We chose food topic because we consume food every day so we were interested in what contains the foods we eat each day. So we chose foods that we eat regularly. Firstly we wanted to work on vegetables to see (a toi de choisir, mais je trouve qu'il manque juste une indication) if lactic acid had an antimicrobial effect by comparing vegetables without lactic acid and others where it was injected. However, for practical reasons, we preferred to work on yogurt that already contains lactic acid. The yogurt contains two types of bacteria: Streptococcus thermophilus and Lactobacillus bulgaricus essential for the production of lactic acid. These two bacteria together make it possible to curdle the milk enclosing the lactose and producing lactic acid.
We chose to make our own yogurts for practical reasons because the yogurts of the shops already contain the two bacterial strains or we wanted to observe yogurts containing only a bacterial strain.
For this, we decided to make three different yogurts:
- Yogurt containing Streptococcus thermophilus and Lactobacillus bulgaricus;
- A second containing only Lactobacillus bulgaricus;
- A third containing Streptococcus thermophilus.
Then we compared the appearance of each yogurt and also their pH.
We can see that during the first survey, the pH of the three yoghurts is around 7.0. Then, 24 hours after manufacture and 3 days after, we can see that the pH of yogurt 2 containing only Lactobacillus has a pH that does not vary, on the other hand the pH of yogurt 1 and 3 have decreased and are in the vicinity of 4.0.
Regarding the yogurt aspect, we were able to see that yogurt 1 and 3 are firm and have a normal appearance. On the other hand, yogurt 2 is liquid.
After having favored the yogurts, we wanted to determine the acidity of the yogurts by carrying out a volumetric dosage in which we used Dornic soda, which has the peculiarity of dosing 10 milligrams of lactic acid per 1 milliliter of soda ash.
To realize the experiment, we proceeded as this scheme. We have adjusted to zero the soda which has a molar concentration of 0.11 mol / L and then in the erlenmeyer we put 25 milliliters of distilled water with 10 grams of yogurt and a few drops of thymol blue which is the indicator of the end of the reaction.
