You've brought all your fresh produce home from the supermarket or farmers' market. Now it's time to unpack.
The task seems easy enough, but you'd be best served to at least put a little thought into it. What you store where, and when, matters in part because of ethylene, a plant hormone that is responsible for ripening and can, over time or under the right circumstances, shorten shelf life and cause spoilage, says Laura Strawn, an associate professor at Virginia Tech's Department of Food Science & Technology.
The presence of ethylene and variations in how fruits and vegetables release or react to it mean that not all produce plays well together. Here's what you need to know, and why it may not be a big deal if you sometimes get it wrong.
Ethylene producers v ethylene sensitive
When it comes to ethylene, "Technically, the generalisation of saying fruit are producers and true vegetables are sensitive is a safe logic to follow," Strawn says.
Bananas, melons (such as cantaloupe, not watermelon), apples, tomatoes and avocados are prime examples of ethylene producers, Strawn says. But it's a little more nuanced than either/or, as many ethylene producers are also sensitive to the hormone – they produce it to trigger their own ripening process. Carrots, broccoli, greens and cucumbers are ethylene sensitive examples that do not produce their own ethylene.
Naturally, the division isn't quite as clear cut once you dig a little deeper. "Humans love to categorise," says Christopher Watkins, a professor at Cornell University's School of Integrative Plant Science Horticulture Section. But there are plenty of exceptions and variations. Production and sensitivity to ethylene can vary by variety or phase of maturity. Some apples produce lots of ethylene and are very sensitive to it (anyone who has had a mushy one can relate), while varieties such as Fuji and Pink Lady produce much less, Watkins says.
If you've ever heard the advice to not store potatoes and onions together because ethylene will cause one or the other to sprout, that's only partially true.
A green tomato will be very sensitive to ethylene since it's not ripe yet, but a red one less so. Watkins notes that heirloom tomatoes from the farmers' market that seem to turn the second you look at them wrong are more sensitive to ethylene than store-bought varieties.
Climacteric v nonclimacteric
There's another way to classify fruit, too, depending on whether they will ripen after they are picked. These are called climacteric, and will respond to the presence of ethylene by producing more ethylene. The ethylene producers I mention above are climacteric, as are stone fruit, pears, kiwi and mangoes. (That is also why items such as bananas and tomatoes are often picked unripe and then either left to ripen on their own or treated with ethylene for sale.) In climacteric fruit, starch continues to be turned into sugar, improving texture and flavour.
Nonclimacteric fruits "don't respond to ethylene with their own escalating ethylene production," Harold McGee says in On Food and Cooking. While they won't get sweeter, other enzymes can turn them softer or improve aroma. Citrus, grapes, cherries, berries (blueberries are somewhere in between), pineapple and watermelon are examples of nonclimacteric fruit.
Keep in mind that just because a type of produce does not respond to ethylene does not mean it is immune to spoilage. Bacteria, moulds, yeast, moisture and temperature can all shorten the shelf life of fruit and vegetables.
What does this all mean for storage?
"When you're thinking about storing fruits and vegetables, in general, you want to avoid storing ethylene-producing produce items near ethylene-sensitive ones," says Alexis Hamilton, a postdoctoral associate in Strawn's lab.
Exposure to ethylene can cause broccoli and cabbage to yellow and carrots to turn bitter, Strawn says. Lettuce and other greens, as well as some herbs, can also discolour or droop in the presence of ethylene.
"Reducing the unwanted effects of ethylene at home is as easy as storing these commodities in separate drawers or bags and in separate places in your refrigerator," Hamilton says.
"Many refrigerators are set up with multiple types of sliding bins that are designed to store fruits and vegetables to preserve their quality and freshness based on humidity needs, but you want to be sure that you control for ethylene production within these, as well."
So consider keeping ethylene-sensitive produce in the vegetable bin of your fridge, where you can help ensure there is enough humidity to prevent things from going limp.
In the fruit bin, where you can typically slide open a vent to allow moisture to escape, you can store your nonclimacteric fruit, and even ripened items, such as peaches.
Apples do well in colder spots, so consider storing them toward the back in a bag, where they will also be separated from more ethylene-sensitive items. The good news is that refrigeration also slows down the production of ethylene.
For items you are storing on the bench that you plan to eat soon or need to ripen, you don't have to worry too much about going to extreme measures to separate items that may be more or less sensitive to ethylene. Watkins says the open air of your kitchen or dining room has plenty of ventilation to allow the ethylene to dissipate, though, as Strawn notes, bananas are particularly susceptible to ripening in the presence of other bananas.
Confined spaces – a bag, cabinet or refrigerator drawer – are somewhat different, as they can trap ethylene and hasten ripening or sprouting. (If you've ever heard the advice to not store potatoes and onions together because ethylene will cause one or the other to sprout, that's only partially true. Watkins says the bigger reason for keeping them apart is to prevent the potatoes from picking up odours from the onions.)
But can trapping ethylene also be a good thing? That's up next.
How to use ethylene to your advantage
You've probably heard advice about putting fruit in a brown paper bag to speed up the ripening process. In this case, the conventional wisdom checks out. By bagging such climacteric produce as kiwis, avocados or mangoes, as Strawn did during her food safety research, you are trapping ethylene gas that will trigger the production of even more and, voila, it will ripen. Bagging alone is often sufficient, though you can add an apple for good measure, if desired.
How much does it really matter?
Much of the advice around storage is based on experiments and observations made in highly controlled scientific environments, Watkins says, but "the real world is very different". Meaning not everyone is going to follow the advice – and maybe it won't necessarily be a problem.
Will the typical person always notice or care about a little yellowing broccoli or slightly tougher asparagus? As Watkins indicated in a report, damage caused by ethylene is most relevant to large-scale growers and markets. A home kitchen is not the same, especially in terms of the amount of produce present and how long it's being kept.
That being said, "At the end of the day, we do want people doing the right thing," Watkins says. In addition to being equipped with a basic knowledge of the differences in produce and the small steps you can take to address them in storage, his top tip is to pay attention to what you have so that you can eat it while it's fresh and ripe, or at least move it to the refrigerator, where appropriate.
"If you're worried about food waste, then your number one priority would be thinking about what's in your fridge."