Reduction in soaking time and anti-nutritional factors by high pressure processing of chickpeas

Abstract

High pressure (HP) treatment was applied to Kabouli chickpeas to reduce soaking time and anti-nutritional factors, and enhance their quality. Chickpeas were subjected to HP treatment at 100–600 MPa with single and multiple cycles (up to 6) with 10 min holding time as soak-treatments with or without prior soaking at 40 °C for 2 h. HP treatment alone resulted in 89.1% hydration while a combination of pre-soaking followed by HP treatment resulted 93.8% hydration; however overnight soaking (12 h) of chickpeas at room temperature resulted only in 42.5% hydration. Texture softness and color brightness were enhanced by HP treatment with or without pre-soaking (2 h at 40 °C) as compared to overnight soaked chickpeas. HP treatment reduced tannin to 25 mg CE/100 g and phytic acid to 0.2% levels which were about one fifth of their content in raw chickpeas and significantly lower than in overnight soaked product. Scanning electron microscopy revealed that 600 MPa HP treated samples showed larger pore sizes and bigger starch granules corresponding with the higher hydration rates. Fourier transformation infrared spectroscopy results also showed a difference between raw and HP treated chickpeas. Overall, HP treatment was effective in reducing the anti-nutritional factors and soaking times and enhanced quality factors.

Introduction

Chickpea is the 3rd most important legume crop worldwide (Bashir and Aggarwal 2017). It is also the 3rd among the processed products of all pulses following peas and lentils (londeau et al. 2003). Global chickpeas production in 2013 was 12,164 metric tons. According to Xu et al. (2016), chickpeas gross composition is 66.8% of total carbohydrates, 25.1% of proteins, 4.7% of fat and 3.4% of ash. The protein fraction is mainly composed of globulin (56%), glutelin (18.1%), albumin (12%) and prolamin (2.8%), and their degree of digestibility are affected by processing method used and anti-nutritional factors. A common practice is to soak them for long time to achieve hydration, to accelerate starch gelatinization during cooking and to reduce anti-nutritional factors such as phytates, tannins and enzyme inhibitors by leaching them out to water. Chickpeas can be cooked with or without prior soaking (Sayar et al. 2001; Turhan et al. 2002). However, soaking helps to hydrate the starch granules and allow them to swell making the gelatinization process during cooking more efficient and to obtain better cooking quality.

Prolonged cooking of chickpeas can reduce the product’s quality by decreasing protein digestibility and losing some essential amino acids (Laguna et al. 2017). Hence soaking is used as a pretreatment for chickpeas. Room temperature soaking takes a long time; hence soaking is generally done at elevated temperatures for few hours for achieving full hydration (Sayar et al. 2009). However, soaking at elevated temperatures can lead to quality deterioration, and therefore, alternate methods of soaking which result in efficient hydration within a short time without deteriorating the quality are on demand. High pressure (HP) as a non-thermal soaking treatment is the focus of this study.

Yu et al. (2016) found that single cycle of HP treatment could shorten cooking time and reduce the hardness of brown rice. Tian et al. (2014) reported that HP treatment can significantly increase the moisture content of normal rice. Yamakura et al. (2005) reported an increase in moisture content is a proof that water effectively penetrated the outer layer of starch granules in rice under HP. Yu et al. (2017) demonstrated that two cycle HP treatment resulted in lower water absorption, better structural properties, higher structural disruptions and softer texture. Multiple cycles of HP have been shown to result in higher digestibility of rice starch (Deng et al. 2015). It is also recognized that regular soaking at atmospheric pressure decreases phytic acid, oligosaccharides and other anti-nutritional components in pulses. Han and Baik (2006) showed that HP during soaking increased oligosaccharides leaching and reduced soaking time required for oligosaccharide content reduction in legumes.

The objective of this study were to a) evaluate the effect of HP treatment with or without pre-soaking (for 2 h at 40 °C) on hydration efficiency, color and texture of chickpeas in comparison with overnight soaked samples, and b) the effect of selected such treatments on the reduction in anti-nutritional factors (phytic acid and tannin content), as well as the resulting influence on microstructure.

Materials and methods

Material

Dried Canadian Kabuli chickpeas (CLIC brand) packed in a heat-sealed clear plastic polyethylene bags weighing 407 g per packet were purchased from a local supermarket (Provigo Distribution Centre, Montreal) and stored at room temperature. Before using, chickpeas were washed thoroughly in water, and damaged, spotted and split pieces were removed.

Sample preparation

About 10 pieces of dried chickpeas was accurately weighed (~ 0.4 g) with a precision balance (± 0.0001 g accuracy) (APX-200 Digital Weighing Balance, Denver Instruments, USA) and placed in 7 oz. low-density polyethylene bag (Whirl-Pak(R), Nasco, Fort Atkinson, WI, USA). The bag was then filled with 25 mL of distilled water (conductance: 18 V, Milli-Q, Millipore, Bedford, USA) and heat sealed and transferred to the HP equipment for treatment.

High pressure treatment

High pressure treatments were given in an HP equipment (ACIP 6500/5/12VB-ACB Pressure Systems, Nantes, France) consisting of a cylindrical pressure chamber of 5L volume. The pressure–time (P–t) treatment was administered using a computer connected to a data logger (SA-32, AOIP, Nantes, France). The pressure transmission medium used was water. The pressurization rate was 5 MPa/s up, upon reaching the target pressure, a 10 min treatment (holding time) was given followed by a rapid depressurization (< 4 s) to atmospheric pressure. Samples were divided into 2 groups: a) the first group samples were HP treated without pre-soaking and while the second group samples were pre-soaked for 2 h at 40 °C before HP treatment. Six HP treatments were given at: 100, 200, 300, 400, 500 and 600 MPa, and at each pressure, a single pressure cycle (pressure come-up, 10 min hold, depressurize) or up to six such pressure cycle treatments were given. In addition, a single 20 min holding time pressure cycle was also included to compare with two 10 min holding time cycles (same total holding times but with one or two cycle pressure treatments). After HP treatment, the samples were analyzed for various quality parameters.

