Use of flaxseed cake as a source of nutrients in the food industry and possible health benefits- a review

Abstract

Oilseed cake is a potential by-product with a rich nutrient profile that can be used to develope value-added products. Flaxseed cake (FC), a by-product of flaxseed oil extraction, has gained considerable attention due to its diverse nutritional profile, bioactive constituents, and potential health benefits. It is an economical source of various bioactives, especially proteins, alpha-linolenic acid, lignan, flaxseed gum, and dietary fiber with potential health benefits. FC also contains residual fat which is a rich source of omega-3 fatty acids. Its high protein content and non-toxic nature make it a potential ingredient for use in human nutrition.

This review paper provides an overview of the nutritional profile of FC which includes macro and micronutrients as well as bioactive components such as lignans, phenolic compounds, and antioxidants along with their health-promoting properties. Due to the presence of antinutrients, processing is required for flaxseed cake which has been discussed. Furthermore, the health benefits linked to the consumption of FC are scrutinized.

Despite the positive attributes of FC, antinutrients namely cyanogenic glycosides, phytic acid, and tannins could restrict the absorption of nutrients. Several processing methods such as soaking, steaming, and fermentation have been proven to be beneficial in improving the nutritional quality of oilseed cake and the bioavailability of nutrients.

Graphical Abstract

Highlights

• Flaxseed cake is a by-product of the oil industry.

• Protein, fiber, residual oil (PUFA), polyphenols, flavonoids, and antioxidants are the major components comprising the profile of flaxseed cake.

• Secoisolariciresinol diglucoside (lignans) are present in abundance in flaxseed cake.

• Processing treatments help to inactivate cyanogenic glycosides which are the major antinutrient in flaxseed cake.

• Feasibility of utilizing flaxseed meal as a potential nutrient-dense by-product in different food has been well explored.

Introduction

Globally, a substantial quantity of oilseed crop was approximately 580 million tonnes in the year 2017–2018 (Singh et al., 2022). The production and processing of oilseed crops lead to the generation of agro-industrial by-products (Mannucci et al., 2019). India is one of the major oilseed crop producers that simultaneously produce oilseed cake after the oil extraction process. In the year 2016, 317 million tonnes of OSC was produced. Projections indicate that by the year 2025, it is anticipated to increase to 386 million tonnes (Kaur et al., 2021). Oilseeds, an essential crop, play a prominent role in augmenting the human diet with high-quality lipids, proteins, and fibers. A flow chart of the oil extraction process is shown in Fig. 1. Various oilseed cakes, including soybean, groundnut, rapeseed, sunflower, cottonseed, and flaxseed, possess considerable nutritional value but are predominantly utilized as animal feed (Stodolak et al., 2017). OSCs can also function as a source of antioxidants that can be consumed by humans after processing. Additionally, OSCs also serve as valuable substrates for the extraction of bioactive compounds, surfactants, enzymes, and vitamins. Despite having a rich nutritional profile, oilseed cake also contains antinutrients. Some non-edible oilseed cakes such as neem, mahua, and castor have a restricted role as agricultural manure (Gupta et al., 2019). OSC can serve as an exceptional starting point for the production of plant-based foods that are rich in protein, dietary fiber, and antioxidants (Kaur et al., 2021). OSC is an affordable by-product that is available throughout the year. Moreover, it may serve as a suitable raw material for the development of different food products with higher nutritional content along with mitigating the burden of industrial waste.

Fig. 1

Flow-chart showing the processing steps for oilseeds

Flaxseed (Linum usitatissimum L.) also called linseed is an oilseed and its global production is 1.2 million tonnes (Łopusiewicz et al., 2021). As shown in Fig. 2, there has been an increase in the production of flaxseed in the past five years. There are two different varieties of flaxseed from which oil is extracted namely fiber flaxseed and oil flaxseed. Flaxseed oil which consists of polyunsaturated fatty acids, monounsaturated fatty acids, and saturated fatty acids, is obtained after seeds are pressed (Goyal et al., 2014; Sun et al., 2021). In addition, flaxseed oil contains fat-soluble vitamins: vitamin A (carotenoids, 57 ppm) and E (tocopherols, 20–70 mg/100 g) (Mohanan et al., 2018). The cold-press technique of oil extraction is one of the techniques used to expel oil. Cold-pressed flaxseed cake (FC) has relatively high levels of soluble fiber, high-quality protein, lignans, minerals, and residual oil which provides health benefits to humans if incorporated in food (Poreda et al. 2017). The abundance of important fatty acids, particularly alpha-linolenic acid and, linoleic acid which are present in residual oil, makes FC suitable for incorporation into a variety of food products (Teh & Birch, 2014a, 2014b). The environmental and genetic variables affect the nutritional profile of flaxseed while processing procedures have an impact on the composition of FC (Kajla et al., 2017). The milling conditions and the techniques used at the time of pressing, relative temperature, and humidity of the mill could cause fluctuations in the moisture content of the seeds which affects the oil as well cake (Ogunronbi et al., 2011). Unlike in the soybean industry where, after oil is extracted from soybean, soybean oil cake is defatted and used to develop value-added protein, in the flaxseed oil industry, defatted FC is either discarded or utilized in animal feed. Due to a lack of knowledge concerning the potential uses for the cake and the presence of antinutrients such as phytates and cyanogenic compounds, FC is overlooked. Although FC has a robust nutritional profile, it also carries several kinds of antinutritional compounds. The protein and carbohydrate fractions of FC possess various functional properties such as water binding capacity, oil holding capacity, and emulsification capacity (Bekhit et al., 2013). This review aims to provide insights regarding the nutritional composition, anti-nutritional factors, removal of anti-nutrients by different processing techniques, applications in various food products, and corresponding health benefits of these products. The network visualization of the content is shown in Fig. 3.

