Effects Of Heat Treatment On Functional Characteristics,Structure And In Vitro Digestion Of Quinoa Protein

Quinoa protein is rich in nutrition and remarkable function,and belongs to high-quality plant protein resources.Heat treatment is one of the most commonly used methods in food processing,and it is also a commonly used technique for protein modification.Studying the functional properties of quinoa protein is of great significance for evaluating its application potential and guiding the process of food production and processing.In this study,‘Long Li No.1'is used as the test material,and the quinoa protein is extracted by the ultrasonic assisted alkali-soluble acid precipitation method optimized by the response surface.Systematic study on the functional properties?solubility,emulsifying properties,emulsifying stability,foaming capacity,foaming stability,water holding capacity,oil holding capacity,flexibility and thermogravimetry?,structure characteristics?thiol and disulfide bonds content,particle size distribution,turbidity,endogenous fluorescence,ultraviolet spectrum,FTIR infrared spectrum,scanning electron microscope and 600 MHz NMR spectrum?and in vitro digestion characteristics?hydrolysis degree,antioxidant activity and change of amino acid content?of quinoa protein isolates by heat treatment at different temperatures and times.The aim of this study is to provide a basis for the processing and utilization of quinoa protein isolates.The research results are as follows:1.Optimization of QPI extraction conditions.Based on the single factor of alkali-soluble acid precipitation and the optimization of response surface,the optimal conditions for extracting QPI by ultrasound-assisted treatment were further optimized.The results showed that under the optimal extraction conditions of ultrasonic time 99 min,material-liquid ratio 1:20 g·mL^-1,ultrasonic temperature 47 ^oC and solvent pH value 10,the QPI extraction rate and purity reached74.67?±1.08?%and 87.17?±0.58?%,respectively,which were 10.18%and 5.49%higher than the traditional alkali-soluble acid precipitation method,and the isoelectric point is 4.5.2.The effect of heat treatment on the functional characteristics of QPI.The effects of heat treatment at 60 ^oC,70 ^oC,80 ^oC,90 ^oC,100 ^oC,and 121 ^oC for 5,10,20,and 30 min on their properties were studied.The results showed that the emulsification,solubility,emulsification stability,water holding capacity,oil holding capacity,foamability and flexibility of QPI increase gradually with the increasing of temperature and time under the condition of 60⁹0 ^oC.They reached a maximum of 58.86%,12.05 m²·g^-1,71.25%,161.47%,173.33%,71.67%and 0.42 when the heat treatment conditions were 90 ^oC and 30min,which were 33.41%,83.97%,27.05%,12.50%,14.18%,34.39%and 27.27%significantly higher than the untreated?P<0.05?.However,under the conditions of 100?and 121 ^oC,they decreased significantly with the extension of the heat treatment time.When the heat treatment conditions were 121?and 30min,they decreased to the lowest of 45.45%,6.16 m²·g^-1,61.76%,145.00%,156.60%,50.00%and 0.369,respectively.The thermal stability of QPI is gradually increased with the increase of heat treatment temperature and time.There is a maximum value when the heat treatment conditions are 121 ^oC and 30 min.The foam stability of QPI is contrary to the foaming property result.When the heat treatment conditions are 90 ^oC and 30 min,the foam stability of QPI is 71.76%,which is 20.81%lower than the untreated.3.The effect of heat treatment on the structure of QPI.With the extension of the heat treatment temperature and time,the content of QPI's free thiol first increased and then decreased,and the maximum was 2.75nmol/mg when the heat treatment conditions are 90 ^oC and 30min.The disulfide bond content decreased first and then increased,and the minimum value was 1.79nmol/mg when the heat treatment conditions are 90 ^oC and 30 min.The average particle diameter D_?4,3?,turbidity,and the maximum absorption wavelength of endogenous fluorescence all showed a gradual increase trend,and the maximum was respectively 49.69?m,0.94,363.4 nm and 3.05 when the heat treatment conditions are 121 ^oC and 30 min,and which were increased by 3.31 times,4.53 times,2.25%compared with the untreated group.In addition,scanning electron microscopy found that heat treatment increased pores,roughness and loose structure of QPI surface.The maximum fluorescence intensity and ultraviolet absorption peak gradually decreased,and the minimum of the maximum fluorescence intensity was 17.49 when the heat treatment conditions are 121 ^oC and 30 min,which was 55.33%lower than the untreated group.The relative content of?-helix and?-fold structure decreases,the relative content of?-turn and random coil structure increases,and the secondary structure tends to be disordered.The 600MHz NMR spectrum showed that the normalized relative percentage of QPI had a minimum value,which was respectively 23.25±0.34 and 34.97±0.82%between3.30³.81 and 0.75¹.26 when the heat treatment conditions are 121 ^oC and 30 min,which was8.46%and 2.47%lower than the untreated group.There are maximum values in the interval of 1.77².28,2.28².79 and 2.79³.30 ppm,which was respectively 10.85±0.07%,2.26±0.38%and 5.42±0.96%,which was 24.57%,4.51 times and 7.54%higher than untreated.4.Effect of heat treatment on the in vitro digestion of QPI.With the increase of the heat treatment temperature,the degree of hydrolysis and antioxidant activity decreased,and the degree of hydrolysis was 25.53%when the heat treatment conditions are 121 ^oC and 30 min,which was 20.64%lower than that of the untreated.ABTS+·clearance rate,total reducing power and DPPH·clearance rate were 68.54%,0.585 and 62.54%,which were 24.77%,28.72%and 21.26%lower than the untreated.The contents of total amino acids and free amino acids were 414.87 mg/L and 27.3 mg/L,which were 27.85%and 60.57%lower than those in the untreated.