Page 147 - The Manga Guide to Biochemistry

How fruits become sweet

Now, let’s get back to fruit. Fruits, such as mandarin oranges and melons, get
sweeter and more delicious as they ripen. Why is that?

Hmm. When I was shopping for strawberries and mandarin oranges at the
supermarket earlier, there was a sign that said, “Sugar Content: 11–12%.”

I suppose if fruit gets sweeter as it ripens, that just means that the sugar
content has increased, right? Biochemically speaking, we must be talking about
a change in the saccharides.

You got it. So...let’s talk about saccharides!
Take a look at the graphs below. Before they’re ripe, citrus fruits, such as

mandarin oranges, contain roughly an equal amount of glucose, fructose, and
sucrose. But as they ripen, the relative amount of sucrose steadily increases. In a
Japanese pear, all three types increase.

Saccharides Saccharides Fructose
(mg/g fresh weight) (mg/g fresh weight)

100 50

sucrose sucrose
Glucose
Fructose Month
Glucose June July Aug Sept
Month Sept Oct Nov Dec Jan Feb Japanese Pear

Mandarin Orange

Source: Saburo Ito, Editor, Science of Fruit, Asakura Publishing Co., Ltd. (1991)

When fruit ripens, polysaccharides, like starch, are broken down into monos­ accharides,
like glucose, and the activity of sucrose-phosphate synthase, an enzyme that synthe-
sizes sucrose in fruit, increases while the activity of invertase, which breaks down
sucrose, decreases.

Biochemistry in Our Everyday Lives 133