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Journal of Food Science and Technology logoLink to Journal of Food Science and Technology
. 2024 Aug 29;62(4):706–715. doi: 10.1007/s13197-024-06060-7

Improving the cooking quality of black glutinous rice by using the low-pressure cold plasma technology

Kamonporn Panngom 1, Patpen Penjumras 2, Wasinee Panjan 1, Jirapong Sornsakdanuphap 1, Pradung Suanpoot 3, Sukree Yoosuk 4, Isara Wattananapakasem 2,✉
PMCID: PMC11914404  PMID: 40109687

Abstract

Black glutinous rice grains are hard cooking even though they have been soaked in water for a long period of time. This study aimed to improve the cooking quality of black glutinous rice by the low-pressure cold plasma. The low-pressure cold plasma significantly increased the lightness of the rice grains, especially the oxygen and air plasma. The water uptake of the rice grains was significantly increased after plasma treatment. No difference among the plasmas generated by Ar, Air or O2 gases. SEM examination showed that the surface structure of rice grains changed from smoothness to roughness throughout the break and reduced the contact angle. Air and O2plasma increased the softness and stickiness of cooked rice. Anthocyanin content was significantly increased in the range of 14.59 to 25.97 mg/100g in treated sample than those of untreated (10.72 mg/100 g). Air and O2plasma could modify the surface structure of rice grains due to increased softness and stickiness while reducing hardness. Therefore, low-pressure cold plasma could be a potential innovative technology for improving the cooking quality process while maintaining the nutritional value of black glutinous rice grains.

Keywords: Black glutinous rice, Anthocyanin, Cooking properties, Low-pressure cold plasma

Highlights

  • Air and O2 plasma modified the surface of rice grains due to increased softness and stickiness.

  • Anthocyanin content of plasma-treated rice grains was increased compared to untreated rice gains.

  • Cold plasma could be a potential innovative technology to improve the cooking quality process.

Introduction

Innovative plasma technology is an emerging alternative technology that could improve and ensure the quality of postharvest productivity (Lopez et al. 2019) and could be applied to food preservation, extending the shelf life, and bioactive compounds maintained in foods (Ganesan et al. 2020). It is well known that low temperature plasmas produce multiple chemical components consisting of charged particles (electrons, negative, and positive ions), radical species (superoxide: O-2; hydroxyl radicals: OH; nitric oxide: NO), and UV photons (Fridman 2008). The application of low temperature plasma to improve the cooking process of brown rice has been reported in previous studies. Park et al. (2020) and Sarangapani et al. (2016) showed the positive effects of cold plasma treatment on germinated brown rice in terms of the length, weight, and phenolic content of the seedling. (Yodpitaka et al. 2019) reported that cold plasma technology increased the percentage of germinated brown rice, phenolic compounds, vitamin E, phytosterols, triterpenoids, and anthocyanins. Furthermore, anthocyanins extracted from purple glutinous rice significantly increased with cold plasma modification (Poomanee et al. 2021). In addition, cold plasma treatment of brown rice could effectively increase the water absorption and change the textural properties by reducing the hardness of the rice grains (Liu et al. 2021; Lee et al. 2016). However, a few previous studies showed the effect of low-temperature plasma generated under low-pressure conditions on physicochemical properties and cooking quality of colored rice. This has not been reported yet about the application of low-pressure cold plasma to increase the cooking quality of the pigmented glutinous rice of Thailand.

Black glutinous rice is a pigmented rice variety and highly valued as a staple food because it is rich in antioxidants (anthocyanin and gamma-oryzanol), fiber, minerals, and several important amino acids that are good for human health (Yamuangmorn and Promuthai 2021). Black glutinous rice contains dark purple pigments, especially anthocyanins, which are found in the aleurone layers of the rice grains (Hu et al. 2003). Due to the high nutritional contents of black glutinous rice, its demand is increasing all over the world, especially among health-conscious people (Yamuangmorn and Promuthai 2021). However, the cooking process of black glutinous rice consists of several steps and requires some time for soaking in water. In some varieties, the black glutinous rice grains are still hard after cooking even though they have been soaked in water for a long period of time. Normally, soaking is an important step to increase the water diffusion to the inside of the rice grains because the rice grains are cooked only briefly. However, the pericarp layers of the rice grains may contain various compounds that would prevent water absorption, such as proteins, fat fibers, and ash (Noppakun et al. 2021). Black glutinous rice grains would need having their surface modified without losing the nutritional value to improve the cooking quality procedure. Several conventional cooking methods, such as steam cooking, superheated steaming but those techniques are lessening the antioxidants, bioactive compounds in rice pigments (Thuengtung and Ogawa 2019; Piyawanitpong et al. 2018). Ultrasonic technology has been previously studied by combining the parboiled rice that enhanced the yield, texture, and quantity of free and bound polyphenols. There is now more interest in alternative technology for raising the caliber of the cooking process. Plasma technology is one of these new generation alternatives that changed the physical and chemical properties of brown rice with an increase in the cooking quality process. (Wu et al. 2023; Charoux et al. 2021; Charoux et al. 2020). Based on the above information, plasma generated from different gases may affect the physical properties of pigmented rice grains differently depending on the enhancement of the cooking process.

