the operation method
Sample processing: Take 25 mL (g) of the sample into a sampling tank or homogenized cup containing 225 mL of sterile water, shake for 30 min, make a 1:10 sample homogenate, and draw 1:10 with a 1 mL sterile pipette. 1 mL of sample homogenate was injected into a test tube containing 9 mL of sterile water, repeatedly sucked 50 times to become a sample homogenate of 1:100, and so on, and a sample homogenate of 1:1000 dilution was made, one for each time. A straw.
inoculation: general food selection 2 to 3 dilutions for testing, liquid samples containing less bacteria (such as drinking pure water and mineral water, etc.) can directly absorb the original solution for testing. Place the mold yeast test piece on the flat test bench, uncover the upper film, and use a sterile pipette to draw 1 mL of the sample solution and slowly and evenly drop it onto the paper. Then slowly cover the upper film and let it stand for about 10 seconds. The medium is solidified. Two tablets were inoculated at each dilution while using sterile water as a blank negative control.
Cultivation: Put the test pieces together and put them back into the original ziplock bag. The transparent surface is placed horizontally in the constant temperature incubator, and the number of stacked pieces is not more than 12 pieces. The culture temperature is 28 ° C ± 1 ° C,
The culture time is 48 to 72 hours.
Counting principle and reporting method
counting
If the average number of colonies for all dilutions is greater than 150 CFU, the test piece with the highest dilution is counted and multiplied by the number of average colonies by the dilution factor (see Example 1).
If the average number of colonies for all dilutions is less than 10 CFU, the average number of colonies with the lowest dilution should be multiplied by the dilution factor (see Example 2).
If all dilutions (including liquid sample stocks) are grown aseptically, multiply by less than 1 by the lowest dilution factor (see Example 3).
Report
When the number of colonies is less than 100, the contract is rounded up according to the principle of “rounding off” and reported by two valid figures.
When the number of colonies is greater than or equal to 100, the first three digits are rounded off by the principle of “rounding off”, taking the first two digits, followed by 0 instead of digits, or by exponential form of 10, according to “rounding off”. After the principle is revised, two valid figures are used.
Case | Dilution and number of colonies | Selected count dilution | Reporting method(CFU/g or CFU/mL) | ||
10-1 | 10-2 | 10-3 | |||
1 2 3 | 多不可计 8 0 | 352 5 0 | 169 1 0 | 10-3 10-1 <1×10 | 1.7×10 5 8.0×10 <1×10 |
Interpretation of the results
The mold and yeast show blue spots on the test piece. The mold colonies show slightly larger spots and the yeast colonies are smaller and smoother. Many molds will have their own unique colors in the later stage of culture. The number of colonies was counted from 10 CFU to 150 CFU, and the average of the two test pieces was multiplied by the dilution factor to obtain the number of molds and yeasts per ml (gram) of the sample.