Drying mushrooms is a wonderful way to preserve them for long-term storage. There are a number of different methods, usually focusing on heat and moving air. I think the best one for you and your situation is drying under vacuum, but before the drying, if they're dirty, wipe them off with a damp cloth rather than soaking them in water. In addition , you can using the low temperature instead of high temperature, you can try it. Good Luck my brother.  

What is the pre-treatment you applied for your mushroom before drying? Usually the cutting or slicing enhance the oxidation due to exposing the polyphenol oxidase that is within mushroom structure which activates the oxidation process due to their exposure to air. From what I know about blackening prevention, is to apply the power of antioxidant, may be you can try use natural antioxidant like ascorbic acid or citric acid.  

Another treatment to prevent browning reactions which is used for plant materials/vegetables is blanching. I must admit, in order to the planned drying step it seems counterproductive but it is worth to try it. The blanching step (normally 60-80 °C, < 5 min) should inactivate the browning enzymes.

In my opinion to prevent browning or darkening of mushroom slices you may soak them in 0.5% ascorbic acid( promptly after cutting )for a few minutes, then steam blanching the slices at 85-95 degrees Centigrade for one minute and finally vacuum or freeze drying to 5% final moisture content before grinding to powder.

You should have action to incatived or control the enzime polyphenol oxidase. In the first instance heat is a popular option, but also can be used other techinologies, like high pressure processing. The enzime can be control using additives to: change pH, reduce oxidative enzimatically yield products, reduce oxigen difusivitivy, chelating agents, etc.

Blanching is the best option to inactivate Phenol oxidase enzyme to prevent  browning before drying. If hot water blanching is done, reduce the pH of blanching water by adding different acid treatments i.e. Ascorbic acid 0.1%, Citric acid 0.1%,  Acetic acid 0.1%, Acetic acid 1%, and  compared with water without adding any acids. Then you can observe the best treatment after drying.

Some of you got it right The darkening is the result of the oxidation of polyphenol to quinones by the PPO enzyme (poly phenol oxidase). In the step the enzyme oxidizes phenols to =O  groups. So pulling a vacuum, or gas flushing with nitrogens works is not going to be dried, . The quinones have 2 ketoses groups that can then react with any amines (amino acids, the terminal Nh2 group on lysine in a protein swell as the Nh2 terminal amine of the whole protein. This reaction is called the Maillard reaction and causes the darkening, like in apple, bananas and shrimp. . You can inhibit the browning by adding sodium sulfite in solution, FDA controls the levels that can be used I thing 2000 ppm solution. Citric acid lowers pH which modifies the protein structure which slows the reaction by competitive inhibition. Or you can blanch Better to use steam and I would sclice the mushrooms first. Steam has a heat transfer coefficient 90 higher than hot water. Also the rate of heating is proportionl to the thickness squared. So on button mushroom slice , thin enough to get 4 from one mushroom you will get a rate of heating that is 16 time faster. So with stem and slices you  can do it in 20 o 30 second without cooking flavor developing. This also speeds rying by the same rate. Vacuum drying is expense and freeze drying is very expensive . More than $3 per pound of water remove. So to get a pound of dry mushrooms you need to dry 10lbs of fresh mushrooms which means the drying cost $27 per dry pound.  I forgot to mention the opta company sell a completive inhibitor for PPO , 6 hexyl resorcinol. Crud fuicin  from figs also works

The very effective method to prevent browning in vegetables (as a result of oxidative enzymes) prior to preservation methods is blanching. I think 95o for 1 or 2 min will be enough. Also I a gree with Theodore P Labuza, Benedict Purschke and Ranjani Amarakoon 

 The very effective method to prevent browning in vegetables and fruit (as a result of oxidative enzymes) prior to preservation methods is blanching. and dry in Vaccum at low temperature. 

You can try Intermittent Microwave Convective Drying 

Topic Browning and drying of white mushrooms

Author : Prof Ted Labuza

Email: [email protected]

There are two reactions that cause browning in dry mushrooms The 1st is called enzymatic browning and is caused by the release of the enzyme poly phenol oxidase (PPO). This enzyme reacts with the -OH groups on polyphenolic oxidizing them to a =O group, essentially a ketose group on the ring. This groups is very reactive with amine groups on free amino acids or with those in a protein (eg. lysine) the 2nd browning step called the Maillard reaction. IT is actually a series of many steps leading to the pro color compound developed. So this is a defense reaction in that some of the 100s of compounds produced have antimicrobial activity and lowers the chances of infection. Other common foods with this problem are bananas, potatoes, and shrimp. Several ways to prevent this or slow it down are to :

(1) Blanch the mushrooms i.e. cook in steam for 30 sec or water at 80°C for one to two minutes . Less time if you slice and do it ASAP.