Thus, the aim of this research was to evaluate the effect of low-pressure cold plasma on improving the cooking quality of black glutinous rice grains (Doi Saket). Three different gases (Ar, air, and O2) were fed to generate plasma under low-pressure, and several time treatments were compared. Pigmented glutinous rice grains treated with low-pressure cold plasma were analyzed for their seed color, water absorption, surface structure, contact angle, and textural properties. The sensory test of the cooked rice was also mentioned in this research.

Materials and methods

Plasma device structure

Figure 1 shows the structure and optical emission spectroscopy (OES) low-pressure cold plasma equipment at Maejo University Phrae Campus, Phrae, Thailand. Figure 1A illustrates the plasma equipment composed of a cylindrical stainless-steel 304 vacuum chamber having a diameter of 32 cm and a height of 60 cm, or the volume of this reactor was about 50 L. The chamber was equipped with several ports for diagnostics and was evacuated to a residual pressure of 10–100 mTorr (0.01–0.13 mbar) by a pumping system composed of a rotary backing pump. The actual pressure in the chamber was determined by a manual leakage needle valve connected to the gas (argon, air, and oxygen) feeding system. During the operation, the pressure inside the chamber was monitored by an active Pirani gauge (EDWARDS® APG high vacuum gauge, USA). The capacitively coupled plasma (CCP) discharged technique was used to generate the plasma by an RF power supply (Kurt J. Lesker, KJLC® Research R601 RF Power Generator, USA), having a frequency of 13.56 MHz. The operating power of the RF varied between 80 and 100 W. Figure 1B-D show the OES measurement of plasma by using AvaSpec-ULS3648 (Avantes Inc., USA) and the color of the different plasma which was generated from the different gas. The color of the oxygen plasma was light gray, the air plasma was red-pink, and the argon plasma was violet.

Fig. 1.

Fig. 1

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The schematic of plasma structure device and optical emission spectroscopy of plasmas. (A) Plasma device structure, plasma light and plasma chamber, (B) The OES of argon plasma, (C) The OES of air plasma, and (D) The OES of oxygen plasma. The OES measurement of plasmas by using AvaSpec-ULS3648 (Avantes Inc., USA)

Preparation of the rice grains and plasma treatment conditions

Black glutinous rice grain cultivar Doi Saket was used for this experiment. Rice seeds were purchased from agriculturist farmer network growing seeds for Lanna Rice Research Center, Chiang Mai University. For preparing the rice grains, the rice seed coat was removed without damaging the grains’ surface and maintaining the purple color with brown rice chilling machine (NW 250, Green bee, Natrawee Technology Co., Ltd). Then, 100 g of rice grains were put in the plasma chamber (Fig. 1A), and a vacuum pump was turned on until the pressure in the plasma chamber was reduced to 50 mTorr (0.067 mbar). The times of the plasma treatment were varied from 10, 20, and 30 min, respectively by feeding three gases comprising oxygen, air, and argon, and the RF power was set at 100 W. The rice grains without plasma treatment were used as the control.

