Mushrooms Solutions

VALORISATION OF THE SPENT MUSHROOM SUBSTRATE (SMS) IS CRUCIAL FOR DEVELOPING A SUSTAINABLE MUSHROOM INDUSTRY IN THE FRAME OF A CIRCULAR-ECONOMY MODEL

The study of the exploitation potential of the Spent substrate from mushroom cultivation has been a topic that has always aroused my curiosity. Many years ago, it has been 20 years since I began my research and experiences in relation to this matter, back in 2003, when almost no one was talking about sustainability, climate change or the circular economy, much less in the field of mushrooms cultivated.

There was a turning point in my research, when in 2006 I attended the 2nd International Symposium held in Concordville, in the state of Pennsylvania (USA), entirely dedicated to the topic of the Spent substrate from mushroom cultivation. The conference addressed some advances, challenges and opportunities for Spent mushroom substrate. This Symposium featured speakers, paper presentations and posters that emphasized the scientific and technical aspects related to the use and valorisation of SMS. At this Symposium I gave considers that the management and valorisation of Spent mushroom substrate was both one of the most important problems facing the mushroom industry and on the other hand a fascinating opportunity.

Since then, I have participated in numerous research projects both at European and national level and I have carried out numerous practical experiences, always with the focus on how to give value to this material, which for me is not at all a residue or waste, nor a spent material but an extremely valuable material with infinite possibilities of use, which is why I preferred to coin the term in Spanish “Sustrato Postcultivo de Hongos (SPCH)”.

I have researched and developed experiences in different aspects of SPCH use: the extraction of relevant enzymes and their efficiency to degrade contaminants resulting from different industrial processes; the development of biofilters for different polluting companies and the analysis of their capacity to eliminate pollutants in air and water; the study of SPCH as a biocontrol agent against diseases of fungal or edaphic origin in different types of crops; In the recovery of soils contaminated with pesticides, it was proven that SPCH can serve as an adsorbent medium for insecticides and other types of pesticides; use of SPCH in vineyards as fertilizer; the recovery through the SPCH of unproductive saline soils, forest soils in climatic recession and highly degraded soils for subsequent agricultural or forestry use; the use of SPCH as a substitute casing material for peat in mushroom cultivation; use of SPCH as a material for the preparation of substrates for the cultivation of different species of mushrooms; use of SPCH in Mycoremediation: to degrade PAHs (polycyclic aromatic hydrocarbons) and other contaminants; biodegradation of soils highly contaminated with heavy metals; recovery of acid mine soils contaminated with heavy metals; studies to obtain biogas, drying and pelletizing experiences of SPCH for use as fertilizers in different types of crops, etc.

The cultivation of mushrooms is related to the conversion of agricultural and agro-industrial waste into foods of high nutritional value and stands out as an environmentally sustainable option.

The Chinese call the resulting material after the mushroom cultivation cycle, mushroom residue, the Americans call it Spent Mushroom Substrate (SMS), the Europeans call it Spent Mushroom Compost (SMC), in Spain we call it Post-Cultivation Mushroom Substrate “Sustrato Postcultivo de Hongos (SPCH).

But what is SMS, SMC or SPCH? It is the residual biomass generated after the harvesting of the fruitbodies of edible/medicinal mushrooms. The removal of SPCH, the main by-product of the mushroom cultivation process, often causes serious environmental problems and incurs high economic costs. Efficient recycling and valorisation of SMS are crucial for the sustainable development of the mushroom industry within the framework of the principles of the circular economy.

The physical properties and chemical composition of SMS are a solid basis for developing several applications, and recent literature shows an increasing research interest in exploiting that inherent potential. This post aims to provide a comprehensive perspective of the exploitation possibilities of SMS and discusses critically recent findings related to specific applications.

Depending on the nature of the materials used for formulating the substrate, the type of production system, and the cultivated species, three to five kg of SMS is generated per kg of fresh mushrooms. In total 64 million tons of SMS were generated worldwide by the mushroom industry in 2018, and this figure could escalate to above 100 million tons by 2026.

Photo of the author: collection of SMS after her cultivation cycle

In my region La Rioja, which is the first mushroom-producing region in Spain, we have a recycling plant for the Post-Cultivation Mushroom Substrate (managed by the company Sustratos de La Rioja, which is made up of the growers) where all the SMS are received.  SMS generated in the different types of mushroom cultivation, these SMS are passed through a large trommel where the plastics that are collected for recycling are separated on one hand and on the other hand the Post-Cultivation Mushroom Substrate comes out, already clean and plastic-free. which will be subjected to a re-composting process also using microorganisms to obtain a material suitable for use in different fields of application.

