Lab-8-Food Microbiology (1)

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Food Microbiology Lab 8: Food Microbiology (100 points) Student Name: Brett Davis Student ID:5869492 Course ID: BIOL202 -Each question on the lab worksheet must be answered completely, thoroughly, in complete sentences and correctly in order to be considered for full credit -If the question asks you to do research or find a source, a reputable, credible and/or scholarly source citation must be included in order to be considered for full credit -If a math formula is required to arrive to an answer, work must be shown otherwise, no credit will be awarded Pre-Lab Questions 1. What is the “danger zone” of bacterial growth that consumers are warned against about holding foods? (5 points) The danger zone is the temperature where bacteria grows the fastest. This is generally between 40-140 degrees fahrenheit. This temperature allows it to grow quickly that is why freezing or refrigerating certain foods maintains their freshness for longer. 2. There are several foods and beverages listed in this lab that are created through the actions of microorganisms. Name five of the foods and list the specific microorganisms, the genus and species, that are used to make each of them. (5 points) Beer, wine, and whiskey are all made by using yeast (Saccharomyces cerevisiae). Yeast is also added to flour, water, and salt to make the dough of bread or pretzels. Yogurt is a fermented milk product that uses the bacteria streptococcus thermophilus and Lactobacillus bulgaricus.

Food Microbiology Experiment 1 Results Tables Table 1: Time Required for Methylene Blue Color Change (10 points) Milk Sample Start Time/Date (Step 10) End Time/Date (Step 11) Time Elapsed (End Time-Start Time) 0 hours 11/21 3pm 11/24 3am 60 hours 1 hour 11/21 3pm 11/23 3am 36 hours 3 hours 11/21 3pm 11/22 11am 20 hours 4 hours 11/21 3pm 11/22 7am 16 hours

Food Microbiology Experiment 1 Post-Lab Questions 1. Which sample took the least time to become white? Why was that the case? (5 points) The 4 hour sample was the quickest to turn white. This is because it was left in the danger zone for longer allowing bacterial growth to occur longer. 2. Which sample took the most time to become white? Why was that the case? (5 points) The fresh sample just out of the fridge took the longest. This is because it had the least bacterial growth out of all of the samples. 3. How does refrigeration affect the number of bacteria present in milk? (10 points) Refrigeration slows the growth of bacteria. This doesn’t stop them from growing or already being present, but ensures the milk stays fresher for longer. 4. How does pasteurization affect the number of bacteria present in milk? What differences would this assay show if performed using raw (unpasteurized) milk? (10 points) Pasteurization kills off the harmful bacteria. This means pasteurized milk will have less within it compared to unpasteurized milk. It would make the assay have more bacteria and a shorter time to turn white. 5. What causes the methylene blue to change from the blue color back to the color of white, the usual color of milk in the assay? What is responsible for this change? (10 points) The methylene blue changes back to white once the oxygen is gone from it. The bacteria within the milk will start to ferment taking the oxygen out of the tubes. Once there is no oxygen the methylene blue will become white because it gets reduced to leucomethylene which is colorless.

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Food Microbiology Experiment 2 Results Table Table 2: Yogurt pH Results (10 points) Sample (Step #) pH 7 7 11 8 15 7 20 4.5 22 4 Experiment 2 Post-Lab Questions 1. Make a graph of the pH changes over the course of the experiment (from Table 2) and insert it here. Explain what causes the pH to change at each step? Why does the pH change at each of the steps? (15 Points) The pH levels change as the milk is heated because it kills the bacteria that is in it. Adding the yogurt brings bacteria back into the milk and allows for fermentation to continue. Once we started to incubate the lactose was broken down into lactic acid. This dropped the level down to about 4.5. Removing the yogurt from the incubator allowed for fermentation to continue and kept the pH level at a stable 4.

Food Microbiology 2. In the protocol you used to make yogurt from milk, why was it necessary to raise the temperature of milk to 85 degrees for two full minutes? (5 points) This allowed for the milk to start pasteurizing. It killed off the harmful bacteria and raised the pH level making it more alkaline. This made it so adding the yogurt back in stabilized the mixture. 3. How does the consistency of the milk change during the production of yogurt? What facilitates this change in the pH? Is this related to the breakdown products of lactose and what about the taste? (10 points) The milk has a breakdown of lactose into lactic acid during the fermentation process. This causes the pH to drop to a more acidic level. Once the milk sufficiently becomes acidic enough, caseins, a protein within the milk, will begin to clump together. This changes the consistency of the milk to form yogurt. These bacteria in the yogurt prevents other bacterial growth, which would usually spoil milk. The taste comes from this and allowing it to ferment for too long will give a strange taste. In order to prevent that the mixture must be cooled to slow the growth of the bacteria. References Pasteurization | Definition, Process, Inventor, & Facts | Britannica Fermentation of Yogurt and the Chemistry Behind It | FoodUnfolded Blues the Clue: Souring Milk for Science (Grades 9-12) | National Agriculture in the Classroom (agclassroom.org)

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