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Desiccants and drying

All you need to know about desiccants and hot to dry mushrooms using them. The text covers the most commonly used desiccants.




There seems to be a lot of these questions lately so here is a post I hope clears up doubts.

There is absolutely nothing new here just information brought together from different sources.

DRYING YOUR MUSHROOMS WITH DESICCANT


A FEW POINTERS BEFORE WE BEGIN

What is a desiccant?

A desiccant is a chemical substance that has a high affinity for water. It absorbs water from the air lowering its relative humidity. This dry air will then extract water from your mushrooms and eventually dry them out.

These chemicals have a set amount of water they can hold. Once that amount has been absorbed they will stop working and you will either have to regenerate them (see later) or buy more. It is important to store your desiccant in airtight containers when not being used. if not, it will absorb water from the air it is in contact with and be less effective when you use it, as some of its drying capability will have been used up.

What kinds of desiccant are there?

The most commonly used desiccants can be divided into two classes, silica gel and calcium salts. We will go into detail on the specific kinds later on in this text.

A NOTE ON INDICATORS

Many desiccants come with an indicator that will tell you when the desiccant is spent. It is a blue substance that turns pink once the desiccant is fully used up.

This substance is cobalt chloride. This is a salt of a heavy metal (cobalt) and everyone knows how toxic heavy metals are.

If your desiccant of choice contains an indicator extra precautions should be followed to avoid any contact at all between your mushrooms and the desiccant.

It is advisable to avoid direct contact with the desiccant at all times even if it does not contain an indicator. Even though the actual chemicals that make up the desiccant are not especially toxic they don't really taste that nice.

DRYING

The keys to good drying of mushrooms are: speed and cracker dry.

Speed

We want to dry the fruits as fast as possible to avoid any rotting and we want them as dry as possible to avoid them spoiling and/or loosing potency.

Cracker dry

Cracker dry is the way to describe correctly dried mushrooms. The stems should be dry enough that when you try to bend them they crack, not bend.

HOW SHOULD I USE MY DESICCANT?

The best way to use a desiccant is to first fan dry the mushrooms and then use the desiccant to extract the last amount of water.


Why should I fan dry first?

Normal, room-temperature air can extract a considerable amount of water from the fruits. By placing them in front of a fan you increase the air circulation and help speed up the drying process. This is the same principle as drying your laundry. It dries a lot faster on a windy day than on a day with no wind.

By fan drying before you are extracting a big part of the water from the fruits so the amount of water that the desiccant needs to absorb is much lower. This allows you to use the same desiccant a lot longer.

Fan drying is also a faster process that drying with desiccant so you end up speeding the process up considerably (and as we have said already, speed is good).
For those of you lucky enough to live in climates with dry enough air (Relative humidity or RH below 40%) you might not even need to use desiccant as you may be able to achieve cracker dry mushrooms with just fan drying. If after 48 hours of drying in front of a fan the mushrooms are not cracker dry you will need to put them into a container with desiccant.

TIP: Even if your mushrooms appear to be cracker dry after fan drying there is often some residual humidity inside them and they will soften up a bit after a few days in storage (not good). It is a good idea to either store the fruits with a bit of desiccant or pop them in a desiccant chamber after fan drying for a few days just to make sure you have got every little bit of water out of the fruits.

So, I have fan dried my mushrooms. What now?


Once you have fan dried the mushrooms you will want to put them in a container with desiccant to finish drying them off. There are many possible designs and examples can be found all over this website but the principle is basically the same.

You want an airtight container. In this container you will put a layer of desiccant and on top some element (mesh, rack...)that will keep the fruits out of contact with the desiccant. On this separator you will place the partially dried mushrooms spread out and not packed too tightly and close the container. In a few days your mushrooms should be cracker dry.

TIP: If you are putting your mushrooms in the desiccant chamber after fan drying them you can pack them in a lot closer than if you are putting them in fresh.
If you cannot fan dry and have to put them in fresh you will need to use a greater amount of desiccant and spread the fruits out well on the rack, making sure none touch.

My mushrooms are now cracker dry. How should I store them?

There are many ways of storing the dried mushrooms and it is up to personal choice which to use. The only thing that you must always do is store them in an airtight container. Adding a small amount of desiccant (for example one of those little packets found in boxes when buying many items) is a good idea to ensure they stay cracker dry.

CHOOSING A DESICCANT

The main kinds of desiccant are silica gel and two kinds of calcium salt: calcium sulphate and calcium chloride.

SILICA GEL

Silica gel is a very porous kind of sodium silicate. Because it is highly porous it has a big capacity to absorb moisture.

Available as: Silica gel, Silica beads, Silica gel cat litter.

Found at: Silica gel is used often in craft work for drying flowers so most craft stores will stock it. The kitty litter version is exactly the same stuff and can be found at many pet stores. It is often sold as low dust or low maintenance cat litter so look at the label to see what it is. As long as it says silica gel, it is the stuff you want.

Price: Moderate

Regenerating: Spread out in a maximum 1 inch thick layer and either use microwave or oven

Microwave: Medium/high setting. Give 3-5 minutes microwaving and then stop for five minutes until the indicator turns dark blue again. If your kind has no indicator the approximate time is 12 minutes microwaving (in the 5 minute bursts) per pound of silica gel.

