Effect of Different Pretreatments and Drying Methods on The Compositional Quality of Oyster Mushroom

This study was carried out to evaluate the effect of pretreatments prior different drying methods and temperatures on compositional quality of the dried mushroom. Oyster mushroom (Pleurotus ostreatus) was cultivated on rice substrate and collected samples were pretreated (steeped or blanched) by different solutions (0.1% sodium metabisulfite and 0.1 and 0.5% citric acid) and dried using oven at 50°C and 60°C, sun-drying at 30-35°C and microwave drying at 2.45 GHz until reached to constant weight. Rehydration ratio and browning index (color) of the dried oyster mushroom samples were the most effective parameter for judging and selecting the pretreatment and the drying process.Blanching pretreatment seems to elongate drying time compared to control or steeped samples. Control and steeped samples recorded the highest rehydration ratio and lowest browning index compared with blanched samples dried at the same drying temperature. Samples steeped in 0.1% sodium metabisulfite or 0.5% citric acid prior drying had the lowest browning index, while those blanched in the same solutions had the highest browning index values. In general, drying processes has affected on the chemical composition of different samples, especially protein.  Moreover, reduction in total microbial counts of dried mushroom samples was recorded. The total amino acid contents of fresh mushroom was in the highest levels followed by steeped pretreatment with 0.5% citric acid prior sun-dried mushroom and oven dried mushroom, while was lowest in microwave dried mushroom. Essential amino acids of fresh mushroom were higher than published by FAO/WHO.In this study, minerals of microwave dried mushroom were in lowest levels. Sun-drying and oven-drying after pretreatments with 0.5% citric acid has no effect on mineral contents in dried mushroom.Total phenol and flavonoids were not affected by oven or sun-drying while there was little effect by microwave drying. On the other hand ascorbic acid was greatly affected by microwave drying. Sensory evaluation revealed that control and samples steeped in 0.5% citric acid prior sun-drying and microwave got higher scores in colour, texture and taste than other oven drying mushroom.