Effects of microwave, ultrasound, and various treatments on the reduction of antinutritional factors in elephant foot yam: A review

Abstract

Elephant foot yam (EFY) includes a high degree of nutritious fiber, carbohydrates, glucose, protein, and sugars. It also has plenty of calcium, sodium, potassium, and vitamin C, and is a promising source of starch. Due to the presence of oxalate and acridity, EFY has been extensively underused. Researchers have employed a variety of ways to decrease calcium oxalate, including boiling and NaCl treatment. A novel technique like ultrasound is a promising technique for the reduction of antinutritional factors effectively by keeping the natural phytoconstituents in it. The reduction of antinutritional factors by ultrasonication resulted from the high temperatures and pressures generated amid the collapse phase of cavitation bubbles. Microwave heating depends on the dielectric properties of the food. The dielectric constant is the ability to absorb the energy, and dielectric loss is the ability to reflect where it is dissipated as heat. Because of its heat‐labile nature and the formation of insoluble complexes, microwave treatment reduces the antinutrient component. This review focused on several studies on yam oxalate concentrations that were conducted using various treatments. This would help researchers and the food industry find more effective strategies to reduce the antinutritional factor using frequency‐controlled power ultrasound.

INTRODUCTION

Elephant foot yam (EFY; Amorphophallus paeoniifolius), widely known as the “King of Tubers,” is a member of the Araceae family and is endemic to Asia (Jogi & Lahre, 2020). It is an underutilized tuber having medicinal and therapeutic properties that will benefit mankind in the food industry (Sreerag et al., 2014). Yam is a tropical tuber crop with a high potential for adoption as a cash crop in tropical countries due to its high production potential and acceptability as a vegetable in a variety of exquisite cuisines. Yam is a South-East Asian crop that grows naturally in the Philippines, Indonesia, Malaysia, and other Southeast Asian countries. Suran or Jimmikand is the common name for it in India, and it has historically been cultivated commercially in Kerala, Andhra Pradesh, West Bengal, Karnataka, and Tamil Nadu (Anuradha & Wadhwa, 2012).

In India's northern and eastern provinces, wild cultivars are extensively used to prepare vegetables, pickles, and local ayurvedic remedies for different ailments (Srikanth et al., 2019). The dry matter composition of the tuber ranges between 17.60% and 25%, starch between 13.83% and 21.63%, sugar between 0.56% and 1.78%, protein between 0.85% and 2.70%, and fat between 0.08% and 0.39% (Nagar et al., 2019). According to scientific data, EFY contains a high degree of nutritious fiber, carbohydrates, glucose, protein, and sugars. It also has a lot of calcium, as well as sodium, potassium, and vitamin C (Sheela Immanuel et al., 2020). EFY research is crucial since it has a variety of medicinal characteristics and is extensively used in Indian medicine, including Ayurveda (Singh et al., 2020). It possesses analgesic, antidiabetic, antibacterial, antifungal, and anthelmintic properties and anticancer properties. The corm is used to treat bronchitis, stomach discomfort, asthma, diarrhea, piles, splenomegaly, elephantiasis, and rheumatic swellings (Hosseini et al., 2015). EFY has medicinal purposes and is used for the treatment of breathing problems, hemorrhoids, tumor conditions, cough, splenic diseases, and prostate disorders (Mizanur et al., 2014).

Various tuber extracts have been shown to exhibit immunomodulatory, analgesic, anti-inflammatory, cytotoxic, anthelmintic, hepatoprotective, and anxiolytic properties (Harshavardhan Reddy et al., 2012). Tuber extracts have been shown to exhibit apoptotic and cytotoxic effects on the sapiens colon cancer cell line HCT-15 (Saha et al., 2013). EFY has a range of nutritious properties, but it also contains antinutritional components such as oxalate, phytate, and saponins, which may be decreased by different treatments before consumption (Sarada & Rajani, 2012). The major problem with consuming EFY is its acridity and oxalate level. Because of these difficulties, EFY has not been utilized as a food crop (Barua et al., 2022). Acridity generates irritative feelings in the mouth and throat (stinging, burning, and itching), which may progress to swelling. Itching may occur when it rubs against the skin, indicating the intensity of the irritation (Kumar et al., 2020). Raphides, which are needle-like oxalate crystals, cause acridity. Oxalate is regarded as antinutritional, toxic, and an irritant (Hosseini et al., 2015). If a person consumes more than 2 g of oxalate, they may die. Oxalates may render minerals such as iron, calcium, zinc, and magnesium unavailable to the body by chelating them (Singh et al., 2021). Consequently, eating oxalate-containing foods may cause a mineral deficiency in the body. Oxalate clusters lodge in the kidney, causing renal stones and, in severe cases, renal failure. Oxalates account for over 75% of kidney stones, and eating oxalate-containing foods raises urine oxalate levels to variable degrees (Pramod et al., 2012). EFY has been widely underutilized because of the presence of oxalate and acridity. On the basis of this, the objective of this review is to discuss various treatments applied to reduce oxalate and different antinutrients from EFY. The application of boiling, NaCl treatment, ultrasonication, and microwave processing to reduce calcium oxalate is discussed in detail.

