Why is the fruit ripe?

1. Ethylene, a plant hormone:

Plant hormones (hormones) such as ethylene, gibberelline, auxine, brassinosteroid, abscisic acid, cytokinine . act at the molecular level and interfere with the regulation of gene expression, affecting all both growth and development stages of plants.

People distinguish two kinds of fruits:

- Climacteric: (peak of respiration): Left off the tree, continued to ripen. This type of fruit strongly respects, producing ethylene. In the first stage, the fruit produces very little ethylene, but the amount of ethylene increases a lot when the fruit starts to age and continues to increase after the fruit has left its branches.

- Non climacteric: Fruit only ripened on the tree: respiration does not affect the ripening process of the fruit (Yang and Hoffman, 1984)

As mentioned above, ethylene is a very important gaseous hormone for plants. It increases the removal of CO ₂ and the consumption of O ₂ gas. It participates in many stages of plant growth and is considered a hormone that controls fruit ripening. In 1979, Adams and Yang discovered that ethylene was made directly from acide-1-aminocyclopropane (ACC).

The final step in the synthetic process of ethylene where fruits are made is by catalytic enzymes called ACC oxydase (ACCO). At birth, this enzyme contains a site (site) that consists of a single atomic ion Fe (II), connecting 2 histidine with 1 aspartate. The mechanism of this reaction remains unclear, but they suspect that at the early stages ACC and oxygène are attached to Fe to form a tertiary complex Fe / ACC / O ₂ . After that, there can be the participation of strong oxidizing substances such as Fe (III) OOH, Fe (V) = O, .

Preparations containing ethylene or lamp soil are still being used in Vietnam, especially in the ripening of coffee beans after harvest.

2. General changes of fruit:

Every fruit changes its state in the aging process with slightly different results:

a. Left position:

That is the result of a balance between sugar and acidity. Strawberry has a sweet, slightly acidic aroma. This taste is due to a balanced volatile substance between acid and sugar. Depending on the fruit, the ratio of sugar varies and varies depending on the ripeness. For bananas, jackfruit, apricots, melons, apples, pears, monks, peaches . the ripening process is initiated and regulated by the synthesis of a hormone called ethylene. One can ripen green fruits by incubating them with this substance. These fruits store starch at the beginning of the development period. They contain a lot of sugar due to the breakdown of starch into glucose, even when fruits are picked at a green age. The case of strawberries or tomatoes is different. Research in the melon shows that ethylene does not work on ripening processes, and that the sugar and color levels of the fruit are not dependent on ethylene. They do not store starch while the fruit grows but the sweet taste is due to the accumulated saccharose breakdown from the early stages of fruit life. This kind of fruit must be picked when it is cooked until it tastes good.

When ripe, fruit shrinks because they contain phenolic compounds, multi-phenols such as tannins . combined with cellular compounds (pectic types) into complex compounds.

b. The color change from green to orange or red:

We call green fruit that means it is not ripe. At this early stage, chlorophylle gives the fruit its green color. Under the action of the enzyme chlorophyllase, chlorophyll decomposes so that other pigments are available in the left. Example of papaya fruit. In parallel with this process, other pigments are synthesized. There are two groups of pigments: the caroténoide group, with beta-carotene that gives orange-yellow color like mango fruit, and the anthocyanes group, which pelargonidol (pelargonidin-glucosine) will give the strawberry fruit red. These pigments often form complex compounds that do not form blue.

c. Aroma:

Fruit scent is complicated because there are hundreds of different compounds to create a fragrance for a fruit. For example, there are about 200 aromatic compounds for strawberry fruits. These volatile compounds (alcohol, aldehyde, ester, cétone, terpenol .) are involved in odor formation. The synthesis of these substances is caused by external factors such as temperature, oxidation . It is too many parameters to produce odors, but it is difficult to know which aromatic compounds are most special for the fruits in when they are volatile, and also felt by each person.

d. Loss of firmness:

The texture of the fruit is operated by many parameters such as cell size and shape, intercellular volume, integrity, thickness of the cell membrane, osmotic pressure . Changes in the parameters These numbers are complex processes that produce large amounts of protein. In addition, cell wall degradation is the result of hydrolytic enzymes synthesized during adulthood, leading to the destruction of cell and tissue structures. Even though the disorder of the cell is seen as the main cause of configuration changes, there are other events observed during adulthood, such as the loss of adhesion between cells and pressure changes. osmosis, also makes the fruit soften.

For grapes, strawberries, cherry, . the ripening process is not based on the synthesis of ethylene. Although the regulatory factors are still unknown, there are several auxine (where strawberries) act in the development of fruit at a young age.

3. Gene transformation:

Understanding the adjustment of gene expression associated with development and fruit ripening helps people to characterize the gene's functional properties, identify genes and isolate them. From these observations, in recent years gene transfer in the laboratory has been a unique way to prove hypothesis on the function of genes. The beneficial genes are transferred into the plant genome and observed changes in fruit. People can also reduce gene expression, stop the biochemical pathway. In France, transgenic experiments on fruits are intended to understand the mechanism, not for commercial purposes. The experimental fruit is tomato. They create coffee varieties that have bigger fruits, better quality and long-lasting damage after harvest.

Vo Thi Dieu Hang