**3. History of electrical stimulation for mushroom fruiting body development**

The application of a pulsed high voltage to improve the yield in edible mushroom cultivation has also been attempted by some research groups. The fruiting capacity of shiitake mushroom (*L. edodes*) was remarkably promoted by applying a high voltage to cultivation bed-log (wood) [3]. This effect was also recognized in *L. edodes* fruiting on a mature sawdust substrate [5, 6]. The fruiting body (sporocarp) yield in the electrically stimulated substrate was observed to be 1.7 times more than that without the electrical stimulation [6]. This effect was also confirmed in the fruiting body development of edible mushrooms: *Grifola frondosa*, *P. microspora*, *F. velutipes*, *Hypsizygus marmoreus*, *P. ostreatus*, *P. eryngii*, *P. abalones* and *Agrocybe cylindracea* [7, 8]. The fruiting body yield in the electrically stimulated substrate was observed to be 130–180% greater than that without the electrical stimulation [7]. The high-voltage stimulation technique was also applied to ectomycorrhizal fungi such as *Laccaria laccata* and *Tricholoma matsutake* [9, 10].

Many types of electrical power supplies have been employed to provide electrical stimulation. A large scale 1 MV high-voltage impulse generator was used to stimulate *L. edodes* log wood [2]. High-voltage AC was used to stimulate an *L. edodes* sawdust substrate [5]. Inductive energy storage (IES) pulsed power generators have favorable features for mushroom-cultivating applications, for example, they are compact, cost effective, light, and have high-voltage amplification compared with capacitive energy storage generators such as the impulse generator [11]. The yield of *L. edodes* fruiting bodies was improved with high-voltage stimulation generated by the IES pulsed power generators. The effect of the pulsed voltage stimulation on some other types of mushroom such as *P. microspora* and *L. decastes* was also confirmed using an IES generator developed for the improvement of yield of mushroom production [8]. The harvested weight from log wood and/or sawdust substrates for mushroom cultivation was increased by applying a pulsed voltage as an electrical stimulation.

The mechanism driving the increase in the fruiting body formation by applying high voltage is not clear, but researchers have suggested two possible explanations. One is that the mushroom hyphae are ruptured by applying a high voltage. Physical damage to the hypha stimulates fruiting body formation in mushrooms [5, 7]. The other explanation involves the activation of enzymes. Some enzymes are activated by applying a high voltage, and consequently, mushroom fruiting bodies develop abundantly [2]. Some effects of the high-voltage stimulation were recognized using microscopic observation and chemical analysis. A scanning electron microscope observation indicated that the synthesis of crump connections was accelerated with electrical stimulation [2, 5]. Some types of enzymes, including *laccase* and *protease*, were activated by the electrical stimulation [3, 5, 9].

in parallel at charging phase. After charging up the capacitors, the connection of the capacitors is changed from parallel to series using the gap switches. As a result, the output voltage is multiplied by changing the connection of the capacitors. Typical output voltage is in range from 250 kV to 1 MV. The rise time of the output voltage is controlled around the microsecond-order as an artificial lightning stroke voltage. The example of the applied voltage to the bet-log is shown in **Figure 3** [2]. The peak voltage of 288 kV is generated by operating the impulse generator. The rise time of the voltage is close to 0.5 μs as shown in **Figure 3**. In experiments, the bed-logs are connected to high-voltage electrode as shown in **Figure 2**. The bed-logs (Konara oak; *Quercus serrata*) have dimension of 1 m length. The bed-logs 5–9 are bundled or connected in parallel as shown in **Figure 4** for the high-voltage stimulation by impulse generator. The impulse high voltages are applied to the bed-logs bundle or top of the bed-logs connected in parallel. After the stimulation, the bed-logs are cultivated for fruiting body formation. The yielding rates of the fruiting bodies on the bed-logs are monitored for each stimulation

High-Voltage Methods for Mushroom Fruit-Body Developments

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**Figure 2.** Photograph of impulse generator at stimulation on shiitake mushroom cultivation bed-log [12].

Typical results of the stimulation on yielding rate of *L. edodes* fruiting bodies are shown in **Tables 1** and **2** for various amplitudes of applied voltage. The numbers of the bed-logs are 24 and 21 for each experimental condition. The number of fruiting body formation and total harvested yield increase by stimulating high voltage. In both cases, the fruiting body

condition.
