The total colony count refers to the total number of bacterial colonies contained in 1 g or 1 ml of the sample obtained after the food has been processed and incubated under certain conditions. The total number of colonies does not represent the total number of bacteria actually present in the sample. The total number of colonies does not distinguish between the types of bacteria present in the sample, so it is sometimes referred to as the number of stray bacteria, the number of aerobic bacteria, and so on. Plate culture counts can be used to determine the total number of colonies in a food product.
Operation method
plate counting method
Principle
Total colony count refers to the total number of bacterial colonies contained in 1 g or 1 ml of the sample obtained after the food has been processed and incubated under certain conditions. The colony count is mainly used as an indication of the degree of contamination of the food, and it can also be applied to observe the dynamics of bacterial reproduction in the food, so as to provide a basis for hygienic evaluation of the sample being examined. The total number of colonies does not indicate the total number of bacteria actually present in the sample, and the total number of colonies cannot distinguish the types of bacteria, so it is sometimes called the number of stray bacteria, aerobic bacteria and so on.
Materials and Instruments
Agar medium Food samples Move I. Inspection procedures Caveat 1. Plate culture counting method can only detect the growth of live bacteria, but cannot detect the total number of bacteria in the sample, which is always less than the actual number of bacteria surviving in the food, this is because there are many kinds of bacteria in the food, and their life characteristics are different, so it is not possible to grow all of them under the uniform culture conditions. However, it can still be used to assess the degree of bacterial contamination of the whole food, so at present this method is commonly used in the hygienic inspection of general food. 2. Plate colony count to determine the total number of colonies in the food, generally are used in the medium temperature culture, especially has been directly for food made food, because of these food hygiene requirements, is to strictly prevent the contamination of gastrointestinal infectious disease pathogens and food poisoning pathogens, these pathogens belong to the thermophilic bacteria, and thus the determination of the number of bacteria, the use of medium-temperature culture is more reasonable. For more product details, please visit Aladdin Scientific website.
Ethanol Saline Sodium hydroxide solution
Electrothermal incubator Refrigerator Constant temperature water bath Tray balance Electric stove Straws Wide-mouth bottles Triangular bottles Glass beads Petri dishes Test tube holders Alcohol lamp Homogenizer Mantle Sterilizer knife Scissors Sterilizer tweezers Alcohol cotton balls Glass crayons Registration film
See Figure 5-1 for total colony count test procedures
II. Sample dilution and culture
1. In aseptic operation, the test sample 25 g (or 25 mL) after cutting, placed in a sterilized glass bottle containing 225 mL of sterilized saline or other diluent (pre-positioned in the appropriate number of glass beads in the bottle) or sterilized mantle, after sufficient shaking or grinding into a uniform dilution of 1:10. After adding the diluent to the solid sample, it is best to put in a sterilized homogenizer at 8,000 r/min-1,0000 r/mln speed for 1 min to make a uniform dilution of 1:10.
2. Use a 1 mL sterilized pipette to suck up 1 mL of 1:10 dilution solution, and inject it into a test tube with 9 mL of sterilized saline or other diluent (note that the tip of the pipette should not touch the diluent in the tube, the same hereinafter), and shake the test tube to mix it well and make it into a 1:100 dilution solution.
3. Take another 1 mL sterilized pipette and make 10-fold incremental dilutions according to the sequence of operations in the above item, so that for each incremental dilution, i.e., replace it with a 1 mL sterilized pipette.
4. According to the requirements of food hygiene standards or the estimation of the contamination of the test samples, select 2-3 suitable dilutions, respectively, in the 10-fold incremental dilution at the same time, that is, to absorb the dilution of the pipette to move 1mL of dilution in a sterilized dish, each dilution for two dishes.
5. Dilution solution into the petri dish, should be promptly cooled to 46 ℃ nutrient agar medium [can be placed in (45 soil 1) ℃ water bath insulation into the petri dish 15 mI a 20 mL, and rotate the petri dish bit mixed well, at the same time, the nutrient agar medium poured into the addition of 1 mL of dilution solution (without samples) in the sterilized petri dish as a blank control.
6. After the agar solidified, turn the plate, put (36 soil 1) ℃ constant temperature box incubation (48 soil 2) h take out the number of colonies in the plate, multiplied by the dilution times, that is, I g (1 mL) samples contain the total number of colonies.
Third, colony calculation method
1. Selection of plate colony numbers
Plates with colony counts between 30 and 300 were selected as standards for colony count determination. Two plates were used for one dilution, and the average of the two plates was selected. One of the plate has a larger piece of colony growth. It is not appropriate to use, and should be no piece of colony growth of the plate count as the dilution of the number of colonies. If the flaky colonies less than half of the plate, and the remaining half of the colony distribution 2 fB uniform, can be calculated after half of the plate multiplied by 2 U on behalf of the number of colonies in the whole dish.
2. Selection of dilution
(1) A dilution with an average colony count between 30 and 300 should be selected and reported by multiplying the dilution multiplicity (see Table 5-2, Example 1).
(2) If there are two dilutions, the number of colonies grown is between 30 and 300, depending on how the ratio of the two is determined. If the ratio is small, the average should be reported: if the ratio is greater than 2, the smaller number is reported (see Table 5-2, Example 2, Example 3).
(3) If the average colony count for all dilutions is greater than 300, it should be reported as the average colony count for the highest dilution multiplied by the number of dilutions (see Table 5-2, Example 4).
(4) If the average number of colonies at all dilutions is less than 30, it should be reported as the average number of colonies at the lowest dilution multiplied by the number of dilutions (see Example 5 of Table 5-2).
(5) If there is no colony growth at all dilutions, report it as less than 1 times the lowest dilution [see Table 5-2, Example 6].
(6) If the average number of colonies for all dilutions is not between 30 and 300, and a portion of them is greater than 300 or less than 30, the average number of colonies near 30 or 300 is reported multiplied by the number of dilutions (see Table 5-2, Example 7).
3. Reporting of colony counts
The number of colonies in 100 to just t1 according to its own number of reports; greater than 100, with two valid numbers, in the two valid numbers after the number of digits, rounding method of calculation. In order to shorten the number of 0's behind the number, can be expressed as an index of 10, see Table 5-2 "Reporting method" column. 