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Journal of Food Science and Technology logoLink to Journal of Food Science and Technology
. 2020 Oct 22;58(10):3844–3852. doi: 10.1007/s13197-020-04845-0

Assessment of black gram milling by-product as a potential source of nutrients

Mohammad Hassan Kamani 1, Manchanahally Shivanna Meera 1,✉
PMCID: PMC8357861  PMID: 34471308

Abstract

The present study was carried out to classify and explore the nutrient distribution of black gram milling by-product, with an intention to find value-added applications. The by-product was classified into two fractions, i.e., fraction BRGA (By-product Rich in Germ and Aleurone) and husk and their nutritional profiles were compared to cotyledon and whole seed (naive and germinated). BRGA found to be the richest source of protein (31.38%), minerals (Mg, Na, Fe, Zn and Mn) with appreciable amount of soluble dietary fiber (3.13%). Husk was the richest source of total dietary fiber (79.62%). Furthermore, both by-products were endowed with valuable essential amino acid and fatty acid profiles when compared to cotyledon and whole seeds. Overall, the present study revealed that the black gram by-product is a promising food ingredient that can be processed to obtain fractions rich in protein, fiber, essential fatty acids and minerals, for developing specialty foods for target population.

Keywords: Black gram, By-product, Recovery, Nutrient composition, Milling, Fractionation

Introduction

Milling is typically used for improving the nutritional and cooking qualities of pulses. It involves removal of the seed coat (husk) and splitting seed into two halves (Sreerama et al. 2009, 2010). Commercial milling efficiency for pulses is estimated about 75% and the substantial remainder is accounted as by-product with no commercial value. Recovery of by-product generated from milled pulses is a serious technological challenge for legumes processing industry. This issue has forced millers to seek value-added applications for these waste residues (Tiwari et al. 2011). Exploring milling by-products for valuable components allows industry for better utilization and management. Literature has been revealed the presence of nutraceuticals (Sreerama et al. 2010), thermostable peroxidase enzymes (Ajila and Prasada Rao 2009), bioactive and antioxidant compounds (Girish et al. 2012) in the by-products derived from legume processing, which could benefit pharmaceutical and food industries.

Black gram (Vigna mungo) is mainly used as cotyledon (without husk), since dehusked form has better cooking and nutritional quality (Sreerama et al. 2009). Processing of black gram into dehusked seed is difficult due to the firm attachment of husk to the cotyledon by a vitreous layer of gums and mucilage. Although several attempts have been made to facilitate dehusking process and maximize milling yield using pre-treatments such as enzyme, oil and water, there is still substantial milling loss accounting to about 20–30% of total seeds (Tiwari et al. 2007, 2011; Sreerama et al. 2009). Currently, this by-product is either discarded or sold as livestock feed (Ajila and Prasada Rao 2009). This by-product is composed of a fine mixture of valuable grain components like germ tissue (embryo), aleurone layer and husk, and may be a promising source of macronutrients (e.g., protein and dietary fibers) and minerals (Tiwari et al. 2011; Sreerama et al. 2010). In view of this, the by-product is thought to possess noteworthy nutritional value, which needs to be explored.

A thorough survey of the literature indicated that limited studies have been carried out to reveal the nutritional composition and biological value of by-products in pulses. India is the world’s largest producer of pulses and generates annually ~ 2.5 million tonnes of by-product from milling process. Among the pulses, black gram is one of the major pulses grown and consumed in India, and hence was selected for this study (Tiwari et al. 2011). In the continuation of our previous work (Kamani et al. 2019), where we established the fractionation of black gram milled by-products, the present study was further planned to comparatively evaluate the nutritional profile of these by-products with cotyledon and whole black gram (native and germinated).

Materials and methods

Chemicals and reagents

Thermostable α-amylase, pepsin, pancreatin, amyloglucosidase, anhydrous dextrose, BF3-methanol were purchased from Sigma–Aldrich Chemical Co. (St. Louis, MO, USA). Ethanol and methanol were obtained from Merck Co. (Darmstadt, Germany) and tris base and sodium phosphate dibasic were purchased from Loba Chemie Co. (Mumbai, India). Sodium hydroxide, sodium chloride, heptane, sodium acetate anhydrous and 3, 5-Dinitrosalicylic acid were purchased from Sisco Research Laboratory (Mumbai, India).

