Ripening- an important process in fruit development

Ripening- an important process in fruit development
Prof. S. N. Naik
Head, Centre for Rural Development & Technology, IIT Delhi
Email: [email protected], hodrdat[at]rdat.iitd.ac.in
Phone: 011-26591162, 26596251
Fruit, which supply a number of vital nutrients essential to body, is an important constituent of human diet
today, more with much awareness towards health and fitness. Ripening, a process in fruits that causes them
to become more palatable by making them sweeter, less green, and soft, physiologically and commercially,
is the most significant phase in their life. During the developmental phase, the fruit acquires a proper shape,
weight, volume, texture, colour, aroma and flavour. One of the most accepted definitions describe the fruit
ripening as a physiological process involving the induction/ acceleration of a variety of metabolic process
most or all enzymatically regulated and catalyzed.
Fig 1 A diagrammatic representation of fruit ripening process
The annual world production of fruits is around 370 million metric ton, which accounts for 8% of the total
fruit production of the world. India is the second largest producer of fruit in the world and largest in the case
of banana and mangoes production. It is also the fourth, sixth and ninth largest producer of pineapple, citrus
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and apple respectively. The process of fruit ripening is chiefly regulated by a gaseous plant hormone called
ethylene (C2H2). This phytohormone is said to regulate the expression of several genes involved in fruit
ripening so as to modulate the activity of various enzymes involved in the process of ripening. These
enzymes act to soften the `skin' of the fruit and also convert complex polysaccharides into simple sugars.
Ethylene is produced and released by rapidly-growing plant tissues. It is released by the growing tips of
roots, flowers, damaged tissue, and ripening fruit. Ethylene is Generally Recognized as Safe (GRAS) by the
United States Food and Drug Administration (FDA), falls within the category of food substances that, when
used for the purposes indicated, in accordance with good manufacturing practice. The ripening speed of
fruits affects both the consumer and the farmer. Farmers want to pick the produce and transport it to the
consumer when it is ready to eat. Consumers want to buy produce and be able to keep it until they want to
eat it. Several factors affect the speed at which fruits ripen. Natural process of fruit ripening is accelerated by
using certain chemicals for e.g. Bananas are picked when green and artificially ripened after shipment by
being gassed with ethylene. Ethylene was used historically as an important anesthetic until less flammable
compounds were developed. It is a colorless gas with a sweet ether-like odor. As an anesthetic, it was used
as a concentration of 85% with 15% oxygen. Ethylene is a hydrocarbon gas and quite flammable and
explosive at concentrations above about 3%. As a fruit ripening hormone, it is produced in varying
quantities depending on the type of fruit but when the concentration of ethylene reaches 0.1-1.0 ppm (parts
per million), the ripening process in climacteric fruits is considered irreversible. One part of ethylene per
million parts of air that's one cupful of ethylene gas in 62,000 gallons of air - is enough to promote the
ripening process in fruits. While there are other factors involved in this "triggering" of the ripening process
by ethylene, it is essentially a universal ripening hormone. The process may be slowed, but it cannot be
reversed once it is truly under way. Additional and externally applied ethylene, provided prior to the time so
that the naturally produced internal concentration reaches the required 0.1 - 1.0 ppm level, will trigger or
initiate the natural ripening process at an earlier time. Climacteric fruits are usually harvested once they have
reached maturity which then undergoes rapid ripening during transit and storage. Important tropical fruits
such as apple, avocado, banana, mango, papaya, pineapple and guava are examples of these fruits. Non-
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climacteric fruits do not ripen after harvest. Thus, in order to attain full ripeness and flavor, these fruits such
as watermelon, strawberries and oranges, are often harvested once they have fully ripened.
Calcium carbide is the chemical used for the production of acetylene gas during gas welding. But nowadays
this process widely used by the Indian farmers or the fruit vendors for ripening of many fruits like mango,
banana, papaya, plums, chiku, apples, avocados, melons, peaches, pears, and tomatoes, pineapples, dates,
persimmons, jujubes etc. are calcium carbide and 2-choloroethyl phosphonic acid- a clear indication that the
action of ethylene is general and widespread amongst a number of fruits. This is very simple and cheaper
technique. In the recent discovery in parts of Asia, a bed of slightly green-harvested mango and a few small
open containers are covered with clumps of calcium carbide (CaC2) with a plastic covering. The moisture in
the air reacts with the calcium carbide to release the gas acetylene, an analogue of ethylene, which acts as an
artificial ripening agent. CaC2 + 2 H2O → C2H2 + Ca (OH) 2, and is said to have the same effect as ethylene.
