Biovalorisation of okara (soybean residue) for food and nutrition
Introduction
The common industrial practice of discarding food processing by-products leads to economic loss and socio-environmental problems, and the search for their alternative uses and value addition has gained much global attention in recent years (Scialabba, 2014). Okara, also known as biji (Korean) or douzha (Chinese), is a food processing by-product derived from soybeans (Glycine max). It is the ground soybean residue remaining after filtering the water-soluble fraction during soymilk or soybean curd production.
For every 1 kg of soybeans used in manufacturing soybean curd, about 1.1–1.2 kg of okara is obtained (Khare, Jha, & Gandhi, 1995). Therefore, large amounts of okara are produced annually, especially in Asian countries with high soybean consumption. The top soymilk-consuming region and country are Hong Kong and Singapore, while non-Asian countries with significant soymilk consumption include Australia and Canada (Starling, 2011). The amount of okara generated by the soybean curd-manufacturing sector is about 800,000 tons in Japan, 310,000 tons in Korea and 2,800,000 tons in China (Ahn et al., 2010, Li et al., 2011, Muroyama et al., 2006). The amount of okara produced annually in Singapore alone is at least 10,000 tons, comparable to that produced in Canada (Khaw, 2013, Soy 20/20, 2005).
Despite the large volumes of okara generated by the food industry, most of it is discarded as the high moisture content in okara makes it very perishable. In the past twenty years, there has been growing interest in ways of reusing the okara. The direct incorporation of okara into animal feed or human food is possible, but it is limited by the presence of enzyme inhibitors (if the soybeans are not heat-treated prior to grinding) and flatulence-causing oligosaccharides in okara, and the undesirable ‘fishy’ and ‘beany’ flavour of fresh okara (Anderson and Wolf, 1995, Li and Ma, 2014, Mateos-Aparicio et al., 2010a). Dried okara can also be used as an ingredient in various foodstuffs, especially baked goods, and a handful of studies on the drying process parameters and technologies have been done. However, the drying process is usually energy-intensive given the large amount of water in okara, and economic analysis has also shown that the cost of drying okara greatly exceeds the value of protein contained within (Soy20/20, 2005).
Hence, the microbial biotransformation of okara provides an alternative to value-add to this soy food processing waste. This review highlights the valorisation of okara by fermentation for food applications, focusing on the research conducted thus far, their main challenges and limitations, and the potential areas for further exploration and development. Other approaches to okara valorisation and microbial production of non-food substances from okara have also been studied, but these areas lie beyond the scope of this review, and interested readers are referred to the review articles by Li et al., 2011, Li et al., 2013
Section snippets
Composition of okara and potential for biotransformation
The composition of okara differs depending on the cultivar of soybean, the method of soymilk processing, and the amount of water soluble components extracted from the ground soybeans. The general composition of okara is shown in Table 1. Soybean cultivars vary in their crude protein and lipid contents, lipoxygenase activities and fatty acid compositions. The sequence of soymilk processing steps also matters, as illustrated in Fig. 1. In the Japanese way of soymilk manufacture, soaked whole
Fungal fermentation of okara
Okara is suitable for fungal fermentation as it offers a physical surface for fungal adherence and growth. Filamentous fungi excrete cellulolytic enzymes, including endoglucanases, exoglucanases and β-glucosidases, breaking down lignocellulosic biomasses like okara. This has two benefits: fungal growth is supported on okara, and the degraded okara has improved digestibility. Several studies on fungal fermentation of okara have been carried out, as summarised in Table 2.
Bacterial fermentation
Studies on bacterial fermentation of okara are summarised in Table 3. Most studies involved the Bacillus species, likely due to their ability to produce extracellular alkaline proteases (Bhunia, Basak, & Dey, 2012) and their common presence in many fermented soybean products. The potential of okara as prebiotics has been evaluated as well.
Yeast fermentation
Yeasts offer great potential for transforming okara owing to their strong metabolic activity and diversity, yet fermentation of okara by yeast has received little research attention with few reports available in the literature. Clearly this area merits more research efforts. Yeast fermentation of okara to date is mostly focused on final consumer products with improved nutrition or flavour, as summarised in Table 4.
The effect of adding okara in the preparation of idli, a traditional Indian
Future perspectives and conclusion
Nowadays, with a more modernised soymilk and soybean curd manufacture industry, the utilisation of high heat treatments and sanitary processing equipment under the HACCP system produces okara with less microflora contamination, resulting in a suitable substrate for biovalorisation. Research has demonstrated the feasibility and potential in the production of a variety of bioactive substances and final consumer products via okara fermentation. An overview linking current developments with
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