This is the third time I’ve written a How Does It Work column (homemade chloroform and Coors Light cold-activated bottles). It’s a lot of fun (and a lot of work) to write these columns, and I’m really enjoying writing them. I have two more ideas for upcoming How Does It Work columns (forest fire fighting, microwave panini “grilling”), but if you’ve always wondered how something (chemical) works, let me know and I’ll try to work it into the queue! On to Fruit Ripening: How Does It Work?!
Have you ever wondered what causes fruit to ripen? Why do we store some fruits in the refrigerator and some on the counter? Why do we have a special fruit crisper drawer in the fridge?
The answer has to do with a plant growth hormone. One plant growth hormone is primarily responsible for the complex transition we call ‘Fruit Ripening.’ So what would you guess that growth hormone looks like? Do you think it looks more like the protein-based human growth hormone (HGH) or bovine growth hormone (BGH)? Like the synthetic hormone zearalanol, or like other plant hormones like auxins? Or none of the above? Answer below the jump.
What does the Fruit Ripening Hormone look like???
Before I tell you the answer, let’s look at the physical changes that occur when a fruit ripens. Before the fruit is ripe enough to eat, the unripe fruit is green, immature, and not as tasty. It is hard, sour, not fragrant, and is starchy. (sometimes we desire some of these characteristics… do you prefer Granny Smith apples over Red Delicious?) These unripe fruits are generally unappealing to humans and animals – the latter being important evolutionarily because animals will eat the fruit and disperse the fruit’s seeds.
What is fruit ripening?
At the right time (or as we’ll see, sometimes at the wrong time), a series of related transformations occur, all caused by the growth hormone we’re discussing today. The fruit becomes sweeter as the starches are converted into simple sugars by amylases. The fruit changes from green to colorful as the chlorophyll (fruit = green) is broken down by hydrolases revealing anthocyanins (fruit = colored). The fruit becomes less tart as the acids are converted to neutral molecules by kinases. The fruit becomes softer as the amount of pectin is lessened by pectinases. And the fruit becomes fragrant as the large organics are converted to volatile aromatic compounds by hydrolases.
Now, without understanding what hormone was involved, agriculturalists have known about the fruit ripening process for thousands of years. Let’s look at what they knew and see if we can draw any conclusions ourselves:
- Ancient Egyptians would gash figs. They noticed this would stimulate ripening. Wounding often stimulates ripening. Even picking an unripe fruit can induce ripening.
- Ancient Chinese would burn incense in closed rooms. This would assist pear ripening.
- Fruit infected with a bacteria or fungus can cause surrounding fruits to ripen quickly.
- In the mid-1800s, people began noticing that some plants were twisted and had abnormally thickened stems. After investigation, it was noticed that only plants near street lights were affected. What do you remember about street lights of the time?
- Has anyone told you to put a banana in the bag with your apples or pears to help them ripen?
- Does the saying ‘one bad apple ruins the bunch’ actually mean anything?
In 1901, after studying the street light phenomenon, Dimitry Neljubow showed that the active component causing the plant’s malformation was a deceptively simple compound: ethylene!
That’s right. The one molecule that is responsible for the entire fruit ripening process is ethylene. Who would have thought?! Such a simple answer it almost seems like it can’t be true. It doesn’t look anything like a hormone. It doesn’t look like a biomolecule at all – it doesn’t even look like it should be soluble in cells! But it works in trace amounts throughout the life of the plant by regulating various processes. Gene, in 1934, discovered that plants can biosynthesize ethylene. In the next section, let’s take a look at how it works!
How does it work?
To discuss how it works, we should mention that there are two types of fruit: climacteric and non-climacteric. Climacteric fruits continue ripening after being picked (which will be accelerated by ethylene gas). Climacteric fruits include: apples, apricots, avocados, bananas, cantaloupes, figs, guava, kiwis, mangoes, nectarines, peaches, pears, plums, and tomatoes. Non-climacteric fruits ripen only while still attached to the plant. Their shelf life is diminished if harvested at peak ripeness. Non-climacteric fruits include: cherries, grapes, limes, oranges, pineapples, and berries (blue-, black-, rasp-, straw-, etc.).
