dc.description.abstract |
This study investigated the potential for use of electronic noses (e-noses) for early
rapid detection and differentiation of bread spoilage before visible signs of growth
occur. After 24 h incubation at 25°C it was possible to distinguish Penicillium
verrucosum, Aspergillus ochraceus, and Pichia anomala from 3 different species of
filamentous fungi before visible growth was observed on unmodified wheat agar using
a conducting polymer based e-nose (BH114). Discrimination of controls was possible
after 48 h. The BH114 e-nose was able to differentiate between Pseudomonas fragi,
Saccharomyces cerevisiae and P. verrucosum growing on 0.97 aw modified flour-based
media after only 24 h. The BH114 e-nose was able to discriminate between P. fragi
and S. cerevisiae growing in broth cultures and between different aw controls in
exponential growth (13.5 h). Discrimination of Staphylococcus aureus growing in
different aw broths and from uninoculated controls was achieved after 4.5 h.
The BH114 e-nose was also able to detect and differentiate microbial spoilage in situ
using bread analogues. Discrimination was improved using an incubation temperature
of 25°C when compared to 15°C. Discrimination of microbial and physiological
(enzymic) spoilage of bread analogues was possible using e-nose technology, cfu
counts and gas chromatograph-mass spectrometry (GC-MS) using an initial population
of 106 spores/cells ml-1. After 48 h differentiation of the spoilage types and between
some of the microbial spoilage organisms was possible using the e-nose. A significant
increase in populations was noted between 24 and 48 h. There were significant
differences between microbial populations detected after 48 and 72 h. Analysis of
volatile compounds produced, using GC-MS, showed that after 24 h P. anomala was
the only treatment to produce 2-propanol, ethyl acetate, and pentanol. P. anomala also
produced greater amounts of 3-methylbutanol when compared to P. verrucosum, B.
subtilis, lipoxygenase spoilage and controls.
Differentiation between toxigenic and non-toxigenic strains of Aspergillus parasiticus
in vitro was not achieved. However, in vitro on unmodified 2% wheat agar it was
possible to differentiate a non-toxigenic P. verrucosum strain from 4 citrinin producing
strains and controls using the BH114 e-nose. On bread analogues it was possible to discriminate two ochratoxin A (OTA) producing P. verrucosum strains after 24 h using
an initial population of 106 spore ml-1. Increased incubation resulted in only controls
being discriminated. Using a lower initial population of 103 spores ml-1 only controls
were discriminated after 24 h. However, after 48 h an OTA producing strain could also
be differentiated.
The potential for use of e-noses as a tool for screening novel antioxidants was also
investigated. It was possible to differentiate between broth samples with and without
the antioxidants propyl paraben and butylated hydroxyanisole using both the
conductance based e-nose (BH114) and a metal oxide and metal ion based e-nose
(NST3220 lab emission analyser). When samples without antioxidant were removed it
was possible to differentiate treatments containing antioxidant that had been inoculated
with micro-organism and those that had not. The e-noses were also able to
discriminate between sample times. Microbial populations and carbon dioxide levels
increased with incubation time. P. verrucosum and P. anomala populations were
greater in treatments without antioxidant whereas B. subtilis populations were greater
in 0.97 aw treatments containing antioxidant. CO2 production was greater in inoculated
treatments without an antioxidant except at 0.95 aw P. verrucosum produced greater
volumes in the presence of the antioxidant.
Using natural bread cross validation studies of 4 unknown contaminants (P. anomala,
P. verrucosum and B. subtilis and controls) was performed. This showed that using
initial populations of 103 spores/cells ml-1 the BH114 e-nose was able to differentiate
between all the unknown treatments after 48 h and the NST3220 lab emission analyser
after 72 h. CO2 production could be used to detect controls but it was not possible to
differentiate between the micro-organisms. |
en_UK |