AOAC Official Method 989
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EE79F8E661934803A0AE87DF9E29DB32 |
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0.03 |
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2 |
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日期： |
2024-7-30 |
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17.3.03,AOAC Official Method 989.10,Bacterial and Coliform,Counts in Dairy Products,Dry Rehydratable Film Methods,(Petrifilm?Aerobic Count Plate?and,Petrifilm?Coliform Count Plate?) Methods,First Action 1989,Final Action 1991,Results of the interlaboratory study supporting the acceptance of the,method:,Aerobic Count,Chocolate milk:,sr = 0.102; sR = 0.177; RSDr = 4.3%; RSDR = 7.5%,Cheese:,sr = 0.113; sR = 0.117; RSDr = 3.6%; RSDR = 3.7%,Nonfat dry milk:,sr = 0.151; sR = 0.230; RSDr = 4.5%; RSDR = 6.9%,Evaporated milk:,sr = 0.193; sr = 0.198; RSDr = 8.3%; RSDr = 8.5%,Ice cream:,sr = 0.180; sR = 0.222; RSDr = 6.9%; RSDR = 8.5%,Coliform Count,Chocolate milk:,sr = 0.164; sR = 0.257; RSDr = 9.2%; RSDR = 14.4%,Cheese:,sr = 0.221; sR = 0.225; RSDr = 10.4%; RSDR = 10.6%,Nonfat dry milk:,sr = 0.197; sR = 0.151; RSDr = 8.5%; RSDR = 4.5%,Evaporated milk:,sr = 0.200; sR = 0.225; RSDr = 13.0%; RSDR = 13.0%,Ice cream:,sr = 0.081; sR = 0.131; RSDr = 4.1%; RSDR = 6.6%,A. Principle,Method uses bacterial culture plates of dry medium and cold,H2O-soluble gel. Undiluted or diluted test suspensions are added to,plates at a rate of 1.0 mL per plate. Pressure, when applied to plastic,spreader placed on overlay film, spreads test portion over ca 20 sq cm,growth area. Gelling agent is allowed to solidify and plates are incubated,and then counted. Pipet, plate loop continuous pipetting syringe,or automatic pipet can be used for test suspension addition for bacterial,count analyses.,B. Apparatus and Reagent,(a) Petrifilm Aerobic Count Plates.—Plates contain standard methods,media nutrients, 940.36A(g) (see 17.1.02), cold H2O-soluble gelling,agent coated onto film base, overlay film coated with gelling agent, and,2,3,5-triphenyltetrazolium chloride indicator. Circular growth area of,single plate contains ca twenty 1 cm squares outlined on film base.,Petrifilm Aerobic Count Plates (Microbiology Products, 3M Center,Bldg. 275-5W-05, St. Paul,MN55144,USA)meet these specifications.,(b) Petrifilm Coliform Count Plates.—Plates contain violet red bile,nutrients conforming to APHA standards as given in Compendium of,Methods for the Microbiological Examination of Foods, 3rd Ed., 1990,American Public Health Association, Washington, DC, USA, cold,H2O-soluble gelling agent, and 2,3,5,-triphenyltetrazolium chloride indicator.,Petrifilm Coliform Count Plates (Microbiology Products, 3M,Center) meet these specifications.,(c) Plastic spreader.—Provided with Petrifilm plates, consists of recessed,side and smooth flat side, designed to spread test portion evenly,over plate growth area.,(d) Pipets.—Calibrated for bacteriological use, or plate loop continuous,pipetting syringe to deliver 1.0 mL. Automatic pipet to deliver,1.0 mL may be used.,(e) Colony counter.—Standard apparatus, Quebec model preferred,or one providing equivalent magnification and visibility.,(f) Dilution water.—See 940.36A(a) (see 17.1.02).,C. Preparation of Test Suspensions,(a) For total plate counts.—Aseptically prepare 1:10 dilution,(11 g/99 mLdilutionH2O). Mix well and plate. Prepare additional dilutions,as required. Ordinarily, 1:10 and 1:100 dilutions are sufficient.,(b) For coliform counts.—(1) Cream, half-and-half, condensed,milk, egg nog, cottage cheese, butter, margarine, and related products.—,Make 1:5 dilution (24.75 g/99 mL dilution H2O). Mix well and,plate 1mLon each of 2 plates. Multiply total of counts on 2 plates by 2.5,to obtain count/g.,(2) Sour cream, dips, and yogurt.—Proceed as in (1) except after dilution,adjust pH to 6.6-7.2 with 1.0M NaOH (ca 0.1 mL/g test portion).,(3) Buttermilk.—Make 1:10 dilution (11 g/99 mL dilution H2O).,Adjust pH to 6.6-7.2 with 1.0M NaOH (ca 0.1 mL/g test portion). Mix,well and plate 1 mL on each of 2 plates. Multiply total of counts on,2 plates by 5 to obtain count/g.,(4) Ice cream, sherbert, and mixes.—Hydrate dry-film plates,with 1 mL sterile dilution H2O and allow at least 1 h for gel to solidify.,Then, lift top film of prehydrated dry-film coliform count plate,(gel will adhere to top film) and dispense 0.5 mL of 2:3 homogenate,(10 g/5 mL dilution H2O) onto bottom film of each of 3 plates. Replace,top film gently over test suspension. Add counts on the 3 plates,to obtain count/g. Alternatively, plate one plate and multiply result,by 3 to obtain count/g.,(5) Cheese.—Proceed as in (1). Do not use citrate buffer to homogenize,test portion.,(6) Chocolate milk.—Proceed as in (1).,D. Analysis,(a) Bacterial colony count.—Use dry-film aerobic count plates.,Place plate on flat surface. Lift top film and inoculate 1mL test suspension,onto center of film base. Carefully roll top film down onto,inoculum. Distribut……
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