COMPOSITION OF EFY

Nutritional composition of EFY

Tubers are a high-value crop in the food chain, and tubers are generally eaten for their calorie content (Santosa et al., 2014). These EFY tubers are abundant in nutrients but poor in protein (Peetabas et al., 2015). The protein level of the tuber was 1.126%, which is within the range of 0.85%–2.7% for EFY tubers obtained from Indian cultivars. The crude fat content of Nigerian yams was higher (0.01%–0.4%) than that of EFY cultivars in India. The phosphorus content of the yams was 1443.33 mg kgG1. In India, EFY had a similar level (20.89–247 mg/100 g) (Suresh Kumar et al., 2020). This tuber's calcium content was 8535.76 mg kgG1, while its magnesium content was 1512.28 mg kgG1. EFY is high in calcium (950 mg/100 g), iron (0.6 mg/100 g), and phosphorus (934 mg/100 g) (Chattopadhyay et al., 2009). EFY is a high-nutritional-value food that may provide a significant amount of a person's daily mineral requirements via diet or feed. Phosphorus and calcium are essential elements the body needs in considerable amounts. Minerals, among other things, have a role in enzyme control, acid–base reactions, bone formation, and muscle stimulation (Jogi & Lahre, 2020).

Antinutrient contents

Antinutrient factors like oxalate, tannins, hydrogen cyanide, and phytate are present in yam. Antinutrient factors can have a variety of effects on animals, including inducing intoxication, causing ephemerality or decreased output, and lowering feed intake. The antinutritional components of yam are listed in Table 1.

Table 1 Antinutrient composition of Amorphophallus Paeoniifolius

Antinutrient factor	Amount present in yam	Chemical nature	Toxicity	Cause	Reduction methods	Reference
Oxalate	2.91–18.50 mg/100 g	Nature is strongly oxidized Has mineral chelating properties To produce calcium oxalate which is insoluble in nature, Oxalate reaction is done with calcium	An amount of 40–50 mg can cause death in adult	Hypocalcaemia Renal/kidney stones Acridity Gastric hemorrhaging	Cooking Boiling Fermentation Sun drying	Rao et al. (2020)
Tannins	0.456–0.560 mg/100 g	Plant polyphenols Ability to generate metal ion complexes Strong affinity for proteins	Protein–tannins complexes are reliable for lowering intake of feed, and growth rate	Depressed growth Decrease in feed productivity Metabolizable energy Digestibility of protein	Fermentation Sun drying	Xu et al. (2014)
Hydrogen cyanide	35.878 ppm	Widespread in plant kingdom Exist in form of cyanogenic glucosides Cyanide concentration is lower than cassava root	Mainly caused due to cytochrome oxidase inhibition Lethal dose for humans is 0.5–3.5 mg for an adult	Cerebral damage Lethargy in animals and humans	Drying Soaking Fermentation	Ansil et al. (2014)
Phytate	0.165%	Chelates with minerals to produce phytate	Reduce bioavailability of minerals if form phytate mineral complexes	For binding metal ions it has strong affinity such as zinc and phosphorus	Cooking Increase in temperature Heating time Sun drying	Kumar et al. (2017)

Oxalate

The chemical formulae for oxalate anion are C₂O₄²⁻ or (COO)₂²⁻. The bitterness and astringency of yam are caused by the presence of oxalate in the plant's mouth, and throat scratching is common in yam eaters (Abraham et al., 2021). Calcium oxalate is a significant component of kidney stones (CaC₂O₄). Oxalate consumption is known to reduce calcium availability in the body, which might be dangerous for women who need extra calcium in their diets. It has been claimed that when dietary oxalate interacts with calcium, magnesium, and iron, it produces insoluble oxalate salts, leading to oxalate stone formation (Sheela Immanuel et al., 2020). Oxalates also inhibit mineral use, rendering them unavailable in the body (Nagar et al., 2019). Figure 1 depicts a schematic representation of the chemical structure.

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Many variables determine the quantity of oxalate in plants, including soil quality, seasonal changes, climatic conditions, water, and the location of the plant (Kurniawati et al., 2016). The oxalate content of yam was approximately 31.876 mg/100 g (Barua et al., 2021). Oxalic acid's mineral chelating effect is owing to its high oxidation. When oxalates react with calcium, calcium oxalate is formed, which is insoluble and hinders calcium absorption (Singh et al., 2022; Suresh Kumar et al., 2020). Oxalate in meals may induce hypocalcemia because calcium oxalate forms clusters in the kidney (Singh et al., 2011). Oxalic acid consumption results in stomach hemorrhaging, oral and gastrointestinal tract corrosion, and renal failure (Makkar et al., 2007). Consumption of less than 2% soluble oxalate in ruminants and less than 0.5% soluble oxalate in non-ruminants may be acceptable (Jogi & Lahre, 2020). Cattle and sheep are less impacted because oxalate is digested in the rumen. The smallest quantity of oxalate (40–50 mg) may cause mortality in adults (Singh et al., 2020). Cooking and fermenting may lower the oxalate content of yam and yam products. Sun drying procedures may lower the oxalate level of the yam tuber by 26%–35% (Ridla et al., 2016).

