Effect Of Cooking Styles On Protein Oxidation And Proteolysis Of Muscle From Sturgeon(Acipenser Gueldenstaedtii)

Sturgeons(Acipenser gueldenstaedtii)are important commercial fish species in Asia.They are highly appreciated for their valuable caviars,while the chunks of flesh are yet to be substantially utilized.In fact,sturgeons enjoy boneless flesh with excellent quality and taste,which have great potential in food processing.Fish is usually consumed after cooking which could trigger oxidation of lipid and protein.The effect of cooking on fatty acids composition and lipid oxidation of sturgeon have been studied.But there is no literature available on protein oxidation and proteolysis of cooked sturgeon.This paper was aimed to evaluate the effects of different cooking methods and different roasting times on lipid and protein oxidation in sturgeon.In the meantime,oxidation and proteolysis of cooked fish meat during in vitro digestion was also investigated.The results of this paper were as follows:1.The effect of different cooking methods(boiling,steaming,microwaving,roasting and frying)on protein and lipid oxidation of farmed sturgeon(Acipenser gueldenstaedtii)was investigated.General indexes for protein oxidation(carbonyls,free thiols and Schiff bases)and lipid oxidation were evaluated by spectrophotometric and fluorimetric methods.Among five cooking methods,two mild ones-boiling and steaming-showed less impact on lipid and protein oxidation,while roasting and frying caused severe oxidation as 400%increase in carbonyls,50%loss in free thiols and 900%increase in Schiff bases were found in those samples.Side-chain modifications of amino acid residues were pinpointed by gel-and MS-based proteomic technology,which is conducive to understand the mechanism of cooking-induced protein oxidation and its complex oxidative products.Cooking could induce protein oxidation in sturgeon fillets with a variety of modifications occurring at the protein primary structure.Oxidized aromatic amino acids and extensive oxidation of lysine to carbonyl-containing compounds such as a-aminoadipic acid(AAS)and a-aminoadipic acid(AAA)were characterized in all the cooked samples.Sulphur-containing amino acids and proline were vulnerable to oxidation as well.In addition,indirect protein oxidation caused by interacting with lipid-derived aldehydes like malondialdehyde(MDA)and 4-hydroxynonenal(HNE)was also observed and much more pronounced in roasted and fried samples,giving a footprint of lipid-protein interaction during cooking.2.The effect of roasting time on lipid and protein oxidation of farmed sturgeon(Acipenser gueldenstaedtii)was studied.Prolonged roasting significantly increased the levels of carbonyls and Schiff bases which were 6 times and 17 times respectively of raw sample after roasting for 30 minutes.While both levels of free thiols and TBARS exhibited a bi-phase evolution(a gradual decrease following a short-time increase)during roasting.Moreover,UPLC-MS/MS results revealed that roasting could trigger extensive oxidation of lysine to form AAS and AAA,with the former being gradually converted to the latter.Besides,oxidation of aromatic amino acids(tryptophan,phenylalanine,tyrosine)were also detected.Advanced glycation end products(AGEs),MDA-and HNE-protein adducts accumulated with increasing roasting time,proving the interaction between protein and carbohydrate and lipid.3.The effect of different cooking methods roasting time on lipid and protein oxidation and their evolution and consequence on proteolysis during simulated digestion of fish fillets were evaluated.Several oxidation markers(TBARS,free thiols,total carbonyls and Schiff bases)were employed to assess the oxidation of fish.SDS-PAGE and TBNS assay for free amino groups were used to study the proteolysis during in vitro gastrointestinal digestion.The results showed that significant lipid and protein oxidative changes occurring in cooked fish fillets were reinforced after gastric digestion and were much more intense after intestinal digestion.The levels of carbonlys and TBARS in preprandial samples could have conspicuous influence on digested counterparts.Free thoils significantly decreased after digestion,but the loss was not affected by cooking methods and roasting time.Besides,close interconnection between lipid and protein was also manifested as the levels of Schiff bases rose to 2-7 times of the undigested samples.Furthermore,protein oxidation induced by cooking methods and roasting time before digestion had a notable impact on proteolysis during digestion.Prolonged roasted and fried samples were less sensitive to pepsin due to polymerization of severely oxidizing protein(50-130kDa).After the whole gastrointestinal digestion,there were still some short peptides(<14kDa)of cooked fish,especially roasted and fried ones,that cannot be digested thoroughly.Free amino groups decreased with prolonged roasting time,signifying protein oxidation before digestion resulted in impaired proteolysis during digestion.