Original article Viability assessment of lactic acid bacteria in commercial dairy products stored at 4 °C using LIVE/DEADÒ BacLight TM staining and conventional plate counts Yolanda Moreno, 1 M. Carmen Collado, 1 M. Antonia Ferru ´ s, 1 Jose ´ M. Cobo, 2 Enrique Herna ´ ndez 1 & Manuel Herna ´ ndez 1 * 1 Departamento Biotecnologı´a, Universidad Polite´cnica de Valencia, Camino de Vera 14, 46002 Valencia, Spain 2 Red INDE, Investigacio´n Nutricional Danone Espan˜a, Barcelona, Spain (Received 15 November 2004; Accepted in revised form 6 June 2005) Summary The viability of lactic acid bacteria (LAB) from commercial fermented milks was studied during storage at 4 °C. The enumeration of total viable bacteria was assessed using fluorescence microscopy. Plate counting on selective media was used to enumerate LAB. Using LIVE/DEADÒ BacLight TM viability staining, it was observed that bacterial counts decreased gradually after expiry dates, the number of viable bacteria remaining above 10 6 bacteria g )1 for all of the products. Viable cell counts estimated by plating onto selective media were lower than those obtained by direct microscopic counting. The viability of LAB contained in acid products decreased during their storage period at 4 °C. All products contain viable LAB ranging from 10 8 to 10 9 bacteria g )1 and could be considered as probiotic, given that the recommended minimum number of probiotic bacteria in such food products is approximately 10 7 bacteria mL )1 product. The number of bifidobacteria in commercial fermented milks declared to contain bifidobacteria varied from 10 4 to 10 7 bacteria mL )1 . This study confirms the usefulness of fluorescent techniques for a rapid and accurate evaluation of bacterial viability in probiotic products. Keywords Fluorescent dyes, lactic acid bacteria, probiotics, viability. Introduction Fermented milks and yoghurts have the property of containing living micro-organisms with organo- leptic and nutritional qualities. These products are considered to be probiotic because, when con- sumed in certain quantities, they have beneficial effects on health such as maintenance of the intestinal gut, a better digestion of lactose, immune and anti-carcinogenic activities and a reduction in cholesterol levels (Sanders, 1993; Schaafsma, 1996a,b). Lactic acid bacteria (LAB) considered to be probiotic include the Bifidobacterium, Lactobacil- lus and Streptococcus species. It has been suggested that they should reach the intestine alive and in a sufficient number (10 6 –10 7 micro-organisms mL )1 ) for their benefits to be appreciated (Kurmann & Rasic, 1991; Bouhnik, 1993). This requires their survival in the food vehicle during its shelf-life and their resistance to the gastrointestinal conditions. In view of these barriers, it is regarded as essential that dairy foods contain at least 10 6 viable probi- otic organisms per mL; for this reason it is necessary to analyse efficiently the viability of micro-organisms probiotics because they must remain viable in the food vehicle (dairy foods) during their shelf-life. A number of studies have investigated the viability of LAB in different fermented milk products (Nighswonger et al., 1996; Shin et al., 2000) by conventional cultivation on a selective growth medium. However, this approach has a *Correspondent: Fax: +34 963877429; e-mail: mhernand@upv.es International Journal of Food Science and Technology 2006, 41, 275–280 275 doi:10.1111/j.1365-2621.2005.01060.x Ó 2005 Institute of Food Science and Technology Trust Fund