| Beijing roast duck is a famous traditional meat product in China that is very popular with consumers and is characterized by tenderness and chewiness.More than 95% of the Beijing roast duck processed by restaurants is roasted in Gua-lu oven at present.Roasting is an important process in the preparation of Beijing roast duck,and the roasting process and the ripeness determination depend mainly on the experience of the operators,and automatic equipments and ripeness measurement standards are lacked,which often causes certain problems such as uneven product quality and unstable quality,and it is difficult to realize large-scale and standardized production.The end-point of roasting has a greater impact on the tenderness and other qualities of meat products,and is the key control point of standardized production.To realize the industrialized production of traditional Beijing roast duck,it is necessary to study the forming mechanism of quality,optimize processing technology and quantify technological parameters.Real-time monitoring of the center temperature during roasting could better determine the ripeness of the product.Most studies on Beijing roast duck focused on hazard analysis and flavor formation,but few studies on the determination of quality.Studies on the formation mechanism of tenderness based on protein interaction have not been reported.Beijing roast duck was taken as the research object,and the real-time monitoring of the temperature of the traditional hanging furnace and the Ansys simulation were used to quantify the thermal field,which was conducive to the formation of tenderness.Low-field NMR,texture analyzer,scanning electron microscope and transmission electron microscopy were used to investigate the tenderness law of traditional Beijing roast duck.Combining with differential scanning calorimetry,fourier transform infrared spectrometer,laser particle size analyzer,transmission electron microscopy and isothermal titer quantitative thermometer,the relationship between myofibrin characteristics and protein molecular interaction and tenderness were discussed,and Discovery Studio software was carried out to illustrate the protein interaction and further verified.Thus,it will reveal the formation mechanism of tenderness in the traditional roasting process of Beijing roast duck.(1)The temperature of the traditional hanging oven for Beijing roast duck at different locations and the temperature of duck meat were quantified to realize the digitization of the thermal field and the prediction of the central temperature.The temperature of the oven was reduced uniformly annular from the heat source outward,and the temperature of the fire source,the furnace mouth,the furnace wall and the air temperature in the furnace were 500-700°C,250-400°C,220-230°C,and 201-254°C respectively,the highest point of the roast duck oven temperature was the heat source at the mouth of the oven.The temperature was up to 718.31°C,and the extreme difference with the lowest point was 537.48°C,the temperature coincidence rate between the highest point and the lowest point was as high as 95% or more.The temperature reduction in X,Y and Z directions was 41.7%,35.3% and 30.8% respectively,and the temperature reduction in X direction was the largest.In order to make the duck billet evenly heated,the fruit trees were layed as evenly as possible along X direction,and the duck billet needed to be moved in time.Ansys simulation verification could effectively quantify the heat field of roast duck furnace.The surface temperature of the duck body rapidly increased from 23.9°C to 174.4°C and then remained stable,and the core temperature rose from 20.6°C to 99.3°C.The surface temperature and center temperature conformed to the tertiary and the quadratic function curve with the roasting time,respectively,which could realize the real-time prediction of the surface temperature and central temperature of the roast duck.The heat field in the roast duck oven promoted the central temperature of the duck breast to rise,providing a heat field environment for the formation of tenderness of Beijing roast duck.(2)The regularity of tenderness change of Beijing roast duck caused by muscle fiber contraction,moisture and fat of traditional hanging oven roast duck was clarified.With the prolongation of roasting time,the shear force of Beijing roast duck decreased significantly,and remained stable after 20 minutes,which was the key point of shear force change.The hardness,masticatory and adhesiveness first increased and then decreased,the elasticity,adhesion and cohesion increased,and the resilience decreased.The hardness,adhesion,adhesion and masticatory properties reached the highest value at 20 min,the elasticity and cohesion reached the highest value at 40 min,and the adhesion and resilience reached the lowest value at 60 min.The shear force is highly negatively correlated with cohesion,chewiness,and stickiness,hardness is highly positively correlated with cohesion,chewiness,and stickiness,and elasticity is highly positively correlated with chewiness.The shear force,hardness and elasticity could be used to represent the tenderness of Beijing roast duck.After roasting,heat will cause damage to the surface of muscle fibers.The structure of muscle fibers gradually gathers and become more dense.At 20～40 min,band I shrinks,Z line,M line and A band thicken,and Z line disappears after40 min,and I band twists and breaks at 60 min,and large holes appear.Sarcomere length and muscle fiber diameter continuously decrease,and 20 min and 40 min are the turning points for the microstructural changes.The tenderness of roast duck is closely related to the microstructure,the shear force decreases,the elasticity increases,the muscle fiber diameter and sarcomere length are constantly decreasing,and the muscle fibers shrink.The moisture in roast duck evaporates when heated,which was released from an immobile state to become free water,water content is constantly reduced.Subcutaneous fat is heated and melted,most of which moves into the meat,the fat content continues to rise,and a small part of the melted fat forms dripping oil,resulting to increasing cooking losses.Shear force is highly positively correlated with water content and immobile water content,while elasticity is highly negatively correlated with water content and immobile water content.The results of low-field MRI and imaging jointly confirm that the tenderness of roast duck is the result of the combined effect of water and fat.