Why Fruits Change Color and Flavor as They Ripen

Why Fruits Change Color and Flavor as They Ripen


Jay G. asks: Why do fruits change colour and
flavour when they ripen? fruit-rainbowFruits and vegetables (see: The
Difference Between Fruits and Vegetables) come in a wide array of colors that change
throughout their ripening process, with the brightest colors often occurring when the
fruit is optimally ripened. But why do fruits change color at all? There are two ways to answer this question-
first, looking at what’s going on internally while the fruit is ripening and second looking
at why these plants have evolved to do this at all, especially considering it’s seemingly
a massive waste of the plant’s energy and resources to produce fruit of any color around
its seeds. While there are many different types of fruits,
speaking generally, many fruits start out some shade of green thanks to an abundance
of chlorophyll, which, with the help of the sun and nutrients from the soil, is used to
produce much of the contents of the fruit, at this stage mainly starch. At a certain point in the fruit’s development,
usually when the seed is nearly fully matured, the ripening process begins, in many fruits
triggered by the naturally produced hydrocarbon gas ethylene. Exactly how ethylene facilitates ripening
is still an ongoing area of research, but, in a nutshell, receptors in the plant end
up binding to the ethylene. This triggers certain genes to turn off and
others to turn on, resulting in the creation of various enzymes that facilitate the ripening
process, such as amylases, which converts the starch to simple sugars, and pectinases,
which breaks down the cell walls of the fruit, softening it to make the inside more accessible. While the enticing innards are being sweetened
and made easier to access, the chlorophyll is broken down via hydrolytic enzymes, ultimately
removing the green color. As this is happening some pigments are getting
synthesized while others that were there all along, but masked by the chlorophyll, are
revealed. The two primary culprits producing the colors
here are carotenoids, which are generally responsible for the orange and yellow colors
in fruits, and anthocyanins, which are usually responsible for the purples, reds, and blues. (Incidentally, a similar process to this is
what’s happening in autumn with tree leaves, Now for why certain plants have evolved to
produce colorful, tasty fruit. Other than in science fiction, plants are
relatively immobile so those that have survived over the millennia have evolved a few ingenious
methods for spreading their seeds. Some have buoyant seeds that float on the
wind or water, others are carried and buried by industrious rodents and mammals, while
still others stick to the fur and feathers of passing fauna and hitch a ride. But perhaps the most common, and certainly
the most scatological, way seeds are spread is when they are eaten, and later pooped out,
by animals. Beyond potentially spreading the seed far
and wide, this also has the side benefit of providing the seed a good starter of nutrition. To facilitate this type of dispersal, fruits
have evolved to become tastier when their seeds are ready and to change to a more noticeable
color as a potentially attractive signal to animals that it is time to eat. Animals that eat fruits are frugivores, including
mammals, birds, reptiles, amphibians and fish. Among these, it appears that birds and mammals
play the largest role in seed dispersal, and, in fact, more than 35% of all terrestrial
birds and 20% of non-marine mammals are frugivores, at least to some extent. Nonetheless, the others have a role to play,
and fish are especially key to dispersing tropical seeds, while tortoises and lizards
are integral to seed dispersal in insular and arid ecosystems.[1] As far back as the 19th century, Charles Darwin
in On the Origin of the Species (1859) noticed that frugivores (or more particularly, birds)
more commonly ate brightly-colored, as opposed to dull-colored, fruits. Subsequent studies discerned that, for European
birds, the fruit they ate was most frequently red, while for those in the New World, and
neo- and sub- tropics, black fruits are the most popular. Interestingly, in those regions where fruit
eating birds were less common, multi-colored fruits proliferated, and subsequent studies
showed that birds were more likely to remove a fruit with more than one color than a mono-colored
fruit. Similarly, in at least one study, scientists
found that multi- and brighter- colored ripe fruits were more common in the understory
(to be visible in the low light), while duller colors were found higher up (where it is easier
for a fruit to be noticed).[2] The hue, brightness and location of the variety
of different ripe fruit colors has led many to conclude that color serves three purposes:
(1) it draws attention to the fruit; (2) it reveals (or camouflages) the fruit’s location
depending on ripening stage (often hiding when not ripe and revealing when the seed
inside is ready for dispersal); and (3) it signals when the fruit is ripe and, thus,
optimally tasty for its target consumer. Of course, there may be other reasons why
fruits have different colors, and one theory holds that blue, brown and black fruits may
have evolved their dark colors as this “absorbs more radiation . . . thereby raising fruit
temperatures [and] increasing metabolic and developmental rates.”[3] Nonetheless, seed dispersal appears to be
the primary purpose of a fruit’s coloring, as is the production of the tasty, juicy pulp,
so sought after by the dispersers. In fact, it has been noted that “the energy
allocated to produce the pulp . . represents a cost to the plant that probably has no purpose
other than to attract seed dispersers and to protect the seeds.”[4] In addition to transporting eaten seeds, it
is believed that disperser ingestion also aids in seed germination. In the disperser’s gut, the pulp, which
if not removed could preclude sprouting, is removed, as often is all or a part of the
tough outer seed coating – the removal of which frequently speeds up germination. Another benefit of ingestion is that, as previously
mentioned, when the seeds finally leave the disperser at a distant location, they are
surrounded by a rich source of potential fertilizer (known to you and me as fecal matter).