Physical properties

Color measurements

The color of the rice grain after the plasma treatment with conditions mention in above and untreated grain (control) was measured using a HunterLab Colorimeter (ColorFlex EZ, HunterLab, USA). Sample of the rice grain, approximately 30 g, were measured the color change on the rice grain surface affect from plasma. For condition measurement of samples, the geometry included directional annular 45° illumination and 0° viewing, the light source was a pulsed Xenon lamp with one flash per measurement, and the number of measurements was 5 replications. In terms of the color, the HunterLab instrument contained L* (lightness ranging from 0 to 100 indicating black to white), a* (+ a*; redness and -a* greenness), and b* (+ b*; yellowness and -b*; blueness). The hue value was also calculated (for the hue (H° ab), 0° indicated a tone of red). The results were expressed in the CIE L*a* andb* values, and a H° angle was calculated.

Water absorption capacity (%)

Water absorption was measured using the modification method of Alomayri et al. (2014). Samples of the rice grain, approximately 3 g, were weighed to an accuracy of ± 0.1 mg. The samples were transferred into a petri dish, then 30 ml of distilled water was added. The petri dishes were then placed at room temperature for 30 min. At specific times, the soaked rice was taken from the water immersion, and tissue paper was used to remove the surface water and subsequently weighed by electronic balance. The percentage of water absorption was calculated from the increased weight of the rice kernels after soaking. The water absorption was calculated as shown in Eq. (1).

graphic file with name M1.gif 1

Scanning electron microscopy (SEM) examination

Scanning electron microscopy (SEM) (Leo, 1450VP, Zeiss, Germany) was used to characterize the surface morphology of the rice grains for the plasma treatment of 20 min and the control. The rice grains after treatment with plasma for 20 min showed an increase in the cooking quality process without damage nutrients. Rice grains treated by plasma and untreated grains were ionized with gold coating for 5 min, and then observation of the surface structure was performed by SEM (LEO 1450VP, Zeiss, Germany).

Contact angle imaging

A sample of black glutinous rice was treated to the oxygen, air, and argon plasma conditions in order to assess the contact angle. A positive control was the untreated plasma. Each condition’s contact angle was measured independently for four times, and the average was calculated. A high-resolution digital camera was used to capture images of a droplet of around 1 µl of DI water that was placed on the surface of the rice grain. With the help of the drop assessment LB-ADSA plug-in, ImageJ 1.52a software was applied to assess the contact angle from each image (Oh and Park 2018).

Anthocyanin

Anthocyanin was determined by high performance liquid chromatography (HPLC; Agilent 1260, Agilent Technologies, Santa Clara, CA, USA) according to the modified method of Sompong et al. (2011). The sample was extracted with 1 M, w/v of citric acid solution for 10 min. A reversed phase with an Agilent ZORBAX SB-C18 column (250 mm×4.6 mm, 5 μm) column and the detector was set at λ = 520 nm for anthocyanin. The anthocyanin was separated in gradient condition using the mobile phase with 1% of formic acid in water (A) and acetonitrile (B) which mobile phase flow rates was set at 0.8 ml/min. Cyanidin 3-O-glucoside chloride was used as a standard reference.

Textural profile analysis (TPA)

A texture analyzer using a texture profiling program was used to compare the textures of the cooked rice before and after the plasma treatment (TA.XTPlus, Texture Analyzer, Stable Micro System Ltd., UK). Samples for analysis were prepared by soaking each 50 g of treated and untreated plasma in sterile water. The samples were cooked at a temperature of 80 °C for 30 min after soaking for 30 min. Then, 30 g of cooked rice was analyzed using a cylindrical probe (P/36R 36 mm in diameter) under the following conditions: A pre-test speed of 10 mm/s, an auto test type, a test speed of 5 mm/s, a force of 5 g, a post-test speed of 0.5 mm/s, a load cell of 50 kg, and a strain of 80% were conducted by using Thirumdas et al.’s (2016) method.

Sensory evaluation

Sensory attributes were evaluated using the modification method of Borowski et al. (2015). Approximately 30 g of rice grains by the plasma treatment and the control were soaked in distilled water for 30 min, then the water was removed, and the rice grains were cooked for 30 min in a steamer that was inverted during the cooking time. Immediately after cooking, 5 g of each cooked rice was put into plastic cups for testing that were labeled with a three-digit random code and covered with a lid. The cooked rice samples were kept in the warming cabinet to control the temperature at 60 °C throughout the test. All samples were prepared 30 min prior to panel evaluation, and the cooked rice was served and tested at 60 °C. Two sessions were carried out with 5 samples being evaluated in each session and 10-min break between sessions. The panelists evaluated the cooked rice in a testing area and were instructed to rinse their mouths with water between samples to minimize any residual effect.

