Table 4 Mortality results of codling moth larvae in walnuts 20 wal-
nuts per run and five replicates Mortality of 3rd and
Treatments 4th-instars in walnuts
Control 43 9 2.5°C 2 min heating
78.6 53 9 3.6°C 3 min heating
100
would require approximately 24 h to achieve a probit 9 efficacy Soderstrom et al., 1996.
An accelerated shelf-life study ASLT was con- ducted to determine effect of the short time RF
treatment 53°C final temperature and 5 min holding time on walnut quality during long-term
storage. Detailed information on the theoretical basis for ASLT can be found in Taoukis et al.
1997. The RF-treated walnuts were held at 35°C for 10 and 20 days, equivalent to 1 and 2 years
storage at 4°C. The measured peroxide value and fatty acid values Table 5 suggest that the RF
treatments did not significantly affect walnut quality. It is, therefore, possible to develop a
HTST continuous process for walnuts and other similar commodities to satisfy the quarantine se-
curity
requirements of
the industry
against codling moth.
6. Need for furture research
Further research is needed to determine the most heat resistant life-stage of target insect pests
of economic importance and obtain the intrinsic kinetics information of this life-stage for develop-
ing treatment protocols. There is also need for studies to obtain fundamental kinetic information
regarding time – temperature effects on quality of commodities. This information would allow us to
establish a quality curve for each commodity e.g. in Fig. 3 and for calculating the z value or
activation energy for use in Eq. 8. Because of complicated nature of thermal degradation, there
may exist different z value for different tempera- ture ranges. Nevertheless, the kinetic information
for insect mortality and commodity quality can be used in Eq. 7 and Eq. 8 in combination with
One hundred in-shell walnuts, infested at one larva per walnut, were heated by RF energy in the
pilot-system to 43 or 53°C, and held in air for 5 min to impart desired lethality to the codling
moth before air cooling at 4°C. The results of the mortality tests are shown in Table 4. The 2 and 3
min RF treatments resulted in 78.6 and 100 mortality, respectively Table 4.
The standard deviations in the final tempera- tures of the walnut kernels suggested that some
walnuts were not heated to a level that would cause 100 mortality after 5 min holding time
e.g. 48°C after 3 min RF treatment. However, the actual kill at these temperatures far exceeded
the mortality rate estimated from the TDT curves Fig. 2. We speculate that this might have been
the result of possible preferential heating in codling moth larvae due to their much larger
dielectric loss factors at the RF frequency, as compared to that of walnuts Fig. 10. Nelson and
Payne 1982 reported 100 kill in the pecan weevil, Curculio caryae Horn, with 40 MHz RF
treatment of shelled and in-shell pecans when the kernel temperature was raised to 53°C in 15 s. A
hot air treatment at 45°C, on the other hand,
Table 5 Quality characteristics of in-shell walnuts treated by radio frequency energy 3 min
Storage time at 35°C day Peroxide value meqkg
Fatty acid RF treated
Control Control
RF treated 0.28 9 0.04
0.08 9 0.01 0.26 9 0.04
0.08 9 0.01 0.08 9 0.01
0.51 9 0.05 0.49 9 0.05
10 0.09 9 0.01
0.93 9 0.05 0.98 9 0.05
0.10 9 0.01 0.08 9 0.01
20
temperature – time history of different treatment methods to select optimized process conditions,
which will provide adequate thermal mortality of target insects and minimum impact on commodity
quality. Validation tests can then be conducted with infested commodities to confirm the efficacy of the
selected treatment protocols. An advantage of this approach will be two-fold: 1 the fundamental
kinetic information regarding insect mortality and quality should help gain general insights into the
physiological nature of the thermal death of the insect and the quality losses of commodities. Al-
though different insects or commodities may have different tolerance to heat, they should share sim-
ilar underlying mechanisms. Kinetic information on one specific insect or commodity may not be
directly applicable to others, but the fundamental trend should be similar; 2 The tests for insect
mortality and quality can be conducted indepen- dent of the final treatment protocols and the
procedures can be standardised to allow laborato- ries in different nations or regions to collaborate on
similar projects. Such a systematic approach should reduce redundant experimentation.
7. Summary
