Ripening, Respiration & Factors Affecting Ripening
Ripening refers to the changes that occur in a mature fruit either before or after harvesting. Ripening makes the product edible. Mature fruits are not edible in most cases. Several physicochemical changes occur in the fruit after ripening which ultimately determines the quality of the fruit purchased by the consumer.
Ripening is a dramatic event in the life of fruit – it transforms a physically mature but inedible plant part into a visually appealing and tasteful one. Ripening marks the completion of a fruit’s development and the onset of old age and is generally an irreversible event.
Ripening is a result of complex changes, many of them possibly occurring independently of each other.
Some fruits ripen on the tree itself, while some ripen only after harvesting. Those that do not usually ripen on the tree fall off the tree after attaining a certain growth stage if not plucked in time.
There are two distinct types of fruit ripening that show different patterns of respiration.
Non-climactic fruit ripening – refers to fruits that ripen only while attached to the parent plant. If they are plucked before they are fully ripe, the respiration rate gradually slows down during growth and after harvest. So the quality of their food is affected because their sugar and acid content does not increase further. Maturation and ripening are a gradual process. eg. Cherry, Cucumber, Grape, Lemon, Pineapple, Grapes, Citrus, Strawberry, etc.
Climactic fruit ripening – Refers to fruits that can be harvest at a stage of maturity before ripening. These fruits are artificially ripened. Ripening begins with a rapid increase in respiratory rate, called the respiratory climax. After the climax, as the fruits ripen, respiration slows down and good edible quality develops. eg. Apple, Banana, Mango, Sapota Melon, Papaya, Tomato, etc.
Changes Occurs during Ripening
During ripening, changes occur in colour, texture, taste, aroma and chemical constituents. These changes continue until maximum food or taste is achieved and then the tissue begins to degrade or break down, rendering it unfit for consumption. These changes are closely related to the rate of respiration of the fruit.
1. Colour: The color change is the most obvious sign of ripeness; It occurs in many fruits and is often used by consumers to determine whether the fruit is ripe or raw. The marketability of fruit largely depends on the attractive color it develops.
The colors arise from carotene, xanthophyll, and anthocyanin pigments. Carotene and xanthophyll are yellow in color and other colors of fruits like red, pink, purple, etc. are provided by anthocyanin pigments.
Climacteric and non-climacteric fruits show rapid loss of green color upon ripening with the attainment of optimum eating quality. For example citrus fruits in temperate climates (but not in tropical climates). The green color is due to the presence of chlorophyll, a magnesium-organic compound. The loss of green color is due to the degradation of the chlorophyll structure.
The disappearance of chlorophyll is often associated with the synthesis of yellow to red pigments. Many of these pigments are carotenoids, which are unsaturated hydrocarbons. Carotenoids are stable compounds and remain intact in the tissue despite extensive degradation. Carotenoids can be synthesized during the growth stages of the plant, but they are masked by the presence of chlorophyll. After the degradation of chlorophyll, carotenoid pigments begin to appear.
Factors influencing fruit coloration are weather, temperature, moisture, carbohydrate accumulation, and actions such as fruit ringing. Fertilization and irrigation also affect the brightness of the color. Excessive nitrogen delays the development of color. Shaded fruits also do not develop good color.
2. Organic acids: Generally, organic acids are reduced during ripening as they are respirated or converted into sugars. The acid can be considered a reserve source of energy for fruits and, therefore, can be expected to decline during the greater metabolic activity that occurs upon ripening.
3. Texture: Terms such as firmness, crispness, mealiness, juiciness, and hardness are all related to the texture of the fruit and are controlled by the wall-to-wall adhesion of the cells. The fruits become soft when ripe, mainly due to the dissolution of pectic substances in the cell wall. The softening is due to the enzymatic hydrolysis of the polysaccharide. The cell wall is made up of cellulose, hemicellulose, calcium pectate, and glycoproteins. The enzyme pectinase breaks down the pectin between fruit cells resulting in softening of the fruit.
4. Taste: Taste depends on the proper ratio of sugars and acids. Therefore, it is convenient to measure taste as a sugar-acid ratio (brix-acid ratio). The acidity and astringency gradually disappear, with the fruit becoming sweeter due to the conversion of starch to sugar during ripening. The starch content in ripe banana fruits decreases from the initial 21% to about 15%. It is accompanied by the accumulation of sugars mainly of sucrose up to 20% of the fresh weight.
5. Aroma: Aroma plays an important role in developing the best eating quality in most fruits. This is due to the synthesis of many volatile organic compounds during the ripening stages. Along with the taste, it builds up the flavour. The aroma usually develops during ripening but sometimes develops in storage.