Fig. 2

Production scenario of flaxseed in past years in India, Canada, the United States of America, China, and the United Kingdom. (FAOSTAT, https://www.fao.org/faostat/en/#data/QCL/visualize)

Fig. 3

Network visualization of bibliographic coupling

FC- a by-product and waste management through circular economy

The current economic model is based upon an unsustainable approach of the grow-use-dispose principle. An increase in human activities causes post-industrial global issues that negatively affect the ecosystem. These problems include mismanagement as well as overexploitation of natural resources and unsustainable consumption patterns (Ancuţa & Sonia et al. 2020). To resolve the issues regarding waste management, the application of a circular economy-based model is necessary which can be accomplished by using the resources wisely and eliminating the concept of waste along with the supply chain. Any substance disposed of by any industry is known as waste. A potential approach for the efficient utilization of waste involves maximizing the concentration of valuable products during extraction (Ancuţa & Sonia et al. 2020; Borrello et al., 2017; Esposito et al., 2020). An inclination towards plant-based foods which have potential health benefits as well as no harmful effect on the environment has been observed. These include plant-based sources of protein which may come from food waste. For example: banana peels, pineapple peels, spent grains, and many more can be used as novel raw materials for the production of various metabolites using different technologies (Tachie et al., 2023). As the world’s population grows it is expected that both the production of food and the associated waste will increase. An increase in waste production might have negative effects on the environment and economy. Managing food waste in the 21st century has become one of the major concerns. According to the FAO, one-third of the food produced is wasted. The primary issue, however, is that the extent of food waste and its effects have not been thoroughly researched and quantified. After a complete understanding of food waste is reached, informed policies and strategies are aimed at reducing food waste can be developed (Tamasiga et al., 2022). Food waste is estimated to be responsible for 8% of the world’s greenhouse gas emissions, which in turn contributes to the increase in global temperature (Vilariño et al., 2017). Although consumption is one of the most crucial components of every economy, particularly those that are developing, it is becoming increasingly prevalent in today’s society and frequently leads to food waste. Food waste is a multifaceted problem which is a current intense issue, especially for society, the economy, and the environment. The notion of a circular economy is noteworthy because when properly used, it may minimize the problem of food waste, which can be viewed from an economic, social, or environmental point of view (Dudziak et al., 2022). By strategically utilizing FC in the framework of a cirlar economy, a holistic approach can be followed. This may involve the application of various techniques and repurposing the oil cake into food products, which would not only minimize the waste but also enhance the efficiency of resource utilization. The flaxseed oil industry’s use of circular economy concepts exemplifies the harmonious fusion of environmental management, economic viability, and innovation, highlighting the possibilities for sustainable development and minimal ecological effects.

Nutritional composition of FC

Flaxseed is known to be a ‘superfood’ and is recognized as GRAS. Flaxseed, an oilseed is recognized as a functional food by virtue of its nutritional profile and various health benefits (Frag et al. 2020). The proximate composition of oil cake depends upon the geographical location of the crop grown as well as seed processing. In addition to possessing a high protein content, FC also contains water-soluble fiber components, which contribute to its nutritional value (Russo & Reggiani, 2016; Mueller et al., 2010). Food products incorporated with FC may exhibit improved nutritional properties and better functional properties (Bekhit et al., 2013). The industrially cold pressed cake contains high residual oil, moderate protein levels, and low moisture levels (Oomah, 2020). FC consists of 36% protein (from which 85% is digestible) and 7–10% residual oil. Table 1 shows the complete composition of FC and its comparison to soybean oil cake. The above-mentioned properties make FC a significant by-product of the oil industry that could be employed as an ingredient in different food products (Singh et al., 2022).

Table 1 Comparison between the composition of flaxseed and soybean oil cake

Macronutrients

Protein

The major protein fraction of flaxseed proteins is called linin which is made up of five polypeptide subunits. The minor protein fraction is called conlinin which has a single polypeptide chain. Major proteins compromise 66% and minor proteins compromise 20–42% of the seed proteins. In addition, these various structurally small proteins (low molecular weight) are also found in flaxseed (Waszkowiak & Mikołajczak, 2020). Flaxseed consists of two storage proteins, which are salt soluble (high molecular weight) and others are water-soluble (low molecular weight) (Mueller et al., 2010). Both salt and water-soluble proteins are present in approximately equal concentrations (Wanasundara et al., 1999). Flaxseed globulin is composed of at least five subunits with molecular masses ranging from 11 to 61 kDa connected by disulfide linkages and has an overall molecular mass of approximately 320 kDa and an isoelectric point of approximately 4.75 (Łopusiewicz et al., 2020). Flaxseed proteins are rich in amino acids such as arginine, aspartic acid, and glutamic acid. Additionally, amino acids including lysine, leucine, and valine are present in FC. In addition to protein nitrogen, FC contains non-protein nitrogen in the form of vitamins, sinapine, choline, and cyanogenic glycosides (Bekhit et al., 2013). The protein content increases after the defatting of the cake (Rani & Badwaik, 2021). The extraction of protein from flaxseed is challenging due to the presence of mucilage which is a highly viscous anionic polysaccharide accounting for 8% of total dry mass. To enhance the effectiveness of protein extraction FC can be soaked in warm water or treated with enzymes for removal of mucilage thereby optimizing the protein extraction process (Sun et al., 2021).

