agronomy Review Use of Spent Mushroom Substrate in New Mushroom Crops to Promote the Transition towards A Circular Economy Diego Cunha Zied 1,* , Jose Ernesto Sánchez 2, Ralph Noble 3 and Arturo Pardo-Giménez 4 1 Faculdade de Ciências Agrárias e Tecnológicas (FCAT), Universidade Estadual Paulista (UNESP), Câmpus de Dracena, Dracena 17900-000, Brazil 2 El Colegio de la Frontera Sur, Chiapas 30700, Mexico; [email protected] 3 Warwickshire College Group, Pershore College, Pershore WR10 3JP, UK; [email protected] 4 Centro de Investigación, Experimentación y Servicios del Champiñón (CIES), 16220 Quintanar del Rey, Spain; [email protected] * Correspondence: [email protected] Received: 23 July 2020; Accepted: 15 August 2020; Published: 21 August 2020 Abstract: The use of spent mushroom substrate (SMS) in new cultivation cycles has already been reported due to its economic and environmental viability. When considering the application of the circular economy concept in the production of edible mushrooms, the re-use of the SMS within the same process is highly attractive, because it allows a better use of the biomass and the energy involved in the process and, therefore, tends to improve energy efficiency and resource conservation. However, this alternative generates important challenges, which derive from maintaining the quality standards of the mushrooms produced and, at the same time, not incurring excessive costs that are detrimental to the process itself. In our opinion, the main difficulty of the process in achieving success is regarding the biological and agronomic parameters that involve the production of the mushroom. It is useless to apply SMS in new cycles if the mushroom harvest is impaired and farms become non-viable. However, numerous examples are reported here where SMS was recycled into new substrates for either the same or different mushroom species without negatively affecting yield compared with using substrates prepared from 100% fresh raw materials. Thus, we suggest that each farm has its own specific technological study, since a small variation in the raw material of the compost, and mushroom cultivation practices and casing layer used, can influence the entire viability of the mushroom circular economy. Keywords: efficiency process; waste reduction; compost production; casing layer; yield 1. Introduction Mushroom cultivation has a relationship with the conversion of agricultural and agro-industrial waste into food of high nutritional value; it stands out as an environmentally sustainable option [1]. This metabolic capacity of fungi takes place through degradative microbiological processes, which to achieve their highest economic viability, and optimal chemical, physical, environmental and technological process/conditions must be controlled (Figure1). Agronomy 2020, 10, 1239; doi:10.3390/agronomy10091239 www.mdpi.com/journal/agronomy Agronomy 2020, 10, 1239 2 of 20 Agronomy 2020, 10, x FOR PEER REVIEW 2 of 21 Environmental MUSHROOM GROWING Physical conditions Temperature, process humidity and Water Casing (optional) holding CO2 content management capacity, porosity, FUNGAL density DEGRADATIVE Production Mechanical ACTIVITY harvest, drip Composting Particle size irrigation, process control and compost process Straw aeration added casing Sugar cane Raw Correction Construction bagasse Material of the C/N of tunnels, Chemical ratio, N Technological Phase II and content and III together Sawdust pH process process Figure 1.1. Scheme of mushroom cultivation and the main chemical,chemical, physical, environmental and technological processprocess/conditions,/conditions, whichwhich cancan influenceinfluence thethe mostmost importantimportant stagesstages ofof thethe production.production. Currently mushroommushroom cultivation cultivation is carried is carried out worldwide, out worldwide, adopting adopting the most diversethe most technologies diverse possible,technologies from possible, the simplest from the to thesimplest most to technologically the most technologically advanced [advanced2]. It should [2]. It be should noted be that noted the degreethat the of degree economic of economic investment investm andent the and local the social local conditions social conditions of each of country each country are factors are factors that a ffthatect theaffect technological the technological degree degree of cultivation of cultivation [3]. Despite [3]. Despite this, the this, entire the production entire production process startsprocess from starts the selectionfrom the selection of the raw of materials the raw materials that will bethat used will for be theused production for the production of the compost of the/ substrate.compost/substrate. There is a specificspecific criterion in the choice of raw materials