MICROBIAL CONTAMINANTS CAUSE SWELLING AND INWARD COLLAPSE OF YOGURT PACKS

A study was carried out on 10 retail samples of yoghurt, belonging to 3 different brands (A, B and C), showing a rarely described alteration consisting of inward collapse of the packs. Microbiological analyses revealed the presence of high counts of different microbial contaminants: in A samples Acetobacter aceti was found at 10(7) cfu/g, while in B and C samples the main contaminants were Mucor hiemalis Mucor racemosus and Penicillium verrucosum var cyclopium varying between 10(2) and 10(5) cfu/g. The responsibility of isolated strains for alteration was confirmed by inoculating cellular suspensions in non-contaminated yoghurt packs kept at 5-degrees-C and 25-degrees-C: samples inoculated with A aceti showed an inward collapse after 20 d at 25-degrees-C; packs inoculated with M hiemalis, M racemosus and P verrucosum var cyclopium kept at 25-degrees-C showed swelling after 5-7 d followed by inward collapse after almost-equal-to 20-35 d. In samples kept at 5-degrees-C the defect did not appear even extending the incubation time to 60 d. The explanation for the evolution of this defect is the following one: contaminating microorganisms consume oxygen contained in head-space of pack and produce CO2 whose re-absorption into the product causes a partial vacuum involving collapse of the pack.