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Fermented foods have burst onto the health scene and are packing the supermarket shelves. What are they? Do the health claims stack up? Here, dietitian Nicole Senior checks them out and Prof Jennie Brand-Miller explains why many are low GI. 

Everywhere you look there is a revival of time-honoured food preserving skills including jams, preserves, pickles and chutneys. What was once a prudent method for prolonging the nutrition and enjoyment of a seasonal harvest has become an uber-cool way to turn your back on mass-produced food and make your own local artisanal, bespoke food with heart.

What is fermentation? Fermentation occurs naturally when bacteria is given an opportunity to transform the carbohydrates into more complex substances. It is a process of using microorganisms such as yeasts, bacteria and fungi as a food production or preserving method. In the case of alcoholic beverages, yeast is used to ferment sugars into alcohol, whereas in yoghurt bacterial cultures are used to ferment lactose into lactic acid. Fungi can also be used to ferment foods, such as the Japanese filamentous fungi Aspergillus oryzae called koji used to make miso. Sourdough bread is another example of a fermented food using wild yeasts in a sourdough culture. The ripening of cheeses involves the introduction of microbes that develop flavour from breakdown products of proteins and fats, and moulds are sometimes introduced such as in blue-vein cheese. Fermented foods tend to be sour tasting but can be very complex in flavour which adds to their culinary appeal. Here are some common fermented foods and beverages:

  • Sauerkraut – fermented cabbage 
  • Kim chee – Korean sauerkraut with added chilli 
  • Tempeh – fermented soybean cake, similar to tofu but with a beanier, lumpier texture 
  • Miso – fermented soybean paste used as a soup base and marinade 
  • Yoghurt – cultured milk using a variety of lactobacillus strains 
  • Kefir – milk drink cultured with bacteria and yeasts – the bacteria is different to yoghurt 
  • Kombucha – an effervescent fermented tea 

Are fermented foods better for you? They seem to have taken on superfood status and this invites some pretty fanciful marketing hype; everything from preventing intestinal gas to preventing cancer. Most evidence for the benefits of fermented foods is for supporting gut health, particularly their probiotic effect of promoting beneficial gut flora, although other benefits are likely. Most research has been done on dairy foods and consuming fermented dairy foods (yoghurt and cheese) is associated with a reduced risk of cardiovascular disease, although more research is needed. But for most fermented foods, there is very little scientific evidence at all except centuries of traditional use and folklore. Enjoy them for their interesting tastes and textures and the nutritional value of the food itself, and any additional effects are a bonus.

Why do many fermented foods have a low GI? Prof Jennie Brand-Miller explains. “One reason many fermented foods are beneficial to health is the production of organic acids such as lactic acid, acetic acid (the same acid as in vinegar), etc. These are by-products of the fermentation process when the bacteria/yeast metabolise the carbohydrates (sugars and starches) in the food or drink. These organic acids not only add distinctive flavours to the food or drink, they also lower the pH, making it difficult for harmful microorganisms to grow. In our stomachs, they slow down a food’s rate of emptying into the intestine, which in turn slows the rate of digestion and absorption of the food’s carbohydrates into the blood stream, lowering the overall GI.

In traditional breads (e.g., sourdoughs), the slow fermentation not only produces the organic acids that create that unique flavour, but also the slow rise of the dough due to the production of gases (e.g., carbon dioxide). This helps the bread develop the bubbly and chewy texture characteristic of a quality bread. The gluten (protein in wheat) matrix slowly develops and traps the bubbles of gas, which is why traditional sourdough breads have a low GI (54), even when they are made of refined white flour.

Yoghurt and fermented milk drinks like kefir, lassi, leben, and Yakult all have a low GI. There are several reasons why.

  • Unique proteins in milk increase insulin production which accelerates the removal of glucose from the bloodstream. 
  • Milk sugar (lactose) has a lower GI (46) than sucrose (65) because the enzyme lactase works more slowly. 
  • Finally, the lactic acid produced by the fermentation of the lactose by various strains of bacteria like of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophiles slows stomach emptying and therefore the rate that food is digested and absorbed. 

While milk itself is low GI (20–34), the GI values of natural yoghurts (the fermented version of milk) are even lower, ranging from 10–19, depending on whether full cream or skim milk is used. Even sweetened yogurts have a GI in the 30s and 40s.