Non-composted grain-based substrate for mushroom production: an update.

Abstract: The button mushroom production industry is actively seeking methods to reduce the environmental impact of their operations on surrounding residents. To address this demand, researchers have focused their efforts in two general areas: (a) developing methods that minimize the effect of compost-based systems on the environment and (b) testing different non-composted substrates for mushroom production.

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Our work has taken the latter path by focusing on grain-based substrates. Until now, studies involving non-composted substrates have focused on the refinement of substrate formulations for increased yield and bioefficiency. Research on refining substrate formulations is valuable but also needed are studies that adopt system-based approaches, whereby the refinement of substrate formulations and the design of alternative mushroom production process go hand-in-hand. This paper presents a summary of our findings on grain-based substrates, including the theoretical design of alternative mushroom production systems for such substrates.

Producing Agaricus bisporus mushrooms on non-composted substrates has been demonstrated, and interest in non-composted substrates has re-emerged because of mounting pressure on the industry to reduce the environmental impact of their operations. The major tribulations encountered by mushroom producers are odor emission and nutrient-rich runoff from composting sites during the substrate preparation and disposal phases. Mushroom compost disposal is another issue of concern, because permits are required for land-application and land is becoming a limiting factor. As a result, studies have been undertaken to mitigate or eliminate the problems ensuing from the composting step within the mushroom production process. Non-composted substrates were shown to be just as productive as composted substrates in terms of mushroom yield and substrate bioefficiency both in our study and in previous studies described below.

Many studies have investigated the refinement of non-composted substrates (Till 1962; Huhnke and Von Sengbush, 1968; Mee, 1978, Sanchez and Royse 2001; Mamiro et al., 2007). Grain-based substrates were pioneered by San Antonio (1971).

Agaricus bisporus fruiting bodies were produced from a cased basal substrate of grain spawn (San Antonio, 1971), and the conclusion derived from the study was that the quantity of mushrooms produced was comparable to that obtained from conventional compost. Since then, little has been done to further improve mushroom yield and substrate bioefficiency from grain-based substrate. Hiromoto (1991) filed a patent for grain-based substrate for the production of Lentinus edodes and extended this to other mushroom producing species.

Before delving into the details of our findings, it is important to note that there are several milestones in the button mushroom production history with relevance to our work that have transformed the industry and our understanding of mushroom cultivation into what it is today. The first milestone was the creation of grain spawn by Sinden (1932). Today, grain spawn manufacturing and use as an inoculum for composted substrates is a mainstay in industry. Spawn makers have extensive experience in processing grain, and unfortunately for science, most of this knowledge is proprietary. Other milestones in mushroom production include the development of the short composting method (Sinden and Hauser, 1950), the development of delayed-release nutrient supplements (Carroll and Schisler, 1976) and the discovery of the role Scyatlidium thermophilum in the promotion and selective growth of A. bisporus in compost-based substrates (Straatsma et al., 1994).

Our work on substrate formulations has focused on two basal materials: commercial grain spawn and grain/oilseed spawn (prepared on-site), and we have determined several factors that significantly influence mushroom yield. We also tested the co-cultivation of S. thermophilum with A. bisporus using the grain/oilseed based substrates. Furthermore, we have developed a theoretical design for two potential alternative mushroom production systems, the Satellite Production System and the Complete On-site System.

