Changes during fruit ripening

Fruit ripening encompasses both catabolic and anabolic changes. Many fruit store the imported products of photosynthesis (e.g. simple sugars) as the polymerized carbohydrate starch. Bitter- or astringent-tasting phenolic compounds are also often present in immature fruit. Cell walls of unripe fruit are ridged, and adjacent cells are held firmly together by pectic substances in the middle lamella between cells. Immature fruit are therefore not sweet, soft or pleasant tasting to potential herbivores (which include humans). As fruits ripen, starch is hydrolyzed to simple sugars, phenolic compounds are removed either by being metabolized or polymerized, and the structure of the cell wall and middle lamella are altered by specific enzymes. These catabolic reactions produce a sweet, soft and pleasant-tasting edible fruit.

A series of anabolic reactions also accompanies these catabolic transformations in many ripening fruits. Ripening is often heralded by a dramatic change in color brought about by both a loss of chlorophyll and the synthesis of specific pigments. The underlying yellow pigments in banana fruits become visible as the masking chlorophyll is degraded, while in tomato fruit the loss of chlorophyll is accompanied by the synthesis of the red pigment lycopene. The production of characteristic aroma and flavor compounds also accompanies the final stages of ripening. Poor postharvest handling practices with abusive temperature, humidity and damage can alter these metabolic changes so that poor-quality fruit result.

Fruit and vegetables should be harvested as close as possible to their maximum quality to ensure maximum quality of the processed product. For fruit, this stage is fully ripe, while the optimum harvest quality varies tremendously for vegetables because the stage of development at which they are harvested depends on the use for which they are destined. For example, cucumbers can be harvested immature and at only a few centimeters long for sweet gherkin pickles, longer for slicing pickles, longer still for fresh market slicers and near fully ripe for seed production. In contrast to vegetables, there are specific criteria that most fruit should exhibit before being harvested. Softness is often a major attribute defining fruit quality, yet soft, fully ripe fruit cannot survive the rigors of harvesting, handling, shipping and marketing. Therefore, many fruit are harvested when they have developed to a sufficient level of maturity to continue to ripen once harvested yet retain sufficient firmness to be undamaged during marketing. The decision to harvest is therefore often a compromise between the potential highest quality and the greatest marketability.

The ripening pattern of fruits can be separated into two broad groups: climacteric and non-climacteric (Table 7.1). Climacteric fruit are characterized by a substantial increase in ethylene production and respiration coincident with the onset of ripening (Figure 7.1). As climacteric fruit transition from immature to mature fruit they acquire the ability for ethylene to stimulate the rate of its own synthesis; this positive feedback of ethylene on ethylene synthesis is termed auto-catalytic ethylene production. Prior to this, ethylene inhibits its rate of synthesis so that a low concentration is maintained in the tissue. This inhibitory or negative feedback of ethylene on ethylene synthesis is also characteristic of vegetative tissue, and non-climacteric and immature climacteric fruits.

Exogenously applied ethylene stimulates the respiration of receptive plant tissue. Most plant tissue, even non-climacteric tissue, responds to ethylene exposure with elevated rates of respiration and senescence. Interestingly, exposure to low levels of ethylene, which do not produce any observable change during application, can significantly reduce the commodities' shelf-life. The upsurge in ethylene production at the start of the

Table 7.1 Some common fruits grouped by whether they exhibit a climacteric or non-climacteric respiratory pattern while ripening (Gross et al., 2005)

Climacteric fruits Non-climacteric fruits

Table 7.1 Some common fruits grouped by whether they exhibit a climacteric or non-climacteric respiratory pattern while ripening (Gross et al., 2005)

Apple

Blueberry

Apricot

Cacao

Avocado

Cherry

Banana

Cucumber

Fig

Grape

Kiwifruit

Grapefruit

Mango

Lemon

Muskmelon

Lime

Papaya

Olive

Peach/nectarine

Orange

Pear

Pepper

Persimmon

Pineapple

Tomato

Strawberry

Watermelon

Ca0.4

Carbon dioxide

Carbon dioxide

Ca0.4

Ripeness score

6

re o

o

s

5

s

s

e

4

c e

cp

3

be

2

6 8 Days after harvest

Fig. 7.1 Changes occurring during the ripening of a representative climacteric fruit. Graphs show rates of ripening, and ethylene and carbon dioxide production by harvested mature-green tomato fruit held at 15 °C in air. The rise in ethylene production precedes or is coincident with the rise in carbon dioxide production.

climacteric process produces a rise in the internal concentration of ethylene in the tissue. This rise, coupled with increased sensitivity to ethylene, is thought to produce the observed climacteric rise in respiration.

Nullifying the climacteric rise in ethylene production with chemical inhibitors or genetic engineering reduces the rise in respiration and ripening. In contrast, exposing mature fruits to phyto-active levels of ethylene stimulates both respiration and ripening. Climacteric fruit include apples, avocados, bananas, mangoes, pears, tomatoes, and many melons. These

fruit can be harvested at a mature, but unripe, stage and ripened after harvest. For example, mature-green bananas and tomatoes are harvested and shipped when mature but not yet climacteric. They are routinely treated with ethylene in specially designed ripening rooms at regional distribution centers to promote ripening. These ripening rooms should be isolated from other storage areas to prevent ethylene from escaping and promoting adverse changes in sensitive fruits, vegetables and ornamentals in adjacent storage areas. Since ethylene also stimulates respiration as well as ripening, these rooms should be equipped with sufficient refrigeration and air circulating capacity to maintain the optimal temperature and relative humidity to produce the highest quality ripe fruit.