Six sensory attributes of the cooked rice (purple color intensity, aroma intensity, softness, stickiness, sweetness intensity and overall liking were evaluated using a five-numeric scale with the lowest intensity (score 1), to the highest intensity (score 5). Thirty untrained panelists were recruited from the students and staff at Maejo University-Phrae Campus. All the untrained panelists were informed on how to evaluate the score of the cooked rice. Different color appearance intensity was observed by visual perception. Aroma intensity of cooked rice was described as a specific aroma related to purple rice by smelling. For texture, softness and stickiness of cooked rice was evaluated during the chewing as the force required to compress and separate the cooked rice adhered to the teeth or palate. Intensity of sweetness attributes like the natural sweetness of sticky rice, compared to sugar, it is not too sweet. Finally, overall liking was acceptance of all sensory attributes of the cooked rice.

Statistical analysis

All data were calculated as the mean and standard deviation (SD) for indicating the number of replicates. Experimental data were carried out using the completely randomized design (CRD) in physical properties analysis and the randomized complete block design (RCBD) in sensory evaluation. For the statistical analysis, the data were subjected to analysis of variance (ANOVA) using SPSS for Windows version 24. In case of any differences in the mean, multiple comparisons were performed using Duncan’s Multiple Range Test (DMRT) at a 5% level of significance (p < 0.05).

Results and discussions

The effect of plasma on the color of the rice grain

The lightness (L* value) of rice grain was greatly affected by plasma treatment (Table 1). There was an increase in the L* value of the rice grains treated with the plasma of oxygen and air, which indicated the increase in the lightness of the rice grains after treatment. Simultaneously, no significant change was investigated for the L* value of the rice sample treated by argon plasma compared to control (p > 0.05). The rice grains’ brightness was not distinguished between the oxygen plasma and air plasma treatments. Additionally, there was no difference within the treatment. Previous research found that the rice grain surface had discoloration by using oxygen plasma that could be due to a degradation of the polysaccharides on the surface of the rice grain (Noppakun et al. 2021, Selzuk et al. 2008). In addition, Thirumdas et al. (2016) reported that the increase in the L* value could be related to etching caused by the plasma species on the bran layer exposing the inner white core to the surface. Although there was no significant change in the a* value, b* value, and hue value for all samples, there was no effect from the increase of the treatment time. A similar result was found by Thirumdas et al. (2015) after the plasma application to Basmati rice.

Table 1.

The color of the rice grain surface after different plasma treatments

Condition L * a * b * ns Hue ns
Control 13.41±1.53c 7.75±2.53ab 11.12±1.15 0.97±0.19
Ar Plasma 10 min 15.46±0.61bc 8.15±0.76a 10.45±2.08 0.90±0.06
20 min 15.56±1.87bc 8.57±0.54a 10.76±2.15 0.89±0.08
30 min 16.23±2.34abc 8.31±1.07a 10.11±0.54 0.88±0.09
Air Plasma 10 min 17.02±1.46ab 7.90±0.33ab 10.87±0.97 0.94±0.05
20 min 16.81±1.41ab 6.59±0.40ab 9.80±0.49 0.98±0.03
30 min 17.88±0.59ab 6.61±1.23ab 9.20±0.13 0.95±0.09
O2 Plasma 10 min 18.95±2.20a 6.24±0.89ab 8.47±2.15 0.92±0.19
20 min 18.07±0.88ab 7.27±3.95ab 7.75±1.87 0.85±0.36
30 min 18.25±2.63ab 4.67±1.52b 8.30±3.55 1.04±0.16
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L* : lightness; a* :+ a*= redness, -a*= greenness ; b*: + b*=yellow, -b*=blue

All the data expressed as mean ± standard deviations (n = 3)

ns within the same column are not significantly different (p > 0.05)

a – c Different letter superscripts in the same column indicate statistical difference (p < 0.05)

The effect of plasma on the water absorption, surface morphology, and contact angle of the rice grain

