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18.1 Fruits 847

18.1.4 Ripening as Influenced by Chemical

Agents

18.1.4.1 Ethylene

Fruit ripening is coupled with ethylene biosynthesis:

Table 18.35. Ethylene production in ripening fruits

Fruit

Ethylene

(µg/l)

Climacteric maximum Avocado 500

Banana 40

Mango 3

Pear 40

Tomato 27

Nonclimacteric Lemon 0.2–0.2

stationary state Orange 0.1–0.3

Pineapple 0.2–0.4

Climacteric and nonclimacteric fruits respond

differently to external ethylene (Fig. 18.10).

Depending on the ethylene level, the respiratory

increase sets in earlier in unripe climacteric

fruits, but its height is not influenced. In contrast,

in nonclimacteric fruits there is an increase in

respiration rate at each ripening stage which is

clearly dependent on ethylene concentration.

The reaction pathway 18.45 is suggested for the

biosynthesis of ethylene (R–CHO: pyridoxal

phosphate; Ad: adenosine).

Ethylene and compounds capable of releasing

ethylene under suitable conditions are utilized

commercially for enhancing the ripening process.

A number of such compounds are known,

e. g., 2-chloroethylphosphonic acid (ethephon;

R=HorCH 2 –CH 2 Cl) (Formula 18.46).

(18.45)

Ethylene increases rapidly but differently in the

case of climacteric fruits. The maximum values

for some fruits are given in Table 18.35. However,

nonclimacteric fruits produce only a little

ethylene (Table 18.35). This gaseous compound

increases membrane permeability and thereby

probably accelerates metabolism and fruit ripening.

With mango fruits, for example, it has been

demonstrated that before the climacteric stage,

ethylene stimulates oxidative and hydrolytic

enzymes (catalase, peroxidase and amylase) and

inactivates inhibitors of these enzymes.

(18.46)

The use of ethylene before picking fruit (as with

pineapples, figs, mangoes, melons, cantaloups

and tomatoes) results in faster and more uniform

ripening. Its utilization after harvesting accelerates

ripening (e. g., with bananas, citrus fruits

and mangoes). Ethylene can induce blossoming

in the pineapple plant and facilitate detachment

of stone fruits and olives. Vine defoliation can

also be achieved.

The activity of propylene is only 1% of that of

ethylene. Acetylene also accelerates ripening but

only at substantially higher concentrations.

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