Development and characterization of functionalized and nonfunctionalized carvacrol-loaded nanoemulsions used for the inactivation of Escherichia coli O157:H7 lux in Romaine lettuce
Abstract
Globalization led to the expansion of food processing, marketing and distribution thereby increasing the risk of foodborne disease outbreaks in different countries. Also, consumers' negative health perception towards synthetic antimicrobial compounds has generated an interest in natural antimicrobial substitutes capable of ensuring food safety without adversely affecting food properties. Essential oils components, such as carvacrol, have been shown to exert antibacterial effects against most common food borne pathogens, including Escherichia coli O157:H7. Their applicability, however, is limited by their poor water solubility, volatile nature and tendency to bind with food components. In this study, the stability and antimicrobial activity against E. coli O157:H7 lux (in-vitro and in Romaine lettuce) of functionalized and non-functionalized nanoemulsions produced by high-pressure homogenization was assessed. Formulation and processing conditions as well as storage time were the main factors considered. Initially, different formulations and processing conditions were evaluated, varying lipid (coconut oil, CO; cocoa butter, CB; and palm stearin, PS), surfactant (Tween 20, TW20; Avanti lecithin, L; and Ultralec lecithin, LU), carvacrol concentration (0-50% w/w of lipid phase); and homogenization pressures (20 and 30 kpsi). Based on stability, the best carvacrol-loaded nanoemulsion (CO/LU/C2% and 20 kpsi) was selected as the base formulation for the preparation of functionalized nanoemulsions. Various concentrations of chitosan (CH: 0.005, 0.0125, 0.025, 0.0375 and 0.05% w/w), and of polyethylene glycol (PEG: 0.5, 2.5, and 5.0 w/w) were used for functionalization. After evaluating their stability, one system of each functionalized group was selected (CH 0.05% and PEG5.0%), and the antimicrobial activity of those systems and one non-functionalized system, against E. coli O157:H7 lux, was tested in-vitro in MSM at three concentrations (500, 750, and 1000 ppm). More than 5.5 log reduction was observed at carvacrol concentrations above 750 ppm, for all systems. When tested in inoculated lettuce, a maximum of 1.7 log reduction was observed when using CH0.5% and 10000ppm of carvacrol. It was observed from the experimental analysis that all nanoemulsion systems were able to reach internalized bacteria, even though low inactivation was obtained; nanoemulsion functionalization did not seem to affect the antimicrobial activity of carvacrol loaded nanoemulsions in-vitro, however CH-functionalization improved interaction of the droplets with bacterial membrane leading to higher inactivation of E. coli O157:H7 lux in cut romaine lettuce.
Degree
Ph.D.
Advisors
San Martin-Gonzalez, Purdue University.
Subject Area
Food Science
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