Microbes and food 1. Menu - you are what you eat
Yoghurts in a shop photo
Picture 1.22a. Yoghurt is available in a huge variety of flavours.
1.22 Yoghurt
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What is it?
Yoghurt is milk fermented with lactic acid bacteria. The milk sugar lactose is converted to lactic acid, which both extends the storage life of the milk and causes it to thicken. The semi-set product can then be sweetened, flavoured or enriched with fruit pieces or puree.
How is produced?
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Look at the flowchart below. Roll over each stage to build up the flowchart.
The solids content of the milk is supplemented by adding milk powder with a fat content appropriate to the final product. Skimmed milk would be used for low fat yoghurt. A solids content of 11-15% is required.
Milk is homogenised to spread the fat globules evenly.
The milk is pasteurised by heating to 80-90°C for 30 minutes to kill all bacteria except for heat resistant spores. This ensures that pathogens are removed and that no other microbes compete with the starter culture. The heating also has useful physical effects.
The milk is cooled to incubation temperature.
Lactic acid bacteria photomicrograph
Picture 1.22b Lactic acid bacteria in yoghurt.

A starter culture of Streptococcus thermophilus and Lactobacillus bulgaricus is added and the fermentation takes place in about 4 hours. This is done either in pots to get set yoghurt or in tanks if a stirred product is wanted.
The bacteria ferment the lactose in the milk, forming lactic acid which reduces the pH from 6.3 to 4.6. At this point the casein coagulates into a gel which traps all the other milk components. Streptococci grow first, but as the pH falls, the lactobacilli dominate. Compounds produced by one type of bacteria also stimulate the growth of the other. The final acid content is about 0.9-0.95%.

Acetaldehyde produced during fermentation gives yoghurt its characteristic flavour.

When fermentation is done, the yoghurt is cooled to 15-20°C. Fruit or flavourings are added and it is packed in sealed containers before chilling and storage at below 5°C.

There are different types of yoghurt:

  • Long life yoghurt is sterilised by heat treatment and keeps at room temperature for several months.
  • Live yoghurt contains the living bacteria that were used to make it. Chill storage prevents these organisms from growing. Yoghurt is sometimes pasteurised, but it is not usually considered necessary as live yoghurt will keep for 3 weeks in the fridge.
  • Probiotic yoghurt has other bacteria, such as bifidobacteria, added to it after processing. Some people believe that these probiotic bacteria give health benefits as they grow in the gut and suppress more harmful microbes. They can be particularly helpful after a bout of diarrhoea or a course of antibiotics which might have changed the natural gut flora.
Mouldy yoghurt photo
Picture 1.22c Mouldy yoghurt.

How does it spoil?
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Yoghurt can be spoiled by acid tolerant microbes such as yeasts and moulds. They can cause the product to ‘blow’ due to gas production, or have off-tastes and a furry appearance. Good manufacturing practice, hygiene and the use of high quality ingredients, coupled with effective refrigeration, ensure that yoghurt keeps well.

Can it be harmful?
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Food poisoning due to yoghurt is rare as pathogens do not grow for the same reasons as spoilage microbes. Contamination after processing is usually the cause of any problems. An outbreak of Clostridium botulinum poisoning was traced back to yoghurt. It proved to be due to spores in the hazelnut puree flavouring which had been incorrectly processed, allowing the bacteria to grow and produce toxin.