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Tannins in Foods: Nutritional Implications and Processing Effects of Hydrothermal Techniques on Underutilized Hard-to-Cook Legume Seeds-A Review
Department of Food Science, Ladoke Akintola University of Technology, Ogbomoso 210214, Nigeria
Correspondence to:This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Prev Nutr Food Sci 2022; 27(1): 14-19
Published March 31, 2022 https://doi.org/10.3746/pnf.2022.27.1.14
Copyright © The Korean Society of Food Science and Nutrition.
Abstract
Keywords
INTRODUCTION
In their natural form, tannins are water-soluble phenolic compounds with molecular weights in the range of 300 to 500 and have the ability to precipitate gelatin, alkaloids, and proteins (Mena et al., 2015). Tannins are secondary metabolites widely distributed in plants: they are polymeric phenolic substances with astringency properties (Agrawal et al., 2012; Lattanzio et al., 2012; Bone and Mills, 2013; Tamokou et al., 2017). Humans consume a number of foods containing considerable amounts of dietary tannins. Tannins are found in a huge variety of plants, including legume seeds, cider, cereals, cacao, peas, some leafy and green vegetables, coffee, tea, and nuts (Lochab et al., 2014; Suvanto et al., 2017; Fraga-Corral et al., 2020). In general, tannins can be classified into one of two classes on the basis of their structural features: hydrolyzable and condensed tannins (Hatew et al., 2016; Bee et al., 2017). In addition to other antinutritional components, such as phytic acid, hemagglutinin, saponin, goitrogen, and trypsin inhibitors, tannins are present in legume seeds at varying concentrations. These antinutritional components interfere with the digestive process and prevent the efficient utilization of nutrients (Luo and Xie, 2013; Sathya and Siddhuraju, 2015; Wu et al., 2016).
Legumes are the good source of nutritionally important nutrients such as mineral elements and dietary protein (Ojo et al., 2016; Ojo et al., 2017a; Ojo et al., 2018). Proteins from plants sources are preferable to animal sources because, unlike animal proteins, plant proteins are not usually associated with the occurrence of cardiovascular diseases. Legume seeds are good alternatives to animal protein. In recent years, there has been an increase in demand and consumption for plant protein, which has stimulated the search for more protein-rich plants (Adebowale et al., 2005; Ojo, 2015; Septiana and Analuddin, 2019). However, overdependence on common legumes has resulted in a sharp increase in prices (Ojo, 2015; Ojo et al., 2017a). Many legumes remain underutilized in South-West Nigeria, where they are planted mainly for subsistence purposes. Some of these underutilized legumes include
The phenomenon of prolonged cooking times and the presence of antinutritional components are prominent challenges facing the use of these underutilized legumes. The problem of prolonged cooking in legume seeds was reported to be alleviated by hydrothermal processing techniques (Ojo, 2018a). In this study, the nutritional implications of tannins in foods, as well as the effects of soaking at varying hydration levels followed by hydrothermal processing, are reported. Information on the effect of hydrothermal techniques on the tannin content of underutilized legume species before and after processing will enhance the current knowledge of legume processing and nutrition and foster economic utility.
HEALTH IMPLICATION OF TANNINS IN DIETS
Traditionally, tannins are considered to have antinutritional properties (Ojo, 2018b). However, recent evidence has shown that the consumption of tannins can have health benefits. The effects of tannin on human and animal biology vary considerably and depend on the composition of the diet and dietary patterns. Tannins have the ability to form complexes with carbohydrates, proteins, and certain mineral ions in foods (Kunyanga et al., 2011). The formation of such complexes depends on a requirement for suitable conditions such as pH, temperature, and concentration. Tannins have greater tendency to form complexes with proteins than carbohydrates and other food polymers because of the strong hydrogen bond affinity of the carboxyl oxygen of the peptide group. Complexes formed by tannins and proteins have been reported to be responsible for growth depression, low protein digestibility, decreased availability of amino acid, and increased fecal nitrogen (Waghorn, 2008; Woodward et al., 2009; Dijkstra et al., 2013; Grosse Brinkhaus et al., 2016).
