Effect of initial pH on pigments yield and mycelial morphology from Monascus purpureus HBSD08

Purpose This study aims to investigate how the mycelium morphology and the biosynthesis of intracellular and extracellular yellow, orange and red pigments from Monascus purpureus (M. purpureus) were regulated by various initial pH values under liquid fermentation conditions. Design/methodology/approach This study was conducted by cultivating M. purpureus HBSD08 under liquid fermentation media with different initial pH values (from 2.0 to 12.0). Scanning electron microscopy (SEM) was used to observe those mycelium morphology by cultivating at various initial pH values, and those yields of intracellular and extracellular yellow, orange and red pigments, as well as total pigment yields, were measured and calculated. Statistical analysis with p-value less than 0.05 was performed to evaluate the significance of yield differences. Findings The maximum yields of total orange pigment (9.08 ± 0.68 AU·mL−1), total red pigment (11.33 ± 0.49 AU·mL−1), extracellular pigments (24.18 ± 1.90 AU·mL−1) and total pigments (30.28 ± 1.90 AU·mL−1) all occurred at initial pH 5.0. The highest yellow pigment production (14.81 ± 1.60 AU·mL−1) was observed at pH 3.0, while intracellular pigments peaked (14.42 ± 0.48 AU·mL−1) at pH 2.0. At pH 2.0, intracellular orange and red pigment yields increased 3.7- and 2.7-fold, respectively, over pH 3.0, whereas yellow pigment decreased by 75%. Notably, orange and red pigment synthesis exhibited synchronous patterns, at pH 12.0, extracellular pigment yield exceeded that at pH 11.0 by 1.5-fold. Pellet diameter increased then decreased with rising pH, with the most robust morphology at pH 3.0–9.0. Pellets were smallest at pH 2.0 and 12.0. SEM revealed fewer surface particles at pH 2.0, correlating with intracellular pigment accumulation, while pH 12.0 promoted larger surface area and more particles, enhancing extracellular secretion. Originality/value This study systematically elucidates the variation patterns of Monascus pigment biosynthesis and mycelial morphology under different initial pH conditions. The findings demonstrate that extreme initial pH values can effectively steer pigment synthesis toward specific types and spatial distributions (intracellular vs extracellular). Moreover, this work enhances our understanding of the mechanistic role of mycelial morphology in pigment biosynthesis, offering a novel strategy for targeted pigment production.