Sample preparation

Black gram (50 kg) were first cleaned and pre-treated by tempering with 10% (w/w) water overnight, followed by drying at 50 °C for 15 h (Tiwari et al. 2007). The samples were then milled and fractionated according to our established method described in the recent study (Kamani et al. 2019), to obtain the different fractions viz., husk (seed coat), cotyledon (dhal), and a BRGA (By-product Rich in Germ and Aleurone). To compare the quality of milled by-products with germinated and native samples, these samples were also prepared as described by Kamani et al. (2019). All samples were then pulverized to pass through 60 mesh B.S. Sieve and stored for the subsequent analyses.

Proximate composition

The moisture, crude fat, crude protein and ash contents were determined according to the AOAC method (2005). The conversion factor used for protein was 6.25. The total carbohydrate content was calculated by weight difference as described by Sreerama et al. (2010).

Determination of soluble, insoluble and total dietary fiber contents

The soluble, insoluble and total dietary fiber contents were measured by the enzymatic–gravimetric method according to the protocol described by Girish et al. (2012). Briefly, the defatted sample was dispersed in 20 mL of sodium phosphate buffer (0.1 M, pH 6.0) and was treated with 100 μL thermo-stable α-amylase at (95–100 °C for 35 min), followed by enzymatic digestion with 100 mg pepsin (pH 1.5) and pancreatin (pH 6.8) at 40 °C for 60 min. The residue, as IDF, was recovered using a filtration unit (VELP® Scientifica CSF 6; Usmate, Italy) and dried at 105 °C overnight and the weights were measured. The filtrate obtained was precipitated with four volumes of 95% ethanol and filtered, dried overnight at 105 °C and incinerated at 550 °C for 10 h to obtain SDF. TDF was then calculated as the sum of values of SDF and IDF.

Determination of total starch content

The total starch content was determined using spectrophotometry method according to the protocol of Holm et al. (1986) with slight modifications. The defatted sample (100 mg) was suspended in 15 mL of distilled water and digested with 100 μL thermo-stable α-amylase (Termamyl) by incubating in boiling water bath for 15 min, followed by cooling. Afterwards, the digested sample was mixed with 15 mL 0.05 M acetate buffer (pH 4.8) and 10 mg amyloglucosidase and incubated at 60 °C for 30 min. Subsequently, the sample was diluted to 100 mL with distilled water and filtered using a filter paper (Whatman No. 1). A portion of filtrate (2.0 mL) was mixed with 2.0 mL 3, 5-Dinitrosalicylic acid (DNS reagent) and incubated at 100 °C for 5 min, followed by dilution with 16 mL distilled water and mixed well. The absorption was determined at 540 nm using a Hitachi spectrophotometer (model U-2900, Hitachi Co., Japan) against the blank sample and the total starch was calculated using a glucose standard curve (R2 = 0.999).

Amino acid profile

The amino acid profile of black gram fractions was analyzed by a SYKAM Amino Acid Analyzer S433 system (SYKAM GmbH, Eresing, Germany) using the method of post-column ninhydrin derivatization (Kanwate et al. 2019) with slight modifications. The defatted dried samples were accurately weighed. To this, 3 mL of 6 N HCl and 0.1% phenol was added and mixed and incubated at 100 °C for 24 h. Afterwards, the solution was filtered and 200 μL of each sample was taken and dried for 2 h by a speed vacuum concentrator. Then 1 mL of sample diluting buffer was added to the dried sample and passed through a 0.22 µm syringe filter before subjecting to amino acid analysis. The samples were analyzed for amino acid composition by separation on a LCA K06/Na cationic column (150 × 4.6 mm), followed by post-column derivatization with ninhydrin at 130 °C using the amino acid analyzer S433. The areas under the peaks corresponding to the amounts of amino acids were compared to the authentic standards. The amino acid composition was expressed as percentage of total amino acids content of each sample.

