Changes Of Sour Meat Protein And Its Effect On Edible Quality During Fermentation

Fermented sour meat is one of the special fermented meat products in China,which has the characteristics of sour mellow,unique flavor and long storage period.Usually,it can be eaten after fermented for about 20 days,the remaining parts are often continued to use sealed fermentation method for preservation.However,for a long time fermentation,it is easy to cause microbial contamination,serious oxidation of fat or protein,resulting in deterioration in quality.Protein as a major component of meat products,studying its changes in the processing is helpful to understand the quality formation and deterioration mechanism of the product.But there is relatively little research on sour meat in this area.In view of this,this paper takes sour meat as the research object,from the perspective of degradation,oxidation and structure characteristic,mainly studies the change of sour meat protein during 180 days of fermentation and its effect on quality,aiming to further understand the mechanism of quality change of sour meat,and provide theoretical basis for controlling the safe production of sour meat.The main contents and results are as follows:?1?With the prolonging of fermentation time,the content of protein and fat decreased gradually,the moisture content decreased first and then fluctuated,and the ash presented a fluctuated trend.The results of color and texture characteristics showed that a*value was first rise after the decline trend,L*value and b*value was first decline after the rise trend,and hardness,cohesion first increased after the gradual decline.However,in the later stages of fermentation,the elasticity was decreased gradually due to the deterioration of protein gel.The value of TVB-N and TBA was on the rise and has a good linear relationship with the fermentation time,especially the TVB-N,which can be used as an important indicator to judge the quality of sour meat.After 20days of fermentation,the organic acids in the sour meat were mainly tartaric acid and lactic acid,and the pH value fluctuated between 4.47 and 4.78 as the fermentation continued,and slightly increased in the later stage.In addition,the rapid growth and reproduction of lactic acid bacteria inhibited the growth of harmful bacteria such as enterobacter.Sensory evaluation showed that the quality of fermented 50⁸0 days sour meat was good.After 140 days of fermentation may be due to the severe oxidative degradation of protein and fat,resulting sour meat produced unpleasant odor,acceptability decreased.By combining the results of TVB-N,we could be preliminarily judged that if the fermentation method was adopted for preservation,the consumption period should be within110 days,not more than 140 days.?2?The results showed that the content of non-protein nitrogen,amino nitrogen and free amino acid increased gradually during the fermentation process.SDS-PAGE showed that the degradation of sour meat protein was obvious,especially after 110 days of fermentation,many protein bands disappeared.But at the same time,the SDS-PAGE of sarcoplasmic and myofibrillar protein emerged new protein fragments may be due to protein cross-linking.In addition,with the extension of fermentation time,the content of sarcoplasmic protein and myofibrillar protein was reduced,while the content of alkali soluble protein continuously increased?P<0.05?,and the turbidity and the average particle size of those proteins increased compared with 0 days,but fluctuated during fermentation.All these results indicated that although the protein denaturation and aggregation occurred during fermentation,it did not affect the continuous degradation of sour meat protein.On the other hand,the results of the related nutritional quality index showed that the total amount of amino acids gradually decreased,and in vitro digestibility was increased first and the decreased?P<0.05?.Correlation analysis found that the in vitro digestibility of pepsin and TCA soluble peptide and alkali soluble protein significantly correlated?P<0.01?,the in vitro digestibility of pepsin+trypsin was significantly correlated with the content of NPN,TCA-soluble peptide,sarcoplasmic,myofibrillar protein and alkali soluble protein.Besides,sarcoplasmic protein and myofibrillar protein were negatively correlated with hardness,cohesiveness and chewiness,and positively correlated with elasticity.While alkali soluble protein was the opposite,and only alkali soluble protein was correlated with sensory scores.?3?The changes of protein oxidation and structure during the fermentation process were analyzed.The results showed that the free sulfhydryl group and sulfhydryl content of sarcoplasmic protein and myofibrillar protein or acid meat protein were decreasing in the whole fermentation process.Ca²⁺-ATPase activity of myofibrillar also decreased significantly,and the activity was basically disappeared at 50 days of fermentation,while the relative content of schiff base increased with the increase of fermentation time.All those results indicated that the degree of oxidation and denaturation of protein increased with fermentation.Especially after 110 days of fermentation,disulfide bond content significantly increased,leading to more protein cross-linking,which could lead to a decrease in digestibility or deterioration of sour meat quality.The results of protein molecular interactions,ultraviolet absorption,intrinsic fluorescence and surface hydrophobicity showed that the electrostatic interaction was gradually weakened.However,hydrophobic and disulfide bonds were significantly enhanced,both of which became the main force affecting the stability of the protein and also the main cause of the decrease of protein salt solubility.In addition,the protein could be induced by acid and salt,a large number of hydrophobic groups exposed,which led to the decline in the surface hydrophobicity.But may be the changes in pH value and protein hydrolysis,the hydrophobic amino acids,such as tryptophan residues were instead exposed to polar environment,making the change of protein tertiary structure was very complicated.Raman analysis showed that the?-helical content decreased,while the content of the?-sheet increased gradually,and protein secondary structure was reorganized at 110 days fermentation.