that is related to the nutrition of mushrooms, which are classifiedclassified asas primaryprimary andand secondarysecondary decomposers.decomposers. Primary decomposersdecomposers require materials with higher CC/N/N ratio and lignin content and lower nitrogen content (sawdust, sugarcane bagassebagasse and and straw), straw), whereas whereas secondary secondary decomposers decomposers require require materials materials with with lower lower C/N ratioC/N andratio higher and higher cellulose, cellulose, hemicellulose hemicellulose and nitrogen and nitrogen contents contents (manure (manure and compost and compost “mixture “mixture of various of agriculturalvarious agricultural wastes”) wastes”) [4–6]. Another [4–6]. specificAnother aspect specific is thataspect the is secondary that the decomposerssecondary decomposers develop in substratedevelop in degraded substrate by degraded bacteria by or bacteria other fungi or other [7]. Evenfungi so, [7]. the Even versatility so, the versatility of mushrooms of mushrooms can allow themcan allow to be them primary to be and primary secondary and decomposerssecondary decomposers at the same at time, the assame is the time, case as of isPleurotus the case ofspp., Pleurotus which canspp., be which grown can in sawdustbe grown/wheat in sawdust/wheat straw or in substrate straw (sugaror in substrate cane bagasse, (sugarBrachiaria cane bagasse,straw, wheatBrachiaria and ricestraw, bran wheat and limestone).and rice bran This and factor limestone). allows species This factor to be disseminatedallows species and to cultivatedbe disseminated in different and regionscultivated with in greaterdifferent ease regions [8]. Therefore, with greater in mushroom ease [8]. cultivation,Therefore, specialin mushroom attention cultivation, should be paidspecial to whichattention agricultural should be or agro-industrialpaid to which wastesagricultural will be or used agro to- attainindustrial successful wastes production will be used and economicto attain viabilitysuccessful in production the activity and [9]. Ineconomic this review, viability we will in the discuss activity the use[9]. ofIn SMSthis review, in new mushroomwe will discuss crops. the use of SMS in new mushroom crops. 2. Circular Economy 2. CircularThe circular Economy economy’s concept is not only to correctly choose the raw materials that will be the basis of the activity, but also to define correctly all the actions involved during each production phase, The circular economy’s concept is not only to correctly choose the raw materials that will be the in order to apply again the wastes generated in the activity on the same company [10]. Using this basis of the activity, but also to define correctly all the actions involved during each production phase, concept in mushrooms production, the main concern would be the spent mushroom substrate (SMS), in order to apply again the wastes generated in the activity on the same company [10]. Using this the principal waste generated in the cultivation [11]. However, some research has already been concept in mushrooms production, the main concern would be the spent mushroom substrate (SMS), developed aiming at the use of SMS as a complementary material to raw materials for new substrate the principal waste generated in the cultivation [11]. However, some research has already been formulations, and as a casing layer for the cultivation of Agaricaceous mushrooms [12–15]. Figure2 developed aiming at the use of SMS as a complementary material to raw materials for new substrate illustrates the dynamics of the process in the mushroom circular economy, where the only waste during formulations, and as a casing layer for the cultivation of Agaricaceous mushrooms [12–15]. Figure 2 illustrates the dynamics of the process in the mushroom circular economy, where the only waste during the crop cycle would be the plastic bags/bottle, which are used to grow certain species (Lentinula edodes, Pleurotus spp., Volvariella volvacea, Flammulina velutipes, and others). Agronomy 2020, 10, 1239 3 of 20 the crop cycle would be the plastic bags/bottle, which are used to grow certain species (Lentinula edodes, Pleurotus spp., Volvariella volvacea, Flammulina velutipes, and others). Agronomy 2020, 10, x FOR PEER REVIEW 3 of 21 Water RAW MATERIALS Compost production Supplements Decrease environmental Limestone and gypsum impacts Heat treatment Inoculation Spawn Decrease CIRCULAR dependence Spawn on raw ECONOMY SMS run materials Soil Peat WASTES Casing (optional) FLUSHES Minimize the Reducing the waste Production (harvest phase) amount of transportation wastes distance FiFiguregure 2. MushroomMushroom circular circular economy economy representing representing the beginning the beginning of the processof the with process