Tannins

Tannins are plant polyphenols that may combine with metal ions to create compounds as well as macromolecules such as proteins and polysaccharides. The tannin content of EFY is 0.456% (V. Kumar et al., 2010). A. paeoniifolius extract contains flavonoids, tannins, proteins, and carbohydrates. Tannins tightly bind to proteins, forming protein–tannin complexes associated with decreased feed intake, feed efficiency, growth rate, protein digestibility, and net metabolizable energy (Umoh, 2013). Tannins given to chicks lowered development, egg production, and mortality (dietary levels of 0.64%–0.84% and 1.0%–2%, respectively) when the tannin content was raised to more than 3% (Udousoro & Akpan, 2014). Tannin levels can be lowered through fermentation. It was observed that putak meal could lower tannin levels in fermented Chromolaena odorata in rumen content (Kurniawati et al., 2016).

Hydrogen cyanide

Hydrogen cyanide was found in EFY at a concentration of 35.878 parts per million. Hydrogen cyanide (HCN) is found in abundance in plants, mostly in the form of cyanogenic glucosides (Sreerag et al., 2014). The EFY has a lower cyanide content than the cassava root, which has a range of 75–350 parts per million (ppm) but can reach up to 1000 ppm depending on the cultivar, plant age, fertilizer application, soil conditions, weather, and other factors (Garima et al., 2017). The suppression of cytochrome oxidase, a terminal respiratory enzyme found in all cells, causes cyanide toxicosis. The cells suffer from fast ATP depletion when cytochrome oxidase is blocked. Labored breathing, staggering, agitation, gasping, convulsions, paralysis, and death are all symptoms of cyanide poisoning. Because of its high oxyhemoglobin level, blood looks brilliant red (Ridla et al., 2016). Highly poisonous hydrocyanic acid (HCN) is produced during the hydrolysis of cyanogenic glucosides by the enzyme linamarase present in the cassava root peel (Shivi et al., 2014). For humans, the lethal dose of HCN is between 30 and 210 mg kgG1 body weight and ranges from 0.5 to 3.5 mg for an adult, depending on body weight and nutritional status; however, the deadly dose for sheep and cattle is 2.0–4.0 mg kgG1 body weight. Using various methods, cyanogens can be removed, including drying, fermentation, and soaking. Hay and silage should be sufficiently cured before being fed to cattle to ensure that the majority of their cyanogenetic ingredients are removed (Santosa et al., 2016).

Phytic acid

Phytic acid functions as a powerful chelator, mineral complexes, and forming protein and is a typical phosphorus storage form in seeds. Phytates are formed when phytic acid chelates with minerals (Susila et al., 2016). The phytate level of EFYs is 0.165%, which is higher than that of sweet potatoes (0.09%) and chips of cassava root (0.09%) and (0.1%) but lower in other crops, such as soybeans (9.2–16.7 mg) and sorghum (5.9–11.8 mg), and the tuber of the false yam (0.39%) (Hosseini et al., 2015).

As phytic acid has a great ability to bind metal ions like zinc and phosphorus, it interferes with their absorption in the small intestine. It disrupts a variety of metabolic processes (Sarada & Rajani, 2012). Monogastric animals do not have access to the phosphorus in phytic acid. Dietary phytate binds to minerals and creates insoluble phytate–mineral complexes, lowering mineral bioavailability. The human small intestine lacks the microbial community, and phytate-degrading enzymes in the upper digestive system are likewise restricted (Giardina et al., 2014). Phytate content in vegetables is reduced when temperature and heating time are increased, which is lowered by 51% in sun-dried fake yams and 11%–25% in Pterocarpus mildbraedii after heating at 90°C (Dey et al., 2017).

Proximate analysis of EFY

EFY extract was made by blending 1 g of yam with distilled water and filtering the resulting filtrate. Distilled water was used to dilute the obtained filtrate by up to 50 ml. This extract was utilized to calculate macronutrients such as carbohydrates, protein, and vitamin C (Jyoti et al., 2015). There are various methods (Table 2) through which the proximate analysis of EFY is done.

