Food oxidation, as a serious concern of food deterioration, can induce food waste. This spoilage process results from free radical propagation in food molecules and introduces nutrient loss, off-flavor, off-odor and even toxicity issues. Food packaging with an antioxidant coating layer can effectively stabilize free radicals in food products. Avocado byproducts (peel and seed), as reliable and economical sources of natural antioxidants, are rich in phenolic compounds, a predominant group of antioxidants. These food wastes can be utilized for the development of antioxidant packaging. To date, there is no information available in the literature about the applications of avocado byproducts in the packaging field.The purpose of this research was to extract phenolic compounds from avocado byproducts and to use the crude extracts for the development of an antioxidant coating for three types of packaging films commonly used for food products, i.e., PP, PET and LDPE. To achieve this goal, 70% aqueous ethanol and 70% aqueous acetone were first used to recover phenolic compounds from avocado byproducts. An unconventional extraction procedure was employed to maximize extracted phenolic content within a limited time span. To polymerize the phenolic extracts on the polymer films, a non-metal contact dip coater was developed for this research. Alkaline saline (pH = 7.8) and laccase assist (pH = 5) coating methods were applied. Based on SEM observations, the coating layer was evenly distributed on the substrates with a thickness of 37.75 ± 0.30 nm; no polymerized clumps were noticed at a high level of resolution.AgNO3, DPPH•, and ABTS•+ assays were three approaches employed for evaluating the antioxidant efficacy of the phenolic coating in food simulants (95%, 50% and 10% aqueous ethanol). The AgNO3 allowed visual inspection for the existence of phenolic content in the coating layer. The experimental results of DPPH• and ABTS•+ assays showed that the alkaline saline coating technique, an inexpensive approach, could generate a phenolic coating layer with greater antioxidant effectiveness than the laccase assist coating method. Bio-based coating layers with different substrates, extract concentrations, extract ratios (i.e., Wpeel extract/Wseed extract) and coated films with different storage times were tested to analyze antioxidant variation; however, no statistically significant differences were found.While stabilizing free radicals in food simulants, phenolic compounds in the coating layer did not depend on a migration or surface release process for free radical elimination. Instead, they remained in the coating layer. Presumably, there was more than one layer of phenolic compounds polymerized on the substrates. After donating hydrogen atoms to quench free radicals, phenolic compounds at the surface layer of the antioxidant coating abstracted hydrogen atoms from their adjacent phenolic compounds in an inner layer of the antioxidant coating to continuously serve as antioxidants. It was also noticed that different temperature environments did not impact the stability of the coating layer. All these experimental outcomes implied a promising potential of this bio-based antioxidant coating in future commercial use.
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