Produce Degradation Pathways and Prevention

142 Produce Degradation: Reaction Pathways and their Prevention

[RRm] = maximum respiration rate (mmol·kg–1·h–1)
Kmapp = apparent Michaelis constant (% O 2 )
PO 2 = effective oxygen transmission coefficient or permeance (mL·m–2·24 h–1·atm–1)

t = time (h)
a = a coefficient 2.24
b = a coefficient 0.4 E-4
V = volume of the container (L)
m = mass of commodity (kg)
A = surface area of packaging over which diffusion can occur (m^2 )

The quantity of carbon dioxide produced by plant tissues (RRCO 2 ) is equal to
that of consumed oxygen multiplied by the respiratory quotient (RQ).
After a certain storage time (equilibration phase), the respiration of plant tissue
will balance diffusive exchanges through the film and Equation (1) can be simplified:

(5.2)

where xs = O 2 concentration at steady-state (%).

(5.3)

where
RR 0 = maximum respiration rate of the plant tissue at 0°C (mmol·kg–1·h–1)
Q10R=Q 10 of respiration
T = temperature (°C)

Combining Equation 5.2 and Equation 5.3 gives Equation 5.4:

(5.4)

This equation shows that, at equilibrium, oxygen concentration does not depend on
the headspace or on the initial gas concentration. The injection of gas when closing
the package or the punnet (actively modified atmosphere) only permits the steady-
state to be attained more rapidly, thereby shortening or avoiding the detrimental
equilibration phase when the produce is sensitive to enzymatic browning or lipid
peroxidation.
In Equation 5.4, xs is the optimal O 2 concentration previously determined by
controlled atmosphere studies and PO 2 is the permeance to O 2 of the film that will
give xs at equilibrium and at temperature T [142]. It is therefore possible to calculate
the permeance of the film at 23°C (usual temperature for measurements of perme-
ability and permeance of packaging films):

a

RRm x

Km x

m

V

b

PA

V

xx

s

app s

O

· es

·

·

·

()

+

=^2 −

RRm RR Q R

T

= 010 ·^10

P

a

b

mRR Q

Ax x

x

xKm

O

T

es

s

s app

2

01010

=

− +

·

··

()

·