Utilization of Okara as a Sustainable Feed Ingredient in Broiler Chicken Nutrition
2025 Volume 13 Issue 3 Pages 76-91
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2025 Volume 13 Issue 3 Pages 76-91
Okara, a by-product of soymilk and tofu production, offers potential as a functional feed ingredient in broiler chicken diets. This review explores okara’s nutritional composition, benefits, challenges, and processing techniques relevant to poultry nutrition. Rich in crude protein, insoluble fiber, and essential amino acids, okara can partially replace conventional protein sources. However, fresh okara’s high moisture content and anti-nutritional factors (e.g., phytic acid, trypsin inhibitors, and lectins) can limit its direct use in poultry feed. Processing methods such as drying, fermentation, or enzymatic treatment are essential to improve nutrient availability and stability. Inclusion of up to 10% dried or fermented okara in broiler diets supports growth performance, feed efficiency, carcass yield, and gut morphology. Additionally, its bioactive compounds, such as isoflavones and phenolics, exhibit antioxidant and immunomodulatory properties, contributing to oxidative balance and immune function in broilers. Overall, okara shows strong potential as a nutritional and functional ingredient for enhancing broiler performance when properly processed and formulated.
Global meat production has steadily increased, with poultry meat accounting for a substantial portion of this growth due to its relatively lower production costs and shorter production cycles compared to other livestock [1]. In ensuring the high quality of mass-produced poultry products, feed needs to accommodate energy and protein sufficiency [2]. Soybean meal (SBM) is notable as a commercial poultry feed ingredient, providing more than 40% of crude protein (CP) for poultry nutrient adequacy. Generally, the feed aspect contributes 60–70% of the total cost of production [3, 4, 5]. Long-term use of commercial feed inevitably increases production costs [4, 5]. Producers seek strategies such as using alternative feed ingredients to maintain the growth performance of birds [6, 7]. However, certain factors must be taken into account when utilizing alternative feeds in broilers. Many studies have explored the use of alternative feedstuffs in broiler production; however, challenges regarding sustainability and nutritional efficacy persist.
The metabolizable energy (ME) content of the diet is a key factor influencing the growth performance of broiler chickens. Studies indicate that broiler chickens require diets with ME levels ranging from 2,800 to 3,100 kcal/kg, depending on their age and growth stage [8, 9]. Insufficient energy intake can lead to poor growth rates and feed conversion ratios (FCR), which are critical metrics for poultry production [9]. Protein is also a key factor for growth performance, since it is vital for muscle development and overall growth in broiler chickens. The CP requirement typically ranges from 18% to 24% in starter and grower diets, respectively [10]. Essential amino acids, which are only obtained from feed—particularly lysine, methionine, and threonine—play crucial roles in protein synthesis and growth. Moreover, the use of protein sources such as SBM, corn gluten meal (CGM), and other plant-based proteins must be carefully balanced to meet the amino acid profile required for optimal growth [11, 12]. On the other hand, SBM and CGM are predicted to be insufficient in the next two decades. As a result, alternative feeds with similar nutritive content to SBM and CGM are needed.
Okara, a by-product of soybean processing, primarily arises during the production of soy milk and tofu. Approximately 14 million tons of soybeans are produced worldwide; however, half of the by-products are discarded, leading to significant waste in the food industry [13, 14]. Okara represents one of the largest quantities of locally available feed in Asian areas, especially in China and Japan [13, 15]. Since okara is substantially more economical than SBM, it can be used as a partial replacement for SBM. Its composition, characterized by high protein and fiber content along with various bioactive compounds, supports its use in enhancing the nutritional quality of food products. The valorization of raw okara presents an opportunity to create value-added products [13]. Despite its potential nutritional content, raw okara contains high water content, which shortens its shelf-life and makes it unsuitable as a poultry feed ingredient. Due to this attribute, raw okara is less favorable as poultry feedstuff [15]. Additionally, anti-nutritional factors (ANFs) could interfere with the poultry digestive process and impact the absorption of nutrients contained in okara [15, 16, 17]. Therefore, it is essential to understand the potential of okara, a soybean by-product, as a viable feed option and how it can improve the growth performance of broilers.
