Impact of dairy cultures redox potential on the growth of spoilage bacteria causing discoloration defects in soft cheeses

Five strains belonging to the species Pseudomonas gessardii, Ps. proteolytica, Serratia marcescens Buttiauxella spp. were isolated and identified as the causative agents of abnormal coloration in Stracciatella and other soft cheeses. Subsequently, 95 wild and commercial lactic acid bacteria (LAB) strains were screened in vitro using culture-spot assay, agar well diffusion, and co-cultivation in litmus milk to evaluate both direct and deferred antagonistic activity against the spoilage stains. Results indicated that antimicrobial activity was strain-dependent and primarily associated both with the acidification and reduction capabilities of the LAB strains. Based on these findings, four LAB mixtures, each composed of four top-performing strains, were formulated and tested during the production, ripening (7 days at 4°C), and temperature abuse storage (7 days at 10 °C) of Crescenza cheese inoculated with the spoilage strains. Among them, Mix 4, containing Lactiplantibacillus plantarum and Lacticaseibacillus paracasei, exhibited the highest reducing activity (Eh: 306.3 mV) and the strongest antimicrobial effect, reducing Pseudomonas counts by more than 2 log units. Furthermore, the application of all the LAB mixtures prevented the pigment formation under abuse storage conditions (10 °C). For the first time, evidence was provided on the contribution of the reducing activity of lactic acid bacteria in counteracting the development of spoilage bacteria in cheese and on the importance of considering this metabolic activity in the selection of LAB strains for the bio-protection of dairy products.