Why Do My Dehydrator Tomatoes Turn Black and Moldy?

My in laws have another bumper crop of fresh garden tomatoes. Even after canning loads of  almost every conceivable tomato recipe – tomatoes, sauce, juice, salsa,  and new this year ketchup! – their tomato plants are still growing more fruit than even the most dedicated gardener can pick.

Which is how I ended up with two big  bag of very ripe garden tomatoes for which I am very grateful. I immediately brought the tomatoes home, sliced the majority of them, and put them in my portable dehydrator that I absolutely can’t live without (you can learn more about it here) like I’ve done many times before with great success. (I am including affiliate links for your convenience.

When I checked the tomatoes in the dehydrator, instead of packaging them up for later I got my first vegetable dehydrator fail. Most of the tomatoes turned black, moldy, or both in the dehydrator.

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Why?

3 Reasons Why Tomatoes Turned Black in the Dehydrator and How to Prevent It

There are three reasons why dried tomatoes can turn black in the dehydrator:

1. Low acid types of tomatoes will turn darker than higher acid tomatoes like Roma tomatoes. You can see from this photo that the bright red tomatoes are Romas and the blackened tomatoes are beefsteak tomatoes.

 At least the compost bin was happy with all of the new bacteria I added to it with my failed dehydrator tomatoes.

2. The tomatoes turned black because you burned the tomatoes by setting the dehydrator temperature too high.


3. The tomatoes turned black because the tomatoes may be extremely ripe.

How to stop tomatoes from turning black in the dehydrator:

• Mix a bit of powdered food grade citric acid like this kind in water and spray it on the low acid tomato variety slices after you load them into the dehydrator.  

• Dry the tomatoes in the dehydrator at 145 degrees (F) for 6 -12 hours until the tomatoes are crisp, tough, or brittle. Your drying time will depending upon how thick you slice the tomatoes, the temperature, and the amount you want to make at a time. You can speed the drying time by rotating the trays every four hours if you like.

 

Two Reasons Why  Tomatoes Mold in the Dehydrator

In addition to loading my dehydrator with very ripe low acid tomato slices, there were two reasons why the tomato slices molded in the dehydrator:

1. There is mold/bad spots in one or more tomato slices which the dehydrator fan circulated throughout the rest of the food trays in the dehydrator.

I’m kicking myself for this one because I thought I had cut away all of the bad spots. Apparently I didn’t. I soaked the dehydrator trays and liners in oxygen bleach to kill any last bit of mold I may have missed when I washed them with soap and hot water.

2. It was humid and you set the dehydrator temperature too low. I forgot to check that I still had the temperature set at 115 degrees for herbs when I loaded it with tomatoes. The low temperature and humitdity in the air (thanks Ohio!) helped number one grow mold with a vengence.

You should dry the tomatoes in the dehydrator at 145 degrees (F) for 6 -12 hours until the tomatoes are crisp, tough, or brittle. Your drying time will depending upon how thick you slice the tomatoes, the temperature, and the amount you want to make at a time. You can speed the drying time by rotating the trays every four hours if you like.



You should find the following helpful when it comes to avoiding dehydrator mistakes:
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