This review aims to provide comprehensive information on the utilization of okara in broiler chicken nutrition, including its nutritional profile, impact on growth performance, intestinal health, and strategies to optimize its inclusion in poultry diets while addressing potential challenges.
The production of soymilk consists of a series of carefully controlled steps that not only influence the quality of the final product but also determine the characteristics of its by-product, okara. As shown in Figure 1, the process begins with the removal of impurities through washing, followed by the soaking of dehulled soybeans at room temperature for approximately 12 hours. This rehydration step softens the beans, facilitating subsequent mechanical processing. Once adequately soaked, the beans are drained, rinsed, and subjected to brief thermal treatment—typically at 98 °C for about 5 minutes—to achieve microbial decontamination and deactivate protease inhibitors like trypsin inhibitors, which hinder protein digestion in animals [18, 19]. The cooked beans are then blended with water, usually in a 1:10 (w/v) ratio, forming a slurry that undergoes filtration. This filtration step separates the liquid soymilk from the residual solid matter—okara—a fibrous, protein-rich by-product generated in considerable quantities.
Soymilk okara contains approximately 27% crude protein on a dry matter basis, along with significant levels of dietary fiber and beneficial fatty acids [20]. However, its high moisture content—ranging from 79.8% to 82.4%—makes fresh okara highly perishable and difficult to handle, particularly in large-scale feed systems [21]. As such, unprocessed okara is prone to microbial spoilage and nutrient degradation, limiting its direct application in animal nutrition. To enhance its shelf-life, stability, and nutritional efficacy, various post-processing strategies are employed, including drying, enzymatic hydrolysis, and microbial fermentation. These methods not only reduce moisture and anti-nutritional compounds such as phytic acid and lectins but also improve nutrient digestibility and functional properties. Through such interventions, okara becomes more viable as a sustainable and cost-effective feed ingredient, capable of supplementing or partially replacing conventional protein sources in broiler diets.
Figure 1 illustrates the typical process of soymilk production in Japan, with a specific focus on the stage at which soymilk okara is generated. The process begins with dehulled soybeans, which undergo soaking to rehydrate and soften the seeds, followed by thermal treatment to deactivate heat-sensitive anti-nutritional factors. The softened soybeans are then ground with water to produce a slurry. This slurry is filtered to separate the liquid soymilk from the solid residue—known as soymilk okara. Importantly, this by-product is collected immediately after filtration and before any coagulation or curd formation steps, which distinguishes it from tofu okara. The use of dehulled soybeans and the relatively early point of okara extraction result in a composition that is generally higher in protein and lower in fiber compared to tofu okara, which is derived from whole soybeans and collected after further processing. As such, Figure 1 not only visualizes the production steps but also underscores the nutritional and structural uniqueness of soymilk okara. This clarification is essential when considering its functional applications in animal feed or other valorization pathways, as its characteristics are directly influenced by its origin within the soybean processing chain.
2.2 Processing of tofu okaraTofu production begins with steps similar to soymilk production—soaking, heating, and grinding of soybeans followed by filtration to extract soymilk—but diverges significantly in its later stages. A key distinction lies in the raw material: tofu production uses whole soybeans, including the hulls, whereas soymilk production typically uses dehulled soybeans [19]. After obtaining the raw soymilk, the process continues by adding coagulants, such as magnesium chloride (nigari) or calcium sulfate (gypsum), to induce protein coagulation under heat. This forms curds, which are then pressed to expel excess liquid (whey), resulting in the final tofu product. The fibrous residue left behind during this pressing stage is known as tofu okara.
The use of whole soybeans and the extended processing steps significantly influence the nutritional composition and physical structure of tofu okara. Compared to soymilk okara, tofu okara tends to contain less crude protein but more dietary fiber, largely due to the inclusion of the soybean hulls and the leaching of soluble proteins into the tofu curd during coagulation [22]. Additionally, the pressing step affects the moisture content, texture, and density of the residual okara. This variation in composition makes tofu okara a unique co-product, with distinct implications for its digestibility, shelf-life, and application in feed or food formulations. While often underutilized, tofu okara is generated in substantial quantities—estimated to exceed seven million metric tons annually in East Asia alone, particularly in China and Japan [15, 23, 24]. Given its nutritional value and volume, tofu okara holds strong potential for upcycling into animal feed, compost, or fermentation substrates, particularly when processed using drying, enzymatic treatment, or fermentation to reduce anti-nutritional factors and improve stability [25].
