I'm no different. My thinking generally goes, "If I buy these now, I'm set on breakfast for a week." Then Thursday comes around, my 'nanners have turned brown, and suddenly Friday's looking like a toaster waffle sort of day. Sometimes I consider baking banana bread and pretending I meant to let them get overripe, but mostly I throw them away and feel bad.
There is another way. A better way. A way that requires nothing more than what is already likely to be in your kitchen.
We're looking specifically at enzymatic browning and the effect of ethylene production here. If you want to dig much deeper, there's a ton of academic research on bananas available online.
"Relationship between browning and the activities of polyphenol oxidase and phenylalanine ammonia lyase in banana peel during low temperature storage" anyone?
(Postharvest Biology and Technology - PDF link)
When fruits or vegetables are peeled or cut, enzymes contained in the plant cells are released. In the presence of oxygen from the air, the enzyme phenolase catalyses one step in the biochemical conversion of plant phenolic compounds to form brown pigments known as melanins. This reaction, called enzymatic browning, occurs readily at warm temperatures when the pH is between 5.0 and 7.0.
(Australian Institute of Food Science and Technology - PDF link.)
Ethylene promotes maturation and abscission of fruits. This has been known since early last century. Since 1934, it is known that plants themselves can produce ethylene. Many climacteric fruits such as apple, banana and tomato show a strong increase in ethylene levels at the late green or breaker stage. As a consequence of high ethylene chlorophyll is degraded and other pigments are being produced. This results in the typical color of the mature fruit peel. Activity of many maturation-related enzymes increases. Starch, organic acids and in some cases, such as avocado lipids, are mobilized and converted to sugars. Pectins, the main component of the middle lamella are degraded. The fruit softens. These metabolic activities are accompanied by a high respiration rate and consequently by high oxygen consumption. Ethylene levels are especially high in the separating tissues resulting in abscission of the fruit.
(Margret Sauter, University of Hamburg.)
Step 1: Preserve the Bunch: Wrap Stems with Plastic Wrap
This method prevents ethylene gas, produced naturally in the ripening process, from reaching other parts of the fruit and prematurely ripening it. This technique is hit or miss, as the coverage from the plastic wrap is unlikely to fully prevent contact with the ethylene gas. It's certainly better than nothing, though.
This explains a few common tricks about using bananas to ripen other fruits like avocados. Or quick-ripening bananas by storing them all in a bag together. Ethylene is actually used in the banana production facilities to induce ripening at just the right time to ensure that you buy a bunch of yellow (or greenish yellow) from your local grocer.
(The next step is my preferred method, and the one that the science appears to back up with the most evidence.)
Step 2: Separate, then Wrap the Stems
Divide and conquer! Separate the ripe fruit from the slightly-less-ripe, wrap their stems in plastic, then enjoy when you're ready.
This should do a couple of things:
- prevent ethylene gas from initiating the ripening process on under-ripe bananas
- fully cover the stem to really forestall the off-gassing
- make your bananas more convenient to grab and enjoy on the go
Step 3: Keep Banana Slices Fresh
Just toss your banana slices in some lemon juice to inhibit enzymatic browning. Full coverage, particularly on the cut sides, will help prevent the slices from turning brown. In addition to lemon juice, vinegar will also work. So would sulfuric acid, for that matter, but you probably don't want to eat it afterwards.
The acid disrupts the enzymatic breakdown process and prevents your sweet, sweet banana slices from turning into mushy little brown hockey pucks.
A dab'll do ya, so keep your acid in the teaspoon range. Or you'll just have sour bananas.