Lipids

Flaxseed characterized by an oil content ranging from 20 to 40% has received substantial attention due to its superior nutritional profile. Approximately 58.03% linolenic acid is present in flaxseed oil (Zhang et al., 2011a, 2011b). Flaxseed oil is a conglomerate of saturated fatty acids (9%), monounsaturated fatty acids (18%) and polyunsaturated fatty acids (73%) which are present at low, moderate and high concentrations respectively (Gutte et al., 2015). Flaxseed oil is considered a rich source of omega-3 fatty acid (Zhang et al., 2011a, 2011b). Apart from their use in cooking and other culinary purposes, various oils are used in the raw form as salad dressing. Currently flaxseed oil is utilized in various food products because of its distinctive flavor (Sun et al., 2021). FC has high levels of ALA (alpha-linolenic acid), which is an essential fatty acid (Zhang et al., 2011a, 2011b). ALA plays an important role in human development. It also has various biological effects such as preventing coronary heart disease, diabetes, some autoimmune diseases and some type of cancer (Gallardo et al., 2014). ALA has also been proven to be a powerful anti-inflammatory agent that can lower serum cholesterol levels (Zhai et al., 2019).

Dietary fiber

Dietary fiber is present in soluble and insoluble forms in FC. Insoluble dietary fibers found in FC include lignan, cellulose, and hemicellulose. It contains approximately 30% of dietary fibers which are mostly water extractable one-third of which are viscous. The dietary fibers present in FC are made up of heterogeneous polysaccharides. These polysaccharides are composed up of a mixture of neutral arabinoxylans and acidic rhamnose (Basiri et al., 2018). These dietary fibers induce satiety, postprandially as they suppress the ghrelin hormone (Ibrügger et al., 2012). The abundant mucilage in flaxseed cake can be freeze-dried or vacuum-dried and then stored to create a stable powder that can be utilized as an emulsifier and thickener in the food industry. The main constituents of the soluble fiber fraction are mucilage gums, while the insoluble fiber fraction mostly consists of cellulose and lignin (Bongartz et al., 2022). Monosaccharides from FC are composed of a mixture of polymers namely; arabinoxylans, galactose and fructose. The dietary fiber present in flaxseed and its cake is in the form of mucilage which helps to ensure a well-functioning gut by functioning as a prebiotic (Kristensen et al., 2012).

Minerals

Minerals are micronutrients that are consumed through the diet. Minerals play a vital role in the metabolism of some major biological processes and, minerals also play a vital role in osmotic pressure and acid-base regulation in the human body. Both macro and micro minerals are present in flaxseed. In macro minerals, magnesium is present in the highest concentration followed by potassium and sodium, whereas for micro minerals zinc is present in the highest concentration followed by manganese, copper, and iron (Kaur et al., 2017). Compared with oil cakes such as soybean, nigella, groundnut, and mustard seed FC is high in minerals such as phosphorous (9.52 g/kg), potassium (12.41 g/kg), manganese (13.73 mg/kg), copper (17.75 mg/kg) (Kolláthová et al., 2019).

Bioactives from flaxseed cake

The utilization of oil cake obtained after extraction depends upon the origin and constituents present in the oilseed. FC is a rich source of valuable components such as leftover proteins, fibers, and lipids. Residue of extraction process, contains secondary metabolites, especially phenolic acids and flavonoids. During the oil extraction process components such as tocopherols and phospholipids (partially) are removed whereas during the extraction process, the phenolic content does not decrease because there is a high content of phenolic compounds in the oil cake (Marczak, 2014). The concentration of phenolic acids and lignans in the defatted flaxseed increases when the oil is removed before extraction. Most phenolic compounds and antioxidants are bound to the cell so they remain insoluble in lipids and do not migrate to oil during the pressing of oilseeds (Singh et al., 2022). In some of the studies, it is observed that owing to the presence of a bioactive component, FC has a powerful antioxidant effect. The bioactive components of FC must be utilized and its overall utilization value must be increased (Sanmartin et al. 2020).

