by Les Saidel - October, 2010

We all know that yeast is a vital ingredient in bread and it is yeast that makes the bread rise.

What most people don't know is that there are different strains of yeast. I'm not referring to the different brand names (such as Shimrit), but to the biological strains of yeast.

Yeast used to make bread can be classified in two major categories - commercial yeast and wild/natural yeast.

Commercial yeast (biological name: Saccharomyces cerevisiae) is the yeast most of us are familiar with and we find it in dry form, in fresh granular form or in fresh cube form. This is the yeast sold in supermarkets (in Israel the most common type of commercial yeast is Shimrit). Commercial yeast was originally derived from the beer brewing industry and has been cloned and genetically engineered to provide the best performance in terms of fermentation power and speed. The greatest advantage of commercial yeast is exactly that - the high speed with which it ferments a dough (between 1 - 2 hours).

Wild/natural yeast belongs to the same family as commercial yeast but cannot be found not on the supermarket shelves. It surrounds us in the very air that we breathe, on and in our bodies and on the skins of fruit, vegetables and other plant matter. There are hundreds of strains of wild yeast floating all around us, invisible to the naked eye. Some of these strains of wild yeast work favourably to ferment food products and some are undesirable and even harmful. It is an easy matter to "capture" such a wild yeast culture by simply leaving a bowl of water and flour covered by a damp towel on your kitchen shelf for a 4-5 days. You will then see small bubbles begin to form in the mixture. This is the wild/natural yeast reacting with (feeding on) the food source (the flour/water mixture). If the smell of the mixture is repulsive, you have captured an undesirable strain. If the aroma on the other hand, is sweet and nutty, you have had the good fortune to capture a favourable strain of wild yeast. This starter (or so it is called) may be used to ferment bread in much the same way as commercial yeast does.

A wild/natural yeast "starter" needs extra tending. Like any pet it needs to be fed regularly with flour and water to keep it alive and active.

Why would anyone bother to use wild/natural yeast when it is so much simpler to rip open a packet of commercial yeast and simply pour it in?

To understand this we must first understand the chemical reaction of yeast -

Enzymes (created by yeast) + sugar (in the flour) => CO2, alcohol, lactic acid, acetic acid and esters

This checmical reaction is the same for both commercial yeast and natural yeast. The only difference being the TIME taken to generate the same quantity of CO2 (carbon dioxide). It is these expanding bubbles of CO2 trapped in the gluten network of the dough that give the "rising" effect. When the bread is baked, the yeast organisms are destroyed by the heat, but their work of aerating the dough has already been accomplished. This is biotechnology at the highest level.

For commercial yeast the time to achieve the required inflation is typically between 1-2 hours as stated above. For natural yeast to produce the same quantity of CO2, it takes typically between 8-12 hours. It takes much longer because the natural yeast is not a "sprinter" like commercial yeast but more a "long distance runner".

In the short time of 1-2 hours very little lactic acid, acetic acid and few esters are created in the chemical reaction. In a 8-12 hour reaction process there is a much higher acid and ester content in the mixture.

It is these acids (lactic and acetic) and the esters which give natural flavour and aroma to the bread. A bread that ferments over a 12 hour period has a complexity of flavour and aroma that is absent in the 1-2 hour commercial yeast reaction. It is akin to a well matured wine compared to a young, immature wine.

Another advantage of a slower fermentation and a higher concentration of acids is that these provide a natural preservative for the bread. Think acetic acid, think vinegar - nature's own preservatives. Breads made with commercial yeast go stale and mouldy much quicker than naturally yeasted breads.

Breads made with natural yeast are also called sourdough breads, in reference to the slight acidic tang provided by the acids in the natural yeast fermentation process.

If France is famous for its baguette, Italy for its pizza and the Middle East for its pita, America is probably most famous for its San Franciso Sourdough. The combination of climatic conditions and indigenous local yeast strains combine in the Bay Area to create a world famous and unforgettable bread.

Sourdough bread has an unmistakable flavour. Once you taste it, you will either love it or hate it, but you will never forget it. It is a flavour that production line bakeries try to imitate by adding extra salt and other flavourings, but they simply cannot reproduce artificially what nature excels at naturally.

A baker's repertoire typically includes "sweet" breads produced with commercial yeast and also sourdough breads to cater for every taste.

In Saidels bakery we use a number of natural yeast cultures that have been captured from around the world (that is a separate article on its own) which have been used for decades and sometimes even centuries to bake bread.

Our soft white rolls, bagels, bialys and sandwich loaf use commercial yeast. Our ryes and whole grain breads use wild/natural yeast cultures.

We welcome you to try both and see which you prefer.

Les Saidel


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