Water absorption

Water absorption capacity of chickpea samples were measured according to Yu et al. (2017). For this specific purpose, individual chickpea samples were weighed and packed in separate plastic bags and HP-treated. Following HP treatment, chickpeas with and without pre-soaking (2 h at 40 °C) were removed from the plastic bags and wiped with a wet towel to remove the excess water on the surface and then weighed. Water absorption of HP treated samples were computed using Eq. (1) and compared with samples soaked at room temperature and at 40 °C for 3, 6, 9, and 12 h. All measurements were carried out in triplicate. Water absorption or hydration capacity (wet basis) was calculated according to following equation:

$$\text{WA}\left(\%\right)=100\times \left[\frac{M_t-M_0}{{\text{M}}_0}\right]$$ 1

where WA (%) is the water absorption percentage of the sample (wet basis), M₀ is the mass of dry sample without treatment (g), and M_t is the mass after treatment either HP or regular soaking (g) treatment for a specific time (t).

Color

Color parameters of raw chickpeas (control), chickpeas soaked overnight at room temperature and HP treated samples with and without pre-soaking were determined in the L, a, b system using a tristimulus Minolta Chroma Meter (Minolta Corp., Ramsey, NJ, USA). The instrument was warmed up for 5 min then calibrated with a white standard prior to use. Ten measurements were taken with each sample in order to obtain the average values of L (lightness), a (green (−) to red (+)) and b (blue (−) to yellow (+)). The ΔE (total color change) was also determined according to the following equation:

$$\mathrm{\Delta }E=\sqrt{{\left(L_0-L\right)}^2+{\left(a_0-a\right)}^2+{\left(b_0-b\right)}^2}$$ 2

where L₀, a₀, b₀ represent the values of dry chickpeas and L, a, and b represent HP treated samples.

Texture profile analysis

Texture properties of raw and HP treated chickpeas were evaluated using a TA.XT.Plus Texture Analyser (Texture technologies corp., Scarsdale, NY, USA) previously calibrated with a 50 kg load cell, a fixed platform and a 25 mm diameter cylindrical probe. The samples were uniaxially compressed at room temperature to 80% of their original height. Each sample was subjected to two subsequent cycles (bites) of compression–decompression. The crosshead speed was set to 5 mm/s for the first and the second bites, respectively. Canned chickpeas samples were analyzed as a reference for commercial level of product texture. The instrument computed different parameters through the software such as hardness (maximum force required to compress the sample), adhesiveness (work necessary to pull the compression anvil away from the sample), chewiness (gumminess × springiness), cohesiveness (area 2/area 1), gumminess (hardness × cohesiveness), and springiness (area a/area b). The target parameters used for this study were hardness and chewiness because they reflect the quality of chickpeas during mastication (Cardello and Segars 1989). The analysis was performed with ten replicates and the average shown in the results. The instrument automatically recorded the force–displacement and converted them to texture profile analysis.

Scanning electron microscopy

The microstructure of HP treated sample without pre-soaking at 200, 400, and 600 MPa for 20 min, overnight soaked samples at room temperature and raw dry chickpea samples were examined after lyophilization through a scanning electron microscope (SEM) (Hitachi Tabletop Microscope, TM3000). SEM was performed to investigate the effect of HP on pores (cavities) and the increase in starch sizes (Cappa et al. 2016; Yu et al. 2017). Cell sizes were determined using image processing software for scientific analysis (ImageJ 1.50i). Each sample was placed on a carbon layer which was in turn stuck to a rotary holder before being scanned and photographed at 500X magnifications.

Phytic acid content

Phytic acid content was measured according to McKie and McCleary (2016) using a Megazyme phytic acid (Phytase/Total phosphorus) assay kit (#K-PHYT, Megazyme International Ireland). The assay’s principle is based on the hydrolysis of phytic acid by phytase as well as alkaline phosphatase into myo-inositol (phosphate) and inorganic phosphate (Pi). After that, Pi reacts with ammonium molybdate, which is reduced later to molybdenum blue in acidic conditions. Absorbance of molybdenum blue at 655 nm measured with UV/VIS spectrophotometer (VWR, Model V-3100PC) is proportional to the amount of Pi presents in the sample.

Tannin content

Tannin was evaluated using the method described by Khandelwal et al. (2010). The principal was based on reacting condensed tannins with vanillin in the presence of acid to produce red color. One gram of high pressure (HP) treated chickpeas from both groups (with and without pre-soaking), soaked overnight samples at room temperature and dry chickpeas were extracted with 20 mL of 1% HCl (ACROS ORGANICS, NJ, USA) in methanol (LC–MS Grade, EMD Millipore Corporation, USA) for 20 min in water bath at 30 °C. The samples were centrifuged at 2000 rpm for 4 min. The supernatant (1 mL) was reacted with 5 mL vanillin solution [0.5% vanillin (99% pure, ACROS ORGANICS, NJ, USA) + 2% HCl in methanol] for 20 min at 30 °C. Blanks were run with 4% HCl in methanol in place of vanillin reagent. Absorbance was read at 500 nm on a UV/VIS spectrophotometer (VWR, Model V-3100PC). A standard curve prepared with catechin (TRC Canada, Toronto, ON, Canada). Tannin content was expressed as mg CE/100 g. Samples were analyzed in triplicate.

