Our Report

This is our lab report where we summarized our process, results, analysis and conclusions, we also answered some questions to help us understand further this phenomenon. 

Results Obtained

RESULTS	IMAGE	EXPLANATION
Purified Milk		We obtained 1 liter of purified milk after boiling it.
Dense Liquid		Yogurt was added to the milk and the mixture was then covered; about 8 hours later we obtained a thick layer on top of the dense substance we could now call yogurt. It was a white substance with no strong smell.
Sweet and Tasty Yogurt		Because the yogurt had no flavor at all, sugar and strawberries were added to obtain a very sweet and tasty yogurt with soft texture.

Results: After obtaining a mixture of milk and yogurt, we then obtain a thick odorless and colorless substance, yogurt.  If we drank this sugar-less yogurt, it would be like drinking light yogurt. Sugar and fruits were added to give it a sweet and pleasant flavor.

Data Analysis

In our experiment we observed how the process of fermentation of lactose by different bacteria, known as “yogurt cultures”, produces lactic acid, which acts on the milk to give the yogurt its famous feel and texture. ^[1]

The first thing we did was to boil our milk. This was done to kill any undesirable bacteria that may affect the final product. We waited for a while for the milk to cool down, but while it was still warm, we poured our yogurt in. But, why didn’t we boil the yogurt as well? The yogurt has the bacteria we need for our process of fermentation. These include Lactobacillus Bulgaricus and Streptococcus Thermophilus. ^[2]

We then covered our jars of milk and yogurt with newspaper and tons of vinipel paper. The reason for this is the fact that the bacteria needed for the production of lactic acid reproduce and work better and higher temperatures ^[3], so we needed to make sure our containers would lose the minimum amount of heat.

After about 8 hours we uncover our containers. What we found was a thick, colorless, odorless substance. It had a dull and unpleasant taste. This was due to the lactic acid. We mixed large quantities of sugar and strawberries so our yogurt would taste better.

At the end, our result was a thick sweet yogurt; it had a very pleasant taste.

• What is happening on the microscopic level of our experiment?

The Lac Operon, found in prokaryotes, is responsible for the production of lactose in organisms. It is by default in the “off” position and is only turned on in the presence of lactose. When our bacteria were exposed to the lactose in the milk, their lac operon turned “on” and production of B-Galactosidase, Permease and Transacetylase began. These 3 compounds are responsible for the breaking down of lactose for the obtainment of energy. By breaking down the lactose and obtaining energy by fermentation, our bacteria release lactic acid, thus giving us yogurt. ^[4]

[1] CULTURES FOR HEALTH: What is Yogurt? http://www.culturesforhealth.com/what-is-yogurt-history (cited 05/11/13)

[2] NCBI: Molecular Analysis of Yogurt http://www.ncbi.nlm.nih.gov/pubmed/18175741 (cited 05/11/13)

[3] UK GOVERNMENT: What Bacteria Need To Grow http://archive.food.gov.uk/hea/1114/plainenglish/part2.html (cited 05/11/13)

[4] JOHNNY CLORE: Lac Operon http://www.youtube.com/watch?v=2sMFswbOgKk (cited 05/11/13)

CONCLUSIONS

•         Bacteria in the milk could have affected the final product, so that is why the milk had to be boiled.
•        The yogurt must NOT be boiled because it has some bacteria that were needed in the fermentation process, without these bacteria our final product would not have been yogurt.
•        We needed to cover our jar to prevent it from losing heat since we needed it to be warm for the bacteria to work at optimal conditions in the fermentation process.
•        In this experiment we learned that while working on these types of experiments we have to be careful, since any change, as insignificant as it may seem to us can affect the final product from a microscopic level.
•       We can take advantage of the Lac Operon found in bacteria for our own needs, as we can take advantage of many other chemical properties.
•        Sugar and other sweeteners are needed for the yogurt to have a pleasant taste, since the lactic acid gives us a tasteless substance.

QUESTIONS

1. What is pasteurization?

Pasteurization is a process that consists of heating food, for a determinate time and heat and immediately cooling it, this is usually for killing bacteria from liquids like milk. It is the most important step to prepare drinkable milk.

2. Compare your “home-made” yogurt to the “store-bought” variety. What effect do some of the other ingredients have on the “store-bought” yogurt?

Yogurts that are bought have more ingredients like probiotics to help your digestion, they may also contain preservatives that make this yogurt last longer. A homemade yogurt has nothing like this. It is simpler and has fewer chemicals.

3.  What are the 2 most common bacteria used in yogurt making? Which one did you use?

The kinds of bacteria that are in yogurt are Lactobacillus bulgaricus and streptococcus thermophilus. These bacteria are needed for the fermentation of milk to produce the product yogurt. Without these bacteria the milk and all lactose products would be impossible to make and wouldn’t exist. We used Lactobacillus Bulgaricus and Streptococcus Thermophilus..

E. FARNWORTH: Yogurt - A Milk Product with More http://www.medicinalfoodnews.com/vol02/issue5/yogurt (cited 05/11/13)


By: Valeria Arias, Laura Ibañez, Carolina Lopez, Juan Felipe Vasquez.

Now, Vale explains what we did in Spanish: