Pouring MYA

Agar, malt and yeast do three different jobs, and pouring a plate is not the same thing as opening one already made.

A floor, not a harvest

A plate is not a little jar of food and it is not a miniature mushroom. It is a gelled surface, with nowhere to hide, on which mycelium has to walk in two dimensions. What decides whether that walk is readable is not only the genetics you set down. It is the medium: how much water it holds, how much sugar it offers, how clear it is, how firm it is under the blade. MYA is the name of a classic medium, malt yeast agar: agar, malt, yeast. The three ingredients are not three ways of saying “nutrient”. They do three jobs that do not stand in for each other.

If the ratio is wrong, the plate lies. Too soft, and bacteria swim in a film you mistake for damp mycelium. Too hard, and the hyphae advance poorly: you read a hostile floor as a weak strain. Too dark, and the start of a contamination stays invisible until it has already sporulated. That is why it is worth knowing what each powder does before you pour, and before you decide whether pouring makes sense or whether you need a plate already made by someone who actually finished the sterilisation cycle.

A plate is for deciding whether a colony deserves grain. It is not for fruiting. A wedge taken from a gel you cannot read carries both the fungus and the doubt into the jar. The pour is the dull part that makes the reading true.

Agar holds, it does not feed

Agar is a polysaccharide taken from red algae. Mycelium, as a rule, does not eat it. It uses it as a floor. In cold water it swells and does not dissolve. It dissolves only when the liquid gets close to boiling, and it becomes solid again when it falls to around 40–45 °C. That delay between melting and setting is why you can sterilise a liquid and pour it while it is still fluid. It is also why, if you wait too long with the bottle in your hand, the medium gels in the neck and will not come out. You do not fix that by adding water to a gel that has already set. You would change the ratio and contaminate the rest.

The dose is a mechanical compromise, not an act of generosity. Too little agar and the gel will not hold a wedge. The blade sinks, the surface tears, water weeps out and wets the rim. Too much agar and the gel becomes a dry elastic: the hyphae struggle to penetrate, and in the fridge the plate cracks and lifts off the base. A crack is not itself a mould. It is a new edge where condensation sits, and where a spore finds a damp corner. The floor should stay continuous, a few millimetres thick, neither a film nor a slice of cake.

Agar does not correct scorched malt and it sterilises nothing. It is also the component that makes “making plates” pointless without it. Dissolving only malt in water gives you a broth. A broth poured into a petri dish is a puddle, and puddles are bacterial territory, not a window onto mycelium. If the gel, once cold, shivers like a cream that is too loose, do not inoculate it hoping the fridge will firm it up. Cold stiffens it a little. It does not give back the structure that was missing.

Malt is the complex food

Dry malt extract, in the light version, is sprouted barley, dried and concentrated. It carries sugars more complex than pure glucose, some amino acids, salts. It is real food, unlike agar. The word light matters for an optical reason before it matters as nutrition. An amber plate hides the gleam of a bacterial colony and the first green speck. Dark malt, the kind for heavily toasted bread, colours the gel and brings sugars already darkened by factory heat. They do not help you read. If one day you formulate on your own, light malt is the one that lets you see the edge.

The amount shifts how the colony behaves. Too little malt and growth stays thin: you can mistake a poor medium for weak mycelium, especially with slow species such as shiitake or lion’s mane. Too much malt and the medium becomes rich in dissolved sugars. Water is less free for the hyphae, while moulds and bacteria that like that sugar do not stop. On top of that, malt sugars held too long around 121 °C darken. The plate comes out darker, sometimes a little inhibitory to the fungus, and you have spent a whole cycle to get a medium worse than the one you weighed. For a half-litre or one-litre bottle, fifteen or twenty minutes at temperature, counted from when the cooker is actually at pressure, is the usual order of magnitude. An hour “to be safe” burns the food.

One distinction saves you useless plates. MYA premix already contains agar, malt and yeast in the ratio meant to be diluted as the pack says. You do not measure it in spoonfuls. You weigh grams per litre, because a premix is a ratio: if you concentrate it, you concentrate the firmness of the gel and the amount of food together. Guessing “a bit thicker, so it holds” is how the floor and the nutrition drift without you knowing which of the two you are reading.

