Regional and seasonal influences on the microbiological quality of raw milk in northwestern Tunisia: insights into total flora, psychrotrophs, Pseudomonas spp., and heat-resistant spores

To our knowledge, this study provides the first comprehensive investigation in Tunisia of the combined influence of region and season on the microbiological quality of raw milk collected from the governorates of Jendouba, Béja, and Kef in northwestern Tunisia. A total of 1224 samples (102/month) were analyzed over a 12-month period from March 2023 to February 2024. Significant regional and seasonal variations were observed. Total aerobic mesophilic count ranged from 10⁶ to 10⁷ CFU/mL in winter, autumn, and spring, reaching 8 log CFU/mL in summer. Psychrotrophic bacteria and Pseudomonas spp. counts were lowest in winter (5-6 log CFU/mL) but increased markedly during warmer months. Heat-resistant spore (HRS) counts ranged approximately from 0.5 to 1.47 log CFU/mL across seasons, with the highest levels observed in spring. The proportion of spore-contaminated samples increased from 58% in April to 100% in August before declining to 87% in March. Psychrotrophic flora were largely dominated by Pseudomonas spp. (77-86% of isolates), with the highest prevalence in autumn. Regionally, psychrotrophic loads remained high across all governorates (7.1-7.3 log CFU/mL), while Pseudomonas counts were significantly higher in Jendouba. HRS contamination remained relatively low (1 log CFU/mL) and consistent across regions. Correlation analysis revealed strong associations between psychrotrophs, total aerobic mesophilic count, and Pseudomonas, confirming their central role in raw milk spoilage. Overall, these findings highlight the pronounced impact of environmental, hygienic, and seasonal factors on raw milk quality. Improved hygiene practices, rapid cooling, and reinforcement of the cold chain, particularly during high-risk seasons, are essential strategies to enhance milk safety and extend shelf life in Tunisia’s dairy sector.

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