Shipping mature, but as yet unripe, fruit has a number of advantages. The unripe fruit are firmer, and more resistant to mechanical injury and pathogens. Their longer shelf-life allows shipping to more distant markets, or the use of less rapid and therefore less expensive forms of transport. Unripe fruit may also be more resistant to water loss, but that trait varies greatly among commodities since the permeability characteristics of the epidermis and cuticle are often greatly modified during ripening. Because the unripe fruit is not yet ripe, but only has the potential to produce a high-quality ripe fruit, it is very sensitive to its environment during transit from harvest to the ripening room. Abusive temperatures, low relative humidities, adverse handling, or delays do not usually produce changes that are readily apparent, but the damage they inflict becomes very obvious when the fruit is called upon to perform the complex sequence of metabolic and compositional changes that will turn it into a high-quality ripe product.

A fruit is classified as non-climacteric if it does not exhibit a rise in ethylene production or respiration coincident with ripening (Table 7.1). These fruit will not ripen or improve in quality after harvest and must be left on the plant until they have developed sufficient quality to be marketed. While ethylene will not stimulate further ripening of non-climacteric fruit, exposure to phyto-active levels of ethylene will stimulate respiration and the onset of senescence (e.g. loss of chlorophyll, excessive softening).

Senescence in both climacteric and non-climacteric fruits is usually just a continuation of the changes associated with ripening (e.g. softening). The difference between a ripe and a senescent fruit is often a matter of personal preference, which varies greatly among consumers. For example, the definition of a 'ripe' banana varies from a fruit that still retains some green areas on the ends, to a fruit covered with brown spots. Other changes characteristic of senescent tissue are reduced respiration, loss of cellular integrity, loss of turgor and increased disease susceptibility. Wounding, water loss, abusive temperatures and diseases all promote premature senescence in many fresh fruits and vegetables.

Most senescence processes are not simply degradative in nature, but require the activation or synthesis of new enzymes, pathways or compounds. Senescence processes result in the production of lower molecular weight compounds that can be translocated from the senescing tissue to growing portions of the plant. Export of these molecular fragments is precluded in harvested commodities because of their detachment from the parent plant, and their retention and accumulation in the harvest commodity may contribute to some of the postharvest changes that are characteristic of high quality (e.g. high levels of simple sugars and/or organic acids).

The loss of sweetness or acidity, or a significant change in their ratio, often results in a loss of taste quality. The conversion of stored sugars to starch or the hydrolysis of starch to sugars are two processes that also alter the sugar and acid content and the sugar-to-acid ratio in tissues. Sweet peas and sweet corn are valued for their sweetness, and that is directly related to their sugar content. The conversion of sugars to starch in these commodities is controlled by rapid cooling and holding at 0 °C. Genetic modifications that reduce the activity of the enzymes responsible for the sugar to starch conversion have produced lines of corn (e.g. Zea mays L., cv. Sugar Queen) that maintain their sweetness even when stored at room temperature for a number of days.

The contrasting conversion of starch to sugar is a hallmark of fruit ripening and imparts sweetness to many fruit that accumulate starch during their growth and development (e.g. apples). However, this conversion is not always desirable. Potatoes undergo 'sweetening' which is characterized by the conversion of starch to simple sugars when they are stored near 0 °C. When cooked at high temperatures (e.g. frying), these sugars spontaneously react with other components of the cell (e.g. proteins and carbohydrates) in the Maillard reaction to produce unwanted changes in flavor, odor and pigmentation. Dark-colored potato chips and 'French' fries are the result of such reactions during the frying of sweetened potatoes. The content of this 'excess' sugar can be reduced by holding the potatoes at 10 °C for a week so the sugar can be metabolized or re-polymerized.

In addition to changes produced by adverse storage conditions, many normal metabolic reactions produce a variety of deleterious activated oxygen species, such as superoxide radicals, singlet oxygen, hydrogen peroxide and hydroxyl radicals. Although the generation of activated oxygen species is a common event during growth and development, their increased production in response to abiotic stresses - such as chilling, heat, drought, pollutants and ultraviolet radiation - can overwhelm the cell's detoxifying capacity.

Plant cells detoxify activated oxygen species with both enzymes and antioxidants. Superoxide radicals are detoxified by the enzyme superoxide dismutase and hydrogen peroxide is destroyed by the enzyme catalase and different kinds of peroxidases (e.g. guaiacol peroxidase). A major hydrogen peroxide-detoxifying system in plants is the ascorbate-glutathione cycle which includes ascorbate peroxidase and glutathione reductase. The synthesis of lipid- and water-soluble antioxidants - such as ascorbic acid, glutathione, a-tocopherol, flavonols, carotenoids, reduced glutathione and phenolic compounds (chlorogenic, isochlorogenic, caffeoltartartic and dicaffeoltartartic acids) - is part of a complex mechanism that involves both restricting the production of activated oxygen species and protection from the activated oxygen species produced. This protection extends beyond the plant to the consumer. The beneficial effect of eating a diet rich in fruits and vegetables has been partially attributed to the increased consumption of phenolic compounds with antioxidant properties. These compounds reduce the oxidative damage that has been linked to arteriosclerosis, brain disorders and cancer.

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