MYA Agar Premix 100g Terreno Micologia Piastre Petri Slant Coltura€9,70

If you start from separate ingredients, light malt extract is only the food fraction. Without agar it does not gel, whatever the label says. Without a small source of nitrogen and vitamins, many colonies still start, but some slow species stay a film that does not tell you whether the strain is tired or the medium is incomplete. Malt dissolved in a plate is not MYA. It is a broth you let cool in the wrong shape.

DME Light 100g Estratto Malto Secco Agar MEA Coltura Liquida Micologia€8,70

The yeast, and the sugar that does not have to go in

The yeast in the name MYA is yeast extract, soluble, not the baker’s cube and not the flakes from the kitchen jar. Flakes do not dissolve the same way, they carry particles and salt, and they leave a sediment you mistake for contamination. Extract carries amino acids and B vitamins: easy nitrogen and cofactors. In the recipe it is the small fraction, and it should stay small. Raising it for “more vigour” makes the plate wet, the mycelium a cotton with no edge, and bacteria find the richness they were after. A medium you mean to read wants a clean margin. A white coat can be healthy, but on an exaggerated dose of yeast you can no longer tell healthy from confused.

Dextrose powder is a different molecule: glucose, a simple sugar. In PDA, potato dextrose agar, it sits beside potato starch as a fast carbon. In a broth for liquid culture it dissolves and the mycelium respires it in suspension, where you do not need an edge to read. In MYA it is not an obligatory fourth ingredient. Adding it to enrich pushes the medium toward bacteria and toward moulds that use free sugar before your fungus does, and it takes the point out of a medium that is already rich because of the malt. If you are making a PDA or a broth, dextrose has a place. If you are pouring MYA from a mix that is already balanced, leaving it in the jar is the coherent choice.

Malt and dextrose are hygroscopic. They take up moisture from the air, they clump, and a lump weighed as if it were dry is partly water. The ratio shifts without a decision. Lid on, dry place, away from steam. A malt smell is normal. A smell of solvent, mould or acid is not: that powder does not go into a plate. Dry agar fears humidity less, but a premix that contains malt should be treated as malt, not as an inert powder. A pouch left open beside the pot has already changed the recipe.

A pressure cooker, or someone has already poured

Sterile, here, means something precise. The medium has seen saturated steam at around 121 °C, about one bar above atmospheric pressure, long enough that the centre of the bottle gets there too. Boiling at 100 °C knocks a lot down and is not enough. The endospores of some bacteria, the ones you later recognise as a glossy film and a sweet smell, survive boiling water. A plate that stays clear for two days can cloud afterwards, when those spores germinate on a sugar that is waiting for them. If your cooker does not hold that temperature, you are not sterilising. You are pasteurising a medium too rich to settle for pasteurisation. Many European kitchen pressure cookers are not autoclaves. If the valve does not reach and stay at that pressure, the cycle is theatre.

For a half-litre or one-litre bottle the useful time is often around fifteen to twenty minutes, counted from when the pressure is there, not from when you lit the flame. Time does not stand in for temperature, and the liquid has to reach that temperature, not only the steam near the valve. A full bottle that is still cold takes longer. The cap should not be screwed tight: sealed glass is a classic way to find it cracked. A heat-safe container, cap loosened a quarter turn or covered with foil, lets pressure equalise. When the cycle is over, you let the pressure fall on its own. Opening the valve suddenly makes the medium boil over, wets the cap, and can pull kitchen air in when a vacuum forms.

Then you pour. Around 45 °C you can hold the bottle and the agar is still fluid. Hotter, and a polystyrene plate warps; condensation becomes a storm on the lid. Cooler, and you pour lumps that stay as opaque islands. You pour in still air: an upturned box, or a low flame that draws air upward, wrists rested, windows shut, no plants on the sill, no sentences spoken over the open lid. Breath is moist. A base of a few millimetres, on the order of twenty millilitres on a 90 mm plate, is a gel that does not dry out in a few days and is not a pool. Lid on at once. Ten lids open “to go faster” are ten plates exposed while you pour the first.

Buying plates already poured is not skipping the lesson. It is moving sterilisation and pouring to a place where the cycle has already been closed. It makes sense if you do not have a cooker that reaches temperature, or if today you have to transfer a white edge and not spend the afternoon pouring. The biological cost is age. A plate poured weeks ago is drier. If the sleeve has been opened, or the seal has taken on humidity and dust, it can already have a tenant you did not invite. You look at the plates inside the sleeve, against the light, before you inoculate them. A speck, a film, a gel that has shrunk and cracked: that one is not used. The advantage of pouring yourself is freshness and a clear medium, just made. The disadvantage is that every second of an open lid is a way to fail. Neither road forgives the bench where you slice bread.

