Research noteInactivation of Lactobacillus fructivorans suspended in various buffer solutions by low-pressure CO2 microbubbles
Introduction
Sake is a traditional alcoholic beverage of Japan. Alcohol-philic and alcohol-resistant lactic acid bacteria are commonly used for brewing unpasteurized sake, because the sake brewing process generally utilizes an open system. However, lactic acid bacteria degrade the quality of sake by producing white turbidity, increasing acid content and generating off-flavor during the aging and distribution of sake. Unpasteurized sake is generally subjected to heat treatment (65–70 °C) to prevent the growth of microorganisms, although heat treatment often causes undesirable changes in the quality of sake. Therefore, new techniques for inactivating microorganisms without affecting sake quality are desired by the sake brewing industry.
Non-thermal processes using supercritical carbon dioxide (SC-CO2) have been widely studied as alternative processes for food pasteurization, because heat may cause undesirable changes in the taste and flavor of food (Ballestra, Da Silva, & Cuq, 1996; Garcia-Gonzalez et al., 2007; Gunes, Blum, & Hotchkiss, 2006; Haas et al., 1989; Lin, Yang, & Chen, 1992, 1993). High-pressure conditions (10–30 MPa) are necessary to generate sufficient SC-CO2 and effectively inactivate microorganisms. However, the heavy-duty equipment required for this purpose is prohibitively expensive from a practical viewpoint. Recently, we developed a pasteurization technique utilizing CO2 microbubbles (MB-CO2) at a pressure lower than 2.0 MPa, which is a pressure that has no bactericidal effects (Oulé, Tano, Bernier, & Arul, 2006), and reported the inactivation of Escherichia coli and Saccharomyces cerevisiae by MB-CO2 treatment (Kobayashi, Hayata, Ikeura, Muto, & Osajima, 2010; Kobayashi et al., 2009). Pressurized CO2 efficiency was influenced by various food components and food properties (Garcia-Gonzalez et al., 2009). Many components in sake, including sugars and acids, possess buffer capacity. Lactobacillus fructivorans is a lactic acid bacterium that degrades the quality of sake (Nagatani & Kikuchi, 1971). Therefore, this study aimed to investigate the effect of MB-CO2 on the inactivation of L. fructivorans suspended in various buffer solutions.
Section snippets
Microbial culture
L. fructivorans s36, one of the most heat-resistant lactic acid bacterial strains (Nagatani & Kikuchi, 1971), was obtained from the National Research Institute of Brewing (Higashi-Hiroshima, Japan). The L. fructivorans suspension was prepared using a previously reported method (Tanimoto et al., 2007). L. fructivorans was inoculated into test tubes containing 10 mL of SI medium (The Brewing Society of Japan, Tokyo, Japan) (Sugama & Iguchi, 1970) with 10 mL/100 mL ethanol and incubated at 30 °C
Results and discussion
The inactivation of L. fructivorans suspended in various buffer solutions by MB-CO2 treatment is shown in Fig. 1. There were no reductions in the numbers of surviving L. fructivorans cells in 0.1 mol/L CA/0.2 mol/L NaP, 0.1 mol/L CA/0.2 mol/L KP and 0.1 mol/L CA/0.1 mol/L NaC buffer solutions by MB-CO2 treatment at 40 °C and 2 MPa for 60 min, although the numbers of surviving L. fructivorans cells in 0.1 mol/L AA/0.1 mol/L NaA buffer solution and deionized water were significantly reduced from
Conclusions
The inactivating effect of MB-CO2 treatment on L. fructivorans was statistically-markedly lower in phosphate buffer solution than in citric acid buffer and acetic acid buffer solutions. Furthermore, the effectiveness of MB-CO2 treatment tended to increase as the concentrations of the buffer components and the pH of buffer solutions decreased. These results suggest that the properties of foods should be considered prior to the application of MB-CO2 treatment to various foods. In future, it is
Acknowledgment
This work was supported by Grant-in-Aid for Young Scientists (B) (23780146) from Japan Society for the Promotion of Science.
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