Table 2 Proximate analysis of elephant foot yam through various methods

Parameters	Analysis method	Mechanism	Reference
Ash content	Standard method	The ash from the 5 g of yam sample was incinerated according to the usual protocol, and the resulting ash was diluted to 50 ml with distilled water. The mineral estimations were done using the ash solution that was collected.	Das et al. (2014)
Moisture content	AOAC method	Five grams of elephant foot yam was dried in the oven at 110°C for 45 min to remove all moisture. The moisture content of the sample is determined by weight loss.	Ghate et al. (2013)
Carbohydrate	Anthrone method	To determine the total carbohydrate, the yam extract was hydrolyzed with concentrated sulfuric acid and anthrone reagent was added to each tube, and the tubes were cooked in boiling water. The total carbohydrate content was determined using the standard Anthrone technique calorimetrically at 630 nm.	Das et al. (2012)
Protein content	Folin–Lowry method	Protein content was evaluated by incubating yam extract dilutions for 30 min with alkaline copper sulfate and Folin–Ciocalteau reagent before measuring colorimetrically at 660 nm.	Sarkar et al. (2013)
Fat Extraction	By solvent extraction method	One gram of concentrated ammonia solution + 1 g of concentrated HCL 25 ml petroleum ether and 25 ml solvent ether or diethyl ether (40–60°C). To acquire the total fat in the sample, dry the leftover fat in an oven at 98–100°C for 1 h. The oils were separated from the extracted lipids using the thin-layer chromatography method.	Panda et al. (2017)
Starch Isolation & characteristics of sugar	By osazones test	In ice-cold water, 50 g of yam sample is cut and finely crushed. The filtrate is stored in an ice bath to settle the starch. To achieve pure starch residue, the filtrate is decanted, washings are supplied, and the residue is washed with alcohol and ether. In boiling water, a pinch of yam starch was dissolved. One milliliter of 2 N HCl was added to it, and the mixture was placed in a water bath for 30 min to allow the starch molecules to completely hydrolyze. Two milliliters of the hydrolysate were mixed with 2 ml of freshly produced diphenyl hydrazine reagent and left to react for 30 min in a water bath. Osazones were obtained and studied under ×450 magnification microscopically.	Behera and Ray (2016)
Reducing sugars	By Benedict method	Benedict's approach was used to volumetrically quantify the total amount of reducing sugars from the yam extract.
Calcium	By EDTA	Calcium is calculated volumetrically using EDTA and Eriochrome Black T as an indicator.	Sharma and Satyanarayana (2013)
Iron	By Wong's method	Iron was measured colorimetrically at 470 nm from ash solution using the usual methodology of Wong's method.	Srikanth et al. (2019)
Phosphorus	By Fiske–Subbarow's	Phosphorous was measured colorimetrically at 660 nm from ash solution using the standard Fiske–Method Subbarow's procedure.	Nagar et al. (2019)
Potassium	By flame photometry	Potassium concentration was calculated colorimetrically from ash solution using a standard methodology of flame photometry.	Das et al. (2014)
Sodium	By flame photometry	The sodium content of the ash solution was determined colorimetrically using the standard technique of the flame photometry method.	Sarkar et al. (2013)
Vitamin C	By dichlorophenol indophenols blue	The conventional methodology of 2-6 dichlorophenol indophenol blue dye method, also known as the Harris–Ray method, was used to quantify vitamin C from the yam extract solution.	Panda et al. (2017)

Phytochemical analysis

Standard qualitative methods are carried out to investigate the resident phytochemicals like flavonoids, phenols, tannins, and antioxidants in yam extracts. Yams are also known as famine foods, and they serve an important role in the diets of small and marginal rural families as well as forest-dwelling populations during times of food scarcity. These species are distinctive in terms of food, medicinal, and economic worth, but their widespread use is limited due to antinutritional compositions. Phytochemical analysis, properties, methodology, and uses are elaborated in Table 3. The bioactive components are secondary metabolites derived from plants that are used in insect and pest defense mechanisms. Phenols, polypeptides, polyphenols, terpenoids, alkaloids, steroids, and essential oils are bioactive components with a variety of pharmacological actions (M. M. Rahman et al., 2013). Bioactive substances such as flavonoids, phenols, alkaloids, glycoside steroids, tannins, saponins, anthraquinones, and others are known to be abundant in yam species. The nutraceutical importance of various phytoconstituents of EFY is classified in Table 4.

