Recycled substrate in specialty mushroom production.

Abstract: Throughout the worldwide mushroom industry the disposal of Spent Mushroom Substrate (SMS) is an ever-growing topic of concern. The removal and disposal of SMS is usually a financial burden that is placed upon the grower and can be a considerable expense. Environmental concerns create liabilities and companies are looking for alternatives to reduce these expenses and responsibilities.

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Specialty mushrooms typically use sawdust or straw-based materials for substrates in commercial production applications. These materials are generally used only once for production purposes. Both sawdust and straw-based spent mushroom substrates contain valuable nutrients that can be recycled and used again for mushroom production.

Analysis of recycled (100 percent), partially recycled (40 percent), and non-recycled (0 percent) substrate completed in conjunction with the Mycology Department at Montana State University showed higher levels of nitrogen and lower levels of cellulose and hemicellulose in the recycled material, resulting in lower C/N ratios. There was little difference in spawn run times, and mushroom production with 40 percent recycled material was slightly higher than for other treatments. At this point, there appear to be no drawbacks to using the recycled material for organic mushroom production and long-term use appears feasible.

As the mushroom industry continues to grow, so will the debate on the proper disposal of SMS. The recycling of these materials provides a simple and long-lasting solution. It is an environmentally sound technique and provides an economic benefit for operators of commercial specialty mushroom production facilities.

INTRODUCTION

Sawdust or straw-based materials for specialty substrates are generally used only once for production purposes. Both sawdust and straw-based spent mushroom substrates contain valuable nutrients, and recycling could be an economical and environmentally sound alternative for growers facing these concerns. The disposal of these substrates is not only costly, but could be unnecessary if material is processed and used on-site as substrate for further mushroom production. The original reasoning behind this research came out of a shortage of usable sawdust in our area. Montana is an isolated state with virtually no usable hardwood sawdust. With no other option but to use recycled sawdust, we began experimenting with this material for Shiitake production. Many areas around the world face similar substrate shortages, and this will only continue as production in these areas increases (Zhanxi 2001).

It has been shown many times that recycled substrates can be used to produce mushrooms, but most literature focuses only on sequential use via different mushroom species (Chang & Miles, 1989; Stamets, 1993). We have developed a protocol for recycling these waste substrates for use with the same species, therefore eliminating or reducing the need for removal or disposal. Concerns amongst others in the industry have arisen about the recycling of this material and quality change over time. This has been the most unanswered question: will this material change over time to a point that it is no longer viable as a productive substrate material? The research presented here will show that the material is not changing to a point where it cannot be used continuously over time. This research compares the use of recycled versus "new" substrate for the production of Shiitake mushrooms on a hardwood chip substrate, in terms of both productivity and nutrient availability.

MATERIALS & METHODS

The methods used for this research were simple and straightforward. Recycled sawdust substrate that has been used as a growing medium continuously for five years was combined at differing rates with fresh substrate material. The material was autoclaved and inoculated with fresh vigorous spawn. Two strains of Shiitake were used for this research. Data were collected on spawn growth throughout the incubation phase and production numbers were recorded to ascertain which combination of material yielded the highest biomass of saleable product. Analysis was done on all combinations of substrate prior to sterilization to determine levels of various nutrients and other components.

Preparation of the recycled material was done using a chipper/shredder. Old, spent Shiitake sawdust blocks were chopped until the material closely resembled sawdust again. Blocks were somewhat dry to add to the ease of chopping. Once chopped this material was combined with fresh alder (Alnus sp.) sawdust in the following formulations: Recycled Sawdust Substrate Formula Recycled Sawdust 80% Millet Supplement 10% Bran Supplement 10%

100% Moisture Content 65-70% Fresh/Recycled Sawdust Substrate Formula Hardwood Sawdust 48% Recycled Sawdust 32% Millet Supplement 10% Bran Supplement 10%

100% Moisture Content 65-70% Fresh Sawdust Substrate Formula Hardwood Sawdust 80% Millet Supplement 10% Bran Supplement 10%

100% Moisture Content 65-70%

Once combined, 2006 lb sawdust blocks were made from each of the various formulas. These blocks were steam sterilized at 15 psi or 250[degrees]F for 4.5 hours. Once cooled to below 80[degrees]F the blocks were all inoculated with Shiitake (Lentinula edodes) spawn. The strains used in this experiment were Garden City Fungi Strains- GCFSC1000 and GCFSB1007.

All cultures were incubated in #14 micro-filtered bags from Unicorn Manufacturing at 68-72[degrees]F for 60-90 days depending upon strain (See Fig. 2). Blocks were kept in incubation through initial colonization, popcorning and browning phases of growth. Once browning was complete, blocks were moved into fruiting chambers and allowed to fruit. Fruiting conditions were manipulated to create an environment that would fluctuate from the mid 50[degrees]Fs to the mid 70[degrees]Fs on a daily basis. Humidity levels were allowed to fluctuate as well from 80-95 percent. C[O.sub.2] levels were kept at 1500 ppm or lower throughout the fruiting phase. Production information over three flushes was collected for each batch.

RESULTS

Analysis of the substrate material was done at Montana State University analytical lab in Bozeman, MT. Four different batches of material were analyzed. Findings of the analysis work are shown in Figure 1.

The results of the analysis showed variation in the four batches of substrate material. The recycled material had much higher levels of nitrogen and lower levels of cellulose and hemicellulose indicating lower levels of carbon compounds. Insoluble ash was measured and proved to be the highest in the recycled batches. Lignin levels were lower in the recycled material suggesting the presence of a white rot fungus, since white rot fungi decompose both lignin and cellulose (van Griensvan, 1988). Lentinula edodes is known to be a white rot fungus.

[FIGURE 2 OMITTED]

Spawn run data were measured in days. Figure 2 shows the slight differences in spawn run time with each strain and on each substrate. With strain GCFSC1000, the 100 percent recycled material was the quickest to complete the spawn run while the substrate containing 40 percent recycled material was the fastest to colonize with strain GCFSB1007. In both cases the substrates containing 0 percent recycled material were the slowest to complete the spawn run.

Production data for both strains are shown in Figure 3. Production of each strain on both fresh and recycled substrates held constant with no significant differences. Block productivity was measured by weighing only saleable mushrooms with a cap diameter size of at least 1.5 inches. No misshapen mushrooms were counted. Throughout each substrate type no measurable differences in mushroom quality were observed as can be seen in Figure 4.

DISCUSSION

Results show that the use of recycled substrate material as a primary component in new substrate media is a viable option for commercial specialty mushroom production. Although this recycled material is altered through decomposition by fungi such as Shiitake (Lentinula edodes), the material when combined with fresh sawdust is still suitable for mushroom production. It has been suggested that higher levels of nitrogen can inhibit fruiting of Shiitake (Lentinula edodes) (Leatham, 1985) but the production data from the fruiting trials shows this not to be the case here. The C/N ratio of the recycled substrate formula is considerably lower than in the fresh sawdust formula, and this may account for the slight increase in production. More available nitrogen seems to be the most important aspect of the recycled material. This increase can be attributed to the fact that chitin, a polysaccharide which makes up the mycelial cell walls contains nitrogen. This nitrogen from the recycled mycelium provides an extra boost of supplement to the substrate material.

[FIGURE 3 OMITTED]

When you take into consideration the costs associated with substrate removal and the recycling of substrate material, it not only makes good scientific sense but also good financial sense to recycle. The costs associated with the preparation of this material are minimal. From an environmental standpoint this technique has many positive benefits and is ecologically sound. Continued research will help to develop protocols to use other substrate materials and will help to reduce unneeded waste in the mushroom industry.

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