Biotechnology to Improve Shelf Life and Quality Traits of Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 321

mechanical damage and optimizing storage temperatures, humidity, and gaseous
atmosphere (O 2 , CO 2 , ethylene). Packaging techniques, food additives, or special
treatments, such as irradiation, may also be utilized. The optimum treatment and
storage conditions vary with products. Furthermore, extended shelf life products can
be generated by traditional transformation technologies such as microbial fermen-
tations and lately by molecular biology techniques. 2,20
Quality of raw and processed fruit and vegetable products are sometimes elusive
factors and may differ from person to person based on individual tastes. However,
there is no doubt that consumers desire high-quality foods in a fast-moving society.
Consumers demand convenient, fresh, light, and nutritious products for their diets.
Fruit and vegetables fit in a healthy diet. The food chain related to fresh and processed
fruits and vegetables must economically deliver high-quality products to consumers. 20
E XTENDING S HELF L IFE
According to estimations by the U.S. Department of Agriculture, nearly half of the
fresh fruits and vegetables harvested annually is lost due to spoilage; such losses
may be much higher in developing countries. This spoilage is mainly due to the
formation of ethylene which triggers fruit ripening. 12 In order to prevent or delay
fruit ripening, sequestrants of ethylene are used, or fruits are harvested well before
they ripen on the plant. Both ways have their disadvantages; early harvest, as a rule,
results in an unpleasant taste and sequestering ethylene may involve the use of
chemicals and increase the price of the fruit. 12 When a tomato is bruised or wounded,
certain genes responsible for ethylene biosynthesis such as LE-ACS2 and LEACS4
(from ACC synthase family) and pTOM13 (from ACC oxidase family), are turned
on while others are turned off. As much as 30% of the tomato crop is lost annually
from bruising during harvesting, shipping, and storage. 17 Ethylene, a plant hormone
that causes aging, is produced when a tomato is bruised and preliminary research
has indicated that it may influence gene expression. 17
Thus, possibilities have been explored aiming at modifying the ethylene forma-
tion or content in plants and fruits. 12 Table 10.3 shows several strategies used in
tomato for the inhibition of ethylene formation. 12,21 Ethylene is formed from S-ade-
nosylmethionine (SAM) via the intermediate 1-aminocyclopropane-1-carboxylic
acid (ACC). The formation of ACC is catalyzed by the enzyme ACC synthase. The
second step leading to ethylene formation is catalyzed by the ethylene forming
enzyme (EFE) or ACC oxidase. The genes encoding the ACC synthase have been
cloned from tomato and squash, 22,23 and those encoding ACC oxidase have been
cloned from tomato. 24 Transgenic tomato plants have been produced showing a
highly reduced level of EFE. This was due to the expression of a chimeric gene
which was under the control of the constitutive 35S CaMV promoter encoding the
anti-sense RNA of ACC oxidase. 12 The anti-sense fruit never ripens naturally; they
have no aroma and do not turn red or soft. But when exogenous ethylene is added
to reverse the inhibition, the fruit becomes indistinguishable from naturally ripened
fruits with respect to texture, color, aroma, and compressibility. 24 Another approach
to inhibit the formation of ethylene was followed by Klee and co-workers from the
Monsanto Company. They identified a bacterial gene encoding an enzyme able to