Fermented foods: An essay on fermentation and food products

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Fermented foods: The benefits and necessity of fermenting as a process
Food article / commentary

Reviewed By

Sue Dyson and Roger McShane

We all love fermented foods! Sushi would be unthinkable without dipping it in the juice that oozes from fermenting soy beans. On a hot day we love to gulp down some fermented wheat or barley. After a long and satisfying meal where we have enjoyed some fermented grapes we finish the evening with a caffeine drink made from fermented berries.
We begin the day with fermented dairy products and maybe some baked fermented flour. And so, it is not just so-called third world countries where the use of fermented products is important. We all use them.
Some fermented foods (such as bread, beer and soy sauce) are very approachable. Others not so.
A few years ago we were working in Malaysia with a very good friend who, like us, loves Asian food. We were dining at the Seri Melayu in Kuala Lumpur where they served a quite reasonable buffet of local foods. In among the curries and the sambals and the rice we spotted a grayish paste which we didn't recognize.
A small sign indicated that the dish was tempoyak (fermented durian). If you haven't tried durian you should - it is an obsession in South East Asia. It is the most awful smelling fruit on the planet, but its sweet creamy texture and deep flavour make it one of the world's great fruits.
So what would this fermented fruit taste like? We hurried back to our table with some of our newly discovered challenge. The three of us each tried a spoonful. Our friend went into paroxysms. She hated it. We took one taste and almost swooned. It was one of the most ethereal tastes we have ever experienced. This experience set us on the road to learn more about fermented foods and why some people love some but hate others.
Very few would recoil from a bowl of miso soup, a lovely Indian dosai or idli or some soy sauce - but tempoyak or Nepalese gundruk or Sudanese fermented cow's milk seem to evoke different reactions.
Fermentation is one of the oldest methods of processing food into a form that is suitable for preservation. For example, making cheese is a good way of preserving milk. Wine is an excellent means for preserving grapes. Kim chi is a perfect vehicle for preserving and enhancing the flavour of the humble cabbage.
Fermentation probably began from baser motives, however. In the hot climates of the Middle East fruit rapidly deteriorates. Many fruits ferment naturally producing acids and alcohol. People eating this fermenting fruit would have noticed a different flavour from the build up of acid and perhaps a slight effect from the small amounts of alcohol present. This would have been enough to set curious minds on the path to discovering the benefits of controlled fermentation to produce alcoholic drinks.
Reliable reports show that alcohol was being produced from fruit in Babylon almost 7000 years ago. The same peoples were among the first to ferment milk as well.
So what are the benefits of this process that make it so widespread in almost every culture throughout the world?
The benefits of fermentation
We have identified many benefits of fermented foods.
One of the most important reasons to ferment foods is that it is a cheap and energy efficient form of preservation.
Fermentation makes food products more interesting by intensifying the flavour through the conversion of sugars into acids.
Fermenting makes grains more digestible. For example, porridge that has been fermented (as is common in Africa and even in Wales) hydrolises starch into shorter chains of glucose and dextrose.
Fermented products often contain higher levels of vitamins (particularly thiamine, nicotinic acid, biotin and riboflavin) and proteins. Examples are the Mexican drink called pulque, the Indian fermented bread called idli, sorghum beer from southern Africa and palm wine from West Africa.
Some fermented products have meat-like flavours and odours which is important for cultures where meat is scarce (eg soy and fish sauces). South East Asian countries had an excess of fish products and a paucity of meat. It made sense to make something that reminded the senses of meat.
Fermentation can reduce naturally occurring toxins in some foods thus rendering them safe to eat. (eg Cassava which is widely eaten in Africa has semi-dangerous levels of cyanide. By fermenting cassava to produce Kawal the cyanide is rendered harmless.)
Fermented foods often contain a higher level of convertible energy than non-fermented foods of the same weight.
There are medical advantages associated with a constant intake of some fermented foods (koumiss is used in Russia to treat tuberculosis).
Now we are going to look more closely at the fermentation process and what causes it to happen. Why does bread rise? What turns barley into luscious, golden beer? How do green soy beans turn into the black, unctuous, oozing liquid that we dip our sushi into? What creates that tingling flavour that we detect in Indian dosas? Why do the Welsh soak their breakfast cereals overnight?
We will attempt to define what fermentation is and then we will look at some of the chemical reactions that take place while food is fermenting.
