Study On The Extraction And Properties Of Quinoa Protein And Starch

Quinoa(Chenopodium quinoa Willd)is a well-known pseudocereal.Its genes are not almost modified,which can contribute to maintain diversity of agro ecosystem effectively.Recently,quinoa has been widely planted in China.However,as a newly-emerged crop,the research on its theory,development and utilization has not yet in depth.Quinoa is rich in starch and the starch granules are small,so it can be used to produce products with special industrial use.Quinoa protein is a kind of high quality plant protein with balanced amino acid composition,which can be used in food processing instead of soy protein.Therefore,the protein and starch of quinoa were separated and extracted in the study.The effects of different separation conditions on the extraction,structural and functional properties and digestion characteristics of quinoa starch and protein were compared.The treatment of removing saponins from quinoa protein was also carried out,which provided a theoretical basis for the deep processing and utilization of quinoa in the future.Firstly,the composition of quinoa protein was studied,and the protein and starch of quinoa were separated by alkali dissolution method.It was found that quinoa protein was mainly composed of albumin and globulin,followed by glutenin,and the content of gliadin was very low.Besides,it also contained about 10.33% insoluble protein,the main force of which was hydrophobic interaction.The optimized conditions for the separation of quinoa protein and starch taking the extraction rate as an evaluation index were as follows: the solid-liquid ratio of 1:12(w/v),pH 11,temperature 35°C,time 2 h.Under the conditions,the extraction rate of protein was 68.11% with the purity of 76.85%;the extraction rate of white starch was 50.54% with the purity of 95.73%;the extraction rate of yellow starch was 33.66% with the purity of 80.03%.Then,the structural and functional properties as well as digestive characteristics of quinoa protein prepared by different pH and temperature were studied.The results showed that within a certain range,increasing the temperature and p H could improve the functional properties of quinoa protein.Though quinoa protein had a poor gel property,its solubility and emulsifying ability were high.The in vitro digestibility of quinoa protein increased after extraction.When the pH increased,the degree of hydrolysis of quinoa protein at the trypsin hydrolysis stage decreased.Besides,the relative molecular mass distribution of the digestible products of the quinoa protein under different separation conditions was similar,and the ABTS+· scavenging capacity of the in vitro digestion products was better.However,the DPPH· scavenging capacity decreased significantly with the increase of pH.Different separation conditions had little effects on amino acid composition of quinoa protein,but with the increase of pH and temperature,the content of AB subunit complexes decreased while the relative molecular mass of 11 k~14 k subunits increased.The extraction temperature had little effect on the secondary structure of quinoa protein.When the extraction pH increased to 11,the content of alpha helix decreased.The separation conditions resulted in the the tertiary structure of quinoa protein unfolding,which was reflected in the decreasing of the thermal denaturation enthalpy as well as the content of the surface mercapto,and when the condition was further strengthened,the protein would aggregate.Nextly,considering the effect of saponins on the nutritional value and palatability of quinoa,the desaponin treatment of quinoa protein was carried out,and the effect of the treatment on the functional properties and digestive characteristics of quinoa protein were explored.After the removal of saponins,the saponin content of quinoa protein reduced from 14.30 mg/g to 1.59 mg/g,while the solubility,emulsification and emulsion stability of the protein all decreased.Meanwhile,the treatment could also reduce the digestibility of quinoa protein.In addition,the surface hydrophobicity and the content of surface sulfhydryl group of quinoa protein decreased while the content of disulfide bond increased with the thermal denaturation enthalpy and the content of alpha helix decreasing,indicating occurrence of protein denaturation and aggregation.Finally,the structural and functional properties as well as digestive characteristics of quinoa starch prepared by different pH and temperature were investigated.The results showed that the separation conditions had no significant effect on functional properties of quinoa starch.After multiple freezing and thawing,the water release rate of quinoa starch was lower than that of corn starch and potato starch,so quinoa starch could be used for frozen food processing.The gelatinization temperature of quinoa starch was about 54°C,which was lower than that of general cereals.Increasing the pH and temperature would hardly affect the gelatinization temperature,but they would cause the viscosity of starch paste to decrease.The starch paste of quinoa had high transparency,as well as high temperature and shear stress resistant,and the content of fast digestible starch(RDS)of quinoa starch was lower than that of common grain starch.Quinoa starch had a polygonal shape with sharp edges and corners.When the temperature or pH was too high,the starch would expand to a certain extent and collapse slighty.