Acidification of Milk

Page 1

Figure 1

Introduction There are two main types of bacteria in milk: Lactic acid bacteria: Normally present in milk and also used as starter culture in the production of cultured dairy products such as cheese and yogurt. Coliforms: Facultative anaerobes which optimally grow at 37°C. Coliforms are indicator organisms; they are closely associated with the presence of pathogenic bacteria. They can cause rapid spoilage of milk, because they are able to degrade milk proteins and ferment lactose, producing acid and gas. In this experiment we follow the pH changes in milk over 30 hours of incubation.

Equipment 

einstein™Tablet with MiLAB or Android /IOS Tablet with MiLAB and einstein™LabMate

1 liter thermos (with an opening that enables a good seal including the pH cable)


Equipment Setup

| Acidification of Milk | Note: This is a long experiment so ensure that the USB cable is connected to the einstein™ LabMate™ for the duration of the experiment, or USB cable is connected to a charging cable. 1.

Launch MiLAB (


Connect the pH electrode and the Temperature Sensor to ports of the einstein™ Tablet or einstein™ LabMate. Assemble the equipment as illustrated in Figure 1. In the Current Setup Summary window choose Full Setup and use the table below to set up the experiment. Make sure that only the pH Sensor and Temperature Sensor are selected under Measurements.

3. 4.


Current Setup Summary Program the sensors to log data according to the following setup: pH Rate:

Every 1 min


2000 sec

Temperature (-40°C to 140°C) Rate:

Every 1 min


2000 sec

Procedure 1. 2. 3.

Warm 750 ml of pasteurized milk and bring it to room temperature. Pour the milk into the thermos. Insert a pH electrode in the milk and cover the thermos gently to prevent any damage to the pH cable.


Tap Run (


After 30 hours of incubation, you can stop data logging by Tapping Stop (


Save your data by tapping Save (

) to begin recording data. ).


Data Analysis 1. 2. 3.

For more information on working with graphs see: Working with Graphs in MiLAB Use the cursors to mark the start and end of the experiment. Determine the extent of change in pH measured in the process. Calculate the extent of change in pH obtained in the process. Use the cursors to mark the start and end of the experiment. Apply a linear fit to the difference graph: a. Use the cursors to mark the start and end of the experiment. b. Select Linear fit. The fit equation will be displayed below the x-axis. c. The slope of the fit line is the rate of pH change.


| Acidification of Milk |


An example of the graph obtained in this experiment is shown below:

Figure 2

Questions 1. 2. 3. 4. 5. 6.

What causes the decrease in the pH of the milk? Did you observe a decrease in the pH immediately after starting the incubation? What could be the reason for this phenomenon? Does the rate of pH change remain constant during the whole incubation period? Why do you think this is? Why was it necessary to warm the milk and bring it to room temperature before transferring it to the thermos? What would be the results of incubating unpasteurized milk in the thermos for 30 hours?

Further Suggestions 1. 2.

Perform a similar experiment with unpasteurized milk or milk from different species (e.g. goat milk). Keep the thermos at different temperatures (in the refrigerator or an incubator).


| Acidification of Milk |