Solid loss determination

Percent of solids lost after soaking overnight or after HP treatment was determined by drying a known amount of the soaking water at 105 °C until constant mass is reached. The percentage of solid loss (SL  %) was calculated from the following equation:

$$\text{SL}\%=\frac{{\text{M}}_{\text{L}}}{{\text{M}}_0}\times 100$$ 3

where M_L and M₀ are the total amounts (g) of solids in the soaking water, and in the raw seeds, respectively.

Fourier transform infrared (FTIR) spectroscopy

The FTIR spectra of dry chickpeas, room temperature overnight soaked chickpeas and HP treated chickpeas samples with and without pre-soaking were obtained by using a Manga System 550 FT-IR Spectrometer (Agilent 5500a, Northen ANI, Solution, USA) over a wavelength range of 400–4000 cm⁻¹ equipped with an OMNIC operating system software (Version 7.3, Thermo Electron Corporation). FTIR has been used previously to observe oligosaccharides reduction (Yoshida et al. 1997). Samples were covered on the surface in contact with attenuated total reflectance (ATR) on a multi-bounce plate of Zn-Se crystal at 25 °C. All spectra were background corrected using an air spectrum, which was renewed after each scan. Each spectrum was collected from an average of 32 scans with a resolution of 4 cm⁻¹ and the results were reported as mean values.

Statistical analysis

Statistical analysis was performed by using SPSS software version 20. Data were expressed as means of at least triplicate and analyzed by a one-way analysis of variance (ANOVA). p value ≤ 0.05 was regarded as significant based on Duncan’s multiple range tests.

Results and discussion

Water absorption

Chickpeas hydration was evaluated before applying HP treatment by soaking them at room temperature and 40 °C for 3, 6, 9 and 12 h. Results showed hydration percentages were 53.1 ± 3.06, 81.8 ± 1.77, 82.7 ± 1.96, and 83.9 ± 1.34, respectively, after 3, 6, 9, 12 h soaking at room temperature and 70.7 ± 3.24, 84.2 ± 1.98, 84.1 ± 3.56, and 84.4 ± 0.78, respectively at 40 °C. These results were used for comparing the soaking performance of HP treated chickpeas.

Figure 1a compares the hydration capacity of single cycle 20 min vs two-10 min-cycle hold time HP treatments. A significant difference in percentage hydration was observed between the two HP treatments at each pressure level employed between 100 and 600 MPa for chickpea samples without the pre-soaking treatment. The two-cycle action resulted in higher hydration than the single cycle treatment (with same total hold time), possibly because of the additional structural disturbance due the two compression-decompression actions in the two-cycle treatment. Hydration of chickpeas during soaking or during the soak-pressure treatment results from the absorption of water and results in swelling of starch granules or imbibition of water by proteins. With soaking treatment of brown rice, Yu et al. (2017) found the opposite results probably because of the difference in the nature and composition of the samples: brown rice vs. chickpeas, which differ in their carbohydrate and protein contents [carbohydrate (78% & 67%) and protein (7% & 20%) in rice and chickpeas, respectively]. Higher protein content in chickpeas influences water retention in moderate pressure intensities (200–500 MPa) since protein aggregation happens which allows more water to be retained in the cavities. Comparing the hydration at different pressure levels, 400–500 MPa treatments resulted in higher hydration difference than lower pressures (200 and 300 MPa). These results lead to the importance cycles and to further explore additional cycles and added pre-soaking treatments.

Fig. 1.

a Hydration % of HP treated chickpeas without pre-soaking at different pressure intensities with two cycles (10 min each) and a single cycle (20 min). b Hydration rate of HP treated chickpeas without pre-soaking subjected to multiple HP cycles (10 min each) at different pressure intensities. c Hydration rate of pre-soaked chickpeas subjected to multiple HP cycles (10 min each) at different pressure intensities

Figure 1b illustrates hydration capacity of multiple cycle HP treated samples chickpea at different pressure levels, again without any pre-soaking. Moisture uptake ranged from 23 to 88% with six consecutive HP 10 min cycles. Single cycle treatment showed different hydration rate at each pressure level as well as each of the multiple cycle treatments, clearly showing a dependence on both pressure level and number of cycles. Higher intensities resulted in higher hydration capacities ranging from about 25 to 40% as the pressure level increased from 100 to 600 MPa. The increased hydration efficiency at higher pressures could be because HP could breakdown cellular structure and allow more free water to penetrate the cell. Also, hydrogen bonding formation between water molecules and starch granules could help in increasing hydration process (Yu et al. 2016; Xue et al. 2018). Until cycle number six, there was a continuous increase through all pressure intensities except for 500 and 600 MPa that showed a slight decrease that might be due to structural change and rupture of the cell wall that will be discussed in the following sections.

Figure 1c shows the influence of pre-soaking (2 h at 40 °C) treatment on the hydration capacity of chickpea samples following HP treatment at different pressure levels (100–600 MPa) and with up to six consecutive cycles 10 min each. Pre-soaking resulted in an average of 70% (± 4.2%) hydration. After HP treatment, the highest increase in moisture content was during the 1st cycle 17% (± 3.0%) followed by slight increase which did not exceed 4% (± 1.2%) during 2nd cycle only. Further cycles displayed a fluctuation in hydration rate and ended up with a general reduction in hydration by 2–7% between 200 and 600 MPa. It is clear from Fig. 1c that the maximum hydration was ≈ 90–93% for almost all intensities which means that samples were fully hydrated at that point. The reason of moisture reduction in almost all cases with the highest at 600 MPa is because pores (cracks) were caused by HP that led to losing of some water after full hydration plus increase in soluble solids extraction (Sayar et al. 2001; Yu et al. 2016). Turhan et al. (2002) showed that chickpeas could reach the maximum hydration ≈ 105% after 10 h soaking at room temperature. In this study, the maximum hydration was reached with 20 min HP treatment after 2 h pre-soaking or 50 min treatment without pre-soaking.