Table 3 Phytochemical analysis, properties, methodology, and uses

S. No.	Quantitative analysis	Properties	Methodology	Uses	Reference
1	Total phenol content	Phenolic substances are plant metabolites that contain several phenol groups.	A 200 L sample was mixed with 1.5 ml of diluted Folin–Ciocalteu reagent (1:10, vol/vol) and incubated at room temperature for 5 min. After that, 1.5 ml of 0.566 M Na2CO3 was added to the mixture. After 90 min of incubation, the absorbance of the combination was measured at 725 nm using a spectrophotometer. The same analysis process as samples was used to create the standard gallic acid range of 0–125 mg/ml. The result was represented in milligrams of GAE per gram of material (Das et al., 2014).	The presence of various phenolic components is known to have therapeutic as well as antioxidant activities.	Das et al. (2014)
2	Total flavonoid content	Flavonoids are a diverse collection of polyphenolic chemicals having health-promoting effects based on their antioxidant activity.	To measure flavonoid concentration, the aluminum chloride colorimetric method was employed with certain modifications. One milliliter of the extract was combined with 3 ml of methanol, 0.2 ml of 10% aluminum chloride, 0.2 ml of 1 M potassium acetate, and 5.6 ml of distilled water at room temperature for 30 min. A sample blank was made in the same method, except instead of aluminum chloride, distilled water was used. At 420 nm, the absorbance was measured. The standard was quercetin (1 mg/ml). The standard curve was used to calculate flavonoid content, which was expressed as quercetin equivalent (mg/g of the isolated molecule).	Anticancer, antiviral, and anti-inflammatory capabilities have been discovered, as well as impacts on capillary fragility and the potential to prevent human platelet aggregation. Flavonoids can be utilized as repellents or poisons directly, or they can be used in plant improvement projects to produce more resistant crops. Act as a bitterness inhibitor and capacity to generate a bitter or sweet flavor.	Ghate et al. (2013)
3	Total tannin content	Tannins are water-soluble polymers with phenolic monomer units. As a result, some tannin-determination methods are comparable to phenol-determination steps.	Swain's method with minor changes was used to determine the percentage composition of tannin in the extract. One milliliter of sample extract was pipetted into a 50-ml volumetric flask containing 20 ml of distilled water, 2.5 ml of Folin–Denis reagent, and 10 ml of 17% Na2CO3. With distilled water, the mixture was brought up to 50 ml, thoroughly mixed, and let to stand for 20 min until a bluish-green hue appeared. Tannic acid solutions in the range of 0–500 ppm were treated in the same way as the sample above. After the bluish-green hue had fully developed, the absorbance of the tannic acid standard solutions as well as the samples was measured using a spectrophotometer at a wavelength of 760 nm. A tannic acid standard curve was used to determine the tannin concentration, which was represented as mg of tannic acid equivalence per 100 g of dried material.	Leather tanning has been practiced for decades, if not millennia, by immersing hides in pits containing tree bark or tannin-rich wood, such as oak.	Sarkar et al. (2013)
4	Antioxidant content	Any agent that directly scavenges reactive oxygen species (ROS) or indirectly helps to up-regulate antioxidant defenses or decrease ROS formation is referred to be an antioxidant.	The 2,2-diphenyl-1-picryl hydroxyl (DPPH) radical scavenging activity of various solvent extracts was tested using the method published by Das et al. (2012), with minor modifications. 1.5 ml of DPPH (0.1 mM) solution in methanol was combined with 1.5 ml of different amounts of test extract (3.125–100 g/ml) in 10% DMSO. As a control, a combination containing an equal volume of DPPH and 10% DMSO was used. The samples were incubated in the dark for 30 min at room temperature, and absorbance was measured at 517 nm against a blank of methanol. Positive controls included ascorbic acid, gallic acid, quercetin, tannic acid, and BHT.	It protects people from diseases like cardiovascular disease, cancer, neurological disease, Parkinson's disease, and Alzheimer's disease. Antioxidant supplements or foods high in antioxidants can help reduce oxidative damage caused by free radicals and active oxygen species.	Das et al. (2012)
5	Yield	Yield experiments usually have both a substantial genotype × environment interaction and a strong main effect.	In 5 g sample, the yield obtained was 2.1 g. Thus, the yield % is approximately 40%.	Useful in industries for more production and productivity. Minimize loss and waste generation.	Nagar et al. (2019)

EFFECT OF VARIOUS TREATMENTS ON OXALATE CONTENT OF EFY

(C₂O₄)²⁻ or (COO)₂²⁻ are the chemical formulae for oxalate anion. Yam's bitterness and astringency are due to the presence of oxalate in the plant. Because acridity, irritation, and scratching of the mouth and throat are common in yam eaters (Abraham et al., 2021). A key component of kidney stones is calcium oxalate (CaCO₄). Oxalate intake is thought to diminish calcium availability in the body, which could be a danger factor for women who need more calcium in their diets. Oxalate, which is dietary based, has been reported to create insoluble oxalate salts when it comes into contact with calcium, magnesium, and iron, resulting in oxalate stone formation (Nagar et al., 2020). Thus, for the reduction of oxalate, various treatments (Figure 2) were used, some methods are conventional, including boiling and NaCl treatment and other is novel, including ultrasound.

Table 4 Nutraceutical importance of various phytoconstituents of elephant foot yam

S. No.	Phytoconstituent	Nutraceutical importance	Reference
1	Alkaloids	Anticancerous, preanesthetic in surgery, childbirth, ophthalmology, menia, and Parkinsonians	S. S. Rahman et al. (2021)
2	Beta-carboline alkaloids	Antidepressant, antitumour, antibacterial, anti-inflammatory, cardioprotective, and assisting detoxifying enzymes	Patel et al. (2012)
3	Flavonoids	Lower blood cholesterol, antioxidants, antihypertensive, antimicrobleeding, and hepatoprotective	S. Kumar and Pandey (2013); Ghasemzadeh and Ghasemzadeh (2011)
4	Phenolics	Anticancerous	Ozcan et al. (2014)
5	Steroids	Antiasthmatic and antidiabetic	Sheikh et al. (2013)
6	Tannin	Immunomodulatory, hypolipidemic, and cardioprotective	Tariq and Reyaz (2013)

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Plant diets contain water-soluble salts containing sodium, potassium, and ammonium ions, as well as insoluble salts containing calcium, magnesium, and iron ions, making these minerals unavailable to animals (Pan et al., 2022). Hot water (80°C) is used to extract the soluble fraction of oxalates from meals, and hot acid (e.g., 2 M HCl, 80°C) is used to remove the complete (soluble and insoluble) fraction. The amount of insoluble oxalate is calculated by subtracting soluble oxalate from the total oxalate content (Sonal et al., 2022). After human consumption, insoluble oxalates are excreted in the feces. Soluble oxalates can bind to calcium and other minerals in the colon under acidic to near-neutral conditions, rendering them inaccessible. Only 2%–12% of total oxalate eaten is absorbed after ingestion, and the remaining free oxalate in the intestinal lumen interacts with calcium to form calcium oxalate, making calcium inaccessible for absorption (Rao et al., 2020). In the intestine, unabsorbed oxalate is expelled as calcium oxalate in the feces (Kamalkumaran et al., 2020). When oxalate binds to cations like calcium, iron, and magnesium in the digestive tract, it forms insoluble salts, limiting the bioavailability of these vital minerals (Abraham et al., 2021). Second, soluble oxalate must be eliminated in the urine once taken into the body. Oxalates can bind to calcium during this process, forming insoluble calcium oxalate, which subsequently builds up in the kidneys. This calcium oxalate is thought to make up about 75% of all kidney stones (Suja et al., 2021).

