Influence of vapours on the electrical properties of ceramic and polymer films, in relation to rapid detection of fruit and vegetable rots.
When foodstuffs are subject to microbial infection a range of volatile organic compounds
(VOCs) are released which can be indicative of both the type and severity of the infection. The
bacterium Erwinia carotovora, the primary cause of soft rot, is a major problem in the bulk
storage of potato tubers. A number of classes of VOCs have been identified above E. carotovora
infected potato tubers, but no disease specific marker has been identified. A number of studies
have concluded that the best marker of E. carotovora infection is a substantial increase in the
concentration of VOCs in the headspace above the tubers. Chemical sensors which are sensitive
to low levels of the VOCs identified in the headspace above infected tubers have been developed.
The aim was to use these sensors as the basis of a system for the early detection of soft rot in
stored potato tubers.
The sensors developed fall into two main categories: those which required heating to elevated
temperatures, and those which were operated at ambient temperatures. The sensors operated at
ambient temperatures included composites of tin dioxide and chemically prepared polypyrroles.
The composites exhibited a high sensitivity to a range of organic vapours (1-100 vpm) and were
more sensitive than either tin dioxide or polypyrrole at room temperature. Composites of
chemically prepared polypyrroles with various thermoplastics were fabricated and were found to
exhibit a high sensitivity to a range of volatile amines. Further studies incorporated chemically
prepared polypyrroles into a printing ink vehicle, and sensors constructed from these films
displayed good sensitivity, high stability and high mechanical strength.
The sensors operated at elevated temperatures included a range of evaporated tin oxide films
doped with Pt, CuO and ZnO, plus a range of thick film sensors based on tin dioxide, zinc oxide
and mixtures of the two materials. The thick film sensors exhibited the highest sensitivity to the
vapours of interest and also gave superior reproducibility of fabrication when compared to the
sensors based on evaporated thin films. A synergistic effect appeared to be in operation where tin
dioxide and zinc oxide were mixed, with sensors incorporating composites of the two materials
exhibiting higher sensitivities than either tin dioxide or zinc oxide alone. A GC-MS study to
elucidate the surface reactions occuring on exposure to the vapour, suggested that the synergistic
effect was in part due to differences in the catalytic activities/pathways of the two materials.
A prototype device was produced based on two evaporated tin dioxide film sensors and one
thick film tin dioxidelzinc oxide (50/50 mlm) sensor. The device was tested to various quantities
of sound tubers with an infected tuber added. The prototype device was capable of detecting one
infected tuber amongst 100kg of sound tubers in a simulated storage crate.