Color change

High pressure treatment of chickpea with and without prior pre-soak treatment (2 h at 40 °C) affected color values significantly at different pressure levels (100–600 MPa) and pressure cycles (1–6) with a 10 min holding time (Table 1). The control samples for treatments with and without prior pre-soak has significantly different color values from treated samples.

Table 1.

Color values of HP treated chickpeas with pre-soaking and without pre-soaking at different pressure intensities and multiple cycles

Cycles	Pressure level	Color L*		Color a*		Color b*
10 min each	MPa	Pre-soaked	w/t pre-soaking	Pre-soaked	w/t pre-soaking	Pre-soaked	w/t pre-soaking
0	0.1	32.5 ± 0.22a	28.7 ± 0.77a	2.0 ± 0.03a	0.6 ± 0.16a	8.9 ± 0.06a	− 2.5 ± 0.63a
1	100	34.2 ± 0.31b	34.6 ± 0.44b	5.8 ± 0.64c	6.0 ± 0.32c	14.1 ± 0.88b	13.5 ± 1.98c
2		34.1 ± 0.30b	35.3 ± 1.47bc	4.7 ± 1.08bc	4.7 ± 0.53bc	13.0 ± 1.66b	9.7 ± 0.42b
3		34.7 ± 0.96bc	36.3 ± 1.16bcd	4.7 ± 0.54bc	4.2 ± 0.31b	13.0 ± 1.53b	9.7 ± 1.86b
4		35.3 ± 0.48 cd	37.0 ± 1.51 cd	4.7 ± 0.81bc	4.5 ± 0.81b	13.3 ± 2.20b	9.8 ± 0.97b
5		36.0 ± 0.43d	37.4 ± 1.46 cd	4.4 ± 2.23bc	4.1 ± 0.78b	13.0 ± 2.72b	12.5 ± 1.04bc
6		36.1 ± 0.32d	37.6 ± 1.63d	3.7 ± 0.36b	3.4 ± 0.95b	12.7 ± 1.64b	13.2 ± 1.44c
0	0.1	32.5 ± 0.22a	28.7 ± 0.77a	2.0 ± 0.03a	0.6 ± 0.16a	8.9 ± 0.06a	− 2.5 ± 0.63a
1	200	34.0 ± 0.56b	34.8 ± 0.66bc	6.8 ± 0.03b	4.6 ± 0.31b	13.4 ± 1.07b	7.3 ± 0.74bc
2		33.9 ± 0.46b	36.3 ± 1.08bc	4.3 ± 2.15a	3.6 ± 0.62b	13.3 ± 2.33b	6.8 ± 0.30b
3		34.4 ± 0.38bc	35.9 ± 0.67bc	4.1 ± 1.90a	3.6 ± 0.93b	13.2 ± 1.06b	6.6 ± 2.04b
4		35.1 ± 0.53bcd	37.7 ± 0.63bc	4.0 ± 0.49a	7.0 ± 1.14c	12.9 ± 1.32b	14.3 ± 3.85e
5		35.7 ± 0.55 cd	38.7 ± 3.58c	3.9 ± 0.44a	6.1 ± 0.37c	13.0 ± 0.99b	10.3 ± 2.97 cd
6		36.0 ± 1.38d	38.4 ± 1.53bc	4.2 ± 0.48a	4.5 ± 1.32b	11.3 ± 0.74b	12.3 ± 0.75de
0	0.1	32.5 ± 0.22a	28.7 ± 0.77a	2.0 ± 0.03a	0.6 ± 0.16a	8.9 ± 0.06a	− 2.5 ± 0.63a
1	300	34.3 ± 0.46b	38.3 ± 0.82b	4.4 ± 0.30b	6.5 ± 1.76 cd	13.2 ± 0.66c	10.6 ± 1.08bc
2		35.1 ± 0.55 cd	38.6 ± 1.29b	4.3 ± 0.17b	5.7 ± 0.44bc	12.0 ± 0.50c	9.9 ± 1.47b
3		35.1 ± 0.46 cd	38.1 ± 1.81b	4.3 ± 0.15b	5.2 ± 0.86bc	11.7 ± 0.43bc	12.7 ± 2.79bc
4		34.8 ± 0.48c	38.5 ± 0.19b	4.2 ± 0.27b	4.3 ± 0.41b	11.5 ± 0.57bc	13.4 ± 2.13c
5		35.7 ± 0.12de	40.0 ± 1.51b	4.0 ± 0.30b	5.5 ± 0.61bc	10.1 ± 2.69ab	16.9 ± 0.77d
6		36.1 ± 0.32e	38.8 ± 1.07b	3.8 ± 0.36b	7.5 ± 0.70d	11.5 ± 0.77bc	19.9 ± 0.45e
0	0.1	32.5 ± 0.22a	28.7 ± 0.77a	2.0 ± 0.03a	0.6 ± 0.16a	8.9 ± 0.06a	− 2.5 ± 0.63a
1	400	36.2 ± 1.27b	38.2 ± 0.47b	3.9 ± 0.85b	7.1 ± 1.14d	11.7 ± 0.83b	18.5 ± 3.09d
2		38.1 ± 0.90b	38.3 ± 1.61b	4.1 ± 1.68b	3.7 ± 1.09b	10.7 ± 1.58ab	16.4 ± 2.08bc
3		40.1 ± 0.66c	38.0 ± 0.19b	3.2 ± 0.15ab	4.9 ± 0.47bc	10.0 ± 0.46ab	15.2 ± 0.55b
4		41.1 ± 1.38 cd	40.2 ± 1.81bc	3.2 ± 0.41ab	4.6 ± 0.29bc	13.8 ± 1.34c	15.5 ± 0.23bc
5		41.9 ± 1.41 cd	41.8 ± 1.44c	3.3 ± 0.59ab	3.9 ± 2.58b	13.8 ± 1.02c	18.3 ± 2.37bc
6		42.2 ± 0.95d	42.2 ± 0.16c	3.2 ± 0.19ab	6.4 ± 1.49 cd	17.8 ± 1.55d	21.4 ± 0.63d
0	0.1	32.5 ± 0.22a	28.7 ± 0.77a	2.0 ± 0.03a	0.6 ± 0.16a	8.9 ± 0.06a	− 2.5 ± 0.63a
1	500	36.4 ± 0.40b	40.1 ± 1.21b	3.2 ± 0.49b	9.8 ± 0.48c	11.9 ± 1.74ab	24.9 ± 0.90c
2		37.2 ± 0.42bc	40.3 ± 0.92bc	2.9 ± 0.39ab	7.8 ± 1.63b	13.4 ± 1.96b	19.1 ± 3.26b
3		38.3 ± 0.40c	40.6 ± 1.48bc	2.7 ± 0.33ab	9.6 ± 1.60b	15.7 ± 2.64b	19.0 ± 1.38b
4		41.0 ± 1.21d	40.5 ± 0.61bc	2.7 ± 0.40ab	6.6 ± 0.37b	15.8 ± 1.77b	19.8 ± 3.01b
5		44.1 ± 0.45e	41.5 ± 1.01bc	2.5 ± 0.32ab	6.6 ± 1.68b	14.8 ± 2.16b	19.3 ± 3.77b
6		45.7 ± 1.83e	42.1 ± 0.92c	2.4 ± 0.55ab	5.8 ± 0.34b	15.7 ± 1.53b	23.2 ± 1.31bc
0	0.1	32.5 ± 0.22a	28.7 ± 0.77a	2.0 ± 0.03a	0.6 ± 0.16a	8.9 ± 0.06a	− 2.5 ± 0.63a
1	600	37.0 ± 1.33b	40.6 ± 0.08b	2.8 ± 0.78a	8.4 ± 1.12 cd	15.7 ± 1.34b	22.3 ± 0.91bc
2		37.6 ± 1.45b	41.0 ± 0.89b	3.1 ± 0.26ab	10.0 ± 3.05d	17.6 ± 1.55b	22.3 ± 4.57bc
3		45.5 ± 0.36b	41.7 ± 1.00b	3.2 ± 0.50ab	7.7 ± 1.12bcd	19.4 ± 4.41b	20.3 ± 0.75b
4		51.5 ± 1.49d	42.3 ± 1.01b	5.4 ± 0.02d	7.4 ± 1.99bcd	27.3 ± 0.45c	23.7 ± 1.92c
5		52.3 ± 0.57d	44.5 ± 1.09c	4.2 ± 0.08bc	6.2 ± 0.77bc	28.5 ± 0.18c	22.5 ± 1.43bc
6		54.3 ± 3.25e	45.4 ± 1.27c	5.2 ± 0.80 cd	5.3 ± 1.10b	31.2 ± 4.77c	21.9 ± 2.38bc

