Physiochemical changes during ripening of fruits and vegetables

A fruit can be said to be physiologically mature when it has reached its last slow stage of growth and has developed the ability to ripe normally after harvest. It may be commercially mature in an earlier stage when sufficient desirable characteristic have been developed to make it edible. Commercial maturity is the time of harvest related to end used of market requirement. It may be any stage between development and senescence (quality degradation of ripen fruit). Physiological maturity is particular stage of plant. It is the maximum developed stage of fruits and vegetables.
During the development of growth period of fruit, there are many chemical and physical changes taking place in them.

The important changes as the fruit goes through the last stages of development on maturation are increase in sugar, decrease in acidity, decrease in starch and increase in oil content in case of avocados and nuts. Changes in pectic constituents that results in softening of fleshy fruits, loss of tannins and astringency, development of volatiles that contribute of flavor and aroma, formation of pigments in skin and flesh and increase in ascorbic acid.
The skin of many fruits develop wax as they mature giving a fruit a shine on bloom and protection from moisture loss. The percentage of soluble solutes increases in fruit sugar content. There is also marked loss of moisture in nuts as they mature. Some of these properties of fruits have been used as index for harvest maturity.
Stages of fruits and vegetables:
Stage 1: Initiation of edible parts
Stage 2: Termination of natural or desirable growth in size
Stage 3: Start of period of usefulness but too immature for most uses
Stage 4: Period of maximum usefulness
Stage 5: Degradation of quality of fruits and vegetables becomes predominant
Stage 6: End of usefulness of for human consumption
Ripening: ripening is the terminal period of maturation in which fruit attains full size and optimum eating quality. Most of the process of maturation takes place while the fruit is still on the tree or plants. When the fruit develops, flavor, texture and aroma that contribute to eating quality is enhanced. Some fruits will not ripen properly while still attached to trees. Notably avocados, pears, mangoes etc. (climacteric fruit). They require certain ethylene concentration for ripening. Ripening normally occurs in fruit after growth ceases but there must be ethylene present to initiate it. During the course of ripening, enzymes are synthesized that are used in ripening process.
Senescence: It is the period following fruit development during which growth ceases and the process of ageing replaces changes of ripening. The certain rise in CO2 output marks the beginning of senescence.

Physiological changes during ripening of fruit and vegetables:
1. Seed maturation
2. Changes in respiration rate: increase in respiration rate
3. Changes in rate of ethylene production: ethylene production incerases
4. Color changes: loss of green color and formation of red, blue, orange and yellow pigment
5. Changes in tissue permeability
6. Changes in texture: fruit becomes soft and delicate
7. Organic acid changes: Decrease in acid content except in lemon and lime.
8. Protein changes: Increase in protein content due to increased biosynthesis of cellulose, pectinolytic enzyme, peroxidase, catalase etc.
9. Production of flavor and volatiles
10. Development of wax in the skin
11. Changes in carbohydrate composition: During ripening there is noticeable decrease in starch, increase in sugar and decrease in hemicellulose and protopectin.
12. Changes in lipid: increases lipid in avocados and nuts while lipid content decreases in other fruits

Determination of harvesting maturity:
1. Computation of days from bloom to harvest:
This method is popularly used for apples. Delicious varieties of apple is harvested after 135 – 140 days after bloom.
2. Measurement of heat unit: This method uses computation of Accumulated heat unit (AHU). Each crop has a base line temperature below which there is no measurable maturation. Heat unit consists of 1° above this base line temperature for each hour or each day. For instance; if base line temperature is 40°F, 1 hour at 41°F is counted as 1 heat unit while 1 hour at 70°F is counted as 30 heat unit.
3. Visual means: Skin color, fullness of fruit, drying part of plant
4. Physical methods: Ease of separation, pressure test, density grading, pulp to peel ratio etc. For pressure test, force (in pound) required to penetrate the fruit by a rod is recorded. Taylor pressure tester having 5/16 inches plunges is used for testing pressure in fruits.
5. Chemical methods: determination of moisture, total soluble solids (TSS), total solids, sugar, acids, sugar to starch ratio, starch content etc. harvesting of sweet corn is done at moisture content of 70 – 80 %, harvesting of grapes is done when TSS is about 16 %. Apple and oranges are harvested at sugar acid ratio of 40:1 and 10:1.
6. Physiological methods: This includes measurement of rate of respiration, color of skin and flesh of fruit in case of tomato and pineapple, fullness of finger in case of banana etc.
7. Alcohol insoluble (AIS): Salt, sugar and other alcohol soluble solids are extracted leaving acid insoluble solids. Example; starch, cellulose, hemicellulose, fiber, pigments etc. Lower the AIS, fruit is considered less matured.