A significant risk factor for this condition is excessive oxalate excretion in the urine (hyperoxaluria) (Kumar et al., 2021). The diet has a key influence on stone development (Nedunchezhiyan et al., 2021), hence limiting calcium oxalate stone formation in the kidney by avoiding high-oxalate diets and eating calcium-rich dairy foods (Singh et al., 2020). Tubers are a high-value crop in Nigeria's food chain. Since they may be taken from the wild and consumed by many of the world's poorest and most food-insecure households, they have played a significant role in the history of human cuisine (Singh et al., 2022). According to available statistics, tubers are mostly consumed for their calorie content, as they include carbohydrates, fat, fiber, and protein, despite being the least nutritious of all protein sources. Because of the presence of an antinutrition ingredient (oxalate), which renders the minerals in the tubers unavailable to consumers, it is difficult to determine if they can be depended upon as good sources of minerals (Barua et al., 2021).

Yams are grown all over the world in tropical climates (Srikanth et al., 2021). More than 2 million people in underdeveloped countries rely on them for energy and nourishment, and they will continue to do so for the next 2 decades. Natural sources of diosgenin can be found in yams. Diosgenin causes sex hormone lowering. As a result, the female contraceptive pill was developed, and it has remained one of the most effective and extensively used methods of birth control (Sunitha et al., 2020). Plants and plant products are the main sources of dietary oxalate. Oxalate can be found in little or high amounts in many plant foods (Immanuel et al., 2020).

REDUCTION OF OXALATE THROUGH VARIOUS METHODS

Conventional method

Boiling

In extra boiling water, the Yam cubes were cooked for 10, 20, 30, and 40 min (Yam: water- 1:6). After heating, the Yam cubes and soak water were separated. The Yam cubes were placed on blotting paper and allowed to cool in the air before being stored at −20°C for further analysis. Other features such as acridity, phenolic content, and 2,2diphenyl-1-picrylhydroxyl (DPPH) activity, as well as oxalate content (through hydrothermal degradation) and sensory acridity, were investigated. The amount of yam solids lost in the soaked water was studied (Kumar et al., 2017).

Effect of boiling on oxalate content of EFY

Sodium, potassium, and ammonium salts are soluble oxalate (water soluble) crystals found in plants (Abiodun & Akinoso, 2014). Oxalic acid is water-insoluble calcium, iron, or magnesium salt that chelates metal ions. The production of calcium oxalate crystals in the kidneys (renal stones) and a reduction in mineral bioavailability are two of the most noteworthy effects of oxalates on the human body. In different Dioscorea species, total oxalate levels ranged from 67 to 104 mg/100 g, while soluble oxalate levels ranged from 37 to 85 mg/100 g (Ansil et al., 2014). In different types of EFY, the amount of soluble oxalate found ranged from 2.94 to 18.60 mg/100 g (Kumar et al., 2014). Boiling is one of the most effective ways for humans to make meals more enjoyable and less hazardous. The amount of oxalate in the food was significantly reduced after it was cooked. The soluble oxalate concentration dropped 40.9% from 12.97 mg/100 g (0 min boiling) to 7.66 mg/100 g (40 min boiling), while the total oxalate content (soluble and insoluble combined) dropped 48.7% from 72.39 mg/100 g (0 min boiling) to 37.14 mg/100 g (40 min boiling). This process also shows that as boiling time increased, both oxalates decreased, with the greatest reduction occurring during the first 10 min of boiling; the oxalate content in yam cooked for longer periods (20, 30, and 40 min) did not differ significantly from that obtained after the first 10 min of boiling. In a range of root crops, including wild yam, Japanese taro, and trifoliate yam tuber, boiling has been demonstrated to reduce oxalates (Guil-Guerrero, 2014). Thermal degradation/breakdown at higher temperatures, as well as oxalates leaching in the cooking water, could have caused the decline (Kumoro et al., 2014). Oxalate leaching is aided by the skin's weakening during the boiling process.

The influence of boiling, a common cooking method, on oxalate and acridity issues in A. paeoniifolius was examined. Boiling lowered soluble and total oxalate levels, as well as sensory acridity (Shimi & Haron, 2014). A decrease in total phenolic content, as well as DPPH activity, and an increase in solids loss in cook water occurred at the same time. Boiling EFY for 10 min was shown to be enough to reduce oxalates to levels much below the suggested safe threshold of 71 mg/100 g (Xu et al., 2014).