L*, the color lightness; a*/− a*, redness/greenness; b*/− b*, yellowness/blueness. 0.1 MPa is sample without pressure treatment (control)

All values are expressed as mean ± SD. Sample means with different superscript letters in the same column of each pressure intensity are significantly different (p ≤ 0.05)

The L values (brightness) were higher for pre-soaked samples than those for samples without pre-soak treatment and they increased linearly with both treatment pressure (brighter color) and pressure cycles. The increase in L could be due to possible surface migration of white endosperm materials, increase in sample hydration or removal of entrapped intercellular gases. Hydration has been considered as the main reason in earlier studies (Yu et al. 2017; García-Parra et al. 2016; Tian et al. 2014).

The a values (redness) of controls and HP treated chickpea samples also demonstrated significant differences. Generally, the control samples had lower a values than treated samples. Redness was the highest in the first cycle then decreased significantly in the second one and stayed stable throughout the additional cycles. Overall, a values of samples without pre-soaking were higher than after pre-soaking. García-Parra et al. (2016) and Tian et al. (2014) studies support these results. The reason of lowering a* with HP treatment as explained by Saikaew et al. (2018) was due to cell rupture which causes enzymatic and non-enzymatic reactions in corn samples.

Yellowness (b) was the lowest in both controls and then increased significantly through the 1st cycle of high pressure (HP). After that they decreased until 3rd or 4th cycle then returned to increase slightly. Overall yellowness was higher in samples that were not pre-soaked. Yu et al. (2017) findings support the decrease b values and reasoned that the reduction may be due to the diffusion of yellow pigments from brown rice samples into water during high pressure treatment.

