International Dairy Journal 9 (1999) 125}134 Evaluation of aroma production and survival of Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus and Lactobacillus acidophilus in fermented milks Fausto Gardini*, Rosalba Lanciotti, Maria Elisabetta Guerzoni, Sandra Torriani Dipartimento di Protezione e Valorizzazione Agroalimentare, Universita % degli Studi di Bologna, Via San Giacomo 7-40126 Bologna, Italy Istituto di Produzioni e Preparazioni Alimentari, Universita % degli Studi di Bari, Sede di Foggia, Via Napoli 25-71100 Foggia, Italy Istituto di Industrie Agrarie, Universita % degli Studi di Catania, Via Santa Soxa, Catania, Italy Received 28 August 1998; accepted 24 February 1999 Abstract The fat, the non-fat milk solid and the inoculum size of Lactobacillus acidophilus IPVR 224 were modulated according to a Central Composite Design. The aim was to evaluate the e!ects of these variables and their interactions on the decrease in pH during fermentation, the qualitative and quantitative composition of the aroma pro"le, as well as the loss in viability of Streptococcus thermophilus IPVR 161, Lactobacillus delbrueckii subsp. bulgaricus IPVR 132 and L. acidophilus IPVR 224 strains during "ve weeks of storage. In addition, the overall acceptability of the fermented milks was assessed by means of a sensory panel. The polynomial quadratic equations obtained allowed to individuate the variables that signi"cantly a!ected the aroma pro"le, organoleptic properties and microbiological counts of the various fermented milks. The survival of the L. acidophilus strain during storage was higher at low concentration of non-fat milk solids and its presence did not a!ect the acetaldehyde content of fermented milks. From the response surfaces analysis it was possible to select optimum conditions for enhanced positive organoleptic traits and for improved survival of the probiotic culture.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Fermented milks; Lactobacillus acidophilus; Modelling; Survival; Aroma compounds 1. Introduction The microorganisms used for the production of fer- mented milks are claimed to impart nutritional and health bene"ts to consumers. The probiotic activity is related to lactic acid bacteria, that are able to proliferate or even survive for a long period in the human intestinal tract (Mital & Garg, 1992). Among the lactic acid bac- teria, Lactobacillus acidophilus is characterized by its capacity to colonise in the intestine even under low surface tension caused by the presence of bile salts (Gilli- land & Speck, 1977). Consumption of acidophilus milk products is considered to promote several bene"cial ef- fects such as control of enteric infections, normalization of gut micro#ora, lowering of serum cholesterol level, prevention of colon cancer, immunomodulation and en- hanced availability of nutrients (Mital & Garg, 1992; Klaver & van der Meer, 1993; Lee & Salminen, 1995; * Corresponding author. Mitsuoka, 1992; Gonc & Akalin, 1995). Moreover, milk inoculated with L. acidophilus has been found to improve lactose tolerance among lactose malabsorbers (Shah, 1994; Montes, Bayless, Saavedra & Perman, 1995) and inhibit enteric and foodborne microbial pathogens (Kilic, Pavlova, Ma & Tao, 1996). In a study of Dettmann (1995), the nutritional bene"ts of using mixed cultures of L. acidophilus and Bixdobacterium bixdum are stressed and among others their predominant production of L( # ) lactic acid. However, several L. acidophilus strains do not grow in milk and survive poorly in fermented prod- ucts. Shah, Lankaputhra, Britz and Kyle (1995) suggested that the number of viable L. acidophilus cells should be greater than 5 log cfu g to have therapeutic bene- "ts. Therefore, the use of probiotic cultures that provide high viable counts during the storage of the product is essential. The sensory characteristics of fermented milks play an important role in product acceptance by consumers. Sev- eral #avour compounds have been isolated from yog- hurt-type products; however only acetaldehyde, ethanol, 0958-6946/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 8 - 6 9 4 6 ( 9 9 ) 0 0 0 3 3 - 3