Influence of γ-irradiation on physicochemical, functional, proximate, and antioxidant characteristics of pigmented rice flours

Abstract

In recent years, food irradiation using γ-rays is one of the most valuable practices for insect disinfestation in rice grains for extended shelf life. In this study, flours from four pigmented rice cultivars were exposed to γ-irradiation using ⁶⁰Co at different doses (0, 5, 10, and 15 kGy). The impact of γ-irradiation on the physico-chemical, functional, and morphological characteristics of pigmented rice flours were analyzed. Results revealed that reduction in amylose content, pH, bulk density, tapped density, and syneresis, while solubility, water absorption capacity, and swelling power values increased significantly (p < 0.05). Pasting characteristics of pigmented rice flours also reduced after exposure to γ-irradiation. Morphological features of pigmented rice flour granules revealed no evidence of physical destruction after irradiation except for black kavuni flour. The structural analysis by FTIR confirms no effect of γ-irradiation on pigmented rice flours. Overall, the study revealed that irradiated pigmented rice flours with enhanced functional properties of less than 10 kGy can be effectively used in the development of value-added rice-based food products considering all the beneficial and safety aspects.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05709-z.

Introduction

Rice is a one of the most important cereal crops that is consumed as a primary stable food by the world’s majority population (Monks et al. 2013). It stands second in utilization next to wheat in the world’s cereal production, and India ranks second in the production of rice in the world (Bhat and Riar 2017). Though rice constitutes a key part of the South Indian diet, traditionally grown rice was not given patronage because of its variable qualities such as cooking quality and poor sensorial attributes (Ocloo et al. 2017). Traditionally, rice has been consumed in the form of whole grain, but after the green revolution, white or polished rice is the most prevalent type of rice consumed worldwide. In recent years, pigmented rice cultivars including brown, black, and red rice has gained wide recognition among consumers, as they are aware of its unique health benefits, such as protecting the human system against microbial infections, cardiac diseases, cholesterol metabolism, and prostate health (Bhat and Riar 2017).

In recent times, pigmented rice varieties can be used as a source of functional foods that can naturally prevent the oxidation process in foods. Pigmented rice has higher nutritional value than non-pigmented due to the presence of numerous bioactive compounds in the embryo and bran sections containing phytochemical compounds, functional lipids, minerals, vitamins, dietary fiber, and gamma-aminobutyric acid (Reddy et al. 2017; Sultan et al. 2018). The pigmented rice flours with flavonoids and phenolics exhibits diverse unique bioactivities, viz., antioxidant, anticancer, antimicrobial, and anti-inflammatory activities, thereby bolstering the overall immune system of the human body, implying that pigmented rice is a source of potential functional food (Bhat and Riar 2017; Reddy et al. 2016).

Recently, pigmented rice flour has been considered the most important cereal flour from a nutritional perspective, and its usage in infant foods, and also for gluten intolerant patients has been increasing (Chinma et al. 2015). Due to its distinctive functional qualities, it has become one of the most vital ingredients in many novel food products like puddings, beverages, tortillas, salad dressings, and gluten-free breads (Falade and Christopher 2015). The utility of rice flour in the food industry for novel food production needs a look at its functional and physicochemical attributes. In modern food systems, functional and physicochemical characteristics of rice play a vital role in establishing the food quality and cooking attributes of rice. However, these properties are subject to change over with storage conditions and post-harvest handling. Modification of flour can be useful in improving the functional and physicochemical attributes of certain components present in food products.

Gamma-irradiation is one of the key modification practices that is widely used in the food processing industry for the modification of biopolymers to their desirable end uses. This irradiation method is mostly preferred as it is fast, low cost, environmentally friendly, and safe (Reddy and Viswanath 2019). FSSAI proposed the best irradiation dose for microbe free sample, to diminish food-borne microbial growth, for cleaning and for commercial sterilization of diverse food groups at about a 0–25 kGy dose range (Zhu et al. 2010). In recent years, the horizon of γ-irradiation has been widely employed worldwide owing to its numerous advantageous effects. This irradiation treatment aids in the inactivation of food-borne pathogens, hinders food allergies, and effectively lengthens the shelf stability of rice flour and rice-based products like rice noodles, murukku, idiyappam, and rice puddings. The use of γ-irradiation is believed as a green technology, which is reliable with the practice of sustainable agriculture. Proximate composition, physical, functional, and antioxidant properties constitute the main criteria for the acceptability and adoption of pigmented rice flour in food systems.

Irradiation processing of foods brings about desired changes in their structural, functional, and physical attributes depending upon dose of γ-irradiation. The influence of gamma treatment was stated on a few attributes of rice flour at specific doses like 0.25, 0.5, 1, 1.5, 5, 10, 15 and 20 kGy (Sultan et al. 2018; Zhu et al. 2010). Ocloo et al. (2017) reported that ionizing radiation showed a fallen trend towards pH, swelling power, and pasting viscosities, while an increase in trend was observed in solubility for rice cultivars at 0 to 1.5 kGy dosage. Improvements in food safety and reduction of postharvest losses could happen during irradiation of some plant foods at determined doses (Zhu et al. 2010). Sultan et al. (2018) studied the effect of γ-irradiation on the functional, physicochemical, and antioxidant properties of brown rice flour. Their results showed a significant reduction in amylose, foaming capacity, foaming stability, and pasting viscosities. Irradiation of pigmented brown rice flour up to 5 kGy increased its total phenolics and antioxidant activities (Sultan et al. 2018). However, a variety of studies have stated the effect of γ-irradiation on the quality characteristics of rice flour. However, to our knowledge, studies have not been done so far especially in these pigmented rice varieties of Southern India. The objective of the present study was to determine the effect of γ-irradiation on changes in morphological, physico-chemical, and functional characteristics, and structural identification of functional groups using FTIR of pigmented rice flours of Southern India.