The percentages of the water absorption and the surface morphology of rice grain by SEM are shown in Fig. 2A and B, respectively. A significant difference (p < 0.05) between the untreated and treated samples with plasmas (Ar, air, and O2) for the percentage of the water absorption was observed in Fig. 2A. This was due to the plasma immanent species like electrons and photons breaking the C-C and C-H bonds on a solid surface that disrupted the structure of the surface then it could increase the water absorption (Sarangapani et al. 2015). The untreated samples showed a normal grain structure without any observable change in the morphology of the bran layer. After the plasma treatment, the roughness on the grain surface of the rice kernel was increased. The result could be clearly seen by the fissures and formations of holes through which water could easily penetrate from the surface into the inside of the grain. Additionally, Fig. 2C shows the hydrophilic properties of the rice grains through the contact angle results after the plasma treatment. When the contact angle values of various plasmas were compared, it was discovered that the oxygen plasma could change the contact angle more than the air or argon plasmas, but there was no significant difference among them. Furthermore, it was discovered that after receiving oxygen plasma treatment for 10, 20, and 30 min, respectively, the water contact angles of the rice grains were reduced to 90.25°, 90.56°, and 90.07°. Various reactive species may also be produced by various gases in plasma (Mir et al. 2016). This had an impact on how the surface structures of the rice grains changed over time. According to certain research, oxygen plasma is made up of reactive species in the hydroxyl, carboxyl, and carbonyl groups. The surfaces of the starch materials that appeared to be hydrophobic may be altered by all of these such that they turn hydrophilic (Thirumdas et al. 2015). According to Thirumdas et al. (2016), the preferential plasma etching was primarily based on the differing etching rates of the polymeric surface when exposed to gaseous plasma. The extent of the etching or ablation could also be altered by changing the gas for a plasma generation, and each gas had a unique ability to alter the surface properties. Furthermore, Fig. 2B shows that argon plasma had a high surface roughness when compared to other gases. According to Zou et al. (2004), glow discharge plasma generated from argon gas could be used to modify starch by generating cross-linking of a-D-glucose due to the argon species colliding with the hydroxyl group. Argon plasma could also change the surface topography and surface chemistry of the powder (Dixit et al. 2018; Noppakun et al. 2021). Typically, a brown rice kernel has bran containing protein, fat fiber, and ash, which would prevent the water uptake into the grain (Khatoon and Gopalakrisha 2004; Noppakun et al. 2021). Consequently, increasing the water absorption rate after the plasma treatment indicated that the plasma eroded the grain surface, thus affecting the acceleration of the water absorption during immersion (Chen et al. 2012).

Fig. 2.

Fig. 2

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The percentage of water absorption, surface structure and contact angle of the rice grains after plasma treatment for 10, 20 and 30 min. (A) The percentage of water absorption of the rice grain after plasma treatment for 10, 20 and min (n = 3). (B) SEM micrograph of the surface structure of the rice grain after plasma treatment for 20 min. (C) The contract angle of the rice grain after plasma treatment for 10, 20 and 30 min (n = 3)

Anthocyanin content

Black glutinous rice contains anthocyanins, which are found in the aleurone layers of the rice grains. Black glutinous rice sample had 10.72 mg/100g of anthocyanin content before plasma treatment. Higher anthocyanin content of plasma treated with oxygen, air and argon at 30 min had 25.97 mg/100 g, 25.38 mg/100 g and 23.15 mg/100g respectively. The anthocyanin content of plasma treated black rice was significantly different (p < 0.05) in comparison with the untreated rice sample (Table 2).

Table 2.

The anthocyanin content in the rice grain after different plasma treatments

Condition Anthocyanin content (mg/100 g)
Control 10.72 ± 0.46a
Ar Plasma 10 min 14.59 ± 0.63b
20 min 19.28 ± 0.39d
30 min 23.15 ± 0.92e
Air Plasma 10 min 16.62 ± 0.80c
20 min 24.33 ± 1.15ef
30 min 25.38 ± 0.54fg
O2 Plasma 10 min 14.63 ± 0.50b
20 min 19.50 ± 1.36d
30 min 25.97 ± 0.96g
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All the data expressed as mean ± standard deviations (n = 3)

a – g Different letter superscripts in the same column indicate statistical difference (p < 0.05)

The anthocyanin content in black glutinous rice was increased after the grafting reaction with the reactive oxygen species (ROS) from plasma treatment. The increased anthocyanin content with plasma generated by free radical such as O, O2, O3, O2+, O3+, NO and N. This could be due to breakdown of vacuoles and chemical degradation of the polyphenols and structural molecules under stress condition with the increasing treatment time (from 10 to 30 min) which could indicate the structural degradation in layered structure could be explained by increasing of anthocyanin under stress conditions. The same trend has been reported in pomegranate juice extracts that showed the amount of anthocyanins and phenolic compounds in were increased after plasma treatment (Kovačević et al. 2015).

