Due to the high cost of wheat importation in countries where the climatic conditions do not favour its cultivation, alternative sources of bread baking flour are required. Maize is a suitable alternative because it is by far the most important crop produced in Africa. However, it lacks gluten, the protein that is formed in wheat dough which is responsible for the desirable quality attributes (high loaf volume, soft and open crumb structure) of wheat bread. Therefore the need arises to improve maize bread quality.
The effects of three types of non-wheat bread methods on the quality of maize bread were investigated. The first was a traditional sourdough method used in Lesotho for making steamed bread. This involved addition of spontaneously fermenting sorghum malt sourdough (equivalent to 15% of the total maize flour) and pre-gelatinization of the starch in the maize flour with boiling water. The second was a Food and Agriculture Organization method which involved pre-gelatinization of the starch in 10% of the maize flour by cooking. The third method was a modern gluten-free sourdough method which involved fermenting 75% of the maize flour with a multiple strains starter culture or Lactobacillus plantarum plus the natural flora in the maize. The modern sourdough method produced maize bread with a more open crumb structure and a significant increase in loaf volume compared to the other methods. This was probably related to the high percentage of fermented maize flour in the recipe, which was probably sufficient to modify the dough properties satisfactorily enough to impact positively on the maize bread quality. Based on these findings, the modern sourdough method was investigated further.
Maize sourdoughs were prepared (as described) and compared to chemically acidified maize dough. Sourdough maize bread had an approx. 25-26% increase in loaf volume and a more open crumb structure with large gas cells. This showed that the maize bread quality improvement was not due to low pH. Confocal laser scanning microscopy revealed a cohesive dough structure in the sourdoughs. Larger cells and a more uniform crumb structure were also observed in maize breads with maize sourdough. This indicated an improvement in the maize dough properties with sourdough. Differential scanning calorimetry showed that maize sourdough had a slightly lower peak temperature than straight maize dough, an indication of starch modification. Rheological analysis showed that maize sourdough had a shorter relaxation time, an indication that it was less elastic. Strain sweep analysis revealed that maize sourdoughs had the lowest elastic modulus, also indicating a less elastic dough. Temperature sweep analysis showed an initial less elastic dough and a final high tan delta, suggesting that the maize dough could withstand gas expansion pressure during baking without crumbling.
The dominant lactic acid bacteria in the sourdoughs were identified as L. plantarum. In the two sourdoughs, the L. plantarum present were gram-positive, catalase negative and exhibited proteolytic activities. However, only the L. plantarum in the multiple strains starter culture fermented maize sourdough exhibited amylolytic activities. It is proposed that proteolytic activity of the L. plantarum degraded the endosperm protein matrix and hydrolysed the proteins soluble in the dough liquid, thereby allowing increased accessibility of water to the starch granules. It is further proposed that the amylolytic activity of the L. plantarum slightly hydrolysed the starch granules, increasing water absorption by the starch granules.
It is proposed that improvement in maize bread quality by sourdough fermentation is due to starch modification (increase water accessibility and water absorption by the starch granules due to the proteolytic and amylolytic activities of the dominant lactic acid bacteria in the sourdoughs) which made the dough less elastic. This in-turn improves the ability of the dough to trap and withstand the pressure of the expanding carbon dioxide in the fermenting dough and bread.