Natural antimicrobial agents as an alternative to chemical antimicrobials in food preservation

Introduction: Microbiological quality of food is of considerable concern to the food industry, since the activity of microorganisms can cause deterioration and, more seriously, affect the health of consumers [1]. Therefore, chemical additives are added to most of the foods with the purpose of maintaining certain properties or prolonging their shelf life [2]. However, there are several health-related problems associated with the use of these additives. Due to the problem of demonstrated or potential toxicity to the consumers, the number of chemical compounds allowed as food preservatives is limited [3]. Although, currently, most preservatives have no side effects and are classified as safe, there have been problems with the safety of some of these chemicals. The objective of this work was to determine the minimum inhibitory concentrations of three natural antimicrobial agents (ethanolic extract of propolis, chitosan and nisin) against 32 microorganisms (15 Gram-positive and 15 Gram-negative bacteria and two fungi) in order to compare with the minimum inhibitory concentrations of the chemical compound sodium nitrite, alone and in combination with sodium chloride. The inhibition of microorganisms was also tested at different pH values and temperatures of incubation to simulate the presence of the pathogens in different foods stored at different conditions. Conclusions: The natural compounds tested had greater inhibitory effect against the various microorganisms compared with the sodium nitrite alone and in combination with sodium chloride, in the different conditions of pH and temperature. Chitosan at 0.65% (w/v) inhibited most of the tested microorganisms. At 10 mg/mL of propolis, fungi, most of the Gram-positive and some of the Gram-negative bacteria were inhibited. At 25 μg/mL of nisin it was possible to inhibit most of the Gram-positives. Sodium nitrite, individually or in combination with sodium chloride, did not inhibit most microorganisms. The minimum inhibitory concentrations of several microorganisms were different depending on the methodology. With these in vitro results, it seems that the natural compounds investigated are good alternatives to chemical antimicrobials used in food production.