Antimicrobial efficacy of Lippia citriodora natural extract against Escherichia coli and Enterococcus faecalis in "Piel de Sapo" melon juice

Synergistic Antimicrobial Effect of a Lippia citriodora Natural Extract with Vanillin against Verotoxigenic Escherichia coli in Refrigerated Piel de Sapo Melon Juice

ABSTRACT

The antimicrobial activity of a commercial Lippia citriodora extract (LCE) at 2,500 μg mL-1 (maximum sensory acceptable level) and vanillin (MIC and 2 MIC) alone and in combination were analyzed in four strains of Escherichia coli (two nonverotoxigenic and two verotoxigenic) in cation-adjusted Mueller-Hinton medium and in Piel de Sapo melon juice (MJ) stored under refrigeration (4°C) for 7 days. The bacterial counts of the four strains together in untreated samples were higher (≈4 log CFU/mL) in cation-adjusted Mueller-Hinton medium than in stored MJ. LCE showed higher antimicrobial activity in MJ than in standard culture broth, but vanillin showed a higher effect in broth. The verotoxigenic strain E. coli O146:H stx2 was the most sensitive to LCE in refrigerated MJ. Combinations of vanillin (at MIC and 2 MIC) with LCE were very effective in reducing E. coli counts either in broth or in refrigerated MJ to undetectable levels. Bactericidal effects were observed for the combinations in all strains in broth and in MJ. Also, these combinations showed an antimicrobial synergistic effect after day 3 of storage in MJ in three of the bacterial strains tested. These results indicate that the combination of LCE (at maximum sensory acceptable levels) and vanillin (at low concentrations) could be considered as a promising natural antimicrobial agent to inhibit verotoxigenic E. coli growth in refrigerated MJ and improve its quality.

HIGHLIGHTS

Assessment of (-) epicatechin as natural additive for improving safety and functionality in fresh "Piel de Sapo" melon juice

Epicatechin (EC) is a very abundant flavonoid in vegetable tissues that presents high antioxidant activity in living systems. The minimum inhibitory concentration (MIC) of (-)EC was determined in three species of bacteria commonly associated with foodborne illness of plant origin: Listeria (L.) monocytogenes, Escherichia (E.) coli -serogroups O157: H7 and O111- and Bacillus (B.) cereus; two strains of probiotic-type lactic acid bacteria (PT-LAB) and two control strains. All 10 strains were assayed under three temperature conditions (30º, 10º, and 4ºC) and at each temperature under two pH conditions (6.7 and 5.5). Mean EC MIC values were generally lower at refrigeration (4º and 10ºC) temperatures and at standard pH (6.7). By inoculating with each of the strains separately, both melon juice (MJ) and MJ supplemented with EC (ECSMJ), at the accepted maximum sensorial limit, and storing them at 4ºC for 10 days; the final counts (CFU/mL) were lower for ECSMJ than for plain MJ both for pathogenic bacteria and for PT-LAB. The presence of EC during refrigerated storage counteracted the ability of MJ as a growth medium for all the pathogenic bacteria. ECSMJ increased the antioxidant activity of MJ significantly to levels similar to those of EC alone. (-) Epicathechin would be a promising ingredient for increasing the functional properties of "Piel de Sapo" MJ (phenolic compounds and antioxidant ability) while contributing to improving the safety of this type of juice during prolonged refrigerated storage at 4ºC.

Comprehensive Analysis of Antioxidant Compounds from Lippia citriodora and Hibiscus sabdariffa Green Extracts Attained by Response Surface Methodology

Phenolic compounds have shown to have a high bioactive potential against various pathologies, postulating as an interesting alternative to manage some diseases. In this sense, both Lippia citriodora and Hibiscus sabdariffa are two botanical sources with a demonstrated high bioactive potential, in which their antioxidant capacity stands out. In this work, the optimization of the extraction conditions for the recovery of phytochemicals from L. citriodora leaves and H. sabdariffa calyces has been carried out using Response Surface Methodologies (RSM) considering their total polar compounds measured by HPLC-ESI-TOF/MS and Folin-Ciocalteu assay, and its antioxidant capacity evaluated by Ferric Reducing Antioxidant Power (FRAP) and Trolox Equivalent Antioxidant Capacity (TEAC) assays. The results showed that to maximize the antioxidant capacity in H. sabdariffa, a moderate temperature and high ethanol percentage are needed, while a low temperature and a high percentage of ethanol are needed in L. citriodora. In addition, with the results obtained in the multiple response analysis, it is possible to affirm the importance of this type of analysis to develop functional ingredients, taking into account both total content of phenolic compounds and their bioactivity. Furthermore, as confirmed in this study, these analyses can be extrapolated in different techniques and in different matrices, with phenolic compounds from different families being important to develop new high added value products for food, pharmaceutical or cosmetic industries.