Study On Improvement Of Preparation Of Rapeseed Protein And Enzymatic Hydrolysates

Rapeseed meal is the by-product of rapeseed oil industry. Rapeseed meal is abundant in China as it has the largest annual amount of rapeseed. Rapeseed meal is rich in quality protein and the enzymatic protein hydrolysates have a variety of biological activities. However, the application of the protein and the hydrolysates are limited and mainly used as fertilizer or feeds due to the presence of anti-nutritional substances, such as glucosinolate, phytic acid and phenolic compounds. The objective of the paper is to improve the preparation of rapeseed protein and enzymatic hydrolysis in order to utilize rapeseed meal more effectively.Results of preparation of protein by alkali-extraction and acid-precipitation showed that the recovery of protein could be increased obviously by the addition of alkali to keep the pH value through the alkali-extraction process. The recovery could reach as high as 91.1% by extraction for two times at pH of 11. The pH of NaOH solution could fall from 11 to 6.5 during the alkali-extraction process without the addition of extra alkali solution and the recovery of protein could drop to 61.8%. Due to the broard isoelectric point range, the rapeseed protein obtained in the acid-precipitation process at pH of 4.5 was about 40% of that in alkaline extracts and the acid-soluble protein (pH4.5) was not recovered.The acid soluble protein could be recovered and the protein recovery could be increased if the rapeseed protein was prepared by the combination of alkali-extraction and acid-precipitation with membrane processing. The content of glucosinolate and phytic acid in rapeseed meal, precipitated protein and soluble protein was analyzed. The content of glucosinolate and phytic acid in rapeseed meal was 1.38 mg/g and 3.68% respectively. Glucosinolate was not detected in precipitated protein but was 2.43 mg/g in acid soluble protein. The content of phytic acid of 0.50% and 0.75% in precipitated protein and soluble protein respectively meant that most phytic acid was left in rapeseed residue. The content of glucosinolate and phytic acid could be reduced by membrane processing, with glucosinolates content of only 0.11 mg/g of acid-soluble protein after membrane separation of CF 4 and DV 3. However, phytic acid was not effectively reduced as it might combine with the soluble protein. The acid-soluble protein after membrane processing showed DPPH radical scavenging activity.The enzymatic hydrolysis of protein in rapeseed meal and especially the effects of addition of sodium sulfite on hydrolytic rate, molecular weight and functional properties of hydrolysates were studied. Effects of the variety of enzyme(ie. alcalase, papain, neutrase and flavourzyme) on hydrolysis rate of rapeseed protein in meal were studied. Results showed that alcalase was more effective than three others. Sodium sulfite could increase hydrolytic rate which was dependent on the variety of protease. The degree of hydrolysis of rapeseed protein at 60min with the addition of sodium sulfite was 1.8,2.5, 2.9 and 1.7 times that without sodium sulfite for alcalase, papain, neutrase and flavourzyme respectively. Chromatograms of the size exclusion chromatography showed that the molecular weights of protein hydrolysates with the addition of sodium sulfite were lower than that without sodium sulfite. The addition of sodium sulfite could improve fat adsorption, but decrease emulsifying activity, foaming capacity and foaming stability of hydrolysates. Fat adsorption and foaming stability of hydrolysates could be improved by ultrafiltration and diafiltration.Acid-soluble (pH4.3) rapeseed protein hydrolysates were prepared by the combination of enzymatic hydrolysis and alkali extraction. Results showed that the solubility of hydrolysates with alcalase was improved and the recovery was increased after further extraction with alkali. Inactivation of enzyme by lowing pH could improve the recovery of protein hydrolysates compared with that by heating. The recovery of protein hydrolysates were 73.4% at the degree of hydrolysis of 10% and at extraction pH of 9 with the inactivation of enzyme by lowing pH. The hydrolysates were soluble at pH of 4.3 which might be used as a nutritional fortifier in acidic foods such as acid soft drink after further study to separate antinutritional factors, such as glucosinolate, and inorganic salts by membrane processing.