Materials and methods

Raw materials

Traditional paddy cultivars (2 brown, 1 black and 1 red rice), namely ‘Kattuyanam’ (KYF), ‘Black kavuni’ (BKF), ‘Red kavuni’ (RKF) and ‘Karungkuruvai’ (KKF) were procured from rice farmers in Tamilnadu, India. Prior to use, the paddy samples were dried, cleaned manually, packed in screw-capped plastic containers, and stored at 4 °C for further analysis. All reagents and solvents used for the experimental study were procured from Himedia, Mumbai and of analytical grade.

Pigmented rice flour preparation

The paddy grains were dehusked with the laboratory Husker (THU-34A, Satake Engineering Co., Japan) to obtain the pigmented rice. Flour was prepared from the pigmented rice varieties by grinding the rice grains using Mini Grain Mill (A11B, IKA Inc, Bangalore, India.) The resulting flours were sieved using 100 µm mesh-sieve and kept at 4 °C for further analysis.

Irradiation treatment

Gamma irradiation was executed at Central Instrumentation Facility (CIF), Pondicherry University, India, and irradiation treatments were given in triplicates. The pigmented rice flours (50 g/bag) were irradiated using Cobalt-60 (⁶⁰Co) γ-irradiator (GC5000, BRIT India) at room temperature (24 ± 1 °C). The irradiation doses of 5, 10, and 15 kGy at dose rate of 2.5 kGy/h were given. Irradiation doses were confirmed by ceric-cerous dosimeter. One of the flour samples from each of the four cultivars was used as control (0 kGy).

Chemical composition of pigmented rice flours

Following the standard AOAC procedures, the fat, protein, ash, and moisture content of pigmented rice flours was quantified. Total dietary fibre (TDF) and amylose content was analyzed, following the procedures described by Savitha and Singh (2011) and Reddy et al. (2017), respectively.

Color characteristics

The color characteristics of pigmented rice flours was analyzed using a Colorimeter (D-25, Hunter Associates Lab, Ruston, USA). Chroma (C) and hue angle was calculated from the obtained Hunter L* (lightness), a* (redness), and b* (yellowness) values and the Eqs. (1) and (2) were given below.

$$C=\sqrt{{(a\ast )}^2+{(b\ast )}^2}$$ 1

$$Hueangle={\mathit{Tan}}^{-1}\frac{b}{a}$$ 2

Bulk and tapped densities

The relationship between the mass and volume ratio was used to figured out the density of flour. Briefly, 6 g of flour was dispersed into an empty calibrated measuring cylinder of 100 mL from a constant height and cylinder was patted gently until no variation in the volume (V₁). Bulk density (p_b) was represented as kg/m³ (Wani et al. 2013). Tapped density (p_t) was analyzed by carefully tapping 10 times without compacting the flour (V₂). The bulk and tapped densities were calculated by the Eqs. (3) and (4) were given below:

$$Bulkdensity\left(p_b\right)=\frac{Massofthesample(W)}{Volumeofthesample(V_1)}$$ 3

$$Tappeddensity\left(p_t\right)=\frac{Massofthesample(W)}{Volumeofthesample(V_2)}$$ 4

pH and carboxyl content

The pH of each flour blend (1 g of flour in 25 ml of water) was determined using a digital pH meter (Hanna, USA) at 25 °C. The carboxyl content of pigmented rice flours was analyzed by the method described by Suriya et al. (2017). The amount of carboxyl content was calculated by the following formula:

$$Percentcarboxyl\left(d,b\right)=\frac{\left(s,a,m,p,l,e,-,b,l,a,n,k\right)mL\times normalityofNaoH\times 0.045}{Weightofdrysample}\times 100$$

Swelling index

The swelling index rice flours was determined by the method of Reddy et al. (2015).

$$Swellingindex(g/g)=\frac{Weightofwetpellet}{Weightofsample(db)}$$

Water absorption capacity (WAC) and water solubility index (WSI)

Flour (0.5 g) on dry basis (db) was suspended with 6 ml of deionized water and kept for incubation in a water bath at 30 °C with continuous shaking for 30 min, then centrifuged at 4000 rpm for 10 min. The supernatant was placed in hot air oven (105 °C) for drying. The wet sediment and the dried supernatant were weighed.

$$WAC\left(\%\right)=\frac{Weightofwetsediment}{Weightofdrysample}\times 100$$

$$WSI\left(\%\right)=\frac{Weightofdriedsupernatant}{Weightofdrysample}\times 100$$

Pasting properties

The pasting profile of flours were performed using Rapid Visco Analyser-S4 (Perten Instruments Pvt. Ltd., Warriewood, Australia) (Reddy et al. 2015).

Morphological characteristics

Morphological characteristics of pigmented rice flours was analyzed by scanning electron microscope (Hitachi, S-3400N, Tokyo, Japan). Flour sample was fixed on aluminum stubs using double-sided adhesive tape, coated with a thin film of gold (10 nm) under vacuum. Images was taken at × 1.10 k magnification using 15 kV voltage.

Assay for total phenols

Total phenolic content (TPC) was executed using the Folin–Ciocalteau spectrophotometric method (Reddy et al. 2017). Gallic acid was used as the standard and the obtained results were expressed as mg GAE/g of rice flour.

Assay for total flavonoids

Total flavonoid content (TFC) was determined using the method of Chen et al. (2017) with slight modifications. The standard flavonoid used was catechin and the results were expressed as mg CE/g.

