Given the choice, there is a fair chance that a child would prefer a slice of orange to a slice of grapefruit…because an orange is not bitter. The bitterness you taste when eating a grapefruit is due to a molecule known as naringenin-7-O-neohesperidoside, otherwise known as naringin. Naringin is a flavanone. Citrus fruit – such as oranges, mandarins and grapefruit – harbour large quantities of flavanones which are either bitter or tasteless. Oranges, like mandarins, house the tasteless ones, while grapefruit accumulate the bitter ones. The bitterness of any citrus fruit is directly proportional to the amount of naringin present.
It is a very thin line between the two tastes. Bitterness depends on the positioning of a sugar – known as rhamnose – with respect to a second sugar on a flavanone skeleton. If rhamnose is bound to the C2 position of the sugar, it will produce the familiar bitter taste. If it binds to the C6 position of the sugar, it will produce no taste at all. And what is responsible for the positioning of one or the other type? An enzyme: a rhamnosyltransferase. In grapefruit, the culprit is known as 1,2 rhamnosyltransferase and transfers the rhamnose entity to the C2 position of the glucose moiety.
1,2 rhamnosyltransferase is a medium-sized protein that acts on its own, as a monomer. It is largely expressed in young fruits, leaves and stems in grapefruit and, as a consequence, produces huge amounts of naringin which can constitute up to 75% of the dry weight of young fruits! The fully ripened fruit, however, does not express 1,2 rhamnosyltransferase anymore and the concentration of naringin is diluted making the mature fruit far less bitter although the sensation is still perceived.
Why is fruit bitter in the first place? Bitterness is probably a means of defense. If a fruit wants to survive development, one way to keep it out of an animal’s mouth is to make it disagreeable to the palate. Watch a young child sink its teeth into un-ripened fruit... Despite its bitterness, naringin – like all flavanones – is known to be good for our health. Flavanones not only participate in keeping our blood vessels and bones healthy but they also act as cancer-suppressing agents as well as anti-allergic, anti-inflammatory and anti-microbial compounds. Consequently, it is hardly surprising that rhamosyltransferases are now the target of bio-engineering, both to manipulate the taste of fruit and to produce flavanones for health benefits. The sweeter side of a bitter taste.