(3)The function law of in the traditional Gua-lu roasting,the heat degeneration and aggregation of myofibrillin lead to the formation of tenderness was detected.During the roasting,the denaturation time of myosin head,myosin tail and sarcoplasmic protein and actin was 20 min,30 min and 40 min respectively.Shear force was highly positively correlated with actin denaturation,while hardness and elasticity was highly negatively correlated with myosin degeneration,and actin degeneration had a more significant effect on tenderness than myosin degeneration.Roasting for 20 min and 40 min was the turning point of thermal denaturation of duck breast myofibrillar protein and the change of tenderness of Beijing roast duck.The surface hydrophobicity of myofibrillar proteins was enhanced,ionic bonds and hydrogen bonds were reduced,and the side chains of hydrophobic aromatic amino acids were exposed,forming hydrophobic interactions.The secondary structure changes from α-helix and β-fold to β-fold,and the protein conformation changes.The turbidity and particle size increased,the charge increased,and the electrostatic interaction changed,the proteins bound together,and gathered more tightly.Hardness and elasticity were negatively correlated with hydrophobic interactions and hydrogen bonds,and shear force was highly negatively correlated with hydrogen bonds,hydrophobic interactions and turbidity.There were interactions between the intermolecular force,water retention and microstructure and the indicators that characterize the tenderness of Beijing roast duck.With the extension of roasting time,myofibrillar protein was denatured by heat,the intermolecular force changed,the molecular structure became loose,which was released from an immobile state to become free water,protein irreversiblely precipitated to form aggregates,forming a three-dimensional network structure,and then finally resulted to the unique tenderness of roast duck.(4)The effect of binding mode of myosin S1 and actin on the tender formation of Beijing roast duck was analyzed by in vitro heating model and molecular dynamics simulation.It was temperature that exerts a positive effect on the interaction of the in vitro heating model myosin S1 with actin.The interactions of myosin S1 and actin belonged to thermodynamically moderate binding strength during20-60℃.After heating,the α-helix content of myosin S1 and actin system decreased,random coil rose,and β-angle content rose first and then decreased.The secondary structure changed from α-helix andβ-fold at 20℃ to β-fold and random coil at 80℃,and the interaction index of α-helix at 20°C and the interaction index of random coil at 80°C were the strongest.There were interactions when β-folding at20 ℃ and 60 ℃ and β-turnat 40 and 60 ℃.The particles of protein became larger and the proteins became dense after heating,forming distinct aggregates at 60℃ and 80℃.The interaction index of myosin S1 and actin at 80℃ was the largest.The interaction between the proteins increased with increasing temperature.The tenderness of Beijing roast duck was closely related to the protein appearance,morphology and secondary structure.Molecular dynamics simulation confirmed that myosin S1 and actin are rigidly docked and the optimized conformation was more stable,of which the hydrophobic amino acidswere mostly helices,and a total of 32 amino acids changed and got one conformation Poses3.The calculation time of molecular dynamics simulation were within 329-344 h,and the average energy gradually increased and the total energy first decreased and then increased,and the interaction at 40℃ was the strongest.RMSD confirmed that the protein simulation systems at 20℃,40℃,and 60℃ reached to equilibrium and RMSF values at 40℃ was higher than others and more significantly fluctuated.The exposure of hydrophobic residues might provide docking sites for the aggregation of myosin S1 and actin,and Met800(S1),Phe798(S1),Leu799(S1),and Val 47(Actin)were the common amino acids.The amino acid residues which could interact were distributed evenly on the two molecules,and the dihedral angle reached their maximum at 40℃.The binding stability of myosin S1 and actin was related to temperature.The binding of myosin S1 and actin was more stable at40 ℃.The shear force of Beijing roast duck was highly negatively correlated with molecular size and its interaction index,and highly positively correlated with α-helix.Hardness was highly positively correlated with β-turn,and elasticity was highly positively correlated with molecular size and random coil,and was negatively correlated with α-helix.The shear force of Beijing roast duck decreased,elasticity and hardness increased,and then tenderness was formed.Molecular size and its interaction index increased,and α-helix decreasedand β-turn and random coil increased.The appearance and secondary structure of myosin S1 and actin were closely related to the tenderness of Beijing roast duck,which could be used to explain the change law of tenderness. |
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