Assay for DPPH radical scavenging activity

100 µl of the pigmented rice extract was mixed with methanol (3.9 ml) and DPPH radical methanolic solution (1.0 ml in 1.0 mM). The absorbance was measured against blank (100 µl methanol in 1.0 ml DPPH radical solutions) at 517 nm after incubating in a dark room for 30 min.

Assay for ABTS radical scavenging activity

ABTS (2,2-azino-bis-(3 ethylbenzothiazoline-6-sulphonic acid) diammonium salt) radical scavenging activity of flour extract was performed spectrophotometrically by the method of Shen et al. (2009). Results obtained was expressed as mmol Trolox equivalents per g dry weight.

Spectral analysis by FT-IR

The functional groups of native and irradiated pigmented rice flours were determined by using FTIR optical spectrometer (Model: 6700, UK) at room temperature. Spectra achieved were smoothed and baselines were corrected using OPUS software.

Statistical analysis

All experimental analysis was carried out in triplicates. The data collected was analyzed and computed statistically using SPSS 22.0 software package (SPSS Institute Inc., Chicago, USA). Values were manifested as mean ± standard deviation. DMRT test was employed to compute the significance of differences between mean values at p < 0.05.

Results and discussion

Effect of γ-irradiation proximate composition

Table 1 showed the proximate values of native and irradiated pigmented rice flours. From the findings, the protein and moisture content of native pigmented rice flours were at around 11.81–14.30% and 7.42–11.85%, respectively. Irradiation of pigmented rice flours resulted in no changes in protein or moisture content. Darfour et al. (2012) also reported that γ-irradiation did not alter the protein and moisture content of cowpea flour. In this study, the moisture content of irradiated pigmented flours was comparatively lower than that reported by Lee and Kim (2018). The moisture content of all rice flours was low, indicating better starch keeping quality as well as the storage stability of rice (Ocloo et al. 2017). In native pigmented rice flours, there were 66.80–72.49% carbohydrates and 7.04–11.88% dietary fibre in them (Table 1). After γ-irradiation, no changes in quantity of carbohydrates and total dietary fibre content of pigmented flours were noted. The fat and ash content of native pigmented flours were around 4.17–5.42% and 2.17–2.85%, respectively. The γ-irradiation process did not significantly (p < 0.05) affect the fat and ash content of pigmented flours (Table 1). Overall, the proximate constituents of pigmented flours were not significantly influenced by the 15 kGy dose of irradiation.

Table 1.

Proximate composition and apparent amylose content of native and γ-irradiated pigmented rice flours

Parameter	Dose (kGy)	KYF	BKF	RKF	KKF
Protein (%)	0	12.64 ± 0.24a	13.20 ± 0.24a	11.81 ± 0.24a	14.30 ± 0.25a
	5	12.66 ± 0.24a	13.25 ± 0.22a	11.86 ± 0.12a	14.25 ± 0.13a
	10	12.68 ± 0.23a	13.29 ± 0.21a	11.93 ± 0.15a	14.41 ± 0.05a
	15	12.70 ± 0.24a	13.33 ± 0.21a	11.88 ± 0.03a	14.44 ± 0.04a
Fat (%)	0	4.17 ± 0.02b	5.29 ± 0.01c	5.42 ± 0.02b	5.28 ± 0.01c
	5	4.20 ± 0.01ab	5.33 ± 0.02b	5.45 ± 0.01b	5.30 ± 0.01c
	10	4.19 ± 0.01ab	5.35 ± 0.01b	5.48 ± 0.02a	5.35 ± 0.02b
	15	4.22 ± 0.01a	5.40 ± 0.02a	5.50 ± 0.01a	5.39 ± 0.02a
Ash (%)	0	2.17 ± 0.01c	2.85 ± 0.02b	2.26 ± 0.06b	2.45 ± 0.03b
	5	2.18 ± 0.01bc	2.88 ± 0.01ab	2.30 ± 0.02ab	2.48 ± 0.02ab
	10	2.20 ± 0.02b	2.89 ± 0.01a	2.33 ± 0.03a	2.50 ± 0.02a
	15	2.23 ± 0.02a	2.90 ± 0.02a	2.37 ± 0.02a	2.51 ± 0.02a
Moisture (%)	0	8.52 ± 0.49a	11.85 ± 0.13a	11.20 ± 0.12a	7.42 ± 0.25a
	5	8.51 ± 0.46a	11.83 ± 0.14a	11.21 ± 0.08a	7.40 ± 0.05a
	10	8.48 ± 0.47a	11.79 ± 0.09a	11.19 ± 0.01a	7.38 ± 0.03a
	15	8.47 ± 0.45a	11.82 ± 0.02a	11.17 ± 0.03a	7.33 ± 0.02a
Carbohydrate (%)	0	72.49 ± 0.57a	66.79 ± 0.27a	69.31 ± 0.29a	70.55 ± 0.33a
	5	72.45 ± 0.52a	66.69 ± 0.26a	69.19 ± 0.19ab	70.57 ± 0.18a
	10	72.37 ± 0.53a	66.63 ± 0.14a	68.93 ± 0.20ab	70.24 ± 0.07ab
	15	72.29 ± 0.47a	66.43 ± 0.23a	68.86 ± 0.03b	70.03 ± 0.07b
Total dietary fibre (%)	0	9.33 ± 0.12a	11.88 ± 0.13a	7.04 ± 0.09b	10.87 ± 0.11b
	5	9.36 ± 0.13a	11.91 ± 0.02a	7.08 ± 0.04ab	10.92 ± 0.02ab
	10	9.40 ± 0.09a	11.87 ± 0.15a	7.13 ± 0.04ab	10.95 ± 0.03ab
	15	9.45 ± 0.11a	11.86 ± 0.07a	7.17 ± 0.02a	10.99 ± 0.02a
Apparent amylose content (%)	0	30.03 ± 0.09a	27.30 ± 0.08a	27.28 ± 0.26a	29.39 ± 0.35a
	5	27.69 ± 0.61b	26.85 ± 0.26b	26.04 ± 0.08b	26.35 ± 0.31b
	10	24.32 ± 0.86c	24.42 ± 0.08c	23.69 ± 0.21c	25.06 ± 0.07c
	15	24.19 ± 0.95c	24.18 ± 0.17c	23.13 ± 0.24d	23.34 ± 0.06d

