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[Edisi Sains Am] How do your favourite fruits ripen?

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Author: appleringo       Show all posts   Read mode

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Post time 6-5-2018 02:09 PM | Show all posts |Read mode
Edited by appleringo at 6-5-2018 02:20 PM

Theres nothing beats the taste of perfectly ripened fruits! Especially your favourite fruits kiddos! It is way differ when you munch your favourite fruits, just slightly a week before they are ripe. Have you tried? You will get a completely different experience. Trust me! Ok, firstly, how do we know that the fruits are already ripened? All of us must have different opinions on the criteria to look upon the ripened fruits. But basically the most common way is to observe the physical appearance or the external features of the fruits. Let us name the first feature is the softnessto the touch. But if you still memorized your very first frustrating moment of munching the unripened fruits of your favourite, you will surely remember that the fruits have hard skin as well as their flesh. So, how is the fruit softness regulated? What is actually happened during the ripening process of the fruits that induce their softness of external features? The firmness or softness of the fruits are determined by the state of none other than the infamous cell walls (Figure 1). Yeah, we learned about cell walls protection in plants and animals during our high schools Biology class. Please refer to Figure 1, in case you already forgotten the basic structure of plant cell walls .

Figure 1: Plant cell walls.

Cell walls surround each plant cell and they consist of a firm layer of sugars, called polysaccharides, which encased each cells membrane. There are 3 main polysaccharides (Figure 2) of the cells wall, which are;

1)  celluloseC made up of hundred of glucose sugars that joined together to form a long chain;
2) hemicelluloseC made of different type of sugars including  glucose, xylose, galactose, and mannose. Non-linear but branched structures.
3)  pectinC long, branched of sugars, made of galacturonic acid, rhamnose, galactose and arabinose.

Figure 2: Plant cell wall structure. A. The cell wall structure of a tomato fruit. A can be viewed under a microscope B. The cells can be seen to be surrounded by a polysaccharide cell wall, which is seen in the circle. C. The cell wall is composed of three main components, called cellulose, hemicellulose and pectin.

Fruits softness is controlled by the activity of a complex of enzymes such as cellulase and pectinase (refer to the previous main polysaccharides names). The activity of these enzymes is directly linked to the shelf life and the texture of the fruit. How do these enzymes function? They dissolve the main polysaccharides on cell wall and lead to the breakdown of the cell wall. The other factor is turgor pressure or briefly means the fluid pressure inside the plasma membrane. It actually keeps the fruit firm, just like air pressure inside a balloon keeps the balloon firm. Do you get it? So, when the fruit is harvested, it will lost fluid (water) causing a decrease in turgor pressure, so the fruits shrivel. The best example is in a strawberry, as it is no longer looks good and might not be picked up by consumers once the fruit loses 6C10% of its fluid. Poor strawberry!

The next external feature we commonly observed is the colourof the fruits. Briefly from dark green turned to bright yellowor orangeor red. We already knew that fruits change colours as they ripen. Its a general knowledge, for God sake! Haha. But do you know what is the mechanism underlying the colour changes? This happens because of the breakdown of a greenpigment called chlorophyl that normally contained in higher amount in unripened fruit. This was then followed by the creation and accumulation of other pigments responsible for red, purple, or blue hues which are known as anthocyanins, or bright red, yellow, and orange hues or scientifically called carotenoids. So, now you already can imagine the real content of the fruits in the figures below right? (Figure 3 a and b).

Figure 3a: A ripening process of a banana

Figure 3b: A ripening process of a tomato

Alright, the other feature is an internal feature that is the sweetness.So, how fruit ripening brings out the flavor of fruitparticularly a fruits sweetness. During ripening, there is an increase in the breakdown of starch inside the fruit, and a corresponding increase in the amount of simple sugars which taste sweet, such as sucrose, glucose and fructose. As previously mentioned that the cell walls structure are well-made of sugars. This process is particularly obvious in bananas as they ripen. Green bananas do not taste sweet at all, and the riper they get, the sweeter they taste. Why? As the fruit ripens, there is also a decrease in bitter plant substances, such as alkaloids as well as decrease in in acidity. This was followed by the production of complex compounds that are released into the surrounding air, giving a ripe fruit its pleasant aroma. This explained why banana smells wonderfully pleasant as it ripens (Figure 3a).

However above all, there is one well-known hormone that actually regulates all the ripening process of fruits from the softness, sweetness to the colour changes of the fruits! It is called ethylene. How ethylene dose them all?! A burst of ethylene production means a common ripening signal of fruits. But what is actually ethylene? In a simple way, ethylene is a simple hydrocarbon gas with a molecular formula of  2HC=CH2.
It was produced by the ripening fruits and later shed into the atmosphere. 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 initiate 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 (Figure 4).

Figure 4: Ethylene functions in the ripening process of fruits

The action of the all those 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. 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 greento 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 (Figure 5).

