Introduction

Legumes, including beans and chickpeas, are rich sources of complex carbohydrates, protein, vitamins and minerals. In the world, chickpea (Cicer arietinum L.) is the 4th largest grain legume crop over the total grain production (FAO STAT 2005). They have some benefits like lower glysemic index for people with diabetes, increased satiation, cancer prevention and protection against cardiovascular diseases due to their dietary fiber content (Hangen and Bennink 2002).

Bulgur is a valuable cereal product and a very famous industrially processed ancient wheat product. And also bulgur is a very important pre-cooked wheat product due to its storability, high nutritional value, ease preparation and low cost (Bayram et al. 2004). Bulgur is usually produced from different cereals by soaking, cooking, dehulling, drying and grinding (Bayram et al. 2004). Generally, durum wheat is preferred for bulgur production, but some researchers choosed to use oats, corn, triticale, barley, rye and soybean instead of durum wheat (Singh and Dodda 1979; Elgün et al. 1990; Köksel et al. 1999; Bayram et al. 2004). Soaking, cooking, dehulling and the other food processing methods not only improves flavor of legumes but also increases the bioavailability of nutrients by destroying antinutritional factors, and improves the protein digestibility (Martin-Cabrejas et al. 2004).

Before the cooking process, soaking is a preliminary step, it causes soften texture and decreases the cooking time. Soaking is widely applied at household and industrial scale. Soaking could decrease the soluble antinutrients which can be eliminated with the discarded soaking solution (Abdus Sattar et al. 1989). Researchers developed the methods to decrease the cooking time of legumes have largely depended on soaking prior to cooking (Pujola et al. 2007; Shimelis and Rakshit 2007).

There is only one published research on chickpea bulgur, and therefore the objectives of this study are to introduce the processing treatments of chickpea bulgur and to investigate the effects of the soaking time and soaking water pH on chemical and nutritional properties of chickpea bulgur.

Materials and methods

Materials

Seeds of Gökçe chickpea variety were obtained from Sarayönü Vocational School of Higher Education in Konya, Turkey.

Preparation of bulgurs

Chickpea samples free from cracks, dust and other foreign materials were soaked at different soaking time (2, 8 and 12 h) and different soaking medium (soaking water pH 4, 6 and 8). The seed-to solution ratio was 1:5 (w/v). Soaking water was prepared with 0.01 N NaOH and 0.01 N HCl solutions. After soaking, the water was drained off and chickpea seeds cooked at 121 °C, 20 min in autoclave (1:2 w/v) (Hirayama AT-HVA-85, Saitama Japan). The cooked materials were dried at 50 ± 5 °C to 10% moisture content in a dryer (Nüve FN-500, Ankara, Turkey). The dried seeds were conditioned with 2% additional water by mixing for 10 min, milled into course grist on the disk mill (Bastak, Ankara, Turkey) so that all the material passed though a 3.55 mm sieve and 1.6 mm sieve, and aspirated to remove bran material.

Physical analysis

One thousand seed weight, 1000 seed volume were determined according to Williams et al. (1983) with some modifcations. Seed weight was calculated as the mean weight of 1000 undamaged chickpea seeds. For the determination of seed volume, seeds were transferred to a 250 ml measuring cylinder, and 100 ml distilled water was added. Seed volume was determined as total volume minus 100. Samples were analyzed for density, using the following formula;

$$ Density = seed\;volume/seed\;weight $$

A digital micrometer (0.001 mm, Mitutoyo, inoto-Ku, Tokyo, Japan) was used to measure three dimensions (length, width and thickness) of the chickpea seeds. The sphericity and diameter ratio were found using the formula by Mohsenin (1968).

$$ Sphericity = geometric\;mean\;diameter/major\;diameter = {\left( {L \times W \times T} \right)^{{1/3}}}/L $$
$$ Diameter\;ratio = length/width = L/W $$

Color measurement was performed using a Minolta Chroma Meter CR-400 (Minolta, Osaka, Japan). The L*, a* and b* color measurements were determined according to the CIE Lab color space system, where L* corresponds to light/dark chromaticity (changing from 0% dark to 100% light), a* to green/red chromaticity (changing from −60% green to 60% red) and b* to blue/yellow chromaticity (changing from −60% blue to 60% yellow). The instrument was calibrated with a white reference tile (L* = 97.10, a* = −4.88, b* = 7.04) before the measurements.