Although the consumption of legume seeds has been reported to confer health benefits, a number of reports have described the harmful effects of tannic acid on the gastrointestinal tissues (Maphosa and Jideani, 2017; George et al., 2020). The toxicity of orally ingested tannins is relatively low: the rectal toxicity of tannic acid is approximately twice its oral toxicity (George et al., 2020; Hassan et al., 2020). The ingestion of tannic acid has been reported to cause hardening of the gastrointestinal mucosa, which results in a reduction in the gastrointestinal absorption of nutrients. Sorghum, which contains tannins, has been implicated in the occurrence of esophageal cancer (Wexler, 2014). It is important to consider that most of the toxicity studies of tannins in experimental animals are conducted using commercially available samples of tannins or tannic acid. There was no evidence of toxicity in sheep and cattle fed diets containing tannin (Wen et al., 2002). It is therefore extremely difficult to extrapolate if tannins in legume seeds used in feeds will have similar toxicities upon ingestion. There are conflicting reports. Recently, tannins have been reported to be useful in the control of zoonotic pathogens, such as
Conversely, tannic acid used to be administered for the treatment of diarrhea and the dressing of skin burn. A precipitant of proteins affecting the colloidal stability of beer, tannic acid removes some metals (such as Al, Zn, Pb, and Fe) and some polyphenols when they are bound to proteins. Tannic acid has been reported to be effective against staling, flavor, and light instability (Wexler, 2014). It has been reported that tannins in
INFLUENCE OF SOAKING ON TANNIN CONTENT OF LEGUME SEEDS
The seeds of some underutilized hard-to-cook legumes were soaked in distilled water at various hydration levels (0, 10, 25, 50, 75, and 100%) for a period of 24 h at an ambient temperature of 23°C to 28°C (Ojo, 2015; Ojo et al., 2017a; Ojo et al., 2018). The changes in the concentrations of tannins in the seeds before and after soaking were determined (Table 1). For each of the legume seeds, there was a significant difference (
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Table 1 . Concentration of tannins in legume seeds before and after soaking at various hydration levels (unit: mg/g)
Legume sample Hydration level 0 10 25 50 75 100 Mallotus subulatus 1 (white variety)28.10±1.22f 27.03±0.10e 26.07±0.14d 25.32±0.21c 24.28±0.07b 24.02±0.11a (3.81) (7.22) (9.89) (13.59) (14.52) Cassia hirsuta 62.31±0.37f 61.93±0.51e 61.37±0.42d 60.17±0.22c 59.48±0.60b 56.87±0.50a (0.61) (1.51) (3.43) (4.54) (8.73) Canavalia ensiformis 27.47±0.44f 26.50±0.12e 26.37±0.21d 26.12±0.03c 24.70±0.10b 23.68±0.11a (3.53) (4.00) (4.91) (10.08) (13.79) Vigna subterranean 1 (mottled colored)31.58±0.31f 30.20±0.13e 28.88±0.24d 26.72±0.17c 25.87±0.25b 25.41±0.10a (4.36) (8.55) (15.39) (18.08) (19.54) Vigna racemosa 28.97±0.32e 28.04±0.10d 27.16±0.02c 26.55±0.13b 24.47±0.24a 24.47±0.24a (3.21) (6.25) (8.35) (15.53) (15.53) Mallotus subulatus 2 (brown variety)38.87±0.41f 36.42±0.21e 35.26±0.32d 34.87±0.24c 34.23±0.10b 33.08±0.12a (6.30) (9.29) (10.29) (11.94) (14.89) Vigna subterranean 2 (cream colored)42.59±0.53e 40.08±0.12d 38.14±0.10c 37.52±0.30b 37.34±0.21a 37.34±0.21a (5.89) (10.45) (11.90) (12.33) (12.33) Sphenostylis sterocarpa 39.88±0.24e 38.24±0.21d 37.06±0.10c 34.35±0.22b 34.18±0.30a 34.18±0.20a (4.11) (7.07) (13.87) (14.29) (14.29) Values are presented as mean±SD (n=3) on dry basis and values in parentheses represent the percentage loss.
