| The flaxseed protein and other small varieties of vegetable protein with high protein contents(over 90%)prepared by green processing technology are increasingly expected by the market.Aqueous medium extraction method is considered to be a vegetable protein extraction technology with a green concept,since no organic solvents are adopted in this process.Currently,problems of difficult separation of protein and lipids in aqueous phase and the poor processing characteristics of flaxseed protein are urgent to be solved.On the basis of understanding the existing state and separation mechanism of flaxseed protein and lipid in the aqueous phase system,the paper studied the process of preparing flaxseed protein with high protein content by aqueous medium method,and further improved the solubility and thermal stability of flaxseed protein through enzyme modification.This paper provides a theoretical basis for the green processing of flaxseed.The main results are as follows:Firstly,the pretreatment method of flaxseed and the aqueous extraction conditions were investigated.The results showed that shelling treatment was more beneficial to the extraction of flaxseed protein and lipids than degumming treatment.After shelling,the extraction rate of protein and lipid increased from 71.45% to 92.05% and from 85.87% to 92.36%,respectively.Grinding with low-speed and long-time,beating in high-speed and increase in pH are all conducive to improving the extraction rate of flaxseed protein and lipid;however,the obtained milk was seriously emulsified with high-speed beating,resulting in a very low removal rate(only 77.00 %)of lipid after low-speed centrifugation(3000g),while grinding with low-speed can effectively avoid the emulsification phenomenon,and the removal rate of lipid at low-speed centrifugation can reach 87.22 %.The existence status of proteins and lipids in flaxseed milk was studied by sequential centrifugation,particle size analysis,confocal laser scanning microscopy(CLSM),protein electrophoresis and lipid composition analysis.It was found that flaxseed milk was a polydisperse system with an average particle size of 1051.13 nm.Oil droplets,formed by combination of most of the neutral lipids(86.95%),a small amount of polar lipids,and oleosins,can be separated by low-speed centrifugation in the form of emulsion layer;while most of the soluble proteins(91.30%)remained in the aqueous phase.After medium-speed(12000g)and high-speed(38900g)centrifugation,the obtained supernatant was mainly composed of flaxseed storage proteins,while the precipitation was insoluble protein-lipid complexes,in which the ratio of oleosins and polar lipids was higher.Thus,the removal of protein-lipid complexes is the key to improve the protein content of products.In order to obtain the method of preparing flaxseed protein with high protein content under low centrifugation condition,to fulfill the feasibility of industrial production,the separation effect of protein-lipid complex was further investigated as a function of pH,temperature and centrifugal force,etc.The results showed that the completely separation of protein and lipid complexes can be achieved by adjusting pH back to 7.0,and centrifuging at 12000 g for 20 min at 20?C;after isoelectric point precipitation,flaxseed protein isolate(FPI)with a protein content of 90.55%,and a recovery ratio of 40.16% was obtained;on the other hand,the precipitate fraction which contained 67.00% of protein with a recovery ratio of 23.83% can be used as flaxseed protein concentrate(FPC).Then the physical and functional properties of FPI products were characterized and the enzyme modification of FPI was studied.The results showed that the main components of FPI protein were globulin(linin)with the relative molecular weight of 20000~37000 and albumin(conlinin)with the relative molecular weight of 10000,as well as a small amount of oleosin and caleosin.Heat treatment could improve the emulsifying activity of FPI,and had no effect on other properties.Flaxseed protein exhibited weak gel properties,and addition of 0.4 mol/L NaCl could further increase gel strength,while the alkaline conditions were not conducive to gel properties.About 32 kinds of volatile substances were detected in FPI including aldehydes,alcohols and ketones,which increased to 42 after heat treatment.The results of enzyme modification showed that the solubility of FPI can be improved from 41.17% to 84.56% after limited hydrolysis with alkaline protease.The recovery ratio and properties of flaxseed oil products determine the overall feasibility of flaxseed aqueous medium processing technology.The results showed that under the optimal demulsification conditions(pH 5.0,freezing at-18?C for 18 h,thawing at 50?C for 1 h),the centrifugal demulsification ratio reached 94% and the oil recovery ratio was 74.10%.The prepared flaxseed oil showed better values in the acid value,phospholipid content,vitamin E content and fatty acid composition as compared to cold drawn oil,except the higher peroxide value.The content of alcohol volatile substances in aqueous medium extracted flaxseed oil was high,while cold drawn oil contained less content of alcohol,and more content of esters and acids. |
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