To evaluate the overall color change, ∆E values were also compared. Single cycle HP treatment at 100 MPa with and without pre-soaking were 6.71 (± 1.08) and 17.9 (± 0.53), respectively, demonstrating a large increase in ∆E. Color change after HP treatment for samples without pre-soaking were sharper than in pre-soaked ones. Extreme treatment conditions (600 MPa, 6 cycles) resulted in ∆E 31.3 (± 0.75) and 29.9 (± 2.85) with and without pre-soaking indicating that the differences could be reduced with HP treatment intensity. However, there was a huge ∆E difference between the minimum and maximum HP treatment. It was clear that ∆E increased with increasing pressure intensity for both categories. García-Parra et al. (2016) also found that higher pressures resulted in larger ∆E than lower pressures for pumpkin.

Texture profile analysis

Hardness

Texture profile analysis was mainly used in this study to see the effect of HP treatment on hardness and chewiness of chickpeas since those are the two most important parameters during mastication. Figure 2a shows hardness results for chickpeas after single-cycle 20 min versus two-cycle 10 min HP treatments at different pressures without pre-soaking. There was a significant decrease in hardness with the two-cycle treatment at all pressure levels except 600 MPa which showed the opposite. De Oliveira et al. (2017) reasoned this could be due to protein aggregation. The maximum drop in hardness was associated with 200 MPa treatment with a 35% drop in value as compared to 10–15% drop for others. Reduction in hardness was considered desirable (softening) in this study.

Fig. 2.

a Hardness of HP treated chickpeas without pre-soaking at different pressure intensities with two cycles (10 min each) and a single cycle (20 min). b Hardness of HP treated chickpeas without pre-soaking at different pressure intensities with multiple cycles (10 min each). c Hardness of pre-soaked HP treated chickpeas at different pressure intensities with multiple cycles (10 min each) during 2 h at 40 °C

Figure 2b demonstrates a general reduction trend in the hardness with an increase in the number of treatment cycles at different pressure levels for chickpeas without pre-soaking. Control sample of chickpeas (raw dry chickpeas) had a hardness of 368 ± 3.77 N which meant that even the HP treatment at the lowest pressure level achieved a massive reduction in hardness. The minimum hardness achieved after six-cycle treatments were 114 ± 4.75 N for 100 MPa and 93 ± 3.91 N for 600 MPa. Softest texture could be accomplished with 200, 300 and 500 MPa which resulted in 72, 70, and 70 N. Texture degradation can be attributed to tissue collapse and weakened hydrophobic interactions of protein matrix and internal redistribution of moisture (Koca et al. 2011). The only pressure treatment that showed an increase in hardness by 5 N was for 400 MPa after the sixth cycle which might have resulted from protein denaturation, aggregation, oxidation or fluids’ loss (de Oliveira et al. 2017; Sun and Holley 2010).

Figure 2c illustrates the effect of HP on the hardness of samples that were subjected to pre-soaking (2 h at 40 °C) before the HP treatment. Pre-soaked samples (control) had a hardness value of 92 ± 1.86 N. As a result, even the mildest treatment showed a significant hardness reduction on an average of 24 ± 3.31 N for all pressure intensities. Overall, the hardness ranged 54–78 N for all HP treated samples, and the two cycle 500 MPa treatment gave the softest texture (48 N) which was lower than the lowest hardness associated with HP treated samples without pre-soaking. The reason of softer texture in these samples could be the hydration achieved during soaking (de Oliveira et al. 2017). Similar observations were made by Yu et al. (2017) and Koca et al. (2011).

A relationship was observed between hydration rate and hardness. By comparing Figs. 1 and 2 (hydration and hardness), it can be observed that higher hydration rate resulted in softer tissues in almost all chickpea samples without pre-soaking. On the other hand, pre-soaked samples had softer tissues after HP treatment up to 400 MPa, but with treatment at higher pressures (up to 600 MPa), they became harder which might be due to protein aggregation.

Chewiness

Chewiness is another parameter in texture profile analysis (TPA) that followed the same trend of hardness (Koca et al. 2011; Chotyakul and Boonnoon 2016). Chewiness of HP treated chickpeas without pre-soaking were influenced by the treatment pressure and number of cycles, but demonstrated much larger variability. Chewiness for control (raw dry chickpeas) has an average of 41.43 N (± 3.03). After HP treatment, it dropped to a maximum of 13 N and a minimum of 5 N which was more than 30% reduction. 100 MPa treatment had the highest values of chewiness, while 400 MPa has the lowest. All cycles of 200, 300 and 500 MPa with its 1^st cycle only had significant changes in chewiness, while other pressures and cycles had slight changes only.

Pre-soaked samples without HP treatment had an average of 14.0 ± 1.5 N for chewiness and dropped significantly after HP processing by more than one third. Chewiness for all pressures and cycles ranged between 2 and 5 N which resulted in a non-significant changes associate with pressure levels and pressure cycles. The effect of HP on springiness, gumminess, chewiness, resilience, fracturability, and adhesiveness were investigated as in many studies (de Oliveira et al. 2017; Koca et al. 2011; Sun and Holley 2010), but there were no consistent data to support a clear effect of HP.

In this study the softest texture was chosen as desirable. As a result, 6-cycle-200 MPa and 5-cycle-500 MPa for HP treated samples without pre-soaking and 4-cycle-200 MPa and 2-cycle-500 MPa for pre-soaked HP treated samples were selected. Selected samples were used for other quality tests (FTIR, solid loss and phytic acid and tannin contents) followed by cooking step and its further quality tests (texture and color).