Polyphenols

Polyphenols are insoluble and are present in vacuoles, lipoprotein bilayers, lignin, hulls, and cell walls (Gutiérrez et al., 2010). Phenolic acids occur as esters, glycosides, and other bound complexes in plants. However, heating processes such as roasting weaken the covalent bond that binds the phenolic compounds to cell walls which in turn increases the extractable phenolic compounds (Zhang et al., 2023a, 2023b, 2023c). Total phenols found in methanolic extracts of flaxseed cake (1.728 mg/g) is found to be higher than those found in flaxseed whole (1.538 mg/g) (Quezada & Cherian, 2012). Table 2 summarizes the phenolic compounds in flaxseed cake. With a substantial concentration of acids such as chlorogenic acid, ferulic acid, coumaric acid, and p-hydroxybenzoic makes up the majority of the phenolic acids in flaxseed. Major phenolic acids present in FC are chlorogenic acid, ferulic acid, and gallic acid. Other, less prevalent phenolic acids include p-coumaric acid glucosides, cinnamic acid, caffeic acid hydroxycinnamic acid glucosides, and 4-hydroxybenzoic acid (Ganorkar et al., 2016; Teh & Birch, 2014a, 2014b). These phenolic acids also exhibit antioxidant activity which is linked to the arrangement of hydroxyl group on ring (Barthet et al., 2014). The phenolic content extracted from FC differs due to differences in the extraction process. A high total phenolic content of HPC (hot pressed cake) extract may be due to the chemical reaction of plant tissues caused by heat treatment, which alters the structure of lignocellulose and increases the availability of phenolic chemicals in plant extracts. Phenolic extracts of supercritical extracted cake were found to be more than hot pressed as in supercritical fluid extraction diffusion coefficient as well solubility of phenolics increased (Sanmartin et al. 2020). Flaxseed cake (FC) exhibits antioxidant activity due to the substantial amount of phenolics present in it (Hao et al., 2020). The antioxidant potential of FC can be assessed through various methods such as DPPH (1,1-Diphenyl 2-picrylhydrazyl), FRAP (Ferric reducing ability power), and ABTS (2.2´-azino-bis(3-ethylbenzothioazoline-6-sulphonic acid) (Table 3). DPPH act as a scavenger of the free radical (Terpinc et al., 2012), FRAP assay is based on the reduction ability to convert Fe(III) to Fe(II) at pH 3.6 (Teh et al., 2014a, 2014b) and ABTS assay is based on radical decolorization (Marczak, 2014).

Table 2 Phenolic compounds of flaxseed cake

Table 3 Antioxidant potential of flaxseed cake

Flavonoids

Flavonoids are a group of secondary plant metabolites with potential antioxidant activity that prevent the oxidative deterioration of food. The accumulation of flavonoids in plants depends upon various factors such as the presence of enzymes required in the flavonoid pathway, the supply of amino acids, and much more (Oomah et al., 1996a, 1996b). Flavonoids are a category of plant phenolics, that are composed of two aromatic rings connected by a 3-carbon bridge in a heterocyclic arrangement. The flavonoids present in FC are flavanols, flavones, and flavanones. These compounds act as strong reactive species scavengers, that act as signal molecules as protective measures against pathogens. The flavonoid content in the methanolic extract of flaxseed is found to be 1.7 mg/g whereas in flaxseed cake is 2.8 mg/g (Quezada & Cherian, 2012). The content of flavonoids is higher in the extract of pressed cake as compared to solvent-extracted cakes. Flavonoids are polar substances retained in pressed cakes during oil production. There is a weak relation between the flavonoid content and oil content in flaxseed.

Lignans

Lignans are phytoestrogens found as minor elements that are responsible for the development of plant cells. Lignans are made up of two units of phenylpropane. Lignans are phytochemical compounds that have anti-carcinogenic and antioxidant properties. SDG scavenges free radical activity which leads to the antioxidant potential of flaxseed cake. SDGs can make complex divalent transition metal cations that are responsible for the health-promoting effects (Prasad, 1997). Lignans are the major polyphenols present in the extract of defatted FC (Teh & Birch, 2014a, 2014b). When SDG is ingested, it is not present in circulation rather it is metabolized into its metabolites enterodiol and enterolactone. Due to ester linkages with 3-hydroxy-3-methyl glutaric acid (HMGA) and glycosidic linkages with phenolic substances, such as hydroxycinnamic acid derivatives and herbacetin diglucoside, lignans found in FC are oligomers. Lignan is composed of monomeric units of SDG which has high biological activity as it has antioxidant and anti-inflammatory properties that prevent cardiovascular diseases and other metabolic syndromes (Tavarini et al., 2021). Flaxseed and FC are considered the richest plant sources of the lignan precursor, secoisolariciresinol diglucoside (SDG) (Rudra et al., 2021). Lignans present at minor levels include matairesinol and pinoresinol (Williams et al., 2007). Lignans are known to exhibit antioxidant potential due to their properties to scavenge free radicals such as hydroxyl ions. Flaxseed lignans have a higher antioxidant potential as compared to vitamin E (Khattab et al. 2012).

Amino acids and bioactive peptides

The majority of studies highlight the benefits of flaxseed for human health such as ALA-rich flaxseed oil, flaxseed lignans, and soluble polysaccharides whereas the protein component remains underutilized. Dietary nitrogen which is assimilated in humans in the form of small peptides has various physiological activities. Food proteins can generate bioactive peptides that are known to have a positive influence on the human body. Currently, there is much research and understanding of how dietary proteins might produce bioactive peptides. Bioactive peptides are unique protein fragments that have a positive impact on the overall health (Wanasundara, 2008).

