Study On Gel Properties And Antioxidant Mechanism Of Protein From Yoghurt Treated By MTG

At present,problems mainly are focused on whey precipitation,short shelf life,safety concerning about the use of food additives and so on in the fermented milk industrthe.Related studies show that the addition of mTG is an effective way to solve the whey seperated.However,the effect of adding mTG on the antioxidant activity of proteins in fermented milk and the mechanism of structure-activity are not clear.In this paper,lactic acid bacteria and mTG were used to produce yoghurt,The main work and achievements of this paper are as follows:1.The effects of mTG addition on the sensory quality of fermented milk were evaluated by titration acidity,whey precipitation,viscosity and water holding capacity.When the addition amount of mTG was at the range of 2U ~ 3U for one gram milk peotein,the gel structure was uniform and exhibited good sensory and texture quality.2.Fermented milk protein(FMP)and mTG treated fermented milk protein(mFMP)were isolated by centrifugal trapping with 30 kDa,10kDa and 3kDa ultrafiltration membranes,which were identified as FMP/mFMPI,FMP/mFMPII,FMP/mFMPIII and FMP/mFMPIV,and the FMP-GI/mFMP-GI was obtained by following gastrointestinal digestion(GI).The antioxidant activity effects of mTG on FMP and FMP-GI in vitro were evaluated by different systems.The results showed that the antioxidant activities of FMP/mFMP and FMP-GI/mFMP-GI were negatively correlated with their molecular weights(p<0.05);The antioxidant capacity of FMP-GI/mFMP-GI was significantly higher than that of FMP/mFMP(p<0.05).3.SDS-PAGE was used to analyze the peptide composition of all FMP/mFMP ultrafiltration components.The results revealed that milk protein new polymers appeared(molecular weight of about 43.0kDa)in mFMPI.The band of mFMPⅡwas darker than FMPⅡ in the molecular weight of about 14.0kDa and 18.0kDa.The bands of FMP/mFMPⅢ and FMP/mFMPⅣ were not obvious.The free amino acid composition of each FMP/mFMP ultrafiltration component was analyzed by amino acid automatic analysis technique.The results showed that the total amount of free amino acids in mTG group was significantly higher than that in blank group(p<0.05).The contents of amino acids with strong antioxidant activity(Phe,His,Tyr,Met)and hydrophobic amino acids(Ser,Thr)were significantly changed(p<0.05).4.Using Sephadex G-25 gel chromatography to isolate and purifie FMP/mFMP producted FMPS/mFMPS-1 and FMPS/mFMPS-2.Their antioxidant activities were determined in different systems.The results showed that the antioxidant activity of FMPS/mFMPS-2 was significantly higher than that of FMPS/mFMPS-1,and the antioxidant activity was FMPSⅣ-2、 FMPSⅢ-2、FMPSⅡ-2、FMPSⅠ-2 from high to low.Further investigation of FMPS/mFMPS-2 amino acid composition,results showed that mFMPⅢ-2 had a large amount of antioxidant activity amino acid Met、Tyr.Analysising mFMPSIV-2 amino acid composition found that it only contains Ser,Cys and Tyr three amino acids.The main antioxidant peptides of mFMP Ⅲ-2 and mFMPS Ⅳ-2 were analyzed by using quadrupole time-of-flight mass spectrometry(Q-TOF).The results showed that little peptide was not detected in mFMPSⅣ-2.IAKYIPIQ,NQFLPYPYYAK,PAAVRSPAQILQWQ and VLPVPQKAVPYPQ antioxidant peptiedes were identified from mFMPⅢ-2.5.The antioxidant activity of IAKYIPIQ,NQFLPYPYYAK,PAAVRSPAQILQWQ and VLPVPQKAVPYPQ was evaluated by the DPPH· scavenging system.The secondary structure of lactopeptide was analyzed by fluorescence spectroscopy and FTIR respectively.The results showed that antioxidant peptiedes all had strong antioxidant activity.DPPH· scavenging rate of NQFLPYPYYAK may be associated with the inclusion of irregular coil structures.