Effect of heat treatment and spray drying on lactobacilli viability and resistance to simulated gastrointestinal digestion

        R. Paéz et al. / Food Research International 48 (2012) 748–754        

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Effect of heat treatment and spray drying on lactobacilli viability and resistance to simulated gastrointestinal digestion

R. Paéz a, L. Lavari a, G. Vinderola b,⁎, G. Audero a, A. Cuatrin a, N. Zaritzky c, J. Reinheimer b

a INTA EEA Rafaela, Ruta 34 km 227, Rafaela, Santa Fe, Argentina b Instituto de Lactología Industrial (INLAIN, UNL-CONICET), Facultad de Ingeniería Química, Universidad Nacional del Litoral, 1° de Mayo 3250, Santa Fe, Argentina c Centro de Investigación y Desarrollo en Criotecnología de Alimentos (CIDCA, UNLP‐CONICET) La Plata, Argentina[pic 2]

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The diversification of the market of probiotic foods relies on the availability of new strains or new formats of probiotic cultures. Until now, fermented dairy products, mainly fermented milks, have been used as the most successful commercial food products for the delivery of probiotic bacteria (Figueroa-González, Quijano, Ramírez, & Cruz-Guerrero, 2011; Saxelin, 2008), being frozen and freeze-dried cultures the commercially available formats of starter and probiotic bacteria. In particular, the production of dried cell cultures is particularly interesting because, unlike frozen cultures, dehydrated cultures demand less storage capacity and lower cost of transport and refrigeration. However, the maintenance of cell viability during drying and storage is a major challenge. Insufficient or too extensive dehydration (moisture >5.0% (wt/wt) or b2.8% (wt/wt), respectively) causes bacterial inactivation (Zayed & Roos, 2004). Presently,

However, freeze-drying is a discontinuous and expensive process with low yields and time and energy demanding (Knorr, 1998; Meng, Stanton, Fitzgerald, Daly, & Ross, 2008). Spray drying is an interesting and promising low-cost alternative because it is relatively inexpensive and allows the continuous production of large amounts of dried cells within short time periods (Gardiner et al., 2000). However, it should be mentioned that cell dehydration may inevitably cause membrane damage and inactivation depending on the technological conditions applied. In spray drying bacterial cultures are exposed to different stresses (osmotic, heat, oxidative) due to the quite harsh conditions of temperature required for product dehydration, which can cause a partial thermal inactivation of cells.

The incorporation of probiotic cultures into fermented dairy products relies almost exclusively on the use of frozen or freeze-dried cultures provided by foreign companies. In particular in Argentina, many medium to big-size dairy industries possess the technological infrastructure for the production of spray dried probiotics. Spray drying can offer a 6 times less expensive alternative every kg of water removed compared to freeze-drying (Knorr, 1998). However it was observed that the success of its application is highly strain specific (Ananta, Volkert, & Knorr, 2005; Corcoran, Ross, Fitzgerald, & Stanton, 2004; Desmond, Stanton, Fitzgerald, Collins, & Ross, 2002a; Gardiner et al., 2000; Lian, Hsiao, & Chou, 2002; O'Riordan, Andrews, Buckle, & Conway, 2001).

The aim of this work was to screen among lactobacilli strains for candidates able to survive to spray drying and to study the effects of a preliminary heat treatment and different food matrices on post-drying survival and simulated gastric acid resistance.

1. Materials and methods

2.1. Strains and cultures conditions

A total of 22 commercial or collection strains of Lactobacillus casei,

Lactobacillus paracasei, Lactobacillus acidophilus and Lactobacillus plantarum was used in this study (see Table 1). The name of commercial strains was changed in order to avoid any conflict of interest. Strains belong to the culture collections of the INLAIN (UNL-CONICET, Santa Fe, Argentina) and CIDCA (UNLP-CONICET, La Plata, Buenos Aires, Argentina). When needed, fresh overnight (16 h, 37 °C) cultures of cells were obtained in MRS (deMan, Rogosa and Sharp) broth (Biokar, Beauvais, France) after three transfers from frozen (−70 °C) stocks maintained in MRS added with 18% (wt/vol) glycerol (Ciccarelli, Santa Fe, Argentina).

2.2. Heat tolerance assay

Heat resistance was evaluated using the conditions suggested by Simpson, Stanton, Fitzgerald, and Ross (2005) in overnight cultures harvested (6000×g, 15 min, 5 °C), washed twice with Phosphate Buffered Saline (PBS) solution (pH 7.1) and resuspended in MRS broth or 10% (wt/vol) skim milk (San Regim, Santa Fe, Argentina). Cell suspensions were placed in a water bath at 60 °C for 5 min and then immediately cooled on an ice bath. Cell counts (MRS agar, 48 h, 37 °C, aerobic incubation) were performed immediately before and after exposure to heat.

2.3. Spray drying in skim milk

L. plantarum com, L. paracasei A13, L. plantarum 8329, L. acidophilus A9 and L. casei Nad were selected for spray drying (justified in the Results section). Overnight cultures in MRS broth were harvested (6000×g, 15 min, 5 °C), washed twice with PBS solution (pH 7.1), re-suspended in 20% (wt/vol) skim milk and a mild (15 min at 52 °C) heat treatment (MHT) was applied or not (NMHT), according to Desmond, Stanton, Fitzgerald, Collins, and Ross (2001). Cell suspensions were spray dried in a laboratory scale spray dryer (Buchi mini spray dryer model B290, Flawil, Switzerland) by using a constant inlet air temperature of 170 °C, an outlet temperature of 85 °C and a flux of 600 l h−1. Spray drying conditions were those previously suggested as adequate for skim milk (Ananta et al., 2005; Gardiner et al., 2000; Gardiner et al., 2002). Cell suspensions were atomized and sprayed into the drying chamber by using a two-fluid nozzle. The product dried almost instantaneously and the residence time was very low. Three independent replicates were performed for each strain. Spray dried powders were vacuum sealed in individual samples of 10 g. Residual moisture (% wt/wt) was determined in triplicate at 101±1 °C (FIL-IDF 26 A: 1993). Cell counts of lactobacilli were performed before and after spray drying on MRS agar (37 °C, 48 h aerobic incubation) and periodically during the storage at 5, 25 or 37 °C for 75 days.

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