Anti-nutritional factors in FC and their mitigation strategies

FC contains several antinutrients that can be minimized by various processing methods, such as boiling, fermentation, and pelleting (Ogunronbi et al., 2011). After the extraction of flaxseed oil, protein and fiber fractions of flaxseed are concentrated (Drozłowska et al., 2020a, 2020b). FC is considered a source of nutritional compounds such as proteins, lignans, and fibers. In addition, various anti-nutrients such as linustatin, neolinustatin, linamarin, and lotaustralin are also present in FC and are known to be cyanogenic glycoside compounds (Russo & Reggiani, 2016). These antinutrients increase as the oil is extracted which can act as a potential health risk factor if consumed directly. The primary anti-nutrient present in flaxseed is cyanogenic glycoside (Sun et al., 2021). Another anti-nutrient present in FC is phytic acid. Both flaxseed (whole) and FC have cyanogenic compounds. The cyanogenic compounds release cyanide in the human gut by acid-catalyzed hydrolysis or hydrolysis by endogenous seed enzymes (Bacala & Barthet, 2007). Various processing methods have been proven to be effective in removing anti-nutrients of FC and depicted in Fig. 4.

Fig. 4

Different processing methods for removing antinutrients from flaxseed (whole) and flaxseed cake

Cyanogenic glycosides (CGs)

Cyanogenic glycosides are the main antinutritional factors present in flaxseed that are concentrated in the cake. When cyanogenic glycosides are digested, hydrogen cyanide is formed which can cause acute toxicity in adults. Cyanogenic compounds are nitrogenous secondary plant metabolites derived from valine and isoleucine which can cause chronic effects if ingested in high quantities (Russo & Reggiani, 2016), synthesized through tonoplast glycosyltransferase that accumulates in vacuoles. After the cell structure is ruptured cyanogenic glycosides decompose to hydrogen cyanide which is catalyzed by β-glucosidase and α-hydroxy nitrile lyase (Russo & Reggiani, 2014). Due to the intestinal β-glucosidase activity, cyanogenic glycosides liberate toxic HCN which is known to have a toxic effect on the respiratory, nervous, and endocrine systems (Zhai et al., 2019). Cyanogenic glycosides are known to be composed of α-hydroxy nitrile-type aglycon and a sugar part (glucose or gentobiose) (Russo & Reggiani, 2016). Additionally, cyanogenic glucosides are unstable at low temperatures (Mannucci et al., 2019). Cyanide decreases the bioavailability of iron, manganese, or copper ions because it can bind minerals. Flaxseed contains cyanogenic compounds in the range of 264–354 mg per 100 g of seeds. Due to damage to plant tissue and the presence of the enzyme, linase, cyanogenic compounds can produce 100–300 mg/kg hydrocyanic acid (HCN) (Imran et al., 2013). According to previous studies 100 mg of CG, if consumed might be toxic to human health (Russo & Reggiani, 2016). These compounds can be removed partially by microwave treatment, autoclaving, and boiling of seeds (Russo & Reggiani, 2016). Cyanogenic glycosides are further fractioned into linustatin and neolinustatin. Linatine, another anti-nutrient binds to vitamin B6 and reduces its absorption which in turn causes its deficiency in humans (Teh & Birch, 2014a, 2014b). If consumed, these compounds form a complex with ferric ions of mitochondrial cytochrome oxidase and cause acute or chronic intoxication which leads to abnormal respiration and nervousness. FC contains relatively high concentrations of hydrogen cyanide (HCN) which has been proven to be a potent respiratory inhibitor (Feng et al., 2003). Vomiting, nausea, dizziness, arrhythmias, headache, and difficulty in vision are some of the symptoms caused by acute intoxication caused by cyanogenic glucoside. CG is found not only in flaxseeds but also in cassava (linustatin, limarin), passiflora (neolinustatin), wheat, and oat (linamarin) (Russo & Reggiani, 2014).

Phytic acid

Phytates are the primary phosphorus storage form in plants. Mineral absorption is hampered by phytic acid. Minerals and proteins are electrostatically bound in complexes by a highly charged phytic acid molecule, reducing their bioavailability (Poreda, 2017). Phytic acid is the primary form of stored phosphate and is a naturally occurring plant inositol hexaphosphate that is frequently detected in seeds. The phytic acid content in flaxseed depends upon the season and geography (Zhang et al., 2023a, 2023b, 2023c). Phytic acid forms insoluble complexes with metals such as calcium, magnesium, zinc, copper, iron and molybdenum which hinders their absorption by the gastrointestinal tract. In addition, various mineral and protein complexes with phytic acid are formed which then decreases their bioavailability. Phytic acid strongly affects the bioavailability of various minerals. In defatted and dehulled oilseed cakes made from oilseeds including soy, peanut, and sesame, phytic acid accounts for approximately 1.5%. In flaxseed, the concentration of phytic acid ranged from 23 g/kg to 33 g/kg. Phytin extracted from flaxseed flour accounts for 13% of organic phosphorus. Approximately 70% of the total phosphorus is present in the form of phytic acid (Oomah et al., 1996a, 1996b). Proteins and starch may also directly interact with phytic acid, reducing its solubility and digestibility (Farag et al. 2012; Oomah et al., 1996a, 1996b).

