Determination of lactic acid bacteria in yogurt
Determination of lactic acid bacteria in yogurt
Active yogurt requires control of the ratio of various lactic acid bacteria, which have complex nutritional requirements, requiring carbohydrates, amino acids, peptides, fatty acids, esters, nucleic acid derivatives, vitamins and minerals for growth. When determining lactic acid bacteria it is necessary to try to detect all the live lactic acid bacteria in the specimen. Satisfactory results can be obtained by using the dilution plate colony counting method to detect various lactic acid bacteria in yogurt. The purpose of this experiment is to understand the principle of isolation of lactic acid bacteria in yogurt and to master the detection method of lactic acid bacteria count in yogurt.
Operation method
Dilution plate colony counting method
Principle
Active yogurt requires control of the proportion of various lactic acid bacteria, and some countries use the content of live lactic acid bacteria as a basis for differentiating product varieties and quality. Since lactic acid bacteria have complex nutritional requirements, growth requires carbohydrates, amino acids, peptides, fatty acids, esters, nucleic acid derivatives, vitamins and minerals, etc., and it is difficult for general broth media to meet their requirements. The determination of lactic acid bacteria must try to detect all live lactic acid bacteria in the sample. To improve the detection rate, the key is to choose a specific good culture medium. Using the dilution plate colony counting method, the detection of various lactic acid bacteria in yogurt can obtain satisfactory results.
Materials and Instruments
Yogurt MRS medium Modified CHALMERS medium M17 medium Move I. Sample Dilution Caveat 1. Indicator color developmentBecause the acid-producing colonies can make CaCO3 around the dissolution circle, acid-base indicator should be acidic color reaction. 2. Mirroring morphologyIf necessary, different forms of colonies can be picked up to determine the microscopic examination of Lactobacillus or Streptococcus lactis. Lactobacillus bulgaricus is rod-shaped, into a single rod, or double bacilli or long filamentous. Streptococcus thermophilus is spherical, in pairs, or in short or long chains. 3. Reference comparisonIt is reported that the detection rate of modified CHALMERS medium is higher than that of MRS medium, and M17 medium is more suitable for the cultivation of Lactococcus lactis, and more than one medium can be used at the same time for comparison during testing. For more product details, please visit Aladdin Scientific website.
Water
Pipettes with glass delta bottles Test tubes Petri dishes Vortex homogenizers Thermostats
The yogurt sample was first stirred well, and 25 ml of the sample was pipetted into a triangular bottle containing 225 ml of sterile water, and shaken well on a vortex homogenizer to ensure that the sample was evenly dispersed, i.e., a 10:1 dilution of the sample, and then diluted to the appropriate dilution according to the estimation of the bacterial content of the sample.
II. Plate-making
Choose 2~3 suitable dilutions, petri dishes labeled with the appropriate label, respectively, aspirated dilutions of different dilutions of 1 ml placed in the petri dish, each dilution for two replicates. Then pour the petri dish with MRS or modified CHALMERS medium which was dissolved and cooled to about 46 ℃, rotate the petri dish quickly to make the mixture homogeneous, and then cool it down to form a plate.
III. Culturing and counting
The petri dishes were inverted and placed in 40 ℃ constant temperature box for 24~48 h. Observe the growth of fine colonies, count the number of colonies, and select 30~300 colonies petri dishes for calculation according to the conventional method.