Enhanced bioconversion of potato byproducts into natural blue pigments by Streptomyces lydicus PM7: bioprocess optimization and biochemical insights

Artificial colorants raise health and environmental concerns, creating demand for sustainable natural alternatives. Blue pigments are particularly scarce due to their structural complexity and instability, with actinorhodin standing out among Streptomyces metabolites. A major challenge for actinorhodin production is to improve yields and reduce costs to enhance process feasibility. Discarded potato, an abundant and underutilized agricultural byproduct, is a nutrient-rich, low-cost substrate for microbial processes. Recently, a Streptomyces lydicus strain was reported to convert this byproduct into actinorhodin, but with relatively low production compared to traditional media and other Streptomyces species. This study aimed to optimize the conversion of discarded potato into actinorhodin-related blue pigments by S. lydicus PM7 and to evaluate biochemical responses that influence pigment production. A Plackett–Burman design identified temperature, agitation, pH, and KH2PO4 supplementation as significant factors among 11 tested variables. Optimization using a central composite face-centered design (CCD) within the framework of response surface methodology (RSM) increased pigment production up to 8000 mg L− 1. Model validation using point prediction identified optimal conditions of 30 °C, 180 rpm, an initial pH of 9, and 0.15 g L− 1 KH2PO4. Growth kinetics under optimized conditions revealed two exponential phases and shifts in α-glucosidase and α-amylase activities, indicating a possible sequential use of carbohydrates. Catalase activity coincided with the onset of exponential growth and pigment production. The optimized process yielded an 8.5-fold increase in pigment production, supporting the use of potato byproducts as an effective and low-cost fermentation substrate. The biochemical responses of S. lydicus PM7 provide initial insight into metabolic features associated with pigment formation. Overall, the findings establish a laboratory-scale proof of concept and a basis for future bioreactor-scale and application-oriented studies on microbial blue pigments by Streptomyces spp.