Validation of a predictive model for growth of gas-producing Leuconostoc spp. in MA-packed deli-meat

Microbial gas formation by heterofermentative Leuconostoc spp. is a major cause of package bulging and food waste in modified-atmosphere (MA) packed deli-meats. This study identified the specific spoilage organisms associated with bulging in commercial products, developed a secondary growth model for Leuconostoc spp. in heat-treated MA-packed deli-meat and linked Leuconostoc growth to CO₂ accumulation and bulging as a function of packaging material. Bulging retail products (n = 28) were collected from Danish supermarkets and characterised for headspace gas, pH, aerobic viable counts and microbiota (16S rRNA amplicon sequencing), followed by isolation and whole-genome sequencing of gas-producing isolates. The microbiota of bulging pork and chicken products were dominated by Leuconostoc. Gas-producing isolates were identified as being mainly Leuconostoc carnosum and Leuc. mesenteroides. For model development, 36 challenge tests with cooked bologna-type deli-meat formulated with different pH, salt-in-water, and added Na-lactate, Na-acetate and nitrite were conducted under 30% CO₂/70% N₂ at 3-8 °C. Growth curves were fitted with the Baranyi and Roberts primary model to estimate V_max, and Na-lactate and Na-acetate were converted to total water-phase lactate and acetate. A temperature-scaled multiple linear model with a centred temperature term (T - T_min, T_min = -1.75 °C) and linear effects of T, pH, salt-in-water, lactate and acetate plus pH-acid interaction terms provided the best description of V_max. Independent validation using 52 in-house challenge tests and 107 literature growth curves (total n = 159) showed acceptable performance for the new Leuconostoc model (bias factor 1.13, accuracy factor 1.44). Visible bulging in low-permeability films occurred mainly when Leuconostoc populations exceeded ∼7 log CFU/g, whereas high-permeability films prevented CO₂ build-up despite similar growth. The new model provides a practical tool for predicting Leuconostoc-driven gas spoilage and for optimising formulation, packaging and shelf life of MA-packed deli-meats.