Diversity in the bacterial spoilage of the french dessert "île flottante"
Résumé
The consumption of chilled dairy desserts is increasing in the European Union. The famous French “île flottante”, consisting in a foam of egg white floating on a custard cream, represents a highly nutritive medium for bacterial growth and for the expression of enzymes susceptible to affect the food shelf-life at chilled temperatures. To avoid such issues, the suppliers may either adjust the processes of heat-treatment -but this means of control is limited due to the high heat-sensitivity of egg proteins- or incorporate food additives. However, there is an increasing consumer demand for natural products without preservatives. The present study aims at better controlling the spoilage events by fully characterising the bacterial biodiversity and spoilage potential during cold storage.
Sixty-five spoiled desserts delivered by two companies were analysed for their (i) sensorial characteristics and (ii) bacterial counts and 16S rDNA sequencing. Five hundred sixty-six isolates were further studied through an in vivo (in pure custard cream) spoilage test at 10°C. An in vitro (in a custard cream-based medium) test allowed assaying the expression of proteases (10°C) and growth (6°C). The expression of lipases was assayed on PCA tributyrin agar (10°C).
The spoilage mainly concerned the custard cream part of the dessert, exhibiting high bacterial counts (> 6 log UFC/mL), pH decrease, thickening, coagulation and appearance of off-odours. Eighty bacterial genera were identified, mainly corresponding to Gram positive bacteria. The contamination by the Bacillus cereus group concerned 80% of the spoiled desserts. Fifty-seven and 8% of the recovered isolates expressed proteolytic and lipolytic activities at 10°C. Thirty four percents was able to grow at 6°C in the in vitro test. A statistical analysis of the whole data highlighted various correlations between the type of bacterium, its growth and enzymatic potentials at low temperature, and the spoilage characteristics of the dessert.