Figure 2 visually depicts the tofu production process, with particular emphasis on the formation and separation of tofu curd from soymilk. Unlike Figure 1, which ends with soymilk extraction, this diagram continues through the coagulation and pressing stages, clearly identifying the point at which tofu okara is produced. The diagram highlights how coagulants are added to the hot soymilk to facilitate the aggregation of proteins into curds, which are subsequently molded and pressed to form tofu. The solid-liquid separation that occurs during pressing is crucial not only for tofu structure but also for the generation of tofu okara as a distinct by-product. The schematic helps contextualize the compositional differences between tofu and soymilk okara by demonstrating how thermal, mechanical, and chemical treatments shape the final residue. This visual representation is essential for understanding the underlying mechanisms that determine tofu okara's fiber-rich profile and its unique processing challenges and valorization pathways.
2.3 Waste management strategiesDespite its nutritional value, a significant portion of okara—approximately 50%—is often discarded as waste, leading to environmental concerns worldwide [25]. Effective waste management strategies are essential to minimize the environmental impact of okara disposal. One approach is to convert okara into value-added products, such as animal feed, which can reduce waste while providing a sustainable protein source for livestock [20]. Additionally, okara can be utilized in composting or as a substrate for the cultivation of insects, such as black soldier fly larvae (Hermetia illucens), which can further enhance waste reduction and provide a high-quality protein source for animal feed. The larvae can efficiently convert okara into biomass, which can then be used as feed, thereby closing the loop in the utilization of this agro-waste [26].
The fresh okara poses challenges for storage and transportation, leading to rapid spoilage and increased handling costs [27]. Therefore, effective drying and preservation methods are crucial for extending the shelf-life of okara and facilitating its use in animal feed [28]. Moreover, there is a need for further research to optimize the inclusion rates of okara in animal diets and to develop processing techniques that enhance its nutritional value [22]. As producer demand for sustainable animal production increases, the valorization of okara presents a significant opportunity for the livestock industry to reduce waste and improve feed sustainability.
2.4 Current uses in animal feedOkara can be utilized in various forms, including fresh, dried, or ensiled, allowing for flexibility in its application within animal diets. Approximately 40% of the produced okara is currently used for animal consumption, primarily in monogastric diets [25]. Research has demonstrated that okara can replace a portion of traditional protein sources, such as SBM, without negatively impacting animal performance [22]. Wicaksono et al. [29] reported that the apparent ME (AME) value of okara can be estimated through in vitro digestibility techniques, providing insight for constructing feed for broiler chickens. The inclusion of dry okara has been associated with improved FCR and enhanced meat quality [30]. In addition to its direct use as animal feed, okara can also be fermented to improve its nutritional profile. Fermentation processes can enhance the digestibility of okara and increase the availability of nutrients, making it a more effective feed ingredient [22, 31]. Furthermore, the fermentation of okara can produce beneficial metabolites that contribute to gut health in monogastric animals [32]. In this review, we also present the nutritional content of okara for monogastric animals, specifically for broiler chickens.
Okara is recognized for its high CP, which typically ranges from 20% to 35% in dry matter (DM) [19, 33]. The protein in okara is primarily composed of globulins, particularly the 7S and 11S fractions, which are known for their favorable amino acid profiles [34]. Protein in okara is rich in essential amino acids, particularly lysine, which is often limiting in cereal-based diets [33, 34, 35]. CP and AME contents of okara vary by source and processing (Table 1). Raw and dried tofu okara had protein levels of 32.89% and 23.97% DM, with AME values of 1,816 and 2,700 kcal/kg DM, respectively [36, 37]. Dried soymilk okara had the highest protein (35.64% DM) and AME (2,972 kcal/kg DM), while fermented tofu okara showed lower protein (21.66–23.28% DM) and moderate AME (2,830 kcal/kg DM) [17, 38, 39]. The actual digestibility of okara protein is relatively high (62.79–74.14%), making it a valuable source of nutrition for monogastric animals [36, 40]. Studies have shown that the inclusion of okara in animal diets can enhance growth performance and feed efficiency [17, 22, 30].