Values are means of triplicates ± standard deviation. Means with the same superscript letters within the same column are not significantly different at p < 0.05 level

Effect of γ-irradiation on apparent amylose content

After γ-irradiation treatment, the levels of apparent amylose (AAC) in all the irradiated pigmented flours were significantly (p < 0.05) reduced (Table 1). As shown in Table 1, the AAC of native and irradiated pigmented flours was observed in the range of 30.03–24.19% (KYF), 27.30–24.18% (BKF), 27.28–23.13% (RKF) and 29.39–23.34% (KKF), respectively. In pigmented rice flours, decline in the volume of amylose might be possible due to the breakdown of amylose polymer into smaller fragments that lower the iodine binding ability of the amylose molecules. Further, γ-irradiation can lower the amount of apparent amylose by interfering with the amylose-iodine bindings in amylose–lipid complexes. Exposure of molecules to γ-irradiation can cause carboxylic acid formation by altering the amylose and amylopectin ratios and can accelerate AAC diminution (Bashir and Haripriya 2016).

Effect of γ-irradiation on optical property

The color values of native and γ-treated rice flours are shown in Table 2. According to the findings, there were significant difference in color values of native and pigmented rice flours with respect to L* (Lightness), a* (Redness) and b* (Yellowness). Results demonstrated that L* value of KYF and BKF was improved with an increase in irradiation dose, while in the case of RKF and KKF, a decrease in trend was observed. The a* value of native flour was 8.01 (KYF), 3.87 (BKF), 8.43 (RKF) and 6.92 (KKF). After γ-irradiation, a decrease in redness (a*) was observed with the rise of the irradiation dose up to 15 kGy. Wani et al. (2015) reported similar findings regarding the decrease in a* value of arrowhead tuber flour with increasing dose of irradiation up to 15 kGy. Hunter color b* value, however, increased significantly (p < 0.05) from 10.86–12.16 (KYF), 4.18–5.95 (BKF), 12.44–13.31 (RKF) and 11.04–12.93 (KKF) with the lowest value for native rice flour and the highest value for 15 kGy. Thus, rather than caramelization, the color change in pigmented rice flour could be attributed to the Maillard reaction between protein residues and sugars or the alteration of residual phenolics (Zhu et al. 2010).

Table 2.

Density and color attributes of native and γ-irradiated pigmented rice flours

Parameter	Dose (kGy)	KYF	BKF	RKF	KKF
Bulk density (kg/m3)	0	501.67 ± 1.66a	428.97 ± 0.49a	417.22 ± 0.32a	398.41 ± 0.66a
	5	497.52 ± 1.13b	425.37 ± 1.45b	411.60 ± 0.64b	395.12 ± 0.95b
	10	494.41 ± 1.02c	421.52 ± 1.02c	408.97 ± 0.61c	392.71 ± 1.12c
	15	492.86 ± 0.36c	419.03 ± 0.95d	406.86 ± 0.57d	389.59 ± 0.56d
Tapped bulk density (kg/m3)	0	528.07 ± 1.75a	474.12 ± 0.55a	455.15 ± 0.35a	416.51 ± 0.69a
	5	524.78 ± 0.59b	470.63 ± 0.82b	450.71 ± 0.45b	413.04 ± 0.96b
	10	520.03 ± 0.18c	467.57 ± 1.16c	448.66 ± 0.39c	409.86 ± 0.68c
	15	516.62 ± 0.62d	465.97 ± 1.00c	445.60 ± 0.62d	406.89 ± 0.78d
Color parameters
L*	0	69.52 ± 1.02b	52.81 ± 0.39c	68.06 ± 0.25a	69.89 ± 0.02a
	5	70.87 ± 1.53ab	59.76 ± 0.10a	67.65 ± 1.43a	67.08 ± 0.51bc
	10	71.94 ± 0.18a	59.32 ± 0.11a	68.04 ± 1.49a	68.19 ± 0.19b
	15	71.53 ± 0.09a	58.29 ± 0.27b	68.21 ± 0.21a	66.11 ± 1.24c
a*	0	8.01 ± 0.03a	3.87 ± 0.05a	8.43 ± 0.04a	6.92 ± 0.01c
	5	7.74 ± 0.08b	3.85 ± 0.01a	7.84 ± 0.25b	7.26 ± 0.07a
	10	7.70 ± 0.05b	3.81 ± 0.03a	7.90 ± 0.02b	7.14 ± 0.02b
	15	7.75 ± 0.01b	3.86 ± 0.04a	8.04 ± 0.03b	7.12 ± 0.07b
b*	0	10.86 ± 0.13c	4.18 ± 0.12d	12.44 ± 0.09b	11.04 ± 0.02c
	5	11.41 ± 0.16b	5.45 ± 0.02c	12.35 ± 0.44b	12.51 ± 0.10b
	10	11.96 ± 0.09a	5.75 ± 0.05b	13.06 ± 0.08a	12.89 ± 0.01a
	15	12.16 ± 0.01a	5.95 ± 0.03a	13.31 ± 0.03a	12.93 ± 0.11a
Hue angle	0	53.59 ± 0.25d	47.17 ± 1.17c	55.87 ± 0.12c	57.91 ± 0.05d
	5	55.87 ± 0.13c	54.77 ± 0.06b	57.59 ± 0.09b	59.85 ± 0.06c
	10	57.22 ± 0.03b	56.45 ± 0.32a	58.81 ± 0.09a	60.99 ± 0.07b
	15	57.49 ± 0.05a	57.05 ± 0.29a	58.87 ± 0.13a	61.15 ± 0.02a
Chroma	0	13.49 ± 0.12c	5.70 ± 0.05d	15.03 ± 0.09bc	13.03 ± 0.02c
	5	13.79 ± 0.17b	6.67 ± 0.02c	14.63 ± 0.51c	14.46 ± 0.12b
	10	14.22 ± 0.10a	6.89 ± 0.04b	15.26 ± 0.08ab	14.73 ± 0.07a
	15	14.42 ± 0.01a	7.09 ± 0.03a	15.55 ± 0.09a	14.76 ± 0.13a

