Effect of Devil's Claw and Moringa oleifera on cold-contact fermentation performance of Metschnikowia pulcherrima for low alcohol marula beer production

Edible medicinal plant powders (EMPPs) have gained interest as ingredients in the production of fruit-based functional beverages. Thus, this study evaluated the effect of Devil's Claw (DC) plant root powder and Moringa oleifera (MO) plant leaf powder on the fermentation performance of Metschnikowia pulcherrima for low alcohol marula fruit beer production under cold-contact fermentation conditions. Changes in cell viability and physicochemical properties were monitored. In addition, the total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activity (AA) were determined. The addition of EMPPs did not result in a significant (p < 0.05) reduction of the specific gravity (SG) of the marula fruit beers over time. Still, it significantly (p < 0.05) increased total sugar (glucose and fructose) utilisation, with MO-treated marula fruit beer showing significantly higher total sugar (glucose and fructose) utilisation (up to 94.27 %) after 312 h. Significantly (p < 0.05) higher free amino nitrogen and yeast assimilable nitrogen utilisation (up to 71.47 %) were observed in DC-treated marula fruit beer at the end of fermentation, highlighting the yeast's metabolic switch to favour nitrogen sources for cell growth and viability. The addition of EMPPs produced acceptable alcohol concentrations (≤ 1.50 % v/v) for low alcohol fruit beers. Phytochemical analysis showed that a significantly (p < 0.05) higher concentration of EMPPs resulted in a higher final TFC at the end of the fermentation compared to the control (Ctrl). Specifically, MO-treated marula fruit beer treatment 2 (MO2) and DC-treated marula fruit beer treatment 2 (DC2) showed a TFC percentage increase of 14.49 % and 62.88 %, respectively. The TPC in the MO-treated marula fruit beers (MO1 and MO2) increased with fermentation after 312 h. A reverse trend was observed in the DC-treated marula fruit beers (DC1 and DC2) after 312 h. Additionally, all the EMPPs-treated marula fruit beers showed higher AA than the Ctrl at the end of the fermentation process. The AA in MO2 and DC1 showed consistent AA of 74 % DPPH inhibition between 24 and 312 h. Overall, the EMPPs enhanced the cell viability of M. pulcherrima by converting carbon and nitrogen sources into cellular components, organic acids, and a limited amount of alcohol. Moreover, the EMPPs enhanced the phenolic compound development and AA of the low alcohol marula fruit beer, with MO showing a more stable and sustained phenolic response compared to DC.