| References |
Basis (sources) |
Water content (%) |
Crude protein (% DM) |
Crude fiber (% DM) |
Apparent metabolizable energy (kcal/kg DM) | Animal subjects |
|---|---|---|---|---|---|---|
| [36] | Raw (Tofu) | 76.71 | 32.89 | 12.73 | 1,816 (Sibbald & Morse Technique) | Pekin Ducks |
| [17] | Dry (Soymilk) | 4.65 | 35.64 | 12.67 | 2,972 (Matterson Technique) | Broiler Cobb |
| [37] | Dry (Tofu) | 7.92 | 23.97 | 16.90 | 2,700 (Hill & Anderson Technique) | White Leghorn Cockerels |
| [38] | Fermented (Tofu) | 8.69 | 23.28 | 17.35 | 2,830 (Tilman Technique) | Broiler CP 707 |
| [39] |
Fermented (Tofu) |
11.18 | 21.66 | 20.26 | 2,830 (Hill & Anderson Technique) | Broiler Arbor Acres |
Okara is also notable for its high dietary fiber content, which can range from 40% to 55% on a dry-weight basis [34, 41]. The fiber in okara is predominantly insoluble, contributing to its functional properties in food applications, such as improving texture and moisture retention [42]. The fat content of okara varies, typically falling between 10% and 14% on a dry-weight basis, with a composition that includes both saturated and unsaturated fatty acids [34, 43]. In terms of carbohydrates, okara contains approximately 30% to 50% insoluble fiber [25, 44]. It comprises less than 1% of sugars derived from sucrose and maltose, which limits energy availability for microbial fermentation in the gut [25]. The balance of protein, fiber, and fat in okara makes it a versatile ingredient for various feed formulations.
3.3 Vitamins and minerals in OkaraOkara is a source of several essential vitamins and minerals, although the concentrations can vary depending on the processing methods and soybean variety. It contains small amounts of B vitamins, including thiamine, riboflavin, and niacin, which are important for energy metabolism [45]. Additionally, okara is rich in minerals such as calcium, magnesium, potassium, and phosphorus, contributing to its overall nutritional profile [46]. The presence of bioactive compounds, including isoflavones and phenolic compounds, further enhances the nutritional value of okara. These compounds are known for their antioxidant properties and potential health benefits, including anti-inflammatory and cholesterol-lowering effects [47]. The combination of proteins, fiber, vitamins, minerals, and bioactive compounds positions okara as a functional ingredient with significant potential for use in both animal and human nutrition.
3.4 Comparison with SBMSBM is a co-product primarily derived from soybean oil extraction, whereas okara is a by-product generated during the production of tofu or soymilk [19, 23]. Although both originate from soybeans, different target products produce distinct by-products with varying nutritional values. SBM offers a high CP, typically ranging from 44% to 48% in DM [48, 49]. Meanwhile, okara provides CP of approximately 20–35% in DM, which, although still lower than that of SBM, is still comparable [23]. In terms of amino acid content, SBM provides essential amino acids such as lysine, methionine, and threonine. Although okara contains lower levels of amino acids compared to SBM, its amino acid digestibility is comparable, suggesting its potential as a viable alternative or complementary protein source in animal diets [16, 48, 49].
Both SBM and okara face challenges. They contain ANFs such as trypsin inhibitors, phytates, and lectins, which can impair nutrient absorption and digestion [50]. During oil extraction, SBM is subjected to high-temperature processing (>100 °C), which effectively inactivates ANFs and enhances protein digestibility and availability. In contrast, okara, a moist by-product of soymilk or tofu production, typically undergoes lower and less consistent heating (<85 °C). This treatment only partially reduces ANF levels and is less effective than the controlled thermal treatment of SBM. Consequently, okara contains higher levels of ANFs compared to SBM [16, 22]. Therefore, additional processing steps such as drying, fermentation, or enzymatic treatment may be necessary to improve the nutritional quality and shelf-life of okara.
