Does boiling vegetables make them tough?

When vegetables are boiled, they become hard and then soft.

When vegetables are boiled, the phenomenon of hardening (hardening phenomenon) occurs mainly at around 50-60°C, and the phenomenon of softening (softening phenomenon) mainly occurs at around 80-90°C.
This is because the structure of pectin, which holds cells together, changes with temperature.
Vegetables become hard around 50-60℃ because the enzyme called pectin methyl esterase contained in vegetables works actively in this temperature range.
On the other hand, when the temperature reaches around 80 to 90°C, this enzyme stops working and the pectin is decomposed by heat, making it softer.

In addition, pectin softens when heated in a solution with a pH of 5 or higher, and softens by hydrolysis even at a pH of 3 or lower. (*This is why it becomes softer when baking soda is added to make it more alkaline.)
In addition, since it is the hardest to soften at pH 4, adding vinegar makes it crispy. (This is why vinegar is added when boiling burdock and lotus root to create a crisp finish.)
*Pectin is one of the substances that make up the cell walls of plants.

From here on it's subjective.

When using the above knowledge in cooking

① Stir-fried vegetables can be fried at a low temperature (low heat) to make them crispy.

②Lettuce used in salads can be made crunchy. (Immersed in hot water of about 50°C for about 3 minutes)

 *This article omits a lot of content, so the actual chemical change is more complicated. Also, in the example of use, the purpose is to make the vegetables hard, and making them delicious is a completely different story.