Application of pumpkin and mung bean flours in wheat sourdough starters for leavening and steamed bun quality

Sourdough fermented using lactic acid bacteria (LAB) and yeast, functions mainly as a leavening agent for breads with limited nutrients. Nutritious non-cereal flours from pumpkin (vegetable) and mung bean (legume) were thus targeted to be used along with the typical ingredient, wheat (cereal) as composite flours to improve sourdough starter and bread. Sourdough starters with up to 20% pumpkin or mung bean flour were developed for the first time by mixing equal amount of composite flour and distilled water, fermented for 14 days at room temperature with daily pH and total titratable acidity (TTA) measurement, followed by backslopping at 1:1:1 mass ratio. As starters developed and gained maturity, a sour aroma developed as pH decreased in overall by > 30.0% and TTA increased more than twofold. Pumpkin and mung bean addition slightly raised the final sourdough pH and TTA to satisfactory pH < 4.0. Based on the stable pH and most excellent leavening, matured composite starters with pumpkin and mung bean flours at 15% were selected together with control for three-stage activation study where leavening performances were modelled as a new attempt to improve the challenging and time-consuming sourdough fermentation processes. The re-parameterised Gompertz equation was used for modelling the hourly changes in volume expansion ratio measured by its height in a beaker against fermentation time. Backslopping was conducted at maximum height separately for each sourdough starter to initiate the second stage, followed by the same process repetitively until the end of third stage. The addition of pumpkin and mung bean markedly increased the maximum volume expansion ratio from 0.40 to 1.72 and 2.03 cm3/cm3 respectively and reduced lag time from 6.39 to 2.13 and 2.35 h following the activation system. The leavening model was predicted accurately with most R2 > 0.97. To tackle the uncertain acceptance of local consumers, the new potential of all three activated starters were explored to make steamed sourdough buns (SSBs). Doughs were initially formed, proofed for 4 hours and mixed until fully developed using pumpkin flour fortification at 0, 5, 10, 15 and 20% for the 1-hour second proofing. The doughs were steamed for 30 minutes. SSB specific volume was significantly raised from 1.05 to 1.88-2.22 cm3/g, whereas spread ratio and hardness significantly dropped from 1.86 to 1.41-1.59 and from 7353 to 2605-2991 gram-force respectively by involving pumpkin and mung bean composite starters. The pumpkin fortification of SSBs was optimum within 5-10% for attractive colour, soft texture and satisfactory sensory scores. Apart from the usual studies on sourdough starters, microbiological analyses were also conducted on sourdough products, SSBs with each composite sourdough starter at 5% pumpkin fortification and control SSB to detect differences of microbiological profiles from starters. Possibly probiotic LAB under Lactobacillus genus was the major bacteria in composite sourdough starters at 94.7-97.1% while the predominance remained at lower abundances of 51.8-53.9% in SSBs. Composite sourdough starters had fungal communities dominated by the Saccharomyces genus at 72.9-82.6% indicating non-cereal addition has encouraged yeast growth that was absent in control, thus improving leavening capability.

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