Essentially all parts of higher plants produce ethylene (stems, roots, flowers, tubers, and seedlings). Ethylene production is induced at several key stages of the plant’s life. Notable for us, ethylene production is promoted during fruit ripening and abscission (dropping) of leaves. However, it is now known that ethylene production can be artificially increased by external factors: wounding of the fruit, environmental stress, and exposure to certain chemicals.
The biosynthesis of ethylene starts with the amino acid methionine. The enzyme met adenosyltransferase converts methionine into S-adenosyl-L-methionine (SAM). The enzyme ACC synthase (ACS) converts SAM into 1-aminocyclopropane-1-carboxylate (ACC). The last step in ethylene biosynthesis involves molecular oxygen. The enzyme ACC-oxidase (ACO, which used to be called Ethylene Forming Enzyme, EFE) converts ACC into ethylene, as well as carbon dioxide, hydrogen cyanide, and water.
The rate of ethylene production is regulated by ACC synthase converting SAM into ACC. Thus, regulation of this enzyme is key for the biosynthesis of ethylene. Manipulation of this enzyme by biotechnology delays fruit ripening. The Flavr Savr tomatoes used this biotechnology. On the other hand, in a sort of positive feedback loop, the biosynthesis of ethylene is upregulated by either endogenous or exogenous ethylene. Producing ethylene causes more ethylene to be produced.
In 1993, the genes involved in the fruit ripening response were identified. The ETR1 and CTR1 genes are turned on until ethylene is produced. Then ETR1 and CTR1 turn off. This initiates a cascade ultimately turning other genes on. These other genes make the various enzymes mentioned earlier (amylases, hydrolases, kinases, and pectinases) needed to ripen the fruit. These changes invite animals to consume the fruit and disperse the seeds. That’s how nature does it. Let’s look at how the fruit industry exploits this knowledge.
Synthetic Ethylene and the Fruit Industry
Since ethylene controls the ripening process, if we can control the ethylene, we can control the fruit. While ethylene is synthesized by plants, it is also prepared commercially. Ethylene is the most produced organic compound in the world (>107 million metric tons in 2005). The petrochemical industry produces ethylene through steam cracking of gaseous or light liquid hydrocarbons by heating to 750-950 °C. Compression and distillation purifies the ethylene. Ethylene is then used for a variety of applications, including the synthesis of PVC and polyethylene plastics.
Here’s the problem for the fruit industry. Ripened fruits don’t ship well. That Chaquita Banana commercial with the happy yellow banana in the comfy bed on the ship being transported to the US… not accurate. In fact, bananas are usually picked when green and artificially ripened after shipment. To ripen the bananas, the company gasses the unripe bananas with external gaseous ethylene. Recall from above that exogenous ethylene upregulates the biosynthesis of endogenous ethylene. The bananas ripen, then are sold.
The picking of unripe fruit and artificial ripening later is not uncommon. In parts of Asia, a plastic cover is placed over unripe harvested mangoes. Calcium carbide is placed in open containers in strategic positions inside the bag. Moisture from the air converts the calcium carbide into acetylene which has the same fruit-ripening effect as ethylene. However, industrial-grade calcium carbide is sometimes contaminated with trace arsenic and phosphorous. The use of calcium carbide to stimulate fruit ripening is illegal in most countries.
More commonly, however, catalytic generators are used to produce the ethylene gas necessary for fruit ripening. The generators allow for control of the overall ethylene concentration in the room. Typically, between 500-2000 ppm of ethylene is administered for 24-48 hours to successfully ripen the fruit.
On the other side of the spectrum, after the unripe fruit is picked, we want it to remain unripe until after shipment. Scientists have researched ways to inhibit ethylene biosynthesis and inhibit ethylene perception. Aminoethoxyvinylglycine (AVG), aminooxyacetic acid (AOA), and silver ions inhibit ethylene synthesis, but this is not always effective because exogenous ethylene can still be perceived by the fruit and stimulate ripening.
