Tagged: lactic acid

Bacteria and Yoghurt: A* understanding for IGCSE Biology 5.7

Biology is a great subject to teach:  you get to introduce young minds to the wonders of the natural world, to show them how evolution has been able to take an ancient planet with a few self-replicating molecules in some deep sea vent and end up today with perhaps 100 million different species all occupying unique niches in an ever changing ecosystem.  You can take students onto a journey into the cell so that they understand and appreciate the complexities of DNA as a coding molecule and how proteins have evolved to carry out the myriad processes of cellular biology.

And you also get to teach the role of bacteria in yoghurt making.  This will be a short post.

This is yoghurt.  It is made from milk.

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Milk contains a sugar, lactose.  Yoghurt is made when a culture of Lactobacillus bacteria is added to the milk.  The two most important bacterial species involved in yoghurt making are called Lactobacillus bulgaricus and Streptococcus thermophilus (although only the former is mentioned in the specification).  Perhaps they didn’t want the excitement levels to get too high….?

 

Lactobacillus use the lactose in the milk as a respiratory substrate producing lactic acid as a waste product.  This lactic acid gives the yoghurt its bitter taste as well as lowering the pH so that the yoghurt takes on its gel like appearance.

Respiration in Lactobacillus:

Lactose ——> Lactic Acid

That’s the limit of my knowledge and interest in this topic:  hope it helps…….

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