Chryseobacterium species have been reported to cause spoilage in food products
such as fish, meat, poultry and dairy products. Some of these Chryseobacterium
species can grow at temperatures below 7 °C which gives them the ability to grow
and cause spoilage in milk kept at refrigerated temperatures. Protease and lipase
enzymes produced by psychrotolerant species in milk are responsible for spoilage by
the production of off-odours and âflavours. The aim of this study was to determine
the growth and spoilage characteristics of Chryseobacterium species in milk and
compare the results to those of Pseudomonas fluorescens which is regarded as the
major organisms causing spoilage in milk.
The specific growth rates and cardinal temperatures for C. joostei, C. bovis and Ps.
fluorescens were determined by optical density measurements. Chryseobacterium
joostei had the highest maximum specific growth rate followed by Ps. fluorescens
and C. bovis. All three organisms were able to grow at 4 °C, but Ps. fluorescens
showed the highest growth rate temperatures below 7 °C. All three organisms can
thus be classified as psychrotolerant mesophiles due to the fact that growth was
observed at 4 °C with optimum temperatures ranging between 25 and 30 °C.
Arrhenius plots for the three species showed that C. joostei had the lowest activation
energy followed by Ps. fluorescens and C. bovis. This means that C. joostei was the
least sensitive to temperature changes that may take place while C. bovis was the
most sensitive of the three tested organisms. The protease activity per mg protein
for the three organisms was determined with the azocasein method.
Chryseobacterium joostei showed the highest activity followed by C. bovis while Ps.
fluorescens showed significantly lower activity than the Chryseobacterium species.
All three species showed protease activity at 4 °C.
Determination of the lipolytic activity of C. joostei and Ps. fluorescens were done by
measuring the amount of FFAs present in fat-free and full cream milk incubated
either at 4 or 25 °C. High levels of lipolysis were observed for all the inoculated
samples. Although differences between the different sample treatments were
observed, these differences were not significant enough to differentiate between the spoilage potential of C. joostei and Ps. fluorescens in the different milk samples.
Secondary lipid oxidation of the two organisms was determined with the TBA
method. Although inoculated milk samples demonstrated higher levels of oxidation
compared to non-inoculated samples, there were no signigicant differences between
any of the inoculated milk samples.
Sensory analysis was done on the samples and C. joostei produced stronger odours
than Ps. fluorescens. All the samples that were inoculated with C. joostei scored
significantly higher spoilage scores than the control samples as well as the Ps.
fluorescens samples. The odours described for the two organisms also showed
distinct differences. In addition to the âsmelly feetâ, âblue cheeseâ and âagarâ odours
described for all the inoculated samples, C. joostei also produced âputridâ and âsourâ
odours while Ps. fluorescens produced odours described as âfruityâ, ânuttyâ and
âbitterâ.
The volatile compounds produced by C. joostei and Ps. fluorescens were identified
with headspace SPME-GC/MS analysis. Ketones, alcohols and fatty acids were the
main compounds produced. Ketones are responsible for the production of fruity and
floral odours. Fatty acids produce sour odours, with isovaleric acid being responsible
for the smelly feat odour. Alcohols do not usually contribute to the odour production
in milk. The results could not be used to differentiate between the two organisms in
the different milk samples.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-08142012-154107 |
| Date | 14 August 2012 |
| Creators | Bekker, Anneke |
| Contributors | Ms L Steyn, Prof PJ Jooste, Prof CJ Hugo |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-08142012-154107/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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