Different letters (a-f) in the same row indicate significant differences (
P <0.05).Data from the article of Ojo (2015), Ojo et al. (2017a), and Ojo et al. (2018).
The percentage reduction in tannin content of the legumes studied was similar to, but comparatively lower than, those of a previous report on
The reduction in the tannin content may be due to leaching of the polyphenols into the soaking water (Wu et al., 2016; Ojo et al., 2018). Tannins are polyphenols and polyphenolic compounds are mostly water-soluble in nature and mostly located in the seed coat. Therefore, it can be inferred that a pre-processing method, such as soaking, can be used to reduce the level of some water-soluble/leachable antinutritional factors, such as tannins (Ojo et al., 2017a; Ojo et al., 2018; Ojo, 2018b).
IMPACT OF HYDROTHERMAL TECHNIQUES ON TANNIN CONTENT OF UNDERUTILIZED LEGUME SEEDS
Four hydrothermal techniques [atmospheric boiling (AB), atmospheric steaming (AS), pressure boiling (PB), and pressure steaming] were employed to process the seeds of some underutilized hard-to-cook legume seeds (Ojo, 2015; Ojo et al., 2017a; Ojo et al., 2018). The effects of hydrothermal processing techniques on the levels of tannins in the seeds are presented in Table 2. The hydrothermal techniques caused a reduction in the concentration of tannins in the legume seeds. All the hydrothermal processing methods (i.e., boiling and steaming at atmospheric pressure, as well as boiling and steaming at elevated pressure) had significant effects (
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Table 2 . Tannin content of legume seeds as influenced by hydrothermal processing techniques (unit: mg/g)
Legume sample Processing condition Raw dried sample Atmospheric boiling Atmospheric steaming Pressure boiling Pressure steaming Mallotus subulatus 1 (white variety)28.10±0.22e 6.85±0.32a 7.84±0.40c 7.57±0.05b 8.15±0.13c (75.62) (72.10) (73.06) (70.99) Cassia hirsuta 62.31±0.37d 13.42±0.33a 15.38±0.32b 13.42±0.61a 16.67±0.80c (78.46) (75.32) (78.46) (73.25) Canavalia ensiformis 27.47±0.44d 9.70±0.41a 10.60±0.34c 10.03±0.20b 10.64±0.32c (64.69) (61.41) (63.49) (61.27) Vignasubterranean 1 (mottled colored)31.58±0.31d 11.49±0.24a 12.82±0.31b 12.31±0.57b 13.19±0.60c (63.62) (59.40) (61.02) (58.23) Vigna racemosa 28.97±0.32c 9.02±0.13a 10.15±0.21b 9.38±0.34a 10.13±0.42c (68.86) (64.96) (67.62) (65.03) Mallotus subulatus 2 (brown variety)38.87±0.41c 9.20±0.18a 10.74±0.52b 9.61±0.29a 10.77±0.61b (76.33) (72.37) (75.28) (72.29) Vigna subterranean 2 (cream colored)42.59±0.53 14.57±0.57a 16.13±0.60b 15.20±0.70a 16.16±0.42b (65.79) (62.13) (64.34) (62.06) Sphenostylis sterocarpa 39.88±0.24d 10.07±0.15a 11.76±0.23b 11.01±0.40b 11.93±0.37c (74.75) (70.51) (72.39) (70.09) Values are the mean±standard deviation (n=3) on dry basis and values in parentheses represent the percentage loss.
Different letters (a-e) in the same row represent significant differences (
P <0.05).Data from the article of Ojo (2015), Ojo et al. (2017a), and Ojo et al. (2018).
