Maybe. I’m about half-way through making a test batch of sun-dried tomatoes, here in humid Virginia.
It’s been a learning experience.
You can’t do that here.
Conventional wisdom says that you can’t sun-dry tomatoes in a humid climate. That is, you can’t preserve tomatoes by placing cut tomatoes outside and letting the sun and the breeze dry them.
Southern Italy? Fine. California and Arizona? No problem. Virginia? Nope.
In theory, the food dries too slowly and will mold before it becomes fully dry. In a humid climate, you need to dry them in some sort of dehydrator (even if it’s a solar-powered dehydrator), or in the oven. Just putting them out under the sun won’t cut it.
Northern Virginia summers are hot and humid. And so, based on what I’ve read, I figured that simple open-air tomato drying wasn’t worth trying, in Virginia.
And if that were the end of the story, this would be a pretty short blog post.
But it’s not cut and dried.
The detailed directions for making sun-dried tomatoes are anything but cut-and-dried. Name any aspect of vegetable sun-drying — how hot, how humid, how long, what end point, what post-processing is needed, and how stable the end product is — and you will find disagreement among seemingly authoritative sources. Some of that is likely due to variations in climate. But some of it suggests that there is no one agreed-upon standard process.
The basic idea is the same, across all sources.
- cut the tomatoes
- lay them on something (preferable a screen)
- put them out in the sun and breeze
- protect them from insects and birds
- collect them when they are dry
How hot and dry must the weather be? The University of Minnesota extension service says you need 20% relative humidity. But Oklahoma State University extension service says you can sun-dry some produce at 85F and 60% relative humidity, or less. I note that the USDA-backed authority in this area — the National Center for Home Food Preservation — agrees with a minimum temperature of 85F and maximum relative humidity of 60%. University of Utah says 98F (!)
How long do they need to dry? Here, without citation as to source, most sources say four days. But, the University of Utah says to leave vegetables in the sun no more than two days, and finish drying in the shade for an unspecified period. And both The Spruce Eats and The Daring Gourmet say anywhere from four days to two weeks.
Handling during drying. Virtually all sources say to take your tomatoes inside at night, turn them daily, and cover them with netting to keep off bugs and birds. But in addition, some sources say that once the tomatoes are partially dried, you should finish drying in the shade. This recommendation comes from Utah, where UV in sunlight is likely quite intense due to high altitude.
What end point? Most extension services say “crispy”. But a lot of non-official sources would have you stop well before that point, with a more raisin-like texture. For example, The Spruce Eats specifically says dry but not crispy. Still other sources specify “leathery or crisp“. None of the directions I have found, so far, actually tells you what the target moisture content should be, in percent.
What post-processing after drying? At least one source (Oklahoma State University) suggests that you pasteurize the end product to kill any insects or insect eggs, either by freezing for two days or heating to 160F for half an hour. That, for any fruits or vine-dried beans. Other sources only mention that treatment for beans and similar produce that is allowed to dry on the vine.
But the biggest post-processing step that is frequently recommended is to preserve them in oil. Apparently this is the traditional Italian approach. However, the USDA specifically says not to preserve in oil If any moist areas remain, they may harbor botulism in the anaerobic environment in the oil bath. One apparently reputable source says that you need to do a separate acidification step if you want to preserve your tomatoes in oil (source). Botulism thrives in a low-acidity anaerobic environment.
All of that makes sense to me, in light of the science on canning fresh tomatoes. Historically, tomatoes were considered a high-acid food, and so did not need to have acid added during canning in order to suppress botulism. But modern tomato varieties (and, by rumor, over-ripe tomatoes) may not be sufficiently acidic to meet the modern standard for canning. And so, you have a dichotomy between what Italian grandmothers have been doing for decades, without incident (just can them), and what the USDA recommends for safest current practice (add acid before canning).
When in doubt, do it the USDA way.
Summary:
- Probably need four consecutive sunny days with 85F temperature and under 60% relative humidity.
