Microbes and food 1. Menu - you are what you eat
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Jars of pickles photo
Picture 1.5a . Assorted jars of pickles.
1.5 Pickles - fermented foods
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What are they?
Dill pickles, olives and sauerkraut are all fermented foods. They are made using microbes, giving the foods their characteristic flavour and extending the storage life of the product.
Lactobacilli photomicrograph
Picture 1.5b Lactobacilli - bacteria that produce lactic acid.
How are they produced?
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Dill pickles are fermented cucumbers. The fruits are held in large vats of brine. The salt leaches out sugars from the cucumbers and bacteria in the normal flora on the surface of the fruit ferment these. A succession of microbes is involved. Streptococci start the process, producing acid. As the pH falls, types of Leuconostoc, Pediococcus and Lactobacillus grow, making more acid. Carbon dioxide is also formed but this is removed from the vats. The fermentation takes several weeks. The combination of salt and lactic and acetic acid preserves the food, preventing spoilage microbes from growing. ‘Dill’ pickles get their name because spices and herbs like dill are added to the fermentation vats to give flavour.

Green olives photo
Picture 1.5c Some green olives.

Olives are stone fruits with a very tough skin. They are inedible until fermented. There are two main types of olives – the Spanish-style green ones and the Greek black ones.

The green olives are picked unripe and treated with a sodium hydroxide solution (lye) to break down a compound called oleuropein which has a very bitter taste and also inhibits lactic acid bacteria. It also softens the olive skins.This takes several hours, after which the caustic solution is washed off thoroughly. The olives are placed in brine and the natural fermentation sequence begins. Sometimes sugar is added to the brine to help start the microbial growth. Due to the low pH and salt, lactic acid bacteria and some yeasts replace the natural flora of the fruits. Air is kept out of the vat so that spoilage moulds and yeasts cannot grow. The process takes some weeks. The product contain 1% lactic acid and has a pH about 4.

The black olives are ripe and are not treated with lye. The fermentation is slower because it takes time for nutrients to diffuse through the olive skins into the brine for the microbes to grow on. A variety of yeasts are responsible for the fermentation process, with significant numbers of lactic bacteria present only if the salt content is less than 6-7%. The final product is much less acid than green olives (pH 4.5-4.8) and so more salt is usually added to prevent the growth of spoilage microbes on storage.

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How do they spoil?
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Provided the correct production procedures are followed and salt and acid levels carefully controlled, picked vegetables and fruits are unlikely to spoil. Yeast overgrowth is the most probable type of spoilage. Food poisoning from such products is very unlikely.