Fatty acid profile

The fatty acid profile was determined on extracted fat using gas chromatography according to the protocol of AOCS Ce 1b-89 (2005). The individual fatty acid methyl ester was identified and quantified using a Supelco 37 component F.A.M.E. mix standard based on the peak retention time and expressed in percentage.

Mineral profile

The mineral profile of flour fractions were analyzed using the method of Spiteri and Attard (2017). For this purpose, the ash of each sample was prepared and subjected to the digestion and further dilution with the 5% nitric acid. The digested samples were then analyzed by a microwave plasma-atomic spectroscopy (Model 4210 MP-AES, Agilent Co., USA). Each mineral (Mg, Mn, Na, Cu, Ca, K, Zn and Fe) was detected at its individual wavelength and calculated with reference to the standard curve and dilution factor. The concentration of each element was express in mg/100 g of sample.

Scanning electron microscopy

The micrographs of the flours were recorded using a Leo scanning electron microscope (model 435 VP, Leo electron microscopy Ltd., UK). The samples were adhered on the sample holder and sputter-coated with gold (1 min, 20 mbar). The images were viewed at 10–15 kV.

Statistical analysis

Data were subjected to analysis of variance (ANOVA) using the SPSS™ software (version 16, SPSS Inc., Chicago, USA). Differences in mean values were tested using Duncan's test at 0.05 significance level. All measurements were carried out three times and the results were presented as mean ± SD.

Results and discussion

Nutrient composition of fractions obtained from milled samples

Milling is typically used to improve the nutritional profile and cooking quality of pulses (Sreerama et al. 2009). After milling, the whole seed was divided into three parts, namely cotyledon, husk and fraction BRGA (a mixture of flour containing fine particles of husk, aleurone and germ tissue). Table 1 shows the proximate composition of fractions obtained from milled black gram. The results indicated that the nutrient composition of whole seed did not significantly change after dehulling (p > 0.05). Similar results reported for the fat and protein contents of dehulled proso millet by Bagdi et al. (2011). Change in nutrients during milling process might be dependent on the type of seed and pre-milling conditions, wherein the seeds are conditioned with water, oil, chemical or enzyme (Rout et al., 2007; Tiwari et al. 2011). No significant difference was observed in the moisture contents among all groups except for BRGA (p < 0.05), but considerable variation was recorded in protein contents. The crude protein was found to be 11.32% for husk fraction, which was higher than the protein reported for chickpea husk (Bose and Shams-Ud-Din 2010). Cotyledon, whole and germinated seed showed higher level of protein contents as compared to husk. BRGA recorded the highest amount of protein (31.38%). This might be due to the presence of small particles of germ tissue (the major storehouse of protein in seed), which increase the protein content in this fraction (Girish et al. 2012; Kadam et al., 1985). The crude fat was found to be 1.68% in BRGA, followed by whole seeds (native and germinated), while it was the least in the husk (0.59%). The present results indicated that fraction BRGA is the main reserve for protein, making it suitable as protein enricher in weight restriction diets. Total carbohydrate was found to be the dominant composition (74.25%) in the husk fraction amongst all groups. This is comparable with the total carbohydrates reported by Sreerama et al. (2010) for chickpea (83.9%) and horse gram (82.6%) husk. Since the husk carbohydrate is the concentrated source of fiber (Tiwari et al. 2011), this fraction might be useful in food formulations needing high fiber content to improve gastrointestinal health (Sreerama et al. 2010). Germination did not significantly (p > 0.05) change the nutrient values of the whole seed. The proximate composition measured in the present study is in agreement with the previous report on different cultivars of black gram (Wani et al. 2013). According to this study, the protein, fat, ash, carbohydrate and moisture contents in whole black gram seed ranged between 24.5–28.4%, 1.1–1.4%, 2.7–3.3%, 54.1–56.5% and 10.3–11.6%, respectively. Although there are reports for nutrient composition of black gram whole seed, to our knowledge, the number of reports on the nutrient composition of milled by-product viz., BRGA and husk is scarce.