3.5 Comparison with cornCorn is another conventional feed ingredient widely used in animal diets, primarily as a source of energy. It typically contains around 8% to 10% protein, with a high starch content (approximately 70% to 75%) [51]. Corn is valued for its digestibility and palatability, making it an essential component of many feed formulations. However, corn lacks certain essential amino acids, particularly lysine, which can limit its effectiveness as a sole protein source [51, 52, 53]. Okara, with its higher protein content compared to corn yet low in energy supply, can complement the amino acid profile of corn-based diets. The inclusion of okara in diets formulated with corn can enhance the overall nutritional value by providing additional protein and essential amino acids [16, 54]. Partial substitution of corn with okara may also mitigate a small portion of the environmental burden associated with corn production [54].
3.6 Factors affecting nutritional qualityThe nutritional quality of okara is influenced by several factors, including the characteristics of the soybean variety utilized, processing methods applied, and conditions under which okara is produced. Different soybean cultivars exhibit varying protein and fat profiles. Stanojević et al. [55] explained that okara derived from six soybean genotypes displayed differing total protein content, with Novosadjanka exhibiting the highest content (40.36%), followed by ZPS-015 (37.32%), Balkan (35.41%), Krajina (35.27%), Nena (35.07%), and Lana (31.81%). Regarding oil content, Sharma et al. [56] reported variation among eight soybean genotypes, with total oil content ranging from 14.0% in SL 869 to 18.7% in SL 688, with intermediate values in SL 831 (14.1%), SL 794 (14.9%), SL 799 (15.0%), SL 768 (18.0%), SL 783 (17.5%), and SL 525 (18.5%). All varieties predominantly contained unsaturated fatty acids, particularly linoleic acid (46.3–58.8%) and oleic acid (20.9–30.4%), with lower proportions of saturated fatty acids such as palmitic (10.6–12.9%) and stearic acid (3.0–4.0%) [56, 57].
3.7 Processing methods and their impactThe processing methods employed in the production of okara significantly influence its chemical composition and functional properties. Traditional methods of soymilk production involve soaking, grinding, and boiling soybeans, followed by filtration to separate the liquid soymilk from the solid okara [58]. Variations in these processing steps can lead to differences in the nutritional quality of the resulting okara. For example, high-pressure homogenization has been shown to improve the extraction yield and alter the biopolymer structure of okara, potentially enhancing its nutritional quality [53]. Similarly, flash-drying techniques can affect the protein structure and solubility of okara, leading to variations in its functional properties [59]. The choice of drying methods, such as air jet impingement or convective drying, can also impact the moisture content and shelf stability of okara [60, 61]. Moreover, the incorporation of fermentation processes can further modify the composition of okara. Fermentation with specific strains of bacteria, such as Lactobacillus plantarum, can enhance the digestibility of okara and increase its antioxidant properties [22, 31]. This not only improves the nutritional value of okara but also expands its potential applications in functional foods and animal feed. Furthermore, understanding the benefits of okara in broiler diets needs to be addressed in the following section.
Weight gain and FCR are pivotal indicators of growth performance in broiler chickens. A study by Diaz-Vargas et al. [62] reported that broiler growth performance—including body weight gain, feed intake, and FCR from days 21 to 42, as well as carcass yield at 42 days of age—was not significantly affected by dietary inclusion of up to 10% okara. This finding is particularly significant given that okara contains 12.7% neutral detergent fiber, a component known to dilute dietary energy and potentially impair broiler development. Okara comprises approximately 20–35% protein, along with essential amino acids that support muscle development and overall growth [17]. Furthermore, the high fiber content of okara can promote gut health, which is crucial for optimal nutrient absorption and utilization [62]. Consequently, broilers consuming diets supplemented with okara meal exhibited improved weight gain and feed efficiency compared to those fed the corn-SBM diet. Additionally, studies have demonstrated that the inclusion of fermented okara in broiler diets can lead to enhanced nutrient absorption and utilization, resulting in improved growth performance [17, 30, 62]. This aligns with findings on the positive effects of dietary fiber on gut health and nutrient digestibility in poultry [30, 62].