The effect of plasma on the textural properties of the cooked glutinous rice

After the plasma treatment with oxygen, argon, and air under a low-pressure condition, the treated sample improved the textural properties which resulted in a decrease in the hardness and an increase in the adhesiveness (Table 3). The hardness of the treated cooked rice was significantly different (p < 0.05) when compared with the control. The adhesiveness of the rice grain after being treated with the plasmas from argon, oxygen, and air was increased, especially, the oxygen treatment that likely improved the stickiness of the cooked rice sample. Therefore, decreasing the rice hardness and increasing the adhesiveness could be related to improving the soft texture of the cooked rice grains. Our findings agreed with those reported by Zhou et al. (2007), who showed that the stickiness and hardness of starch granules were related to how much water they absorbed. During the cooking process, the rice grains would absorb water and then expand. The hydrogen bonds between the starch molecules of the starch rice grain and the absorbed water were broken and destroyed as a result of the rice grains expansion, which made the grains swell. This had an impact on the gelatinization level, which had the consequence of making the cooked rice grains less hard and stickier.

Table 3.

The texture properties of the cooked rice grain after different plasma treatments

Condition Hardness (g) Adhesiveness (g.sec)
Control 55784.21 ± 4874.69a -57.77 ± 20.76ef
Ar Plasma 10 min 41693.63 ± 3618.21b -60.75 ± 21.83ef
20 min 33124.59 ± 1540.64c -91.39 ± 17.13cdef
30 min 36029.56 ± 2025.75c -82.11 ± 10.36def
Air Plasma 10 min 36858.43 ± 2569.58bc -52.81 ± 11.5f
20 min 33177.31 ± 2974.57c -98.72 ± 33.80cde
30 min 32301.56 ± 2800.56c -104.82 ± 30.86cd
O2 Plasma 10 min 35568.20 ± 6514.12c -162.24 ± 32.09b
20 min 32728.15 ± 2646.78c -131.59 ± 44.37bc
30 min 33352.01 ± 3892.45c -202.45 ± 17.36a
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All the data expressed as mean ± standard deviations (n = 3)

a – f Different letter superscripts in the same column indicate statistical difference (p < 0.05)

Sensory evaluation

The effect of plasma on the sensory quality of the cooked glutinous rice

The sensory characteristic was investigated using a five-numeric scale. The low-pressure cold plasma affected the sensory characteristics of the rice sample (Table 4). For the color of the cooked rice samples, it was found that untreated rice grains and plasma treated rice grains were not significant (p > 0.05). The softness and stickiness were the important parameters which were considered for the texture evaluation and overall preference between the untreated and treated samples. There was an increase in the softness and the stickiness score with the increasing of the treatment time on the sample with treated plasma especially the treated plasma with air and oxygen, which indicated the decreasing of the hardness and increasing of the stickiness of the rice sample. In addition, increasing of the stickiness and the sweetness intensity following the treatment time on the sample with air plasma were found. This could be due to the increasing of the water absorption, which affected the increase in the rice softness (Mohapatra and Bal 2006; Noppakun et al. 2021). However, the increasing time of treated argon plasma had a negative impact on the stickiness score. This could be due to the increase in the surface roughness of the rice grain after the plasma treatment followed by providing too high softness and the cause of agglomeration of the rice samples. Moreover, the increase in the time treating with the oxygen and air plasmas provided a higher score of the sweetness intensity. This could be due to an increase in reducing the sugar content by the plasma treatment that affected the higher sweetness intensity in the treated samples. The increase in reducing the sugar of the plasma treated rice was found by Lee et al. (2018) after the plasma application to the cooked brown rice. Likewise, Sarangapani et al. (2016) reported that reducing the sugar content of the parboiled rice flour increased after treatment with low-pressure plasma. This increase in the amount of reducing the sugar occurred due to the dextrinization of the starch with plasma (Lii et al. 2002). The findings of this study demonstrated the existence of an important and considerable value that could be utilized and scaled up in the industry; nevertheless, further research into the physical and chemical properties would be required for large-scale applications.

Table 4.

