Enzymatic extraction of sulfated polysaccharide from the skin of rainbow trout (Oncorhynchus mykiss) and evaluation of its chemical, antioxidant and functional properties

Bioactive compounds are substances found in small amounts in food. In addition to their influence on human development, these compounds also play a crucial role in reducing diseases in human. Polysaccharides are a group of bioactive compounds that come from a variety of sources. Polysaccharides are macromolecules that are usually composed of more than ten monosaccharides. The constituent monosaccharides are arranged linearly or branched together through glycoside bonds, depending on the length of the chain and the number of constituent monosaccharides. They also have different molecular weight. Polysaccharides, like other essential macromolecules such as proteins and poly-nucleotides in the body, are essential for the flaxseed body's daily activities and play an influential role in cell-cell communication, cell adhesion, and the identification of molecules in the immune system. A group of polysaccharides derived from marine sources are sulfated polysaccharides. These polysaccharides are a broad branch of the resulting polysaccharides. In industrial quantities, sulfated polysaccharides are produced from pig skin and pig bone, and there are some restrictions on the use of these products in some countries. The limitations on the use of these products made from pig waste are the risks of transmitting influenza, as well as the prohibition of pork in some Islāmic countries. In this regard, by-products from seafood processing, which account for about 20 to 50 percent of the initial weight of raw material, are one of the sources that researchers are considering to extract these compounds. 

After preparation of the by-product, it was covered with ice in a ratio of 1 to 3 and transferred to the laboratory of Tarbiat Modares University. The sample was then washed and then ground. Finally, it was packed in plastic bags and kept in the freezer at -18 ° C until the day of experiment. Then, the enzymatic hydrolysis method and precipitation by ethanol were used to get sulfated polysaccharides. Chemical analyses were performed to determine carbohydrates, sulfates, proteins, and uronic acid content. The FTIR spectrum of extracted sulfate polysaccharide was determined using an FTIR spectrophotometer in the range of 400-4000 cm-1. Evaluation of antioxidant properties of obtained sulfate polysaccharide was assessed by DPPH free radical scavenging activity, ABTS free radical scavenging activity, and ferrozine tests. Emulsifying and foaming properties were also evaluated as functional properties. 

In the present study, sulfated polysaccharide was extracted from Rainbow trout (Oncorhynchus mykiss) skin by pepsin enzyme and its FTIR spectrum, carbohydrate, sulfate, uronic acid and protein were analyzed. The results of the chemical analysis of the extracted polysaccharide showed that the extraction efficiency was 3.23± 0.02%, as well as the percentage of carbohydrate and protein of the obtained polysaccharide was 57.03± 2 2.56, 7.78± 0.43% respectively. Also, the amount of sulfate and uronic acid were 6.54± 0.77 and 3.86± 0.43, respectively. The results of infrared spectroscopy showed the presence of a broad peak in the range between 3350 and 3450 cm-1 for the –OH group and the S=O sulfate flexural band in the range of 1245 cm-1. An increasing and significant trend was observed in different concentrations used for the DPPH test (p <0.05) which had the highest neutralizing power (38.85%) at a concentration of 2 mg/ml. The highest percentage of ABTS radical chelating was observed at a concentration of 4 mg/mm of distilled water with 71.70% (p <0.05). The chelating results of the extracted polysaccharide against ferrous ions showed that the highest chelating percentage was 98.43% (p <0.05). The foaming capacity, stability properties of the foam, and the emulsifying ability of the studied sample showed a trend of increasing the concentration coefficient of the sample (p <0.05), and the concentration of 10% used sulfated polysaccharide had the highest foaming percentage (72/22%) and foam stability (62.22%) (p <0.05). The emulsifying property of extracted sulfate polysaccharide against soybean oil was higher in all concentrations used than sunflower oil (p <0.05), and the highest value of that was related to the concentration of 10% with 86.57% and 92.59% against sunflower oil and soybean oil (p <0.05). The obtained results demonstrated that the fish skin extracted polysaccharide can serve as a natural antioxidant and functional agent in the food industry