Why are ripe fruits sweet?

Why is ripe fruit sweet to eat? The same is true of many fruits. Mangoes, pears, oranges, grapes, melons, and watermelons—all are delicious when ripe. Oranges are sour to eat when they are still slightly green when unripe. But it is sweet when ripe.

Seasonal ripe watermelon is also very sweet and delicious! But why? This is because various fruits and berries are high in three sugars—sucrose (table sugar), fructose (fruit sugar), and glucose (grapeose).
Glucose, sucrose and fructose — these have different chemical structures. First let’s know more about these sugars. Monosaccharides are the basic unit of sugar.

Monosaccharides are made up of at least three carbons linked together. Glucose and fructose are also monosaccharides. They consist of six carbons. Two or more monosaccharides are linked together to form oligosaccharides.
Although sucrose is an oligosaccharide, it is made up of two monosaccharides. Hence they are also called disaccharides. There are also sugars composed of many monosaccharides. They are quite large molecules and have complex branching. These are called polysaccharides.
Potatoes and rice are loaded with sugars called starch. This starch is a polysaccharide.
Several glucose molecules join together to form starch. Plants store the glucose produced in photosynthesis as starch. Our bodies (all animals) also have a stored sugar like plant starch. Its name is glycogen. Excess glucose molecules are stored as glycogen in the liver and muscles. Later, when there is a lack of glucose in the body, this glycogen is broken down and glucose is supplied. There are also other types of polysaccharides such as cellulose and chitin. Cellulose is the main component of plant cell walls. And chitin is found in the hard shells of various crustaceans, such as shrimp and crabs. Mushroom bodies also contain chitin as a structural component.

How sweet is fruit? What is the meaning of this ‘ripe’ word in the language of life chemistry? As the fruit ripens, its sugar content increases. Biochemically speaking, we should talk about the change in the amount of these sugars. Citrus fruits such as oranges contain roughly equal amounts of glucose, fructose, and sucrose before ripening. But when ripening begins, the amount of sucrose gradually increases. In Japanese pears, all three are increased. What is the reason? As the fruit ripens, polysaccharides such as starch are broken down to form various monosaccharides such as glucose, while the activity of the sugar-phosphate synthase enzyme increases. This enzyme combines glucose and fructose to form sucrose. On the other hand the efficiency of invertase enzyme decreases. The function of this enzyme is to break down sucrose.

As the amount of sucrose increases, the fruit becomes sweeter. Glucose, fructose and sucrose are all sweet sugars. Of the three sugars, fructose is the sweetest, followed by sucrose, and glucose the least. For example, the best time to harvest citrus fruits is winter. As they ripen, the amount of sucrose increases. It is very tasty and sweet to eat. In Japanese pears, sugar turnover is even greater—as the fruit ripens, polysaccharides break down to produce more fructose and sucrose. Like citrus fruits, the sweetness of watermelon depends on the amount of sucrose. The higher the sucrose content, the sweeter the watermelon.