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Babla Shingha Barua (PhD): E-mail: drbsbarua@gmail.com

Producing exotic mushroom substrate (EMS) for cultivation can be a challenging process, even for experienced growers. Several factors can affect the success of mushroom cultivation, including temperature, humidity, and substrate quality. Most EMS is prepared from wood sawdust and supplemented with agro-food wastes. Agro-food wastes are generated in large amounts locally and globally, which requires significant expenditure and energy for its disposal. However, food waste has a high nutritional value and almost no toxic components. It can, therefore, be used as a medium for exotic mushroom EMS. The food that we consume can then be produced from agro food waste, enabling the recycling of agricultural waste streams. This article aimed to produce new forms of EMS using agro-food waste for mushroom cultivation.

Exotic mushroom substrate (EMS) refer to the materials used as a growing medium for mushrooms. The substrate provides the necessary nutrients and physical support for the fungi to grow. In essence, substrates act as a source of food and water for the mushroom, and also provide structural support for the fruiting body to develop. Different types of mushrooms require specific substrates for optimal growth, and the type of substrate used can impact the yield, quality, and flavor of the harvested mushrooms.

Agricultural wastes are considered a good source of nutrients for mushroom substrate. The most commercially used agricultural wastes for edible mushroom production include wheat bran, cottonseed hulls, sawdust, sugarcane bagasse, corncobs, hardwood sawdust/chips, and rice and wheat straw. Several researchers have demonstrated the use of grass, banana stalks, and coffee husks as substrates for the cultivation of oyster mushroom. Rice straw as a substrate has been utilized for the production of straw mushrooms (Volvariella volvacea) and oyster mushrooms.

Different mushroom species have different preferences when it comes to substrates. In general, these are the most popular exotic mushroom species and their ideal substrates:

1) Pleurotus spp: including king oyster (eryngii) are commercially important edible mushrooms and are cultivated globally. Oyster mushrooms prefer substrates that are rich in lignin and cellulose like rice straw, wheat straw, coffee grounds, barley straw, maize straw, sugarcane bagasse, maize stem residue, wheat stalk, cotton waste, corn husk, rice husk, banana leaves, elephant grass, bamboo leaves, soybean straw, and sawdust. Sawdusts from tropical tree species such as mango (Mangifera indica), Jackfruit (Artocarpus heterophyllus), Coconut (Cocos nucifera), Kadom (Anthocephalus sinensis), Mahogony (Swietonia macrophylla), Shiris (Albizzia spp.), Jam (Syzygium spp.), are also utilizable as substrate. In this case, it is good practice to add 5-10% calcium carbonate or lime to control pH at around 6.5-7.0, and also serve as a pest control. In Japan, various innovations have provided a method of utilizing the corncob for Pleurotus spp cultivation.

2) Grifola frondosa (Maitake): this mushroom grows best on a hardwood substrate. The preparation of this substrate is crucial for successful growth. Start with hardwood sawdust, commonly from oak, maple, or elm, as these are the natural growing environments for maitake. Some substrates use an 18-20% supplementation rate with rice bran or wheat bran but I would recommend adding okara (soybean pulp, a byproduct from the production of tofu) for best yield and 1-3% calcium carbonate to enhance the nutrient content of the substrate and maintain pH at 5.5-6.0.

3) Lentinula edodes (Shiitake): is one of the most popular mushrooms in the world. When it comes to cultivation, shiitake mushrooms prefer hardwood sawdust, supplemented with wheat bran or rice bran, and a pH between 5.5-6.0. Oak, beech, and sweet gum are some of the commonly used hardwoods, though acacia or eucalyptus woods can also be used for shiitake cultivation. 5-7 mm sawdust size will result in a better spawn run. The author has tested shiitake cultivation on mango and Jackfruit sawdust, and shiitake fruits well with these substrates.