Substrate Formulations

The first non-composted grain substrates we tested were composed of millet grain, perlite and calcium carbonate. We showed that mushroom yield was comparable to treatments with composted substrates (8.7 kg/[m.sup.2] and 7.7 kg/[m.sup.2] respectively- [Bechara et al., 2006a]). Replacing the millet grain with commercial rye grain spawn yielded lower (5.3 kg/[m.sup.2]). However, adding delayed-release supplements to the commercial grain spawn substrate increased yield from 1.4 kg/[m.sup.2] to 7.6 kg/[m.sup.2], and the addition of an underlying layer of perlite below the substrate further increased mushroom yield to 13 kg/[m.sup.2] (Bechara et al., 2005). The highest yield obtained from a commercial grain spawn substrate was 14.28 kg/[m.sup.2]. Otherwise, a sterilized mixture of cereal grain and cracked roasted soybean colonized with A. bisporus and then supplemented with a delayed-release supplement (Full House -S41) yielded 16.9 kg/[m.sup.2] compared to 8.7 kg/[m.sup.2] for a substrate composed of commercial millet grain spawn supplemented with a delayed-release supplement (Bechara et al., 2006b). Further refinement of the cereal grain and oilseed substrate increased mushroom yield to 25 kg/[m.sup.2] with observed bioefficiencies greater than 200% (unpublished data). Figure 1 depicts the fruiting of A. bisporus on a grain/oilseed substrate.

In summary, we have determined that mushroom yield and substrate bioefficiency for commercial grain spawn and grain/oilseed substrates are influenced by the following:

[FIGURE 1 OMITTED]

* type and rate of delayed-release supplement

* type and rate of oilseed

* addition of a water-holding material beneath the substrate mixture

* grain spawn age

* grain moisture content

* casing formulation and addition of activated carbon

There are various advantages of using a non-composted grain-based substrate. Primarily, grain-based substrates are a concentrated nutrient source producing high substrate bioefficiency values. As a result, one can anticipate lower substrate processing volumes, a shorter substrate processing time and reduced waste generation. In addition, the production capacity of growing rooms could greatly increase, because substrate depth is much lower compared to compost-based substrates, allowing more beds per growing room. Finally, the spent substrate could be used as a bioenergy feedstock or as animal feed.

Inoculating Grain-based Substrate with S. thermophilum

Initially, we observed that the time for the complete colonization of the grain-based substrate was quite lengthy and could probably be reduced with a higher inoculation level and number of inoculation points. However, carbon dioxide is known to stimulate the vegetative growth of A. bisporus. Hence, co-cultivating S. thermophilum as a source of C[O.sub.2] with A. bisporus may prove to have several advantages over a monoculture of A. bisporus. First and foremost, the carbon dioxide output of S. thermophilum could be used to stimulate the vegetative growth of the mushroom fungus. Furthermore, the enzymes produced by S. thermophilum may improve the bioefficiency of the grain-based substrate specifically for grains with high fiber content. To date, we have observed the following:

1) output of C[O.sub.2] from S. thermophilum was substantial even at 16[degrees]C,

2) S. thermophilum does improve the bioconversion of high fiber grains such as oats,

3) preliminary results for S. thermophilum conferring substrate selectivity have not been validated.

Satellite Production System

The Satellite Mushroom Production System uses commercial grain spawn and delayed-release nutrient supplements as the basal substrate for mushroom production. Mushroom growers would purchase fully colonized grain spawn, delayed-release supplements, peat moss and lime from appropriate suppliers. The substrate components (commercial grain spawn + delayed-release supplements) would be mixed on-site and added to mushroom trays containing an underlying layer of perlite or other water-holding material, and then cased with conventional peat-based casing. Equipment necessary to perform all of these tasks is basically the same as that used for conventional compost-based substrates. The filled mushroom trays are then transported to environmentally controlled chambers and cropping proceeds as in the traditional system. Spent grain substrate would be collected by separation from the perlite and sold as bioenergy feedstock or as an animal feed ingredient and the perlite is an inert material that may be discarded or perhaps reused.

Complete On-site System

The Complete On-site Mushroom Production System differs from the Satellite Mushroom Production System in that the grain-based substrate is processed using a continuous aseptic processing system and colonized with the mushroom fungus on-site. The aseptic processing system would be formed of four sections that are combined to provide continuous production of mushrooms. The aseptic processing system described is based on segmented-flow technology, patented by Penn State (Walker, 2002). Table 1 provides a brief description of the different sections of the aseptic processing unit.