Prepared by
Nam Sun Wang
Department of Chemical & Biomolecular Engineering
University of Maryland
College Park, MD 20742-2111

Table of Contents


To demonstrate the use of microorganisms in food processing by using yogurt as an example.


Actually this experiment has already been performed. One may have noticed in Experiment No. 1 that mushy substance formed during the prolonged precuring process in cheese manufacturing in which the natural action of lactose fermenting culture originally resident in butter milk was utilized to acidify milk. Of course, this custard-textured substance was none other than yogurt, sometimes spelled yoghurt or yoghourt.

Other than cheese, buttermilk, and yogurt, lactic starter cultures are also used to help prepare or manufacture a wide variety of food products such as sour dough bread, pickles, and sausages. As implied by the name "lactic cultures," they belong to a category of microorganisms that can digest the milk sugar lactose and convert it into lactic acid. For the cells to utilize lactose, deriving carbon and energy from it, they must also possess the enzymes needed to break lactose into two components sugars: glucose and galactose. Some representative strains are Streptococcus lactis, S. cremoris, thermophilus, Lactobacillus bulgaricus, L. acidophilus, and L. plantarum. These cultures can be purchased directly from local health food and drug stores in tablet form. These tablets, taken orally during the intake of dairy products, help those people who have digestive tract disorder and cannot tolerate lactose. The major steps involved in a large scale production of lactic starter cultures are the following: media preparation (constitution, mixing, straining, sterilization), inoculum preparation, fermentation, cell concentration by centrifugation, liquid nitrogen freezing, and packaging.

In summary, commercial yogurt production is composed of the following steps: pretreatment of milk (standardization, fortification, lactose hydrolysis), homogenization, heat treatment, cooling to incubation temperature, inoculation with starter, fermentation, cooling, post-fermentation treatment (flavoring, fruit addition, pasteurization), refrigeration/freezing, and packaging. For set yogurt, the packaging into individual containers is carried out before fermentation. In addition to the above steps, the starter culture is propagated in parallel. Although a batch process is followed in this illustrative experiment, the commercial production of yogurt is carried out in an automated continuous fermentation process. A good strain of starter culture not only affects the flavor and aroma, it can also speed up the process and thus reduces the effective equipment cost.

List of Reagents and Instruments

A. Equipment

B. Reagents


  1. Heat 1 liter (approximately 1 quart) of milk in a beaker slowly to 85 ºC and maintain at that temperature for 2 minutes. This step kills undesirable contaminant microorganisms. It also denaturizes inhibitory enzymes that retard the subsequent yogurt fermentation. If you are attempting this procedure at home with a sauce pan, use caution so as not to allow the milk to boil over and make a mess on your kitchen stove. See Note 1.
  2. Cool milk in a cold water bath to 42-44 ºC. The cooling process should take about 15 minutes.
  3. Add 5 g of starter culture to the cooled milk and mix with a glass rod. See Note 2.
  4. Cover the container to minimize the possibility of contamination. Incubate at 42ºC for 3 to 6 hours undisturbed until the desired custard consistency is reached. Yogurt is set when the mixture stops flowing as the container is tipped slowly. Fluid yogurt results if the mixture is stirred as the coagulum is being formed. See Note 3.
  5. The fresh made yogurt is ready for consumption when it is set. However, you may want to refrigerate it first if you are not accustomed to warm yogurt. Refrigeration also stops the growth of the lactic acid culture, which is thermophilic. (Thermophilic cultures grow best at high temperatures.) See Note 4.
  6. Use of Lactobacillus acidophilus: Grind 4 yogurt tablets (about 1 g) into fine powder. Repeat Steps 3-5.
  7. For entrepreneurs or simply hungry/thrifty students: You can recycle a small part of the finished product as the starter culture for the next batch. Theoretically, you can multiply or maintain your supply of yogurt indefinitely. However, in actuality, extended recycling is not recommended because the composition of the mixed culture will gradually deviate from the ideal one, and hence the flavor.


