ambient temperature in an excess of HCl 0.25 M (with a solid-to-liquid ratio of about 1/40 (w/v)). The deproteinization is conveniently obtained in NaOH 1 M within 24 h at a temperature close to 70 degrees C with no incidence on the molecular weight or the DA. In these conditions, the residual content of calcium in chitin is below 0.01%, and the DA is almost 95%.

Aquatic humic and fulvic acids can increase the permeability of biological membranes to lipophilic solutes. In in vivo experiments, passive diffusion of fluorescein diacetate (FDA) into the green alga Selenastrum capricornutum increased in the presence of Suwannee River humic and fulvic acids at pH 5 (humic > fulvic) but not at pH 7. The observation of enhanced diffusion at the lower pH is consistent with adsorption measurements, which showed that the association of humic and fulvic acids with the algal surface was greater at pH 5 than at pH 7. Permeability experiments were also performed on model membranes to investigate the interaction of these humic substances with membrane lipids. In these in vitro experiments, we followed leakage of the fluorescent probe sulforhodamine-B (SRB) that had been encapsulated within 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) vesicles; this model phospholipid is representative of those found in the plasmalemma of green algae. Release of SRB from the vesicles was markedly accelerated in the presence of Suwannee River humic and fulvic acids (humic > fulvic); for the humic acid, lowering the pH from 7.6 to 5.7 enhanced this surfactant-like effect. The demonstration that humic substances can alter the permeability of phytoplankton and model membranes at natural concentrations and pH values has potential implications for the uptake and regulation of toxic and essential solutes by the phytoplankton community.

The aim of this paper was to contribute to the interpretation of the mechanism of shrimp shell deproteinization. We used amino acid analysis to quantify the amount of proteins remaining in chitin. NaOH 1 M was added to a demineralized shrimp shell powder with a solution-to-solid ratio of 15 mL/g at ambient temperature. Because of the limited precision of the technique, after 24 h the protein content measured by elemental analysis had to be considered as negligible. However, with the use of amino acid analysis, it was still possible to determine with precision this content down to 0.25%. We also showed that among the peptides remaining linked to chitin after deproteinization, acidic amino acids were always in proportion higher than alkaline ones, but the balance between the two kinds of residues increased in favor of the latter with time. The kinetic study of the deproteinization clearly revealed a three-step mechanism with very different rate constants. The variation of these constants with temperature was used to calculate the energies of activation and the frequency factors of collision, thus allowing us to propose a new interpretation of the mechanism of deproteinization.