Study on the browning mechanism of betel nut (Betel catechu L.) kernel

Abstract

Taking betel nut (Betel catechu L.) kernel as raw materials, to analyze the browning mechanism of betel nut kernel. First, we extract and separate the phenolic substances and polyphenol oxidase (PPO) from the betel nut kernel and then find out how O 2 penetrates into the kernel, study the above 3 key factors of enzymatic browning so as to prove the possibility of enzymatic browning, and further study the browning products to clarify the mechanism of browning of betel nut kernel during storage process.The results showed that 11 kinds of phenolic compounds were isolated and identified from the betel nut kernel by liquid chromatography-mass spectrometry, among which chlorogenic acid was the highest, followed by dopamine and L-epicatechin, and other contents were relatively low. Meanwhile, PPO was also separated from the betel nut kernel by DEAE-Sepharose Fast Flow and Phenyl-Sepharose 6 Fast Flow column chromatography. On this basis, scanning electron microscopy showed that the damage of the betel nut tissue was aggravated with the prolonged storage time, the wax was gradually decomposed, and the stratum corneum of the peel is destroyed in a honeycomb-shaped; the lignification of the flesh was aggravated, and the interstitial space was increased; and the crack of the kernel membrane was also enlarged. These changes of structure contribute to increase gas exchange within and outside the organization, including the entry of O 2 . Finally, the oxidation products generated from simulated reaction of chlorogenic acid, dopamine, and epicatechin with purified PPO under aerobic conditions in vitro were compared with the products extracted from naturally brown betel nut, and the same absorption spectra were found. Therefore, it indicates that the browning of betel nut kernel is an enzymatic browning caused by the reaction of phenolic substrates were oxidized by PPO. K E Y W O R D S betel nut kernel, enzymatic browning, O 2 , phenolic compounds, PPO 1 | INTRODUC TI ON Betel nut (Betel catechu L.) is the fruit of the Betel catechu tree. Approximately 700 million individuals regularly chew betel nut worldwide, which is one of the world's three largest oral addicting food (betel nut, chewing gum, cigarette), the fruit contains a variety of beneficial substances and nutrients required by the human body and has various functions such as refreshing and relieving pain and helping | 1819 GUO et al.

Enzymatic browning in relation to permeation of oxygen into the kernel of postharvest areca nut under different storage temperatures

“…At the initial stage of storage, the mesocarp tissue of areca nut arranged in a neat, orderly, and tight manner with small cell space (Guo et al., 2020). After storage of 20 days at 5°C, the mesocarp tissue structure of the areca fruit became disordered, in which the connections between tissues relaxed, mesocarp tissue separated from others and resulted in increased mesocarp tissue space (Figure 5b).…”

“…The initial stage of storage, the kernel shell structure was intact and smooth (Guo et al., 2020). After 20 days of cold storage at 5°C, the cracks appeared at cell membrane in kernel shell of areca nut (Figure 5c).…”

“…In case of lignification in mesocarp tissue, the blocking effect on permeability of external gasses may be greatly impaired because lignified cells are dead cells (Singh et al., 2010). With the development of mesocarp lignification of the areca nut fruit, the arrangement of mesocarp tissues became disordered, the connection between tissues loosened gradually, the connected parts reduced gradually, and the separation from other tissues led to an increase in mesocarp tissue space (Guo et al., 2020). The present results demonstrated that the mesocarp tissue of the areca nut was lignified during natural senescence and chilling stress, as indicated by higher lignin content at 25 and 5°C, which might increase oxygen infiltration into interior tissue.…”

“…Unlike internal flesh browning of in pineapple and pear fruits, the internal browning of the areca nut occurs mainly in the shell‐enclosed kernel, although such compact structure seems to make oxygen permeation difficult. By using the SEM, we recently observed a continuous disintegration in different areca fruit tissues (including pericarp, mesocarp, and kernel membrane) with the prolongation of storage at ambient temperature (25°C) (Guo et al., 2020). In addition, some studies suggested that browning of the areca nut kernel could be possibly attributed to chilling injury (CI) (Li et al, 2016; Zhang, Li, et al., 2017).…”

“…The fruit contains a variety of beneficial substances, providing human body with a variety of positive effects, such as refreshment, pain relief, and digestive aid (Peng et al., 2015). Although the market is dominated by dry areca nut products, a portion of consumers in China and Southeast Asia countries prefer eating fresh areca fruit (Guo et al., 2020). As areca fruit is harvested from August to December each year in China, the storage of the products is essential to adjust the supply during the off‐season and satisfy the demands of consumers.…”