Tannins

Tannins are categorized as phenolic compounds with molecular weights of 500–3000 Da. Tannins are water-soluble compounds that are found in vacuoles and can bind with protein and form soluble or insoluble tannin-protein complexes. Based on their structure and molecular weight, tannins can be divided into two categories, namely condensed and hydrolyzable tannins. The commercial use of flaxseed is limited due to the presence of various toxic compounds, including tannins, which not only decrease micronutrient bioavailability but also hinder the protein digestibility. Additionally, tannins are compounds that impart an astringent taste to the oil cakes (Imran et al., 2014).

Effect of processing methods on flaxseed (whole) and FC

It has been scientifically proven that proper processing operations are required to remove the antinutrients present in FC to make it suitable for human consumption (Bekhit et al., 2018). Some methods of removing antinutrients are enzymatic hydrolysis, fermentation, roasting, microwave, and extrusion. Heating processes such as steaming and boiling can reduce the antinutrient factors (Xu et al., 2022). High temperature and short time combinations used in extrusion technology are employed to reduce the concentrations of anti-nutrients in soybean, canola and cottonseed (Imran et al., 2013). The extraction process is not sufficient to remove cyanogenic compounds completely, apart from the fact that some beneficial compounds are also removed during the extraction process (Zhai et al., 2019; Yamashita et al., 2007). The process of heating decreases the nutritional value and it is difficult to remove residual solvent through the solvent extraction process. Fermentation is known to be the ideal treatment for the removal of antinutrients. The fermentation process has been known to decrease the HCN content (Zhai et al., 2019). Fermentation is also known to increase phenolic content, radicle scavenging activity and bioavailability (Stodolak et al., 2017), (Zhai et al., 2019). When Aspergillus niger and Candida utilis were used in combination, HCN levels decreased and nutritional content was more bioavailable because of the utilization of cyanide compounds by these microorganisms (Zhai et al., 2019). Other physical methods are also employed to remove the antinutrient factors to acceptable levels. The effects of different processing methods on antinutrient factors of FC and flaxseed (whole) are presented in Table 4.

Table 4 Various processing methods for removing antinutrients from flaxseed cake and flaxseed (whole)

Functional properties

Research on the nutritional qualities and potential applications of protein from affordable, underutilized high-protein oilseeds and legumes has become important due to the rising costs and insufficient availability of animal proteins. To facilitate their incorporation into food formulations, protein flours from unique sources must exhibit the required interaction characteristics with other food components (Khattab & Arntfield, 2009). The nitrogen solubility of different protein-rich products is regarded as a reliable indicator of functional properties. Different solvents are used to extract them which significantly affects the functional properties as hexane-extracted cakes showed 9.7 times higher adsorption of water than their weight. It was also observed that hexane-extracted cake adsorbed 2.6 times more soybean oil than their original weight whereas cakes extracted with methanol-ammonia-water increased fat adsorption capacity by 3.2 times (Wanasundara & Shahidi, 1994). To develop processed foods such as baked goods, sauces, soups, and similar food products with improved quality and yield, several functional qualities are crucial. Compared with mustard and soybean oil cake, FC has the highest oil absorption capacity (OAC). The OAC depends upon the hydrophilicity of the protein and its retention for binding to lipids. Bulk density depends on the intermolecular forces, which are essential for wet processing and development of new products. FC and the defatted cake had moderate bulk densities. Foaming capacity is always related to the content of soluble protein as well as polar and non-polar lipids in any matrix, FC accounts for a moderate amount of foaming capacity when compared to the soybean and mustard oilseed cake as well as defatted cake. Emulsification is also a property that directly depends upon protein content, as flaxseed has a moderate protein content compared to soybean and mustard, and the emulsification property of flaxseed is also moderate, whereas emulsion capacity was found to be highest in FC (Bárta et al., 2021). A previous study reported that the water holding capacity (WHC) changes with changes in plant variety, geography, cultivation temperature, and extraction method (Bekhit et al., 2013). Alkali-treated oilseed cakes such as hemp and canola had the highest water-holding capacity followed by acid-treated oilseed cakes due to changes in the protein structures of hemp and canola as a result of alkali and acid extractions, exposing more hydrophilic binding sites that allowed the proteins to bind more water whereas a study done showed that FC has highest water absorption capacity (WAC) compared to canola and soybean oil cake because of the presence of mucilage polysaccharides which have a high tendency for absorption and swelling (Khattab & Arntfield, 2009). The emulsifying activity was found to be different reason being the physicochemical characteristics such as protein and ionic strength (Mueller et al., 2010). Different processing methods such as microwave processing, autoclaving, boiling, soaking, and solvent extraction lead to alterations in the functional properties of defatted flaxseed flour. Defatting leads to an increase in the water-holding and oil-holding capacity of the flour. This may be due to the increased availability of hydrophilic components such as mucilage or carbohydrates and the increased ability to hold oil by the capillary attraction process. The swelling indices of other functional groups increase due to the elimination of non-polar groups which act as interfering components in the interaction between flour and water. The foaming capacity depends upon the pH and protein structure, and defatted flaxseed flour shows better foaming properties at pH 2 because protein denaturation occurs at a particular pH. Denaturation of proteins is caused by whipping and aggregation formation in liquid-air interphase formation. A higher protein content is associated with better emulsion capacity and emulsion stability. The defatting of flaxseed flour leads to an increase in protein concentration content which positively impacts the emulsion properties as it forms a protective layer around the oil droplets and minimizes the interfacial tension (Safdar et al., 2020).

