Phytase supplementation has become an efficient tool to improve bioavailability of P present in feedstuffs and to reduce the amount of phytate-derived P excreted to the environment by animals. Phytase-mediated hydrolysis of phytate also releases several other essential minerals (Lei et al., 1993a,b,c). Soybean meal is a common ingredient to be mixed with corn and other cereals for the swine and poultry ration. Various sources of plant and microbial phytases, along with other feed supplements such as citric acid, vitamin D, and strontium, have been tested to enhance utilization of P and other nutrients in the corn-soybean meal based diets (Han et al., 1998; Snow et al., 2004; Pagano et al., 2007b). Findings from these experiments have been used to improve performance and health of commercial herds and spare costly non-renewable sources of inorganic P.
Soybean serves as an important component of many dishes oriented to human nutrition. It is consumed as cooked, sprouted, and processed into soy milk, tofu, miso, tempeh or natto. Industrial processing of soybean is derived not only by its nutritional properties, but also by its chemical characteristics. Soy proteins contain lipophilic, polar, non-polar, and negatively and positively charged groups that enable them to be associated with many different types of compounds (Endres, 2001). Representing the major industrial products, soy oil and soybean meal are produced through a solvent extraction process. Crude soybean oil is further processed in to a variety of products, whereas soybean meal can be further processed to protein concentrates, protein isolates, or textured protein products for preparations of comminuted meat products or meat analogs. Although phytase may be used to improve the nutritive utilization of soybean by humans, much less research in this regard has been done than that in animals.
2. Nutrient and non-nutrient composition of soybean
Soybeans are important dietary sources of protein, lipids, minerals, vitamins, fiber, and bioactive compounds. The chemical compositions of soybean and most of its derived products are characterized by high protein content that ranges from 33 to 43% (Grieshop et al., 2001; Karr-Lilienthal et al., 2004; Rani et al., 2008; Saha et al., 2008). After soy oil and hull are removed during processing of soybean meal, protein contents of the resultant products may rise to 47-59% (NRC, 1998; Grieshop et al., 2003). Higher protein contents may be achieved in specific extraction products like soybean protein concentrate or soybean protein isolate (64 and 85%, respectively; NRC, 1998). Soybean manifests an excellent amino acid profile, with only lysinine and methionine as the limiting amino acids (for swine). The potential availability of soybean amino acids can be affected by the extent of thermal processing conditions (Grieshop et al., 2003). Parameters like KOH protein solubility, protein dispersibility index (solubility in water) or urease activity (trypsin inhibitor activity) are used to assess the quality of soybean protein and the processing appropriateness of soybean products (Grieshop et al., 2003; Karr-Lilienthal et al., 2004). Oil and fiber are the other two major components of soybean. Acid-hydrolyzed fat is in the range of 13-15%, and may reach 22% (Achouri et al., 2008; Rani et al., 2008; Saha et al., 2008; Yuan et al., 2009). Soybean oil is mainly composed of polyunsaturated fatty acids followed by monounsaturated and saturated fatty acids. The major fatty acid is linoleic acid, although soybean oil has considerable amounts of oleic and linolenic acids (Nwokolo, 1996; NRC, 1998; Yuan et al., 2009). The presence of lipoxygenase can give rise to the appearance of off-flavors and aroma at different stages of processing, which may negatively affect the organoleptic properties of soybean oil. The major food uses of soybean oil are as salad or cooking oil, part of mayonnaise and dressings, and margarine or shortenings. Dietary fiber in soybean comprises from 11.3 up to 30% of the total seed content (NRC, 1998; Grieshop & Fahey, 2001; Karr-Lilienthal et al., 2004; Jiménez-Escrig et al., 2010). The fiber content of soybean meal is considerable, unlike lipids which are in very low proportion due to an initial extraction process. Soybean and its by-products are also a good source of nutritionally essential macro- and micro-minerals (Raboy et al., 1984; NRC, 1998; Giami, 2002; Karr-Lilienthal et al., 2004; Rani et al., 2008), although their availability can be seriously compromised by the presence of phytic acid, polyphenols, and oxalate, or by the specific structure of soybean proteins (Lynch et al., 1994).
A variety of non-nutritional components in soybean may interfere with its nutrient availability (Liener, 1994). Among these components, protease inhibitors and lectins decrease protein digestion, cause systemic effects on the digestive tract, and inhibit animal growth. Heat processing for the production of soybean meal and reduction of disulphide bonds using a NADP-thioredoxin system can inactivate such components and alter the compact structure of soybean proteins, thus improving the nutritional value of soybean containing foods (Liener, 1994; Kakade et al.,. 1972; Marsman et al., 1997; Giami et al., 2002; Olguin et al., 2003; Karr-Lilienthal et al., 2004; Faris et al., 2008). Meanwhile heat-stable components including phytate (Raboy et al., 1984; Han et al., 1988; Kumar et al., 2005; Yuan et al., 2009), polyphenols, saponins (Giami, 2002), oxalate (Ilarsan et al., 1997; Al-Whash et al., 2005) or α-galactoside oligosaccharides may interfere with the bioavailability of protein, lipids or minerals present in the diet or induce flatulence (Suarez et al., 1999). Al-Whash et al. (2005) have reported that a strong correlation exists between oxalate and phytate content of soy foods and a significant correlation, based on molar basis, between the divalent ion binding potential of oxalate plus phytate and Ca plus Mg content. Nevertheless, some of these so-called anti-nutritional factors like saponins, phytic acid and polyphenols are also responsible for certain beneficial health effects related to soybean consumption (Rao & Sung, 1996; Porres et al., 1999; Kerwin, 2004; Kang et al., 2010; Zhang & Popovich, 2010). In general, the proximate composition of soybean or soybean meal is highly dependent on genetic, environmental, and processing conditions (Grieshop & Fahey, 2001; Karr-Lilienthal et al., 2004; Kumar et al., 2006). Improvement of soybean cultivars for specific characteristics such as early maturity, high yield, desired seed quality and resistance to pests has been carried out through plant breeding, and new lines of soybean have been developed (Giami, 2002). In addition, contents and properties of the major anti-nutritional components in soybean like trypsin inhibitors, polyphenols or phytic acid are not altered in the advanced lines tested.
