Abstract
Rice husk is a by-product produced abundantly in rice production but it has low commercial value and causes environmental pollution. This study was conducted to examine different extracting solvents and conditions to optimize the efficacy of antioxidant and antimicrobial potentials, and chemical components in rice husk. By the use of distilled water at 100 °C, the ethyl acetate (EtOAc) extract was potent in both total phenolic content (TPC), total flavonoid content (TFC), and DPPH scavenging activity. The treatment of either ethyl acetate (100 °C, 1 h), combined with MeOH 100%, showed the highest percent of lipid peroxidation inhibition (LPI) (86%), meaning that the strongest antioxidant activity was by the β-carotene bleaching method. The treatment of distilled water at room temperature possessed the strongest antioxidant activity in the assay of the reducing power. The use of dried samples at 100 °C for 2 h, combined with methanol (MeOH) 10%, provided the most potent antimicrobial activities against Escherichia coli, Klebsiella pneumoniae, Listeria monocytogenes, Bacillus subtilis, and Proteus mirabilis. The results suggested that the EtOAc extract from rice husk could be a potential source of natural antioxidants. In general, the use of temperature 100 °C for 2 h, combined with either EtOAc or 10% MeOH, can optimize chemical components and antioxidant and antimicrobial capacities in rice husk. Principal constituents putatively identified by gas chromatography–mass spectrometry (GC–MS) revealed the presence of momilactones A and B (MA and MB, respectively), phenols, phenolic acids, and long-chain fatty acids, although yields of these compounds varied among extracts. The bioactive MA and MB were found in most of the extracts, except distilled water and MeOH ≤ 50%, at any temperature. Findings of this study provided optimal conditions for future production at an industrial scale for rice husk to exploit its potent biological properties. It thus helps to increase the economic value and reduce the disposal burden and environmental troubles caused by rice husk.
Highlights
Rice husk is an agricultural waste that farmers have experienced difficulties in disposing of in large quantities
The contents of total phenolics and flavonoids in EtOAc extracts prepared from rice husk varied from 63.8 to 5962.3 mg GAE/g DW, and from 21.8 to 571.6 mg RE/g DW, respectively (Table 2)
The M17 method was potent in both total phenolic content (TPC) and total flavonoid content (TFC), and DPPH scavenging activity
Summary
Rice husk is an agricultural waste that farmers have experienced difficulties in disposing of in large quantities. Rice husk is traditionally disposed of in landfills, and has recently been used for electricity generation, but simultaneously a large number of greenhouse gases are being produced [2]. Others are cellulose (35%), hemicellulose (25%), crude protein (2%), and ash (17%) [3], it is a challenge to develop rice husk to be a value-added byproduct, as it has hard surface, high silica content, small bulk density, and is not fermented by bacteria. Rice husk has elevated ash and lignin contents, it is not appropriate to use as animal feed raw material. Rice husk has been reported to be a potential source for bioethanol production [1]; furfural, one of the top value bio-based chemicals, has been recently synthesized from rice husk [2,6]. The manufactured price appears to be difficult for consumption in markets because of its non-competitive price, as compared to another renewable source
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