The pH of milk changes when making yogurt due to the production of lactic acid by bacteria during fermentation. This process reduces the pH of milk from its natural value of around 6.5 to a range of 4.0 to 4.5, making the milk more acidic. The decrease in pH causes the casein proteins in milk to coagulate, leading to the formation of a gel-like structure characteristic of yogurt.
Lactic Acid Production
During the fermentation process, bacteria such as Lactobacillus acidophilus and Bifidobacterium bifidum convert lactose (milk sugar) into lactic acid. This increase in lactic acid concentration is the primary reason for the decrease in the pH of milk when making yogurt.
pH Decrease
The production of lactic acid reduces the pH of milk from its natural value of around 6.5 to a range of 4.0 to 4.5. This decrease in pH is essential for the coagulation of casein proteins and the formation of yogurt.
Casein Coagulation
When the pH of milk approaches the isoelectric point (IEP) of casein, the electrostatic repulsion between casein molecules decreases, allowing them to aggregate and form a gel-like structure. This coagulation is responsible for the thickening and curdling of milk into yogurt.
Reversibility
The clotting of milk can be reversed by increasing the pH, for example, by adding a base like caustic soda. This reverses the coagulation process, and the yogurt returns to a liquid state.
Importance of pH Control
Maintaining the correct pH range during fermentation is crucial for the quality and safety of yogurt. Incorrect pH levels can lead to discoloration, excessive free whey, and variations in tartness.
Factors Affecting pH Change
Several factors can influence the pH change during yogurt production, including:
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Starter Culture: The type and amount of bacteria used in the starter culture can affect the rate and extent of lactic acid production, and thus the final pH of the yogurt.
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Fermentation Time: Longer fermentation times generally result in a lower pH as more lactic acid is produced.
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Temperature: Higher fermentation temperatures can accelerate the production of lactic acid, leading to a faster pH decrease.
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Milk Composition: The composition of the milk, such as the protein, fat, and lactose content, can influence the buffering capacity and the rate of pH change.
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Additives: The addition of ingredients like stabilizers, sweeteners, or fruit can affect the pH of the final yogurt product.
By understanding the factors that influence the pH change during yogurt production, manufacturers can optimize the process to achieve the desired texture, flavor, and safety characteristics of the final product.
Conclusion
In summary, the pH of milk changes when making yogurt due to the production of lactic acid by bacteria during fermentation. This decrease in pH leads to the coagulation of casein proteins, resulting in the characteristic gel-like structure of yogurt. Maintaining the correct pH range is crucial for the quality and safety of the final yogurt product.
References:
- Byju’s. (n.d.). Fresh milk has a pH of 6. When it changes into curd yoghurt, will its pH value increase or decrease? Why? Retrieved from https://byjus.com/question-answer/fresh-milk-has-a-ph-of-6-when-it-changes-into-curd-yoghurt-will-its/
- Oregon State University Extension Service. (n.d.). Yogurt Made Simple. Retrieved from https://extension.oregonstate.edu/sites/default/files/documents/8836/fs173emakingyogurt.pdf
- Cole-Parmer. (2013, February 7). Food Quality: Why Measure the pH of Yogurt? Retrieved from https://www.coleparmer.com/blog/2013/02/07/food-quality-why-measure-the-ph-of-yogurt/
- Chemistry Stack Exchange. (2016, May 11). Why does milk clot while making yogurt? Retrieved from https://chemistry.stackexchange.com/questions/51022/why-does-milk-clot-while-making-yogurt
- NCBI. (2014, March 11). Conventional and Innovative Processing of Milk for Yogurt Manufacture; Development of Texture and Flavor: A Review. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5302305/.