The lowest value of tannins (6.85 mg/g) was recorded for the white variety of
As recorded in Table 2, pressure processing, both boiling and steaming, resulted in relatively smaller losses of tannin than the atmospheric processing of boiling and steaming. This observation is true for all the legumes studied, except for
CONCLUSION
The processing of these underutilized legume seeds must be performed such that the tannins will be reduced to a safe level/concentration. Although there is no standardized safe limit for legume tannins, it is well established that tannins form complexes with nutritionally important compounds, such as proteins and some mineral elements, making them unavailable for absorption. Soaking and hydrothermal techniques reduce tannin concentrations in legume seeds and hence improve the bioavailability of protein and mineral elements. Although the toxicity of synthetic/commercially available tannins has been demonstrated in experimental animals, legume seed tannins have not been reported to cause any known toxicities or physiological disorders upon ingestion. Considering the recent evidence showing the health benefits, the classification of tannins that have been hitherto regarded as an antinutrient should be reconsidered. Therefore, the provision of information on the importance of tannins in legume nutrition is anticipated to foster economic utility, encourage cultivation, and prevent the imminent extinction of these lesser known legume food crops. Increasing the consumption of nutritionally important underutilized legume seeds will alleviate the problem of protein energy malnutrition in developing nations.
FUNDING
None.
AUTHOR DISCLOSURE STATEMENT
The author declares no conflicts of interest.
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Article
Review
Prev Nutr Food Sci 2022; 27(1): 14-19
Published online March 31, 2022 https://doi.org/10.3746/pnf.2022.27.1.14
Copyright © The Korean Society of Food Science and Nutrition.
Tannins in Foods: Nutritional Implications and Processing Effects of Hydrothermal Techniques on Underutilized Hard-to-Cook Legume Seeds-A Review
Department of Food Science, Ladoke Akintola University of Technology, Ogbomoso 210214, Nigeria
Correspondence to:Moses Ayodele Ojo, E-mail: mayoojo2006@yahoo.com
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
Tannins, water-soluble phenolic compounds, have been reported to have the ability to form complexes with nutritionally important nutrients such as protein and mineral elements thereby making them unavailable for absorption and utilization. Toxicity of tannin has been demonstrated in experimental animals although no deleterious effect of ingestion of legume tannin on human physiology has been reported. This report highlights the processing effects of soaking and hydrothermal techniques on some underutilised hard-to-cook legume crops and the importance of tannin in legume nutrition. Soaking and hydrothermal processing reduce the tannin content of processed legume seeds and hence improve the availability of protein and mineral elements. In view of the recent findings of the health benefits, classification of tannin which is traditionally regarded as an antinutrient should be reconsidered. Provision of these information will enhance knowledge of legume nutrition and economic utility. Increasing consumption of underutilized nutritionally important legume seeds, it is hoped, will alleviate the problem of protein energy malnutrition in many developing nations.
Keywords: hydrothermal techniques, legume seeds, nutritional implication, tannin
INTRODUCTION
In their natural form, tannins are water-soluble phenolic compounds with molecular weights in the range of 300 to 500 and have the ability to precipitate gelatin, alkaloids, and proteins (Mena et al., 2015). Tannins are secondary metabolites widely distributed in plants: they are polymeric phenolic substances with astringency properties (Agrawal et al., 2012; Lattanzio et al., 2012; Bone and Mills, 2013; Tamokou et al., 2017). Humans consume a number of foods containing considerable amounts of dietary tannins. Tannins are found in a huge variety of plants, including legume seeds, cider, cereals, cacao, peas, some leafy and green vegetables, coffee, tea, and nuts (Lochab et al., 2014; Suvanto et al., 2017; Fraga-Corral et al., 2020). In general, tannins can be classified into one of two classes on the basis of their structural features: hydrolyzable and condensed tannins (Hatew et al., 2016; Bee et al., 2017). In addition to other antinutritional components, such as phytic acid, hemagglutinin, saponin, goitrogen, and trypsin inhibitors, tannins are present in legume seeds at varying concentrations. These antinutritional components interfere with the digestive process and prevent the efficient utilization of nutrients (Luo and Xie, 2013; Sathya and Siddhuraju, 2015; Wu et al., 2016).