4) Pholoita nameko (Nameko): prefers hardwood sawdust, supplemented with 20-22% wheat bran or rice bran and also is an excellent species to cultivate with cherry, sweet gum, sugar maple, ironwood, box elder, and 2-needle pine species. It is necessary to maintain pH at between 5.5-6.5. Nameko mushroom sometimes do not get a high yield due to changing internal mechanisms, such as shifting from two nuclei to a single haploid nucleus during incubation periods. The mycelia grow normally in this case. So this phenomena is speculated to relate to incubation and the spread of nameko mycelium within the fruiting blocks, therefore the substrate ratio and selection is very important.

5) Hypsizygus marmoreus (Bunashimeji): this is one of the most popular mushrooms in Japan. Bunashimeji prefer cotton hulls, rice bran or wheat bran as nutrition supplements, and lime, gypsum powder, or calcium carbonate are used to stabilize pH at 7.0-7.5. But in my research work I have found that the use of only cotton hulls leads the resulting fruiting body to taste a little bit more bitter. Corncob is most suitable, supplemented with rice bran or wheat bran to make the substrate, though wood sawdust is also highly suitable. Standard trees used in bunashimeji include sugar maple and beech, though a number of other hardwood sawdusts are compatible. When using hardwood sawdust, ground corncob, with rice or wheat bran is used as a nutrition supplement. Farms then add lime, gypsum, or calcium carbonate to stabilize pH at 8.0.

However, all approaches to mushroom farming are using agricultural wastes as a bio-conversion for the cultivation of edible mushrooms in a controlled way. Therefore, mushroom cultivation is a highly-efficient and economically viable biotechnology that can help solve environmental problems by using agricultural wastes as a raw material. Recently, the simultaneous increases in the generation of domestic food waste, leftovers, and/or trade refuse per capita due to population bulge, affluence, and urbanization pose significant problems for disposal. Every year one-third of food produced for human consumption is wasted from farm to plate.

The author’s previous research on edible mushroom cultivation using food waste (FW), including barley tea residue (BR), coffee residue (CR), and rice hulls (RH) is currently limited. This study, therefore, focuses on producing mushroom spawn using rice hulls (RH) with rice bran (RB) and starch (St) on different substrates for mushroom cultivation using agro-food waste.

Spawn substrates were produced using rice hulls with rice bran and starch for spawn. Spawn 2 did not use starch. Material percentages were calculated as a dry wet basis, and rice hulls, rice bran, starch, and calcium carbonate were mixed well for fixed pH 5.5-6.0 and moisture was set at 59_endash_61%. 400_endash_450g of substrate was then filled into 850cc polypropylene (PP) bottles. The bottles were sterilized at 121°C for 30 minutes in an autoclave. After cooling at a clean bench, the bottles were inoculated with pieces of a mycelia mat from a PDA plate medium culture on over bed surfaces and then incubated for 20_endash_30 days at 25°C in the incubation room. The spawn mycelia was colonized well inside and on the surfaces of bottles and then stored at 5ºC ±1C for further inoculation into mushroom cultivation bags.

A total of eight different substrates (T1_endash_T8) were used including barley tea residue, coffee residue, rice hulls, food waste, including vegetable and fruit peels, a diverse range of leftover food including meat, eggs, fish, and vegetable dishes, rice bran, and okara with calcium carbonate to adjust pH (Table 1). After mixing the materials well, 500g of media with 62_endash_63% moisture were put in a polyethylene bag (38× 15cm) and autoclaved at 121°C for 60 minutes. After being cooled at the clean bench, the bags were inoculated over substrate surfaces (top-spawning) using 5_endash_7g of the mother spawn and then incubated for approximately 20_endash_30 days at 25°C in the incubation room until the formation of mushroom primordia.

Both mushrooms did not demonstrate differential mycelial colonization after inoculation on any of the spawn substrates during the incubation periods. However, a thicker mycelial density was observed in the spawn substrate containing rice bran with starch, spawn 1, than in spawn 2, which did not contain starch. Among the tested substrates, a higher mushroom yield was obtained through inoculation with spawn 1, which contained rice bran and starch, and substrate inoculated with spawn 2 had a lower yield.