Table 1.

Nutrient composition of black gram and its milled fractions (%)

Sample Whole Germinated Cotyledon Husk BRGAa
Moisture 11.05 ± 1.07b 10.61 ± 0.1b 11.27 ± 0.42b 10.56 ± 0.11b 9.69 ± 0.12a
Protein 27.75 ± 0.86b 29.29 ± 2.64bc 26.43 ± 0.07b 11.32 ± 1.12a 31.38 ± 1.07c
Fat 1.1 ± 0.14bc 1.23 ± 0.23c 0.87 ± 0.11b 0.59 ± 0.01a 1.68 ± 0.1d
Ash 3.68 ± 0.35bc 3.72 ± 0.2bc 3.58 ± 0.19b 2.96 ± 0.08a 4.02 ± 0.19c
Carbohydrate 56.3 ± 1.42bc 55 ± 2.02ab 57.74 ± 0.48c 74.25 ± 0.74d 53.04 ± 1.02a
Dietary fiber
Soluble 1.74 ± 0.81a 2.04 ± 0.54a 2.8 ± 0.26a 2.57 ± 0.16a 3.13 ± 0.89a
Insoluble 34.48 ± 0.23b 23.85 ± 1.42a 21.63 ± 1.88a 77.04 ± 0.37c 36.29 ± 0.53b
Total 36.23 ± 1.04b 25.9 ± 0.87a 24.43 ± 2.14a 79.62 ± 0.28d 39.42 ± 0.36c
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Different superscripts within each row are significantly different at p < 0.05

aBRGA, By-product Rich in Germ and Aleurone

Dietary fiber content

Dietary fibers present various beneficial physiological effects in reducing the risk of heart disease, obesity, diabetes and some forms of cancer (Girish et al. 2012). As presented in Table 1, the total dietary fiber (TDF) content in different fractions varied from 24.43% (for cotyledon) to 79.62% (for husk). These values were almost comparable with the values reported by Naidu et al. (2011) for husk and cotyledon of fenugreek after milling process. TDF in whole seed of black gram was found to be 36.23%, which was higher than the TDF reported for chickpeas, green gram and lentils legumes, but lesser than soy bean (Tiwari et al. 2011; Wang et al. 2009). Germination reduced TDF significantly in whole seed (p < 0.05). This is in line with the findings of Chitra et al. (1996), who reported TDF for germinated chickpea, pigeon pea, and mung bean and attributed the decline to increase in alpha-galactosidase activity. Insoluble dietary fiber (IDF) in different fractions varied from 21.63 to 77.04%, while soluble dietary fiber (SDF) was found to be much lower ranging from 1.74 to 3.13%. Comparison between SDF and IDF contents indicated that the SDF constituted a small portion of total dietary fiber in all milled fractions, which was comparable to the ratio reported by Sreerama et al. (2010) for milled chickpea and horse gram. IDF includes cellulose, some hemicelluloses and lignin, whereas SDF includes natural gel-forming fibers like gums, pectins, mucilages, some hemicelluloses and storage polysaccharides. In terms of health benefits, both IDF and SDF complement each other, and each one exerts specific physiological effects (Tiwari et al. 2011). In view of the obtained results, it is quite evident that the by-product fractions derived from black gram are the rich source of dietary fibers and can be efficiently used to formulate fiber-fortified foods for diet management.

Starch content

Starch is the principal component of total carbohydrate in almost all beans and mainly composed of amylose and amylopectin (Tiwari et al. 2011). According to Fig. 1, the starch content ranged from 8.26 to 40.32% in different fractions. Starch content in cotyledon was significantly greater than the other groups (p < 0.05), representing this fraction as the main storage source of starch. The whole seed showed 33.06% starch content, followed by germinated seed, however was non-significant. The literature reported a range from 32.3 to 47.9% for starch content of black gram (Nwokolo and Smartt 2012). No significant reduction observed as a consequence of germination (p > 0.05). The husk showed the minimum starch content (8.26%), implicating that non-starch polysaccharides such as cellulose and hemicellulose are the predominant carbohydrates (Arulnathan et al. 2013). To our knowledge, no study has yet reported the starch content of husk/seed coat in any pulse; however the present value for husk starch, was comparable to the starch content reported for wheat husk (Bledzki et al. 2010). The present results also revealed that the main by-product-BRGA contained a considerable level of starch (19.7%); therefore, from technological viewpoint, this by-product can be a valuable ingredient for improving textural quality of emulsion-type foods.

