Packaging and Produce Degradation 121
described by Varoquaux et al. [9] and Benkeblia et al. [10] consists of two airtight
stainless steel, cylindrical vessels and a temperature-controlled bath with an accuracy
of ± 0.1°C. Plant tissues are placed in one of the vessels, the lid of which is tightly
secured. The head space of the whole instrument is flushed for 0.5 h (repeated after
CO 2 equilibration, if necessary) with a ternary gas mixture of preset composition.
Once the target atmosphere has been achieved, the vents are shut and the program
is run. Internal gas is pumped through a gas analyzer to measure CO 2 and O 2 partial
pressures. If this partial pressure is 0.1 kPa higher than the preset value of CO 2
partial pressure, the computer activates an electronic valve that directs the head space
atmosphere through a CO 2 trap filled with a 0.1 N sodium hydroxide solution until
the initial CO 2 concentration in the sample vessel is restored. The trapping of excess
CO 2 results in a proportional decrease in pressure, which is detected by a highly
sensitive differential pressure probe. The pressures in each vessel are then balanced
by the injection of pure O 2 through a mass flow meter into the vessel containing the
plant tissue samples. The CO 2 trapped in the sodium hydroxide is continuously
measured by conductivity. The computer logs the O 2 injection times and the changes
in conductance of the carbonated sodium hydroxide. At the end of a run, when the
O 2 consumption has been constant for at least 2 h, the computer calculates O 2 and
CO 2 respiration rates in millimoles per kilogram per hour. The instrument (like all
other respirometers) is not able to achieve headspace levels of 0% CO 2 , since the
minimum CO 2 partial pressure at steady-state is reached when the rate of CO 2
production by the plant tissue equilibrates with the CO 2 trapping rate. The minimum
CO 2 partial pressure that can be obtained is about 0.3 kPa in the normal configuration
of the instrument.
5.2.1.2 Influence of Temperature
The respiration rate of plant tissues increases exponentially with temperature. Gore’s
law (log RR is proportional to temperature), an approximation of Arrhenius’ law,
allows the calculation of Q 10 (Figure 5.2). Q 10 is the multiplication factor for the RR
FIGURE 5.2Changes in the natural log of RR (mmoles per kilogram per hour) as a function
of temperature in degrees Celsius in litchis. (From Varoquaux, P. et al., Fruits, 57, 313, 2002.)
log(RR) = 0.0317*T(°C) − 0.6441
R^2 = 0.9335−0.6−0.5−0.4−0.3−0.2−0.10.00.10 5 10 15 20 25Temperature (°C)log(RR)log RRO 2
log RRCO 2Q 10 = 1010*0,0317
Q 10 = 2.07