Growth and spoilage characteristics of Chryseobacterium species in milk

[摘要] English: 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 –flavours. 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. fluorescensshowed 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 activationenergy 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 inoculatedsamples. 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 showeddistinct differences. In addition to the 'smelly feet, 'blue cheese and 'agar odours described for all the inoculated samples, C. joostei also produced 'putrid and 'sour odours while Ps. fluorescens produced odours described as 'fruity, 'nutty and 'bitter.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.

[发布机构] University of the Free State