Synergistic antimicrobial activity of food-grade compounds in wax coatings on fruits during wax drying

Wax coatings are used in various fruit and vegetable products, including citrus, apples, peaches, cucumbers, and avocados, to improve the shelf life of fresh produce. In many industries, particularly citrus and apple, wax coatings are air dried using hot air. Although wax coatings improve the shelf life by reducing the loss of moisture, they have a limited impact on the inactivation of microbes on the fruit surface. To enhance the reduction of bacteria on fruit surfaces and to reduce the risk of cross-contamination during the wax coating process, this proposal aims to develop a synergistic antimicrobial approach that can be combined with wax coating and drying. The specific goals of this project are to (a) Assess the synergistic interaction of LAE and other food-grade compounds or extracts with mild heat to achieve rapid inactivation of bacteria inoculated in a wax suspension; (b) Assess the role of synergistic treatment) in the inactivation of the pathogens inoculated in a wax composition and pathogens inoculated on the surface of apples and citrus fruits; and (c) Evaluate the influence of the optimal synergistic treatments on the quality, microbial load, and shelf life of fruits.

This project aims to enhance the inactivation of bacteria on wax coatings and fruit surfaces during the wax coating drying process. The proposed enhanced inactivation of bacteria on wax coating and fruit surfaces will be achieved using a synergistic antimicrobial approach proposed in this research. This synergistic inactivation of bacteria is based on the combined effect of selected GRAS compounds added to commercial wax compositions with mild heat-assisted drying of wax coatings. Overall, this research is motivated by the needs of the fresh fruit industry to improve the safety of fresh fruits during postharvest processing. This research project focuses on both citrus and apple fruits. The specific goals of this project are to (a) assess the synergistic interaction of LAE (Ethyl lauroyl arginate) and other food-grade compounds or extracts with mild heat to achieve rapid inactivation of bacteria inoculated in a wax suspension; (b) evaluate the role of synergistic treatment in the inactivation of pathogens inoculated in a wax composition after coating on citrus fruits and apples; (c) demonstrate the efficacy of synergistic treatment to inactivate pathogenic inoculated on the surface of apples and citrus fruits; and (d) evaluate the influence of the optimal synergistic treatments on the quality, microbial load and shelf life of fruits. The success of this research will reduce the cross-contamination risks from wax compositions and brushes and improve fresh fruit safety by reducing the bacterial load on the fruit surface.

(1) Assess the synergistic interaction of LAE and other food-grade compounds or extracts with mild heat to achieve rapid inactivation of bacteria inoculated in a wax suspension (2) Assess the role of synergistic treatment (optimal combination of food-grade compounds in a wax coating and mild heat identified in Aim 1) in the inactivation of the pathogens inoculated in a wax composition on the surface of apples and citrus fruits (3) Measure the influence of synergistic treatment in the inactivation of the pathogens inoculated on the surfaces of apples and citrus, including the stem and calyx regions (4) Evaluate the influence of the optimal synergistic treatments identified in Aims 2 and 3 on the quality (including color), microbial load (endogenous), and shelf life of fruit during storage

Summary of Findings

Strong inhibitory activities of carnauba-based wax products (Objective 1)

Carnauba-based wax formulations demonstrated intrinsic activity to inactivate bacterial cells in wax suspension, with morpholine-supplemented waxes showing stronger inhibition than non-morpholine formulations.

Gram-positive bacteria were more susceptible than Gram-negative bacteria.

Heating wax formulations to 40–50°C further enhanced the inhibitory activity.

Strong synergistic effects with olive pomace extract (OPE) or propyl gallate (PG) combined with mild heat in wax formulations (Objectives 2 & 3)

On orange skins and whole oranges, OPE- or PG-supplemented wax combined with 55°C drying achieved greater than 3-log reductions within 2–5 minutes, including reductions on fruit surfaces, including stem and calyx regions.

Strong synergistic activities were observed on citrus but were limited in apple-specific wax formulations.

However, heated OPE alone achieved greater than 4-log reductions on apple peels, indicating a promising alternative strategy for apples. Based on this approach, sequential OPE treatment resulted in 2-log inactivation of bacteria on dip-inoculated apples. The reduction was lower as compared to oranges but significantly improved with the sequential treatment process.

Sequential OPE application to apples resulted in ~1.1 log CFU reduction in total aerobic plate counts and a ~0.5 log CFU reduction in yeast and mold (YM) counts, compared to ~0.5 log CFU and ~0.3 log CFU reductions in total aerobic plate counts and YM counts, respectively, achieved with the 55°C water treatment followed by wax coating.

Improved fruit quality and shelf life (Objective 4)

Oranges treated with OPE-supplemented wax combined with mild heat showed slower microbial buildup during 4 weeks of storage at 4°C compared to control oranges with standard wax coating.

Oranges treated with OPE-supplemented wax combined with mild heat showed reduced discoloration (about 2-fold lower ΔE*) and reduced weight loss (about 1.9-fold lower) during 8 weeks of storage at 4°C compared to controls.

During 12 weeks of storage, total aerobic plate counts and YM counts on OPE-treated apples remained relatively stable. In contrast, other treatment groups exhibited a general reduction in total aerobic plate counts and a slight increase in YM populations over the storage period.

Recommendations for Industry

Evaluate OPE- or PG-supplemented carnauba wax combined with 50–55°C drying in commercial citrus operations.

For apples, consider heated plant extract pre-treatment approaches prior to wax coatings.