Chemistry of enzymatic browning in longan fruit as a function of pericarp pH and dehydration and its prevention by essential oil, an alternative approach to SO₂ fumigation

Abstract

Background Longan fruit is a rich source of bioactive compounds; however, enzymatic browning of pericarp and microbial decay have limited its postharvest life. SO2 has widely been used to overcome these limitations; however, due to safety and regulatory concerns, alternative means should be identified. In this study, antioxidant and antimicrobial properties of thymol (TH) essential oil were investigated against the enzymatic browning and decay of longan fruit. Methods Fruits were coated with TH (4%) for 5 min, sealed in polyethylene (PE) packages and stored at 4 °C for 42 d. Fruits immersed in distilled water (DW) and stored in PE were used as control. Results TH extended the postharvest life of longan to 42 d than 28 d in DW. TH residues decreased from 142 to 11.17 mg kg–1, while no residues were found at day 42. TH significantly (P ≤ 0.05) reduced the respiration rate, inhibited polyphenol oxidase (PPO) and peroxidase (POD) enzyme activities, sustained high phenols/flavonoids and prevented pericarp browning (BI) than DW. TH also effectively (P ≤ 0.05) maintained the color values, firmness of peel and aril, total soluble solids (TSS), titratable acidity (TA), inhibited decay incidence (DI) and resulted in lower ethanol content than DW. BI as a function of pericarp pH was highly correlated; pH and BI (r = 0. 97), with PPO (r = 0.93) and with water loss (r = 0.99). A high coefficient of correlation of BI was found with the pericarp pH, enzymes, phenolic, water loss and decay incidence with ethanol. TH could be the best alternative to SO2 and other synthetic preservatives.

Effects of Various Polymeric Films on the Pericarp Microstructure and Storability of Longan (cv. Shixia) Fruit Treated with Propyl Disulfide Essential Oil from the Neem (Azadirachta indica) Plant

“…This trend of high phenols, reduced enzymes activity levels, and consequently low BI levels was also seen in our previous study, when longan fruit cv. Daw was treated with plant essential oil (thymol) [ 7 , 8 ]. Valero et al [ 31 ] fumigated table grapes with thymol, eugenol, and menthol, and reported that these essential oils maintained the phenol contents and high color values better than the control.…”

“…The antioxidant effect of PD on the inhibition of enzymes activity levels was determined using the pericarps of 15 fruit samples. Crude enzyme extract was obtained according to the method used by Khan et al [ 7 ] and Duan et al [ 22 ]. The reaction mixture used to assess the PPO activity was prepared by following the protocol used by Khan et al [ 7 ] and Jiang [ 23 ].…”

“…Crude enzyme extract was obtained according to the method used by Khan et al [ 7 ] and Duan et al [ 22 ]. The reaction mixture used to assess the PPO activity was prepared by following the protocol used by Khan et al [ 7 ] and Jiang [ 23 ]. Absorbance was recorded at 410 nm for 5 min with a UV–visible spectrophotometer (SPECORD 50 Plus, Analytik Jena, Germany).…”

“…One unit of enzyme activity was defined as the activity that caused a change of 0.001 in the absorbance per min. The reaction mixture for POD activity was prepared following the protocol used by Khan et al [ 7 ] and Zhang et al [ 24 ] and absorbance was measured using a UV–visible spectrophotometer (SPECORD 50 Plus, Analytik Jena, Germany). One unit of enzyme activity was defined as the amount that caused a change of 0.01 in the absorbance per min.…”

“…Further, due to the unique pericarp structure of longan fruit, the dehydration and microbial invasion expedite the senescence and browning, consequently shortening its postharvest life [ 1 ]. Several studies have been conducted on treatments such as chlorine dioxide [ 2 ]; adenosine triphosphate (ATP) [ 3 ]; hydrogen peroxide [ 4 ]; chitosan [ 5 ]; SO 2 , ClO 2 , or their combination [ 6 ]; thymol coatings and thymol fumigation [ 7 , 8 ]; and many other studies in order to preserve the quality and extend the shelf life. However, due to consumer awareness of the health concerns regarding the residues of the synthetic compounds and the resistance of the microbes to the existing preservatives, there is a need to develop other preservatives that are safe to humans and the environment.…”