Application of FC in the food industry

Currently, increased awareness of health issues among consumers is observed which has led to a spike in the demand for new products. Additionally, as a result of the expanding population, there is an increased need for plant proteins to meet dietary needs. It is perceived that plant-based food products are a blend of antioxidants, vitamins, dietary fiber, and minerals (Łopusiewicz et al., 2020). Oilseed cake is an exceptional raw material that can be incorporated into food products because it contains high levels of antioxidants, vitamins, proteins, minerals, and dietary fiber. FC can serve as a possible delivery system for bioactive substances, opening the door for the production of premium goods with improved nutritional profiles and desired health benefits. FCis a source of numerous compounds such as antioxidants, omega-3 fatty acids, bioactive proteins, soluble and insoluble fibers, lignans, and minerals such as Mg, K, Na, and Zn) (Łopusiewicz et al., 2020). Over the past few decades an increase in the value of flaxseed, an essential oilseed crop, has been observed due to its distinctive nutrient profile. The potential application of FC as an additive to enhance the nutritional profile of food products is shown in Fig. 5. The presence of antinutrients (cyanogenic glucosides and phytic acid) should be considered, but these compounds can be removed by various methods. Researchers are more interested in utilizing oilseed cake to develop new functional food products. FC has been used to develope various functional foods including beverages, bakery, extruded and fermented products. Potential food applications of FC are listed in Table 5.

Fig. 5

Potential applications of flaxseed cake in the food industry

Table 5 Applications of flaxseed cake in different food

Health benefits

Various health benefits are associated with plant-based diets (Lea et al., 2006). Various components of FC such as lignans, proteins, and dietary fiber are beneficial for treating cardiovascular diseases, cancer, and diabetes (Kumar et al., 2019). Various studies have reported a positive influence of flaxseed consumption on human health (Drozłowska et al., 2020a, 2020b). An equal quantity of FC and oil significantly improved the body weight and feed efficiency in broiler chickens (Lee et al., 1991). Flaxseed is known to have polysaccharides that have mucoadherent properties and therapeutic benefits (Łopusiewicz et al., 2021). Polysaccharides from FC are known to have anti-tumor effects due to their antioxidant properties (Gutiérrez et al., 2010). FC also possesses anti-microbial properties due to the presence of chitooligosaccharides against pathogens such as Candida albicans, Penicillium chrysogenum, Fusarium graminearum, and Aspergillus flavus (Pag et al., 2014). Polyphenols present in flaxseed exhibit various therapeutic effects. In addition, FC has been known to have anti-microbial, anti-inflammatory, anti-thrombotic, anti-allergenic, and antioxidant properties (Akl et al., 2020). FC also has tocopherols and these are known to show beneficial effects on human health which include anti-inflammatory qualities, neoplastic transformation prevention, artery wall protection, and LDL oxidation prevention (Mannucci et al., 2019). Lignans and phenols have outstanding biological properties as antioxidants, anti-bacterial agents, and anti-carcinogenic substances (Kajla et al., 2015; De Silva & Alcorn, 2019). This valuable laxative property is due to the high fiber content of flaxseed and its defatted cake has been known to reduce the symptoms of irritable bowel syndrome as it stimulates fecal output during constipation (Xu et al., 2012). SDG is also known to have antimicrobial effects on bacteria and fungi. In a study performed on Fischer 344 male rats, a 15% decrease in tumor formation and size was observed when FC was substituted for corn cake and flaxseed oil was substituted for corn oil by 15% each respectively (Bommareddy et al., 2009). Flaxseed is recognized for its significant positive impact on digestive health. ALA plays a positive role in decreasing gastrointestinal tract inflammation (Plissonneau et al., 2022). Omega-3 fatty acids play an important role in human growth and development because it reduces blood pressure and other inflammatory diseases (Coorey et al., 2015). The results of different studies related to the health benefits associated with the consumption of FC are summarized in Table 6.

Table 6 Animal studies showing health benefits of flaxseed oil cake

Cardioprotective

Cardiovascular heart disease (CHD), also known as ischemic heart disease, stroke, and peripheral vascular disease, is the leading cause of death in industrialized nations (Prasad et al., 1994). Flaxseed is the richest source of lignans (secoisolariciresinol diglucoside, SDG) and has high antioxidant activity. In addition, it also helps in preventing hypercholesterolemic atherosclerosis (Prasad, 2000). In a study of 73 people suffering from type-2 diabetes type were subjected to lignan supplements for 12 weeks and the other 36 subjects started placebo. After 12 weeks a significant reduction in HbA_1c was observed (Pan et al., 2007). Omega-3 fatty acids show cardioprotective effects, and in addition, they protect against lipid abnormalities and atherosclerosis (Austria et al., 2008). Lignans from FC prevent complications such as atherosclerosis, hyperlipidemia, ischemia, hypertension, and cardiotoxicity (Zanwar et al., 2014). When oil is extracted from flaxseed, lignan secoisolariciresinol remains which is known to have the ability to suppress atherosclerosis. This suppression is due to antioxidant activity resulting from a decrease in the serum lipid concentration. In a study of a rabbit model, the reduction in the development of hypercholesterolemic atherosclerosis was 73% due to flaxseed lignan complex and 34% due to SDG (Prasad, 2009).

