The effect of starter cultures on the pH, bacterial population and volatile flavour compounds in ‘ugba’ during fermentation

Abstract

Traditionally, ugba of acceptable quality made from African oil bean (Pentaclethra macrophylla Benth) is characterized by its typical flavour and aroma. In this study, aroma profile and volatile flavour compounds released by spontaneously fermented ugba and five samples of ugba inoculated with starter cultures (A - Bacillus subtilis; B - B. safensis; C- Mixed culture of B. subtilis and B. safensis; D - Mixed culture of B. subtilis, B. safensis, B. clausii, B. licheniformis, and B. xylanilyticus; and E - B. cereus sensu lato) were monitored at 24 hour interval during 72 hours fermentation using solid-phase microextraction (SPME) and gas chromatography-mass spectrometry (GC-MS). Commercially prepared ugba is the control. The total heterotrophic bacterial count (THBC) of the samples were determined using Standard Microbiological Methods; the pH involved the use of standard method. Increase in the THBC and pH of all the samples of fermenting ugba were within the range of 1.4 × 104 to 1.7 x 1012 CFU/ml and 5.89 to 8.24, respectively. A total of 31 compounds which comprise of 7 ketones, 7 alcohols, 5 organic acids, 4 aldehydes, 2 pyrazines, 1 ester, 1 furan, 1 sulfur and 3 other compounds were identified as aroma compounds in the fermenting samples of ugba. Flavour compounds categorized as ester, furan and sulfur compound were consistently detected in all the samples of ugba prepared in the laboratory. Ugba prepared using starter cultures including the spontaneously fermented sample compare favourably with commercially produced ugba with regards to volatile flavour compounds.