- Use netting to keep bugs off, take inside at night, flip tomato slices each day.
- Expect this to take four days, but it can take up to two weeks.
- The final product can be leathery or totally crisp..
In my case, I’m probably going to finish these up in my electric dehydrator, and get them completely crisp. I have found that my batches of tomatoes dry unevenly, mostly because I have a mix of all the types and varieties that I am growing. Given that, I think I want to force them to a uniform crispness, rather than have to test each piece to see if it is sufficiently leathery.
Plus, I have no clue what “sufficiently leathery” means.
I just need a string of good drying days.
Source: Weather underground.
To try sun-drying tomatoes in Virginia, all I need is four good drying days in a row. That is, days with sunshine, heat, breeze, and low relative humidity. Per the bulk of directions, I just need days with an extended period where the temperature is at least 85F and relative humidity is below 60%.
Even in humid Virginia, we occasionally get a stretch of days like that during the summer. So, for sure, I can’t reliably sun-dry tomatoes. But maybe I can occasionally sun-dry tomatoes, if the weather cooperates.
We’re in the middle of a string of days like that, right now, as shown above. Temperatures are going to top out in the high 90s F, and relative humidity will bottom out around 40% during those periods of peak temperatures. So I’m having a go at a test batch of sun-dried tomatoes.
For the first two days, I used a fan to ensure constant air flow, and to keep insects off.
By the start of the third day, the tomatoes finally started to look dryer. Below, the same tray at the start of days 1, 2, and 3.
So for days three and four, I simplified the rig. No fan, just a piece of tulle over the food to keep insects and birds off it. This has the big advantage of being portable, so I am able to keep this facing the sun from mid-morning to late afternoon.
Getting over the ick factor.
Source: Gencraft AI
Arguably, the hardest part of this, for me, is getting used to the idea of having cut tomatoes simply sitting around, wet. My mom was big on food safety, and nothing that needed refrigeration stayed out of the fridge any longer than it absolutely had to. So this is a little hard to get used to.
Parenthetically, my mother’s concern for food safety was completely rational, given her upbringing. My mother was a registered nurse who grew up in the era before antibiotics or household mechanical refrigeration. People frequently died of literal “food poisoning”, that is, from the consequences of ingesting food tainted with pathogenic bacteria. If you look at historical cause-of-death data, in the 1920s, about 3% of all deaths were attributed to “diarrhea, enteritis, ulceration of the intestines” (Source: CDC). By contrast, food poisoning now accounts for perhaps 3000 deaths a year, or around 0.1% of all deaths in the modern era.
I am trying to take solace in two facts.
First, everybody says it takes four days. So everybody who does this has wet, sliced tomatoes sitting around for four days. So, in this context, this is normal.
Second, it’s only safe to do sun-drying of foods that are either high sugar, high acid, or both. Tomatoes, being a generally high-acid food, fall within the class of foods for which this is a relatively low-risk preservation method. If this were some low-acidity food like, say, pumpkin, the ick factor would be real.
But for tomatoes, despite how much I dislike having wet sliced tomatoes sitting around, this, apparently, is not dangerous from a food safety perspective.
I just have to get over my 20th century suburban fear of un-refrigerated food.
A final note on relative humidity.
Source: Omnicalculator.
Relative humidity depends strongly on air temperature, falling as the temperature rises. Typically then, relative humidity starts off high in the morning and falls throughout the day, until things begin to cool in the evening.
If the weather for the day is stable, you can predict your afternoon relative humidity based on morning conditions. Go to any on-line relative humidity calculator, and fill in the (known) AM temperature and relative humidity from (e.g.) the Nationl Weather Service forecast for your area. The calculator will then fill in the dew point. Now erase the relative humidity, type in the known dew point, type in the afternoon’s projected high temperature, and the calculator will give you a projection of relative humidity for that time.
Today, my relative humidity was 61% when it was 83F this morning. In this case, I can expect late-afternoon relative humidity of around 40 percent.
A good drying day. Or, at least, good enough. I hope.
