Abstract: The invention discloses a method for preparing water soluble chitosan oligosaccharide from squid sheath. The method is characterized by comprising the following steps of: crushing the squid sheath, adding 6 to 9 percent HCl solution, ensuring that a material/solution (w/v) ratio is 1:14, decalcifying for 20 to 60 minutes, washing to neutrality, adding 10 percent NaOH solution, ensuring that a material/solution (w/v) ratio is 1:10, removing protein, filtering and washing to neutrality, and drying to obtain beta-chitin; adding 30 to 50 percent NaOH solution into the beta-chitin, ensuring that a material/solution ratio (w/v) is 1:10, performing ultrasonic bath reaction at the temperature of between 70 and 100DEG C for 2 to 4 hours, filtering, washing to neutrality, and drying to obtain chitosan; and adding 3 to 12 percent H2O2 into the chitosan, ensuring that a material/solution ratio (m/v) is 1:15-1:25, reacting at the temperature of between 45 and 75DEG C for 2 to 5 hours in the assistance of ultrasonic waves, concentrating filtrate, adding ethanol in an amount which is 3 to 5 times volume of the concentrated filtrate, precipitating and filtering a product, and drying to obtain the chitosan oligosaccharide, wherein the chitosan oligosaccharide has the molecular weight of less than 1*10<4>Da, the DPPH free radical removing capacity (EC50) of between 0.3 and 1.3mg/mL and the cholic acid combining capacity of between 20.47 and 63.53mg/g. The invention provides technical support for comprehensively utilizing leftovers, namely the squid sheath in aquatic product processing and industrially producing the water soluble chitosan oligosaccharide.
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