Brewery spent yeast medium for <i>Serratia</i> sp. bio-beads improves <i>Chenopodium quinoa</i> Willd. growth in the Northern Altiplano of Bolivia

Quinoa (Chenopodium quinoa Willd.) is a climate-resilient Andean crop with high nutritional value and strategic importance for food security in high-altitude regions. However, its productivity in low-input farming systems remains limited. This study developed scalable strategies for propagation and formulation of a Serratia sp. strain as a biofertilizer, using brewery spent yeast (BSY) as growth substrate. Microwave-assisted extraction (MAE) at 1200 W for 15 min significantly (P ≤ 0.05) enhanced soluble protein release from BSY, and MAE-treated media with a C:N ratio of 24:1 supported optimal bacterial growth. Carrageenan-based bio-bead formulations produced at 40°C with 96 g L⁻¹ carrageenan yielded the highest bacterial viability and moisture retention. In a field trial in the Bolivian Altiplano, bio-beads containing Serratia sp. applied at branching stage increased quinoa yield by up to 3.4-fold (P ≤ 0.01) compared with the control. The formulation control also substantially improved yield (2.2-fold), indicating that both the carrier matrix and bacterial inoculation contributed to growth enhancement. These findings demonstrate the potential of biofertilizer technologies based on agri-food by-product valorization to improve crop performance under extreme and resource-limited agricultural conditions.