Sensory acceptance scores for all attributes of the cooked rice grain after different plasma treatments

Condition Sensory attributes
Color ns Aroma Softness Stickiness Sweetness Overall liking
Control 2.80 ± 0.82 2.35 ± 0.76cd 1.52 ± 0.54a 1.38 ± 0.61a 2.13 ± 0.39a 2.00 ± 0.69a
Ar Plasma 10 min 2.90 ± 0.84 2.78 ± 1.06f 2.10 ± 0.84b 3.23 ± 1.05b 2.25 ± 0.50ab 2.43 ± 0.85bc
20 min 2.73 ± 0.82 2.23 ± 0.77c 2.23 ± 0.89b 3.03 ± 1.01b 2.20 ± 0.51a 2.32 ± 770ab
30 min 2.85 ± 0.84 2.60 ± 0.91df 2.60 ± 1.15cd 2.98 ± 0.93b 2.48 ± 0.65bc 2.95 ± 0.93d
Air Plasma 10 min 3.08 ± 1.17 2.47 ± 0.98cdf 2.33 ± 0.60bc 2.95 ± 0.91b 2.57 ± 0.65cd 2.67 ± 0.68cd
20 min 2.98 ± 1.03 1.87 ± 0.72ab 2.93 ± 0.84f 3.03 ± 0.97b 2.77 ± 0.85d 2.92 ± 0.98d
30 min 2.83 ± 1.17 2.13 ± 0.89abc 3.15 ± 1.02f 3.20 ± 0.82b 2.68 ± 0.81cd 2.83 ± 1.17d
O2 Plasma 10 min 3.08 ± 1.14 2.45 ± 0.92cdf 2.35 ± 0.58bc 2.93 ± 0.94b 2.58 ± 0.67cd 2.72 ± 0.72cd
20 min 2.97 ± 1.06 1.83 ± 0.79a 2.83 ± 0.87df 3.03 ± 0.96b 2.80 ± 0.84d 2.92 ± 0.93d
30 min 2.83 ± 1.12 2.18 ± 0.85bc 3.03 ± 1.12f 3.13 ± 0.89b 2.68 ± 0.83cd 2.78 ± 1.09d
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All the data expressed as mean ± standard deviations (n = 60)

ns within the same column are not significantly different (p > 0.05)

a – f Different letter superscripts in the same column indicate statistical difference (p < 0.05)

Conclusion

In summary, it appears that low-pressure plasma had an effect on how effectively black glutinous rice cooked. As the rice grains were treated with plasma composed of oxygen and air gases, the low-pressure plasma could increase the whiteness of the rice grains. Additionally, the low-pressure plasma treatment on the rice grains could cause the surface structure to change from smooth to rough due to an increase in the water absorption and a decrease in the contact angle (from hydrophobic to hydrophilic). The textural feature could also be observed to increase the adhesiveness while decreasing the hardness. Furthermore, the sensory evaluation showed that the cooked rice grains increased the softness and stickiness score after the plasma treatment. When comparing the effects of the various gases on black glutinous rice grain, oxygen and air gases generated plasma outperformed argon. Moreover, plasma treated rice sample demonstrated the increase in anthocyanin content. The results revealed the data from the water absorption, SEM, contact angle, textural properties, sensory score, and anthocyanin content thus demonstrating that plasma treated rice sample, especially the oxygen and air plasma source were more effective in improving the cooking of black glutinous rice. Our results demonstrated that low-pressure cold plasma could be useful for improving the black glutinous rice grain’s cooking process and properties.

Acknowledgements

This study was financially supported by office of agricultural research and extension Maejo University (Grant No. MJ.1-62-022.1).

Author contributions

Kamonporn Panngom and Patpen Penjumras performed the experimental design, data collection, wrote, and manuscript editing, Wasinee Panjan and Jirapong Sornsakdanuphab performed the experimental design and data collection, Pradung Suanpoot and Sukree Yoosuk reviewed and edited the manuscript, Isara Wattananapakasem conceived the idea, supervised the work, wrote, and edited the manuscript

Funding

Funder: The office of agricultural research and extension Maejo University (Grant No. MJ.1-62-022.1).

Data availability

The data that supports the findings of this study are available within the article.

Code availability

SPSS license by Maejo University’s server license.

Declarations

Ethical approval

Not applicable.

Consent to participate

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