Wednesday, April 5, 2017

Fermented foods - tasty, safe and good for you.

Recently, one of my regular readers watched a BBC television programme on 

"Which foods can improve your gut bacteria?" narrated by Dr. Michael Mosley.  She wrote and asked me  to comment on the safety of such foods, how they should be stored, plus potential risks of manufacturing them at home.

Traditional, naturally fermented foods have been around for hundreds, if not thousands, of years.  These days, we tend to relate the term "Fermented food" to sauerkraut, kimchi, tempeh, or perhaps yoghurt and cheese.  However, the Egyptians knew about wine production 5,000 years ago (though they didn't know that yeasts were involved).

Fermentation is a natural process, carried out by bacteria, yeasts and occasionally moulds, that actually preserves raw foods, often by production of acid or alcohol, which prevent spoilage.  Of course, the preserved food sometimes bears little resemblance to the original.  We enjoy these foods because the fermentation changes the flavour and texture of the raw material, often increasing the complexity.

We now know that our gut microflora, now called the gut biome, can be influenced for the better by consumption of some fermented foods.  The microorganisms that carry out the fermentation can sometimes pass through the very acid conditions of the stomach and may be able to colonise the gut wall, perhaps displacing undesirable bacteria, though some just pass through, having a transitory effect on the gut biome.  They can help with traveller's diarrhoea or lactose intolerance.

Dr. Mosley and his team investigated the claims for such benefits by setting up three groups to consume various fermented products for four weeks and looked for changes in their gut bacteria.  You can read about it in the link above.

The team also tested the products to determine the range of bacteria in them.  Home-made fermented foods tended to contain a diverse range of bacteria, while commercial products, which may have been pasteurised, often contained very few.  Thus the flavours and textures would be similar, but the potential health benefits of the microbial populations had been removed.

Over many years of teaching food microbiology, I made sauerkraut with my students.  It's an easy fermentation to carry out, requiring only finely shredded cabbage and about 2 - 2.5% salt packed in a suitable container.  Air must be excluded to prevent the growth of moulds on the surface of the kraut. We followed the fermentation daily, counting and identifying the bacteria taking part, and measuring the titratable acidity and pH.  

We always found a succession of bacteria.  The undesirables, like Escherichia coli and other coliforms disappeared within the first couple of days as lactic acid bacteria produced lactic acid and reduced the pH.  Leuconostoc  species, found on the inside leaves of the cabbage, would begin the acid production, giving way to Lactobacillu plantarum and eventually L. brevis.  The final pH of the sauerkraut is around 3, which is plenty low enough to prevent the growth of pathogens.

The kraut can be stored in the refrigerator for a short time, but must be protected from oxygen to prevent spoilage.  If you would like to make some at home, download the original recipe, written by Carl S. Pederson in 1939.  He studied the fermentation extensively over many years.  There are many other, more recent recipes available on-line and much has been published in the scientific literature on the microbiological changes that occur during fermentation.  You can also find many recipes for kimchi on-line.

Are home-made fermented vegetables safe to consume?  Yes, provided that the fermentation has produced sufficient acid to lower the pH to around 3 to 3.5, which will prevent the growth of pathogenic bacteria.  The finished product must not be allowed to go mouldy, since moulds can metabolise the acid and raise the pH to levels at which pathogens could grow.  My son, who is an electronics engineer, has successfully made sauerkraut and kimchi at home.  You don't need to be a microbiologist or food technologist to make safe, tasty fermented foods!