Ripe for the picking: Which fruits keep ripening after harvest?

“Will my peppers continue to ripen? How about my eggplants?”  It is common knowledge to most gardeners (and home cooks) that tomatoes will ripen on the kitchen counter, as will bananas and several other fruits.  You know that one day your bananas look perfectly ripe and the next they’re a brown mush But does this work for all fruits?   We often get questions about whether specific fruits will continue to ripen after picking.  And the answer is….. it depends.

How green were my peppers…

One of these fruits is not like the other

The answer as to whether a fruit will continue to ripen after harvest depends on which one of two groups it falls into.  These groups are climacteric and non-climacteric fruits.  In short, climacteric fruits are the ones that will continue ripening after harvest and non-climacteric fruits are ones that don’t ripen after harvest.

Image result for ethylene

This refers to the “climacteric phase” of fruit ripening where there is an increase in the gaseous plant hormone ethylene and an increase in respiration, which drives the ripening process. It is the climacteric fruits that will keep ripening once they’ve been harvested, thanks to ethylene.  The only stage of maturity for non-climacteric fruits after harvest is…..compost.

 

As long as you’re green, you’re growing.  As soon as you’re ripe, you start to rot. -Ray Kroc

Almost all fruits produce ethylene, but non-climacteric fruits produce them at much lower levels and do not rely upon it as the main driver of ripening.  I’ll go into a bit more detail in a bit, but first – which fruits are climacteric and which are non-climacteric?

 

Common Climacteric Fruits Common Non-Climacteric Fruits
Apple Brambles (raspberry, blackberry, etc).
Apricot Citrus (oranges, lemons, limes, etc.)
Avocado Eggplant
Banana Grape
Blueberry Melon (including Watermelon)
Cantaloupe / Muskmelon Pepper *
Cherry Pumpkin
Fig Squash (summer and winter)
Kiwi Strawberry
Mango
Papaya
Pawpaw
Peach
Pear
Plantain
Plum
Tomato
Cherry
*Some evidence of climacteric ripening in hot peppers

Image result for avocado ripe meme

The ripening process

Ripening is genetically programmed – meaning that it is highly dependent on processes that are regulated by genes and it specific to each species.  Parts of the process are started and stopped due to the transcription and translation of genes, which are in turn controlled by signals such as chemical compounds, physiological stages of the plant, climate, and so on.  These ripening processes have a lot of end results – sugars accumulate in the fruit, pigments develop, some compounds that have pleasant flavors develop while others that are unpleasant are broken down, some of the pectins in the fruit break down to make it softer, and on and on.

Tomatoes – the classic climacteric fruit
Getting close…

 

 

 

 

 

 

 

Research shows that ethylene, the simple little gaseous hormone plays a crucial role in the ripening of climacteric fruits by altering the transcription and translation of genes responsible for ripening.  Ethylene is the dominant trigger for ripening in these plants.  Ethylene receptors in the cells are triggered by the presence of the gas which leads to cascade effect.  This is why ethylene can be introduced from other fruits to trigger ripening in fruits that aren’t ready to ripen.  If you’ve heard of the tip to put an apple in a bag full of some other fruit to get it to ripen, it actually works – as long as it is a climacteric fruit.

The same ripening processes happen in non-climacteric fruit as well, but they are not dependent on the presence of ethylene.  In fact, these pathways are also present in climacteric fruits – the ethylene-dependent processes are just the dominant (and faster) way that they ripen.

Controlling ripening

The dependence on ethylene for a vast majority of fruits to ripen has been used by farmers and the food industry for a long time to keep climacteric fruit more stable for shipping.  These fruits are harvested “green” before they ripen and shipped unripe since they are much firmer and much less likely to get damaged in transit.  These days, bananas, tomatoes, and other climacteric fruits are likely to be given a treatment that temporarily inhibits the ethylene response before harvest or shipping to extend their shelf life further.  Once they’re close to their final destinations they’ll either be allowed to ripen on their own or given a treatment of ethylene to speed back up the ripening process.

What we gain in shelf-life and reduced food waste we do lose in a bit of flavor.  Since the fruits are no longer attached to the plant when they ripen they don’t have the chance to transport more sugars and flavor compounds from the mother plant.  So “vine ripened” fruits do have a bit more sweetness and flavor than those that are picked green.  Having just gotten back from Rwanda, a country where bananas are a common staple food I can attest that the ones that ripen on the plant are much sweeter than those we get shipped in to the US – you know, the ones that will ripen next week sometime if you’re lucky.  There were even some in our group that don’t care for bananas here that loved the ones we had at breakfast every morning.

Grapes must stay on the vine to ripen

One possible direction for biotechnology is the engineering of plants to alter or eliminate the ethylene ripening response to reduce food waste and spoilage.  Since many genes that are responsible for ethylene production such as enzymes that catalyze the production of ethylene precursors, or proteins that serve as ethylene receptors have been identified, work is being done to develop delayed ripening by altering or knocking out these genes in a variety of crops.

Sources

Alexander, L., & Grierson, D. (2002). Ethylene biosynthesis and action in tomato: a model for climacteric fruit ripening. Journal of experimental botany53(377), 2039-2055.

Pech, J. C., Bouzayen, M., & Latché, A. (2008). Climacteric fruit ripening: ethylene-dependent and independent regulation of ripening pathways in melon fruit. Plant Science175(1-2), 114-120.

Lelièvre, J. M., Latchè, A., Jones, B., Bouzayen, M., & Pech, J. C. (1997). Ethylene and fruit ripening. Physiologia plantarum101(4), 727-739.

Cornmeal magic – the myth that will not die

Way back in 2010 (and then again in 2012) I wrote about a bizarre belief that cornmeal could be used to treat fungal diseases, from lawn spot to athlete’s foot. Rather than rehash what’s already been written, I’ll invite readers to read those posts for background. And of course look at the comments, which are…interesting.The weird thing is that this post from 2010 is the single most popular post on the blog. (Our stats are only for the last two years since we migrated the web site – who knows how many there were before May 2017?)

Blog stats over two years

The consistent popularity for the topic spurred me to publish a university fact sheet on the use of cornmeal and corn gluten meal in home landscapes and gardens. This fact sheet reviews the pertinent literature, and makes recommendations that are pretty much the same as those I made almost 10 years ago. Nothing has changed in the research world to support cornmeal as a fungicide.

But wait, there IS something that’s happened since 2010! Now cornmeal is being touted as an insecticide! In fact, if you go to Google and search for “cornmeal” and “insecticide” you’ll find thousands of hits.  As you might expect, there’s no research to support this notion: researchers in Maine, for instance, found no effect of cornmeal on fire ants. However, it is used as a bait to deliver actual insecticidal chemicals.

Way back in 1937.

But facts don’t get in the way of home remedies, such as Lifehacker’s eyebrow-raising advice.

Hmmm…

By refining the search to only include university websites (use “site:.edu” to do this), and swapping out “ants” for “insecticide,” you’ll find at least one Master Gardener group happily (and illegally) recommending cornmeal as an ant killer. The popular mode of action is either (1) they can’t digest cornmeal and starve or (2) the cornmeal absorbs water in their gut and they explode.

Boom!

This reminds me of yet another food product – molasses – recommended for killing ants. Since you’re already here, you might as well check out Molasses Malarkey parts 1, 2, and 3 too.

Might I recommend everyone use their cornmeal and molasses to make bread or cookies or pancakes? There are some delicious recipes on the internet.

Yum!