The fermentation process
There isn't common agreement on a definition of fermentation. It all depends on who is defining it and what their perspective on the process is.
A food aid person might define it as a relatively low-cost means of using bacteria, yeasts and moulds in preserving food and enhancing its flavour and nutritional value.
A food technologist, however, might see it in a different way. To them, fermentation is a process by which bacteria, yeasts and moulds convert sugars and carbohydrates into less complex products such as carbon dioxide and alcohol.
A key point about fermentation from a chemical perspective is that relatively complex organic compounds are split into simpler chemicals.
We exclude from the definition processes such as drying, canning, freezing and pasteurising for two reasons. The first is that they are all about excluding bacteria, yeasts and moulds. Secondly, the equipment required is usually (with the exception of drying) high cost.
Everyone knows what bacteria are, but not everyone realises that there are ‘good' bacteria as well as ‘bad' bacteria. Some of the fermentation processes actually encourage the presence of bacteria to break down inedible or harmful food products and render them safe for consumption. A good example is the introduction of bacteria into milk products to produce yoghurt.
Yeasts and moulds are less well understood by the general community. Both are funghi and are by nature parasites. They must feed off other substances such as soil, wood, leaves or fruit. Yeasts chomp away at the barley malt to produce the alcohol for beer. Moulds are injected into cheeses to produce the fabulous flavour of a gorgonzola or stilton.
A fundamental chemical underpinning of fermentation is that an acidic environment is created. Many harmful organisms cannot exist in an acidic solution.
All of this has been known for a long time. Georg Stahl developed the theory of fermentation back in 1697 although the chemistry was detailed by the famous Lavoisier in 1787.
Bacterial fermentation
We all know about bacteria spoiling our food. Just leave a glass of milk in the sun for a couple of hours and those tiny microbes will have had more fun than Denzil Washington at the Oscars.
But bacteria can be harnessed for good as well. Bacteria added to milk in the presence of rennet starts the process of cheese production. Bacteria added to chopped cabbage produces the acidic environment required for the creation of sauerkraut in Europe, kim chee in Korea. A similar process is used for cucumber pickles in the United States, khalpi and gundruk in Nepal, hum choy in China and torshi khiar in Egypt.
Bacteria are either single or multi-cell organisms that are found just about everywhere in nature. Often, but not always, they contribute to food spoilage. They are voracious eaters. They do this by sending enzymes out through their cell walls on raiding parties into the surrounding environment. These enzymes attack food sources and break them down into particles that can be absorbed by the cell through osmosis.
Some bacteria are particularly fond of carbohydrates found in dairy products and can rapidly convert them into lactic acid. Far fewer are very keen on the sugars in fruit and break them down to produce acetic acid. The reason that not so many bacteria are successful at fermenting fruits is that growing fruits are usually high in acid – the nemesis of many bacteria.
Not so with vegetables, however. Most vegetables are low acid and very susceptible to bacterial fermentation. Therefore the fermentation of vegetables into a high acid environment (sauerkraut, pickles, kim chee) is a perfect way of preserving them.
The acids produced by fermentation are the slightly sour taste that you will recognise in many of our favourite foods including buttermilk, yoghurt, sauerkraut, pickles and even olives. As the acid levels increase, harmful bacteria are killed off.
Lactic acid bacteria go to work on carbohydrates found in flour, grain, dairy products and vegetables to produce an acidic environment that is both suitable for the preservation of the food and for changing the nature of the flavour of the food.
Thus, lactic acid bacteria are responsible for the highly desirable sour taste in sourdough bread. They are responsible for the acidic nature of sauerkraut. They are even responsible for nullifying the harmful cyanide substances in cassava enabling the production of the widely eaten gari and fufu in Africa. In this case the lactic acid bacteria turn the cyanide compounds into cyanic gases which then escape from the fermenting food, thus rendering it harmless.
By producing the acidic environment they cause other, harmful bacteria to die, thus making the food safe for storage and subsequent consumption.
In some cases the process is quite complex. For example, the production of sauerkraut requires the presence of at least three different types of lactic acid bacteria as well as salt and pressure to release the sugary juices from the cabbage.
