Control of Listeria monocytogenes on avocado and avocado contact surfaces during dry packing by acetogenins naturally present in avocado seed

Avocado seeds are rich in acetogenins, a group of lipid derivatives that have a range of biological benefits, including antimicrobial activity against Listeria monocytogenes (LM). Using avocado seed extracts offers an opportunity for the avocado industry to obtain effective, natural antimicrobial compounds that can be used for adding an extra element of safety to avocado packing, while adding value by utilizing the seeds. Avocado packers could use acetogenin-enriched (ACE) extracts against LM as a complement rather than a replacement to the strict sanitation and verification programs already existing in the avocado packing industry. Acetogenins are abundant in avocado seed and can be extracted and mixed with propylene glycol and avocado oil to make a food-grade compound that can be used as coating for avocados during packing. Since this ACE extract coating is non-aqueous, its use should not interfere with the dry packing system currently used by Mexican avocado packing plants that process avocado for export to the US. This proof-of-concept study focuses on conducting challenge studies to determine the feasibility of reducing LM by coating avocados with this compound. If successful, this would be a ground-breaking contribution not only for avocado safety but for the fresh produce industry in general.

Lauraceous acetogenins are a group of fatty acid derivatives found in avocado, that possess diverse types of bioactivity, from antimicrobial (mainly Gram-positive bacteria and molds) to antitumoral activity. The greatest concentration of acetogenins is in the seed, compared to skin or pulp. In this proposal, the antimicrobial effect of acetogenins,will be evaluated as a potential control measure to reduce Listeria monocytogenes (LM) during avocado packing. An avocado seed extract, which is rich in acetogenins will be mixed with propylene glycol and avocado oil to prepare a food-grade coating agent for which the acetogenins content can be adjusted to the desired concentration to provide levels that will reduce LM potentially present on the avocado surface. The objectives of this proof-of- concept study are to: 1. Evaluate the ability of the acetogenin-enriched (ACE) extract to reduce LM on avocado surfaces in a simulated dry packing operation. 2. Evaluate the effectiveness of the ACE extract at reducing cross-contamination with LM after contact with contaminated packing equipment. 3. Study metabolic responses of avocados to the coating with ACE extract (rich in acetogenins) that are uniquely present in the same avocado and their effect on ripening. These objectives will be achieved by conducting inoculated challenge studies on both avocados and on food-contact surfaces of simulated packing machinery. First, avocados will be inoculated with LM and then sampled for LM counts to determine the reduction by coatings with increasing concentrations of acetogenins when comparing to a non- coated control. The coatings will contain The LM counts will also be conducted during storage after coating to determine any long-term antimicrobial effect of the ACE extract. The project also includes inoculating food-contact surfaces and determining the transfer rate of LM from contaminated equipment to the fruits, and the effect of the ACE coating to reduce LM after this transfer. Lastly, the coating effect on fruit ripening and metabolic activity will be determined by metabolomic and chemical tests to ensure that the coatings have no effect on the quality of avocados when arriving to retail establishments. Avocado packers could use the ACE extract as a natural, food-grade compound with specific activity against LM this method as a potential “kill” step that is complementary to adequate sanitation. Being a non-aqueous compound, it would be adequate for the dry packing method frequently used in avocado packing. If successful, the opportunity will be open for testing in other similar commodities such as other tropical fruits and melons.

The aim of this study is to evaluate the effect of the antilisterial lipid derivatives contained in the ACE extract on the reduction of, and prevention of cross-contamination with LM, on avocado surfaces, and to develop an effective method to apply the ACE extract coating onto avocados during dry packing. 

Specific objectives are: 

1. To evaluate the ability of the acetogenin-enriched extract to reduce L. monocytogenes on avocado surfaces in a simulated dry packing operation. In this objective, variables such as the levels of acetogenin concentration in the ACE extract, the method used to coat avocados with the ACE extract and the type of skin surface (rough [var. Hass] vs. smooth [green skin varieties]) will be compared to determine their effect on the effectiveness of the to reduce LM under simulated packing conditions. 

2. To evaluate the effectiveness of the ACE extract at reducing cross-contamination with LM after contact with contaminated packing equipment. 

Objective 2 will involve the exposure of avocados to food contact surfaces that have been inoculated with LM. Factors such as the level of inoculum, strength of attachment of LM cells to the food contact surface and surface moisture in the avocado will be compared for their effect on the ability of LM to relocate from the contact surface to the avocado. 

3. To study metabolic responses of avocados to the coating with ACE extract. The effect of is objective will consist of pilot plant studies to determine the impact of coating avocados with high concentrations of acetogenins, which are uniquely present in the same avocado, on the ripening process and the expression of specific metabolites associated with the expected quality in retail operations.

This project is ongoing. A final report will be provided when the project is finished.