Effect of different blanching methods on the quality of vegetable soybeans

To clarify the advantage of new pre-processing techniques on the quality retention of frozen vegetables, the effects of blanching, thermosonication and microwave treatments on the sensory characteristics and nutritional quality of vegetable soybean were studied by using a variety of analytical methods, such as LF-NMR and GC-MS.The results showed that blanching significantly reduced the L*value, a*value and ascorbic acid content, but significantly increased the b*value of vegetable soybean ( p < 0.05) .Compared with conventional blanching, thermosonication and microwave treatments were better to retain the ascorbic acid content and chlorophyll content of vegetable soybean. Results showed that there were four distinct water populations in the data of multi-exponential fitting of vegetable soybeans LF-NMR T2 transverse relaxation time, which were T21 ( < 1 ms) , T22 ( 1~8 ms) , T23 ( 40~50 ms) and T24 ( > 250 ms) , respectively.Compared with untreated samples, the values of T22 and T23in vegetable soybean after blanching were decreased significantly ( p < 0.05) .The relative content of free water ( M24) in samples treated by thermosonication was significantly less than that in both microwave and conventional blanching samples ( p < 0.05) . Amino acid analysis showed that vegetable soybean was a kind of high quality protein source, since it contained 7kinds of essential amino acids, and the ratio of essential amino acids/total amino acid was up to 35.2%.However, there was no significant difference in the kind and content of amino acids in vegetable soybean treated by different pretreatment. GC/MS results showed that there were significant differences in the types and content of volatile substances of samples pre-treated by different blanching methods.The values of 1-octen-3-ol and 2-heptenal in samples treated by thermosoncation, which were the main flavoring substances of vegetable soybean, were up to 5.70% and 9.64%, respectively.