4.2 Meat yield and qualityThe impact of okara meal on meat yield and quality is another important aspect of its use in broiler diets. For instance, broilers fed diets containing okara meal have shown increased breast meat yield compared to those on a corn-SBM basal diet [17, 30, 62]. This is attributed to the high protein content of okara, which supports muscle growth and development [30]. In terms of meat quality, studies have reported that okara meal can positively influence various quality parameters, including tenderness, juiciness, and color [17, 62]. The presence of bioactive compounds in okara, such as isoflavones and phenolic compounds, may contribute to improved meat quality by enhancing oxidative stability and reducing lipid oxidation [62, 63]. Sinha et al. [64] reported that up to 25% okara meal, along with supplementation of non-starch polysaccharide-degrading enzymes, did not negatively impact the overall parameters of growth and meat. Other studies reported that 10% inclusion of dried okara without any additional enzymes is not detrimental to the growth performance and meat quality of broilers [17, 62].
4.3 Impact on gut microbiotaThe gut microbiota plays a crucial role in the overall health and performance of broiler chickens. A balanced and diverse gut microbiome is essential for optimal digestion, nutrient absorption, and immune function. The incorporation of okara meal into broiler diets has been associated with beneficial changes in gut microbiota composition. Research indicates that the high fiber content of okara can act as a prebiotic, promoting the growth of beneficial bacteria in the gut [65]. The fermentation of dietary fibers by gut microbiota leads to the production of short-chain fatty acids (SCFAs), which serve as an energy source for intestinal cells and contribute to gut health. Studies have shown that broilers fed diets supplemented with okara exhibit increased populations of beneficial bacteria, such as Lactobacillus and Bifidobacterium, while reducing the abundance of pathogenic bacteria [66]. This shift in microbial composition can enhance gut health, improve immune responses, and reduce the incidence of gastrointestinal diseases. Furthermore, the bioactive compounds present in okara, such as isoflavones and phenolic compounds, may also contribute to the modulation of gut microbiota. These compounds possess antioxidant properties that can help mitigate oxidative stress in the gut, promoting a healthier microbial environment [67].
4.4 Digestibility and nutrient absorptionThe digestibility of feed ingredients is a critical factor influencing the growth performance of broiler chickens. okara meal has been shown to enhance nutrient digestibility and absorption, which can lead to improved growth performance. The high protein content of okara, combined with its favorable amino acid profile, contributes to its effectiveness as a feed ingredient [17, 30, 55, 62]. Studies have demonstrated that the inclusion of okara meal in broiler diets can improve the digestibility of key nutrients, including proteins, fats, and soluble fibers [17, 30, 62]. The presence of dietary fiber in okara can stimulate the secretion of digestive enzymes, enhancing the breakdown of nutrients and facilitating their absorption in the intestine [63]. Additionally, the fermentation of okara can further improve its digestibility by breaking down ANFs that may inhibit nutrient absorption [68]. The impact of okara on nutrient absorption is also reflected in the morphology of the intestinal lining. Research has shown that diets containing okara can lead to increased villus height and surface area in the intestines, which enhance the absorptive capacity of the gut [17, 58, 69]. This morphological improvement is associated with better nutrient utilization and overall growth performance in broilers [17, 62].
4.5 Effects on immune responseThe immune system of broiler chickens plays a crucial role in their overall health and productivity. Dietary components can significantly influence immune function, and the inclusion of okara meal has been suggested to enhance the immune response in poultry [70, 71]. Research indicates that okara, owing to its rich protein content and bioactive compounds, can stimulate the production of immunoglobulins and enhance the activity of immune cells, such as macrophages and lymphocytes [70, 72, 73]. Studies have demonstrated that broilers fed diets supplemented with okara exhibit increased levels of immune cell populations, which are essential for adaptive immunity [71, 73]. This enhancement in immune cell populations can lead to improved resistance against pathogens and a reduced incidence of diseases [72, 73]. Furthermore, the presence of dietary fiber in okara can promote gut health, which is closely linked to immune function [73, 74]. A healthy gut microbiota contributes to the development of the immune system and helps in the prevention of infections [73, 75]. The immunomodulatory effects of okara may also be attributed to its antioxidant properties, which help mitigate oxidative stress and inflammation in the body [73, 74, 75, 76]. By reducing oxidative damage, okara can support overall immune function and improve the resilience of broiler chickens to environmental stressors [73, 74].
