Antimicrobial Effects of Chitosan, Mastic Essential Oil and Citric Acid, and Their Combinations on the Spoilage Microbiota of “Guacamole”: An Avocado-Based Salad

Guacamole is an avocado-based Ready-to-Eat (RTE) salad product, consumed globally and increasingly popular due to health trends. Its composition, characterized by a relatively neutral pH and stable water activity, creates favorable conditions for microbial proliferation, leading to spoilage, and resulting in a limited shelf-life of the product. Natural antimicrobials, such as essential oils (EOs), organic acids, chitosan, etc. have the potential to control microbial populations, therefore delaying spoilage and consequently providing a shelf-life extension. This study evaluated the effects of different concentrations of the selected natural antimicrobials [citric acid, mastic essential oil (EO), chitosan] added, either singly or combined, on the indigenous microbial ("spoilage") association of guacamole during two storage conditions (chill, mild) for a period of 7 days. Results showed that of all the species enumerated in the present study, yeasts and molds were the predominant species, followed by Lactic Acid Bacteria (LAB), <i>Enterobacteriaceae</i> and <i>Pseudomonas</i> spp., at populations ranging from 3.6 to 2.0 and 2.65 to 1.45 log CFU/g, at 4 and 10 °C, respectively, in the control (CNL) avocado-based salad samples. Reductions in the range 1 to 2 log CFU/g were obtained for <i>Pseudomonas</i> spp., <i>Enterobacteriaceae</i>, and yeasts and molds under the triple antimicrobial treatment (citric acid, mastic EO, chitosan; CACHM), whereas interestingly, for LAB the highest reduction (1.74 log CFU/g) was achieved by chitosan and mastic EO (CHM), followed by CACHM (1.5 log CFU/g). Refrigeration (chill, 4 °C) as a hurdle acted as an additional barrier delaying microbial growth in all samples. To our knowledge, this is the first study to (a) evaluate the effect of natural antimicrobials (added, either singly or combined), namely citric acid, mastic EO, and chitosan on the microbiota of guacamole and (b) assess the possible application of the aforementioned natural antimicrobials in potentially increasing the shelf-life of a RTE avocado-based product (guacamole, in this study).