Microorganisms Contaminated in the Process of Cultivation and Their Effect on the Production of Shiitake

Abstract

Contamination has been a major problem for the shiitake production in Taiwan. To eliminate the contamination to a level as low as possible is very important. In this study, the contaminated microorganisms were identified and the sources of contaminants in the sawdust, spawn, inoculation and incubation rooms, and cultivation yard were described. Attempts were also made to explore the effects of the contaminants on the mycelial growth and/or the yield of shiitake. The effect of medium pH values on the mycelial growth of shiitake and contaminants were also reported. Results indicated that species of Trichoderma, Penicillium, Neurospora and Aspergillus were the most frequently isolated fungi from the sawdust spawn, while Trichoderma, Bacillus and Aspergillus were often isolated in the wheat grain spawn. The mycelium of shiitake grew more quickly in the wheat grain medium than it did in the sawdust medium. More microorganisms were trapped in the cultivation yard than in the incubation room. Unused sawdust piled outdoor for 6 months contained more microorganisms than the cultivating one. During marketing or transporting process, population of microorganisms on the surfaces of shiitake fruiting bodies increased greatly. Less amount of microorganisms were found in the normally grown sawdust than in the contaminated sawdusts. The pH value of contaminated sawdust was higher than the normal one, but not much difference was observed in the water contents of the two kind of sawdusts. The pH value of Castanopsis hytrix saw-dust was very low within 2 months of piling, but it gradually increased with time and reached ca. 6.0 within 3-4 months which will be suitable for medium perparation. The pH values of Acacia confusa and broad leaf trees also increased during piling. The mycelia of shiitake grew best in the acidic conditions when the pH value was between 4.5 and 6.0, while Trichoderma grew very well in almost all tested pH values except 7.5 The growth of Neurospora was relatively slow in all pH values with pH 6.5 as the most suitable one. When the air temperature was equal to or greater than that of sawdust, less microorganisms were found in the sawdust, however, when the air temperature was lower, more microorganisms were found. In spite of pH values, more shiitake mushrooms were produced in the sawdust medium pasteurized 8 hrs at 98℃ than that autoclaved 90min. at 121℃ (151b/in^2). The quality of shiitake mushrooms harvested from the former was also better than those form the latter. In the pasteurized medium, yields of the medium with pH values at 5 and 6 were higher than those of the medium with pH 7. Autoclaving completely killed the microorganisms in the sawdust medium while pasteurization only killed those harmful ones.