2009_Book_FoodChemistry

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.