Legumes are the good source of nutritionally important nutrients such as mineral elements and dietary protein (Ojo et al., 2016; Ojo et al., 2017a; Ojo et al., 2018). Proteins from plants sources are preferable to animal sources because, unlike animal proteins, plant proteins are not usually associated with the occurrence of cardiovascular diseases. Legume seeds are good alternatives to animal protein. In recent years, there has been an increase in demand and consumption for plant protein, which has stimulated the search for more protein-rich plants (Adebowale et al., 2005; Ojo, 2015; Septiana and Analuddin, 2019). However, overdependence on common legumes has resulted in a sharp increase in prices (Ojo, 2015; Ojo et al., 2017a). Many legumes remain underutilized in South-West Nigeria, where they are planted mainly for subsistence purposes. Some of these underutilized legumes include
The phenomenon of prolonged cooking times and the presence of antinutritional components are prominent challenges facing the use of these underutilized legumes. The problem of prolonged cooking in legume seeds was reported to be alleviated by hydrothermal processing techniques (Ojo, 2018a). In this study, the nutritional implications of tannins in foods, as well as the effects of soaking at varying hydration levels followed by hydrothermal processing, are reported. Information on the effect of hydrothermal techniques on the tannin content of underutilized legume species before and after processing will enhance the current knowledge of legume processing and nutrition and foster economic utility.
HEALTH IMPLICATION OF TANNINS IN DIETS
Traditionally, tannins are considered to have antinutritional properties (Ojo, 2018b). However, recent evidence has shown that the consumption of tannins can have health benefits. The effects of tannin on human and animal biology vary considerably and depend on the composition of the diet and dietary patterns. Tannins have the ability to form complexes with carbohydrates, proteins, and certain mineral ions in foods (Kunyanga et al., 2011). The formation of such complexes depends on a requirement for suitable conditions such as pH, temperature, and concentration. Tannins have greater tendency to form complexes with proteins than carbohydrates and other food polymers because of the strong hydrogen bond affinity of the carboxyl oxygen of the peptide group. Complexes formed by tannins and proteins have been reported to be responsible for growth depression, low protein digestibility, decreased availability of amino acid, and increased fecal nitrogen (Waghorn, 2008; Woodward et al., 2009; Dijkstra et al., 2013; Grosse Brinkhaus et al., 2016).
Although the consumption of legume seeds has been reported to confer health benefits, a number of reports have described the harmful effects of tannic acid on the gastrointestinal tissues (Maphosa and Jideani, 2017; George et al., 2020). The toxicity of orally ingested tannins is relatively low: the rectal toxicity of tannic acid is approximately twice its oral toxicity (George et al., 2020; Hassan et al., 2020). The ingestion of tannic acid has been reported to cause hardening of the gastrointestinal mucosa, which results in a reduction in the gastrointestinal absorption of nutrients. Sorghum, which contains tannins, has been implicated in the occurrence of esophageal cancer (Wexler, 2014). It is important to consider that most of the toxicity studies of tannins in experimental animals are conducted using commercially available samples of tannins or tannic acid. There was no evidence of toxicity in sheep and cattle fed diets containing tannin (Wen et al., 2002). It is therefore extremely difficult to extrapolate if tannins in legume seeds used in feeds will have similar toxicities upon ingestion. There are conflicting reports. Recently, tannins have been reported to be useful in the control of zoonotic pathogens, such as
Conversely, tannic acid used to be administered for the treatment of diarrhea and the dressing of skin burn. A precipitant of proteins affecting the colloidal stability of beer, tannic acid removes some metals (such as Al, Zn, Pb, and Fe) and some polyphenols when they are bound to proteins. Tannic acid has been reported to be effective against staling, flavor, and light instability (Wexler, 2014). It has been reported that tannins in
INFLUENCE OF SOAKING ON TANNIN CONTENT OF LEGUME SEEDS
The seeds of some underutilized hard-to-cook legumes were soaked in distilled water at various hydration levels (0, 10, 25, 50, 75, and 100%) for a period of 24 h at an ambient temperature of 23°C to 28°C (Ojo, 2015; Ojo et al., 2017a; Ojo et al., 2018). The changes in the concentrations of tannins in the seeds before and after soaking were determined (Table 1). For each of the legume seeds, there was a significant difference (
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Table 1 . Concentration of tannins in legume seeds before and after soaking at various hydration levels (unit: mg/g).