NaCl treatment

For 60 min, yam slices were soaked in three different NaCl solutions (5%, 10%, and 15%). For the control (P0), yam that has not been soaked, either soaking NaCl and water, P1 (soaking in a NaCl 5% solution), P2 (soaking in a NaCl 10% solution), and P3 (soaking in a NaCl 10% solution) (soaking in a solution of NaCl 15%). Mayasari's optimization of the yam's oxalate content decrease resulted in the desired concentration (Mutaqin et al., 2021).

The reduction of calcium oxalate is greatly helped by NaCl solution. Soaking yam in different amounts of NaCl solution yielded significant changes. The different treatments have varying Ca oxalate content, as well as different percentage decreases in Ca oxalate content, according to the research content of Ca oxalate (ppm). The sample without NaCl solution soaking (P0) had the highest average Ca oxalate content of 102.44 ppm, whereas the sample with 10% NaCl solution soaking had the lowest average Ca oxalate content of 78.92 ppm. The average decrease in the highest Ca oxalate level from P2 was 22.89%, whereas the average decrease in the lowest Ca oxalate content from P1 was 13.61%. The results showed that soaking purple yam in a 10% NaCl (P2) solution is the most effective strategy to reduce Ca oxalate levels. The highest oxalate concentration, 22.89%, can be reduced using this solution. Because the average reduction in oxalate content is lower than P2, which is 20.96%, increasing NaCl concentrations above 10%, that is, 15% (P3), has no discernible effect on the oxalate content drop percentage. Mayasari (2010) found that flouring reduced oxalate concentration in Bogor taro by soaking in a 5% NaCl solution for 30 min and soaking in a 7.5% and 10% NaCl solution for 60 min. The best results came from soaking taro in 10% NaCl for 60 min, which reduced the oxalate concentration by 96.83%. The addition of a salt concentration greater than 5%, i.e., 7.5%, for 30 min resulted in a 5% drop in oxalate level, which was not substantially different. While soaking in 7.5% NaCl can reduce oxalate by 62.73%, the reduction was less when compared to soaking in 5% NaCl, which can reduce oxalate by 72.47% (Mutaqin et al., 2021).

Novel techniques for reduction of ant nutrient components of yam

Application of microwave for reduction of ant nutrient components

A microwave-aided extraction is a cutting-edge approach that is currently being researched. Microwave energy is used in microwave-assisted extraction to concentrate plant metabolites with solvents. Because of its ease of handling and understanding, this approach has proven to be safe for the vast majority of specimens (Ozcan et al., 2014). Exploration of the functional application of microwaves for business development of phytoconstituents is still ongoing, however it is still in its early phases (Simha et al., 2016). Microwaves create dipole rotation in organic molecules as well as heating, which breaks hydrogen bonds. This causes ion activity, which causes a heating effect owing to the greater kinetic energy of the ions, as well as friction between ions due to their rapid movement and direction changes (Himani & Dahiya, 2014). The breakdown of hydrogen bonds also enhances the penetration of solvents into the plant matrix (Ghasemzadeh et al., 2011). In microwave-assisted extraction, microwaves are part of the electromagnetic spectrum of light, with wavelengths ranging from 1 cm to 1 m and frequencies ranging from 300 MHz to 300 GHz (Afoakwah et al., 2012). Two perpendicular oscillating fields make up these waves, which serve as energy and information carriers. The interaction of microwaves with particular materials that may absorb a portion of their electromagnetic energy and convert it to heat is the first application of microwaves. Commercial microwaves use 2450 MHz of energy, which is roughly equivalent to 600–700 W, for this function (Bimakr et al., 2011).

Blanched slices of EFY were heated using a microwave oven (Model: SAMSUNG, Model-CE1041DFB) at varying power levels of 300, 600, and 900 W for 1, 1.5, and 2 min, respectively (Javed et al., 2014). The microwave-heated EFY slices were then transferred to a hot air tray dryer and dried at a continuous temperature of 60°C until they reached a final moisture content of 10% (db) (S. Kumar & Pandey, 2013). A schematic representation of microwave-assisted extraction in a closed vessel is depicted in Figure 3. Microwave heating depends on the dielectric properties of the food, where the dielectric constant is the ability to absorb the energy and dielectric loss is the ability to reflect where it is dissipated as heat (Meda et al., 2017). The two phenomena for the microwave heating of food material were “dipole rotation” and “ionic conduction” (Orsat et al., 2017). Thus, the generated heat inside the food by these mechanisms results in the decrease of antinutritional factors because of their heat-labile characteristics, including the hydrolysis of peptide bonds, deamidation (splitting of covalent bonds), destruction, or interchange of disulfide bonds (Rahate et al., 2021; Suhag et al., 2021).

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Precisely, microwave action might moderately diminish phytic acid because of its heat-labile characteristic and its insoluble complex development between phytate and other components (Kakati et al., 2010). At the same time, tannins are water-soluble and thermally unstable phenolic compounds that can be treated with microwaves for reduction, and before microwave processing, the sample soaked has the ability to dissolve tannins in aqueous media and may simply be decreased by microwave processing (Yang et al., 2014). The degradation in oxalate with respect to the processing by microwave may be due to heat stress which abolishes the total oxalate (Randhir & Shetty, 2004).