(2) keep the mushrooms in a refrigerator The slows the browning rate by about 4 fold slower.

(3) freezing the mushroom stops the reaction but damages the cells such that they release PPO and on slow thawing you will get browning.

(4) Commercially with  gulf shrimp, they are soaked for several minutes in Sodium bi-sulfite, the sulfur group reaction with the =O group on the polyphenol thus preventing the further reaction with the -NH2 amine group . (

5)  Many fruits are packed in a high concentration sugar solution. This creates a diffusion barrier for oxygen which slows the reaction.

 (6) Recently  Simplot  Co. have produced a potato which  has been genetically modified by deleting the PPO gene . Potatoes still will have the 2nd step and the potato has free glucose sugar  with a reactive aldehyde (C=O) which can react with main amines to produce the desired browning of french fries and POTATO CHIPS. One reaction is with asparagine a storage amino acid which eventually form acrylamide  a carcinogen. The latter has an EPA has a limit of 0.5 ppm (5 micrograms) in water as it is a carcinogen at a cancer risk of 1 in 1  million. WHO suggest we limit food intake to no more than 1 micro-gram of high temperature (deep fat frier) fried foods FDA surveys show that french fries have the highest levels.  Risk assessments show that soma children consuming chips and/or french fries  daily  put the children at over  80 X the WHO limit. Another firm has developed a white mushroom in which the PPO gene has been knocked. The FDA has also suggested that the industry and restaurants should lower the frying temperature to be below 170°C (338 °F) as should home cooks

(7) And now to drying.. There are critical chemical and physical chemical and engineering  paradigms in drying foods including mushrooms. First it to slice to a thin a mushroom as much as possible. The drying time is proportional to the slice half thickness squared, i.e. it takes 4 time longer for a 1/2 inch thick slice as compared to a 1/4 ink slice. My family has a prune farm in California and before the buyer will take them from us we need to dry them to about 26% water from the original of > 90%. People eat whole pitted prunes which take us 2.5 to 3 days to dry in a very warm but dry climate. This is done in a long tunnel drier  in which we pull out a rack of dried prunes from the end, then push all the other racks forward and put a new rack in at the entrance and then close the doors at both end and turn the propane heater and fan blows hot air through the tunnel.  Note that   is hard to dry in humid conditions unless you go to very high temperatures which can damage the food. For mushrooms to maximize shelf life you need to dry a much lower moisture to maximize shelf life.

Below is a diagram of the water content the dry food would have if held at different humidities (Water activity is %RH/100) so a scale of 0 to 1 vs 0 to 100%. Most dry foods should be dried to a water activity of about 0.2 to 0.3. At the corresponding moisture there is not enough water to allow enzyme activity and other water requiring reactions like the Maillard reaction. If we dry too much below this water activity range we have an increased rate of the oxidation of unsaturated lipids leading to off odors like dry hay. Above this optimum water activity both the reaction rates of lipid oxidation and water phase reactions double or triple for each 10% increase in the food water activitynlowering shelf life, Thus the shelf life of mushroom dried to a water  activity of 0.5 instead of 0.2 the optimum will have a shelf life of 2x2x2 = 8 times less. Just drying right and keeping the finished condition it a tightly sealed glass jar to prevent moisture pick up and at room temperature is important. You can measure the water activity by placing a small humidity gauge in the jar and seal it and after 1 day read the gauge's %RH and divide by 100 to get the water activity. If the water activity is over 0.6 you will have another problem of the mushrooms getting moldy.  For your additional information, one other factor of crisp dry foods (water activity of 0.2) is that they loose crispness once they pick moisture to go above 0.4 to 0.5 water activity, namely storage at or above 40% RH. So they should be kept in a tightly sealed package with low water transmission rates. Many plastic containers advertising that they are airtight are wrong We have tested them and the plastic thickness is not enough to privet water pickup. Most houses in Minnesota during the winter have a %RH of about 25 to 30% in the winter and 35% in the summer. This year with the warner temperature and the heavy rains raised the inside to 50%RH or more even with a good central air conditioner system which can remove the inner moisture. So in a few days my open bowls of pretzels and chips turned soggy. Good luck on drying your mushrooms.

The attachment has the graph refferd to in the above text

That was my answer on October 2015!!

Shahram Dokhani

Hi Dr Dokhani

I'm surprised too !! :D

I had never seen that before !!!!