Effect of Processing on Certain Antinutrients in Seeds of Şeker Variety (P. vulgaris L.) Grown in Turkey

Abstract

The effects of different cooking/autoclaving methods were investigated on sucrose, raffinose, stachyose, phytic acid, and tannins content of Şeker bean seeds (Phaseolus vulgaris). All these oligosaccharides and antinutrients decreased under various cooking/autoclaving treatments. Among the different cooking/ autoclaving treatments, 18 h sodium bicarbonate solution soaking followed by autoclaving procedures were the most effective for removing oligosaccharides (65–72%), phytic acid (51%), and tannins (100%). These conditions could be recommended to remove undesirable sugar contents and antinutrients of the Şeker bean used for culinary purposes.

Keywords:

• Dry beans
• α-Galactosides
• Phytic acid
• Tannins
• Soaking
• Cooking
• Autoclaving

Introduction

Legumes, especially beans are considered important and inexpensive protein and dietary fiber sources in human nutrition. Further, beans contain a considerable amount of vitamins and minerals. However, their wide acceptability is adversely affected by the presence of α-galactosides, phytic acid, tannins, and other antinutritional substances.1 2 Phytic acid is considered the main factor causing impaired absorption of minerals from legumes.3 4 The ability of phytate to interact with proteins and minerals has long been recognized.5 These interactions may result in a reduced bioavailability of minerals as well as in a modification of plant protein properties after intestinal digestion.6 Food legume tannins are reported to interact with proteins to form tannin–protein complexes resulting inactivation of digestive enzymes and protein insolubility. Bean tannins decrease protein digestibility, either by inactivating digestive enzymes or by reducing the susceptibility of the substrate proteins after forming complexes with tannins and absorbed ionizable iron.7 α-Galactosides content, among which raffinose, stachyose, and verbascose are generally undesirable due to flatus factors.8 9 10 Because the human alimentary tract is deprived of α-galactosidase capable of hydrolyzing the α-1→6 galactoside linkage, these oligosaccharides are not digested and accumulate in large intestine where they undergo anaerobic fermentation by bacteria. Thus, some gases are produced owing to fermentation, such as carbon dioxide, hydrogen, and methane.11 12 13

Traditional treatments such as soaking, cooking, and autoclaving have been used to improve nutritional quality of legumes, dry beans.10 It has been shown that antinutritional factors are removed considerably by utilizing these simple and inexpensive processes. Although there are a number of researches associated with antinutritional factors of dry beans, unfortunately, there is no research about these factors of the dry beans grown in Turkey and the removal procedures before consumption. The present study has been planned to understand to what extent, and by which treatments, the antinutritional compounds in dry bean Şeker variety can be eliminated/reduced.

Materials and Methods

Dry bean (variety Şeker) samples used as research material were supplied from East Anatolia Institute of Agricultural Research. The damaged beans were segregated from main population, and then raw bean material were soaked in distilled water and 0.5% sodium bicarbonate solution (3 times by weight) for 12-h and 18-h periods at room temperature (20°C±2°C). After draining the soaking water and washing, the soaked beans were cooked by boiling for 60 min and autoclaved (121°C, 152 kPa) for 30 min in 4 times water by weight. Beans were dried at 55°C, and finally ground. The traditional treatments of cooking beans are given Chart 1.

Chart 1. Treatments

I	Cooking for 60 min, autoclaving for 30 min without soaking.
II	Soaking in distilled water, for 12 h, and then cooking for 60 min and autoclaving 30 min
	Soaking in distilled water, for 18 h, and then cooking for 60 min and autoclaving 30 min
III	Soaking in sodium bicarbonate solution, for 12 h, and then cooking for 60 min and autoclaving 30 min
	Soaking in sodium bicarbonate solution, for 18 h, and then cooking for 60 min and autoclaving 30 min

Chemical Analyses

Total solids, ash, protein contents, and pH of the samples were determined according to AOAC,14 while the determination of starch amount was done modified polarimetric method.15

Extraction and Identification of Oligosaccharides

Raw and soaked-cooked dry beans were subjected to oligosaccharide extraction with 100 mL (80%) ethanol according to Ref.16. The extracts and washings were combined and concentrated to 100 mL under vacuum at 50°C. The oligosaccharides were separated and identified by thin-layer chromatography,16 while the sugars (raffinose and stachyose) were identified by comparison with the reference standards obtained from Nestec (S.A. Centre de Recherche, Sweden) and sucrose from Fluka (Fluka Chemie AG CH-9470 Buchs).

Quantitative Analysis of Oligosaccharides

Sucrose, raffinose, and stachyose contents of the beans were quantitively determined by using guide-strip technique. Sugar spots on chromatograms were scraped off and extracted with 2 mL of distilled water in a test tube overnight at room temperature. Then oligosaccharides were determined with thiobarbutiric acid reaction with the reference standards containing 10–100 µg/mL of each oligosaccharide.16

Tannin Analyses

Tannins were assayed according to the modified vanillin-HCl method.17 A 2 g sample was extracted with 50 mL 99.9% methanol for 20 min at room temperature with constant agitation. After centrifugation for 10 min at 653× g, 5 mL of vanillin-HCl (2% vanillin 1% HCl) reagent was added to 1 mL aliquots and the color developed after 20 min at room temperature was read at 500 nm. Correction for interference from natural pigments in dry bean was achieved by subjecting the extract to the conditions of the reaction, but without vanillin reagent. A standard curve was prepared using cathechin (Sigma Chemical, St. Louis, MO) after correcting for blank, and tannin concentration was expressed in mg catechin equivalents/100 g.