Photo of the author: recycling plant for Post-Cultivation Mushroom Substrates

The large quantities of generated SMS, currently regarded as a waste product with little inherent value, present a major challenge to mushroom producers due to the need to find suitable disposal sites and to cope with the high cost incurred for the transportation of a bulky material with high moisture content and low density; drying fresh SMS is an energy-intensive activity (although there are novel solutions to make it feasible). Moreover, SMS handling/disposal is of primary environmental concern due to the emission of greenhouse gases from spontaneous anaerobic digestion (often occurring in the piles formed during provisional storage), foul odors, and leachate drainage to water receptors causing pollution and eutrophication. Landfilling has traditionally been the chief disposal strategy for SMS, but it is now banned in the European Union by a Council Directive on landfilling of biodegradable wastes. The current linear ‘take, make, dispose of’ approach, where SMS is regarded as waste, threatens the future development of the mushroom-growing sector. Valorisation of SMS is crucial for developing a sustainable mushroom industry in the frame of a circular-economy model. It is essential to investigate SMS characteristics to identify appropriate valorisation alternatives.

At the time of writing this post there are more than one and a half million search results in relation to Mushroom Post-Cultivation Substrate. This post presents different uses of SMS that can serve as inspiration, some applications have been scientific experiments, but others have been put into practice on a larger scale.

SMS composition and properties are mainly associated with the type of raw materials and supplements used to prepare the initial mushroom substrate. During cultivation, substrate components are enzymatically degraded, and the resulting nutrients (together with others existing in the substrate) are used for fungal growth and mushroom production. In the end, SMS composition strongly depends on the nature of the initial substrate and the cultivated species. Therefore, SMS primarily consists of plant cell-wall components (lignin, hemicelluloses, cellulose) and residual fungal mycelium, as well as non-cell-wall carbohydrates, proteins, and minerals.

Figure 1. Mushroom cultivation cycle and possibilities of using SMS (Georgios I. Zervakis and colb.)

 There are different valorisation routes for SMS, here I will only briefly name an updated overview of possible SMS applications and products related to:

1.- Reusing spent mushroom substrate for new cultivation of mushrooms:

The reuse of SMS in new mushroom crops seems to have considerable potential since it can support high yields and is both financially feasible and environmentally sustainable. The elements and organic compounds existing in SMS constitute valuable sources of energy and nutrients, which can partially or entirely cover the needs of additional cultivation cycle(s) after suitable treatment or supplementation.

• Reuse of spent mushroom substrate (SMS) for the cultivation of various mushroom species:  in this sense, certain factors that affect the cultivation parameters and the use of SMS in new mushroom cultures must be taken into account.
  
  
  
   
• Use of SMS as a component of casing soil: attempts focusing on exploiting SMS as casing material alone or mixed with farm yard manure or sphagnum peat in the cultivation of Pleurotus eryngii and A. bisporus, respectively. In the use of SMS as casing soil for the cultivation of A. bisporus I have carried out numerous practical experiences both on an experimental and commercial scale. Several authors have reported the success of using SMS as a casing layer. The environmental benefit of replacing peat with SMS in whole or in percentage represents numerous environmental and economic benefits. However, SMS should not be used directly as a cover layer, it must be subjected to some processes (heat treatment, maturation, washing).
  
  
  Photo of the author: Casing soil obtained after the SMS processing
  
  
  
  Photo by the author: trials of using SPCH from the cultivation of A. bisporus mixed with different percentages of peat for use as casing soil for mushroom cultivation.

The result of this experiment showed that addition of SMC to peat positively affected the mushroom dry matter and protein content compared to control. In addition, mushrooms of better general quality were obtained with less bacterial blotch, cobweb, dry and wet bubble.

2.-Use of Spent mushroom substrate as feed:

The need for animal feed production is predicted to increase significantly, and the feed industry must look for additional/alternative means to cover the respective demand. Exploiting suitable bioresources (e.g., SMS) could contribute toward this direction by readily providing material to be used as feed supplement.

Consequently, the high nutritional value of SMS is the main factor for its inclusion in the diets of poultry, ruminants, and monogastric animals, and, recently, in fish and edible insects. There are numerous experiences on the reuse of SMS as animal feed.

