Optimising the sourdough process by fermenting bran with yeast and lactic acid bacteria can produce mildly sour wheat bread with improved flavour, texture and nutritional value, say food researchers in Finland.
Concluding a seven year research project, Kati Katina, a scientist at the Technical Research Centre of Finland (VTT Biotechnology), demonstrated that lactic acid bacteria fermented sourdough can produce high-fibre wheat bread with a longer shelf life, as well as improving the microbiological safety of the bread.
"Sourdough is a potential pre-treatment method for cereals and cereal fractions in the improvement of flavour, texture and the nutritional value of cereal products," says Katina.
The application of new starters and the combination of sourdough and enzymes provides fascinating future prospects for this traditional biotechnological method, she adds.
The VTT scientist reports that a '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. And the influence of yeasted sourdough (made only with yeast) on flavour was modest.
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.
VTT explains that traditional methods for baking rye bread include the use of sourdough. But 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," writes VTT.
"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.," the institute reports.
The best results for volume and softness during storage were obtained with mildly acidic sourdough - made with white wheat flour and fermented for 12-14 hours (lactic acid bacteria fermented sourdough) or for 6-8 hours (yeasted sourdough).
In addition, the scientists report that the production of high-fibre wheat bread with 'excellent volume and shelf life' is possible by fermenting bran with yeast and lactic acid bacteria.
The bran sourdough is of even greater quality if used in combination with enzymes, they add.
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, VTT reports.
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 less than 4 and the pH of bread than 5.
The same sourdough did not prevent rope spoilage if the pH of sourdough was in excess of 4.5.