Microbiology Lecture 9

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Microbes and Food

Microbiology is involved in many aspects of food production.

Fermentations – beer, wine, bread, chocolate, soy sauce, yogurt, cheese, sauerkraut etc.  

Spoilage – what is food for us is also food for microorganisms. This can be advantageous eg in food fermentations, or can cause spoilage eg acid production in wane and beer, soured milk, rotten fruit etc.

The science of preservation requires a knowledge of the factors that promote microbial growth (nutrients, moisture, warmth, time to grow etc) so that these can be restricted to reduce growth (drying, salting, freezing etc) and extend the shelf life of products.

Food safety is not all about microbiology (eg importance of prevention of contamination by chemicals), but microorganisms that cause foodborne disease (Salmonella, Campylobacter…….) and/or produce toxins in food (Staphylococcus, Clostridium botulinum….) must be tested for and excluded from food.

Food fermentation= ‘the conversion of carbohydrates into alcohols or acids under anaerobic conditions used for making certain foods’

Why ferment milk?

  • Stability of product and extend shelf life
  • Increase microbiological safety by reducing pH
  • Organoleptic properties
  • Increase acceptability and nutritive value


  • Whole, skimmed, or fortified with milk powder
  • Cows, goats, sheep
  • Must be free of anti-microbials  eg antibiotic residues that cattle were treated with,  or sanitisers from cleaning equipment (Why?)
  • Homogenisation to reduce size of fat globules below 2µm diameter
  • Increased viscosity, appears whiter
  • Heated to 80-90oC for 30 minutes (Why?)

Heating eliminates most contaminating microflora that would compete with starter cultures. Also destroys immunoglobulins and expels oxygen, and promotes interactins between casein and whey proteins that helps prevent the milk separating out. 

Starter Cultures

  • Milk cooled to 40-43oC
  • Total of 106-107 bacteria per ml added
  • Streptococcus thermophilus (optimum 39oC)
  • Lactobacillus delbreuckii (optimum 45oC)
  • Fermented for 4 hours either stirred in tanks or for set yoghurt in retail pack
  • pH drops to 4.6 as lactose ferments to lactic acid


  • Streptococci grow fastest to begins with
  • When pH drops below 5.5, lactobacilli take over
  • Final bacterial growth count is 108 per ml
  • Flavour compounds such as acetaldehyde are also produced
  • Product cooled to 15oC and fruit etc is added
  • Can be stored at 5oC for about 3 weeks

Lactic acid bacteria (LAB)

  • Examples are Lactobacillus and Streptococcus thermophilus
  • Gram-positive, non-sporeforming rods or cocci
  • Most are aerotolerant anaerobes (they can tolerate presence of oxygen, but do not use it)
  • They derive energy from fermentation of carbohydrate to produce mainly lactic acid
  • Two types – homofermenters and heterofermenters

Heterofermenters and homofermenters can be distinguished in the lab by the ability of heterofermenters to produce carbon dioxide. 

Heterofermenters produce twice as much ATP  than homofermenters per molecule of glucose, so get a greater growth rate and yield. 


  • Fermented cabbage (can ferment most veg)
  • Simple process, but with interesting chemistry and microbiology
  • Provide conditions for growth of LAB naturally present on leaves by adding salt


  • Cabbage leaves shredded to 1mm thick

Increases surface area

  • Packed into vats with 2-3% salt

Extracts moisture by osmosis to produce brine 

Inhibits some natural…


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