Values are means of triplicates ± standard deviation. Means with the same superscript letters within the same column are not significantly different at p < 0.05 level

The hue angle of the native rice flour was 53.59 (KYF), 47.17 (BKF), 55.87 (RKF) and 57.91 (KKF), which was increased in irradiated flours (Table 2). The values for irradiated flours showed an increased trend from 53.59–57.49 (KYF), 47.17–57.05 (BKF), 55.87–58.87 (RKF) and 57.91–61.15 (KKF), with the highest value being recorded at 15 kGy dose. The chroma value of native flour was noted as 13.49 (KYF), 5.70 (BKF), 15.03 (RKF) and 13.03 (KKF), respectively, which was enhanced to 14.42, 7.09, 15.55, and 14.76 at 15 kGy doses. Reduction in lightness (L*) and redness (a*) with an increase in yellowness (b*) and chroma intensity indicates that pigmented flours were darker with an enormous amount of yellow tint, which in turn was caused by the caramelization of simple carbohydrates during high energy exposure (Abu et al. 2006).

Effect of γ-irradiation on functional properties

Bulk and tapped densities

Bulk density and tapped density of flour are key factors in the design of devices used for grain handling and storage, such as silos and hoppers (Nalladurai et al. 2002). The bulk density of the native rice flour samples (KKF < RKF < BKF < KYF) showed significant difference ranging from 398.41 − 501.67 kg/m³ respectively (Table 2). The bulk density of the irradiated rice flour samples decreased (KKF < RKF < BKF < KYF) from 395.12 kg/m³ to 497.52 kg/m³ at 5 kGy dose. Thereafter, the bulk density was reduced significantly (p < 0.05) with an increase in irradiation dose, and the least values were recorded as 492.86 kg/m³ (KYF), 406.86 kg/m³ (RKF), 419.03 kg/m³ (BKF) and 389.59 kg/m³ (KKF) in the samples irradiated with 15 kGy. The tapped density of the native rice flours (0 kGy) samples order as follows: KKF < RKF < BKF < KYF. The tapped density and bulk density of the irradiated flour was decreased significantly (p < 0.05) when compared with the native rice flour with respect to the increase in dosage. From Table 2, it is clear that flour treated with gamma-irradiation diminishes the bulk and tapped density of flour by up to 15 kGy, which is significantly lower than standard flour, which might be because of potent degradation of starch molecules present in the flour samples (Rombo et al. 2001).

Carboxyl content and pH

The levels of pH values and carboxyl content for native and irradiated pigmented rice flours are given in Table 3. Results demonstrated that carboxyl content increased significantly with an increasing dose of γ-irradiation. However, the percent increase in carboxyl content was higher in RKF when compared to other flours. The highest carboxyl content was found for 15 kGy dose treated flour. The increase in carboxyl content of native and γ-treated flours might be possible not only due to free radical oxidation breakdown, but also to starch molecule breakdown (Reddy and Viswanath 2019). This shift in carboxyl content could also be ascribed to variations in the generation of carboxylic acids due to free radical action (Suriya et al. 2017). The pH values of native pigmented rice flours are 6.31 (KYF), 6.61 (BKF), 6.65 (RKF) and 6.70 (KKF), respectively (Table 3). An increase in dosage noted a reduction in pH of the flour blends. The decrease in pH values was mainly owing to the partial interruption of starch fragments by γ-rays, thereby inducing the increase of carboxylic acid (Suriya et al. 2017).

Table 3.

