LOW INPUT TECHNOLOGY FOR GROWING MUSHROOMS: SELF-HEATING PASTEURIZATION
José E Sánchez, Lilia Moreno and René Andrade
El Colegio de la Frontera Sur. Km. 2.5 carretera al Antiguo aeropuerto,
Tapachula, Chiapas. 30700 México. <firstname.lastname@example.org>
Principle and description
Self-heating pasteurization refers to use of heat, naturally generated by microbial activity under controlled conditions, in sufficient quantity to pasteurize raw materials. Mixed raw materials must be piled and isolated from the environment, avoiding anaerobic conditions. The technique seeks to produce and retain heat in an aerobic simple and efficient way for providing a temperature/time treatment suitable for inhibiting or killing potentially harmful organisms during mushroom cultivation.
Figure 1. Fundamentals of self-heating pasteurization
1) Heat produced by microbial activity in the substrate is sufficient to kill thermosensitive organisms.
2) A container with an appropriate thermal insulation minimizes convection, evaporation and conduction heat losses. It helps retain most of the generated heat.
3) Heat favors natural ventilation because of differences in air pressure and density.
4) Substrate is remixed to improve aeration and homogeneity of the process.
5) High temperature is maintained for sufficient time to reach substrate pasteurization.
6) A slightly alkaline pH improves substrate selectivity for mycelial growth and development.
KEY ASPECTS FOR SELF HEATING PASTEURIZATION
The basis of self-heating pasteurization is to take advantage of heat produced by microbial activity in the substrate. Heat increases substrate temperature to sufficient levels for killing thermo sensitive contaminant organisms. The key point is to provide special conditions for producing heat and the ability to retain it. However, temperature is a complex variable defined by different parameters and variables of the system, for instance: substrate mass, volume, composition, pH, particle size and moisture. Also environmental conditions, aeration, and container insulation are important.
The microbiota in the substrate develops and multiplies by degrading carbon sources, mainly carbohydrates and lipids thereby producing heat. This is a natural phenomenon observed in a minimal volume of wet material, i.e., appropriate mass and volume of wet substrate is needed to produce the necessary heat for increasing piled material up to 60-70ºC (Figure 2). The energy produced not only destroys thermally sensitive microorganisms, but also heats up the air inside the substrate and causes (because of pressure and density differences) air circulation from the bottom of the container to the upper part, and then to the atmosphere. The space left by this air is replaced by new surrounding air. A natural flux of fresh air is drawn into the substrate providing oxygen perpetuating an aerobic environment. Anaerobic processes would be harmful for the process.
As it can be observed in Figure 2 and according to Overtijns (1981), who indicated that 55-60ºC during 6-10 h is lethal for the development of most white button mushroom competitors, the self pasteurization technique is able to provide the substrate with the necessary heating for assuring a pasteurization treatment. Furthermore, an alkaline pH (8-9) at the end of the treatment contributes to the specificity of the substrate by limiting development of contaminants.
Figure 2. Temperature profile during self-heating pasteurization of a 220 kg batch of Pangola grass (65% moisture) containing 2% lime, in a square wooden box of 1 m per side, with a preheating chamber and a 2 cm layer of polyurethane insulation. Nomenclature: N = Levels 1, 2, and 3 (upper, middle and lower levels in the box), CP= Preheating chamber, Ts= Outside temperature.
Content of the substrate
Nutrient content of substrate is very important for mushroom growth and development (Múez-Ororbia and Pardo-Nuñez 2001, Ziombra 2000, Stölzer and Grabbe 1991). Also, nutrients are important during self-heating because they provide fuel for producing heat. In general, materials with a high C/N ratio are preferred. Additionally, substrate moisture is especially important for the development of microbiota and for producing heat during pasteurization. If one varies the moisture content of an 80 kg Pangola grass pile between 55-70%, temperature profile is affected substantially during self-heating. Higher temperatures are observed at 60-65% moisture. In general, greater differences are observed at the lower level of the substrate where temperatures are lower than the rest of the substrate batch (Fig 2). Temperatures in this lower area may be critical for successful treatment. For instance, in Figure 3, the lower level of substrate does not reach 40ºC when 55% moisture is used.
Figure 3. Temperature profile of the lower section (15 cm above the bottom) of a batch of 80 kg Pangola grass during self-heating pasteurization in a wooden crate at different moisture contents (55, 60, 65 and 70%) (Extracted from Sánchez et al. 2011).
