Chitosan from Penaeus monodon prawn shell waste by Bacillus subtilis fermentation-assisted deproteinization and its evaluation for turbidity removal from water and postharvest coating of grapes

The crustacean shell waste generated by the shellfish industry poses a significant threat to the environment. Extracting chitin from invertebrate exoskeletons can reduce shell-waste disposal costs. This study aimed to convert Penaeus monodon shell waste into chitosan via a fermentation-assisted route and to test one batch of the product as both a coagulant for turbid water and a postharvest coating for grapes. Waste shells of the prawn Penaeus monodon were the starting material in the present study: conventional chemical extraction yielded chitin, fermentation with Bacillus subtilis removed the residual protein, and three days of deacetylation in 65% sodium hydroxide at room temperature then yielded chitosan. The chitin yield was 32% on dry raw shell, and the chitosan yield was 48% on the chitin charged to deacetylation, using two different bases. XRD showed crystalline peaks at 2θ = 11.38°, 18.04° and 22.6°; FT-IR showed the characteristic bands of chitosan, and SEM showed a smooth, compact surface without cracks. The as-prepared chitosan had moisture and ash contents of 4.1% and 1.9%, respectively. Viscosity was 55.933 cP, and the degree of deacetylation was 53.44%, below the range usually reported for commercial grades. In jar tests on silt-laden water, removal was greatest at pH 5, where a 0.2 mg/L chitosan solution removed 80.2 ± 1.2% of the turbidity and 76.1 ± 1.0% of the total solids (mean ± SD, n = 3); larger doses lowered removal through re-stabilisation. As a coating on grapes stored at ambient temperature, the chitosan cut the seven-day weight loss from 64.0 ± 0.9% to 41.7 ± 1.1%, about one-third less than in uncoated fruit, and limited the rise in juice pH (4.93 ± 0.02 against 5.26 ± 0.03). These results, from triplicate runs on one batch of chitosan, indicate that low-DD chitosan from prawn shell waste can act as both a coagulant and a postharvest coating.