New sourdough process developed in Finland

07/09/05 Recent research carried out at the Technical Research Centre of Finland (VTT) indicates that sourdough, which is traditionally used for baking rye bread, can also be used for making tasty, high-fibre wheat bread with a prolonged shelf life. A seven-year research project conducted at VTT shows that optimising the sourdough process by fermenting bran with yeast and lactic acid bacteria makes it possible to produce mildly sour wheat bread with improved flavour, texture and nutritional value.

In her recently published doctoral thesis, Kati Katina, Research Scientist at VTT Biotechnology, has demonstrated that lactic acid bacteria fermented sourdough can be used to produce tasty, high-fibre wheat bread with a longer shelf life, as well as improving the microbiological safety of bran-rich wheat bread.

Industrial bread types are often disappointing for consumers due to the lack of taste and poor shelf life. Furthermore, bread that is manufactured from white flour and only contains the inner part of cereal kernel is not the best option from the nutritional point of view, due the low fibre content of such products. The use of the whole kernel or the outer layers of kernel is, however, a challenge for microbiological safety because many spoilage microbes are located in the kernel's outer layers.

The use of sourdough is a natural, additive-free method for improving the flavour, texture and nutritional value of bread. Sourdough results from a fermentation process in which part of the baking flour is pre-fermented with lactic acid bacteria and/or yeast. During the fermentation process organic acids are formed and components of flour are partly degraded and modified.

In addition, precursors of flavour (such as amino acids) and flavour active components (such as volatile compounds) are formed. Although traditional methods for baking rye bread include the use of sourdough, in wheat baking the use of sourdough is less common and more challenging because the mild flavour of most wheat breads does not tolerate strong acidity - typical for traditional sourdough breads. In her thesis, Katina clarifies the influence of parameters in the sourdough process the ash content of flour, fermentation time and temperature and different microbial strains - on the biochemical activity of sourdough and the subsequent flavour and texture of bread.

Mildly acidic sourdough containing high levels of amino acids improves bread flavour.

Katina demonstrates that levels of process parameters and types of microbes have a very significant impact on the biochemical activity of sourdough as well as the flavour and texture of sourdough bread. Balanced, stronger flavour was obtained when sourdough was made from flour with high ash content, fermented with lactic acid bacteria for 20 hours at 24 °C. Such sourdough was mildly acidic and contained high levels of amino acids. The influence of yeasted sourdough (made only with yeast) on flavour was modest.

The best results for volume and softness during storage were obtained with mildly acidic sourdough - made with white wheat flour and fermented for12-14 hours (lactic acid bacteria fermented sourdough) or for 6-8 hours (yeasted sourdough).

Production of high-fibre wheat bread with excellent volume and shelf life is possible by fermenting bran with yeast and lactic acid bacteria. This bran sourdough is of even greater quality if used in combination with enzymes.

The use of the combination process was close to preventing the hardening of the bread during storage and provided volume as good as in white wheat bread.

Lactic acid bacteria fermented sourdough also prevented rope spoilage of bread, if the acidity of the sourdough and subsequent bread was low enough: the pH of sourdough should be < 4 and the pH of bread < 5. The same sourdough did not prevent rope spoilage if the pH of sourdough was > 4.5.

Kati Katina's thesis is available online at: http://www.vtt.fi/inf/pdf/publications/2005/P569.pdf