4.6 Antioxidant propertiesOkara, a by-product of soybean processing, is rich in bioactive compounds, such as isoflavones and phenolic compounds, that possess strong antioxidant properties [77, 78]. These antioxidants play a vital role in neutralizing free radicals and reducing oxidative stress, which can adversely affect the health and performance of broiler chickens [79, 80, 81]. Oxidative stress is known to impair immune function, decrease growth performance, and negatively impact meat quality [79, 81]. The antioxidant properties of okara can enhance the oxidative stability of broiler meat, improving its shelf-life and quality [79]. Studies have shown that dietary supplementation with okara can lead to lower levels of malondialdehyde (MDA), a marker of lipid peroxidation, in the meat of broilers [77, 78]. Moreover, the inclusion of okara in broiler diets has been associated with increased activity of antioxidant enzymes, such as superoxide dismutase and glutathione peroxidase, which play crucial roles in protecting cells from oxidative damage [77, 81]. By promoting the activity of these enzymes, okara can help maintain redox balance and support the immune system, further enhancing the health and performance of broiler chickens [79, 81].
Phytic acid is one of significant ANFs in okara (Table 2). It binds minerals like calcium, magnesium, iron, and zinc, reducing their bioavailability and absorption in the digestive tract [82]. The presence of phytic acid can cause mineral deficiencies in broiler chickens when okara is a major component of their diet. One study found that low-phytic acid soybean mutants had a 50–75% reduction in phytic acid content compared to wild-type soybeans [83]. The selection of soybean varieties with naturally lower phytic acid levels could be beneficial for reducing phytic acid levels in okara. The effectiveness of fermentation in reducing phytic acid is attributed to the activation of endogenous phytase enzymes [83, 84]. Phytase hydrolyzes the phytic acid molecule, liberates bound minerals, and improves their bioavailability [84]. The duration and conditions of the fermentation process, ideally in five days with pH 7.3 under room temperature [85]. Phytase enzymes produced during fermentation degrade phytic acid, releasing bound minerals and improving absorption [86]. Besides phytic acid, okara contains lectins and tannins, which can negatively impact nutrient utilization and health in broiler chickens (Table 2) [87]. Lectins interfere with nutrient absorption and may cause gut inflammation, while tannins reduce protein digestibility and affect feed palatability [88]. Various processing techniques like drying treatment, fermentation, and enzymatic hydrolysis can reduce these ANFs in okara, enhancing its nutritional quality as a feed ingredient for broiler chickens [89]. The selection of soybean varieties genetically deficient in specific ANFs can effectively mitigate their impact without requiring extensive processing. Generally, heat processing during tofu and soymilk production diminishes the impact of ANFs on by-products. Furthermore, dehulling the outer skin of soybeans can reduce the levels of ANFs such as phytic acid and protease inhibitors [57].
| Anti-nutritional factors | Description | Potential impact on broiler health | References |
|---|---|---|---|
| Phytic Acid | Binds essential minerals (calcium, zinc) and reduces their bioavailability | Mineral deficiencies, reduced bone development, and lower feed efficiency | [22, 24] |
| Lectins | Proteins that bind to the gut lining and interfere with nutrient absorption | Intestinal damage, decreased nutrient absorption, and impaired growth | [24] |
| Tannins | Polyphenols that can form complexes with proteins minerals | Reduced protein digestibility, impaired growth, and reduced feed intake | [22] |
| Trypsin inhibitors | Proteins that inhibit trypsin enzyme activity, reducing protein digestion | Reduced protein digestibility, impaired growth, and nutrient absorption | [19, 24] |
Trypsin inhibitors are one of the primary ANFs present in okara (Table 2). These compounds can interfere with protein digestion by inhibiting the activity of trypsin, an essential enzyme involved in the breakdown of proteins in the digestive system [55]. The presence of trypsin inhibitors in okara can lead to reduced protein digestibility and nutrient absorption, ultimately impacting the growth performance of broiler chickens [90]. Research has shown that the levels of trypsin inhibitors in okara can vary depending on the soybean variety and processing methods used during soymilk production [55]. For instance, drying treatment during the processing of soybeans can significantly reduce the activity of trypsin inhibitors, thereby improving the nutritional quality of okara [19]. Therefore, the processing methods employed can play a crucial role in mitigating the effects of trypsin inhibitors and enhancing the overall digestibility of okara when included in broiler diets. To address the challenges posed by trypsin inhibitors, dietary strategies such as fermentation or enzymatic treatment can be employed. Fermentation has been shown to reduce the levels of trypsin inhibitors in okara, improving its nutritional profile and making it more suitable for inclusion in animal feed [19].
