Survival of probiotic microflora in Argentinian yoghurts during refrigerated storage
Introduction
In recent years, there has been a worldwide increasing interest in the addition of intestinal bacterial species (Bifidobacterium spp. and Lactobacillus acidophilus/casei) to fermented milks. The addition of probiotic bacteria is made not only because of certain claimed health-promoting effects in the intestinal tract (Klaver, Kingma & Weerkamp, 1993) but also because of the sensory aspects as well as the expanding variety of products that can be formulated with them (Kneifel, Jaros & Erhard, 1993). The main “probiotic effects” attributed to these bacteria are: enhancement of immunity against intestinal infections and improvement in lactose utilisation, prevention of diarrhoeal diseases, colon cancer, hypercholesterolaemia and upper gastrointestinal tract diseases, stabilisation of the gut mucosal barrier (Kailasapathy & Rybka, 1997), formation or reconstruction of a well-balanced indigenous intestinal microflora, improvement of calcium absorption and vitamin synthesis and pre-digestion of proteins (Nakasawa and Hosono, 1992, Wood, 1992).
To perform their probiotic action these bacteria must arrive at the intestinal tract alive. This requires their survival in the food used as a vehicle during its shelf-life and after consumption, and their resistance to the acidic conditions of the stomach as well as to bile salts in the small intestine (Kailasapathy & Rybka, 1997). Taking into account all these barriers, it is regarded as essential that: (a) the carrier food contains at least 106 viable cells of the probiotic microorganism per gram, (b) the species are of human origin (L. acidophilus, L. casei, B. bifidum, B. longum, B. adolescentis, or B. infantis), and (c) the total intake per week of the product is approximately 300–400 g (Samona & Robinson, 1994).
Some studies have been carried out with the objective of monitoring the survival of the constitutive microflora and the intestinal probiotic bacteria added to the different fermented milks (Beerens, 1996, Martin & Chou, 1992, Lapierre et al., 1992, International Dairy Federation (1995), Lim et al., 1995, Rybka and Kailasapathy, 1996, Pacher and Kneifel, 1996, Dave and Shah, 1996, Silvi et al., 1996, Nu-trish Cultures Catalog. (1996), Reinheimer and Vinderola, 1998).
In Argentina, there is still a tendency to produce yoghurts with a relatively high acidity level (pH values ranging from 4.0 to 4.5). Lactic acid starters are used with direct (DVS) and semidirect inoculation. On the other hand, a minimal content (106 CFU g−1) was established for bifidobacteria added to fermented milks by regulations recently approved (Pagano, 1998) by the countries of MERCOSUR (Argentina, Paraguay, Brazil and Uruguay). Regulatory levels for L. acidophilus in these products have not yet been established. However, the high acidity of Argentinian fermented milks generates doubts about the viability of the probiotic microflora.
The aim of this work was to evaluate the survival of lactic acid and intestinal probiotic bacteria in Argentinian commercial yoghurts during refrigerated storage, as well as the effect of the lactic acid content on cell viability.
Section snippets
Starters and probiotic cultures
Two commercial lactic acid starter cultures (lyophilised form) were used for reduced-fat (liquid) and full-fat (set) yoghurts. Direct inoculation was done with SID (Centro Sperimentale del Latte, Italy) and indirect inoculation was performed with SISD (Centro Sperimentale del Latte, Italy). Both cultures contained S. thermophilus and L. delbrueckii subsp. bulgaricus. The lyophilised probiotic cultures used were B. bifidum BBI and L. acidophilus LAI, from our collection. These cultures are
Changes in microflora and pH in yoghurt
Fig. 1, Fig. 2 show the viability of the lactic starter- and probiotic bacteria in reduced-fat and full-fat yoghurts, respectively, as well as the changes in pH, during the refrigerated storage. Yoghurts produced with the starter SID (direct inoculation) contained a lactic acid bacteria concentration higher than 107 CFU ml−1. S. thermophilus counts were higher —by at least 1 log order — than those for L. delbrueckii subsp. bulgaricus . In yoghurts produced with the lactic culture SISD
Discussion
The addition of probiotic bacteria (Bifidobacterium, L. acidophilus and L. casei) to fermented milks is a practice widely adopted by dairy industries. However, it is recognised that there are some physicochemical factors that might condition the survival of probiotic microflora in fermented dairy products, the most important being: yoghurt acidity, dissolved oxygen, species interaction, inoculation practice and storage conditions (Kailasapathy & Rybka, 1997).
In Argentina, the production of
Acknowledgements
This work was supported by the Agencia Nacional de Promoción Cientı́fica y Tecnológica from Argentina (Project PICT No 09-00000-00747).
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