Browsing by Subject "cold storage"
Now showing items 1-2 of 2
-
(2016)The cold storage, utilized to increase the shelf life of raw milk, favors the growth of psychrotrophic bacteria. Psychrotrophs are considered as problematic because they produce heat-stable spoilage enzymes. In addition, some pathogenic bacteria are capable of growing at cold storage temperatures and it has been observed in previous studies that the psychrotrophs in raw milk can be resistant to several antibiotics. In the literature review section of this study, the microbial composition of raw milk and the effect of cold storage, nitrogen gas (N2) treatment and activation of the lactoperoxidase system (LPS) on the raw milk bacteria was reviewed; moreover, the evolution, spread and mechanisms of bacterial antibiotic resistance and the antibiotic resistance on dairy farms and of raw milk bacteria was reviewed. Also the methods to study raw milk bacteria and their antibiotic resistance were documented. The aim of the experimental part was to study the effects of cold storage, N2 gas treatment and LPS treatment on the levels of antibiotic resistant bacteria in raw milk. Untreated, N2-treated and LPS-treated raw milk samples were stored at 6 °C and the counts of bacteria resistant to gentamicin, ceftatzidime, levofloxacin and trimethoprim-sulfamethoxazole were determined in the beginning of the experiments and after three and seven days of cold storage. The antibiotic resistance and spoilage features of bacterial isolates selected from different plates were compared. In addition, DNA fingerprints of selected isolates were obtained by rep-PCR method and for a couple of isolates 16S rRNA gene partial sequencing was performed. The presence of certain antibiotic resistance genes for the isolates partially identified was investigated using a PCR-based method. The length of cold storage affected the proportions of antibiotic resistant bacteria in raw milk. The proportions of antibiotic resistant bacteria in the control milk sample were generally, with the exception of TS-resistant bacteria in some cases, at their lowest after seven days of cold storage when the milk was, though, microbiologically unacceptable. There was a lot of variation in the prevalence of bacterial antibiotic resistance between the milk samples treated differently. The proportions of resistant bacteria were on average clearly higher in LPS samples than in control samples, whereas the effect of the N2 treatment on the proportions of resistant bacteria varied between the experiments. The bacterial isolates resistant to all the considered antibiotics produced less frequently protease and phospholipase than the isolates resistant to fewer antibiotics. The isolates from LPS samples were more frequently resistant to all the antibiotics studied and produced less frequently protease and phospholipase than the isolates from control and N2-treated samples. On the basis of the rep-PCR fingerprints, there were both similar and different strains among the bacterial isolates. Based on the 16S rRNA partial gene sequence, two isolates were identified as Pseudomonas spp. and two isolates as Stenotrophomonas spp. The targeted antibiotic resistance genes were not detected among the considered isolates. Possible reasons can be variations in the gene sequences, suboptimal PCR conditions or that the isolates lacked the genes studied. A PCR product was obtained for one Pseudomonas isolate using mexA-F/R primers, but according to the database the gene sequence did not show homology with antibiotic resistance genes. Further analyses would be required to confirm if the gene studied is linked to antibiotic resistance.
-
(University of HelsinkiHelsingin yliopistoHelsingfors universitet, 2009)The quality of easily spoiling minced meat is regularly inspected by food control authorities. The quality of minced meat near the sell-by date has often been found to be poorer than could be wished for. In previous studies psychrotrophic lactic acid bacteria have been found to be significant spoilage bacteria in modified atmosphere packed meat but the spoilage bacteria of minced meat have not been studied. An industrial manufacturer delivered 20 packages of modified atmosphere packed minced meat which were studied on the sell-by day (±1 day). The concentrations of lactic acid bacteria and enterobacteria in the minced meat samples were quantified on MRS agar and VRBG agar respectively. The appearance and smell of raw minced meat was judged organoleptically. 349 lactic acid bacteria isolates were identified using ribotyping based on numerical analysis. The DNA of the lactic acid bacteria was isolated and digested with the restriction enzyme HindIII. The DNA fragments were separated by agarose gel electrophoresis and transferred to a nylon membrane where the fragments were hybridized with a labelled probe. Thus the ribopatterns of the isolates could be visualised on the membrane. The ribopatterns were compared with corresponding patterns in the research group's database which comprises the ribopatterns of over 7000 strains. Identification of the isolates is based on the locations of type strains in the clusters created in the numerical analysis according to similarity of ribotypes. The lactic acid bacteria counts in the minced meat samples were 1,1 × 107 – 4,9 × 108 cfu/g (on average 1,9 × 108 cfu/g). Enterobacterial counts were 9,0 × 102 – 9,0 × 104 cfu/g (on average 1,4 × 104 cfu/g). Observations in the sensory evaluation included grayness and off-odours of varying strength. The off-odours were described as rancid and buttery. The most common species of lactic acic bacteria were identified to be Leuconostoc gasicomitatum (58 % of lactic acid bacteria isolates) and Leuconostoc gelidum (20 %). Other species were Carnobacterium divergens (10 %), Carnobacterium maltaromaticum (4 %), Lactococcus spp. (3 %), Leuconostoc mesenteroides (2 %), Lactobacillus algidus (1 %), Lactobacillus sakei (1 %), and Leuconostoc carnosum (1 %). In addition to these, 15 isolates were identified as Brochothrix thermospacta which appeared to grow on MRS along with lactic acid bacteria. Lactic acid bacteria, especially L. gasicomitatum and L. gelidum, were recognised as the spoilage bacteria of industrially manufactured, modified atmosphere packed minced meat. These organisms have been shown to grow on modified atmosphere packed meat during cold storage and to be connected with the formation of sour and buttery off-odours. L. gasicomitatum and L. gelidum are not part of the natural microbiota of slaughter animals and therefore the significance of the production environment as their source should be examined.
Now showing items 1-2 of 2