Physico-chemical Properties and Drying Kinetic Evaluation of Hot Air and Vacuum Dried Pre-Treated Oyster Mushroom under Innovative Multi-mode Developed Dryer

Mushrooms are characterized as the fruiting bodies of fungi and are fruitful source of high-quality protein and vitamins with low calories. Among the three most cultivated species, the oyster mushroom stands with limited shelf life (2-3 days at refrigerated conditions). Elevated quotients of browning reaction and restricted shelf life obliges the preservation of species as a matter of concern. A suitable pre-treatment along with the drying method is very important to retain the bioactive compounds of oyster mushroom. Pre-treatment optimization prior to oyster mushroom drying was carried out in two steps which involved individual and combined effects of blanching (70 to 90 oC) and chemical treatments. Thereafter, it was dried in a recently developed multi-mode novel drying unit under hot air and vacuum drying (50-70oC) conditions. Pre-treatment with citric acid and blanching at 80 oC for 2 min resulted in the lowest residual activity of polyphenol oxidase. Retention of phenolics, flavonoids, ascorbic acid and antioxidant compounds were higher in the samples dried using vacuum drying. Increase in temperature from 50 to 70 oC significantly decreased the bioactive compounds and colour of vacuum dried samples. In samples dried using hot air-drying, the higher retention of bioactive compounds and colour was obtained at 60 oC as compared to 70 oC and 50 oC. Page model was found to be the best fitted model among the different models studied. Analysis inferred the usefulness of optimised pre-treatment and vacuum drying technique at low temperature for drying of oyster mushrooms.Abbreviations: AA, Ascorbic acid; BI, Browning index CA, Citric acid; FRAP, Ferric reducing antioxidant power; KMS, Potassium metabisulphite; MR, Moisture ratio; PPO, Polyphenol oxidase; RR, Rehydration ratio; RSA, Radical scavenging activity; TFC, Total flavonoid content; TPC, Total phenol content