A more productive method of inhibiting ripening is to inhibit ethylene perception. This can be done by gassing the molecules with 1-methylcyclopropene (1-MCP). 1-MCP binds tightly to the ethylene receptor and blocking the effects of ethylene (competitive antagonist). 1-MCP is sold commerically as SmartFresh and is approved and accepted for use in more than 34 countries (including the EU and US). It is used in the fruit industry to prevent premature ripening, but it is also used in the horticultural industry to maintain the freshness of ornamental flowers. While there are benefits to consumers (fresher produce, lower cost due to increase supply), some have concerns that consumers may be purchasing fruit older than expected.
Current practice for longer-term fruit storage includes cold temperatures and charcoal scrubbing of the atmosphere to absorb ethylene and keep the concentration of ethylene very low. But what can we do at home to help our already-purchased fruit from over-ripening?
What to do at home
The first thing to do to keep your produce fresh is determine if the fruit is climacteric (naturally ripens after picking) or non-climacteric (doesn’t naturally ripen after picking). The best aspect you can control is properly deciding which fruit to purchase. Don’t buy fruit that’s bruised or where the skin has been cut (unless you’re going to use it right away). Wounding a fruit will increase ethylene production and cause the fruit to ripen faster. Then you’ll probably want to take them out of the produce bag when you get home. Keeping the fruit in the bag will increase the concentration of ethylene in the air in the bag and cause fruit to ripen faster. At least perforate the bag and don’t tie it closed.
Aside from that, tips on keeping fruit fresh really depend on what fruit you’re talking about. Non-climacteric fruits will only get worse with time (over-ripen, spoil…) – get them in the fridge to last as long as possible. For climacteric fruits, some produce a lot of ethylene, and others are very sensitive to exogenous ethylene – don’t store these together in a closed container. There are too many individual scenarios to list out here, but here are three good websites which can tell you the best way to store your favorite fruits (and some veggies).
Now, you can use the incompatibility of heavy ethylene producers to your advantage. If you bought pears or apples that are unacceptably unripe, you can put them in a closed paper bag with a banana to help them ripen faster so you can eat them. The paper bag helps stagnate the air and build up the ethylene concentration produced by the banana to induce the pear to ripen.
Other cool facts
- One bad apple DOES spoil the bunch. In the “good old days,” after apples were picked, they were packed into barrels and stored underground in a root cellar. The root cellar would stay cool and keep apples from ripening during the winder (at least until the family wanted to eat the apple, then they would be removed from the cellar). But if any wormy or fungus-infected apples were packed into the barrel, the wounded apple would off-gas considerable amounts of ethylene. In a month, all the apples would be too ripe, too mushy, too soft and too bruised to eat.
- Ethylene affects more than just fruit ripeness. It also affects flowering (see the 1-MCP discussion above). But ethylene also acts on other parts of the plant besides the fruit. The same actions that we call fruit ripening also occur in the pedicel near where the fruit (or leaf) attaches to the stem of the plant. This attachment point is often called the ‘abscission zone’ because this layer of cells will eventually separate and the fruit (or leaf) will drop from the plant (abscission). These cells respond to the ethylene signal from the ripening fruit, too, and the amount of pectin is diminished by the action of pectinases (just like it is in the fruit). Less pectin makes the cells less ‘glued together’ (pectin is what we add to make jams more jelly-like in viscosity) and causes the cells to detach and slip past each other easier. When the cells have weakened enough, the weight of the fruit (or leaf) will cause it to fall from the plant. This makes it easier for a hungry animal to eat the fruit and disperse the seeds.
- Ethylene also affects flowering and some flowers most affected by ethylene include carnations, geraniums, petunias, roses, and many others. 1-MCP can keep flowers from aging prematurely, or a banana (or synthetic ethylene gassing) can help induce flowers to bloom faster.
- As hinted above, the ripening process occurs not just in fruits, but in leaves, too. This help explain what happens in autumn. Nights get longer and cooler and induce ethylene to be produced. Chlorophyll breaks down (as do other compounds in the leaves) and the leaves lose their greenness and change color. The subunits of these molecules are transported by phloem toward the roots for the duration of winter. The abscission zone softens until the weight of the leaf (or a gust of wind) causes me to have to rake my lawn AGAIN this weekend.