Fig. 1.

Fig. 1

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The starch content of different fractions of milled black gram. BRGA: By-product Rich in Germ and Aleurone

Amino acid profile

Data presented in Table 2 represents the presence of 16 amino acids (8 essential, 4 conditionally essential and 4 nonessential) among the 22 amino acids found in nature. The most abundant amino acids were Glx, and Asx in all the fractions. The percentage of essential amino acids was found to be higher than the non-essential group, indicating that black gram might be a suitable source to meet proteinous requirements of humans. The present results are in agreement with the previous findings on the amino acid profile of whole black gram (Zia-Ul-Haq et al. 2014). Table 2 indicates no significant change (p > 0.05) between the amino acid profile of cotyledon and whole seed, except a slight difference in Cys. It implies the negligible impact of milling on amino acids composition of black gram seed. Germination slightly decreased Met, Lys and Glx, which is partly in conformity with El-Adawy (2002) in germinated chickpea seed. From the nutritional viewpoint, the most important aspect of a protein, is its essential amino acids since human body cannot synthesize them and must be provided through the diet (Tiwari et al. 2011). BRGA, the richest protein by-product, had a comparable essential amino acid composition to the whole seed. Husk fraction, the low-in-protein by-product, also showed a suitable profile with respect to the essential and conditionally essential amino acids. Therefore, the amino acid profiles of these by-products are appreciable and can be beneficial for food fortification in order to overcome the issue of essential amino acid deficiency. However, other parameters such as protein digestibility corrected amino acid score (PDCAAS) needs to be further examined to fully assess the protein quality of these fractions. In addition, these by-products contain considerable hydrophilic amino acids (~ 61%), which from a functional standpoint, is helpful to utilize in food dispersion system.

Table 2.

Amino acid composition (%) of defatted black gram and its milled fractions

Amino acid Whole Germinated Cotyledon Husk BRGAa
Essential
Met 2.03 ± 0.06b 1.84 ± 0.04a 2.09 ± 0.08b 2.78 ± 0.07c 2.21 ± 0.06b
Val 4.82 ± 0.03bc 4.79 ± 0.06b 4.89 ± 0.07bc 4.96 ± 0.04c 4.65 ± 0.05a
Ile 4.57 ± 0.06b 4.6 ± 0.05b 4.59 ± 0.05b 4.37 ± 0.14ab 4.31 ± 0.12a
Leu 8.76 ± 0.16b 8.7 ± 0.39b 8.86 ± 0.26b 7.81 ± 0.24a 8.37 ± 0.4ab
Phe 7.45 ± 0.23b 7.48 ± 0.04b 7.55 ± 0.16b 6.58 ± 0.19a 6.86 ± 0.04a
His 4.57 ± 0.12a 4.42 ± 0.05a 4.67 ± 0.01a 9.53 ± 0.33b 4.62 ± 0.08a
Lys 8.49 ± 0.04b 7.56 ± 0.14a 8.1 ± 0.07b 8.52 ± 0.29b 8.46 ± 0.28b
Thr 3.23 ± 0.02a 3.12 ± 0.07a 3.20 ± 0.02a 3.64 ± 0.33b 3.43 ± 0.02ab
Total 43.92 42.51 43.95 48.19 42.91
Conditionally essential
Tyr 4.75 ± 0.05a 4.96 ± 0.37ab 4.73 ± 0.08a 6.25 ± 0.12c 5.39 ± 0.32b
Cys Trace 0.55 ± 0.03a 0.62 ± 0.03a Trace 0.74 ± 0.06b
Arg 7.98 ± 1.07a 8.4 ± 0.21a 8.51 ± 0.06a 8.01 ± 1.06a 8.89 ± 0.02a
Gly 2.8 ± 0.53a 2.85 ± 0.04a 2.24 ± 0.06a 4.49 ± 0.07b 4.34 ± 0.42b
Total 15.53 16.76 16.1 18.75 19.45
Nonessential
Asx 13.43 ± 0.03c 15.62 ± 0.17d 13.52 ± 0.05c 11.63 ± 0.04a 12.81 ± 0.03b
Ser 4.3 ± 0.28a 4.26 ± 0.09a 4.07 ± 0.07a 4.29 ± 0.07a 4.1 ± 0.09a
Ala 2.92 ± 0.07a 3.08 ± 0.06abc 3.04 ± 0.12ab 3.27 ± 0.09c 3.25 ± 0.05bc
Glx 19.5 ± 0.07c 18.21 ± 0.26b 19.63 ± 0.34c 15.44 ± 0.06a 18.02 ± 0.09b
Total 40.15 41.17 40.26 34.63 38.18
Amino acid with different characteristics
Hydrophobic 35.3 35.45 35.75 36.02 35.04
Hydrophilic 61.5 62.14 62.32 61.06 61.07
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Different superscripts within each row are significantly different at p < 0.05. Asx: Asp + Asn; Glx: Glu + Gln. Hydrophobic: The sum of aliphatic (Ala, Ile, Leu, Met and Val) and aromatic (Phe and Tyr) amino acids groups; Hydrophilic: Ser + Thr + Cys + Asx + Glx + Arg + His + Lys (Damodaran and Parkin, 2017)