Ultrasonication applications for reduction of ant nutrient components

Ultrasound is a nonthermal food processing procedure that can help with food preservation, increased mass transfer, thermal treatment assistance, texture alteration, and food analysis, among other things (H. Wang et al., 2020). Sound waves having frequencies between 20 and 100 kHz are known as ultrasonic waves (sometimes known as supersonic waves) (Rahaman et al., 2021). Cavitation in liquids, pressure changes in gaseous media, and liquid movement in solid media are all caused by ultrasound (Manzoor et al., 2020). It's a type of high-frequency vibration that results in microscale fluid mixing and shear forces (Cui & Zhu, 2020). Ultrasonic cavitation has found favor in a range of applications, including chemical reaction amplification, oil emulsification, chemical and biological pollutant elimination, microbe inactivation, and so on (Barua et al., 2021; Ojediran et al., 2020).

The ultrasonication waves produce two different pressures alternatively into the liquid, namely compressions (high pressure) and rare fraction (low pressure) cycle with rates liable on the frequency (Zhao et al., 2020). In the rare fraction cycle, vacuum bubbles or voids of smaller size are generated in liquid as a consequence of high-pressure ultrasonication waves. The formed bubbles attain a maximum size at which they can no longer absorb energy and then destructively blast during compression, resulting in the formation of microjets (Shojaeiarani et al., 2020). The degradation of particles during ultrasonication resulted from the high temperatures and pressures generated amid the collapse phase of cavitation bubbles. Furthermore, the formation of reactive radical species such as hydroxyl radicals from water molecules could subsequently outbreak and decrease other composites in the matrix (Huynh et al., 2020).

Tannin has a habit of precipitating proteins in food and, consequently, turns into an antinutritional factor that reduces the bioavailability of essential nutrients (Diouf et al., 2019). The increase in ultrasonication amplitude and the time of treatment have the ability to reduce the tannin content of the foods. Thus, the ultrasound assists in converting hydrolyzable tannic acid into gallic acid, and encourages leaching out of the condensed tannin from the sample, resulting in declining the total tannin content of the sample (Bhangu et al., 2018).

The ultrasound amplitude and soaking time of the sample negatively influence the phytate contents. At higher ultrasound amplitude, heat is created in the sample, thus resulting in a rising temperature. The heat promotes the chemical deterioration of phytate to lower inositol phosphate, which helps in declining the phytate content of the sample (Yadav et al., 2021). Higher temperatures and increased treatment time normally cause higher solubility and degradation of polar organic compounds like oxalate. These compounds are oxidized by the hydroxyl radicals diffused out from the homolytic fission of water in cavitation bubbles This oxidation procedure is typically improved by rising temperatures which enables the diffusion course. Furthermore, ultrasonication waves were also skilled in degrading fibers which can bind with oxalic acid and minerals to produce complexes containing fiber, oxalate, and mineral. Therefore, the breakdown of these fiber complexes might lead to further solubility of oxalic acid released in the liquid during ultrasonication (W. Wang et al., 2018). The process of ultrasound and factors affecting ultrasonication is shown in Figure 4.

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CONCLUSION

As an alternative to traditional procedures, several unique techniques have been developed that offer advantages in terms of extraction time, solvent usage, extraction yields, and reproducibility. Because oxalate and acridity are common problems in A. paeoniifolius, an effort has been made to reduce calcium oxalate from yam using boiling, NaCl treatment, microwave, and ultrasound. While traditional methods such as boiling and NaCl treatment are effective for oxalate reduction, they also cause phytochemical loss, making them ineffective. Ultrasound, a revolutionary food technology technique, is successful in preserving numerous bioactive components while significantly lowering oxalate levels. The diversity of EFY in India must be investigated in terms of its potential for food and health benefits. The antioxidant activity of the yam can help to reduce the detrimental effects of free radical reactions, which is good for consumers. The existence of promising levels of phenolic compounds as well as a reasonable total flavonoid content percentage back up the prior assertion. EFY is high in caloric content and a good source of nutrients. Thus, EFY can be used as feed or food, but it must be treated before eating due to its antinutritional value. When compared to other physical and chemical food processing activities, ultrasonication is regarded as a clean technique with a high potential for customer acceptability. It is a physical alteration procedure that may or may not result in significant chemical changes. If its potential for the development of new goods is fully realized, ultrasound might have a significant presence in the food industry.

AUTHOR CONTRIBUTIONS

Shivangi Srivastava: Data curation; formal analysis; methodology; validation; visualization; and writing – original draft. Vinay K. Pandey: Data curation; investigation; methodology; resources; software; validation; visualization; and writing – original draft. Poornima Singh: Data curation; formal analysis; methodology; and software. Gurajala Venkata Siva Bhagya Raj: Methodology; resources; and software. Kshirod K. Dash: Conceptualization; data curation; project administration; resources; supervision; validation; and writing – review and editing. Rahul Singh: Conceptualization; project administration; resources; software; and supervision.

ACKNOWLEDGMENT

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

ETHICS STATEMENT

This study does not involve any studies involving animal or human subjects.

References

Abiodun, O. A., & Akinoso, R. (2014). Effect of delayed harvesting and pre‐treatment methods on the antinutritional contents of trifoliate yam flour. Food Chemistry, 146, 515–520. [DOI: https://dx.doi.org/10.1016/j.foodchem.2013.09.098]