Yogurt Making (Microbiology Term Paper)

Joe Albrecht

Term Paper

Dr. DeOndarza

             Yogurt has been used as a preferred dairy food for people across the globe for approximately 8,000 years (1).  While many view yogurt a simple go-to snack, there are significant bacterial processes involved in its production which mostly go unforeseen.  Analyzing the specific types of bacteria helps to determine how yogurt is made and why it possesses certain key properties. 

            Depending on which culture of bacteria is used, yogurt can have distinct flavors, textures and other noticeable characteristics.  For example, mesophilic or “medium-loving” cultures thrive best at room temperature (70°-77° F) and produce a thinner yogurt than that of thermophilic or “heat-loving” cultures (2).  Some examples of mesophilic bacteria include those dwelling within animals bodies (Enterobacter, Citrobacter, E. coli, Klebsiella…).  Thermophilic bacteria often thrive at temperatures between 122 and 140° F, and can be found in sun-lit soil as well as hot-springs.  These organisms have endospores (a protective coating which allows for survival in high heat conditions).  In addition, the preparation methods used effect the final outcome of how the yogurt will result.  If one uses the direct set cultures, they often are added to milk to produce a single batch of yogurt.  Some yogurt may be used as a starter for a new batch but after a few times, a new powdered starter must be used.  In contrast, heirloom cultures may be continually used.  In this process, a portion of the yogurt is conserved in each batch to make more yogurt (2). 

            The specific metabolic processes used by different bacteria create different effects in regards to human food consumption.  For instance, certain members of the Aspergillus species, notably, A. flavous and A. parasiticus, create aflatoxins in their metabolism.  This generally occurs in various grains, nuts and oil seeds.  The most prevalent and dangerous aflatoxin is known as AFB1 and is classified as a human carcinogen, or cancer-causing agent (3).  Livestock in the dairy industry can break down AFB1 in their GI-tract, creating a new aflatoxin, AFM1.  AFM1 has also been listed as a human carcinogen, thus requiring limitations of 0.5 micrograms per liter of dairy product in the U.S. (3).  The effects of microbial processes in regards to food are constantly analyzed to assess how adequate certain foods are for human consumption.  Some good news here is that other microbes can help negate of these negative effects.  Lactobacillus bulgaricus and Streptococcus thermophiles, for example, showed the ability to bind AFM1 at 90% and 70% respectively, during in-vitro analysis.  As a result, these bacteria are commonly used in yogurt production to remove AFM1 (3).  Exactly how these bacteria eliminate the AFM1 is unknown, however, it has been suggested that the toxin attaches via cell-wall components instead of being metabolically reduced (3). 

            Enzymes are essential components of yogurt production.  One key enzyme, known as β-galactosidase is a catalyst for reducing lactose to glucose and galactose.  After this takes place, glucose goes through a cycle known as glycolysis to yield pyruvate, a molecule that controls the rate of metabolism.  After this occurs, pyruvate undergoes lactate fermentation.  This results in the production of lactic acid, which accounts for yogurt’s composition and texture (4).  Some examples of lactic acid-fermenting bacteria include L. bacillus, L.  bulgaricus, L. plantarum, L.caret, L. pentoaceticus and L. brevis.  In terms of human consumption, yogurt has shown to be beneficial in promoting maintain adequate growth rates and strengthening the health of the gastro-intestinal tract.  Some evidence also implies that yogurt can reduce the prevalence of vaginal yeast infection caused by Candida (4).  An increased feeling of fullness (satiety) was noted by researchers at the University of Washington in Seattle among those who consumed a 200 calorie yogurt snack instead of various fruit and dairy snacks of the same caloric amount.  This is perhaps due to the high protein content of yogurt, which causes increased satiety (5).

Many of yogurt’s specific health properties stem from the various probiotics it contains.  Probiotics are either uniform or mixed batches of living microbes which benefit the host by improving its gut bacteria, also known as microflora (4).  These important microbes fight disease by restoring balance to the body’s natural gut bacteria.  The relationship between human gut bacteria and health status is becoming more prominent as research expands in this field.  Infectious disease physician, Ravi Kamepalli, MD, noted this balance with using active bacteria in healthy stools to treat common pathogens of the colon like C. difficile: "Human beings are 90 percent bacteria and once that balance is altered with antibiotics, opportunistic infections can cause serious problems. All we are doing with this treatment is resetting the balance.”(6). Furthermore, common microbes in yogurt, specifically members of the Bacillus species, have demonstrated potential to bind and eliminate heavy metals from the body.  Researchers specifically attributed these benefits to the “high peptidoglycan and teichoic acid content in their cell walls” (7).  The composition of peptidoglycan is a continuous disaccharide joined by polypeptides to form a protective coating for the whole cell.  Teichoic acid mainly consists of an alcohol and a phosphate (8).  The beneficial lactobacilli also help to enhance digestion of lactose.  This is why some lactose-sensitive individuals may be able to tolerate yogurt with ample amounts of live and active cultures to assist the digestive process. 

Our bodies are exemplary of a storehouse, said to have at least 10¹³ total bacteria belonging to approximately 30-40 different species (11).  Depending on which bacteria one consumes, the ratios of various types of microflora will increase and/or decrease.  For example, using gel-electrophoresis (a test to analyze microbial DNA) methods to study stool samples, researchers found an increase in lactic acid bacteria and a simultaneous reduction in Bacteroides (11).  The total amount of  Bacteroides normally present is close to 1 billion per gram of feces.  These anaerobic organisms can also be found in the gingival crevice.  They are gram-negative immotile and lack protective endospores (8). 

