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Yogurt Fermentation Lab

Caleb Sheirer

Kathy Hom

Discussion Cont.

Discussion

  • The gram stain was also a point of contention. We left the counterstain on for about 5 or 6 seconds too long, but were able to find both Lactobacillus and Streptococcus, in the form of gram positive rods and gram positive cocci respectively.

  • Regardless, there was still quite a large amount of pink on the gram stain, which is apparently a large amount of protein, casein, that stains pink (3).

  • After doing a bit of research, we discovered that yogurt typically contains Streptococcus thermophilus and either Lactobacillus acidophilus or Lactobacillus bulgaricus (4).

Bibliography

1. Walton, AG. Nov 25, 2013. Yogurt May Cut Type 2 Diabetes Risk. Forbes. Nov 23, 2015:1.

2. Slonczewksi, JL, Foster, JW. 2014. Products of Catabolism, p. 513. In Anonymous Microbiology: An Evolving Science, 3rd ed., . W. W. Norton & Company, New York.

3. Fankhauser, DB. January 12, 2010. Milk Fermenters. November 23, 2015:1.

4. Amirdivani, S, Baba, AS. 2011. Changes in yogurt fermentation characteristics, and antioxidant potential and in vitro inhibition of angiotensin-1 converting enzyme upon the inclusion of peppermint, dill and basil. LWT-Food Science and Technology. 44:1458-1464.

Conclusion

  • We ended up deciding that our second tube of yogurt was the best one. It contained 5mL of milk and 5mL of yogurt, incubated at room temperature.

  • We didn't choose the first tube as the smell was a little bit too potent.

  • The serial dilution platings did not work out properly, no countable plate was found. It is believed that we killed the bacteria with heat by not waiting long enough.

  • As such, the "best" plate we had was the 10^-6 plate, yielding tentatively 3x10^6 bacteria as the OCD, from the yogurt that we cultured previously.

  • The 10^-3 plate was the best plate from the commercial yogurt, with a miniscule value of 2x10^3 for the OCD, completely inconsistent with the values we would have expected.

  • The hypothesis was not supported entirely, while incubation at room temperature did yield a better sample of yogurt than the higher two temperatures, the coldest temperature did not yield the best yogurt.

  • The two proper species of bacteria, Lactobacillus and Streptococcus were both found.

Procedure Cont.

  • Six additional centrifuge tubes were filled with 900µL of diluent.

  • Then, dilution were performed with 100 µL of the 10^-2 sample.

  • The centrifuge tubes were then plated with 100µL from each tube from 10^-3 to 10^-8.

  • Then, incubated for 48 hours and observed for countable plates.

  • Lastly, a gram stain was run using standard procedure.

Procedure

  • The tubes were incubated for 48 hours and observed for these traits: smell, texture, consistency, color and pH.

  • Serial dilutions were performed. 9mL of diluent was added to a test tube with 1mL of the best sample of yogurt.

  • The sample was vortexed and 1mL was added to a centrifuge tube.

Results Cont.

Gram Stain

Commercial Yogurt

2x10^3 OCD

Tube 2

3x10^6 OCD

Results

  • There were 8 test tubes used, split into two groups of four.

  • The first batch contained 5mL of whole milk and 5mL of Yoplait yogurt.

  • The second batch had 2mL of whole milk and 8mL of Yoplait yogurt.

  • One tube from each batch was incubated at one of four temperatures: 4°C, 25°C, 30°C, and 43°C.

Each quality was rated on a scale from 1 to 5, 1 being the worst and 5 being the best.

Hypothesis

Hypothesis Explanation

Samples incubated at lower temperatures will produce better results.

  • The hypothesis was tested by varying the temperature and the amount of yogurt added in each sample.

  • Fermentation works in a number of different ways but in this case, its lactose fermentation via bacteria that produce lactic acid (2).

Introduction

  • What amount of yogurt and temperature will produce the best sample of yogurt?

  • What kind of bacteria are present in yogurt?

  • How will we determine the amount of bacteria present?

  • How is fermentation used to make yogurt?

(1)

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