Anti-carcinogenic

Lignans are the major component contributing to the anti-carcinogenic properties of these plants. In a study, lignan showed dose and time-dependent anti-proliferative effects on acute myeloid leukemia cells (Tannous et al., 2020). The early stages of carcinogenesis are inhibited by flaxseed, which is a rich source of the mammalian lignan precursors secoisolariciresinol-diglucoside (SDG) and alpha-linolenic acid (ALA). The initiation (before carcinogen), promotion (after carcinogen), and progression (visible tumors) stages of carcinogenesis have all been the subject of animal model research using dimethylbenzanthracene (DMBA) and a rat mammary cancer model. DMBA was used as the carcinogen, and it was discovered that 1.5 mg of SDG supplied daily at the beginning stage (1 week after DMBA for 20 weeks) caused 46% fewer tumors per group and 37% fewer tumor multiplicities (Thompson et al., 1996). Secoisolariciresinol diglucoside (SDG), which is present at the highest concentration in flaxseed, and its cake has been known to have various health benefits. The various properties of SDG are anti-cancer, including reducing the growth and proliferation of cancerous cells (Wang et al., 2005).

Anti-diabetic

Diabetes mellitus, commonly known as diabetes is a chronic metabolic disorder caused by increased blood sugar levels. A study conducted on diabetes-induced rats revealed a 64.62% decrease in glucose levels, 48 h after SDG intake (Moree et al., 2013). In another study where BB (BioBreeding) rats were fed SDG which reduced the development of diabetes by 71%, the onset of diabetes was moderately delayed (Prasad, 2000). In a study, 60 individuals were fed with flaxseed gum incorporated Chapatis. A decrease in fasting blood sugar and a decrease in cholesterol by 11.6% and 10.4%, respectively was observed. Flaxseed gum is a potential ingredient for improving the blood profile of people suffering from type 2 diabetes (Thakur et al., 2009).

Conclusion

The purpose of this review was to summarize and highlight the importance of the valorization of FC. In conclusion, oilseed cake is a valuable by-product of the oil industry and includes a variety of nutrients, making it an economically viable ingredient for use in numerous foods to improve their nutritional profile. Nutritional analysis of FC revealed that it is rich in proteins, polyphenols, lignans and antioxidants which are essential for human nutrition. In addition to having a rich nutritional profile, FC also possesses several functional properties, including the ability to bind both water and oil (water and oil holding capacity) as well to swell and emulsify (swelling capacity and emulsification). Various treatments such as fermentation can improve these characteristics. Several antinutrient factors are found therefore cake needs to undergo several pretreatments. The intake of FC can be followed by various processing techniques such as boiling, autoclaving, microwaving, fermentation and others to remove antinutrient factors such as cyanogenic glucosides, phytic acid, and tannins which hinder the absorption of essential nutrients. The applications of FC in food products are diverse and promising because it acts as a plant-based protein source. It can act as a potential functional ingredient in baked goods, cereals, snacks, and beverages enhancing its nutritional properties and providing desirable textural properties. Additionally, FC which is rich in dietary fibers helps to develop fiber-rich food. The health benefits of FC are multifaceted. Lignans and antioxidants which are found in abundancee in FC help in the management of several chronic diseases such as diabetes and cancer. By including FC in the diet, the consumption of plant-based proteins increases, which would be a positive step toward human health, the environment, and sustainability.

Further research and development in this area is required to make flaxseed oil cake more palatable. Additionally, research on treating it in such a way that cake is devoid of antinutrients and has an extended shelf life is needed.

Challenges and future trends

A by-product of the extraction of oil from flaxseed is FC. Proteins, fibers, and several other components make up its profile. Due to its composition and processing needs, FC can create some difficulties when used as a culinary ingredient. The utilization of FC presents a confluence of intricate challenges. Despite being a by-product of extraction, it is an amalgamation of residual PUFAs, proteinaceous components, fibrous matter and other bioactive substances. However, its incorporation is a challenge. Due to the presence of PUFA, their shelf life and stability are limited. Other obstacles that surround the incorporation of FC into the culinary landscape include minimizing allergenic potential, dealing with regulatory frameworks, and improving the distinctive sensory qualities that it imparts to different food products, accentuating the complexity of the process.

As the global food industry is growing, it is inclining more towards sustainable approaches to combat the requirements. Overcoming the challenges of innovation and sustainability FC has emerged as a promising raw material that may fulfill protein requirements. A spike in the demand of plant-based protein has been seen, which aligns seamlessly with FC and its ability to satisfy the nutritional profile. FC has a compelling future in the culinary world due to the convergence of health-conscious consumer preferences, sustainable food practices, and dynamic adaptability.