3. Phytase: Enzymology and dietary efficacy
Phytases are phosphohydrolytic enzymes that initiate the stepwise removal of phosphate groups from
The stomach seems to be the major site of action for the histidine acid phosphatases isolated from
4. Synergism of soybean and phytase in nutrition
Due to its excellent protein quality, soybean meal has been extensively used as a common protein supplement in swine and poultry ration (Lei et al., 1993a, 2003b; Fernandez-Figares et al., 1997; Stahl et al., 2003; Boling et al., 2000). Different strategies have been employed to improve its nutritional value. These include supplementing its limiting amino acids or mixing with corn, and wet feeding (Liu et al., 1997), supplementing organic acid (Ravindram & Kornegay, 1993), or treating with enzymes. Supplementations of exogenous carbohydrases, proteases or phytases enhance the dietary utilization of essential nutrients that otherwise would be lost to the animal and excreted to the environment. Soybean is also being increasingly used in aquaculture to replace the scarce and expensive fishmeal protein in diets for fish, crustaceans, and shellfish (Yan et al., 2002; Pham et al., 2010; Brinker & Reiter, 2011). In addition, substitution of 50% or even 100% of fish meal by a mixture of soybean meal and wheat gluten in trout diets counteracted the pathological alterations in the liver that were often related to highly-energetic fish meal diets (Brinker & Reiter, 2011).
Addition of phytase to the corn-soybean meal based diets for farm animals (Lei & Porres, 2007) improves phosphorus retention and bone metabolism not only in P-deficient, but also in P-adequate pigs. Pagano et al. (2007b) has observed increments in bone breaking strength (11-20%), mineral content (6-15%) and mineral density of metatarsals and femur of pigs fed P-adequate diets supplemented with the
Although several low-phytate barley or corn lines have been developed and tested for nutritional applications (Sugiura et al., 1999; Baxter et al., 2003; Applegate et al., 2003; Overturf et al., 2003), the low-phytate soybean line has shown a reduced seedling emergence (Meis et al., 2003; Oltmans et al., 2005; Trimble & Fehr, 2010). In contrast, Yuan et al. (2009) have recently developed two new low phytic acid mutants and studied their nutritional properties. Their mutants showed good agronomic performance, reduced phytic acid, and increased inorganic P concentration in all tested environments without changes in the crude protein content, amino acids, total oil, and individual saturated fatty acids despite variations in oleic and linoleic acid contents. Furthermore, low phytic acid lines had a higher content of isoflavones than their parental wild-type lines.
Nevertheless, phytase supplementation is still the most feasible method for improving the phytate-mediated low availability of essential minerals in the corn soybean-based diets. In fact, Stahl et al. (1999) found that phytase was effective in releasing phytate-bound Fe and P from soybean meal
Developing phytase transgenic crops represents another strategy to improve the availability of P and other minerals in soybean. Li et al (1997) have shown the secretion of active recombinant phytase from soybean cell suspension cultures that displayed biochemical properties indistinguishable from the commercially available fungal phytase. Denbow et al. (1998) have observed an improved bioavailability of phytate-P from soybeans transformed with a fungal phytase gene to broilers. Bilyeu et al. (2008) have reported that the cytoplasmic expression of an active
5. Future perspective
The demonstrated and potential health benefits of soybean foods have rendered these products as functional foods. Numerous new health claims associated with these products are being evaluated worldwide. Several non-nutritional components in soybean have proven to be beneficial in the prevention and nutritional treatment of chronic diseases. Phytic acid is considered to be antioxidative due to its ability to chelate transition metals like Fe that may induce oxidative stress (Porres et al., 1999). Fiber and isoflavones represent other major beneficial components of soybean. Dietary intakes of soy isoflavones may be associated with lower incidences of atherosclerosis, type 2 diabetes, and coronary heart diseases, decreased risk of certain types of carcinogenesis, improved bone health, and relieved menopausal symptoms (Blum et al., 2003; Xiao, 2008; Messina et al., 2009). It is interesting to mention that those components are metabolized in the large intestine by specific bacterial populations that are present in a relatively low percentage of Westerners (Lambe, 2009; Messina et al., 2009). Such metabolism gives rise to products like equol that are just as effective as or even more effective than daidzein intrinsically present in soybean (Setchell et al., 2002). Novel benefits of soybean protein hydrolyzates have been recognized in the treatment of hypertension and hypertension-derived renal injury (Yang and Chen, 2008). Another new finding is the ability of soy isoflavones to up-regulate the expression of genes critical for drug transport and metabolism. Of especial interest is the stimulation of several phase I and II metabolizing enzymes that may act in the chemoprevention of cancer, or the activation of CYP family of enzymes that play an important role in bile acid metabolism (Appelt & Reicks, 1997; Li et al., 2007; Bolling & Parkin, 2008). Therefore, it will be fascinating to explore potential synergism between phytase and soybean in improving human and animal health beyond nutrition.