In P. ostreatus, differential mycelial colonization was observed with spawn 1 inoculations with any of the substrates, excluding T2 and T5 containing food waste and rice bran or okara with other agro waste which demonstrated somewhat slower mycelial colonization among the other substrates. Faster mycelial colonization was observed in substrates containing sawdust with rice bran or okara. The mushrooms grew well in any substrates when inoculated with spawn 1, and did not lead to significantly different fruiting body yields, even in substrates containing rice bran or okara, excluding T7_endash_8 which contained sawdust. The highest yield of 104_endash_133.3g was obtained in T2_endash_T3, which contained food waste supplemented with rice bran. The yield of T1, which did not contain food waste, was lower than T2~T3, at 86 grams. Higher yields were obtained in T2_endash_T3 than either T5_endash_T6 substrates containing okara or T7-T8 substrates containing sawdust with rice bran or okara. The lowest yield of 55_endash_75g was obtained in T7_endash_T8 which contained sawdust with rice bran or okara (Fig 2A-H). The mushroom yield did not demonstrate any significant difference in substrates T4_endash_T6 containing food waste with okara. However, the substrates T5 and T6 demonstrated relatively higher yield than substrate T4 as well as substrates T7 and T8.

Among substrates innoculated with spawn 2, the highest yield of 91.6_endash_98g was obtained in substrate T2_endash_T3 containing food waste with rice bran, followed by T5_endash_T6 with okara. The lowest yield obtained was 35_endash_50g in T7 and T8 containing sawdust with rice bran, or okara (Fig 3I-P). Among the substrates T4_endash_T6 containing okara, a relatively higher yield was obtained in substrates T5_endash_T6, that contained food waste, than substrate T4, which did not contain food waste. Although spawn 1 demonstrated a higher yield, the highest yield was obtained in substrate T2_endash_T3 through both the spawn inoculations in the mushroom cultivation substrates.

There was no significant difference observed in the mushrooms grown on the cultivation substrates, except for substrate T2 with food waste and rice bran. The yield was significantly different from other mushroom cultivation substrates through the inoculation of spawn 1. The highest yield was determined to be 115g on the cultivation substrate T2 and the lowest yield was determined to be 70_endash_73.3g in T4_endash_T6 containing food waste with okara and T7_endash_T8 on sawdust with rice bran and okara when inoculated with spawn 1 (Fig 4 A-H). Among the substrates containing okara, the yield of substrate T5_endash_T6 was relatively higher than substrate T4 which did not contain food waste. Through the inoculation of spawn 2, the highest yield was found to be 77_endash_85g in substrates T2_endash_T3 and the lowest was 46_endash_56g in T4_endash_T6, containing food waste supplemented with okara, and T7_endash_T8 containing sawdust with rice bran or okara. However, the mushroom yield of spawn 2 was lower when compared with spawn 1 (Fig 4 I- P) in all of the tested substrates. Both of the two spawns demonstrated their highest performance in yield on substrate T2_endash_T3, consisting of food waste supplemented with rice bran.

In conclusion

Proper utilization of agro-food waste for mushroom cultivation is economically viable and can contribute to reducing global environmental pollution by recycling agricultural and food system waste streams. This study demonstrated that spawn substrates comprising rice hulls, and rice bran with starch may produce higher yield with substrates comprising agro/food waste for the cultivation of Pluerotus spp., than substrates containing wood sawdust. Not only can agricultural waste products such as rice hulls, rice bran, and starch be used to produce high-quality spawn for P. ostreatus and P. citrinopiteatus, but food waste, barley tea residue, and coffee residue can then be repurposed as a high-yield substrate for mushroom cultivation.

These findings suggest that spawn 1 with substrates T2_endash_T3 comprised of rice hulls supplemented with starch and a high ratio of food waste, including a proper ratio of barley tea residue and coffee residue, are suitable for mushroom cultivation and minimize incubation time. These findings suggest that growers may not only optimize their production substrates to take advantage of the marketplace but also solve relevant social issues such as reducing food loss and recycling agro/food waste generated in their regions. Further research is necessary to apply these findings to other mushroom species such as shiitake, maitake, nameko, bunashimeji etc., and to determine the yield obtained from different agro/food waste substrates when cultivating these species.