Question

5. Remember, you can change this test to target other bacteria!!!! Here is a hypothetical scenario: You work in the food industry. You make a neutral pH juice, like carrot juice. You supply this fresh to the consumer so there is no heating to kill microbes present.

You wash and sanitize the carrots prior to juicing so microbe levels will be low initially. Let’s say you want to know the MPN of homolactic and heterolactic fermenting, lactic acid bacteria (review the differences from lecture).

How would you design your experiment? Remember, in the coliform MPN there is lactose supplemented in…that sugar won’t help you here. So how would you change the media and how would you interpret the results? The same MPN table would be used. Just make the assumption that only lactic acid bacteria are present in your carrot juice.

Answer 1

Most probable number(MPN) is a method used to estimate the concentration of viable microorganisms in a sample by means of replicate liquid broth growth in ten-fold dilutions.

Here is the method to test MPN of coliforms in carrot juice.

Materials.

5 Double strength lactose Durham tubes,Micropipettes,Tips,Carrot juice.

Day 1;

1.Carrot juice samples in your lab bench.

2.Each part of the test have 5 tubes (5 for 0.01ml,5 for 0.01ml,and 5 for 1.0ml).

3.Incubate the tubes at 37degree for 24 hrs.

Day 2;

Count the positive tubes you have the value on the table corresponds to your tube count. This will be an estimate of Coliforms per/Ml of carrot juice.

1.Yellow tubes indicates lactose fermentation .

2.Gas in the tube indicates gas production.

3.Tubes that are yellow and have gas present is indicative for E-coli.

4.This is used as a Persumptive positive for the presence of E-coli in carrot juice.

How to report this data

You report put all the results with yellow tubes +bubbles as premitive E-coli. Then you do a count of all yellow tubes and report that out as coliforms(with or without bubbles,count them all).You count all yellow tubes for second part because E-coli are still coliforms(but not all coliforms are E-coli).