Production of organic acids, titratable acidity and pH-development during fermentation of cereal flours

ABSTRACT Following the household method of fermentation used in making the Ethiopian bread injera, fermentation of a flour/water slurry was carried out. The fermentation experiments were done at temperatures of 25 and 35 °C with whole grain wheat flour, whole grain tef (Eragrostis tef) flour and commercial bakery flour of 70% extraction rate. The slurry was made by mixing 300 g flour and 600 ml water. Backslopping was done by adding a portion of the slurry which was fermented in a previous batch as a starter (0.1%, 1.0%, and 10%) to the next batch to be fermented. During the fermentation, samples were taken at definite intervals of 3, 6, 9, 12, 18, 24, 48 and 84 hours respectively and kept frozen until they were used for analysis. The pH and titratable acidity were recorded and the organic acids were determined by HPLC using an Aminex® HPX-87H column, The growth of lactic acid bacteria including the typing of species/strains was also determined. Samples of the fermenting slurry were taken for analysis and the final concentration of lactic acid was found to be approximately 1 g per 100 g. A higher amount of inoculum gave a quicker lowering of pH, and also a lower initial pH. A smaller inoculum amount showed a delayed start in the lowering of pH. A higher temperature increased the production rate of lactic acid. The dominant (100% at 25°C) species of the final sample of fermented tef slurry was found to be Lactobacillus plantarum. Commercial bakery flour showed a comparatively low buffering capacity, less than 0,1 g lactic acid/100 g was needed to reduce pH to <5. Tef, a cereal with very small seeds contributing a larger portion of outer parts of the seeds in the flour, had the highest buffering capacity. In the region pH 6 to 4, approximately double the amount of lactic acid was required in comparison to the commercial bakery flour in order to attain the same pH. This indicates that at the same pHlevel, a fermented food item made of tef, and also of whole grain wheat, probably has a higher food safety than a food item made of commercial bakery flour. Thus, the use of whole grain flour improves not only the nutritional quality, but also the food safety of a fermented food, which is of importance under household conditions of low-income countries. At normal injera fermentation (tef, 1% backslopping at 25°C) it took 12 h t