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The aim of this study was to produce protein isolates from rainbow trout (Oncorhynchus mykiss) and to investigate its nutritional, structural, functional and sensory properties. Fish protein isolates were produced by pH change method using alkaline and acidic pHs (2.5, 3.5, 11 and 11.5). The extracted proteins were evaluated for nutritional value, and functional properties, sensory properties, color, and structural changes of proteins. The extracted proteins showed relatively high nutritional value in the amount of essential amino acids. The solubility, gelling and foaming capacity of the proteins depended significantly on its extraction pH. So that the percentage of solubility and foaming capacity of proteins extracted under alkaline conditions showed a higher rate than acidic conditions while the least gelation concentration was related to pH 11.5. Examination and comparison of color (L, a, b and whiteness) and sensory properties of isolated proteins showed that proteins at acidic pH had a brighter and whiter color and better sensory properties than those from alkaline pH. The change in pH caused extensive protein degradation as shown by FTIR and SEM. The above results showed that the protein isolated from rainbow trout waste has good functional and nutritional properties in alkaline conditions, while the sensory properties and color of acidic conditions showed better results.

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ABSTRACT
The aim of this study was to produce protein isolates from the head of the Siberian sturgeon (Acipenser baerii) and study its functional and structural properties. Fish protein isolates were prepared by the pH shift method using alkaline pHs 10.5, 11, 11.5, 12, and 12.5. The results showed that the efficiency of protein extraction and its amount of essential amino acids at pH 11.5 were higher than other treatments. Furthermore, the results of determination of functional properties such as Water Holding Capacity, Oil Holding Capacity, emulsifying properties, foaming and solubility of the isolated proteins showed that as the pH increased, the functional properties improved and the protein isolates at pH 11.5 compared with other treatments was significantly higher. (p < 0.05). Comparison of the color characteristics (L, a and b) of the isolated proteins showed at pH 12 were more bright (higher L parameter) than those isolated in other treatments tested. In addition, the amount of red (parameter a) and yellowness (parameter b) of the isolated proteins decreased with increasing pH. The results of the organoleptic examination of the smell and taste of protein isolates at pH 11.5 are the most common among other treatments. According to the observed cases, the results show that the resulting Siberian sturgeon isolate protein has favorable functional properties and that the use of alkaline pH changes can lead to improved functional properties and parameters of color protein isolates.

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 In the current study, fish-isolated protein (FPI) was extracted by alkaline pH method from the wastes of the head and fins of Siberian sturgeon (Acipenser baerii) and Iranian sturgeon (Acipenser persicus) and its composition, functional and structural characteristics were investigated. The results showed that the extraction efficiency according to the wet weight, the protein recovery of FPI, and the dry matter recovery of FPI from the Siberian sturgeon was higher than that of Iranian sturgeon. The results showed 91-94% protein in FPI of two species. The results did not show any significant differences between the proximate analysis and TVB-N (P> 0.05). The water absorption capacity, foaming capacity, and stability of FPI were shown at 30, 50, and 50%, respectively. The whiteness and transparency index of the FPI extracted was low. The results of particle size and zeta potential (P< 0.05) showed that the size of FPI particles in both samples is about a micron and they have a negative charge, so they can be easily used in colloidal systems. FTIR spectroscopy in both samples showed all the absorption peaks related to the main bonds of the protein structure. In general, considering the large amount of sturgeon waste in processing, and the amount of protein, functional properties, and nutritional value of FPI extracted from two species of sturgeon, there is the possibility of using them in human food and sports supplements.