Ripening is the process by which fruits attain their desirable flavour, quality, colour, palatable nature and other textural properties.
Ripening is associated with change in composition i.e. conversion of starch to sugar. On the basis of ripening behavior, fruits are classified as climacteric and non-climacteric fruits.
Fruit ripening is a genetically programmed stage of development overlapping with senescence. The fruit is said to be ripe when it attains its full flavour and aroma and other characteristics of the best fruit of that particular cultivar. The words ―mature ―and ―ripe‖ are essentially synonymous when used to describe these fruits that ripe on the plants known as non-climacteric. However, in case of climacteric fruits a mature fruit require period before attaining a desirable stage of edibility.
List of climacteric and non-climacteric fruits
Apple, Apricot, Avocado, Banana, Ber, Mango, Melons, Pear, Peach, Kiwifruit, Cherimoya, Fig, Guava, Plum, Persimmon, Papaya, Tomato, Sapota, Passion fruit.
Carambola, Cherries, Citrus, Grape, Litchi, Loquat, Olive, Pineapple, Pomegranate, Strawberry.
Changes During Fruit Ripening
1. Cell Wall Changes
- Cell wall consists of pectic substances and cellulose as the main components along with small amounts of hemicellulose and non-cellulosic polysaccharides.
- In cell wall, the changes particularly in the middle lamella which is rich in pectic polysaccharides are degraded and solubilised during ripening.
- During this softening, there is a loss of neutral sugars (galactose and arabinose-major components of neutral protein) and acidic pectin (rhamnogalacturonan) of all cell wall.
- The major enzymes implicated in the softening of fruits are pectinesterase, polygalacturonase cellulase and β- galactosidase.
- During fruit ripening sugar levels within fruit tend to increase due to either increased sugar importation from the plant or to the mobilization of starch reserves within the fruit, depending on the fruit type and whether it is ripened on or off the plant.
- With the advancement of maturity, the accumulated starch is hydrolysed into sugars (glucose, fructose or sugars) which are known as a characteristic event for fruit ripening.
- Further breakdown of sucrose into glucose and fructose is probably mediated by the action of invertase. In vegetables like potato and peas on the other hand, the higher sucrose content which remains high at fresh immature stage, converts into starch with the approach of maturity.
3. Organic Acids
- With the onset of fruit ripening there is downward trend in the levels of organic acids.
- The decline in the content of organic acids during fruit ripening might be the result of an increase in membrane permeability which allows acids to be stored in the respiring cells, formation of salts of malic acid, reduction in the amounts of acid translocated from the leaves, reduced ability of fruits to synthesize organic acids with fruit maturity, translocation into sugars and dilution effect due to the increase in the volume of fruit.
- With the approach of maturation, the most obvious change which take place is the degradation of chlorophyll and is accompanied by the synthesis of other pigments usually either anthocyanins or carotenoids.
- They can give rise to a wide range of colours (from red to blue).
- The chloroplasts in green immature fruit generally lose chlorophyll on ripening and change into chromoplasts which contain carotenoid pigments.
- Carotenoids are normally synthesized in green plant tissue a major product being 3 carotene.
- However, in many fruits additional – carotene and lycopene is synthesized during ripening.
5. Flavouring Compounds
- Although fruit flavour depends on the complex interaction of sugars, organic acids, phenolics and volatile compounds but the characteristic flavour of an individual fruit or vegetable is derived from the production of specific flavouring volatile.
- These compounds are mainly esters, alcohols, aldehydes, acids and ketones. At least 230 and 330 different compounds in apple and orange fruits have been indicated respectively.
6. Ascorbic Acid
- L-ascorbic acid (Vitamin C) is the naturally occurring ascorbic acid in fruits.
- A reduced amount of ascorbic acid is noticed in pome, stone and berry fruits at the time of harvest.
- An increase in ascorbic acid content with the increase in fruit growth has been and the levels declined with the advancement of maturity and onset of fruit ripening in pear, sweet potatoes, potato, asparagus and okra during the course of post harvest handling.
The phenolic content of most fruits declines from high levels during early growth to low levels when the fruit is considered to be physiologically mature and thereafter susceptible to the induction of ripening.
8. Amino Acids and Proteins
- Decrease in free amino acid which often reflects an increase in protein synthesis.
- During senescence the level of free amino acids increases reflecting a breakdown enzymes and decreased metabolic activity.
9. Ethylene Production and Respiration
Physiological events responsible to ripening process are as follows
(1) Ethylene production
(2) Rise in respiration
In climacteric fruits such as mango, banana, ethylene production increase and causes:
• Rise in respiration
• Rise in temperature
• Rise in activity of hydrolytic enzymes. Ethylene is produced from an essential amino acid — methionine.
Following the steps as below:
SAM — Methionine S adenosyl methionine (ACC synthase)=>Amino cyclopropane (ACC oxidase)=>Carboxylic acid (ACC)=>Ethylene=>Perception by ethylene receptor=>Signal transduction=>Switching or ripening genes
Rise in respiration
Respiration is required for releasing energy and the substrate for synthesis of several organic compounds required in the ripening process. During ripening in climacteric fruits, there is rise in respiration called climacteric. The climacteric peak is obtained very fast when temperature is relatively high.
Respiration is a most deteriorating process of the harvested fruits and vegetables which leads to the oxidative breakdown of the complex materials (carbohydrates or acids) of cell into simpler molecules (CO2 and water) with the concurrent production of energy required by the cell for the completion of chemical reactions. In brief, the process of respiration can be summed up with the following reaction:
C6H12O6 + 6O2 6 CO2 + 6 H2O + energy
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