Food Chemistry

18.1 Fruits 847

18.1.4 Ripening as Influenced by Chemical
Agents

18.1.4.1 Ethylene

Fruit ripening is coupled with ethylene biosynthe-
sis:

(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. How-
ever, nonclimacteric fruits produce only a little
ethylene (Table 18.35). This gaseous compound
increases membrane permeability and thereby
probably accelerates metabolism and fruit ripen-
ing. 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.

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).

(18.46)

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 acceler-

ates 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.