| Okra?Abelmoschus esculentus?is a kind of versatile and high-value herb that can be divided into green okra and red okra depending on the color of the fruit.It has been widely planted in many provinces throughout the country since its introduction into China in the 1990s.Okra has large production and abundant resources.At present,the okra is mainly eaten,but its pods contain a large amount of pectin resources that can be exploited.This experiment used green okra and red okra as raw materials,and extracted pectin by the method of ultrasonic-assisted acid.In the single factor experiment,the influence of acid types,solid-to-solvent ratio,pH value,extraction temperature,ultrasonic time and ultrasonic power on the extraction rate of pectin was explored and analyzed.The Box-Benhnken design was used to optimize the optimum extraction conditions and compare the physicochemical properties,rheological properties and emulsifying properties of the pectin.The main findings are as follows:1.The optimized extraction conditions of pectin from green okra were as follows:solvent pH 2.0,hydrochloric acid,extraction temperature 70?,solid-to-solvent ratio1:40 g·mL-1,ultrasonic time 41 min,ultrasonic power 350 W,while 15.87%of the pectin yield was achieved under these conditions.The optimized extraction conditions of pectin from red okra were as follows:solvent pH 2.0,hydrochloric acid,extraction temperature 70?,solid-to-solvent ratio 1:36 g·m L-1,ultrasonic time 42 min,ultrasonic power 350 W,which enable the pectin yield to reach 15.03%.2.The results from physicochemical analyses and Fourier transform infrared spectroscopic?FTIR?showed that theall indicators of green okra pectins and red okra pectins are better than the national standards.78.72%and 72.92%of the content of galacturonic acid,66.95%and 68.68%of esterification degree were detected in the pectins from green okra and red okra,respectively.Both pectins are high methoxyl pectin and the gel texture characteristics of hardness,elasticity,cohesiveness,stickiness,chewiness and recovery have been significantly improved after adding two kinds of pectin.3.All pectin samples showed features of shear thinning fluids under different treatment conditions.Pectin concentration,pH value,sucrose content,temperature and heat treatment time had different degrees of influence on the apparent viscosity of the two okra pectin.Under the same conditions,the viscosity of red okra pectin was significantly greater than that of green okra pectin.In the determination of dynamic viscoelasticity,pectin concentration and p H value are the main factors that affect the viscoelastic properties of the two okra pectin.The red okra pectin has different viscoelastic properties at pH 4 than other conditions,but the sucrose concentration had no obvious effect on the viscoelastic properties of pectin.4.Okra pectin has good emulsifying properties under acidic conditions.The pectin stability first increased and then decreased.Emulsion particle size,Zeta potential and microscopic observation all showed that the pectin emulsification of green okra at pH5,the stability of the liquid was the highest,while the emulsion stability of red okra pectin first decreased and then increased.At pH3,the stability of the emulsion was the best.With the increase of pH,the emulsification activity of pectin of okra first increased and then decreased,and the emulsification activity of pectin of red okra gradually increased.The viscosity of the red okra pectin emulsion was obviously greater than that of the green okra pectin,and the pH value had a greater influence on the pectin creaming index of the red okra than that of the green okra pectin,and the time was an important factor affecting the creaming index. |
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