Functional properties of native and γ-irradiated pigmented rice flours

Parameters	Dose (kGy)	KYF	BKF	RKF	KKF
Carboxyl content (%)	0	0.53 ± 0.01d	0.49 ± 0.01d	0.78 ± 0.02d	0.69 ± 0.01d
	5	0.61 ± 0.01c	0.58 ± 0.01c	0.89 ± 0.01c	0.74 ± 0.01c
	10	0.62 ± 0.01b	0.58 ± 0.01b	0.94 ± 0.02b	0.78 ± 0.01b
	15	0.69 ± 0.01a	0.60 ± 0.01a	0.99 ± 0.01a	0.90 ± 0.01a
pH	0	6.31 ± 0.05a	6.61 ± 0.01a	6.65 ± 0.03a	6.70 ± 0.01a
	5	6.21 ± 0.04b	6.56 ± 0.01b	6.53 ± 0.03b	6.59 ± 0.01b
	10	6.19 ± 0.06b	6.47 ± 0.04c	6.52 ± 0.01b	6.42 ± 0.01c
	15	5.98 ± 0.05c	6.09 ± 0.01d	6.20 ± 0.04c	6.18 ± 0.01d
Swelling power (g/g)	0	7.85 ± 0.28d	7.61 ± 0.15c	8.39 ± 0.25d	5.78 ± 0.03d
	5	10.09 ± 0.10b	8.05 ± 0.05b	10.91 ± 0.03a	9.07 ± 0.03b
	10	10.38 ± 0.02a	8.19 ± 0.01b	10.28 ± 0.03b	9.44 ± 0.03a
	15	8.75 ± 0.06c	9.37 ± 0.03a	8.79 ± 0.01c	7.78 ± 0.03c
Water absorption capacity (%)	0	240.96 ± 1.67c	247.33 ± 1.92b	253.61 ± 0.54c	250.09 ± 0.69d
	5	232.32 ± 0.71d	252.64 ± 0.99a	261.87 ± 0.88a	266.37 ± 0.86b
	10	251.21 ± 0.33a	254.83 ± 0.87a	257.95 ± 0.85b	269.63 ± 0.45a
	15	248.46 ± 1.08b	238.41 ± 1.49c	237.04 ± 1.08d	264.53 ± 1.10c
Water solubility index (%)	0	2.96 ± 0.02c	3.19 ± 0.01c	3.69 ± 0.01b	3.11 ± 0.02d
	5	3.86 ± 0.01a	3.94 ± 0.02b	3.63 ± 0.01c	3.34 ± 0.01b
	10	2.98 ± 0.02c	4.16 ± 0.02a	4.37 ± 0.01a	3.56 ± 0.01a
	15	3.57 ± 0.02b	3.92 ± 0.02b	4.37 ± 0.01a	3.29 ± 0.01c

Values are means of triplicates ± standard deviation. Means with the same superscript letters within the same column are not significantly different at p < 0.05 level

Swelling index

The swelling index of the native and irradiated pigmented rice flours is illustrated in Table 3. The swelling index of rice flour increased significantly with the rise of irradiation dose and revealed a possible reduction at 15 kGy dosage. Wani et al. (2015) found that degradation of amylopectin chains by gamma-irradiation caused a diminution in the swelling of flour, whilst the swelling power of rice flour granules is presumably due to the amylopectin fraction rather than the amylose molecules. De Kerf et al. (2001) found a significant decrease in amylopectin fragments of irradiated starches. The texture of the cooked food would be better because the swelling power would be less. This could be a good thing because the shattering of starch could be slowed down.

Water absorption capacity (WAC) and solubility

In Table 3, the WAC and solubility of native and irradiated pigmented rice flours are depicted. The WAC of pigmented rice flours increased significantly (p < 0.05) upon irradiation. The WAC of native rice flours is 240.96% (KYF), 247.33% (BKF), 253.61% (RKF) and 250.09% (KKF) respectively. Higher WAC of gamma-treated rice flours could be attributed to partial degradation of starch to dextrin, disaccharides, and monosaccharides with higher water affinity (Abu et al. 2006). The high WAC of gamma-treated flours is advantageous for use in the formulation of food products such as breads, sausages, and buns, extending freshness and shelf life (Wani et al. 2015). The highest WAC was recorded for 10 kGy treated flour (Table 3). However, further increments in irradiation dosage cause a reduction in the WAC of flour. This decrease in WAC might be due to polymer cross-linking that occurred concurrently with chain scission at a 15 kGy dose.

With an increase in irradiation doses for the flour samples, there was a significant increase in water solubility index (WSI) (Table 3). Wani et al. (2015) stated that the presence of soluble fragments like albumin, sugars, and amylose aids the solubility of flours. As a result, gamma irradiation depolymerizes the starch molecules into smaller fragments and hence it increases the smaller chain production, which can be easily dissolved by surrounding water molecules. As a result, it produces a large number of hydrogen bonds, which increases the solubility of flour.

Pasting properties

The pasting behaviors of the native and irradiated flours are shown in Supplementary Table 1 and Supplementary Fig. 2. Significant reductions (p < 0.05) in peak viscosity, hold, final, setback and breakdown viscosities were noted in pigmented flours by dose-dependent manner with increasing irradiation dose. The peak viscosity of native rice flours was 2040.0 cP (KYF), 1190.7 cP (BKF), 2069.33 cP (RKF) and 1378.33 cP (KKF), respectively. Chung and Liu (2010) stated that a substantial reduction in the peak viscosity of flours after irradiation might be possible due to the depletion of starch swelling index and degradation of amylose and amylopectin through chain scission. The disruption of starch particles caused by shearing and heating resulted in a reduction in hold viscosity. The re-ordering of leached amylose and long-linear amylopectin shortening is substantially responsible for the setback and final viscosities. Drastic reduction in the values of setback viscosity results in disruption of starch molecular structure that causes inability to develop longer helical networks and association between starch polymers, hence the induction of minimal starch retrogradation (Ocloo et al. 2017). The decrease in setback value also indicates that the food products produced from irradiated flour have a softer texture. Breakdown viscosity, which measures the ability of starch molecules to resist shear force during heating, decreased after irradiation. Diminution in breakdown viscosity stipulates that the irradiated flours are highly resistant to shear thinning during cooking. Pasting temperatures of native flours were 80.0 °C (KYF), 82.0 °C (BKF), 79.90 °C (RKF), and 87.23 °C (KKF), and decreased significantly after irradiation, indicating that starch granules have an increased ability to expand freely. Gamma irradiation causes a reduction in rice viscosities, which makes it unsuitable as a thickener but suitable for foods like porridges and any other food in which solids need to be added to enhance the quantity and quality of the child’s nutritional aspects as well as be used as fillers in starch matrix (Ocloo et al. 2017).