aBRGA, By-product Rich in Germ and Aleurone

Fatty acid profile

Fatty acids play a significant role in human diet. The GC profile, represented in Table 3, shows linolenic acid (C18:3), linoleic acid (C18:2), oleic acid (C18:1) and palmitic acid (C16:0) as major fatty acids in black gram fractions. Among of, linolenic acid was recorded as the most abundant fatty acid. A comparable profile has been previously reported by Zia-Ul-Haq et al. (2014), who detected palmitic, oleic, linoleic, linolenic acids as principal fatty acids in different varieties of black gram. Apart from the mentioned major fatty acids, myristic (C14:0), pentadecanoic (C15:0), arachidic (C20:0), behenic (C22:0), lignoceric (C24:0), cis-10-heptadecenoic (C17:1), and erucic acids (C22:1) were also detected in small quantities. Table 3 also indicates that the amounts of unsaturated fatty acids were much higher than total saturated groups, implicating the suitability of this pulse for nutritional applications. The results showed that milling significantly increased the linolenic acid (from 57.16 to 64.44%) and also caused a slight decline in palmitic acid (p < 0.05). However, in sharp contrast to milling process, germination showed a vice versa effect in terms of these fatty acids. BRGA, the main by-product fraction, contained only 20% saturated acids and its dominant part was unsaturated with the majority of PUFAs (56.59%). Husk, the second by-product, also had suitable fatty acid profile with respect to MUFAs and PUFAs. It is generally well known that the intake of unsaturated fatty acids helps to maintain good health. On the other hand, black gram lipids, in particular, were shown to positively have cholesterol-reducing effect (Devi and Kurup 1972). Therefore, this by-product may be regarded as a potential source of health-promoting ingredient for human nutrition.

Table 3.

