Why do you think some apples are sweet and others sour?

The sweet taste of apples (and other fruits) is given by carbohydrates, mostly simple ones (monosaccharides – glucose, fructose and others). Complex carbohydrates don’t taste sweet. Carbohydrates and other nutrients enter the fruit – this is how the plant stores the seed with all the elements necessary at first for development. If the fruit has not yet had time to ripen – to accumulate the necessary substances, then it will be sour. Also, each variety of apples has its own individual concentration of carbohydrates in the fruit – in the vacuoles of plant cells there is cell juice with one or another sugar content.