Morphological characteristics

The microscopic images of native and irradiated flours are displayed in Fig. 1. The size of the starch granules was varied from small to large, with different sizes and shapes, and agglomerated with some molecules. These molecules were the incidence of fibre, minerals, and protein bodies shattered during milling (Wani et al. 2015). The granules revealed spherical, polyhedral, and irregular shapes with varying sizes. There was no significant difference observed after irradiation of the flour samples except for BKF. In BKF, the surface of the native flour was smooth in texture, whereas in irradiated flour, the surface became rough, tightly packed, and more agglomeration was observed in 10 and 15 kGy treated flour. Flour did not show any superficial changes in shape and size after irradiation.

Fig. 1.

Scanning electron micrographs of native and gamma-irradiated pigmented rice flours. A-KYF 0 kGy, B-KYF 5 kGy, C-KYF 10 kGy, D-KYF 15 kGy, E-BKF 0 kGy, F-BKF 5 kGy, G-BKF 10 kGy, H-BKF 15 kGy, I-RKF 0 kGy, J-RKF 5 kGy, K-RKF 10 kGy, L-RKF 15 kGy, M-KKF 0 kGy, N-KKF 5 kGy, O-KKF 10 kGy, P-KKF 15 kGy

Total phenolic content

The effect of γ-irradiation on total phenolic content (TPC) in all native and irradiated rice flours is showed in Table 4. TPC values for native pigmented flours are 5.99 mg GAE/g (KYF), 3.33 mg GAE/g (BKF), 5.89 mg GAE/g (RKF), and 4.59 mg GAE/g (KKF), respectively. Results demonstrated that the γ-irradiation significantly (p < 0.05) altered, either reduced or enhanced the phenolic content in all rice flours as compared to their native counterparts. For KYF and KKF, γ-irradiation significantly increased their TPC as compared with native flour in a dose-dependent manner. However, in RKF, γ-irradiation at 5 kGy attained the highest TPC (10.23 mg GAE/g) as compared to native counterpart. For BKF, irradiation significantly (p < 0.05) improved their TPC, but their growth was not in a dose-dependent manner. Zhu et al. (2010) reported that γ-irradiation at specific doses enhance the antioxidant activities of flours. The variance in phenolic content of rice flours might be possible due to the stimulation of specific enzymes during irradiation treatment, resulting in improved or reduced the synthesis of phenolic compounds (Sultan et al. 2018). Thus, it is plausible that γ-irradiation may disrupt the phenolic acids resulting in a reduction of the content and activate some enzyme inducing the synthesis of the phenolic acids Zhu et al. 2010). In addition, the variance in phenolic content of rice flour might be possible due to release of phenolic compounds from the glycosidic compounds and with the degradation of larger phenolic compounds into smaller ones by γ-irradiation (Santos et al. 2022).

Table 4.

Antioxidant properties of native and γ-irradiated pigmented rice flours

Parameters	Dose (kGy)	KYF	BKF	RKF	KKF
Total phenolic content (mg GAE/g)	0	5.99 ± 0.11c	3.33 ± 0.02c	5.89 ± 0.20d	4.59 ± 0.11d
	5	8.48 ± 0.06b	3.99 ± 0.05a	10.23 ± 0.10a	5.48 ± 0.07c
	10	8.71 ± 0.19b	3.45 ± 0.13c	7.59 ± 0.16b	7.82 ± 0.11b
	15	9.40 ± 0.25a	3.77 ± 0.11b	7.14 ± 0.11c	8.99 ± 0.21a
Total flavonoid content (mg CE/g)	0	71.10 ± 2.36a	44.08 ± 0.38a	84.40 ± 0.95a	51.33 ± 2.02b
	5	60.60 ± 2.07b	42.93 ± 2.01a	66.18 ± 0.85b	47.30 ± 0.70c
	10	51.41 ± 1.61c	36.30 ± 2.68b	54.35 ± 1.60c	60.23 ± 1.17a
	15	51.78 ± 2.05c	42.33 ± 0.96a	53.63 ± 1.39c	58.23 ± 2.57a
DPPH (%)	0	99.45 ± 0.14a	95.48 ± 0.24a	99.52 ± 0.00a	99.61 ± 0.04a
	5	98.59 ± 0.51ab	93.24 ± 0.46b	98.86 ± 0.03b	98.94 ± 0.03b
	10	98.68 ± 0.21ab	91.99 ± 0.63c	98.51 ± 0.44b	98.85 ± 0.04c
	15	98.07 ± 0.73b	92.63 ± 0.10bc	98.53 ± 0.32b	98.71 ± 0.03d
ABTS (mmol TE/g)	0	453.43 ± 1.15a	207.80 ± 2.30c	452.39 ± 0.01a	433.14 ± 0.63a
	5	399.93 ± 5.39c	226.65 ± 2.92b	364.32 ± 3.59bc	305.70 ± 1.20d
	10	435.15 ± 4.74b	230.39 ± 4.41b	354.29 ± 1.61c	381.09 ± 1.94c
	15	446.31 ± 5.50a	247.79 ± 2.18a	373.24 ± 2.15b	414.27 ± 1.07b

Values are means of triplicates ± standard deviation. Means with the same superscript letters within the same column are not significantly different at p < 0.05 level. mg GAE/g, mg gallic acid equivalent per gram; mg CE/g, mg catechin equivalent per gram; DPPH, 2,2-diphenyl-1-picryl hydrazyl; ABTS (mmol TE/g), 2,2-azino-bis-(3 ethylbenzothiazoline-6-sulphonic acid) diammonium salt (mmol trolox equivalent per gram)

