A.Gashe Berhanu (PhD)Mahari Tadesse2018-07-112023-11-082018-07-112023-11-081988-06http://etd.aau.edu.et/handle/123456789/7973An attempt has been made in this study to isolate A identify thermoduric A psychropivilic bacteria, which are the most serious spoilage organisms, from milk obtained from tin? Addis Ababa Oairv Hovelpoment Enterprise (AADDE ). An effort was also made to trace the sources A origin of these organisms so as to prevent or at least reduce their access into milk A hence increase the shelf -life of the pasteurized milk A milk products. The standard plate count { SPG) ranged between 4 X 10? A 3 X 109 bactoria/ml for raw milk, between 6 X 10'* A 3 X 10^ for plant pasteurized milk A between 3 X 103 A 2.5 X for laboratory pasteurized milk. The high standard plate count for the plant pasteurized milk, as compared to that of the laboratory pasteurized milk, has been found to bo due to the ability of many organisms to gain access to the pasteurized milk as post pasteurization contaminants. Psychrophilic bacteria wore the most dominant organisms in both raw (98.1%) A plant pasteurized (53.0%) milk samples. Laboratory pasteurized milk, on the other hand, contained predominantly thermoduric bacte. During the course of study, bacteria belonging to 14 genera were isolated, the most dominant being: Streptococcus ( 27% ) , Staphylococcus (19,5%), micrococcus (13.9%) , Bacillus ( 12.9%) A Arthrobactor (11.3%),. Out of the 185 representative isolates identified 50% were cocci A 40% rods. As many as 16 species of bacteria belonging to 9 genera were isolated from raw milk, while 20 species of bacteria belonging to 14 gonora wore isolatcd from tho plant pasteurized milk. This indicated that bacteria , other than those found in raw milk have been introduced into the pasteurized milk as post pasteurization contaminants from the bulk tanks, fillers A their pipelines and the plastic bags that contain pasteurized milk. This fact has also been supported by data from keeping quality tests performed at different sampling sites. Sodium hydroxide ( dnOH ) A sodium carbonate ( NaC'03) , chemicals regularly used for rinsing A disinfecting utensils at the Addis Ababa hairy development Enterprise , were found to be ineffective in inactivating bacteria at concentrations as high as 1:100 at 50-90 minutes holding time. Other disinfectants tested such as Savlon , A Chlorox, on the other hand, were found to i> effective in eliminating the same group of bacteria at lower concentrations ( 3 :HOO ) at 15-30 minutes holding time. Based on this study , therefore, recommendations are made by which the Dairy Plant can improve milk handling A processing conditions A produce milk A milk products with improved shelf -life.enBiologyThesis