Emily Johnson
Mushrooms are a staple in the human diet and an important source of nutrition worldwide. However, they are susceptible to rotting, which can render them unsafe for consumption. Various factors can cause mushrooms to rot, including moisture, bacterial growth, and specific types of fungi. The presence of mushrooms on trees can also indicate the presence of wood rot, posing potential hazards. This highlights the importance of understanding the causes and signs of mushroom rot to ensure the safety and quality of this valuable food source.
| Characteristics | Values |
|---|---|
| Type | Brown rot, soft rot, white rot |
| Cause | Fungi |
| Fungi types | Serpula lacrymans, Armillaria (honey fungus), Fibroporia vaillantii (mine fungus), Coniophora puteana (cellar fungus), sulfur shelf, Phaeolus schweinitzii, Fomitopsis pinicola |
| Fungi diet | Carbohydrates, lignin, starch, sugars |
| Environment | Moisture, rain |
| Wood appearance | Brown discoloration, cracks, reddish-brown and brittle wood |
| Mushroom appearance | Slimy skin, wrinkles, dark spots, discolouration |
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What You'll Learn
- Brown-rot fungi: breaks down hemicellulose and cellulose, causing reddish-brown and brittle wood
- White-rot fungi: degrades all wood components, including lignin, causing wood to turn white
- Soft-rot fungi: breaks down cellulose and hemicellulose, but not lignin
- Moisture: Mushrooms require moisture to thrive, but too much can cause them to rot
- Refrigeration: Keeping mushrooms in the fridge for too long can make them slimy and cause bacterial breakdown
Brown-rot fungi: breaks down hemicellulose and cellulose, causing reddish-brown and brittle wood
Wood-decay fungi are categorised into white rots and brown rots. Brown-rot fungi, unlike white-rot fungi, do not attack lignin, the substance that acts as the “glue” holding the cells together. Instead, brown-rot fungi break down hemicellulose and cellulose, which form the wood structure.
The breakdown of hemicellulose produces hydrogen peroxide (H2O2), a small molecule that can diffuse rapidly through the wood. This leads to a decay that is not confined to the direct surroundings of the fungal hyphae. As a result, the wood shrinks, turns brown, and cracks into roughly cubical pieces, a phenomenon called cubical fracture. The wood becomes dry and crumbly, sometimes incorrectly referred to as dry rot.
The initial stage of brown rot is non-enzymatic, involving chemical agents such as oxalic acid and hydrogen peroxide that snip the chains of cellulose and hemicellulose into smaller pieces. Carbohydrates become partly soluble, and enzymes release sugars that are slowly absorbed by the fungus. This process results in the extensive demethylation of lignins, causing the reddish-brown colour associated with brown rot.
Brown-rot fungi can effectively degrade cellulose, hemicellulose, and some polysaccharides, but they have a limited impact on lignin components due to the lack of a specific lignin-degrading enzyme system. They can, however, cause oxidative demethylation, side-chain oxidation, depolymerization, and potential repolymerization of lignin. Certain types of brown-rot fungi remove cellulose compounds from wood, contributing to the brown colour.
Brown rot is caused by various fungi, including Serpula lacrymans (true dry rot), Fibroporia vaillantii (mine fungus), and Coniophora puteana (cellar fungus). These fungi can attack timber in buildings and are of significant economic concern.
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White-rot fungi: degrades all wood components, including lignin, causing wood to turn white
Wood-decay or xylophagous fungi are species of fungi that digest moist wood, causing it to rot. There are three main types of wood-decay fungi: brown rot, soft rot, and white rot.
White-rot fungi are a group of fungi that secrete lignin-modifying enzymes that transform organic compounds into non-harmful carbohydrates. They are the most well-known type of wood-rot fungi. White-rot fungi are capable of degrading all wood components, including lignin, causing wood to turn white and have a fibrous texture. Lignin accounts for 30% of the organic carbon on Earth. White-rot fungi are the most efficient lignin-degrading organisms, playing a critical role in the movement of carbon from the atmosphere to organisms and vice versa.
The process of lignin biodegradation by white-rot fungi relies on the catalysis of laccase, lignin peroxidase (LiP), and manganese peroxidase (MnP) to produce chemically unstable lignin-free active intermediates, free-radical-based chain reactions, and a series of spontaneous degradation reactions. Lignin polymers are first oxidized by enzymes produced by white-rot fungi to generate aromatic radicals. Different processes, including C-4-ether decomposition, aromatic ring cleavage, Cα-Cβ decomposition, and demethylation oxidation, occur during this process.
White-rot fungi have been explored for their potential in mycoremediation applications, which focus on removing organic pollutants from the environment. They have been found to degrade chlorinated aromatic hydrocarbons (CAHs), DDT, lindane, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, PCP, and polychlorinated dibenzo(p)dioxins. White-rot fungi are also commercially grown as a source of food, such as shiitake mushrooms, which constituted approximately 25% of total mushroom production in 2003.
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Soft-rot fungi: breaks down cellulose and hemicellulose, but not lignin
Soft-rot fungi are known to soften wood, but they are less studied compared to other wood-rot fungi. Soft-rot fungi break down cellulose and hemicellulose, but not lignin.