Fatty acid composition (%) of the oil extracted from black gram and its milled fractions

Fatty acids Whole Germinated Cotyledon Husk BRGAa
Saturated acids (SFA)
Myristic acid (C14:0) 0.13 ± 0.05a 0.26 ± 0.02b 0.13 ± 0.01a 0.32 ± 0.02b 0.17 ± 0.03a
Pentadecanoic acid (C15:0) 0.16 ± 0.02a 0.3 ± 0.02b 0.14 ± 0.01a 0.34 ± 0.01b 0.17 ± 0.01a
Palmitic acid (C16:0) 15.45 ± 0.6b 23.42 ± 0.11d 13.13 ± 0.08a 18.61 ± 0.46c 16.51 ± 0.81b
Arachidic acid (C20:0) 0.77 ± 0.06b 0.97 ± 0.03c 0.48 ± 0.03a 1.95 ± 0.08d 1.83 ± 0.07d
Behenic acid (C22:0) 0.52 ± 0.06a 0.87 ± 0.07ab 0.29 ± 0.01a 1.76 ± 0.13b 0.91 ± 0.85ab
Lignoceric acid (C24:0) 0.46 ± 0.17ab 0.69 ± 0.07bc 0.21 ± 0.01a 1.29 ± 0.07d 0.73 ± 0.06c
Total 17.49 26.51 14.38 24.27 20.32
Monounsaturated acids (MUFA)
Cis-10-heptadecenoic acid (C17:1) 0.35 ± 0.04a 0.71 ± 0.01b 0.28 ± 0.02a 0.6 ± 0.05b 0.4 ± 0.06a
Oleic acid (C18:1) 17.96 ± 2.56abc 12.94 ± 0.11a 15.73 ± 0.03ab 24.01 ± 2.96c 22.12 ± 3.72bc
Erucic acid (C22:1) 0.28 ± 0.04a 0.24 ± 0.01a 0.22 ± 0.01a 0.5 ± 0.09b 0.32 ± 0.01a
Total 18.59 13.89 16.23 25.11 22.84
Polyunsaturated acids (PUFA)
Linoleic acid (C18:2) 6.57 ± 0.83b 13.72 ± 0.12c 4.81 ± 0.51a 23.23 ± 0.1d 26.72 ± 0.57e
Linolenic acid (C18:3) 57.16 ± 2.53d 45.69 ± 0.1c 64.44 ± 0.42e 26.34 ± 0.8a 29.87 ± 1.44b
Total 63.73 59.41 69.25 49.57 56.59
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Different superscripts within each row are significantly different at p < 0.05

a BRGA, By-product Rich in Germ and Aleurone

Mineral profile

Table 4 shows the mineral contents in different milled fractions of black gram. The whole seed was found to range from 67.89 to 659.32 mg/100 g for the major minerals (Ca, Mg, K and Na) and from 0.43 to 7.2 mg/100 g for the trace minerals (Fe, Zn, Cu and Mn). The results for whole seed are somewhat comparable with the values reported in USDA database (2019) in terms of type of detected minerals, however, considerable variations existed with respect to their concentrations. This variability is mainly due to various environmental factors like soil condition, level of fertilizer and type of processing at post-harvest stage (Arulnathan et al. 2013; Hall et al., 2017; Damodaran and Parkin 2017). After removing the husk from whole seed, a significant decrease was observed in the contents of Ca, Mg, Na and Fe in cotyledon, whereas the other elements did not show any change. Similarly, Wang et al. (2009) reported a reduction in Ca, Mg and Fe as a consequence of dehulling of lentils. Germination significantly increased K, Zn, Cu and Mn (p < 0.05), while the other elements remained with no change except for Na, which decreased. The results revealed that the BRGA had the highest concentration of Mg (157.54 mg/100 g), Na (89.26 mg/100 g), Fe (10.26 mg/100 g) and Zn (3.48 mg/100 g) when compared to other fractions. In addition, this by-product was a good source of Ca (73.09 mg/100 g), K (650.08 mg/100 g), Cu (0.95 mg/100 g) and Mn (1.58 mg/100 g). Husk was also found to be good source of both major and trace minerals. According to the U.S. Food and Drug Administration (FDA), Ca, Mg and Na have vital effects on bone formation, nervous system, blood pressure regulation and hormone secretion in human body. Fe, Zn, Cu and Mn are also linked with essential biological functions like wound healing, energy production and growth (FDA 2018). Therefore, in view of high concentration of minerals in BRGA and husk fractions, these by-products have promising potential to be utilized for fortification purposes in order to overcome the prevalence of minerals deficiency in food products.

Table 4.

