Biochemistry of Fruits - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 19

Chapter 3

Biochemistry of Fruits

Gopinadhan Paliyath and Dennis P. Murr

3.1 Introduction

Several metabolic changes are initiated after the harvest of fruits and vegetables. In the
case of vegetables, harvesting induces stress responses through reduced availability of
water and nutrients, wounding and exposure to shelf life, enhancing storage methods such
as cooling. In most cases, these changes help the produce to enhance the shelf life. In the
case of fruits, an increase in the biosynthesis of the gaseous hormone ethylene serves as the
physiological signal for the initiation of the ripening process. In general, all plant tissues
produce a low, basal, level of ethylene. During the ripening process, some fruits evolve
large amounts of ethylene, sometimes referred to as an autocatalytic increase in ethylene
production, which occurs in conjunction with an increase in respiration referred to as the
respiratory climacteric. Fruits are generally classified into climacteric or nonclimacteric
types on the basis of the pattern of ethylene production and responsiveness to externally
added ethylene. The climacteric fruits characteristically show a marked enhancement in
ethylene production and respiration, as noticeable by the evolution of carbon dioxide. By
contrast, the nonclimacteric fruits emit a considerably reduced level of ethylene. (For a
list of fruits showing climacteric or nonclimacteric pattern of ripening, see Kays (1997),
General Reading.) In climacteric fruits such as apple, pear, banana, tomato, and avocado,
ethylene evolution can reach 30-500 ppm/(kg h) (parts per million, microliter/L), whereas
in nonclimacteric fruits such as orange, lemon, strawberry, and pineapple, ethylene levels
usually range from 0.1 to 0.5 ppm/(kg h) during ripening. Climacteric fruits respond to
external ethylene treatment by an early induction of the respiratory climacteric and accel-
erated ripening in a concentration-dependent manner. Nonclimacteric fruits, on the other
hand, show increased respiration in response to increased levels of ethylene concentration
without showing acceleration in the time required for ripening. Vegetables produce very
low amounts of ethylene most of them with less than 0.1 μ L/(kg h), with slightly higher

levels as in cassava (1.7 μ L/kg h), breadfruit (1.2 μ L/kg h), and cucumber (0.6 μ L/kg h)

when measured at 20-25 ◦ C.
In all plants, ethylene biosynthesis occurs through a common pathway that uses the
sulfur amino acid methionine as the precursor (Yang, 1981; Fluhr and Mattoo, 1996)
(Fig. 3.1). The first reaction of the pathway involves the conversion of methionine to S -
adenosyl methionine (SAM) mediated by the enzyme methionine adenosyl transferase.
SAM is in turn converted into 1-aminocyclopropane-1-carboxylic acid (ACC) by the en-
zyme ACC synthase. The sulfur moiety of methylthioribose generated during this reaction

19