Nutritional improvement of rice husks

Abstract

The primary objective was to explore the chemical (NaOH treatment), physical (steam treatment, ground and whole rice husks), and biological (nitrogen supplements and solid-state fungi fermentation) methods of enriching the nutritional value of whole and ground rice husks. Rice husks were compared for their response to alkali and steam treatment and solid-state fermentation by Pleurotus sajor-caju. Treatment with 40 g/L NaOH for 21 days, followed by washing to pH 7, increased the in vitro digestibility (IVD) of ground husks from 0.16 to 0.49 and reduced neutral detergent fibre from 794 to 607 g/kg; and similar values were found for whole husks. Treatment with 40 g/L urea, steamed at 1 atm pressure for 60 min, or 20 g/L NaOH for 48-h followed by washing to pH 7, did not elicit these improvements. Fermenting husks for up to 25 days with or without organic nitrogen supplements (rice bran (RB) or palm kernel cake (PKC)) or fermenting ground husks after steam or 20 g/L NaOH for up to 21 days, promoted fungal growth and improved IVD and crude protein (CP) content without organic matter loss. However, supplementing with urea did not promote growth. Fermenting with supplements RB and PKC was better than without these supplements, and differences between ground and whole husks were small. The IVD after 20 g/L NaOH was larger than after steam treatment. The IVD of ground husks after 21 days pre-treatment were 83 and 23% larger than whole husks for the 20 g/L NaOH and steam treatments, respectively. Cost and environmental concerns preclude the use of NaOH. Although fermenting with supplements substantially enriches the quality of rice husks, its acceptance as a feed may require further cost-efficient improvements because the nutritional gains found were not sufficient to warrant using rice husks as an animal feed.

Introduction

Rice cultivation yields three by-products, rice straw, the vegetative residue after grain harvest, and rice husks and rice bran, the residues after grain milling. Rice bran has commercial value as a protein supplement in pig and poultry rations but rice straw and rice husks continue to remain unutilized in Malaysia. Both residues are potential animal feeds but unlike rice straw, which has been studied for its varietal differences (Vadiveloo, 1995), chemical treatment effects (Tuen et al., 1991, Vadiveloo, 2003a) and response to solid-state fermentation (Vadiveloo, 2003b), there have been no parallel studies on rice husks.

Based on an annual grain production of 2.2 million tonnes, the output of rice husks is about 0.44 million tonnes in Malaysia. The residue has a variety of applications, whole husks as fertilizer, energy source or animal feed, husk ash (silica) in steel, cement and ceramic manufacturing and as filler in lignocellulosic fibre-thermoplastic composites. The silica (20% of ash) is present in the outer epidermal cells which are thick, highly convoluted and lignified (Park et al., 2003). This constitutes a major limitation to its use as an animal feed, apart from low protein (20 g/kg, DM) and high lignocellulose (580 g/kg, DM) contents (Beg et al., 1986).

Physical treatment, such as steaming under pressure or chemical treatment with acids and alkalis, breaks the covalent association between lignocellulose components, hydrolysing hemicellulose and de-polymerising lignin (Saha, 2003). With steaming, however, high temperatures and long residence times release furfural and soluble phenolic compounds which depress microbial activity (Castro et al., 1993, Liu et al., 1999) as well as to ease lignin biodegradation (Chen et al., 2002). Biological treatment of lignocellulose by solid-state fermentation with white-rot fungi can potentially generate a product with an improved digestibility and nitrogen content (Villas-Boas et al., 2002, Vadiveloo, 2003a). Lignocellulose degradation is mediated by lignin-modifying and auxiliary enzymes which expose the cellulose and hemicellulose to enzymatic degradation (Leonowicz et al., 1999).

The objective of the present investigation was to explore chemical, physical and biological methods of enriching the nutritional value of rice husks.