Different types of fruit have different aroma-enhancing compounds but all of them are volatile. The aroma of fruits is not due to anyone chemical compound but it is a combination of them. The aroma is derived from aliphatic compounds, alcohols, acetates, ketones or esters, and terpenoids. In most fruits, the biotransformation of aromatic compounds increases with the advent of ripening.
6. Abscission: During ripening, the pectinase enzyme reduces the adhesion of cells to the abscission zone (the layer of cells in the pedicel often referred to as the abscission zone). So, the cells in this region get weakened and the fruits drop from the plant due to their weight.
7. Development of surface wax: A delicate waxy or powdery substance develops on the surface of some fruits, such as grapes and berries, during ripening.
8. Respiration rate: This is essential for ripening as it provides the energy needed to drive many reactions and changes. If respiration is disrupted, the ripening of fruits is also disrupted. On the basis of respiratory characteristics, fruits and vegetables can be divided into climacteric and non-climacteric. In non-climacteric fruits, the rate of respiration during ripening remains relatively low but constant. Fruits often do not have much carbohydrate reserves and ripen only when attached to the plant.
In contrast, respiration declines during the late stages of maturation in climacteric fruits, with a rapid peak in respiration followed by a decline in respiration as the ripening process progresses.
9. Chemical changes: Starch is hydrolyzed in to sugars (Glucose and fructose), pectin’s become soluble, acids disappear and tannins responsible for astringency are eliminated by the action of enzymes.
Factors influencing ripening
Temperature: Fruits harvested at the right time generally ripen at any temperature between two critical limits. In some circumstances, a cold temperature treatment may be required before the fruit can be placed in the temperature range for ripening.
Temperature affects the rate of synthesis of specific pigments and their final concentration in the fruit. The appropriate and maximum temperature for the synthesis of a specific pigment varies between species. For example, lycopene synthesis in tomato is inhibited at 300C whereas in watermelon; Synthesis occurs only at high temperatures and cannot be stopped until the temperature of the fruit is above 37 °C.
Carbon-di-oxide: high levels of CO2 will inhibit ripening due to decreased respiration.
Oxygen: Low levels of oxygen inhibit the ripening of fruits and vegetables.
The use of increased CO2 and decreased O2 levels in cold storage is called controlled atmospheric storage. Oxygen is essential for the carotenoid synthesis and an increase in oxygen concentration increases the synthesis of this pigment.
Radiation: Radiation may act as an inhibitor or stimulator of ripening. Grapes ripen more quickly than treated with ‘infrared’ radiations. Bananas irradiated with ‘X’ rays showed a decrease in softening but an increase in skin darkening.
Air humidity: The relative humidity and velocity of the air surrounding the fruit affect ripeness, especially in the development of flavor. Saturated air hinders the development of good flavor in pears. Apples show the darkening of the core.
Volatile: Non-ethylene volatiles may encourage ripening. Pre-climacteric apple ripening is slowed by air purification in a recirculation system with activated carbon, H2SO4, and NaOH. Carbon (activated) slows ripening in both cases.
Growth regulators: These sometimes stimulate the ripening of collected fruits. The treatment seems to be effective, especially when it is treated immediately after picking. 2, 4,5 -T and to a lesser extent 2,4-D when sprayed with a wax emulsion cause a late development of yellow color in lemon peels during storage. Storage life also increases.
Application of ethephon promotes early ripening in grapes, tomatoes, coffee, pears, plums, peaches, and citrus. Smoking is commercially used to hasten de-greening bananas and mangoes and to accelerate ripening. Calcium carbide releases acetylene which on hydrolysis accelerates the ripening process. Spraying of ABA 1ppm, Thio-urea 20%, CCC 4000ppm, Etherel 200-300ppm one week before the crop is beneficial.
Auxins can slow or sometimes even speed up the ripening process. The formation of ethylene is inhibited by auxin and therefore auxin is broken down by peroxidase (IAA oxidase) to control fruit ripening. With ripening, there is an increase in auxin degrading enzymes. Gibberellins also prevent color change in fruits like bananas. The accumulation of abscisic acid (ABA) is also associated with ripening.
Chemicals that delay ripening and senescence:
(8) Metabolic Inducers
(a) Cycloheximide, Actinomycin-D
(c) Maleic acid,
(d) Ethylene Oxide,
(9) Ethylene absorbents
(b)Fumigants like methyl bromide
Harvesting: The degree to which certain fruits are climacteric or non-climacteric at harvest is an important factor influencing ripeness. Only the act of picking can affect the rate of ripening of some fruits. Detachment accelerates the ripening of fruits such as avocados and apples. It is believed that when fruits such as apples and avocados are attached to the tree, an inhibitory auxin is given off by the leaves. Bruising in fruits such as citrus, apples, bananas, and avocados often encourage ripening. so; The fruits are harvested with great care.