Scanning electron microscopy (SEM)

Scanning electron micrographs of chickpea samples soaked overnight and treated with 0.1, 200, 400 and 600 MPa for 20 min are illustrated in Fig. 3 (500× and 250×). Starch granules were oval and round-shaped in untreated and overnight soaked samples. HP treated samples retained the same oval and round shape, but with swelling as also observed by Ahmed et al. (2016a). An average of 0.34, 0.38, 0.38, 0.37 and 23.88 μm for raw chickpeas (0.1 MPa), soaked overnight, treated with 200, 400, and 600 MPa, respectively. So, a significant change in granule diameter was observed with all samples. The main purpose of soaking is to help absorption of water which facilitates easy gelatinization of starch during cooking. It can be achieved either through conditioning below the gelatinization temperature then cooking or through direct cooking in water above the gelatinization temperature. During high pressure treatment, water is forced into chickpea samples and causes more rapid hydration which is driven by the external pressure applied. This results in rapid swelling even at ambient temperatures. Swelling is a result of the increase in hydrogen bonding between water and starch which promotes water uptake (Yu et al. 2016; Turhan et al. 2002; Sayar et al. 2001). Raw and overnight soaked samples showed compact tissues. On the other hand, HP treated samples showed bigger pores sizes and aggregated granules especially for samples treated with 600 MPa. Those aggregates might explain the reason for the relatively harder texture that was discussed previously. Big pore sizes were the reason for enhanced hydration and swelling of granules due to easily water diffusion. Surface and cell wall of overnight soaked samples was compact and intact compared to 600 MPa treated one. Cell wall of 600 MPa treated sample was almost disappeared and destroyed with very big size pores that proves the damage cause by HP (Denoya et al. 2016; Yu et al. 2017; Yu et al. 2016; Ahmed et al. 2016b).

Fig. 3.

SEM images of raw chickpeas without HP treatment (a), soaked overnight without HP treatment (b), HPT for 20 min at 200 MPa (c), HPT for 20 min at 400 MPa (d), HPT for 20 min at 600 MPa (e), cross section of chickpea’s membrane soaked overnight (f), and cross section of chickpea’s membrane with HPT at 600 MPa (g)

Fourier-transform infrared spectroscopy (FTIR)

FTIR is an important technique to observe whether any change happened to the structure of the sample when applying any processing such as pressure, thermal treatment, chemical treatment or any other types of treatments. Figure 4a illustrates spectrum of HP treated chickpea samples, soaked overnight and raw chickpeas in the spectral region 2800–3700 cm⁻¹. Control (soaked overnight) sample and HP-treated samples showed similar main peaks with just a little difference in the amplitude of peaks which confirmed that all of them absorbed water but with different levels.

Fig. 4.

a FTIR spectra of HP treated chickpea samples, soaked overnight and raw chickpeas. Dark Blue = raw chickpeas without any treatment; Dark purple = 500 (5) (HP treated chickpeas without presoaking at 500 MPa for 5 cycles each 10 min); Light purple = 500 (2) (Pre-soaked chickpeas treated with HP at 500 MPa for 2 cycles 10 min each); Light Blue = 200 (6) (HP treated chickpeas without presoaking at 200 MPa for 6 cycles each 10 min); Red = soaked overnight chickpeas; Green = 200(4) (Pre-soaked chickpeas treated with HP at 200 MPa for 4 cycles 10 min each). b FTIR spectra of treated chickpea samples, soaked overnight and raw chickpeas. 1 = 200(4) (Pre-soaked chickpeas treated with HP at 200 MPa for 4 cycles 10 min each); 2 = soaked overnight chickpeas; 3 = 200 (6) (HP treated chickpeas without presoaking at 200 MPa for 6 cycles each 10 min); 4 = 500 (2) (Pre-soaked chickpeas treated with HP at 500 MPa for 2 cycles 10 min each); 5 = 500 (5) (HP treated chickpeas without presoaking at 500 MPa for 5 cycles each 10 min); 6 = raw chickpeas without any treatment

On the other hand, raw sample differed considerably in the major peak at 3276 cm⁻¹ in which it had lower intensity compared to HP treated samples with a higher intensity peak at 2923 cm⁻¹ and an extra peak at 2853 cm⁻¹ that was absent for other samples. Higher intensities are an indicative of crystallization suggesting strong hydrogen bonding whereas the absence of the band showing an amorphous structure which indicated the effect of HP on chickpeas carbohydrates structure (Wolkers et al. 2004). The reason of the difference between raw and processed samples was the degree of crystallinity by relatively sharp absorption bands shown in raw sample, whereas broader absorption bands (2932 cm⁻¹) were visible in amorphous HP treated chickpeas. Crystalline structure had also a shoulder peak in the OH stretching region that was around 3000 cm⁻¹ which is an indicative of hydrogen bonds (Wolkers et al. 2004).

Figure 4b is mainly focused on carbohydrates with the carbohydrate fingerprint region 900–1200 cm⁻¹. A considerable difference between raw sample and HP treated ones can be noticed. Like previous figure, raw chickpeas had crystalline structure than HP treated samples since it contained sharper and higher peaks’ intensities. Present results of overall carbohydrate peaks shapes of chickpeas are supported by Sun et al. (2014). Wolkers et al. (2004) reported that bands’ shift to lower wavenumbers means dehydration in addition to broader peaks which is proved by present results having almost all peaks of raw dry chickpeas broad and shifted to lower wavenumber. Major bands absorption at 994 cm⁻¹ shifted to around 1000 cm⁻¹ with increasing pressure which is an indication that the crystalline structure of starch disappeared through HP treatment since HP affects starch gelatinization (Ahmed et al. 2016b). The signal at 1740 cm⁻¹ in raw chickpeas represented vibrations of ester groups in pectin. Since this peak is absent in HP treated samples, then it might be either the cell wall destroyed with HP or an overlap took place with the strong water band centered at 1640 cm⁻¹ (Chen et al. 1997). It was reported that peaks at 1146 cm⁻¹ and 1100 cm⁻¹ are assigned to the PO₂ stretching modes, the P–O–C anti-symmetric stretching mode of phosphate ester, and to the C–OH stretching of oligosaccharides. The decrease in intensity of 1146 cm⁻¹ peak was attributed to the decrease in oligosaccharide content (Yoshida et al. 1997). As a result, raw chickpeas had higher oligosaccharides than pressurized and overnight soaked samples.

