Effect of Freezing-thawing Cycle on the Quality of Prefabricated Mushroom Soup

To investigate the influence of freezing-thawing cycle on the quality of prefabricated mushroom soup, the differences in the basic indicators of mushroom soup including microbial quantity, color, soluble protein, reducing sugar, and total phenolic content were compared among unfrozen samples (control) and repeated freezing-thawing samples. Besides that, the equivalent umami concentration (EUC) values and volatile flavor compounds in those prefabricated mushroom soup were further analyzed by high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS), respectively. The results showed that the first freezing-thawing treatment comprehensively affected the quality of prefabricated mushroom soup by decreasing the content of basic components (reducing sugar, total phenolics and total amino acids) and flavor (EUC value and volatile components) of mushroom soup obviously. The effect of freezing-thawing cycles (1~5 cycles) on the quality of mushroom soup was further compared. When the mushroom soup was frozen and thawed for three times, the content of soluble protein in the soup decreased significantly (P<0.05), in addition, the EUC value greatly decreased from 0.441~0.450 g MSG/100 g (1~2 freezing-thawing treatments) to 0.407 g MSG/100 g. Microorganisms began to be detected in the third freezing-thawing prefabricated mushroom soup. In the subsequent freezing-thawing treatments (4~5 times), the content of total phenolics, total amino acids, and EUC value of the mushroom soup were relatively stable, while the content of nucleotides, soluble proteins, and volatile components continuously decreased, together with the bacteria kept proliferating. Consequently, the freezing-thawing times of prefabricated mushroom soup could be monitored in twice for final consumption.