Chemical analysis

The AACC methods were used for the determination of moisture (method 44–19), crude ash (method 08–01), crude fat (method 30–25), crude fiber (method 32–10), and crude protein (method 46–12) contents of the raw material and bulgur samples (ACCC 1990). Starch contents were determined by using the polarimeter (Elgün et al. 2005). Gross energy (kcal) was estimated by multiplying the percentages of crude protein, lipid and carbonhydrates by the factors 4, 9 and 4, respectively.

In the analysis of minerals, a microwave system was used for acid digestion of all the samples. The samples were dried in a forced stove until dry weight. For the determination of mineral concentrations, the samples were preliminarily digested by means of a closed pressurized system microwave oven, using MARS 5 CEM Corporation. Within 15 min, completely clear and colourless solutions were obtained which were subsequently diluted with double-distilled water. Samples were prepared in triplicate runs (Anonymous 1998). Total mineral and HCl extractable mineral concentrations were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) (VARIAN-CCD Simultaneous ICP-AES, Australia) with an automatic sampler system. HCl extractabilities of the minerals were carried out according to Saharan et al. (2001). HCl extractable minerals and HCl extractable ash were extracted in 0.03 N HCl for 3 h at 40 °C and digested in diacid mixture.

Trypsin inhibitor activity was essentially determined according to Kakade et al. (1974). TIA (trypsin inhibitor activity) was calculated as units/mg sample, and one TI unit was defined as an increase in absorbance of 0.01 absorbance unit at 410 nm in 10 min by the reaction mixture of volume 10 ml.

Phytic acid content was determined by the method of Haugh and Lantzsch (1983). In-vitro protein digestibility (IVPD) was determined by the methods given by Bookwalter et al. (1987). The total phenolic content was determined by the Folin-Ciocalteu method using a Shimadzu UV 160A spectrophotometer.

Statistical analysis

A commercial software program (MINITAB and MSTAT) was used to perform statistical analyses. Data were assessed by analysis of variance. Duncan’s multiple-range test was used to separate means. Significance was accepted at P < 0.05 throughout the analysis. The data reported in all the tables are an average of triplicate observations.

Results and discussion

Analytical results

The average 1000 kernel weight, 1000 kernel volume, density, lenght, width, thickness, sphericity, diameter ratio, L*, a* and b* values were found as 417.12 g, 328.38 ml, 1.27 g/ml, 9.72 mm, 7.80 mm, 7.62 mm, 0.857, 1.247, 51.56, 6.03, 15.65, respectively. The sphericity values were higher than the findings of Zia-Ul-Haq et al. (2007); Kaur et al. (2005). The density values were similar with reported by Zia-Ul-Haq et al. (2007) and Kaur et al. (2005).

Chemical properties of chickpea bulgurs

The effects of bulgur processes as soaking time and soaking water pH on the chemical properties on chickpea samples are shown in Table 1 (A). Soaking time significantly (p < 0.05) reduced the ash content between 32.7 and 37.8% when compared with the raw chickpea seed. Similar results have been reported by some researchers who showed that soaking and cooking reduce the ash content because of the certain minerals diffusion into the soaking and cooking water (Köksel et al. 1999; Wang et al. 2010). The lowest ash content was obtained with soaked in pH 8 soaking water. The protein content tends to rise, while the starch, crude fiber and crude fat tend to fall compared to raw material. This is in agreement with data given by Wang et al. (2010) but contradict the results of Habiba (2002). The starch content was found to decrease from 49.2 to 46.93% with increase in soaking time from 2 to 12 h. Energy values of chickpea seeds decreased with bulgur process.

Table 1 Changes in chemical and nutritional composition of chickpea bulgur samples due to soaking treatments
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Nutritional properties of chickpea bulgur samples