Phytic acid and tannin contents and solids loss

Table 2 shows the results for phytic acid and tannin content and solid loss in selected HP treated samples to compare the quality of samples after HP treatment as compared to overnight soaked samples and raw dry chickpeas. Soaking overnight reduced phytic acid content significantly to almost one third of that in the raw sample. HP treated samples reduced the phytic acid levels to more than overnight soaked samples. The most powerful way of reducing the phytic acid content was HP processing of pre-soaked samples at 500 MPa for two consecutive cycles. Soaking prior to HP treatment is advantageous from phytic acid reduction point of view. Other studies (Deng et al. 2015; Linsberger-Martin et al. 2013) have shown the significant effect of HP treatment on phytic acid reduction in beans, peas and buckwheat compared to untreated samples. The reduction might be attributed to the hydrolytic activity of the enzyme phytase which gets activated during soaking (Deng et al. 2015; Sinha and Kawatra 2003).

Table 2.

Phytic acid, tannins and solid loss of raw, soaked overnight, HP treated chickpeas with and without pre-soaking

HP treatment	Pressure intensity (MPa) & cycles	Phytic acid (%)	Tannins (mg CE/100 g)	Solid loss (%)
Raw	NA	1.19 ± 0.115a	116.44 ± 3.262a	NA
Soaked overnight at room temp	NA	0.30 ± 0.150b	47.11 ± 1.596b	2.17 ± 0.118a
HP treated without Pre-soaking	200 (6)	0.22 ± 0.059b	26.17 ± 1.434 cd	2.35 ± 0.276a
	500 (5)	0.21 ± 0.076b	34.75 ± 1.841 cd	3.69 ± 0.377ac
HP treated with pre-soaking (2 h at 40 °C)	200 (4)	0.22 ± 0.072b	36.17 ± 2.321 cd	4.39 ± 0.533bc
	500 (2)	0.17 ± 0.032bc	26.83 ± 3.472c	4.61 ± 0.859bc

All values are expressed as mean ± SD. Sample means with different superscript letters in the same column are significantly different (p ≤ 0.05)

NA = not applicable; 200 (6) = HP treated chickpeas without presoaking at 200 MPa for 6 cycles each 10 min; 500 (5) = HP treated chickpeas without presoaking at 500 MPa for 5 cycles each 10 min; 500 (2) = Pre-soaked chickpeas treated with HP at 500 MPa for 2 cycles 10 min each; 200(4) = Pre-soaked chickpeas treated with HP at 200 MPa for 4 cycles 10 min each

Overnight soaking reduced tannins content of raw chickpeas by more than half and as observed with phytic acid (Table 2); HP treatment contributed to further lowering up to 24% of the initial level. Among the 4 samples treated with HP, the one treated with 200 MPa for 6 consecutive cycles reduced tannin content slightly more than the other 3 samples. Deng et al. (2015) reported that HP processing at 600 MPa for 30 min could reduce tannins by about 20%. The difference might be due to the use of multiple cycles in the present study which helped to reduced tannins to a greater extent. General reason of tannin reduction is that they are water soluble, so they leach out into the water (Uzogara et al. 1990).

Solids loss is another quality parameter that should be taken in consideration since part of the present study was to better understand the HP treatment for hydration of chickpeas. Solids loss and water absorption can occur simultaneously during any soaking treatment. When water penetrates chickpeas, it can solubilize some carbohydrates and proteins in addition to causing some vitamins and minerals to leach out into water (Sayar et al. 2009). The lowest solid loss was associated with overnight soaking. Since no literature could be found to compare HP effect on solids loss, comparison of normal soaking between present study and other studies was done. 2.2% was the solid loss for 12 h soaking at 25 °C in present study versus 2.5% for 15 h at 20 °C in a study conducted by Sayar et al. (2009) and 0.7% for 6 h at 25 °C in another study (Johnny et al. 2015). Both studies were on Kabuli chickpeas.

Solids loss in HP treated samples were significantly higher than in overnight soaking except for the one treated with 200 MPa for 6 cycles. It had a solids loss average of 2.4%. Generally, HP processed samples that were pre-soaked had significantly higher solid loss percentage than the ones without pre-soaking regardless pressure intensity or number of cycles. It is logical because presoaking and high pressure considered as two soaking steps rather than one in HP processing. In addition, pre-soaking temperature was 40 °C and it is known that the higher the temperature the more solid loss (Johnny et al. 2015).

Conclusion

High pressure treated samples improved chickpeas quality by reducing tannin content around 26.7% and phytic acid content around 16.7% from initial levels in addition to enhancing their textural properties. Pre-soaked HP treated samples had better effect than direct HP treatment of dry chickpeas in soak water although both enhanced chickpeas quality significantly over overnight soaked samples. HP treatment allowed to reach the desired hydration percentage (≈ 90–93%) in less than an hour where similar results could be reached with overnight soaking without HP processing. HP soaking with multiple cycles resulted in higher hydration rate, brighter color and softer texture, 48 N for pre-soaked HP treated samples and 70 N for HP treated samples without pre-soaking compared to 368 N of untreated samples, which are important for consumers’ acceptance. SEM and FTIR support the effect of HP on chickpea samples.