Characterization And Antimicrobial Evaluation Of Food-grade Glycerol Monolaurate-based Microemulsion Systems

Microemulsions are colloidal nanodispersions of oil and water stabilized by an interfacial film of surfactant molecules,typically in conjunction with a cosurfactant. The technique has been widely used in pharmaceutics,cosmetics,enhanced oil recovery,but its application in foods hadn't caught much attention before 199th ACS (American Chemical Society) symposium in 1990.As a potential delivery system for bioactives in food,microemulsions recently have been of increasing interest to researchers and exhibited great potential on its industrial applications.The physicochemical characterization of food-grade microemulsion systems with glycerol monolaurate as oil and the antimicrobial activities against foodborne pathogens have been studied in this paper.The main research findings are as follows:(1) The effects of short-chain alcohols,organic acids,polyols,nonionic surfactants and salts on the phase behavior of the food-grade microemulsions have been elucidated.Results showed that the food-grade microemulsions were difficult to formulate from a three-component systems based on water,oil and single surfactant, but it is possible to formulate these microemulsions by the addition of short-chain alcohols,organic acids,polyols and salts.The oil solubilization was dramatically improved in the presence of short-chain alcoho-ls,organic acids and polyols,and organic salts contributed to the improved oil solubilization as hydrotropes.Tween 20 being the most hydrophilic surfactant in Tweens solubilized the maximum oil. However,it was hard to form microemulsions in the presence of sucrose esters even when mixed with ethanol.The oil solubilization capability in these systems was clearly reflected in the phase behavior of these systems.Viscosity and pH results showed that along the dilution lines,at a certain composition the systems inverted from water-in-oil to oil-in-water microemulsions, demonstrated as U-type microemulsion systems capable of being infinitely and progressively aqueous phase diluted.(2) Three different food-grade microemulsion systems(glycerol monolaurate/ethanol/Tween 20/sodium lactate/water microemulsion system,glycerol monolaurate/ethanol/propylene glycol/Tween 20/sodium benzoate/water microemulsion system and glycerol monolaurate/ethanol/Tween 80/potassium Sorbate/water microemulsion system) were established and their antimicrobial activities against Bacillus subtilis,Stenotrophomonas maltophilia,Aspergillus niger and PeniciIlium italicum respectively were examined.Results showed that the food-grade microemulsion systems with glycerol monolaurate as oil was able to sotubilize different antimicrobial salts and exhibited effective inhibitory activities against foodborne pathogens,including Gram-positive,Gram-negative bacteria and fungi.It was found that in the antimicrobial activities of these microemulsion systems, glycerol monolaurate made a major contribution,followed by the solubilized salts. These results confirmed the feasibility of solubilizing at least two food preservatives in a microemulsion system and the antimicrobial effects on foodborne pathogens.(3) The U-type microemulsion system with glycerol monolaurate as oil, propionic acid as cosurfactant,sodium benzoate as salt and Tween 80 as surfactant was characterized and the broad-spectrum antimicrobial activities were examined.The minimum bactericidal concentrations of the microemulsion against Staphylococcus aureus,Escherichia coli and B.subtilis were 500μL/L,250/μL/L and 250μL/L,respectively.Kinetics of killing results showed that the microemulsion at minimum bacterial concentrations killed over 90%bacteria in 5 min.The antimicrobial activities of the microemulsion against E.coli were stronger than those against S.aureus and B.subtilis,since after 60 min the microemulsion caused a 4 log reduction in S.aureus and B.subtilis titre while a complete loss ofE.coli viability.The minimum fungicidal concentrations of the microemulsion against A.niger and P.italicum were 500μL/L and 1000μL/L,respectively.Kinetics of killing results showed that the microemulsion at minimum fungicidal concentrations killed over 99%A.niger and P.italicum spores in 15 min and 30 min respectively.The antimicrobial activities of the microemulsion against A.niger were stronger than those against P.italicum,since after 120 min the microemulsion caused a 5 log reduction in A.niger spores titre while a 3 log reduction in P.italicum spores titre. (4) Based on the broad-spectrum antimicrobial activities,the antimicrobial mechanisms of the microemulsion against E.coli representing foodbome bacteria and A.niger representing foodborne fungi were examined.The cell surface hydrophobicity and membrane permeability experiments were in good agreement with the kinetics of killing results,and indicated that the interaction between the antimicrobial microemulsion and bacterial membranes significantly decreases the bacterial cell hydrophobicity and induces the quick release of 260 nm absorbing materials,leading to the rapid loss of bacterial viability.The membrane permeability experiment and microstructure observation using light microscopy and scanning electron microscopy were in good agreement with the antifungal activities by agar dilution method and broth dilution method,and indicated excellent growth inhibition and sporicidal activities against A.niger,These findings suggested that the observed antimicrobial activities may be attributed to a disruption and disfunction of fungal cell walls and biological membranes.