Legume sample Hydration level 0 10 25 50 75 100 Mallotus subulatus 1 (white variety)28.10±1.22f 27.03±0.10e 26.07±0.14d 25.32±0.21c 24.28±0.07b 24.02±0.11a (3.81) (7.22) (9.89) (13.59) (14.52) Cassia hirsuta 62.31±0.37f 61.93±0.51e 61.37±0.42d 60.17±0.22c 59.48±0.60b 56.87±0.50a (0.61) (1.51) (3.43) (4.54) (8.73) Canavalia ensiformis 27.47±0.44f 26.50±0.12e 26.37±0.21d 26.12±0.03c 24.70±0.10b 23.68±0.11a (3.53) (4.00) (4.91) (10.08) (13.79) Vigna subterranean 1 (mottled colored)31.58±0.31f 30.20±0.13e 28.88±0.24d 26.72±0.17c 25.87±0.25b 25.41±0.10a (4.36) (8.55) (15.39) (18.08) (19.54) Vigna racemosa 28.97±0.32e 28.04±0.10d 27.16±0.02c 26.55±0.13b 24.47±0.24a 24.47±0.24a (3.21) (6.25) (8.35) (15.53) (15.53) Mallotus subulatus 2 (brown variety)38.87±0.41f 36.42±0.21e 35.26±0.32d 34.87±0.24c 34.23±0.10b 33.08±0.12a (6.30) (9.29) (10.29) (11.94) (14.89) Vigna subterranean 2 (cream colored)42.59±0.53e 40.08±0.12d 38.14±0.10c 37.52±0.30b 37.34±0.21a 37.34±0.21a (5.89) (10.45) (11.90) (12.33) (12.33) Sphenostylis sterocarpa 39.88±0.24e 38.24±0.21d 37.06±0.10c 34.35±0.22b 34.18±0.30a 34.18±0.20a (4.11) (7.07) (13.87) (14.29) (14.29) Values are presented as mean±SD (n=3) on dry basis and values in parentheses represent the percentage loss..
Different letters (a-f) in the same row indicate significant differences (
P <0.05)..Data from the article of Ojo (2015), Ojo et al. (2017a), and Ojo et al. (2018)..
The percentage reduction in tannin content of the legumes studied was similar to, but comparatively lower than, those of a previous report on
The reduction in the tannin content may be due to leaching of the polyphenols into the soaking water (Wu et al., 2016; Ojo et al., 2018). Tannins are polyphenols and polyphenolic compounds are mostly water-soluble in nature and mostly located in the seed coat. Therefore, it can be inferred that a pre-processing method, such as soaking, can be used to reduce the level of some water-soluble/leachable antinutritional factors, such as tannins (Ojo et al., 2017a; Ojo et al., 2018; Ojo, 2018b).
IMPACT OF HYDROTHERMAL TECHNIQUES ON TANNIN CONTENT OF UNDERUTILIZED LEGUME SEEDS
Four hydrothermal techniques [atmospheric boiling (AB), atmospheric steaming (AS), pressure boiling (PB), and pressure steaming] were employed to process the seeds of some underutilized hard-to-cook legume seeds (Ojo, 2015; Ojo et al., 2017a; Ojo et al., 2018). The effects of hydrothermal processing techniques on the levels of tannins in the seeds are presented in Table 2. The hydrothermal techniques caused a reduction in the concentration of tannins in the legume seeds. All the hydrothermal processing methods (i.e., boiling and steaming at atmospheric pressure, as well as boiling and steaming at elevated pressure) had significant effects (
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Table 2 . Tannin content of legume seeds as influenced by hydrothermal processing techniques (unit: mg/g).
