Elaboration and characterization of pectin-gellan films added with concentrated supernatant of Streptococcus infantarius fermentations, and EDTA: effects on the growth of Escherichia coli, Staphylococcus aureus and Listeria monocytogenes in a Mexican cheese medium, and physical-mechanical properties

Abstract Films were prepared with 0.2% (w/v) gellan gum, 1% (w/v) citric pectin, 0.5% (w/v) glycerol, 5 mM CaCl2, 0.05 M Ethylenediaminetetraacetic acid (EDTA), and different concentrations of an antimicrobial activity-concentrated supernatant (AMC) obtained from fermentations of Streptococcus infantarius, which produces bacteriocin-like inhibitory substances (i.e., treatments F1, F2 and F3 with 75, 90 and 120 arbitrary units (AU)/mL, respectively). The treatments were based on a minimum-inhibitory-concentration (i.e., AMC, 90 AU/mL, plus EDTA, 0.05 M) for Escherichia coli, Staphylococcus aureus and Listeria monocytogenes growing in brain-heart-infusion medium at 35 °C. The films hindered the bacterium growth in selective media: E. coli -MacConkey, S. aureus-Baird Parker and L. monocytogenes -Oxford, during 30 days at 25 °C. Moreover, the F2 films entirely inhibited the growth of the tested bacteria in a Mexican cheese medium, in 7-day cultures at 35 °C; contrariwise, controls with no film exhibited bacterial growths in the range 107 -109 CFU/g. The physical-mechanical properties of the films were changed by the addition of EDTA-AMC, being (F2 film)/(control-film with no AMC nor EDTA): Young’s modulus (MPa), 1,394/707; Elongation at break (%), 1.9/9.3; Stress at break (MPa), 5.7/12.6; Water Vapour Permeability (10-11 g m Pa-1 s-1 m -2), 3/20, and Oxygen Permeability (10-12 g m Pa-1 s -1 m-2), 1.9/1.2.

摘要 本研究制备的可食膜以0.2%（质量体积比，w/v）结冷胶（gellan gum）、1%（w/v）柠檬酸果胶（citric pectin）、0.5%（w/v）甘油、5 mM氯化钙、0.05 M乙二胺四乙酸（Ethylenediaminetetraacetic acid, EDTA）以及不同浓度的抗菌活性浓缩上清液（antimicrobial activity-concentrated supernatant, AMC）为原料，该上清液取自产类细菌素抑制物质的婴儿链球菌（Streptococcus infantarius）发酵液，对应处理组F1、F2、F3的AMC浓度分别为75、90、120任意单位（arbitrary units, AU）/mL。上述处理组的设计基于大肠杆菌（Escherichia coli）、金黄色葡萄球菌（Staphylococcus aureus）及单核细胞增生李斯特菌（Listeria monocytogenes）在脑心浸液培养基（brain-heart-infusion medium）中于35℃培养时的最低抑菌浓度，即AMC 90 AU/mL与0.05 M EDTA联合使用的浓度。该可食膜可在25℃条件下于选择性培养基中抑制上述细菌生长达30天，所用选择性培养基分别为大肠杆菌的麦康凯培养基（MacConkey medium）、金黄色葡萄球菌的Baird-Parker培养基以及单核细胞增生李斯特菌的牛津培养基（Oxford medium）。此外，F2组可食膜在墨西哥奶酪培养基中，于35℃下7天培养时可完全抑制受试细菌的生长；反之，不含可食膜的对照组的细菌菌落数为10^7~10^9 菌落形成单位（colony-forming units, CFU）/g。添加EDTA-AMC后，可食膜的物理机械性能发生改变：以F2组可食膜与不含AMC及EDTA的对照膜的性能比值计，杨氏模量（MPa）为1394/707；断裂伸长率（%）为1.9/9.3；断裂应力（MPa）为5.7/12.6；水蒸气透过率（10^-11 g·m·Pa^-1·s^-1·m^-2）为3/20；氧气透过率（10^-12 g·m·Pa^-1·s^-1·m^-2）为1.9/1.2。