---

  This study was performed to determine proximate composition and fatty acids in muscle of common carp and grass carp that are commonly consumed in Iran. There is no significant difference in the content of protein, lipid and ash samples of in these two fish species (p>0.05). Grass carp had significantly higher moisture contents than common carp. The fatty acid composition of common carp and grass carp were found to be 32.86 and 34.06% saturated (SFA), 39.43 and 25.02% monounsaturated (MUFAs) and 24.13 and 37.54% polyunsaturated fatty acids (PUFAs), respectively.  In both fish, palmitic acid C16:0 and oleic C18:1 n-9 acid were the principal saturated and monounsaturated fatty acids, respectively. The dominant poly unsaturated fatty acid (PUFA) was Docosahexaenoic acid (C22: 6n-3) in grass carp and Linoleic acid (C18: 2n-6) in common carp. The content of n3 PUFAs was 8.6% for common carp and 25/3% for grass carp. Arachidonic acid (20:4n6) content was 2.55% in grass carp and 1.33% in common carp. Muscle fatty acid composition of common and grass carp showed that there are significant difference in among all the fatty acids except for 22:0 and SFA(p<0.05).

---

The present study was conducted with the aim of protein isolate production from the muscle of grass carp (Ctenopharyngodon idella) and investigation of the effect of extraction pH on different properties of isolated protein. Proteins were produced by pH-shift method in two treatments including dissolution in alkaline pH (11.5), and acidic pH (3), and then evaluated in terms of nutritional value, functional properties, and structural changes. According to the results, the yield of protein production was significantly higher in alkaline conditions than in acidic conditions. The evaluation of emulsifying and foaming capacity and water absorption capacity showed that the protein obtained from alkaline pH was better than acidic treatment. Also, the extracted proteins contained all essential amino acids within the recommended limit for daily consumption of adults. The evaluation of color indices showed that the protein obtained from acidic pH had a brighter and whiter color than alkaline pH. The images obtained from the scanning electron microscope and the FTIR spectra of samples showed that the pH-shift method did not lead to extensive destruction of the protein structure and both protein isolates had all the absorption peaks related to the main bonds of the proteins structure. In general, it can be concluded that the pH-shift method is an efficient method for extracting high quality protein from grass carp tissue, and different alkaline and acidic conditions lead to the production of proteins with different characteristics that can be used based on the application that is intended for the final product.