Microbes and food 1. Menu - you are what you eat
Fizzy drinks label
Picture 1.11a .A bottle of lemonade, showing the contents, including citric acid.

1.11 Fizzy drinks
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What are they?
Fizzy (or carbonated) drinks are a mixture of fruit juice, flavourings, water and preservatives such as benzoic or citric acid. The gassy CO2 also acts as a preservative because it makes the drink acid. The product is usually also heat treated.

Some types of soft drinks are stable without heat treatment such as fizzy cola because of its low pH (2.6), high CO2 level (3.5%) and the antimicrobial activity of orthophosphoric acid and components of the cola oils.

How are they produced?
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If the drink contains citric acid, then this is probably a microbial product. Most of the citric acid made today is produced by fermenting molasses or other sugar substrates with the mould Aspergillus niger. The strains of the fungus used in the commercial processes are a carefully kept secret! A high yield of citric acid can be achieved by controlling the conditions, such as sugar type and concentration, trace metal content, pH, aeration, phosphate concentration and the amount of ammonium salts. Up to 90% of glucose can be converted to citric acid in the process.

Citric acid can be made by two different fermentation processes:

  • the surface process
  • the submerged process.

Surface fermentation is usually carried out in shallow aluminium trays filled with nutrient medium. Fungal spores are spread on the surface and sterile air is blown over the trays. A mat of mycelium forms, and the fermentation is complete in 7-15 days.

Submerged fermentation is easier to automate. The fermentation is carried out in either stirred tanks or aerated tower fermenters made of stainless steel. The fungal mycelium develops into small pellets and the process takes 7-10 days. In both processes the culture broth is filtered; washing of the mycelium is also necessary in the surface process to remove trapped citric acid. Citric acid is extracted from the broth and then crystallised out and packed ready for use in the food and beverage industry.