There are also bacteria that are responsible for acetic acid fermentation rather than lactic acid fermentation. These are the bacteria that work on fruits, ciders and wine to produce vinegars of all types (and also ruin wine if it is left open for too long). It should be noted, however, that vinegar production actually involves both bacterial and yeast fermentation. The yeasts start the process and produce ethyl alcohol and carbon dioxide. The bacteria then take over and, in the presence of air (hence oxygen) turn the alcohol into the required acetic acid.
So the chemcial process for vinegar production is:
C6H12O6 + Yeast ===> 2CO2 + 2C2H5OH
C2H5OH + O2 + Acetobacter ===> H2O + CH3COOH
Yeast fermentation
Yeasts can be found everywhere. They are in your garden, they are on your hands, they are in the air, they are on the surface of some fruits. They are members of the funghi family.
Yeast is used in the production of bread, beer, cider and wine. In Belgium with some of those lovely, rich beers are produced without any added yeast at all. The vat are simply left open to enable the air-borne yeasts to find their way in.
Yeasts are single-celled organisms that tragically reproduce asexually. They are generally larger than bacteria. The cell wall allows oxygen to pass inwards and waste products such as alcohol and carbon dioxide to pass out through it.
Yeasts release enzymes (an important one for fermentation is zymase) on search and destroy missions aimed at sugars. The enzymes induce, and control the rate of, the chemical reactions necessary to break down sugars to produce alcohol and carbon dioxide.
For example, in flour there are abundant starches. These are simply lots of glucose molecules joined together.
When the yeasts have done their work we kill them. Thus the action of baking bread kills the yeast that has produced the carbon dioxide that has helped the bread to rise. Or when they produce alcohol in a closed environment the alcohol eventually kills them.
Yeasts (the best are from the Saccharomyces family, particularly Saccharomyces cerevisiae) convert carbohydrates:
Yeast + carbohydrate or sugar = CO2 + alcohol
In some processes, bacteria compete with yeasts in the race for the sugar. If the bacteria win the race then an acidic solution results with no alcohol present. Therefore the yeast must win.
So we must keep bacteria away. However, if yeasts are to produce alcohol there is another enemy, namely oxygen. If oxygen is present then aerobic fermentation occurs and the result will be the production of carbon dioxide and water. This may be OK if you like non-alcoholic fizzy drinks but is useless if you are dying for a beer!
The reaction you get is:
Sugar + Oxygen ===> Carbon dioxide + Water
Or in the more technical jargon.
C6H12O6+ 6O2 ===> 6CO2 + 6H2O
Aerobic conditions are what yeasts like best and if left to their own devices they will happily convert all available sugars to carbon dioxide and water, not alcohol.
But scientists long ago discovered that yeasts are particularly flexible. They don't particularly mind what conditions they live in provided they get to eat!
Most living things die without access to oxygen. Yeasts are living creatures, however they don't particularly mind if there is no oxygen around. They can still munch on sugars in this absence.
Sugar + No oxygen ===> Carbon dioxide + ethanol
C6H12O6 ===> 2CO2 + 2C2H5OH
As can be seen from the reaction above, when yeasts are denied oxygen they produce carbon dioxide plus ethanol which is an alcohol!
However, a word of warning here! If you produce alcohol in this way it must remain sealed from the air otherwise the resulting bacteria will digest the ethanol in the presence of the oxygen and acetic acid (CH3COOH) will be the by-product. This is why wine goes ‘off' if opened for too long.
And so this is why bread rises and beer fizzes - it is because the carbon dioxide is trapped. In the case of bread it is trapped in the cellular structure of the dough itself. The baking gradually kills the yeast and evaporates the alcohol. In the case of beer and cider the carbon dioxide is kept within the closed containers.
Mould fermentation
Moulds can be preservers or spoilers depending on the type and how they are treated. The aspergillus mould is often associated with food spoilage, for example.
Moulds, do however, impart characteristic flavours and produce enzymes such as amylase for bread making.
Moulds from the penicillium genus help ripen and flavour cheese. If you have eaten a Stilton or Roquefort or Gorgonzola then you have eaten the Penicillium roquefortii mould.
While we have not had the opportunity to explore the caves holding the Roquefort cheeses, we can vividly remember walking through deep cellars in Burgundy, trailing our hands along the ceiling feeling the deep, soft, furry, black thickness of penicillium mould that has been growing for centuries and that is carefully guarded to ensure that it imparts the right ‘atmosphere' to the cellars.
Moulds generally do not assist with the fermentation of fruit or vegetables. They are better with dairy products.
So we hope we have explained the process of fermentation and highlighted the benefits. We strongly believe that fermentation is a low-cost, low-energy process for efficient food preservation and production.
Copyright: Sue Dyson and Roger McShane, 2009

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