5.3 Shelf-life and storage requirementsThe fresh okara is highly perishable, leading to rapid spoilage and microbial growth if not processed promptly [91]. The short shelf-life of fresh okara necessitates immediate drying or preservation to extend its usability as a feed ingredient [60, 91]. Flash drying offers rapid and efficient moisture removal for raw okara in a matter of minutes. However, the choice of drying method can affect the nutritional quality of okara, as excessive heat may lead to the degradation of heat-sensitive nutrients [86]. Furthermore, flash dryers are complex and costly equipment involving large components such as long drying chambers, powerful fans, and sophisticated heating systems, which limits their adoption in widespread practical settings [59, 61]. Other researchers were exploring other alternative preservation techniques, such as fermentation and the use of natural antioxidants, which may be more suitable for enhancing the shelf-life of okara without compromising its nutritional and sensory qualities [79]. Proper storage conditions are also critical for maintaining the quality of dried okara meal. It is essential to store okara in a cool, dry environment to prevent moisture absorption and the subsequent growth of mold and bacteria [92]. Packaging in airtight containers can further protect okara from moisture and oxygen, which can lead to rancidity and nutrient loss [93].
5.4 Determining safe and effective inclusion levelsEstablishing the optimal inclusion level of okara meal in broiler diets involves careful consideration of its nutritional composition and the potential effects on growth performance. Research indicates that okara can be included in broiler diets at varying levels, typically ranging from 5% to 30%, depending on the specific formulation and the nutritional needs of the birds [17, 30]. Studies have shown that lower inclusion levels (up to 10%) of okara meal can be safely incorporated into broiler diets without negatively impacting growth performance, FCR, or overall health [17]. As the inclusion level increases, it is essential to monitor the effects on growth metrics and nutrient digestibility. For instance, replacing up to 75% of SBM with okara meal did not adversely affect the performance of broilers, indicating that higher levels can be safely utilized [30]. It is important to note that the presence of ANFs in okara, such as trypsin inhibitors and phytic acid, can influence the effective dosage. Processing methods such as fermentation or drying treatment may allow for higher inclusion rates without detrimental effects on growth performance [23]. Therefore, ongoing research is necessary to establish specific guidelines for safe and effective dosages of okara meal in broiler diets.
5.5 Case studies and experimental trialsNumerous studies have investigated the effects of okara meal on the growth performance, health, and meat quality of broiler chickens. For instance, one study demonstrated that replacing up to 25% of SBM with okara meal did not adversely affect weight gain or FCR, indicating that okara can serve as a viable protein source in broiler diets [94]. Similarly, diets supplemented with okara improved nutrient digestibility and overall growth performance, suggesting that okara enhances the nutritional profile of broiler diets [30]. In another study, broiler diets including sunflower meal and okara led to comparable growth performance to that of traditional SBM diets [42]. These findings support the notion that okara can effectively replace conventional protein sources without compromising growth metrics. Additionally, the inclusion of okara meal improved the overall health status of broilers, as evidenced by enhanced immune responses and better intestinal morphology [46]. This aligns with findings that dietary supplementation with various plant meals positively influences the immune response of broiler chickens, further supporting the use of functional feed ingredients [95].
In conclusion, the role of okara meal in sustainable broiler poultry production is multifaceted, offering long-term benefits for nutrition as well as reduction of by-product disposal. As research continues to explore its applications and benefits, okara meal holds strong potential as a sustainable feed ingredient, supporting the advancement of a more efficient and future-ready poultry industry.
