Fruit Ripening

In prior discussions, we have learned how the fruit grows to achieve full size, and that there are a diversity of fruit types among the flowering plants. Now we will focus specifically on fleshy fruits. The fruit is full size but it is green and immature...events must now happen to cause it to ripen so that it is attractive and rewarding for an animal to carry it off...dispersing the seeds.

In the diagram above on the left is an unripe fruit. It is hard, green, sour, has no smell, is mealy (starch present), and so on. Sometimes we crave such a combination of characteristics...a 'Granny Smith' apple has many of those. Such fruits are similar to celery or other vegetables, and so are not appealing at other times.

My pet peeve is unripe pears. Most cafeterias serve pears to students that are absolutely green, hard, sour, mealy, and lacking aroma. These are better for driving nails than for eating...in my opinion. Worse is the fact that many cafeterias will not let you take such a fruit back to your dorm room to let it ripen! They actually expect you to eat it like that. (More later!).

The way fruits ripen is that there is commonly a ripening signal...a burst of ethylene production. Ethylene is a simple hydrocarbon gas (H2C=CH2) that ripening fruits make and shed into the atmosphere. Sometimes a wound will cause rapid ethylene production...thus picking a fruit will sometimes signal it to ripen...as will an infection of bacteria or fungi on the fruit. This ethylene signal causes developmental changes that result in fruit ripening.

New enzymes are made because of the ethylene signal. These include hydrolases to help break down chemicals inside the fruits, amylases to accelerate hydrolysis of starch into sugar, pectinases to catalyze degradation of pectin (the glue between cells), and so on. Ethylene apparently "turns on" the genes that are then transcribed and translated to make these enzymes. The enzymes then catalyze reactions to alter the characteristics of the fruit.

The action of the enzymes cause the ripening responses. Chlorophyll is broken down and sometimes new pigments are made so that the fruit skin changes color from green to red, yellow, or blue. Acids are broken down so that the fruit changes from sour to neutral. The degradation of starch by amylase produces sugar. This reduces the mealy (floury) quality and increases juiciness (by osmosis, a process we will study later). The breakdown of pectin, thanks to pectinase, between the fruit cells unglues them so they can slip past each other. That results in a softer fruit...at an extreme, pectin losses may make a fruit "pithy". Also enzymes break down large organic molecules into smaller ones that can be volatile (evaporate into the air) and we can detect as an aroma.

If you think of this process in pears, the ethylene signal causes the fruit to change from green to yellow, from hard to soft, from mealy to juicy, from tart to sweet, from odorless to fragrant. If you have never experienced a ripe pear, you have really missed a sensory delight! It is marvelous.

How can you assist fruit ripening? First let me tell you that bananas are shipped to the US as hard, green, sour, unripened fruits. They ship better that way. They arrive into a distributor's warehouse without bruises. The bananas are put in a room and gassed with ethylene. They all begin to ripen. You buy them at the store and within a few days the ripening process is so rapid that the bananas are "over the hill" before you can eat them all.

By the way, you can allow the bananas to ripen to the stage you like them and then put them in the refrigerator. This slows the ripening process down drastically. For several days after that you can take bananas from the refrigerator and enjoy the fruit inside. Please note: the skin will turn very dark in color after only a short time in the refrigerator...you can ignore that...the fruit inside remains just as it was before you put the banana into the refrigerator.

The ripening bananas produce so much ethylene that you can use them as a tool to ripen other fruits. Take those green pears home and put them on the shelf in a paper bag with a banana. The banana at room temperature produces ethylene that will signal the green pears to start ripening immediately. The paper bag holds the ethylene in stagnant air around the fruits, yet allows oxygen to go into the bag for respiration in the fruits...needed to make the enzymes! In just a few days the pears should be ready to eat! You can do the same with avocados.

Finally this stimulated ripening process helps explain the old phrase, "one bad apple spoils the bunch." In the olden days, apples were packed into barrels for storage in a root cellar. The cool temperature of the cellar helped keep the apples from ripening until the family wanted to eat them during the winter. However in packing the barrel with apples you had to be careful not to put any wormy or fungus-infected apples with the bunch. Why? Because the wounded, infested apple would produce ethylene inside the barrel. If you came back in a month, all the apples in the barrel would be too-ripe (bruised by weight, soft, mushy, etc.) because they responded to that ethylene!

Today we store fruits in cold temperatures and cycle the atmosphere in the warehouse through charcoal filters to adsorb any ethylene being made by any "bad apples" in the building! This way we can enjoy apples year-round.

Fruit Abscission

The fruit-ripening process described above, also occurs in a layer of cells in the pedicel near the point of attachment to the stem of the plant. This layer of cells in the pedicel is often called the abscission zone because this layer will eventually separate and the fruit will drop from the plant.

Just as the cells inside the fruit, the cells in this cross sectional layer in the pedicel get the ethylene signal from the ripening fruit. Reception of the signal causes new enzymes to be made. The cells "ripen" and pectinases unglue the cells of the abscission zone. When the cells have weak-enough connections, the weight of the fruit will cause it to fall from the plant. Maybe Newton discovers gravity?

In this way then, the ripening process is used for a second function in the plant...to make the fruit drop from the tree so an animal can pick it up and carry it off to disperse seeds. Plants are modular organisms, and use the same genetic and physiological processes for different processes in the various modules.

Another use for the ripening process is in leaves in the autumn. The lengthening nights and the cool night temperatures of early autumn are the cue. Ethylene is produced. Enzymes are made. Chlorophyll breaks down as do a range of other compounds in the leaves. The subunits of these molecules are loaded into phloem and transported toward the roots for winter. The leaf blade changes color...just as do carpel walls in fruits! This leads to yet another use of the ripening process...in the abscission zone of the petiole. Just like the abscission zone in the pedicel of the fruit, the petiole has cells that get the same signals as the leaf blade, produce the enzymes, come unglued from each other. Now the weight of the blade, and perhaps some wind forces, cause the leaf to fall from the plant. Sometimes we call autumn "fall" for this reason.

 

This page © Ross E. Koning 1994.

 

 

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