Abstract
The inactivation effects of high pressure CO2 + nisin (simultaneous treatment of HPCD and nisin, HPCD + nisin), HPCD→nisin (HPCD was followed by nisin), and nisin→HPCD (nisin was followed by HPCD) treatments on Bacillus subtilis spores in aqueous solutions were compared. The spores were treated by HPCD at 6.5 or 20 MPa, 84–86°C and 0–30 min, and the concentration of nisin was 0.02%. Treated spores were examined for the viability, the permeability of inner membrane (IM) using flow cytometry method and pyridine-2, 6-dicarboxylic acid (DPA) release, and structural damage by transmission electron microscopy. A synergistic effect of HPCD + nisin treatment on inactivation of the spores was found, and the inactivation efficiency of the spores was HPCD + nisin > HPCD→nisin or nisin→HPCD. Moreover, HPCD + nisin caused higher IM permeability and DPA release of the spores than HPCD. A possible action mode of nisin-enhanced inactivation of the spores was suggested as that HPCD firstly damaged the coat and cortex of spores, and nisin penetrated into and acted on the IM of spores, which increased the damage to the IM of spores, and resulted in higher inactivation of the spores.
Highlights
Bacterial endospores are metabolically dormant, and extremely resistant to the treatments such as heat, desiccation, UV, and γ-radiation, and some bactericidal chemicals because of their unique structures (Setlow, 1995, 2006; Setlow and Johnson, 2012)
It not clear how nisin enhanced the inactivation of the spores by HPCD, and it was necessary to figure out possible action mode of nisin in the synergistic inactivation effect
It is reported that nisin cannot act on intact spores because its access to the inner membrane (IM) was blocked by the coat and cortex, but after the spores germinated and degraded their coat and cortex, nisin could act on them and inhibit their outgrowth by forming pores in the IM of spores (Gut et al, 2011)
Summary
Bacterial endospores are metabolically dormant, and extremely resistant to the treatments such as heat, desiccation, UV, and γ-radiation, and some bactericidal chemicals because of their unique structures (Setlow, 1995, 2006; Setlow and Johnson, 2012). As spores of a number of Bacillus and Clostridium species are agents of food spoilage and food borne diseases (Brown, 2000; Logan, 2012; Setlow and Johnson, 2012), inactivation of spores has been receiving great attention in the food industry. Thermal processing at relative high temperature (121◦C or higher) is an efficient way to eliminate spores. The high temperature compromises organoleptic properties and causes some detrimental effects to the nutritional quality of heat-sensitive food. There is a requirement for new ways of mild processing procedures to inactivate spores. The inactivation effect of high pressure CO2 (HPCD) was firstly shown on Escherichia coli in 1951 (Fraser, 1951). A number of reports indicate that HPCD at
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