Yogurt making varies based on which cultures one intends on using.  Using pasteurized mesophilic yogurt doesn’t require any heat, but pasteurized thermophilic yogurt must be heated to 160°F, before being cooled to 110°F to add the starter culture. In addition, a thermophilic culture of yogurt doesn’t require as much culturing time as a mesophilic culture would (5-12 as opposed to 12-18) (2).  This may be attributed to the increased activity of enzymes (which serve as reaction catalysts) from the higher temperature.  One of the key bacterium used in direct-set thermophilic is Bifidobacterium lactis (an exception is a vegan variety which instead uses bifidobacterium bifidum).  . lactis is a member of the beneficial bifidobacterium family. Currently, it is classified as a subspecies of . animalis.  Belonging to the Actinobacteria in volume 5 of Bergey’s manual, . Lactis is Gram positive, with a high % G + C (Guanine + Cytosine) content.  Bifidobacterium use anaerobic respiration, their structural composition resembling branching rods.  For metabolism, these organisms yield lactic acid through the fermentation of sugars (8). 

To begin yogurt production, the milk for the starter batch must be heated at approximately 85-90°C.  This serves two key functions: to kill off pathogenic microbes and to cause denaturation of the milk proteins, yielding a gelatin texture.  This new gelatin form results from the batch holding in moisture (4).  After this has occurred, one must then choose from various starter cultures available, depending on what type of yogurt they want to make.  Starter cultures can affect the ultimate flavor, texture and consistency of the yogurt produced.   The milk must cool to around 42°C before the starter culture is added.  It is maintained around this temperature, while the pH is measured at different intervals to determine how much lactic acid is being made (a sign of metabolically active bacteria).  A pH of 4.5 indicates adequate lactic acid production, at which point fermentation is inhibited by having the batch is cooled to around 7°C (4). 

In regards to the preservation of yogurt, new methods are constantly developing to maintain its shelf life for a longer duration.  One such method uses a powdered form of yogurt in a process known as foam mat drying.  This technique involves adding maltodextrin (a starch polysaccharide) to fresh milk with lactic-acid fermenting bacteria.  The mixture is then spread out on an aluminum baking pan before heating and drying at 52°C .  The newly dried product is then blended and shifted through a mesh sieve for the finalized yield.  Obtaining the appropriate moisture content is essential for ensuring optimum conditions.  In one such process for making yogurt powder, researchers measured the adequate moisture content to be 10.3% (9).  Nutritionally, the final powdered yogurt product was measured to be 31.2% protein and 36.2% fat content in the optimum conditions.  In addition, the yogurt contained key minerals such as sodium, calcium, potassium, magnesium and iron (9).  This demonstrates that yogurt could be made in many different forms without sacrificing the nutritional quality.   It’s important to note how environmental conditions can impact the production process.  Higher temperatures and increased airflow can lead to quicker yogurt drying by increasing the evaporation rate (9).   

While yogurt production possesses unique bacterial properties, other popular foods rely on fermentative microbes as well.  Some of these include cheese, sourdough, kombucha, soy products and kimchi. Sourdough uses lactobacilli combined with yeasts, in a symbiotic relationship undergoing a long fermentation process.  Soy products vary in what type of microbes are used.  Miso (soy paste) uses koji bacteria, tempeh uses a certain fungus and soy sauce uses aspergillus bacteria (2).  The diversity of these foods demonstrates how different outcomes can be depending on time, temperature, pH, the type of microbes used, and other ingredients used in combination. 

How microbes influence the foods we eat continues to be a subject of avid research.  Bacterial characteristics of yogurt contribute to the brand-name products we see on the shelves and the internal environment of our gut microbiota.  While yogurt can be made in many different varieties with as many different methods, the underlying mechanisms remain the same: microbes working underground to yield significant changes in the structure and chemistry of this popular snack.  

References 

1.      Dairy goodness.  “The History of Yogurt”.  www.dairygoodness.ca/yogurt/the-history-of-yogurt. 9/14/2014.

2.      Cultures for Health.  “Choosing a Yogurt Starter”.  http://www.culturesforhealth.com/choosing-a-yogurt-starter-culture

3.      Andre El Khoury, Ali Atoui, Joseph Yaghi.  “Analysis of aflatoxin M1 in Milk and yogurt and AFM1 reduction by Lactic acid bacteria used in Lebanese industry.”  Food Control 22 (10), 1695-1699.  2011.

4.      Microbe Wiki.  “The Role of Bacteria in the Health Potential of Yogurt.”  https://microbewiki.kenyon.edu/index.php/The_Role_of_Bacteria_in_the_Health_Potential_of_Yogurt. 9/14/2014

5.      Web MD.  “The Benefits of Yogurt.”  http://www.webmd.com/food-recipes/features/benefits-yogurt?page=4

6.      Cancer Active.  “Gut bacteria and colorectal cancer-less diverse and different varieties.”  http://www.canceractive.com/cancer-active-page-link.aspx?n=3516

7.      http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3426676/

8.      Tortura, Funke, Case.  Microbiology: An Introduction (2013).  (pg. 84).

9.      Microbe Wiki. “Bifidobacterium.” http://microbewiki.kenyon.edu/index.php/Bifidobacterium

10.  Anang Catur Sulaksono et. al.  “Production and Processing of Yogurt Powder Using Foam-Mat Drying.”  Food and Public Health 3 (5), 235-239.  2013.

11.  Raimundo Garcia-Albiach et. al. “Molecular analysis of yogurt containing Lactobacillis delbruekii subsp. Bulgaricus and streptococcus thermophilis in human intestinal microbiota.”  Amercan Journal of  Clinical Nutrition 87 (1) 91-96.  2008.