However, acetylene is not nearly as effective for ripening as is ethylene, and acetylene is not a natural plant
hormone like ethylene. All the calcium carbide used in India is of industrial grade and acetylene, when
generated from calcium carbide can contain toxic impurities such as traces of arsenic and phosphorous,
which can be quite harmful for the health and can lead to various ailments. There are many reports to
support the fact that, when the carbide is used on very raw fruit, the amount of the chemical needed to ripen
the fruit has to be increased. This makes the fruit even more tasteless, unhealthy, and possibly toxic.
Acetylene is believed to affect the nervous system by reducing oxygen supply to brain; affect the eyes, skin,
lungs, and memory; and leads to prolonged hypoxia (lack of oxygen supply). However, it has been shown
that, in practice, acetylene is not sufficiently reactive to affect consumers and there are no data available on
the health effects of chronic exposure to acetylene alone, in Humans. Dissolved acetylene is a colorless gas
that is non-poisonous and non-irritant to the skin and mucous membranes. There are other factors that
affects the ripening process, concentrations of carbon dioxide gas (CO2) is one of them. It can delay fruit
ripening by lowering the concentration of ethylene. On the other side phosphonic acid is available under
brand name, Ethrel that produces ethylene gas and is known to trigger the process of fruit ripening. Usually
the apples are ripened by this. Another chemical used for ripening was Ethephon (2-chloroethylphosphonic
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acid), which is a plant growth regulator, mainly used for apples, currants, blackberries, blueberries,
cranberries, morello cherries, citrus fruit, figs, tomatoes and sugar beet , is known to penetrates into the fruit
and decomposes to ethylene and has the same harmful effects. When the Food and Drugs Control
Administration (FDCA) banned its use in India, fruit traders resorted to ethylene and calcium carbide as a
cost effective chemical.
Traditional practices that have been followed for fruit ripening for centuries in various parts of India, needs a
second look for consideration in fruit ripening process as a replacement of harmful chemicals currently in
use. Brief views of various practices that have been in use are mentioned here. For Banana fruit ripening,
they are stacked in pits and lined with pieces of cut pseudo stem on the sides A porous container having
pieces of fuming charcoal is placed at the top center. Finally pits are closed air tight with dried leaves. In
another practice banana hands are stacked in a tall copper vessel called ‘Anda’. Few fuming incense sticks
are kept inside and covered with lid for 24 hours. In an another practice, Neem leaves are put inside a vessel
containing banana hands for ripening of fruits that takes about four days. To induce quick ripening, the
leaves of Basella alba (Basale)are put into the baskets or gunny bags and bunches are stacked inside and
covered air tight. Another way for quick ripening of banana fruits, is sprinkling of lime solution over
bunches. Application of Vaseline, a layer of clay or cow dung ball to the cut end of stalk prevents rotting
during ripening and storage. For uniform mango ripening in a week, fruits are spread on paddy straw on the
floor and covered with straw and the room is kept closed. Branches of 'Aavaram' (Cassia auriculata) plant
are also used for mango ripening.
One can distinguish the artificially ripenened fruit by the uniform skin colour in fruits like tomato, mango,
papaws, etc and in the case of banana, yellow colour fruit with dark green stem.
Banana
apple
plum
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mango
Fig. 2 Picture of some climatric fruits undergoing ripening process
Catalytic Generators are used to produce ethylene gas, simply and safely. Ethylene sensors can be used to
precisely control the amount of gas. It is also possible to precisely control RH and CO2, and even gas the
fruit without the ripener being onsite. Ethylene serves as a hormone in plants. It acts at trace levels
throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, and
the abscission (or shedding) of leaves. Commercial ripening rooms use "catalytic generators", to make
ethylene gas, from a liquid supply of ethanol. Ethylene production can also be induced by a variety of
external aspects such as mechanical wounding, environmental stresses, and certain chemicals including
auxin and other regulators.
Scientists have been working to delay fruit ripening so that farmers will have the flexibility in marketing
their goods and ensure consumers of “fresh-from-the-garden” produce. Ripening delaying SmartFresh is a
technology useful to maintain fresh-picked quality of whole fruits and vegetables. 1-Methylcyclopropene,
Ethylbloc, (1-MCP 0.14%) works with the ripening process to dramatically slow down ethylene production
and prevent over-ripening and problems associated with it.
Iodine (I) can be used to determine whether the fruit is ripening or rotting by showing whether starch in the
fruit has turned into sugar. For example, a drop of iodine on a slightly rotten part (not skin) of an apple will
turn a dark-blue or black color, since starch is present. If it stays yellow, then most of the starch had
converted to sugar.
Awakening to the gravity of the problem, the health ministry sent a circular to all state food authorities
under the Food Safety & Standards Authority of India (FSSAI) stressing the need for legal action for
violation of the Prevention of Food Adulteration Act (PFA) that prohibits the use of chemicals. According to
the circular, the presence of calcium carbide in godowns / wooden crates / premises with fruits could be used
as circumstantial evidence in courts by the enforcement authorities in states to establish that the vendor has
resorted to malpractices.
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