Ripening is when fruit becomes easier to eat. Some fruits, like the tomato or the mango, will continue to ripen after they have been picked. Other fruits, like the cherry or fig, stop ripening once they have been picked.
Climacteric fruits undergo a number of changes during fruit ripening. The major changes include fruit softening, sweetening, decreased bitterness, and colour change. These changes begin in an inner part of the fruit, the locule, which is the gel-like tissue surrounding the seeds. Ripening-related changes initiate in this region once seeds are viable enough for the process to continue, at which point ripening-related changes occur in the next successive tissue of the fruit called the pericarp. As this ripening process occurs, working its way from the inside towards outer most tissue of the fruit, the observable changes of softening tissue, and changes in color and carotenoid content occur. Specifically, this process activates ethylene production and the expression of ethylene-response genes affiliated with the phenotypic changes seen during ripening. Colour change is the result of pigments, which were always present in the fruit, becoming visible when chlorophyll is degraded. However, additional pigments are also produced by the fruit as it ripens.
In fruit, the cell walls are mainly composed of polysaccharides including pectin. During ripening, a lot of the pectin is converted from a water-insoluble form to a soluble one by certain degrading enzymes. These enzymes include polygalacturonase. This means that the fruit will become less firm as the structure of the fruit is degraded.
Enzymatic breakdown and hydrolysis of storage polysaccharides occurs during ripening. The main storage polysaccharides include starch. These are broken down into shorter, water-soluble molecules such as fructose, glucose and sucrose. During fruit ripening, gluconeogenesis also increases.
Acids are broken down in ripening fruits and this contributes to the sweeter rather than sharp tastes associated with unripe fruits. In some fruits such as guava, there is a steady decrease in vitamin C as the fruit ripens. This is mainly as a result of the general decrease in acid content that occurs when a fruit ripens.
Different fruit have different ripening stages. In tomatoes the ripening stages are:
- Green: When the surface of the tomato is completely green
- Breaker: When less than 10% of the surface is red
- Turning: When less than 30% of the surface is red (but no less than 10%)
- Pink: When less than 60% of the surface is red (but no less than 30%)
- Light Red: When less than 90% of the surface is red (but no less than 60%)
- Red: When the surface is nearly completely red.
Images for kids
This lemon turns yellow as it ripens.
Ripening Facts for Kids. Kiddle Encyclopedia.