Ripening Fruit - Its a Gas!

I am sure you have heard the saying; “One bad apple can spoil the whole bunch!” Well it is true. Fruits and vegetables emit an odorless, colorless gas called Ethylene. The detection of Ethylene by the plant triggers the ripening process and the production of more Ethylene. A rotten apple will emit a lot of Ethylene gas, which will cause the other apples to start emitting the gas, and soon all the apples will be ripe, and then before long rotting!

Ethylene is a small hydrocarbon compound with the formula C₂H₄ or H₂C=CH₂. It is a colorless, flammable gas with a faint "sweet and musky" odor and is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). It is widely used in the chemical industry, and its worldwide production exceeds that of any other organic compound. Ethylene is also an important natural plant hormone, and is used in agriculture to force the ripening of fruits. It acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, the loss of chlorophyll (changing the color of the leaves), and the shedding or dropping of leaves.

Ripening is a process in fruits that causes them to become more palatable. In general, a fruit becomes sweeter, less green, and softer as it ripens. Ethylene is responsible for the changes, acting as the aging hormone in plants. It is synthesized from the amino acid methionine in essentially all plant tissues. Even after harvest, fruits, vegetables and flowers are still alive, continuing their biochemical processes, including ripening and the generation of Ethylene gas. It is a natural product all fruits produce to stimulate their ripening process. The problem is, once started, it never turns off and eventually aids in the over ripening of fruit too.

As is often the case, the role of Ethylene and its effects on produce was discovered accidentally. Lemon growers stored green, newly harvested lemons in sheds warmed by kerosene heaters until the lemons turned yellow and ripened enough to ship to market. When new more modern heating systems were introduced, the lemons no longer turned yellow as quickly. Researchers soon determined that the small amount of Ethylene gas given off by the burning kerosene was a key factor in the ripening process.

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If You Can Control the Ethylene Gas
You Will Preserve the Freshness

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Ethylene gas is used commercially to ripen tomatoes, bananas, avocados, and a few other fruits postharvest. Ethylene gas turns bananas yellow, tomatoes red and makes avocados soft and ready to eat. Controlling Ethylene gas is essential in giving the consumer the best possible product.

If you can control Ethylene gas levels, you will preserve freshness.

Climacteric fruits are able to continue ripening after being picked. Non-climacteric fruits can ripen only on the plant. During the ripening process of climacteric fruits, the production of Ethylene dramatically increases as much as 1000 times that of the basal Ethylene levels. Climacteric fruits include apples, apricots, avocados, bananas, cantaloupes, figs, guavas, kiwis, mangoes, nectarines, peaches, pears, persimmons, plums, and tomatoes. Many of these fruits are picked prior to full ripening, which is beneficial, since ripened fruits do not ship well. A bunch of bananas will stay green for a long time unless the Ethylene concentration in the air around them becomes high enough. Just before delivery, the fruit is gassed with Ethylene to kick-start the ripening process. This helps make sure none of the fruit will ripen too early, which would be problematic at the grocery store.

Credit: Steve Hopson, www.stevehopson.com

To extend the shelf life of your produce you should not store ripe fruit in plastic bags. Your produce can deteriorate as much as 50% faster. Of course, if you need to speed up the ripening process, putting the fruit into two plastic bags (one inside the other) and adding an apple will accelerate the ripening because apples produces an extremely high amount of Ethylene gas. Check the bag daily and once the fruit has ripened remove it from the plastic bags. The plastic bags need to be removed at the right stage or they will reduce the shelf life or hamper the ripening process. Punching a few pencil-sized holes in your veggie bags too will help your produce last longer in the fridge.

Additional externally applied Ethylene (gassing) merely accelerates the normal ripening process. Numerous studies have shown that there are no important biochemical, chemical, or physiological differences between fruit that has ripened naturally and mature but unripe fruit that has been triggered to ripen with externally applied Ethylene. For example, tomatoes are not and cannot be "artificially reddened" by Ethylene. The normal tomato ripening process, which includes pigment changes - the loss of green chlorophyll and conversion of carotenoids into red lycopene pigments - can be accelerated and brought about earlier by gassing with Ethylene, but this still happens via natural ripening.

Different kinds of plants react to ethylene differently. Ethylene producing items (such as apples, avocados, bananas, melons, peaches, pears, and tomatoes) should be stored separately from ethylene-sensitive ones (broccoli, cabbage, cauliflower, leafy greens and lettuce). Most sensitive to Ethylene gas are the leafy vegetables, even if the gas is present in very low quantities. Lettuce begins to wilt quickly when exposed to Ethylene gas at low temperatures —even in your refrigerator. Put spinach or kale in the same crisper bin as peaches or apples and the greens will turn yellow and limp in just a few days. Other foods that are sensitive to Ethylene gas, such as fresh peas and bananas, will spoil quickly if they are stored in the same area as avocados, peaches, or apples, which are prolific ethylene producers. The following table shows the relative production and sensitivity of certain fruits and vegetables to Ethylene.

Fruits & Vegetables Apples Apricots Asparagus Avocados Bananas Berries Broccoli Brussel Sprouts Cantaloupe Carrots Cherries Cucumbers Eggplant Grapefruit Grapes Kiwifruit Lemons, Limes Lettuce (*2) Mangoes Melons (*3) Nectarines Onions, Garlic Oranges Papaya Peaches Pears (*5) Plums, Prunes Potatoes (*6) Tomatoes Watermelons	Rate of Ethylene Production VH H VL H M L VL VL H VL VL L L VL VL L VL VL M M H VL VL H H H M VL M L	Level of Ethylene Sensitivity H H M H H L H H M L L H M-H M L H M H H H H L M H H H H M H H	Principal Reaction to Ethylene Gas Scald (*1) Decay Toughness Decay Decay Mold Yellowing Yellowing Decay Bitterness Softening Yellowing Brown Spots Mold Mold Decay Mold Russet spotting Decay Decay Decay Odor, sprouting Mold (*4) Decay Decay Decay Decay Sprouting Shrink, decay Lose firmness
VL = Very low, L = Low, M = Moderate, H = High, VH = Very High
*1. Loose crunch *2. Leafy greens *3. Crenshaw, Honeydew, Persian		*4. Rind breakdown *5. Anjou, Bartlett, Bosc *6. Processing, Seed

Source: Fresh Produce Manual for 1997 from the Produce Marketing Association and the 1991 Sea Land Shipping Guide for Perishables.

If you take care to store your fruits and vegetables to regulate the effects of Ethylene gas, you can keep your produce fresher longer and have it ripe and ready when you need it.

Psalm 92:14-15 (NIV) - They [the righteous] will still bear fruit in old age, they will stay fresh and green, proclaiming, "The LORD is upright; He is my Rock, and there is no wickedness in Him."