The rubber tree Hevea brasiliensis is indigenous to the Brazilian Amazon region and is widely cultivated in Malaysia, India, Indonesia and Thailand (Lourith et al., 2014; Reshad et al., 2015). Hevea trees are mainly cultivated as a natural source of rubber, which is used in various products. Rubber seeds are an abundant by-product in rubber plantations, with annual production between 136–2000 kg/hectare (Zhu et al., 2014); however, only 25% of rubber seeds are used for seeding and the residual 75% are wasted (Indonesian Directorate Generale of Plantation 2010). Therefore, the full utilisation of rubber seeds can provide a significant source of additional revenue from rubber production.
The composition of fresh rubber seeds is 42–48% shell and 52–60% kernel (Reshad et al., 2015). The dry matter of the kernel contains 40–50% oil and 17–20% protein (Ikwuagwu et al., 2000; Widyarani et al., 2017).
Several studies have investigated the use of rubber seed oil to produce biofuels (Reshad et al., 2015; Sai Bharadwaj et al., 2019; Samart et al., 2019). Rubber seed oil applications in production of cosmetics (Chaikul et al., 2017; Lourith et al., 2014), alkyd resin (Ikhuoria et al., 2007) and polyurethane resin (Bakare et al., 2010) also merit attention. Mechanical pressing and solvent extraction are two common methods to separate oil from rubber seeds. Santoso et al. (Santoso et al., 2014) reported that a maximum 31.88% oil yield can be obtained by hydraulic pressing. Higher yields (43.00–49.36%) could be obtained by using n-hexane as a solvent for extraction (Onoji et al., 2016; Reshad et al., 2015). Alternative oil separation methods like supercritical carbon dioxide extraction produced lower yields, ranging from 21.47–33.65% (Lee et al., 2013; Mohd-Setapar et al., 2013). Widyarani et al. (Widyarani et al., 2014) demonstrated that the aqueous enzymatic extraction yielded an oil recovery as low as 34%.
Rubber seed meal (cake), by-product created during oil extraction, is rich in protein, and contains abundant glutamic acid, aspartic acid and leucine. Previous investigations have suggested that plant proteins could be replaced by rubber seed meal as an alternative protein source in animal feed (Deng et al., 2017; Suprayudi et al., 2017). The levels of most essential amino acids in rubber seed meal could provide adequate nutrition supplements for humans and livestock (Oyewusi et al., 2007). Despite the recognised potential of seed protein, studies on rubber seed protein extraction are limited. Widyarani et al. (Widyarani et al., 2014) reported a maximum protein recovery of 71% obtained by alkaline extraction and assessed a one-step combined oil and protein extraction using an aqueous enzymatic method, which produced a protein recovery of 67% and a lower oil recovery of 34%. Alkaline and enzymatic extraction methods have been previously investigated to extract protein from agricultural residues such as brewer’s spent grain, rapeseed press cakes and rice bran (Forssell et al., 2008; Niemi et al., 2013; Phongthai et al., 2016; Rommi et al., 2015).
To achieve the most efficient use of rubber by-products, the present study aims to simultaneously obtain high oil and protein recovery from rubber seeds. The effects of different pre-treatments and extraction methods on oil and protein recovery were investigated. Specifically, we evaluated the optimum rubber seed kernel particle size for screw pressing. Sequential alkaline and enzymatic treatments were used for protein extraction. Additionally, the influence of several processing parameters on oil quality was assessed.