Packaging and Produce Degradation 119
and amino acids. The metabolic pathways involved in aerobic respiration are glyc-
olysis, the tricarboxylic acid cycle (or Krebs cycle), and the electron transport system
[4]. Within the cells, the energy potential (ATP + 1/2 ADP)/(ATP + ADP + AMP)
remains constant even in the case of energy stress. When cells consume their
constitutive proteins and membrane lipids [5]. The damaged membranes will grad-
ually lose their active transfer properties and the cell will die. King and Morris [6]
reported that the aerobic catabolism of broccoli kept at 20°C results in a large loss
of sugars (up to 30%), organic acids, and even proteins during the first 6 h of storage.
After 12 h, the warning signs of energy stress appear (i.e., increase in the pool of
free amino acids such as glutamine and asparagine).
The potential shelf life of plant tissue after harvest is closely related to its
respiration rate (Figure 5.1). The simplified relationship shown in this figure takes
into account only spoilage of physiological origin. The shelf life of plant tissue may
be limited by other mechanisms such as dehydration or microbial proliferation.
Moreover, accurate assessment of the respiration rate and other respiratory
parameters of fruits and vegetables to be packed under modified atmosphere is
essential for the optimization of MAP systems.
FIGURE 5.1Relationship between the respiration rate of some vegetables and their potential
shelf life. (From Varoquaux, P., Les films à perméabilité aux gaz ajustable: Application aux
fruits et légumes, in Les Emballages Actifs, Gontard, N., Ed., Lavoisier, Tec & Doc, Paris,
2000, Chap. 5.7.)
Initial respiration rate at 15°CPotential shelf-lifePotatoCarrotGreen tomatoLettuce
Brussels
sproutsAsparagus
SpinachMushroom