Foreword
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This standard replaces GB 4789.2-2016 National food safety standard - Food microbiological examination: Aerobic plate count
The following main changes have been made with respect to GB 4789.2-2016:
——Annex B is added;
——equipment and materials are changed;
——culture media and reagents are changed;
——the test procedure is changed;
——the operation steps are changed;
——Annex A is modified.
National food safety standard
Food microbiological examination: Aerobic plate count
1 Scope
This standard specifies the determination method for aerobic plate count.
This standard is applicable to the determination of aerobic plate count in food.
2 Terms and definitions
2.1
aerobic plate count
total number of colonies formed in each g(mL) of food samples processed and cultured under certain conditions (such as culture medium, culture temperature, culture time, etc.)
3 Equipment and materials
In addition to the conventional sterilization and culture equipment in microbiological laboratory, other equipment and materials include:
a) constant temperature incubator: 36℃ ± 1℃, 30℃ ± 1℃
b) refrigerator: 2℃~5℃
c) thermostatic apparatus: 48℃±2℃
d) scale: 0.1g
e) homogenizer
f) oscillator
g) sterile pipette: 1mL (with a scale of 0.01mL), 10mL (with a scale of 0.1mL) or micropipettor and sucker
f) sterile conical flask: with volume of 250mL or 500mL
i) sterile Petri dish: with a diameter of 90mm
j) pH meter or pH colorimetric tube or precision pH test paper
k) magnifier or/and colony counter
4 Culture media and reagents
4.1 Plate count agar culture media, see A.1.
4.2 Aerobic count plate, which shall meet the quality control requirements of plate count agar media in GB 4789.28, and the main nutritional ingredients shall be consistent with the formula of plate count agar medium.
4.3 Sterile phosphate buffer, see A.2.
4.4 Sterile saline water, see A.3.
5 Test procedure
For the test procedure of aerobic plate count, see Figure 1.
Figure 1 Test procedure of aerobic plate count
6 Operation steps
6.1 Dilution of test sample
6.1.1 Solid and semi-solid samples: weigh 25 g samples and put them in a sterile homogenization cup filled with 225 mL sterile phosphate buffer or sterile physiological saline, homogenize them at 8000r/min ~ 10000r/min for 1min ~ 2min, or put them in a sterile homogenization bag filled with 225 mL diluent, and beat them with a beating homogenizer for 1min ~ 2min to make a 1:10 sample homogenate.
6.1.2 Liquid sample: pipette 25mL sample with an sterile pipette and put it into an sterile conical flask (with appropriate quantity of aseptic glass beads placed in advance) containing 225mL sterile phosphate buffer or sterile physiological saline, shake well, or put them in a sterile homogenization bag filled with 225 mL diluent, and beat them with a beating homogenizer for 1min ~ 2min to make a 1:10 sample homogenate. When the aerobic plate count in each gram of sample is required, follow 6.1.1.
6.1.3 Pipet 1mL 1:10 homogeneous sample solution with a 1mL aseptic pipette or a micropipettor, slowly inject it into an sterile test tube (note that the pipette or sucker shall not contact with the diluent) containing 9mL diluent along the tube wall, oscillate the test tube make a 1:10 sample.
6.1.4 Follow 6.1.3 to prepare 10-fold serial dilution sample homogenate. For each incremental dilution, use 1mL sterile pipette or sucker.
6.1.5 Select 1 ~ 3 sample homogenates with appropriate dilutions (liquid samples may include stock solution) according to the estimation of sample contamination, and pipet 1 mL of sample homogenates into sterile Petri dishes, and make two Petri dishes for each dilution. Besides, pipet 1 mL of blank diluent into two sterile Petri dishes as blank control.
6.1.6 Pour 15ml ~ 20ml of plate count agar media cooled to 46℃ ~ 50℃ (which may be kept in a constant temperature device at 48℃±2℃) into the culture dish in time, and rotate the culture dish to mix it evenly.
6.2 Culture
6.2.1 Keep the agar still, and after it is solidified, turn the plate over and culture at 36℃±1℃ for 48 h±2 h. For aquatic products, culture at 30℃±1℃ for 72 h±3 h. If the sample may contain colonies spreading and growing on the surface of agar media, cover a thin layer of plate count agar media (about 4 mL) on the surface of the solidified agar medium, and turn the plate over after the medium is solidified.
6.2.2 If aerobic count plate is used, it shall be used according to the relevant technical regulations provided.
Foreword I
1 Scope
2 Terms and definitions
3 Equipment and materials
4 Culture media and reagents
5 Test procedure
6 Operation steps
7 Results and report
Annex A Culture media and reagents
Annex B Examples