Oven: 150ºC/300F for 1 1/2 hours per litre/quart of silica gel.


Toxicity: Non toxic (but the indicator is). Avoid breathing in the dust as it is not good for the lungs.
 
Additional info:

Silica gel is not the most powerful of desiccants, it only lowers the RH to just under 40% but does the job extremely well especially with fan dried mushrooms. If using fresh mushrooms it might be a good idea to try one of the other desiccants.



                     INDICATING SILICA GEL                                                     NON INDICATING SILICA
      


                             SILICA CAT LITTER





CALCIUM SALTS

The calcium salt kind of desiccant have a few things in common. These salts like to absorb water. If they contain no water at all they are in their anhydrous form. An = no Hydro from the ancient Greek for water (dehydrated, hydroelectric...)

As they absorb water they pass through different hemihydrated forms (hemi=half, partially) and keep on absorbing water until they reach their hydrated form. In the hydrated form they can no longer absorb any water. It is this property of the anhydrous form absorbing water to hydrate itself that makes these chemicals a desiccant.

Because the water is chemically bonded to the salt it requires higher heat to take it out of the hydrated form and regenerate it to the anhydrous.



ANHYDROUS CALCIUM CHLORIDE

The anhydrous calcium chloride slowly absorbs water and turns first into a mush and eventually into a liquid brine.

Available as: Damp rid
Comes in sealed containers.

Found at: Just about everywhere. In bulk at hardware/paint stores.

Price: Cheapest

Regenerating: This stuff is a lot harder to regenerate. Once it has turned into liquid, attempting to regenerate will end up with a useless solid block of material. You can get some results if you regenerate when it is mushy, before it dissolves. Place in a shallow pan in a layer 1 inch deep and put in the oven at 230ºC/450ºF for two hours

Toxicity: Non toxic (but the indicator is). Even though it is not toxic it does taste awful and will make you incredibly thirsty when you ingest it. None of this is very nice if you are about to trip, so take extra precautions to ensure that there is no contact between mushrooms and desiccant as this stuff really loves water and will stick to mushrooms at the slightest chance and get inside them.
 
Additional info:

This is strong desiccant and will lower the RH in the chamber to below 25%.
Even though it is very cheap there is a cheaper alternative. Calcium chloride is sold for melting snow in big sacks at a cheap price. It is advisable to regenerate it before use as the manufacturer is not interested in its desiccant properties and will have been less careful to ensure that it has not absorbed some water during storage.


DAMPRID BRAND ANHYDROUS CALCIUM CHLORIDE



ANHYDROUS CALCIUM SULPHATE

Available as:  Drierite.
Comes in granules in sealed containers.

Found at: Online at their website or in hardware/chemical supply stores.

Price: Highest

Regenerating: Place in an oven tray in a layer 1 granule thick and heat in the oven for 1 hour at 210°C/425F. Place back into an airtight container while still hot. Be careful not to overheat or you will lose the special structure of the granules and lose effectivity.

Toxicity: Non toxic (but the indicator is). Can be bought with or without the indicator.
 
Additional info:

Anhydrous calcium sulphate is the most powerful desiccant out of the three. It will reduce the RH in the container to below 1% easily.

A cheaper alternative to drierite is either plaster of Paris (gypsum plaster) or powdered gypsum. They are both calcium sulphate in the hemihydrate and hydrated form respectively. So you need to regenerate them before using them as a desiccant. To regenerate place in a shallow pan in a layer 1 inch deep and put in the oven at 230ºC/450ºF for two hours. Place in an airtight container while still hot.

This cheaper substitute will not be as powerful as it does not have the special granular shape and will contain a higher number of impurities but it will still do an excellent job.


                               TWO KINDS OF INDICATING DRIERITE




             PLASTER OF PARIS


NOTES

When giving the prices I have just compared them, none of the desiccants are extremely expensive.

REGARDING TOXICITY: WHEN I SAY A DESICCANT NOT TOXIC I MEAN THAT IF A MUSHROOM TOUCHES THE DESICCANT BRIEFLY THERE IS NO NEED TO THROW IT AWAY. THAT DOES NOT MEAN YOU SHOULD NOT STILL AVOID CONTACT AT ALL COSTS. WHILE A SMALL AMOUNT WON'T KILL YOU THEY CERTAINLY ARE NOT GOOD FOR YOU.


SUMMARY

SILICA GEL
  •  Intermediate price (10$-20$ for five pounds)
  • Very convenient to regenerate (can use microwave)
  • Is not the strongest of desiccants so is best avoided for drying fresh shrooms without fan drying.
  •  Very long lasting if regenerated correctly

ANHYDROUS CALCIUM CHLORIDE (damp rid)

  • Very cheap (3$ for 3lb)
  •  Intermediate effectivity
  • Hard to regenerate, it is easier to just use once.
  • Bad taste



ANHYDROUS CALCIUM SULPHATE (Drierite)
  • Most expensive (20$ a pound)
  • The most powerful
  • Can be regenerated

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