Sterile 90 mm petri dishes, vented, arrive in a bag of twenty. They are the vessel, not the medium. Sterile when the bag is opened does not mean sterile after an hour on the table with the lid sitting badly. You take them from the bag with hands just washed and wiped with 70% alcohol, you pour, you close them. You do not rinse them. A rinse is non-sterile water on plastic that was already clean.

Piastre Petri Sterili Ø90mm Ventilate – Sacchetto 20 Pezzi per Coltivazione Funghi€18,00

Some people add antibiotics to agar to “keep it clean”. An antibiotic is not a substitute for steam. It hides bacteria instead of stopping them getting in, and a plate that looks white can hand you, into the grain, a mycelium living with a silent guest. When the antibiotic dilutes in the jar, the guest wakes up. If a plate is contaminated you discard it. You do not medicate it.

Condensation is not a mould

On the first day the lid fogs. That is physics. Hot agar has loaded the inside air with vapour, the lid is cooler, the water turns liquid again. It is not a contaminant and you do not dry it with a tissue slipped under the rim. That gesture is an invitation. Keep the lid upward while the gel sets, so the surface stays flat. Stacking plates just poured, in still air, keeps the lids warm and cuts condensation. It works if the stack does not wobble and if you do not move the plates until the gel holds.

When the gel is cold and firm, turn the plate over: agar upward, lid underneath. Condensation stays in the lid and does not rain on the colony. Do not turn it over while the agar is still liquid. You would get a gel stuck to the lid and an empty base, and the writing you put on the rim would no longer know which way the medium faces. Repeated drops on the mycelium are the real problem. They wet the hyphae and give bacteria a liquid bridge. A drop sitting on the lid can be ignored. A puddle on the surface, before you have even inoculated, is a plate to discard, or to leave closed at room temperature for a day, lid up, to see whether the water goes back into the gel. If it stays a lake, you do not set a wedge you care about onto it.

Do not put plates that are still hot into the fridge. The shock multiplies condensation. On a gel that has not settled, it pulls water onto the surface exactly where tomorrow you will want a dry edge. Room temperature first, then the cold. A bubble in the gel, if the medium was clear, is usually just air caught in the pour. A dark grain on the base, appearing after some days, is not a bubble.

Parafilm, vents, weeks in the cold

Vented plates have small rests that hold the lid a fraction raised. They are not holes to widen with a blade, and they are not an invitation to leave the lid ajar “so it can breathe”. That minimal gap exchanges a little gas, so the colony is not sitting in a dome with no turnover, and in return the plate dries sooner than a plate with a flat lid. Parafilm, a single strip around the join, slows the drying and keeps spore dust out without sealing the way an impermeable tape pulled tight in several turns would. One turn. Not a cocoon. Tape that lets nothing through also holds the condensation in, and it can slow a colony that had only that gap to unload carbon dioxide.

Do not leave the rim bare in the fridge for weeks. Fridge air moves past food, and the vented gap is enough for a spore if nothing slows it down. Label the base, never only the lid. Lids get swapped the moment you open two of them. Species, date, and which sector a transfer came from if you know. When the colony has covered the surface, the plate goes between 2 and 8 °C, never in the freezer. Freezing crystallises the water in the gel and turns it into a sponge you can no longer inoculate. In the fridge a well-closed plate stays a copy for weeks. It is not a slant, the tilted tube that lives for months. The plate is wider, it dries sooner, and you can read it better. If the gel has pulled back from the rim, changed colour, or shows specks that were not there on pour day, you do not clean it with a blade. You throw it away closed, inside another bag.

Dry premix and dry malt live outside the fridge, in the dark and in the dry. A poured plate lives in the cold. A plate forgotten somewhere warm, after the colony has closed the circle, either dries or speckles, because old agar resists the spores that land on the gap less well. Cold slows the mycelium you want to keep, too. That is fine. A copy does not have to run. It has to stay recognisable on the day you need a young edge to transfer.

What this guide is not

It is not health advice and it is not a promise of white colonies. It is the mechanics of a gel: a floor that does not feed, a complex food that should not darken, a small fraction of yeast, and a heat that either reaches sterilisation or does not count. If the plate turns green, you do not open it to smell it. You seal it, bag it, and discard it. How to read the edge, once the medium is honest, is a different job.