Effect of melatonin postharvest application on nutritional quality and shelf life of fresh-cut mushrooms

Button mushroom (Agaricus bisporus) is approved in the global food market, with regards for 30% of total mushroom production in the world, and high content of bioactive constituents including vitamins, minerals, essential amino acids as well as valuable source of polysaccharide (Meng et al., 2012). The market for sliced fresh mushrooms is increasing rapidly due to consumer demand for prepared ready-to-use vegetables and convenience foods. Button mushroom was considered as a highly perishable food due to high respiration rate, and actually no natural physical barrier on its surface to protect it, which leading to texture softening, moisture loss and browning during postharvest (Brennan et al., 2000). For sliced fresh mushrooms, the shelf life is even shorter because of the effects of the washing and cutting processes. Melatonin (N-acetly-5-methoxytryptamine) is an endogenously beneficial indole amine synthesized by all cellular organisms, which can reduce the physiological detrimental effect of abiotic stresses in plants (Zhang et al., 2018). With this regard, melatonin can play prominent roles, as a highly efficient antioxidant, scavenging molecule, and as a signaling molecule in regulating or adjusting ROS scavenging enzyme activities (Zhang et al., 2018). Recently, numerous studies reported that exogenous melatonin treatment besides storage at low temperature has been improved postharvest preservation of several fruit and vegetables (Hu et al., 2018). Since there is no information about the response of fresh-cut button mushrooms to oxidative stress following exogenous melatonin treatment during postharvest cold storage, we evaluated the effects of exogenous melatonin on qualitative attributes of fresh-cut button mushroom with regard to cap browning, weight loss, membrane permeability indicators and phenol metabolism during postharvest low temperature storage.

Button mushrooms were selected on the basis of the uniformity of maturity and size and any mechanical wounded, and were then hand sliced using a sharp knife (4 mm wide). The sliced fresh mushrooms were immersed in 0, 1, 10, 100 and 1000 µmol/L of melatonin solution for 5 minutes at 20 °C, then removed from melatonin solution and air dried at room temperature for 1 hr. After that, sliced mushrooms of each replication were packed in 500g pet plastic mushroom box (40 sliced mushrooms per box) and stored in darkness at 4 °C with 90-95% relative humidity. During storage period (5, 10 and 15 days) sliced mushroom caps per replication of each treatment were used for estimating of traits. The button mushroom weight loss was determined during storage period. To calculate the browning index (BI), photographs of edible mushroom samples were taken on sampling days and L, a and b were calculated in Photoshop software environment. Weight loss was measured according to the method of Nasiri et al (2017). A spectrophotometric procedure described by Terada et al (1978) used for the determination of ascorbic acid content. Total phenolics was measured with using the Folin–Ciocalteu reagent by Singleton and Rossi (1965) method. The texture of the mushroom cap was used to measure electrolyte leakage content and determined essentially as described by Meng et al (2012). Malondialdehyde accumulation was measured with using the thiobarbitoric acid reagent according to the method of Hodges et al (1999). DPPH radical was used to measure of scavenging activity according to the method of Dokhanieh and Aghdam (2016). The extraction and activity of PAL were determined using method described by Nguyen et al (2003). PPO was extracted with acetone according to Nguyen et al. (2003). One-way analysis of variance and Duncan’s multiple range test at P<0.05 were used for multiple mean comparisons with SAS software.

Our results showed that weight loss and cap browning index of fresh cut mushrooms following exogenous melatonin treatment (10 µM), was significantly declined to lower amount than the control during cold storage at 4 °C for 15 days. According to our results, exogenous melatonin treatment significantly slowed process of senescence in strawberry fruit as exhibited by reduced weight loss and cap browning softening of during storage (Liu et al., 2018). At the end of storage, application of endogenous melatonin at 100 µmol/L led to an increase in total phenol and ascorbic acid in fresh cut button mushrooms. Similar enhancements in total phenol and ascorbic acid along with quality retention have also been reported in exogenous melatonin treated of peach and strawberry fruit during storage time (GAO et al., 2016; Aghdam & Fard, 2017). In this study, exogenous melatonin treatment obviously declined the increasing rate of electrolyte leakage rate as well as MDA contents compared to the control, demonstrating that the effect of melatonin in reduction of membrane oxidative damage might lead to slow down browning and senescence in fresh cut mushrooms. In agreement with this suggestion, suppression of oxidative stress due to retarded senescence and quality deterioration has been described in melatonin treated cassava root and strawberry and pomegranate fruit (Ma et al., 2106; Liu et al., 2018). DPPH scavenging capacity of melatonin treated fresh cut mushrooms steadily remained higher than the control during cold storage. Higher phenylpropanoid pathway activity in response to exogenous melatonin treatment during storage, might lead to accumulation of phenolic compounds which in result provided free radical scavenging capacity up-regulation (Zheng et al., 2019). As shown in this study, higher PAL and lower PPO enzymes activity in fresh cut mushrooms followed by exogenous melatonin treatment, may be associated with molecular signaling of H2O2 pool due to enhanced NADPH oxidase enzyme activity, result in triggering phenylpropanoid pathway might lead to higher total phenol content and higher total antioxidant scavenging activity, as well membrane lipids protection from peroxidation which indicated by enhanced PAL/PPO enzymes activity accompanying with lowering cap browning (Rastegar et al., 2020).

In summary, results of this study revealed the functional effects of postharvest exogenous melatonin application at 10 µM in preserving fresh cut mushrooms quality by retarding senescence process in A.bisporus as demonstrated by attenuating weight loss and cap browning during cold storage at 4 °C for 15 days. Bioactive constituents such as ascorbic acid and total phenolics notably enhanced in response to exogenous melatonin, which is accompanied with higher PAL and lower PPO activities in melatonin treated fresh cut mushrooms during storage time. Melatonin application maintained cell membrane stability as exhibited by lowering electrolyte leakage, MDA values as well as enhancing total antioxidant activity. In general, our result showed exogenous melatonin treatment can be used as a safe and beneficial method to maintain button mushrooms quality, reduce the cap browning and prolong postharvest life the mushrooms.