  1. Any type of milk may be used. Use nonfat or lowfat milk you are watching your fat intake. For example, one cup of nonfat dry milk powder dissolved in one liter of hot water may be convenient. The consistency and the flavor of the final product depend on the type of milk used. You may experiment at home to find your favorite recipe.
  2. The yogurt in a local market usually contains an active culture. Thus, if a starter culture is not readily available, it can be easily derived from plain store-bought yogurt. In this case, a few teaspoonfuls of the store-bought yogurt will adequately act as as the starter culture. (Make sure the label on the package indicates that it indeed contains an active culture.) The culture in fresh yogurt is healthier and more active than that in an outdated one. A stale one is also more likely to be contaminated with undesirable microorganisms, so check the expiration date. If possible, choose the "All-Natural" variety, because stabilizers and additives, included to suppress microbial activities, are generally harmful to the culture. If one is making yogurt at home, it is more convenient to pour the mixture into smaller containers before incubation; drinking glasses are just about the right serving size. Seal the glasses with a lid or plastic food wrap. Place all the glasses in a baking pan for easy handling.
  3. At home, a household electric or gas oven is an ideal substitute for the incubator. The middle shelf, slightly away from the direct heat, usually gives the most even temperature. The temperature can be controlled better if a pan of warm water is placed on the bottom rack.
  4. You may add your favorite fruits, fruit preserves, puree, jam, or sweeteners to enhance the taste, or you may add equal part of water to make a yogurt drink. Many types of yogurt differ mainly in the post-incubation processing. For example, the yogurt may be frozen, spray-dried or freeze-dried, carbonated, or concentrated.


Yogurt originated in the Balkans and the Middle East; it is now quite popular in Europe and America, as well. The microorganisms used in the production of yogurt accomplish two tasks: production of lactic acid and flavor components. The secret to tasty yogurt is in the proper control of the temperature at various stages. If the temperature is too low, the culture grows too slowly to adequately acidify milk and to achieve a good texture. The commercial starter is a mixed culture of thermophilus and L. bulgaricus. The culture is killed if the temperature is too high. In addition, there is a subtle difference in the taste because the formation and secretion of metabolites which contribute to the overall taste are dependent on the growth rate. The window of proper fermentation is quite small, i.e. from 42 ºC to 44 ºC. In general, as the temperature is raised up to 44 ºC, the rate of culture metabolism is higher, and the yogurt is sweeter. Faster growth also prompts the yogurt to set faster. When the desired acidity is reached, yogurt is quickly cooled to halt further fermentation and metabolic activity. This cooling step is quite critical in industrial yogurt production; it must be done quickly to control tightly the acidity of the yogurt, which has a profound effect on the taste.


  1. Compare the texture and taste of yogurt made from different sources of starter cultures. Also compare your "homemade" yogurt to commercial brands.
  2. What was the cost of "homemade" yogurt? Compare this to the market price of a comparable item. Did you make a profit?
  3. The yogurt starter culture is a mixed population of S. thermophilus and L. bulgaricus, both competing for the common substrate lactose. How does the principle of competitive exclusion apply here?
  4. Can the human intestinal tract be infested with lactic acid cultures? If yes, why has this method not been employed to to treat lactose-intolerant consumers who cannot intake dairy products without the usual gastro-intestinal discomfort? If no, what make these strains different from, for example, Escherichia coli, normal flora of digestive tract?
  5. Why is the shelf life for unpasteurized yogurt longer than that for pasteurized milk?
  6. Comment on ways to improve the experiment.


  1. Tamime, A. Y. and Deeth, H. C. Yogurt: technology and biochemistry, J. Food Protection, 43, 939, 1980.
  2. Driessen, F. M., Ubbels, J., and Stadhouders, J., Continuous manufacture of yogurt. I. Optimal conditions and kinetics of the prefermentation process, Biotech. Bioeng., 19, 821, 1977.

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Yogurt Fermentation with Lactobacillus Cultures
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Nam Sun Wang
Department of Chemical & Biomolecular Engineering
University of Maryland
College Park, MD 20742-2111
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e-mail: nsw@umd.edu