Ripening- an important process in fruit development

Prof. S. N. Naik Head, Centre for Rural Development & Technology, IIT Delhi Email: snn@rdat.iitd.ac.in, 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

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 mango

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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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