Total flavonoid content

Flavonoids are a group of secondary metabolites with potent antioxidant capacity and metal chelating properties. TFC of native flours is 71.10 mg CE/g (KYF), 44.08 mg CE/g (BKF), 84.40 mg CE/g (RKF), and 51.33 mg CE/g (KKF), respectively (Table 4). Reduction in the TFC of rice flour samples was observed with an increase in irradiation doses. For KKF, a significant increase was noted in 10 kGy dosage when compared to their analogous native sample. Verma et al. (2016) also reported a significant decrease or increase in TFC in different mungbean cultivars upon irradiation. The generation of free radicals during γ-irradiation modifies the structure of flavonoids, which consequently decreased solubility in the extraction medium and hence a decrease in trend was observed in TFC. The creation of novel flavonoids by breakdown of bigger flavonoids could explain the increase in TFC in 10 and 15 kGy dosages for KKF.

ABTS radical scavenging activity

The antioxidant potential of native and irradiated flours was analyzed by using the ABTS radical scavenging method (Table 4). As a result, γ-irradiation at various dosage levels resulted in a significant difference (p < 0.05), although not in a dose-dependent manner. A significant reduction in radical scavenging activity was observed for KYF, RKF, and KKF, respectively. But the BKF rice cultivar showed an increasing trend with increasing irradiation dose, but not in a dose-dependent manner. According to Chawla et al. (2009), the maillard reaction may be responsible for the development of novel derivates due to increased antioxidant activity in rice flour samples.

DPPH radical scavenging activity

In Table 4, the DPPH radical scavenging activity (RSA) of native and gamma-treated rice flours is depicted. A decrease in absorbance at 517 nm defines the reaction between DPPH and phytochemicals, as well as the antioxidant reducing ability of pigmented whole grains (Bhat and Riar 2017). With increasing irradiation dose, a significant (p < 0.05) reduction in RSA of rice flour was observed, but not in a dose-dependent manner. The decrease in RSA of rice flours could be owing to the decline of phenolic compounds. The native KKF recorded the highest antioxidant activity (99.61%). The irradiated pigmented rice flour showed an increase in antioxidant activity at a 5 kGy dosage (98.94%). This increase in antioxidant activity may be the result of the maillard reaction, which would have formed novel derivative compounds. Reduction in RSA of samples might be owing to the development of free radicals and higher doses of γ-irradiation in samples.

Fourier transform infrared spectroscopy

The FTIR spectroscopy proved to be a dynamic tool for identifying the presence of functional groups as well as substantiating the reduction of starch double helical crystalline order and glycosidic bond disruption. Spectrum of native and irradiated flours of various rice cultivars is depicted in Fig. 2. After irradiation, the spectrum remained unchanged, and no new functional compounds were formed. This confirms that the flour treated with gamma irradiation did not produce any new compounds. The band 3395 cm⁻¹ in the area of 3300–3600 cm⁻¹ corresponds to O–H stretching complex with strong intensity. Peak at 2931 cm⁻¹ represents the C–H stretching vibration mode. Two prominent peaks 1545 cm⁻¹ and 1645 cm⁻¹ primarily correspond to coupling of NH deformation (amide II) and C=O stretching (amide I) especially in proteins where the hydrogen bonding abilities of amide linkages partly take place in the protein secondary structure. Band at 1412 cm⁻¹ represents C–N in primary amide (Amide II). Sharp band at 1155 cm⁻¹ and 1080 cm⁻¹ are usually assigned to the coupling of stretching C–C and C–O and bending C–O–H band, respectively. Absorption IR peak at 1022 cm⁻¹ is associated to amorphous starch structure (Suriya et al. 2017). IR peaks below 1000 cm⁻¹ (called as fingerprint region) with bands at 930 cm⁻¹ (amylose (α-1,4) linkage), 859 cm⁻¹ (C–O–C ring mode and C1-H deformation mode) and 764 cm⁻¹ (C–C stretching), respectively.

Fig. 2.

FTIR spectrum of native and gamma-irradiated pigmented rice flours

Conclusion

The current study revealed that pigmented rice flour irradiated at a dosage of 5–15 kGy exhibited significant changes with enhancement in irradiation dosage in various characteristics like amylose, bulk density, tapped density, color attributes, carboxyl content, pH, syneresis, swelling power, water absorption capacity, water solubility, total phenol, total flavonoid and ABTS radical scavenging activity. At the same time, significant changes were not observed in morphological characteristics and spectral analysis using FTIR. The pasting viscosities of the irradiated rice flours reduced drastically which is advantageous in formulation of novel baby food products. Further, pigmented rice flour irradiated less than 10 kGy helps to minimize the loss of phenolic acids and can be effectively used in the development of value-added rice-based food products considering all over beneficial and safety aspects.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

SM: Investigation, Methodology, Writing – Review and Editing; MK: Conceptualization, Methodology, Writing—original draft, Data curation; SH: Conceptualization, Supervision, Writing—Review and Editing; JD: Formal analysis, Visualization, Investigation; CMB: Formal analysis, Visualization, Investigation; PG: Formal analysis, Investigation; CKR: Writing—Review and Editing. The article has not been submitted for publication in any other journal and this submission is with the approval by all the authors.

Funding

Pondicherry University, Puducherry, India.

Data availability

The data generated during this research work is available with the authors.

Code availability

Not applicable to the study.

Declarations

Conflict of interest

The authors have no conflicts in the submission and publication of the work.

Consent to participate

All the authors have given the consent to participate in the work.

Consent for publication

The authors have no objection in the publication of data in tables or figures presented in this paper.