Aeration of substrate
Self-heating pasteurization treatment must be aerobic to avoid offensive odor emissions and to avoid substrate acidification. This may be possible by promoting a controlled air flux. This is facilitated by drilling holes at the bottom of the container and having an air exhaust at the upper part of the substrate; however, this may not be sufficient. It is necessary to remix the substrate at least once during pasteurization. Turning the substrate means to carefully remove it from the container, aerate it and fill it back to the container. It is important to place the substrate in a different manner than it was before removing it for a homogenous pasteurization. Turning may be done by aseptically removing the substrate with a shovel and placing it on a clean surface outside then filling it back into the container. It is important to separate the upper, lower and middle sections of the substrate, so when filling the substrate into the container, the previous upper level is placed at the bottom of the container, while the bottom layer is placed in the middle and the middle layer is now placed at the top.
Turning must be applied when the upper layer of substrate has been pasteurized. This happens around 27-30 h after filling the box. Turning allows the lower sections of substrate (Level 3 with lower temperature) to reach pasteurization temperatures because this layer is placed in the middle section after turning. Turning the substrate for aeration allows water evaporation and cooling of substrate and represents a contamination risk. Because of this, turning must be applied carefully, aseptically and quickly to reduce these effects.
A simple, moist substrate pile on the ground 50 cm height and 50 cm wide allows for pasteurization of the center of the pile (Villa Cruz et al. 1999); however, to pasteurize in a controlled manner the whole mass, a larger volume is necessary, and preferable using an insulated container to avoid heat losses (Figure 4). Determination of the minimum size of the container depends upon environmental conditions. Air temperature and relative humidity influence the temperature profile during substrate pasteurization. Trials made so far indicate that with a wooden box 1m per side, with 49 holes (0.81 cm diameter) uniformly distributed in the bottom, with insulated walls, bottom and cover with a 2 cm polyurethane layer, it is possible to pasteurize substrate containing 380 kg milled corn cobs, 220 kg Pangola grass (65% moisture) and variables quantities of sawdust and coffee pulp, in an environment with 22-28ºC (Hernández et al. 2003, Barrios Espinoza et al. 2009, Sánchez et al. 2011, Avendaño-Hernández and Sánchez 2013).
Figure 4. Outside and inside view of a self-heating pasteurization container.
Some elements may improve this system, for instance, installing a chamber for preheating the air entering the container. This is a kind of drawer made from the same material as the container, also insulated and placed below the bottom of the container. It may have 11 holes 2.54 cm diameter around the center of the chamber. This chamber keeps the heat produced at the bottom of the pasteurizing container and thus preheats the entering air. By using a chamber like this, it is possible that 20 h after beginning pasteurization, the air entering the pasteurization container is 5ºC warmer than the surrounding air.
In addition, it is recommended to think about the hall or room where self-heating pasteurization is performed. The general idea is to minimize possible heat losses presented at the interphase between substrate and environment. The insulation material is a key element to retain heat produced by microbial activity.
Materials and insulation of the container
Evolution of temperature inside the crate is affected by environmental conditions, mainly relative humidity and temperature. This is due to heat exchange at the interphase between the substrate and surrounding air. Moreover, this system requires a relatively low flow of fresh air entering at the bottom of the crate, traversing the substrate and exiting at the top. This fresh air provides oxygen for microbial growth and helps to minimize anaerobic conditions that could reduce the production potential of the substrate. The cooler the air, the less temperature increase in the substrate.
To reduce heat losses, the crate should be insulated. Insulation helps to decrease heat losses and also to reduce temperature differences between the core of the substrate mass and the periphery (Avendaño Hernández y Sánchez 2013).
Figure 4 shows the internal and external aspect of a crate used for self-heating the substrate. It is worth mentioning that wood is not the only material that may be used to build a container. Cement, brick, fiberglass, pvc, and even certain corrosion resistant metallic materials may be used.
Results so far (temperature profile and biological efficiency of pasteurized substrate) have been obtained in locations where average daily temperature is 22-28ºC. A 1 m3 wooden crate insulated with a 2 cm polyurethane layer has been used. It is likely that in places where the ambient temperature is cooler, some adjustments might be considered; for instance, the insulation used for both the crate and the site (the hall or room) where the pasteurizer is located. Also the mass and volume of the substrate used may be considered.
Quality of spawn used is of great importance for a successful cultivation of mushrooms. Spawn used so far has been of good quality, but developed for the traditional cultivation in steam-pasteurized substrates (Chang 1982). Perhaps, spawn developed specifically for self-pasteurized substrates may improve strain performance. In fact, it has been observed that substrates pasteurized by self-heating are different from steam pasteurized substrates. The fiber seems less soft and has an abundant and certainly specific microbiota. If, for preparing spawn, these conditions are taken into account, a spawn ready to display the enzymatic capability of the mushroom for growing in such a substrate would help to obtain better results.
