Postharvest Management of Fruits and Vegetables: Maturity and Ripening Process

The corky layer stops the inflow of food material from the plant, making the fruit dependent on its own reserves.

During ripening, carbohydrates are dehydrated, sugars accumulate, typical flavors develop, and characteristic colors emerge.

Harvesting at the correct maturity stage facilitates proper ripening, allows for distant transportation, and maximizes storage life.

Immature fruits may develop white patches or air pockets during ripening, and lack the proper sugar-acid ratio, taste, and flavor.

Fruit ripening is a genetically programmed stage of development that overlaps with senescence.

Over-mature or fully ripe fruits are easily susceptible to microbial and physiological spoilage, and have a considerably reduced storage life.

Carotenoids give rise to a wide range of colors in fruits, from red to blue.

Additional compounds synthesized during fruit ripening are beta-carotene and lycopene.

Volatile compounds, such as esters, alcohols, aldehydes, acids, and ketones, primarily contribute to the characteristic flavor of individual fruits and vegetables.

The ascorbic acid content in fruits decreases during maturity and ripening, after initially increasing during growth.

The phenolic content of most fruits declines from high levels during early growth to low levels when the fruit is physiologically mature and susceptible to ripening.

During senescence, the level of free amino acids in fruits increases, reflecting a breakdown of enzymes and decreased metabolic activity.

During fruit ripening, accumulated starch is hydrolyzed into sugars like glucose and fructose, increasing sugar levels. In contrast, in vegetables like potatoes and peas, higher sucrose content at the immature stage converts into starch as they mature.

The accumulated starch is hydrolyzed into sugars (glucose, fructose, or other sugars), which is known as a characteristic event for fruit ripening.

There is a downward trend in the levels of organic acids during fruit ripening. This decline could be due to increased membrane permeability allowing acid storage in respiring cells, formation of malic acid salts, reduced acid translocation from leaves, reduced ability to synthesize acids with maturity, translocation into sugars, or dilution effect from increased fruit volume.

With the approach of maturation, the most obvious change is the degradation of chlorophyll, accompanied by the synthesis of other pigments, usually either anthocyanins or carotenoids.

In fruits that ripen on the plant, sugar levels tend to increase due to increased sugar importation from the plant. In fruits that ripen off the plant, sugar levels increase due to the mobilization of starch reserves within the fruit.

The decline in organic acid content during fruit ripening might be the result of an increase in membrane permeability, which allows acids to be stored in the respiring cells, reducing their levels in the fruit.

A ripe fruit attains its full flavour, aroma, and other characteristics of the best fruit of that particular cultivar.

For non-climacteric fruits, 'mature' and 'ripe' are essentially synonymous. However, for climacteric fruits, a mature fruit requires a period before attaining a desirable stage of edibility (ripeness).

The major enzymes implicated in the softening of fruits during ripening are pectin esterase, polygalacturonase, cellulase, and $\beta$-galactosidase.

The main components of the cell wall are pectic substances and cellulose, along with small amounts of hemicellulose and non-cellulosic polysaccharides. During ripening, the pectic polysaccharides in the middle lamella are degraded and solubilized, leading to a loss of neutral sugars and acidic pectin from the cell wall.

Apple is climacteric, cherry and citrus are non-climacteric, and pineapple is non-climacteric.

The changes in the middle lamella, which is rich in pectic polysaccharides, are significant because the degradation and solubilization of these pectic substances contribute to the softening of the fruit during ripening.

Potassium permanganate (KMnO4) can absorb ethylene gas, which helps minimize ripening and increase shelf life when used at a concentration of 100 ppm soaked in filter paper.

Packaging in the absence of oxygen, either under vacuum or with an oxygen scavenger, leads to an extension of shelf life in terms of color and odor stability due to low oxidation rates.

Mixtures of iron powder and sodium chloride, as well as activated carbon, are extensively used as oxygen absorbents in fruit and vegetable packaging.

Sodium ortho-phenyl phenate (SOPP) is an antifungal agent that provides protection against molds and stem-end rot, particularly in the form of diphenyl wraps.

MH (1000-2000 ppm) is used as a post-harvest treatment for mango, while 2-dimethyl-hydrazide (10,000 ppm) is used for apples. These inhibitors help extend the shelf life of these crops.

Auxins like 2,4-D (5 ppm for grapes) and 2,4,5-T (25 ppm for figs, 100 ppm for mangoes) can be used to advance ripening and potentially increase shelf life when applied at specific concentrations and stages (pre-harvest for grapes and figs, post-harvest for mangoes).