Optimization of the Development of a Lactic Acid Bacteria Inoculum in Pumpkin Juice for Potential Applications in the Fermentation of Plant-Based Matrices

The objective of this study was to evaluate the effect of sucrose supplementation, powdered milk supplementation, the proportion of pumpkin in the activation medium, and heat treatment on the viability of the total culturable LAB population of a commercial mixed starter culture containing Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, in order to develop an inoculum adapted to a plant-based matrix. The experimental process included a screening design, path of maximum ascent, and response surface optimization, supplemented by principal component analysis (PCA). Cell viability was determined by microbiological count (CFU/mL) and expressed as the logarithmic increase in cell viability (LVI). The refined screening model was statistically significant (p = 0.003). SS and PMS showed significant positive effects on LVI, whereas PJ showed a negative effect close to the significance threshold. The optimization predicted a maximum LVI of 5.01 with 9.41% SS and 5.84% PMS, with PJ fixed at 7.28%. The model had an R2 of 95.13%, and experimental validation showed a relative error of 7.663% between the predicted and experimental responses. PCA explained 79.81% of the variability through two components (PC1 = 61.02% and PC2 = 18.80%), associating PC1 with conductivity, salinity and suspended solids, and PC2 with pH, total solids and titratable acidity. PCA described the joint variability of the physicochemical parameters across the screening treatments.