season
As expected we found that rocket produced in warmer field condition in summer contained higher bacterial loads compared to the rocket produced in winter (Table 3.5). Similar observations were reported by other researchers (Caponigro et al., 2010;Garrido et al., 2015). Garrido et al. (2015) reported that higher counts of psychrotroph bacteria in baby spinach were found in samples grown in spring (April, mean temperature 13.4
oC) compared to baby spinach grown in winter (January, mean temperature 10.2 oC) in
Spain. They also found that leaves harvested in the evening of spring crop contained higher number of bacteria, but this phenomenon was not observed in winter crop. However, this was only true for psychrotrophs and not with Pseudomonas. Similarly, Caponigro et al. (2010) reported that bacterial loads in several species of ready-to-eat salads (lettuce, rocket, spinach and lamb’s lettuce) produced in summer and autumn were higher than those produced in winter and spring. They reported that E. coli were found
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in 27% of the samples. No salmonella or Listeria monocytogenes was found. Overall, total viable bacterial counts varied from 5.01x101 cfu.g-1 in December to 7.94x103 cfu.g- 1 in April. Lactic acid bacteria (LAB) were not detected in December but the counts
increased to 7.94x105 cfu.g-1 in July and September (Capgnigro et al., 2010). Overall, the
counts reported in their study were lower compared to our data. This could be due to difference in handling procedures which related to distance from farm to retailers. The study conducted by Capgnigro et al. (2010), was carried out in Italy using locally grown fresh produce whereas in our case, rocket leaves imported from Italy were used. Long distance transportation which would take a longer time allows bacteria to proliferate.
The increase in bacterial counts reported by previous researchers were similar to the findings in this thesis, but at this point of time our results were based on total plate counts of a pool of bacteria without being sorted by different species or genera of bacteria. A more detailed analysis of the genera and species of bacteria found in fresh RTE rocket is be reported and discussed in Chapter 5.
Results presented in Table 3.5 clearly show that leaves in both seasons subjected to high (25 oC) and moderate (13 oC) temperatures encouraged the growth of bacteria resulting higher bacterial loads as seen in the samples HSSDUD, HSSDUD+2, NPDUD and NPDUD+2. Brief exposure of the rocket to high temperature followed by long term storage at appropriate cold temperature keeps the bacterial loads low. This result implies that temperature abuse by consumer during transit before being stored under domestic refrigeration would not risk the increase in bacteria loads in rocket. In contrast, temperature abuse by supply could be dangerous if the leaves contain pathogenic bacteria
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because after exposing the produce at higher temperature, the leaves will be displayed at moderate temperature (13 oC) that would not stop the bacteria from growing.
The variation in bacterial counts normally correlated well with gas composition sampled from the headspace of the bags. The significance of the gas composition in bagged fresh RTE produce and its relationship with bacterial loads, VOCs and its quality is well acknowledged. Generally, after long storage, especially at an abused temperature, the production of O2 will be reduced and at the same time the production of CO2 will be
elevated (Table 3.5; Deza-Durand et al., 2014). Differences in growing environment as experienced by crops grown in different seasons, markedly affected all gas composition measured, CO2, O2 and ethylene. Summer grown rocket was found to have a lower CO2
and ethylene concentrations, but have a higher O2 in their head space than those in the
bags of winter rockets. The results recorded here seem to be surprising if we relate them to the overall bacterial loads. High overall CO2 and low O2 are actually associated with
high concentration of the gases in four out of 11 treatments, i.e. in HSSDUD, HSSDUD+2, NPDUD and NPDUD+2 of winter rocket. In these four treatments, at a certain stage, the leaves were exposed to high (25 oC, HSSDUD, HSSDUD+2) and moderate temperature (13 oC, NPDUD, NPDUD+2). High CO2 of the leaves in these
treatments would attributable to high respiration suggesting that the winter grown crops that have been acclimatized to low field temperature might have poor adaptability to relatively high temperature during handling and storage phases, causing high respiration rates (Amodio et al., 2015).
In some treatments of winter rocket, the O2 level have dropped to a very low level that
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followed by storage at moderate temperature (13 oC) (HSSDUD, HSSDUD+2) and the samples that continuously stored at 13 oC (NPDUD, NPDUD+2). The results indicate that
the condition in the respective bags could have reached an anaerobic condition, thus affecting the aroma of rocket following the generation of off-flavour volatiles (Deza- Durand et al., 2014; Section 3.5.3).
The release of ethylene from rocket leaves was higher at higher storage temperature, but this response normally can only be observed in the first days in storage (Koukounaras et al., 2007). Koukounaras et al. also reported that, during storage, the concentration of ethylene was found to peak on day one. Similar results were obtained in our study whereby the highest ethylene concentration were recorded in control samples, as well as in gas in bags containing leaves of treatments HSS24 and HSC24, regardless of growing seasons. It is generally known that ethylene is linked to the loss of chlorophyll resulting in the senescence of leaves (Able et al., 2003). Even though the concentration of ethylene was higher in the above treatments, our results did not show any marked differences in chlorophyll index of the leaves across treatments. In the study, measurement of chlorophyll was performed immediately after the treatments ended (within 24 h) and therefore there could be insufficient time for the chlorophyll to degrade.
3.5.5 Handling and storage condition in the supply chain affected the occurrence