Microbiological, physicochemical and sensory changes throughout ripening of an experimental soft smear-ripened cheese in relation to salt concentrations.

To evaluate the sodium chloride content effect on microbiological, biochemical, physicochemical and sensorial characteristics, Munster cheeses were prepared from pasteurized milk seeded with 3 yeasts (Kluyveromyces marxianus, Debaryomyces hansenii, Geotrichum candidum) and 5 ripening bacteria (Arthrobacter arilaitensis, Brevibacterium aurantiacum, Corynebacterium casei, Hafnia alvei, and Staphylococcus equorum). Experiments were performed under 1.0%, 1.7% and 2.4% NaCl levels in cheese in triplicate. Ripening (d2 - d27) was carried under 12°C and 96% RH. These kinetics were both reproducible and repeatable at 99% confidence level. For each microbial, biochemical and physicochemical parameter, 2 kinetic descriptors (maximal or minimal rate and its occurrence time) were defined. On d2 the physicochemical variables (water activity, dry matter, water content) were strongly dependent on the salting level. From d2 to d27 K. lactis was insensitive to salt while D. hansenii was stimulated. G. candidum growth appeared very sensitive to salt in cheese: at 1.0% NaCl G. candidum exhibited overgrowth, negatively impacting rind appearance, underrind consistency and thickness and off-flavor flaws. Salt concentration of 2.4% induced death of G. candidum. Four bacteria (A. arilaitensis, B. aurantiacum, C. casei, and H. alvei) were moderately sensitive to salt while S. equorum was insensitive to it. Salt level in cheese had a significant effect on carbon substrate consumption rates. Lactate consumption rate in 1.0% salted cheeses was approximately twice higher than under 2.4% NaCl. Data analysis of microorganism, biochemical, and physicochemical kinetics and sensory analysis showed that the best salt level in Munster-type cheeses to achieve an optimum balance between cheese characteristics, sensory qualities and marketability was 1.7% NaCl.

Survival of cheese-ripening microorganisms in a dynamic simulator of the gastrointestinal tract.

A mixture of nine microorganisms (six bacteria and three yeasts) from the microflora of surface-ripened cheeses were subjected to in vitro digestive stress in a three-compartment "dynamic gastrointestinal digester" (DIDGI). We studied the microorganisms (i) grown separately in culture medium only (ii) grown separately in culture medium and then mixed, (iii) grown separately in culture medium and then included in a rennet gel and (iv) grown together in smear-ripened cheese. The yeasts Geotrichum candidum, Kluyveromyces lactis and Debaryomyces hansenii, were strongly resistant to the whole DIDGI process (with a drop in viable cell counts of less than <1 log CFU mL(-1)) and there were no significant differences between lab cultures and cheese-grown cultures. Ripening bacteria such as Hafnia alvei survived gastric stress less well when grown in cheese (with no viable cells after 90 min of exposure of the cheese matrix, compared with 6 CFU mL(-1) in lab cultures). The ability of Corynebacterium casei and Staphylococcus equorum to withstand digestive stress was similar for cheese and pure culture conditions. When grow in a cheese matrix, Brevibacterium aurantiacum and Arthrobacter arilaitensis were clearly more sensitive to the overall digestive process than when grown in pure cultures. Lactococcus lactis displayed poorer survival in gastric and duodenal compartments when it had been grown in cheese. In vivo experiments in BALB/c mice agreed with the DIDGI experiments and confirmed the latter's reliability.