Anti-microbial and anti-browning effects of some edible plant extracts for shelf-life extension of fresh-cut fruits

Abstract

Edible plant such as Traditional Chinese Medicine (TCM) herbs and edible flowers, good source of phytochemicals, possesses antioxidants and antimicrobial functions. The main objective of this research was to explore these functional plant extracts can be a potential natural antimicrobials and anti-browning agents for food preservation to extend the shelf life of fresh-cut fruits. Five common edible plants, lonicera japonica (honeysuckle flower), chrysanthemum, aloe vera, mangosteen and safflower, were selected for the evaluation of the antioxidant activity, tyrosinase inhibition activity and antimicrobial activity, then applied to examine its anti-browning capacity on fresh-cut apples as the model. To achieve the objectives, selected edible plants were extracted by ethanolic method and carefully prepared for the examination of the DPPH radical scavenging assay, tyrosinase inhibition screening assay and modified agar dilution method. Quantification of the phenolic compound of chlorogenic acid in honeysuckle flower was analysed by the High Performance Liquid Chromatography (HPLC). Preparation of edible plant extracts was conducted differently due to its unique characteristic and methods used with considerations of maximise the extraction with most of its water soluble bioactive phenolic compounds. Plant extracts were used for the assessment of the anti-microbial and anti-browning capacity on fresh-cut apples. Colour changes of the anti-browning process on the surface of apple wedges were precisely measured by Chroma meter for comparison of the L, a, b values colour spaces. Results showed that the antioxidant activity of the five plant extracts was compared with standard ascorbic acid in the DPPH assay, four plant extracts had a lower IC50 and the safflower has the highest at almost nine times. In tyrosinase inhibition screening assay, mangosteen peel extract and honeysuckle flower extract had a relatively higher percentage of relative inhibition while safflower extract showed low inhibition. Safflower extract also had the lowest minimum inhibitory concentration (MIC) against Escherichia coli and Aspergillus niger, in contrast, honeysuckle flower had a higher MIC than safflower, against both bacteria. High content of phenolic compound of chlorogenic acid was found at 1.4% in dehydrated honeysuckle flower extract. In the examination of anti-browning and anti-microbial capacity with plant extracts mixed anti-browning agents of ascorbic acid, 4-hexyresorcinol and magnesium chloride, honeysuckle flower extract was also found to have highest antimicrobial effect on inhibitory growth of Aspergillus. niger. Aloe vera and mangosteen were found to have potential to be applied as the natural anti-browning agents and able to prolong the browning process in the treated fresh-cut apples. In particular, much potential application of mangosteen peel extract is of great interest for further investigation to make uses of its functional properties to serve as a natural antioxidant, natural anti-browning agent and natural colorant in food industry.