Mushroom strains already tested
Several mushroom strains have been tested so far at the pilot plant level. Also, an experience at the community level has been reported (Avendaño and Sánchez 2013). They all were cultivated according to the guidelines for each species (Estrada et al. 2009, Wuest 1982, Zadrazil and Schneidereit 1972). Among the genus Pleurotus the following strains have been cultivated: P. citrinopileatus ECS-1338, P. ostreatus ECS-0152, ECS-1121, ECS-1122 and ECS-1123, P. pulmonarius ECS-0190, P. djamor ECS-0123, ECS-0127, ECS-0142 and ECS-0149 and P. eryngii ECS-1258. However, the potential seems greater, because several strains belonging to other genera were also cultivated, like: Auricularia fuscoscuccinea ECS-210, Agaricus bisporus ECS-305 and ECS-331 and Agrocybe aegerita ECS-1009. Self-heating pasteurization is a technique that certainly does not work for all mushrooms. Under tested conditions Ganoderma lucidum ECS-0501 and Lentinula edodes ECS-0401 did not produce mushrooms; however, the range is wide enough and worth trying new strains to expand the field of use (Barrios et al. 2009, and Morales and Sánchez 2016).
Drawbacks of self-heating pasteurization
Like any other beneficial treatment, this technique has some characteristics that under certain conditions may be considered as disadvantages relative to steam pasteurization or alkaline soaking (Ali et al. 2007, de Siqueira et al. 2012):
- A minimal mass of substrate is necessary for the process to proceed.
- The length of time may be considered as long (40-48 h).
- It is absolutely essential to facilitate ventilation (turning the whole mass, approximately 30 h after starting the process).
- Once the process is finished, the substrate must be removed from the unit and cooled down to stop thermogenesis.
- During spawn run, a slight increase of substrate temperature is possible.
According to different cultivation tests performed during the evaluation of the self-heating pasteurization technique, the most valuables features are: 1) possibility of cultivating several strains of different mushroom genera, 2) at least for the case of Pleurotus, no decrease in production is observed in regard to steam pasteurization, 3) no need for external source of energy, 4) it is an ecologically friendly treatment using its own energy and 5) through the precise control of particle size, volume, moisture, pH, substrate content and aeration it is possible to control temperature and prepare a selective substrate for several mushroom strains.
Self-heating pasteurization works aerobically as composting does; however, these two techniques are different: composting is a long process seeking to produce humic substances and produce high quality organic fertilizer for plant use (Martin 1991). Self-heating pasteurization, on the contrary, lasts a shorter time (2 days) and material bioconversion is only desirable to the extent that produces heat, not an objective itself. Other than bioconversion, the goal of treatment is to promote a rapid increase in temperature to kill undesirable organisms. Once the temperature profile reaches desirable standards (60 ° C for at least six hours), the treatment is stopped. Of course, there are several changes in substrate composition due to consumption of sugars and a change in C/N ratio, however, these changes are not the target, but rather produce and retain heat for pasteurization purposes.
The self-heating pasteurization technique should be further studied. If conditions are improved, the duration of treatment may be reduced. Also, optimizing aeration would facilitate removal of the mass and reduce heat loss during turning. Finally, another line for improvement could be to study substrate composition for optimizing thermogenesis. For instance- it has been observed that reducing sugars may have an influence on temperature profile. These adjustments may allow its application in areas with cooler environmental temperatures than those tested so far. The microbiota that develops during self-heating pasteurization has been poorly studied and it appears to have a decisive role (Torres et al. 2016).
The technique described here is suitable for small growers because a crate of 1m3 can process about 220 kg of grass or 380 kg corncobs with 65% moisture. If processing a larger amount of substrate is required, it would be possible to increase the volume of the container. However, the question rises to what extent it would be feasible, referring to steam pasteurization. The larger the mass, the greater the heat generated, but also the greater the effort required to obtain a uniform temperature throughout the whole substrate.
The present research was financially supported by “Fondos Mixtos, Conacyt” through the project FOMIX-13149 “Design, construction, equipment and start up of a state center for innovation and technology transfer for the development of coffee growing in Chiapas, Mexico” and through the project MT-11063 of Ecosur “Social and environmental innovation in coffee growing areas for reducing vulnerability”. This text is a section of the chapter entitled “substrate protection” in the book edited by Sánchez JE and Royse DJ. Biology, cultivation, and nutritional and medicinal properties of oyster mushrooms, Pleurotus spp (in Spanish) that will be published soon. The authors wish to thank Dr. Daniel J. Royse for editing the English version of this document.
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