Minerals contents of black gram and its different milled fractions (mg/100 g)

Element/sample Whole Germinated Cotyledon Husk BRGAa
Ca 86.58 ± 4.4c 75.95 ± 3.09bc 27.13 ± 2.95a 66.21 ± 4.4b 73.09 ± 4.83b
Mg 123.01 ± 3.08b 130.02 ± 3.58b 91.10 ± 3.07a 138.36 ± 1.32b 157.54 ± 13.85c
K 659.32 ± 1.39b 811.41 ± 9.72c 683.76 ± 18.02b 496.93 ± 15.46a 650.08 ± 16.49b
Na 67.89 ± 6.16d 25.31 ± 2.38b 10.41 ± 0.29a 48 ± 3.67c 89.26 ± 8.35e
Fe 7.2 ± 2.14bc 4.38 ± 1.78ab 2.59 ± 0.15a 8.38 ± 2.1bc 10.26 ± 0.43c
Zn 1.37 ± 0.17a 2.36 ± 0.15b 1.64 ± 0.1a 1.48 ± 0.22a 3.48 ± 0.35c
Cu 0.43 ± 0.05a 0.77 ± 0.1b 0.44 ± 0.1a 0.89 ± 0.1b 0.95 ± 0.19b
Mn 0.87 ± 0.1a 1.33 ± 0.19b 0.89 ± 0.07a 0.92 ± 0.1a 1.58 ± 0.15b
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Different superscripts within each row are significantly different at p < 0.05

aBRGA, By-product Rich in Germ and Aleurone

Microstructure of black gram fractions flours

The morphological structures of different milled fractions of black gram are shown in Fig. 2a-e. It can be observed that the starch granules associated with protein and other non-protein components. The micrographs displayed that the starch granules had various dimensions and shapes, mainly spherical/oval and round shapes. This structure reported earlier for black gram by Wani et al. (2013). The micrograph in whole and cotyledon samples mainly showed the presence of intact starch granules, however, in BRGA and husk fractions, the aggregation of these granules with other components like fiber is more visible. It has been reported that the size of starch granule in black gram ranged between 12.8 to 14.3 μm (Singh et al. 2004), which is relatively in line with the present micrographs. The SEM image of the germinated sample exhibited a slightly eroded and rougher surfaces for starch granules (Fig. 2e), which might be due to degrading the granules resulting from increase of α-amylase activity after germination (Simsek et al. 2014). Figure 2c represents that the starch granules in husk are entirely embedded with protein matrix and many non-uniform sized and irregular fragments. This may be due to the overlapping dense fibers with the starch granules in the husk surface, which formed such unique structure.

Fig. 2.

Fig. 2

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Scanning electron micrographs of a whole seed; b cotyledon; c husk; d fraction BRGA; e germinated seed. BRGA, By-product Rich in Germ and Aleurone

Conclusion

Black gram is widely used in the form of cotyledon (without husk) for various food formulations. During milling of whole seed into cotyledon, considerable amounts of by-product produced with no commerial use. This study reveals that this by-product namely, fraction BRGA and husk are good sources of valuable compounds and have relatively equal nutritional level with cotyledon and whole seed. Of the different fractions, BRGA, as major by-product, showed high protein and starch contents, whereas husk fraction was rich in dietary fibers. Further, the results indicated that the by-products are suitable sources of essential amino acids, fatty acids and minerals. Therefore, these fractions may be used as a new promising source to balance foods according to nutritional requirements with desired characteristics. Lack of knowledge for the utilization of waste residue of pulse processing is a serious concern for processers. Hence, the results of present study might be a response to this issue and would add value to pulse milling by-products.

Acknowledgments

The authors are thankful to the Director CSIR-CFTRI, Mysore for providing necessary facilities. The first author also wishes to thank the Council of Scientific and Industrial Research (CSIR), India and the Third World Academy of Sciences (TWAS), Italy for the award of CSIR-TWAS Postgraduate Fellowship.

Compliance with ethical standard

Conflict of interests

The authors declare that they have no conflict of interest.

Footnotes

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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