Soft-rot fungi produce cellulase enzymes such as endo-l,4-glucanase, exo-l,4-β-glucanase, and 1,4-β-glucosidase. They can attack cell-wall materials in the surrounding environment through hyphae secreting enzymes and other degradation metabolites, resulting in cavities in the cell wall. Soft-rot fungi can also degrade pectin, which is beneficial for biomass conversion. However, they are less commonly used in biomass conversion compared to white-rot and brown-rot fungi.
Soft-rot fungi are classified as Ascomycota wood decay, where the surface of wood material is colonized by green or dark mould fungi. These mould fungi are incapable of advancing to the heartwood in dead tree trunks but can decompose cellulose and hemicelluloses on wounded or cut wood surfaces and timber. Soft-rot fungi can also efficiently decompose plant litter.
While soft-rot fungi do not break down lignin, they can cause its degradation to some extent. Observations show extensive removal of wood cell wall materials surrounding cavities produced in the secondary walls of both hard and softwoods in advanced stages of decay. However, there are generally always large amounts of residual electron materials surrounding and attached to the hyphae, representing partially degraded lignin.
Soft-rot fungi, along with brown-rot and white-rot fungi, are key species in forest ecosystems. They play an important role in decomposing dead wood, creating new habitats for other species, aiding in nutrient recycling, and providing food for other species.
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Moisture: Mushrooms require moisture to thrive, but too much can cause them to rot
Mushrooms require moisture to grow and thrive. Fungi, which produce mushrooms, rely on moisture to multiply in wood and form mycelia, which then parasitize wood and cause it to rot. This process is essential for the decomposition of dead wood, creating new habitats and providing food for other species. However, excessive moisture can also be detrimental to mushrooms.
Mushrooms have a high water content, and when fresh, they are smooth and plump. If stored for too long, especially in a humid environment, mushrooms can become overhydrated and begin to rot. This is evident when the mushrooms develop a slimy skin, indicating that bacteria are breaking them down. Slimy mushrooms are often the result of excessive moisture and prolonged storage in the refrigerator.
To maintain the optimal moisture content in mushrooms, it is crucial to store them properly. While they can be kept in the refrigerator for a short period, they should be used as soon as possible to prevent overhydration and spoilage. When mushrooms are exposed to air for extended periods, they can also become dehydrated, leading to wrinkling and shrinkage. Therefore, finding a balance between adequate moisture and excessive moisture is essential for preserving the quality and freshness of mushrooms.
Additionally, the type of rot that fungi cause in wood, such as trees, can be categorized into brown rot and white rot. Brown rot fungi, including Serpula lacrymans, Fibroporia vaillantii, and Coniophora puteana, are known to produce reddish-brown and brittle wood. They effectively degrade cellulose and hemicellulose but have a limited impact on lignin due to the lack of a specific lignin-degrading enzyme system. In contrast, white rot fungi, such as those responsible for the white rot of trees, are capable of degrading all wood components, including lignin, cellulose, and hemicellulose. While white rot occurs at a slower pace than brown rot, both types of rot are challenging to eradicate once they have started in the wood.
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Refrigeration: Keeping mushrooms in the fridge for too long can make them slimy and cause bacterial breakdown
Mushrooms are best stored in the fridge, which extends their freshness by three to seven days. However, keeping mushrooms in the fridge for too long can cause them to become slimy and discoloured. This slime is an indication that the mushrooms are starting to rot, and that bacteria are breaking them down. While eating a couple of slimy mushrooms may not make you sick, it is not recommended, as they could be contaminated with pathogenic bacteria, which could make you ill.
Mushrooms have a high water content, and when fresh, they are smooth and plump. If stored in the fridge for too long, they can become wrinkled, which means they are getting old and dehydrated. If they have just started to wrinkle and are not slimy, smelly, or discoloured, they can still be used, but if they have lots of wrinkles and look very shrivelled, they are too far gone, and it is safer not to eat them.
To prevent mushrooms from becoming slimy, it is best to store them in a paper bag in the fridge, as this allows for airflow and prevents excess moisture from being trapped, which creates an environment perfect for microbial growth, including bacteria and moulds. Mushrooms stored in plastic packaging are more likely to become slimy.
If you do find that your mushrooms have become slimy, it is possible to wash the slime off and cook them, but only if they do not smell bad. If they smell off, it is time to throw them out.
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Frequently asked questions
Mushrooms are a type of fungus that can be decomposed by other fungi, such as wood-rot fungi.
Wood-rot fungi are a collective term for the various fungi that multiply in wood, causing it to rot.
There are two main types of wood-rot fungi that can decompose mushrooms: brown-rot and white-rot fungi.
Brown-rot fungi break down the cellulose and hemicellulose in mushrooms through autocrine cellulase and hydroxyl-free radicals. However, they have a limited impact on lignin components due to the lack of a specific lignin-degrading enzyme system.
White-rot fungi have a strong ability to degrade lignin, cellulose, and hemicellulose in mushrooms by secreting enzymes. They are known to produce low yields of molecules with demethylated functional groups.
