Produce Degradation Pathways and Prevention

124 Produce Degradation: Reaction Pathways and their Prevention

respiration rate. It is therefore possible to measure RR as a function of O 2 partial
pressure and to calculate the appKm. The respiration rate is represented in Figure
5.5 as a function of oxygen concentration in double reciprocal coordinates (1/RRO 2
= f(1/O 2 )). A linear regression, fitted to the experimental points, allows the calcula-
tion of the appKm. The appKm values of litchis at 20°C and tomatoes at 10°C are
very high (37 and 23.2% O 2 , respectively). Hertog et al. [28] reported the interme-
diary appKm values of 3.76% for apples and 2.7% for chicory chicons. Peppelenbos
et al. [29] found higher appKm for apples (6.26%) and similar values for tomatoes
and chicory chicons. However, modified atmospheres will not affect the respiration
rate of some plant tissues, such as mushroom and lettuce, because their appKm is
much lower than 0.1% [9]. Benkeblia et al. [10] claimed that the respiration of
onions did not follow the Michaelis-Menten equation because the appKm varied

FIGURE 5.4Changes in oxygen concentration in the respirometer storing litchis at 20°C as
a function of time (h). The experimental data and the polynomial fitting equation are super-
imposed. (From Varoquaux, P. et al., Fruits, 57, 313, 2002.)

FIGURE 5.5Apparent Michaelis constant (appKm in percentage of O 2 ) of litchi at 20°C and
0% CO 2. Lineweaver-Burk double reciprocal coordinates. (From Varoquaux, P. et al., Fruits,
57, 313, 2002.)

y = 2E-06x^3 + 0.0025x^2 − 0.4491x +19.828

R^2 = 0.9999

0

5

10

15

20

25

0204060

Time (h)

O

(kPa) 2

80

y = 13.795x + 1.7125

R^2 = 0.9985

0

2

4

6

8

10

12

14

16

18

20

−0.5 0 0.5 1 1.5

1/O 2

1/RR