Figure 5: The ripening process of a pear. Photo courtesy of Agyle (Own work) via Wikimedia Commons.

From the big roles of ethylene in ripening process of fruits, we can manipulate it to some other processes. How about we try some simple experiments in fruits ripening?

1)try to speed up the ripening process of unripened pear by putting it together with a ripened banana in a bag and seal it properly. How will the mechanism works in this ripening process? As mentioned previously, ethylene triggers the ripening process. Ethylene acts like an enzyme by speeding up the conversion of starch to sugar. Bananas make lots of ethylene as they ripen. The ethylene gas that comes out of the banana is trapped in the bag, exposing the other fruits to extra ethylene. The extra ethylene then triggers the ripening process for the pear and the apple. Apples also produce a lot of etylene, and that's why they can also be used to ripen other fruits (Figure 6).

Figure 6: The unripened pear and ripened banana were kept together in one sealed bag. Photo courtesy of everydaysciencefun.blogspot.

2)   Try putting a green lemon with a ripening banana in a paper bag, as above, and see what happens to the color of the lemon. Mechanism: Ethylene is also used to de-green citrus, by triggering the breakdown of the green pigment (chlorophyll), resulting in orange and yellow coloration of the peel. No loss of flavor is caused because this is merely a continuation of the natural plant process.

However, here is the main tips! Ripening is best conducted at room temperature, around 20C, because low temperatures can inactivate important fruit-ripening enzymes. So, it is best to try this outside of the refrigerator.

But what happens if fruit is exposed to too much ethylene?
Fruits that make ethylene to trigger ripening do not stop making ethylene once the fruit is ripe. They will continue to make ethylene, and the fruit will continue to get softer and softer, with more and more sugar, and will eventually be rotten.

Owing to the enormous effects of ethylene that would eventually lead to the fruits to be rotten, people involve in post-harvest industry are working hard to control or suppress the ripening process. This would be helpful to make sure fruits that are to be delivered to the customers in the market are ripened in a perfect time. But how can we control the ripening process of fruits? Yes, exactly! One major way to slow down ripening is to block the action of ethylene. It can be blocked by using synthetic compounds, such as 1-methyl-cyclo-propene (1-MCP). 1-MCP is a chemical substance with a molecular formula or C4H6, is also used to maintain the freshness of cut flowers. Commercially, 1-MCP is formulated as a stable powder in which it is complexed with -cyclodextrin, so that 1-MCP is easily released as a gas when the powder is dissolved in water (Figure 7).
Figure 7: A commercialised 1-MCP known as EthylBloc.

Biochemically, 1-MCP will works by binding tightly to the ethylene receptor sites in fruits tissue, thereby blocking the effects of ethylene. Once binded, 1-MCP will not be released, so molecules of ethylene failed to bind to their specific receptor site. Blocking of ethylene receptor by 1-MCP will slow the ripening process of fruits (Figure 8).

Figure 8: Mode of action of 1-MCP to block the activation of ethylene. Photo courtesy of Merisko-Liversidge and Liversidge 2011.

Other than 1-MCP, some chemicals can function as ethylene absorbers namely potassium permanganate.
Through these changes, fruits ripen and become sweet, colored, soft and good-tasting. With this brief yet precise explanation, I am definite that we will have meaningful thoughts while munching our favourite ripened fruits as we have a clearer knowledge on their ripening process.




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Post time 6-5-2018 02:56 PM From the mobile phone | Show all posts
My god, even fruits can be si complicated
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 Author| Post time 6-5-2018 02:59 PM | Show all posts
noraidil_06 replied at 6-5-2018 02:56 PM
My god, even fruits can be si complicated

oh is it complicated? hehe..I found it fascinating..
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Post time 6-5-2018 03:06 PM From the mobile phone | Show all posts
appleringo replied at 6-5-2018 02:59 PM
oh is it complicated? hehe..I found it fascinating..

I read first n 2nd para. N x fhm.
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 Author| Post time 6-5-2018 03:21 PM | Show all posts
noraidil_06 replied at 6-5-2018 03:06 PM
I read first n 2nd para. N x fhm.

oh sorry. i will try to simplify the words later ok
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Post time 6-5-2018 03:26 PM From the mobile phone | Show all posts
appleringo replied at 6-5-2018 03:21 PM
oh sorry. i will try to simplify the words later ok

Eh its ok. Hihihi
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Post time 6-5-2018 04:57 PM From the mobile phone | Show all posts
Nice article.. and i lagi suka buah kurma yg muda bergetah tu.. tak betul2 ripen. Rangup kelat2 manis
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 Author| Post time 7-5-2018 02:51 PM | Show all posts
najib888 replied at 6-5-2018 04:57 PM
Nice article.. and i lagi suka buah kurma yg muda bergetah tu.. tak betul2 ripen. Rangup kelat2 mani ...

thank u..sedapnya kurma lagi2 nak dekat bln puasa ni kan...
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