Effect of tagatose on growth dynamics of Salmonella Typhimurium and Listeria monocytogenes in media with different levels of structural complexity and in UHT skimmed milk

• The growth of Salmonella Typhimurium is limited by tagatose, low temperatures and solid structures.
• Tagatose facilitates the growth of planktonic cells of Listeria monocytogenes at very low temperatures.
• Increasing structural complexity of the media enriched with tagatose, has a mild effect on the growth of L. monocytogenes.

Tagatose is a novel low-calorie sweetener which addition changes food product properties, e.g., aw, and formula, that may influence food-microbial growth dynamics in both liquid and solid products. The aim of this work was to study growth dynamics of Salmonella Typhimurium and Listeria monocytogenes in liquid and solid media enriched with tagatose (1.5, 4.5 and 7.5% (w/v)), at 4, 8 and 20 °C. More specifically, liquid media were chemically defined minimal medium, general medium and UHT skimmed milk; gelatine and a gelatine-dextran mixture were the gelling agents used, leading respectively a homogeneous and heterogeneous solid system. At regular intervals, cell concentration was determined and the Baranyi and Roberts (1994) model was fitted for growth parameter estimation. Results show a reduced and even no growth of S. Typhimurium with increasing tagatose concentrations, especially at low temperatures and when increasing media complexity. The behaviour of L. monocytogenes is not affected by tagatose, except in liquid at 4 °C, where tagatose facilitates the growth in general culture media. Varying responses of the studied bacteria to changes in media formulation should be considered for future product design.