Fruits are more delicious when they are ripened fully and sometimes when they are at a certain level of ripening. So what makes them delicious? Why the fruits ripen and what happens in them as they do? Why they change color? It’s these simple questions from our everyday life, excites us to explore the nature. And as the result of such exploration, you are about to read some fascinating facts about fruit ripening.
The hormone responsible for ripening
Fruit ripening is the result of the hormonal signal from the respective plant. The hormone responsible to carry this signal is the biosynthesized ethylene. It is produced throughout the plant’s life by all parts of the plants and is regulated throughout the phases of its growth. Ethylene is also responsible for various responses in plants like abscission and germination of seeds. At the time of ripening (and at normal synthesis), ethylene is synthesized by a complex process of converting amino acid methionine with the help of various enzymes. For the chemistry nerds, the complete biosynthesis of ethylene can be explained by the following Methionine cycle (Yang cycle) illustration.
What happens during ripening?
When it’s the right time for a fruit to ripen (influenced by the natural or artificial environmental regulatory factors), the bio-synthesized ethylene is produced more and this ‘air-borne’ hormone triggers the ripening process. There are several enzymes in play when the fruit ripens. The corresponding enzymes and their process are as follows.
The taste of the fruit changes when it ripens. At the initial stage, the fruit is a little tart or sour due to the presence of acids. When the fruit ripens, kinase enzymes turn the acidic fruit to a neutral one by converting them to neutral molecules. The fruit turns sweet when it ripens, because of the enzyme amylase that converts all the starch present in the fruit to sugars as it ripens.
The color of the fruit changes from green as they ripen. The color has a significant effect on attracting animals to help the plant in seed dispersal. The coloring pigments are revealed when the chlorophyll is broken down by hydrolase enzymes. This new revealed pigments gives the new color of the fruit, which would be green before it ripens.
Hydrolases are also responsible for converting large molecules into smaller aromatic compounds. This aroma also helps in attracting animals, which helps in seed dispersion later.
Unripe fruits are usually hard. This hardness is due to the presence of pectin in the primary cell wall. The pectin is broken down by pectinase and pectinesterase enzymes, (separates cells) making the fruit softer while it ripens.
And when these enzymes play all together, as they receive the signal via the ethylene hormone, you get your sweet and delicious ripened fruit.