Phytic Acid Analyses

Phytic acid was extracted in bean samples with 20 mL 0.2 N HCl for 1 hour at room temperature. After filtration on Whatman No. 1, 0.5 mL of extract was pipetted into a test tube fitted with a ground-glass stopper and added 1 mL of ferric solution. The tube was covered with the stopper and fixed with a clip, then heated in a boiling water bath for 30 min. After cooling in ice water for 15 min, it was allowed to adjust to room temperature. Once the tubes have reached room temperature added 2 mL of 2,2-bipyridine solution and mixed the contents. The absorbance was measured after 1 min at 519 nm.18

Results and Discussion

The results of the chemical analyses of the raw Şeker dry bean variety are presented in Table 1. Moisture and protein contents of the bean were 9.61 g/100 g and 20.11 g/100 g respectively, but these values were reported as 8.52 g/100 g and 16.36 g/100 g for the same bean.19 The differences seen in this variety are usually due to the several factors such as ecology, soil quality, cultivation techniques, maturity levels, transportation, and storage conditions.20

Table 1 The results of chemical properties of raw Şeker dry bean variety (g/100 g, dry bases)

Dry matter	90.39
Moisture	9.61
Protein	20.11
Starch	38.21
Ash	4.11
pH	6.45

The sucrose, raffinose, stachyose, phytic acid, and tannins contents of the bean are given in Table 2. Autoclaving brought about a greater reduction in the level of oligosaccharides compared with cooking on the unsoaked samples. Higher losses of oligosaccharides and sucrose have been reported for unsoaked autoclaved legume seeds.9 In contrast, cooking caused more reduction in the level of phytic acid compared with autoclaving. Other workers21 22 have also reported autoclaving causes less loss of phytic acid content compared to cooking in cowpeas. A long time is required for destroying phytates. Greater time interval would cause a pronounced effect on the destruction of phytate.23 Decreases of tannins content of unsoaked, cooked, and autoclaved samples were very high (79%, 85%, respectively). Tannins were not detected as a result of the other treatments. These losses could be the result of, reduced extractability, actual removal, or change in chemical reactivity.24

Table 2 Mean±SD of sugar (g/100 g), phytic acid (mg/g), and tannins (mg eq cat/100 g) contents of Şeker bean variety (dry bases)

Treatment	Sucrose	Raffinose	Stachyose	Phytic acid	Tannins
Raw	3.91±0.06	1.86±0.24	3.84±0.69	15.54±0.45	45.22±0.59
Cooking for 60 min	3.37±0.01 (14)	1.52±0.11 (18)	3.12±0.22 (19)	13.32±0.29 (14)	9.28±0.34 (79)
Autoclaving for 30 min	2.88±0.06 (26)	1.16±0.22 (38)	1.55±0.45 (59)	14.00±0.28 (10)	6.70±0.35 (85)
12-h soaking in distilled water and cooking for 60 min	3.04±0.30 (22)	1.08±0.04 (42)	2.74±0.13 (29)	11.09±0.49 (29)	0.00 (100)
18-h soaking in distilled water and cooking for 60 min	2.73±0.06 (30)	0.69±0.06 (63)	1.93±0.01 (50)	8.82±0.58 (43)	0.00 (100)
12-h soaking in NaHCO3 solution and cooking for 60 min	2.90±0.02 (26)	0.94±0.03 (50)	1.87±0.08 (51)	8.34±0.26 (46)	0.00 (100)
18-h soaking in NaHCO3 solution and cooking for 60 min	2.59±0.11 (34)	0.63±0.03 (66)	1.85±0.08 (52)	6.58±0.32 (58)	0.00 (100)
12-h soaking in distilled water and autoclaving for 30 min	2.86±0.09 (27)	0.86±0.01 (54)	1.62±0.69 (58)	10.54±0.60 (32)	0.00 (100)
18-h soaking in distilled water and autoclaving for 30 min	2.27±0.40 (42)	0.79±0.35 (58)	1.87±0.05 (52)	9.51±0.57 (39)	0.00 (100)
12-h soaking in NaHCO3 solution and autoclaving for 30 min	2.07±0.20 (47)	0.74±0.04 (60)	1.16±0.06 (70)	9.25±0.49 (40)	0.00 (100)
18-h soaking in NaHCO3 solution and autoclaving for 30 min	1.38±0.21 (65)	0.53±0.10 (72)	1.15±0.28 (71)	7.63±0.70 (51)	0.00 (100)

Note: Values in parentheses indicate the percent loss under different treatments.

Cooking and autoclaving the samples after soaking in sodium bicarbonate solution causes more decrease in the sugar and phytic acid contents than that of the samples soaked in distilled water. Similar results were also reported that legumes soaked in alkali medium gave lower sugar content compared to other soaking conditions.9 Soaking in the 0.5% sodium bicarbonate solution may increase softening of the testa and cotyledons, so it can increase the sugars extraction.25 A greater reduction in phytic acid content was attained by soaking beans in 2% NaHCO₃ than distilled water soaking for 12 h.26 The loss of phytic acid during soaking in legumes may be attributed to the activity of phytase and diffussion. When the soaking time increased, the sugar and phytic acid contents of the samples were decreased. Similar results were also reported by some other researchers.23 27

Conclusion

Different cooking and autoclaving methods decreased antinutrients from Şeker bean to various extents. It may be concluded that soaking for 18 h in sodium bicarbonate solution followed by autoclaving causes losses of sucrose, raffinose, stachyose, phytic acid, and tannins to the extent of 65%, 72%, 71%, 51%, and 100%, respectively. Adoption of cost-effective combining processing methods to remove the flatus-producing and antinutritional factors may further enhance the utilization of Şeker dry beans.