Fruiting bodies were obtained in all treatments assayed including the untreated treatment. We considered the latter treatment as a “positive control”. We expected that the straw would have been rapidly colonized with other contaminant fungi, such as green molds as Trichoderma or Aspergillus but this did not occur. Bags were completely colonized by the white mycelium of Pleurotus; only two bags presented some small spots of black and green color, which were fungal contaminants. Anyway, this situation did not reduce mean yields (Table 1). The low contamination observed could be due to good quality of the wheat straw. It had low water content at the moment of the wheat cropping period; bales were made and were immediately kept in a barn in dry conditions. This situation defines a substrate of excellent quality. When contaminants are scarce in the substrate they do not offer a competence for the mycelium of Pleurotus which quickly colonized it. The proportion of inoculum of Pleurotus against contaminants is much higher.

Mushrooms were collected in all treatments at day 8 after the start of fruiting conditions with the exception of chemical treatment in which the first collect was obtained at day 12. The use of carbendazim extended the time of harvest in 4 days compared to the other treatments. Anyway, yields were not affected resulting in the highest yield obtained in this experience (BE: 106.93%) although without significant differences among untreated (104%) and sterilized substrates (94.91%). BE obtained were similar to those reported by Omarini et al.,19 Lechner and Albertó,15 for the cultivation of P. ostreatus using wheat straw without supplements, and remarkably higher than those obtained by Shah et al.22

The lower yield obtained with the immersion in hot water treatment was also accompanied by a decrease in the number of basidiomes (Table 1). The PR also significantly decreased with this treatment.

The lowest BE was obtained with immersion in hot water treatment (75.83%). The latter treatment was the only one that produced 2 flushes (Table 1). Probably, the third flush was delayed because of the loss of nutrients in IHW treatment. When the distribution of the crop along flushes was observed we found that for the strain and the substrate used, the first flush was the most important with 76.7–80.82%. The second flush varied from 5.4 to 14.9%.

The treatment of immersion in hot water produced a reduction of at least 20% in yields. We supposed that some nutrients, which were important for P. ostreatus production were lost during immersion in hot water treatment. It produced a dark brown water extract, which was enriched with some soluble compounds presented in the straw. When the residual water extract was analyzed we found the presence of sugars (which are expressed as it equivalent to g of glucose), proteins and minerals (Table 2). The content of N and P was 2.99mg/L and 16.98mg/L, respectively. Values of K found in the extract were 36 times higher than those we found in TAP water. Nitrogen is very important in the growth of all organisms. In mushrooms, it is necessary for nucleic acids, protein, and for the chitin of cell walls. The values here found are low if we compare them with the concentration normally employed in culture media. It is also very low if we compare it with values found in wheat straw which has 0.62%.23 Anyway, Ginterová and Maxianova9 stated that P. ostreatus has the ability to fix atmospheric N. If we consider this fact, probably deficient N content may not be so important. Glucose is important for mycelium growth; biomass production of P. ostreatus increases when glucose concentration is increased in the culture media.10

Houdeau et al.12 considered that the immersion of substrate in water can have different consequences according to the type of raw material. They pointed out that there is a “nutrients washing” effect that can be negative when old raw material is used, but useful in new raw material because there is a decrease of soluble sugars that can prevent the development of antagonistic microorganisms. The raw material herein used can be considered “new” type. The method probably helped to decrease contamination as Hodeau et al.12 pointed out but as a negative effect, has significantly decreased yield.

To evaluate if the water residual extract was important for mycelium growth we made a medium of culture using it and we compared it with 2 other culture media: water agar and glucose agar (0.1%). Results showed that the extract improved the mycelia growth showing that the compounds which were extracted from wheat straw were important and useful for fungal metabolism (Fig. 1). This medium with glucose produced a low mycelium growth, comparable to water agar, showing that sugar was not the only compound responsible for the improvement in the mycelium growth and that the rest of compounds, many of them unknown for us, also helped mycelium development. If we compared the mycelium growth in GA with WA we found that there were no significant differences in the diameter of the colony (Fig. 1). Guillén-Navarro et al.10 compared the production of biomass of P. ostreatus at different concentration of glucose. They found that biomass production was increased with the increase of glucose concentration. Although the concentration of 1g/L was one of the lower concentrations assayed they found that the fungal biomass increased at low glucose concentration with the higher efficiency (measured as g of biomass by g of glucose). When we compared the aspect of the colony of mycelium obtained (data not shown) we observed that it was very different for each medium. The aspect of the colony of mycelium obtained in REA was dense and cottony showing a good mycelium biomass development. The mycelium obtained in WA was lax and weak. The aspect of the mycelium of GA was intermediate between REA and WA.

Fig. 1.

Mycelium growth of P. ostreatus in 3 different agar media. WA: water agar; RWEA: residual water extract agar; GA: glucose agar.

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To evaluate if the water residual extract was important for mushroom production, a third experiment was carried out. We used the immersion residual water to hydrate the wheat straw and we compared it with straw hydrated with tap water (TW). Results showed that the latter treatment produced significantly higher yields (BE: 94.54%) that REW treatment (70.02%) after 68 days of crop (Fig. 2). We expected to obtain higher yields with REW treatment but it did not happen. Probably, it could be due to the low concentration of nutrients present in the residual water used to hydrate the wheat. In fact, the volume of water needed for the immersion treatment was major (twice) that the volume needed to obtain 70% of final humidity on the substrate. As a consequence, we used only part of nutrients which were extracted during the immersion process in relation to the mass of wheat straw used. There could probably have been an effect of transformation or loss of available nutrients in the water or effect of toxicity due to heating (first time at 80°C during 90min, and second time autoclaved 2h at 120°C.), causing polymerization/ionization of nutrients extracted from the substrate. López et al.16 determined the quality of water from pasteurizing substrate (coffee wastes), for P. ostreatus cultivation. They found out that residual water could become highly polluted. As a consequence, this factor may have affected mushroom yield.

Fig. 2.

Production of P. ostreatus in wheat straw. REW: using residual water for hydration; TW: using tap water for hydration.

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We have proved that immersion in hot water treatment of substrate reduces yields at least 20% when compared to other straw treatments, as steam, chemical or untreated wheat straw. Compounds which are hydro-soluble are lost during wheat straw immersion in hot water. The loss of these nutrients would be the cause of yield decrease.

Although this method is inexpensive and easy to implement, the reduction of crop is very important causing significant loss, especially when the majority of Pleurotus farmers in Latin America, India or Africa use this methodology to treat the substrate. Additionally, other import factor to take into account is that this method uses a high amount of water, which could be a negative factor due to scarcity of this resource in some areas.

In this experience, the chemical treatment produced higher yields. Anyway the use of this treatment has to be discussed since its success depends on the microbiology population of the substrate. It is a selective fungicide which does not control all possible contaminants as some fungi or bacteria. The use of pesticides is unpopular and mushrooms are considered as an “organic product”, environmentally and pesticide free.

Alternative methods to the use of immersion in hot water as treatment of the substrate should be considered to reduce economical loss especially in mushroom farms with large production.

The authors declare that they have no conflict of interest.