Legume sample Processing condition Raw dried sample Atmospheric boiling Atmospheric steaming Pressure boiling Pressure steaming Mallotus subulatus 1 (white variety)28.10±0.22e 6.85±0.32a 7.84±0.40c 7.57±0.05b 8.15±0.13c (75.62) (72.10) (73.06) (70.99) Cassia hirsuta 62.31±0.37d 13.42±0.33a 15.38±0.32b 13.42±0.61a 16.67±0.80c (78.46) (75.32) (78.46) (73.25) Canavalia ensiformis 27.47±0.44d 9.70±0.41a 10.60±0.34c 10.03±0.20b 10.64±0.32c (64.69) (61.41) (63.49) (61.27) Vignasubterranean 1 (mottled colored)31.58±0.31d 11.49±0.24a 12.82±0.31b 12.31±0.57b 13.19±0.60c (63.62) (59.40) (61.02) (58.23) Vigna racemosa 28.97±0.32c 9.02±0.13a 10.15±0.21b 9.38±0.34a 10.13±0.42c (68.86) (64.96) (67.62) (65.03) Mallotus subulatus 2 (brown variety)38.87±0.41c 9.20±0.18a 10.74±0.52b 9.61±0.29a 10.77±0.61b (76.33) (72.37) (75.28) (72.29) Vigna subterranean 2 (cream colored)42.59±0.53 14.57±0.57a 16.13±0.60b 15.20±0.70a 16.16±0.42b (65.79) (62.13) (64.34) (62.06) Sphenostylis sterocarpa 39.88±0.24d 10.07±0.15a 11.76±0.23b 11.01±0.40b 11.93±0.37c (74.75) (70.51) (72.39) (70.09) Values are the mean±standard deviation (n=3) on dry basis and values in parentheses represent the percentage loss..
Different letters (a-e) in the same row represent significant differences (
P <0.05)..Data from the article of Ojo (2015), Ojo et al. (2017a), and Ojo et al. (2018)..
The lowest value of tannins (6.85 mg/g) was recorded for the white variety of
As recorded in Table 2, pressure processing, both boiling and steaming, resulted in relatively smaller losses of tannin than the atmospheric processing of boiling and steaming. This observation is true for all the legumes studied, except for
CONCLUSION
The processing of these underutilized legume seeds must be performed such that the tannins will be reduced to a safe level/concentration. Although there is no standardized safe limit for legume tannins, it is well established that tannins form complexes with nutritionally important compounds, such as proteins and some mineral elements, making them unavailable for absorption. Soaking and hydrothermal techniques reduce tannin concentrations in legume seeds and hence improve the bioavailability of protein and mineral elements. Although the toxicity of synthetic/commercially available tannins has been demonstrated in experimental animals, legume seed tannins have not been reported to cause any known toxicities or physiological disorders upon ingestion. Considering the recent evidence showing the health benefits, the classification of tannins that have been hitherto regarded as an antinutrient should be reconsidered. Therefore, the provision of information on the importance of tannins in legume nutrition is anticipated to foster economic utility, encourage cultivation, and prevent the imminent extinction of these lesser known legume food crops. Increasing the consumption of nutritionally important underutilized legume seeds will alleviate the problem of protein energy malnutrition in developing nations.
FUNDING
None.
AUTHOR DISCLOSURE STATEMENT
The author declares no conflicts of interest.
