In the early stage of fruit development, starch accumulates in the peel instead of sucrose, resulting in poor texture.
For most fruits, Vitamin C gradually accumulates before they ripen, but once mature, the Vitamin C content drops significantly. Unripe fruits are actually more nutritious.
Yiran, Ph.D. in Botany from the Institute of Botany, Chinese Academy of Sciences
People who often visit the market probably know that if you wait a week or two, fresh peas of the same harvest will taste completely different. The peas that just come to market are tender and sweet. After a week or two, they become old and mealy.
Sweetness is perhaps the flavor that excites the most taste buds. Fruits and vegetables are generally judged by their "sweetness," making "sweet" a popular marketing slogan at markets. When we talk about sweetness, the first thing that comes to mind is sugar. That's right; the source of sweetness in plants is sugar.
Besides being important materials for plant cell construction and energy storage, their properties are vastly different—glucose is the direct product of photosynthesis and is moderately sweet; anyone who has had glucose powder will understand this, as you need to add many spoonfuls to get a sweet taste. Fructose is glucose's close relative; just by moving an oxygen atom, its sweetness is greatly enhanced. When a glucose and a fructose molecule join, they form sucrose, which is the universal transport form of sugars in plants, with a sweetness level between glucose and fructose. The sweetness of peas depends on the content and ratio of these sugars.
In the early development of the pea seed, a large amount of sucrose is continuously transported to it. Initially, the construction project of the seed is quite demanding. On the other hand, to reduce the concentration of sucrose, it is broken down into glucose and fructose. Theoretically, fructose would increase the sweetness of the seed, but they are quickly used to synthesize cellulose and other parts to support the seed. Therefore, newly formed seeds have little sweetness. As the construction of the seed progresses, the main structure is complete, but raw materials continue to arrive. Sucrose, glucose, and fructose all reach a state of "relative surplus," making the seed unavoidably sweet.
However, the ultimate purpose of accumulating these sugars is not to turn the seed into a sugar jar. The space in a seed is limited, so to store enough energy for future germination, these sugars must be "compressed" into starch, which is high in energy and low in volume. Broadly speaking, starch is also a sugar. It has little sweetness despite its name, but it can fill you up. As starch accumulates, some mineral nutrients also build up. From this perspective, older seeds are more nutritious.
While the pea seed is busy, the fruit's peel, which encloses the seed (including what is commonly called the "peel" and "flesh"), is also active. In the early stage of fruit development, starch accumulates in the peel instead of sucrose, resulting in poor texture. This is a clever trick to prevent animals from "stealing a bite" and damaging the immature seeds. After the seed is fully prepared, sucrose and fructose in the peel begin to accumulate in large quantities. Some of the starch that was stored earlier is even broken down into sucrose and fructose, greatly increasing the fruit's sweetness. This is why everyone thinks fully ripe fruits taste better.
In addition to changes in sweetness, the nutritional value of the fruit also changes. For most fruits, Vitamin C gradually accumulates before they ripen, but once mature, the Vitamin C content drops significantly. Therefore, unripe fruits are a more nutritious choice.
The same fruits and vegetables can taste very different depending on when they are eaten, which ultimately leads to the wisdom of Confucius: "Do not eat out of season." However, taste and nutrition are sometimes like fish and bear's paw—how to choose is a matter of personal perspective.