The Science of Yogurt

By Ashton Yoon | July 4, 2017 10:00 am

Guest post by Earlene Mulyawan

Yogurt is an ancient food that has been around for several millennia. One theory of the discovery of yogurt is that during 10,000 – 5,000 BC, when Herdsmen began the practice of milking their animals, they stored their milk in bags made of the intestinal gut of the animals. The intestines contain natural enzymes that cause the milk to curdle and sour. The herdsmen noticed that this method of storing milk extends its shelf life and preserves it. When they consumed the fermented milk, they enjoyed it and so they continued making it. Whether or not this theory is true, the consumption of fermented milk has survived into modern times, and spread throughout the world. The word “yogurt” is believed to have come from the Turkish word “yogurmak,” which means to thicken coagulate, or curdle. Today, the FDA defines yogurt as a milk product fermented by two bacterial strains: a lactic acid producing bacteria: Lactobacillus bulgaricus and Streptococcus thermophiles.


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To make yogurt, milk is first heated to 180 degrees Fahrenheit for 30 minutes to denature the whey proteins; this allows the proteins to form a more stable gel. The yogurt is pasteurized before adding the cultures. The pasteurization process kills any pathogens that can spoil milk as well as to eliminate potential competitors of the active cultures. After milk pasteurization, the milk is cooled down to 108 degrees Fahrenheit, the temperature for optimal growth of yogurt starter cultures. Last, the yogurt starter cultures (probiotics) is added into the cooled milk and incubated until a pH of below 5 is obtained. This is called the fermentation process, whereby lactose in the milk is converted to lactic acid, which lowers the pH. When the pH drops below pH 5, micelles of caseins, which are amphiphilic proteins, loses its tertiary structure due to the protonation of its amino acid residues. The denatured casein proteins reassemble by interacting with other casein proteins, and these intermolecular interactions result in a network of molecules that provides the semisolid texture of yogurt (Zourari, Accolas, & Desmazeaud).

The fermentation process is also essential for the tangy flavor of yogurt: the production of lactic acid by Lactobacillus bulgaricus imparts a sour acidic and refreshing taste. A mixture of various carbonyl compounds like acetone, diacetyl and acetaldehyde are also major contributors to the tarty yogurt flavor. Yogurt has a high concentration of acetaldehyde due to the low utilization of this metabolite by yogurt bacteria, which lack alcohol dehydrogenase, the main enzyme needed to convert acetaldehyde into ethanol (Lees, G. J., and G. R. Jago). During fermentation, acetaldehyde is produced directly from lactose metabolism as a result of pyruvate decarboxylation. However, lactic acid bacteria also have alternative metabolic pathways that can produce acetaldehyde (Figure 1):


Figure 1: Pathways to acetaldehyde production (Chaves, A.C.S.D. et al. 2002)

Yogurt may provide potential health benefits by enhancing nutrient absorption and digestion (Fernandez). However, most commercialized yogurts that are sold in grocery stores contain added sugars, artificial ingredients and fillers, and only a marginal amount of probiotics. (Mercola). Homemade yogurt is an excellent way of getting probiotics into your system without the unnecessary chemicals and flavorings found in commercialized yogurts. All you need is a high quality starter culture and milk! Epicurious‘ step by step on how you can make yogurt at home can help you get started. Or you can check out The White Moustache, the website of Homa Dashtaki, a guest lecturer during the Spring 2017 Science and Food undergraduate course.




A Zourari, Jp Accolas, Mj Desmazeaud. Metabolism and biochemical characteristics of yogurt bacteria. A review. Le Lait, INRA Editions, 1992, 72 (1), pp.1-34.

Fernandez, M. A., and A. Marette. “Potential Health Benefits of Combining Yogurt and Fruits Based on Their Probiotic and Prebiotic Properties.” Advances in nutrition (Bethesda, Md.). U.S. National Library of Medicine, 17 Jan. 2017. Web. 03 July 2017.

Lees, G. J., and G. R. Jago. “Formation of acetaldehyde from threonine by lactic acid bacteria.” The Journal of dairy research. U.S. National Library of Medicine, Feb. 1976. Web. 11 Apr. 2017.

Mercola, Joseph. “Benefits of Homemade Yogurt Versus Commercial.” Organic Consumers Association. N.p., n.d. Web. 03 July 2017.

  • Not_that_anyone_cares, but…

    Any idea why they spell out ‘108 degrees Fahrenheit’ rather than using 108℉ ?

    • Uncle Al

      Stylistic. Separate the number from the units, 108 °F. This is supremely dysfunctional font for a science blog.

      • Amit Mitra

        “Scientific” blog? What kind of “Scientific” entity uses non metric nomenclature?
        On a different point just wondering how is Yogurt different from Curd?

        • Kathleen Dobek

          Curd is formed when rennet or lemon juice is added to milk, causing it to coagulate. Yogurt needs a culture of a bacterial strain added, causing it to ferment.

        • Bugiolacchi

          Ohh, Amit, it’s a lost battle. Even the international NASA website does not use metric as standard (sometimes they generously put the converted value in brackets). With former UK colonies turning their backs to this archaic system, like India, and joining the rest of the world to metrics (from Russia to China to Korea to Canada etc.) I wonder if in 20 years time the US is going to be the only country in the world with their ‘own’ measuring systems!

      • David White Dennis

        Heat milk to 180F in CLEAN pot to kill any pathogens…let milk cool by setting the pot of milk on a heating pad…After temperature has reduced to about luke warm to the touch (with clean hands), add store bought capsules of multi-strain Acidophilous (B. bifidum, L. salivarius etc) …stir and let sit over night resting on heating pad on LOW heat. The next morning you will have Yogurt. That’s how it’s done my friend.

  • Uncle Al

    Yogurt has a high concentration of acetaldehyde” Does it trigger reformed alcoholics?

  • WLU

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Science & Food brings you content on food and science including but not limited to: the scientific and culinary aspects of food that you eat; how knowledge of science and technology can be used to make better food; how science is integral to understanding the impact of food on our health and environment; as well as profiles of scientists and chefs that are advancing the frontiers of science and food.

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