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Table 1 . Concentration of tannins in legume seeds before and after soaking at various hydration levels (unit: mg/g)
Legume sample Hydration level 0 10 25 50 75 100 Mallotus subulatus 1 (white variety)28.10±1.22f 27.03±0.10e 26.07±0.14d 25.32±0.21c 24.28±0.07b 24.02±0.11a (3.81) (7.22) (9.89) (13.59) (14.52) Cassia hirsuta 62.31±0.37f 61.93±0.51e 61.37±0.42d 60.17±0.22c 59.48±0.60b 56.87±0.50a (0.61) (1.51) (3.43) (4.54) (8.73) Canavalia ensiformis 27.47±0.44f 26.50±0.12e 26.37±0.21d 26.12±0.03c 24.70±0.10b 23.68±0.11a (3.53) (4.00) (4.91) (10.08) (13.79) Vigna subterranean 1 (mottled colored)31.58±0.31f 30.20±0.13e 28.88±0.24d 26.72±0.17c 25.87±0.25b 25.41±0.10a (4.36) (8.55) (15.39) (18.08) (19.54) Vigna racemosa 28.97±0.32e 28.04±0.10d 27.16±0.02c 26.55±0.13b 24.47±0.24a 24.47±0.24a (3.21) (6.25) (8.35) (15.53) (15.53) Mallotus subulatus 2 (brown variety)38.87±0.41f 36.42±0.21e 35.26±0.32d 34.87±0.24c 34.23±0.10b 33.08±0.12a (6.30) (9.29) (10.29) (11.94) (14.89) Vigna subterranean 2 (cream colored)42.59±0.53e 40.08±0.12d 38.14±0.10c 37.52±0.30b 37.34±0.21a 37.34±0.21a (5.89) (10.45) (11.90) (12.33) (12.33) Sphenostylis sterocarpa 39.88±0.24e 38.24±0.21d 37.06±0.10c 34.35±0.22b 34.18±0.30a 34.18±0.20a (4.11) (7.07) (13.87) (14.29) (14.29) Values are presented as mean±SD (n=3) on dry basis and values in parentheses represent the percentage loss.
Different letters (a-f) in the same row indicate significant differences (
P <0.05).Data from the article of Ojo (2015), Ojo et al. (2017a), and Ojo et al. (2018).
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Table 2 . Tannin content of legume seeds as influenced by hydrothermal processing techniques (unit: mg/g)
Legume sample Processing condition Raw dried sample Atmospheric boiling Atmospheric steaming Pressure boiling Pressure steaming Mallotus subulatus 1 (white variety)28.10±0.22e 6.85±0.32a 7.84±0.40c 7.57±0.05b 8.15±0.13c (75.62) (72.10) (73.06) (70.99) Cassia hirsuta 62.31±0.37d 13.42±0.33a 15.38±0.32b 13.42±0.61a 16.67±0.80c (78.46) (75.32) (78.46) (73.25) Canavalia ensiformis 27.47±0.44d 9.70±0.41a 10.60±0.34c 10.03±0.20b 10.64±0.32c (64.69) (61.41) (63.49) (61.27) Vignasubterranean 1 (mottled colored)31.58±0.31d 11.49±0.24a 12.82±0.31b 12.31±0.57b 13.19±0.60c (63.62) (59.40) (61.02) (58.23) Vigna racemosa 28.97±0.32c 9.02±0.13a 10.15±0.21b 9.38±0.34a 10.13±0.42c (68.86) (64.96) (67.62) (65.03) Mallotus subulatus 2 (brown variety)38.87±0.41c 9.20±0.18a 10.74±0.52b 9.61±0.29a 10.77±0.61b (76.33) (72.37) (75.28) (72.29) Vigna subterranean 2 (cream colored)42.59±0.53 14.57±0.57a 16.13±0.60b 15.20±0.70a 16.16±0.42b (65.79) (62.13) (64.34) (62.06) Sphenostylis sterocarpa 39.88±0.24d 10.07±0.15a 11.76±0.23b 11.01±0.40b 11.93±0.37c (74.75) (70.51) (72.39) (70.09) Values are the mean±standard deviation (n=3) on dry basis and values in parentheses represent the percentage loss.
Different letters (a-e) in the same row represent significant differences (
P <0.05).Data from the article of Ojo (2015), Ojo et al. (2017a), and Ojo et al. (2018).
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