Produce Degradation Pathways and Prevention

Packaging and Produce Degradation 141

concluded that there was an inverse relationship between internal color and CO 2
concentration for a given film type. This relationship was confirmed by Lopez-
Briones et al. [140].
Green salads, especially fresh-cut ones, are highly sensitive to enzymatic brown-
ing. Lowering O 2 concentration to less than 2% will reduce discoloration. Polyphenol
oxidase is not active at O 2 concentrations below 1%, but this hypoxic condition may
trigger anaerobic catabolism. It is also possible to inject 5 to 10% CO 2 into the bag
at sealing (actively modified atmosphere). CO 2 inhibits polyphenoloxidase activity
and prevents discoloration during the equilibration phase of the passively modified
atmosphere. The optimal gas composition for the storage of lettuce depends on many
parameters, including the cultivar, season of growing, pedoclimatic conditions, etc.
A 2% O 2 and 5% CO 2 atmosphere may be recommended. Higher CO 2 concentrations
may be used for lettuce insensitive to russet spotting. [141]
Increasing CO 2 and/or decreasing O 2 concentration in the storage atmosphere
may have opposite effects on different quality attributes of the commodities. The
optimum gas composition will therefore depend on the market requirements.

5.3 PACKAGING CONDITIONS

5.3.1 MODIFIED ATMOSPHERE PACKAGING

Knowing the respiratory parameters of plant tissues (respiration rates RRO 2 and
RRCO 2 , respiratory quotient RQ, Q 10 of respiration, and apparent Km), the conditions
of packaging, and the optimal gas concentration for controlled atmosphere storage,
it is possible to determine the optimal gas transmission rate of packaging film (in
milliliters of O 2 or CO 2 per square meter per 24 h per atmosphere) with several
mathematical models that were extensively reviewed by Fonseca et al. [8]. Among
all available semiempirical models, those based on a Michaelis-Menten type equation
are the most commonly used [142]. CO 2 inhibition is also considered in some of
them, but for practical reasons the interference of CO 2 is most often skipped due to
the lack of available literature data and the limited effect of CO 2 variation that can
be tolerated by most plant tissues. As stated by Fonseca et al. [8], this simple
Michaelis-Menten model is a simplification that tends to fit the experimental data
well.
In all models gas diffusion through a film follows Fick’s laws, and oxygen
concentration within a pack is governed by the following equation:

(5.1)

where
x = concentration of oxygen within the pack at any given time t (%)
x 0 = initial concentration of oxygen (%)
xe= concentration of oxygen outside the pack (generally 21%)

xx a

RRm x

Km x

m

V

dt b

PA

V

xxdt

app

t

O

= − e

+

(^0) ∫ + −
00
· * 2 · ()
tt
∫