Table 1(b) summarises the nutritional properties of chickpea bulgurs. IVPD values were significantly affected by soaking treatments and increased with bulgur processes which include soaking, cooking and dehulling. The results agree with those of previous studies, Shimelis and Rakshit (2007) and Nestares et al. (1996), who reported that soaking and cooking increased the IVPD of beans and chickpeas. Soaking time had a significant effect on TPC (total phenolic content) (p < 0.01), phytic acid (p < 0.01), HCl-extractable ash (p < 0.01), and soaking water pH had a significant effect on TPC (p < 0.01) and HCl-extractable ash (p < 0.01). The average trypsin inhibitor activity value in chickpea seeds was 8.27 mg/kg. TIA was completely eliminated after bulgur process. The reduction in TIA in chickpea bulgurs would be due to destruction of protease inhibitors during pressure cooking. Phytic acid content decreased by 47.4 to 55.71% with increase in soaking time from 2 to 12 h. Changing soaking water pH did not affect the phytic acid content of the samples significantly. Bilgiçli (2009) reported that phytic acid reduction ratios from raw seed to bulgur were 25.2–32.0% for common bean bulgur and 31.2–39.5% for chickpea bulgur, respectively. The lowest TPC content was determinated in the chickpea bulgur samples with soaked at 8 h and soaked in pH 8 soaking water. This is in agreement with data given by Nergiz and Gökgöz (2007), but contradicts the result of Siddhuraju et al. (2002), who reported that total phenolic content of Sesbania ssp and Vigna radiata seeds increased by the soaking process. The HCl-extractability of ash content of chickpea bulgur samples was increased by bulgur process. The highest HCl-extractability of ash content was measured in the bulgur samples with soaked pH 6 soaking water. Figure 1 points at the effect of soaking time and soaking water pH on the HCl-extractability of ash content. In pH 8 soaking water, HCl-extractability of ash content significantly increased between 2 and 8 h soaking time because of the destroying the antinutritional factors such as phytic acid. As observed by Habiba (2002), cooking resulted in decreasing total and HCL-extractablity of ash in peas.

Fig. 1
figure 1

Effect of soaking time and soaking water pH on the HCl-extractability of ash content (g/100 g) (n = 3)

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Total mineral content and HCl-extractability of minerals

Total mineral content and HCl-extractability of minerals of raw chickpea and chickpea bulgur samples are presented in Table 1(c and d). Bulgur process resulted in decrease of all minerals. The minerals leached from chickpea samples into soaking and cooking water during soaking and cooking treatments. As observed by some researchers, cooking (in boiling water and autoclave) caused great losses of K (20–24%), Ca (11%), P (6%), Mg (21%), and Fe (8–19%) (Haytowitz and Matthews 1983; and Mubarak 2005). The P, Ca, Mg and K values decreased with increasing soaking time. Duhan et al. (2002) reported that Fe content of pea samples decreased while soaking time increased. But in this study, Fe content of chickpea seeds increased while increasing soaking time. And the lowest P, Ca, Mg, Fe and K values of chickpea bulgur samples were measured by soaked with pH 8 soaking water in all different soaking water pH. Fagbemi et al. (2005), reported that boiling resulted in 16.3 to 44.0% losses of total P content. Habiba (2002) reported that cooking resulted in decrease total phosphorus in peas. The HCl-extractability of P, Ca, Mg, Fe and K present in chickpea bulgur samples were significantly (p < 0.05) higher than that of the raw whole chickpea seeds. Soaking the chickpea seeds for 12 h enhanced the HCL-extractability of Fe, Ca, P by 33.06; 7.15; 16.21 g/100 g over the raw samples, respectively. Similar results have been reported by Saharan et al. (2001), who reported HCl-extractability of Ca content of faba bean samples soaked at 12 h increased by 4 g/100 g. The average value of HCl-extractability of Mg (78.56 g/100 g) of chickpea bulgur samples had the highest values over the minerals.

Conclusions

The present work about chickpea bulgur samples made by different soaking treatments has demonstrated chemical differences, nutritional differences among the chickpea bulgur samples. Turkish people may esteem legume products because of the lower cost and higher protein content of the legumes, higher prices of animal products and the reduced incomes of majority of Turkish people. Bulgur process affected the composition of chickpeas. The protein content rised, while the starch, crude fiber and crude fat decreased by bulgur process. Energy values decreased with bulgur process. Soaking time significantly (p < 0.05) reduced the ash content of chickpea bulgur samples. IVPD values were significantly affected by soaking treatments and increased with bulgur process which includes soaking, cooking and dehulling. TIA was completely eliminated after bulgur process. Soaking time had a significant effect on TPC (p < 0.01). The P, Ca, Mg and K values decreased with increasing soaking time. The HCl-extractability of P, Ca, Mg, Fe and K present in chickpea bulgur samples were significantly (p < 0.05) higher than that of the raw whole chickpea seeds.