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The degradation of chitosan by high-intensity ultrasound (HIU) as affected by ultrasound parameters and solution properties was investigated by gel permeation chromatography coupled with static light scattering. The molecular weight, radius of gyration, and polydispersity of chitosan were reduced by ultrasound treatment, whereas chitosan remained in the same random coil conformation and the degree of acetylation did not change after sonication. The results demonstrate that (1) the degradation of chitosan by ultrasound is primarily driven by mechanical forces and the degradation mechanism can be described by a random scission model; (2) the degradation rate is proportional toM_w³; and (3) the degradation rate coefficient is affected by ultrasound intensity, solution temperature, polymer concentration, and ionic strength, whereas acid concentration has little effect. Additionally, the data indicate that the degradation rate coefficient is affected by the degree of acetylation of chitosan and independent of the initial molecular weight.

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The effect of sonication during chitin extraction from freshwater prawn shells on yield, purity, and crystallinity of chitin was investigated. Dry prawn shells were suspended for 4 h in 0.25 M HCl at 40 °C while they were sonicated for 0, 1, and 4 h. Demineralized shells were lyophilized, resuspended in 0.25 M NaOH, and sonicated again for 0, 1, and 4 h for protein removal. The yield of chitin decreased from 8.28 to 5.02% for nonsonicated and sonicated samples, respectively, which was attributed to losses of depolymerized materials in the wash water. The application of ultrasound enhanced the removal of proteins. In nontreated shells, the amount of protein was 44.01% and was reduced to 12.55, 10.59, and 7.45% after 0, 1, and 4 h of sonication treatments. The glucosamine content slightly decreased with sonication probably because of losses due to depolymerization. The crystallinity indices of chitins decreased as the time of sonication increased. The degree of acetylation of chitins was unaffected by sonication, but the degree of acetylation of chitosans produced from sonicated chitin decreased from 70.0 to 68.7 and 61.4% for 1 and 4 h sonicated samples, respectively.

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The influence of sonication during extraction of chitin from North Atlantic shrimp (NAS) shells (Pandalus borealis) on chitin yield, purity, and crystallinity was investigated. Shells were peeled, washed, lyophilized, ground, and suspended for 4 h in 0.25 M HCl (1:40) at 40 °C followed by ultrasonication at 41 W/cm² for 0, 1, and 4 h, respectively. Demineralized shells were lyophilized, resuspended in 0.25 M NaOH (1:40), and ultrasonicated at 41 W/cm² for 0, 1, and 4 h to remove proteins. The yield and mineral and protein contents were determined after each processing step. The purity of extracted chitin was determined from the total amount of glucosamine. The crystallinity index and size of crystals were calculated from wide-angle X-ray scattering measurements. Scanning electron microscope images were recorded to evaluate morphological changes in samples. The yield of chitin from NAS decreased from 16.5 to 11.4% for 0 and 1 h sonicated samples, respectively, which was attributed to increased concentrations of depolymerized materials in the wash water. Sonication did not enhance the removal of minerals. The application of ultrasound enhanced the removal of proteins from 39.8 to 10.6, 8.3, and 7.3% after 0, 1, and 4 h of sonication treatments. The crystallinity index of chitin decreased from 87.6 to 79.1 and 78.5% after 1 and 4 h of sonication, yielding chitosans with crystallinity indices of 76.7, 79.5, and 74.8% after deacetylation, respectively. Fourier transform infrared spectroscopy scans indicated that the degree of acetylation of chitins was unaffected by sonication. Comparison of the extraction results of NAS with that from freshwater prawns indicated that more impurities were left in NAS chitin, suggesting that composition and structural arrangement of chitin in shells influence the efficiency of ultrasound-assisted extraction.

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The influence of high-intensity ultrasonication on the molecular weight and degree of acetylation of chitosan was investigated. High-molecular weight shrimp chitosan was purified by alkaline precipitation and dialysis from aqueous solution. A 1% (w/v) chitosan in 1% (v/v) aqueous acetic acid was sonicated for 0, 0.5, 1, 5, 15, and 30 min at 25 °C at power levels of 16.5, 28.0 and 35.2 W cm⁻². Degree of acetylation was determined by high-pressure liquid chromatography with photodiode array at 210 nm, monitoring acetyl groups released after complete hydrolysis and deacetylation of samples. Average molecular weight of ultrasonicated chitosan was determined by measurements of intrinsic viscosity of samples. The degree of acetylation of purified chitosan was 21.5%. Results indicated that neither power level nor sonication time altered the degree of deacetylation of chitosan molecules. Intrinsic viscosity of samples decreased exponentially with increasing sonication time. Rates of intrinsic viscosity decrease increased linearly with ultrasonic intensity. Results were analyzed in terms of molecular weight decreases. The Schmid polymer degradation model was used to analyze the molecular weight decay as a function of sonication time. A first order relationship for the dependence of the reaction rate on the ultrasonic intensity was suggested. Results of this study indicate that high-intensity ultrasonication can be utilized to reduce molecular weight of chitosan while maintaining the degree of acetylation.