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（CaCl₂）、0.05 M乙二胺四乙酸（Ethylenediaminetetraacetic acid, EDTA），以及不同浓度的婴儿链球菌（Streptococcus infantarius）发酵液来源的抗菌活性浓缩上清液（antimicrobial activity-concentrated supernatant, AMC）制备可食性薄膜；该菌株可分泌类细菌素抑制物质。本研究设置F1、F2、F3三组处理，其AMC浓度分别为75、90、120任意单位（arbitrary units, AU）/mL。 上述处理方案基于针对大肠杆菌（Escherichia coli）、金黄色葡萄球菌（Staphylococcus aureus）及单核细胞增生李斯特菌（Listeria monocytogenes）的最低抑菌浓度（minimum-inhibitory-concentration）：即AMC（90 AU/mL）联合0.05 M EDTA，受试菌在脑心浸液培养基（brain-heart-infusion medium）中于35 ℃条件下培养。 该薄膜可在25 ℃下于选择性培养基中抑制受试菌生长达30天，其中大肠杆菌检测采用麦康凯培养基（MacConkey medium）、金黄色葡萄球菌采用鲍-金培养基（Baird Parker medium）、单核细胞增生李斯特菌采用牛津培养基（Oxford medium）。 此外，F2组薄膜可在35 ℃下培养7天的墨西哥奶酪培养基中完全抑制受试菌生长；与之相反，不含薄膜的对照组细菌菌落数为10⁷~10⁹ CFU/g。 添加EDTA-AMC可改变薄膜的物理机械性能，以F2薄膜与无AMC及EDTA的对照薄膜为例，其性能参数如下：杨氏模量（Young’s modulus）分别为1394 MPa与707 MPa；断裂伸长率（Elongation at break）分别为1.9%与9.3%；断裂应力（Stress at break）分别为5.7 MPa与12.6 MPa；水蒸气透过率（Water Vapour Permeability）分别为3×10⁻¹¹ g·m·Pa⁻¹·s⁻¹·m⁻²与20×10⁻¹¹ g·m·Pa⁻¹·s⁻¹·m⁻²；氧气透过率（Oxygen Permeability）分别为1.9×10⁻¹² g·m·Pa⁻¹·s⁻¹·m⁻²与1.2×10⁻¹² g·m·Pa⁻¹·s⁻¹·m⁻²。