Lactobacillus

Yoghurt is a thick, slightly sour food made from milk. The change from runny milk into thick yoghurt is caused by a microorganism — a type of bacterium called Lactobacillus.

A few points to fix in your mind from the start:

• Lactobacillus is a bacterium (not a fungus or a virus). Bacteria are very useful microorganisms in food production.
• The bacteria are added to milk and, in the right conditions, change it into yoghurt.
• This is an example of using microorganisms on a large scale to make a useful product — a key idea in this part of the Double Award course.

The whole process depends on the bacteria being alive and active, so the conditions (temperature, food supply and cleanliness) must be just right for them.

Exam tip. Make sure you can name the microorganism as Lactobacillus and state that it is a bacterium. 'Yeast' is a common wrong answer — yeast makes bread and alcohol, but Lactobacillus makes yoghurt.

Before the bacteria can do their job, the milk has to be prepared carefully. The steps are:

1. Pasteurise the milk. The milk is heated (pasteurised) to kill any unwanted or harmful bacteria that are already in it. This stops other microorganisms from spoiling the yoghurt or competing with the Lactobacillus.
2. Cool the milk. The milk is then cooled before the Lactobacillus is added. This is important because if the milk were still hot it would kill the Lactobacillus (high temperatures denature the bacteria's enzymes).
3. Add the Lactobacillus. Once the milk is at the right temperature, the Lactobacillus bacteria are added (this is sometimes called the 'starter culture').
4. Keep it warm. The mixture is then kept warm, at the bacteria's optimum temperature (around body warmth), so the bacteria are most active and reproduce quickly.

Why does the temperature matter so much? Like all living things, Lactobacillus has an optimum temperature at which its enzymes work best:

• Too cold → the bacteria are slow / inactive, so fermentation is very slow.
• Too hot → the bacteria's enzymes are denatured and the bacteria are killed.
• Just right (optimum) → the bacteria are most active, so the yoghurt forms in a reasonable time.

Exam tip. A very common question asks why the milk is cooled before the bacteria are added. The answer: if the milk were too hot it would kill the Lactobacillus (denature their enzymes). Saying simply 'to cool it down' is not enough — link it to the bacteria surviving.

Once the warm milk contains active Lactobacillus, the key chemical change begins. This is called fermentation.

Milk naturally contains a sugar called lactose (often called 'milk sugar'). The Lactobacillus bacteria use this lactose as their food / energy source and ferment it. The product of this fermentation is lactic acid:

$\textbf{lactose} \;\xrightarrow{\;\text{Lactobacillus}\;}\; \textbf{lactic acid}$

So the bacteria break down the lactose (milk sugar) and convert it into lactic acid. As more and more lactose is fermented, the amount of lactic acid in the milk rises.

You can picture the process like this:

Exam tip. Use the precise words: Lactobacillus ferments lactose (milk sugar) into lactic acid. Naming the sugar (lactose) and the acid (lactic acid) scores the marks — vague answers like 'the bacteria turn the milk sour' miss the chemistry.

The lactic acid made during fermentation is what actually turns the runny milk into thick yoghurt.

Here is what happens:

1. The lactic acid builds up in the milk.
2. This lowers the pH of the milk — it becomes more acidic.
3. The lower (more acidic) pH makes the milk proteins clot (curdle / coagulate). The proteins clump together and thicken the mixture.
4. The runny milk is now thick yoghurt.
5. The lactic acid also gives the yoghurt its characteristic sour / tangy taste.

So the two effects of the lactic acid are easy to remember:

• It clots/thickens the milk proteins → yoghurt becomes thick.
• It makes the yoghurt taste sour.

The finished yoghurt is then usually cooled to slow the bacteria down, and flavourings or fruit may be added before packaging.

Exam tip. A frequent 'explain why the milk thickens' question wants this chain: lactic acid → lowers pH → clots (coagulates) the milk proteins → milk thickens into yoghurt. Remember it is the proteins that clot.

Throughout yoghurt production, everything is kept hygienic and as close to aseptic (sterile / free from unwanted microbes) as possible.

Why does this matter?

• If other (unwanted) microorganisms got into the milk, they could compete with the Lactobacillus, spoil the yoghurt, or even make it harmful to eat.
• Pasteurising the milk at the start kills the bacteria already present, and using sterilised / sterile equipment stops new ones being added.
• Keeping the process clean means only the chosen Lactobacillus grows, giving a safe, good-quality, consistent product.

This is the same reason hygiene is stressed in industrial fermentation generally: you want only the microorganism you added to grow, and nothing else.

Exam tip. If asked why aseptic/hygienic conditions are used, link it to preventing contamination by unwanted microorganisms that would spoil the yoghurt or compete with the Lactobacillus.