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Lactic acid fermentation refers to a biological process where sugars such as fructose, glucose, and sucrose undergo conversion to form cellular energy lactic acid. The process is an anaerobic kind of fermentation reaction and takes place in a number of bacteria as well as in animal cells like the muscle cells (Campbell & Reece, 2005). The application of lactic acid fermentation is broad especially in the food industry (Tempeh, 2008). Some of the bacteria that are involved in the production of lactic acid include pediococcus cerevisiae, leuconostoc mesenteroides, bifidobacterium bifidus, and streptococcus lactis (Tempeh, 2008). The process is also commercially used especially in the production of sauerkraut and yogurt. The genus Lactobacillus is mainly used in the production of yogurt while genus Leuconostoc is used in the production of sauerkraut (Aneja, 2003).
The experiment intended to determine the how adding yogurt effects the pH and texture of the lactic acid fermentation product of milk. The experiment will seek to answer the question whether pH and the texture of a sample where yogurt is added is different from those of a sample where no yogurt was added. The experiment also aimed at determining the type of bacteria present in each sample.
Methods
Two test tubes were labeled with the identifications #1 and #2 as well as group identification in order to enable retrieval of the test tube. During preparation, cleanliness was maintained to avoid contamination of the milk with bacteria from the environment. In each of the test tubes, 10ml of fresh milk and 2ml (1/2 teaspoon) of powdered milk were added ad mixed well. Both tubes were placed in a water bath (beaker with water) and a thermometer added in one of the test tubes. The beaker with the test tubes was placed on a hot plate. The temperature of the milk was raised to 88OC and maintained for 15 minutes. The milk was then cooled to 45OC by placing the beaker in a cool water bath (a second beaker with cool water). The pH of the milk mixture of each tube was tested by placing a small sample on a piece of pH paper (2cm long) and the results were compared to the chart, and the results recorded.
In the test tube #1, ½ a teaspoon of water was added while a ½ teaspoon of yogurt that contained live cultures was added in the test tube #2 and the content mixed well. The pH and texture for the content in each test tube were noted. The test tubes were covered using a stopper and placed in the test tube rack that was in a water bath at 45OC for at least 8 hour incubation. After the incubation period, the test tubes were returned to the refrigerator.
Yogurt analysis
A small sample of the mixture was taken in each tube and tested for pH as well as the texture, and the results recorded. A wet mount of the yogurt was prepared to observe the living cultures by placing a small drop of water from the tap on a slide and placing a minute drop of Ethylene blue dye at the edge of water on the slide. A small sample of the yogurt was taken and mixed into the water/ dye mixture on the slide. A cover slip was put on the sample, and the sample observed under 1000X magnification. The shape and the arrangement of the two bacterial types were drawn as they appeared. After the experiment, the slides were placed in the disinfectant bath, or the biohazard bag and the test tube with yogurt returned to the test tube rack.
Results
The results for the texture, and pH, of the samples before incubation and after incubation were recorded in the Table 1 below.
The appearance of the two bacteria in terms of their shape and the arrangement was as shown in figure 1 below.
The two main bacteria that are found in yogurt are the Streptococcus thermophilum and Lactobacillus bulgaricus. Streptococcus thermophilus refers to the bacteria that are gram-positive, non-motile and is made up of coccus or circular structure and are seen in pairs or chains (Tarek, 2010). From these characteristic, the bacterium that closely match these characteristics is the bacterium in tube #2. The cell wall of these bacteria is composed of N-acetylmuranic acid and N-acetylglucosamine that are bonded using an ether bond. The unique structures that these bacteria have enable them to endure high levels of temperature a feature that is important in dairy fermentations. On the other hand, the Lactobacillus Bulgaricus, which is also known scientifically as lactobacillus delbrueckii, is useful in fermenting milk into yogurt (Tarek, 2010). The bacteria are anaerobic, non-motile and just like any other bacillus bacteria they are rod shaped. The bacteria in the test tube #1 are the one that gave such characteristics.
Heating during yogurt production is necessary to set the optimum temperature for the organisms involved and thus eat up the sugar in milk more quickly. Heating also prevent other bacteria, which are not thermophilic in nature from attacking the milk. Heating also enhance the texture of the yogurt where the whey protein is denatured and coagulates to enhance the texture and viscosity of the yogurt. The cooling process during yogurt production is essential in controlling tightly the level of acidity in the yogurt. Cooling stops the process of fermentation as well as the metabolic activities by the bacteria (Wang, 2009).
The main sugars that are used in the production of lactic acid are glucose and lactose while others such as the cane sugar molasses have been used in this production (Dumbrepatil, et al., 2008). The level of pH in the two tubes changed slightly in the different stages of the experiment. In tube #1, the pH at the beginning was 6.4 which did not change after water was added into the solution. The main reason for lack of change after water was added may be because of the low amount of water added and due to the fact that the pH level of water is very close to this amount. After incubation, the pH level went down to 6.0 which may have been due to the acid nature of the lactic acid released in the process of fermentation. In tube #2, the pH level dropped from 6.4 to 6.0 after yogurt was added. This is because the yogurt added was more acidic than the content in the tube and thus reducing the pH of the mixture. The pH remained the same even after incubation since the content was already acidic and the produced content was at the same pH level. Tube #1 did not have a lot of bacteria and thus fermentation was not intense as that of tube #2. This explains why the texture of the mixture in tube #2 was intense than that of tube #1.
Conclusion
The experiment intended to determine the how adding yogurt effects the pH and texture of the lactic acid fermentation product of milk. The experiment answered the question whether adding yogurt to milk would affect the pH and texture of the product formed. There was a significant difference between the products that had yogurt added from that where no yogurt was added. The experiment also aimed at determining the type of bacteria present in each sample. From the arrangement and their shape, the bacteria in tube #1 and tube #2 were Lactobacillus bulgaricus and Streptococcus thermophilum respectively.
Reference List
Aneja, K. R., 2003. Experiments In Microbiology, Plant Pathology And Biotechnology. 4th ed. New Delhi: New Age International.
Campbell, N. & Reece, J., 2005. Biology. 7th ed. San Francisco: Benjamin Cummings.
Dumbrepatil, A. et al., 2008. Utilization of Molasses Sugar for Lactic Acid Production by Lactobacillus delbrueckii subsp. delbrueckii Mutant Uc-3 in Batch Fermentation. Appl. Environ. Microbiol., 74(1), pp. 333-335.
Tarek, A. I., 2010. Production of Yohurt by Locally isolated starters: Streptococcus thermophilum and lactobacillus bulgaricus. Journal of Al Azhar University-Gaza, Volume 12, pp. 56-68.
Tempeh, 2008. Lactic Acid Fermentaton. [Online] Available at: http://www.tempeh.info/fermentation/lactic-acid-fermentation.php[Accessed 11 June 2013].
Wang, N. S., 2009. Experiment No. 8: Yogurt Fermentation With Lactobacillus Cultures. [Online] Available at: http://www.eng.umd.edu/~nsw/ench485/lab8.htm[Accessed 11 June 2013].