3.- Use of spent mushroom substrate in agriculture:

Adopting sustainable agronomic practices, including the development of novel environment-friendly and cost-effective biofertilizers and biopesticides, is of high priority. In line with that approach, the SMS’s physical properties, its high content of bioactive compounds, and readily available macro- and trace elements make it a promising candidate for several agricultural applications, the most important of which are presented in the following paragraphs:

• Use of spent mushroom substrate as biofertilizer and soil conditioner: the use of SMS as soil amendment has beneficial effects on soil fertility and structure. SMS presents a promising potential for substituting, at least partially, the use of mineral fertilizers in continuous crops and thus contributes at mitigating soil secondary salinization and acidification, and at avoiding nutrient imbalances and accumulation of toxic allelochemicals.
  
  
  Photo of the author: application of the Spent Mushrooms Substrate in viticulture
• Use of spent mushroom substrate for plant-disease control (biocontrol): The results so far indicate that SMS richness in antimicrobial compounds in concomitance with its natural microbiome, including organisms suppressing soil-borne plant pathogens, are essential prerequisites for developing relevant plant-disease control products. However, further experimentation, including evaluation in large-scale greenhouse and field trials, is required to fully benefit from that potential toward a solid sustainable agriculture model.
  
  In this field, we have been conducting trials for some time using A. bisporus compost teas to control some of the fungal diseases that affect its crops with promising results.
• Effects of SMS on nutritional value and secondary metabolites production in plants:
  
  SMS application affects the content of secondary metabolites in plants.  Although the scientific data on the effects of SMS on the nutritional value of edible and medicinal plants are still limited, the available results reveal SMS potential to increase the content of specific elements and secondary metabolites in plants.

4.- Spent mushroom substrate as source of enzymes and bioactive compounds:

Producing enzymes (SMS is a source of various enzymes that can be recovered by extraction with different solvent systems. Furthermore, SMS can be used as substrate for the cultivation of enzyme-producing microorganisms) and different bioactive compounds (SMS contains bioactive compounds of different functionality and origin. The fungal mycelium contains polysaccharides, sterols, proteins, polyphenols, vitamins, and other bioactive molecules. Mycelial growth throughout the surrounding environment also results in the secretion of potentially useful bioactive compounds) is a reasonable way of SMS valorisation. SMS-derived enzymes are of interest in industrial sectors, such as brewing, baking, starch-processing, leather, and textile industries, as well as in bioremediation and the emerging biofuel and biorefinery business. SMS-derived bioactive molecules have also the potential for application in the pharmaceutical, biomedical, feed, and food sectors.

5.- Spent mushroom substrate valorisation as part of cascade use of plant biomass:

The generation of plant biomass resources by agriculture and forestry takes a long time and requires considerable land areas; thus, their utilization should be rational and efficient. 

For bioeconomy development in a resource-efficient way, cascade use of plant biomass should always be considered. Cascade use, is a complex interaction of material flows used as a strategy to increase resource efficiency in biomass processing. Cascade use occurs when biomass is processed through a series of material uses (Figure 2), by reuse and recycling, before finally being used for energy recovery.

Applying cascade uses to processing plant biomass by mushroom cultivation combined with SMS valorisation through biochemical conversion and other approaches, is expected to maximize the cost-effectiveness of a value chain of variable potential products. The cascade-use concept also results in minimizing resource loss and environmental impacts. Following a cascading approach, SMS, as the primary by-product of mushroom cultivation can be re-used as raw material for new processes, extending total biomass availability within the system. That is a rational approach, where different valuable biomass constituents are recovered and converted into value-added products. Energy uses of residual biomass are considered only at the end of the life cycle when all higher-value products and services have been exhausted. There are different possible examples of multi-stage cascading uses for SMS valorisation.

Figure 2. Schematic illustration of the cascade-use concept of bioresources (based on Vis et al. 2016, p. 6, https://data.europa.eu/doi/10.2873/827106).

Cultivation of edible and medicinal mushrooms is a very dynamic business, with an impressing development during the last decades. However, increased mushroom production leads to the generation of high quantities of spent mushroom substrate (SMS). The accumulation of non-used SMS, or its limited or not high-added value applications, undermines the future of pertinent commercial activities. Therefore, achieving an efficient valorisation of SMS – beyond its current low-value use – is of paramount importance for the sustainable development of the mushroom industry. The analysis carried out in this post shows the enormous potential of SMS as a source of products and services.

For more information contact the author of this post:

MARISA PEÑA DIAZ is consultant with 35 years of experience in the cultivated mushroom sector and CEO of the company MUSHROOMS SOLUTIONS. She offer consulting and specialized training services for the mushroom sector internationally. For more information about my activities, consult my website:

www.mushrooms-solutions.com