How to Determine Acidity in Yogurt

Objective

To determine the titratable acidity of yogurt  

Background

Yogurt is a fermentation product made from milk and bacteria. Bacteria may sound gross, but not all bacteria are bad. Some are good for you. Good bacteria live naturally in our gut and are essential for our health. The bacteria in yogurt are good bacteria. If you look at a yogurt container in the ingredients section, you will see “live bacteria culture”. These bacteria are generally Lactobacillus bulgaricus, Streptococcus thermophilus and Lactobacillus acidophilus. They work together to convert the lactose sugar in milk to produce lactic acid and flavors. This lactic acid in turn cause the proteins in the milk to coagulate to form a thick gel. The production of lactic acid is essential for texture, taste and shelf life, as acidic environments inhibit pathogen growth.  In this lab, you will determine the titratable acidity of various commercial yogurt samples. Titratable acidity is an approximation of the total acidity in a substance. It determines how much of a base (NaOH) is required to neutralize an acid. The base, also known as the titrant, is of known concentration. In this experiment, a color indicator will be used to determine the end point of the acid-base reaction. The indicator is phenolphthalein. Phenolphthalein is colorless in acid and turns pink in basic solutions. Therefore, when the yogurt turns pink, it means that all of the acid has been neutralized. You can then apply the following formula to calculate the total acidity.  

Where, N = normality of titrant; 90 = Equivalent weight for lactic acid

Materials

1. Three containers of plain yogurt representing different commercial brands
2. 1 N sodium hydroxide
3. Phenolphthalein indicator
4. Scale (Capacity = 0-100 g)
5. Burette
6. Magnetic stir plate
7. Magnetic stir bar

Method

1. Fill the burette with 1 N NaOH
2. Collect 15 g of yogurt sample in a 250 ml beaker (in duplicate)
3. Add approximately 4 drops of phenolphthalein indicator
4. Titrate by adding titrant (NaOH) to the sample gradually with continuous mixing on stir plate until the solution changes from colorless to a persistent light pink.
5. Record the amount of titrant that was needed to neutralize the lactic acid
6. Calculate titratable acidity

Lab Questions

1. Create a flow diagram showing how yogurt is made (5 points)
2. Report the titratable acidity values of the yogurt samples (3 points)
3. Based on the process you have outlined above, explain what difference in production conditions may have resulted in differences in titratable acidity among the samples (10 points)
4. Identify a common procedural error that usually occurs during titration tests and indicates how it can be prevented. (2 points)

Reference: Nielsen, S. S. Food Analysis. (2003). New York, NY: Springer Science+Business Media, LLC.