LWT - Food Science and Technology 63 (2015) 452e460
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Effect of low-pressure plasma on physico-chemical properties of parboiled rice Chaitanya Sarangapani a, Yamuna Devi a, Rohit Thirundas a, Uday S. Annapure a, *, Rajendra R. Deshmukh b a b
Food Engineering and Technology Department, Institute of Chemical Technology, NM Parekh Marg, Matunga, Mumbai, Maharashtra 400 019, India Department of Physics, Institute of Chemical Technology, NM Parekh Marg, Matunga, Mumbai, Maharashtra 400 019, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 September 2014 Received in revised form 28 February 2015 Accepted 10 March 2015 Available online 18 March 2015
Cold plasma is one of the emerging novel technologies in the food processing sector due to the ability of plasma to manipulate surface properties without deterring the product quality. The aim of our research is to study the effect of low pressure cold plasma on proximate composition, cooking and textural properties of parboiled rice under various treatment time and power. It was found that low pressure cold plasma increased the water absorption of parboiled rice and could reduce the cooking time upto 8 min. Textural properties were improved after treatment as hardness and stickiness decreased with increase in power and time. Also the surface morphological changes in parboiled rice were investigated using contact angle, surface energy measurements and SEM micro-graphs. After low pressure cold plasma treatment the surface energy of parboiled rice increased and contact angle decreased. We could conclude that low pressure cold plasma treatment improves cooking properties and quality of parboiled rice. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Cold plasma Parboiled rice Cooking time Water uptake ratio SEM
1. Introduction Rice (Oryza sativa L.) is one of the leading food crops and also staple food of over half the world's population. Rice is the second highest produced grain worldwide. Asia alone contributes 90% of total rice production in which India, along with China, accounts for 55% of production. After harvesting, about 20% of the total production is further milled and processed into parboiled rice worldwide. Parboiling improves the Head Rice Yield (HRY). Parboiling is a hydrothermal treatment, it consists of three different operations such as soaking, heating (wet or dry) and drying of the rough or brown rice (Delcour & Hoseney, 2010). Parboiled rice is more nutritious than milled rice as it retains more protein, fat, ash, crude fiber, reducing sugar and amylose. Parboiling is advantageous in better recovery after milling, making grain translucent, resistant to breakage and spoilage by insects and mold, inactivates enzymes, improves biological sanitation, easy removal of hull during milling (Bhattacharya, 1985; Bhattacharya & Subba Rao, 1966; Bhattacharya & Subba Roa, 1966; Ramesh, Ali, & Bhattacharya, 1999), better grain swelling during cooking, less starch in the cooking water, changes taste and texture of rice (Sujatha, Ahmad, &
* Corresponding author. Tel.: þ91 22 3361 2507. E-mail address:
[email protected] (U.S. Annapure). http://dx.doi.org/10.1016/j.lwt.2015.03.026 0023-6438/© 2015 Elsevier Ltd. All rights reserved.
Rama Bhat, 2004). Physico-chemical properties of rice are affected by parboiling (Rao & Juliano, 1970) such as starch polymorphism, starch gelatinization hardens the grain (Islam, Shimizu, & Kimura, 2001, 2002; Jagtap, Subramanian, & Singh, 2008; Miah, Haque, Douglass, & Clarke, 2002; Rao & Juliano, 1970) which consequently needs a longer time to cook to a soft consistency without disintegration of the cell walls. Several studies to reduce cooking time by pre-soaking, partially milling, enzymic polishing of rice (Das, Banerjee, & Bal, 2008) has been conducted. Several chemical treatments were used to make quick cooking rice (Smith, Rao, Liuzzo, & Champagne, 1985). A recent study has shown that a low pressure plasm treatment can reduce cooking the cooking time of long grain rice and brown rice and improve the cooking and physico-chemical properties (Chen, Chen, & Chang, 2012). Plasma technology is known for its excellent antimicrobial and surface engineering properties in various field like bio-medical, textile and polymer industries (Laroussi, 2005; Lopattananon & Jones, 2000; Stone & Barrett, 1962). Etching and deposition of electronics, bonding of plastics, dying in textiles and sterilization are the industrial plasma processing procedures (Korner, Beck, Dommann, Onda, & Ramm, 1995; Naebe et al., 2010; Vlachopoulou et al., 2009). Various aspects and applications of cold plasma in food processing have been reviewed by Misra, Tiwari, Raghavarao, and Cullen (2011), Niemira (2012), Sanguansri et al. (2010), Thirumdas, Sarangapani, and Annapure (2014).
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Plasma technology is based on a simple physical principle. The additional energy is fed into a gas by means of electrical discharge, the gas will turn into the energy-rich plasma state, the fourth state of matter. Plasma is a partially or wholly ionized state which consists of positively and negatively charged ions, free electrons, free radicals and intermediate highly reactive species, atoms, molecules and UV photons (Kogelschatz, 2007) with a net neutral charge. These immanent species are responsible for surface etching and ndez, Shearer, Wilson, & Thompson, microbial inhibition (Ferna 2012; Song et al., 2009). These reactive species diffuse into the surface, undergo several physical as well as chemical modifications (Mozeti c, 2001; Poncin-Epaillard, Brosse, & Falher, 1999). Plasma based on energy levels, plasma can be classified as thermal and non-thermal plasma. Nonthermal plasma are referred as nonequilibrium plasma (Tendero, Tixier, Tristant, Desmaison, & Leprince, 2006) due to the different temperatures of the plasma species. In cold plasma electron temperature is not equilibrated with resulting ion temperature while energy supplied is enough to maintain flow of electrons with partial ionization of gas. Cold plasma can be generated under both atmospheric and low pressure conditions in the order of 10e100 Pa (0.1e1 mbar) (Vlachopoulou et al., 2009). In cold atmospheric plasma, provision of short nanosecond bursts of excitation energy is responsible for low temperatures. The low gas pressure, results in only a few collisions in the plasma causing inefficient energy transfer, leading in formation of heavy particles and high energy electrons. Starch undergoes depolymerization and carboxylic groups are formed on surface by low pressure air glow plasma (Lii, Liao, Stobinski, & Tomasik, 2002a, 2002b). Modification of starch by cross-linkage using glow discharge plasma (Zou, Liu, & Eliasson, 2004). Reduction in cooking time, improving the cooking properties of brown rice and modification of surface properties of seed and cereal grains have been reported (Chen et al., 2012; Dhayal, Lee, & Park, 2006). The major composition of rice is made of starch and its functionality. The gelatinization, thermal breakdown of starch and the recrystallization (retro gradation) of the starch with some lipideamylose inclusion complexes are the major factors that cause the textural change and increases hardness in parboiled rice (Mahanta & Bhattacharya, 1989; Ramesh, et al., 1999). So cold plasma decreases hardness and improves other properties of brown rice (Chen et al., 2012) which gave a scope to present study. Therefore effect of varying power and treatment time of low pressure plasma treatment under vacuum on physico-chemical properties of parboiled rice along with cooking and textural properties is to be studied in detail. 2. Materials and methods 2.1. Materials The raw materials, i.e., Sb Boiled Aiyre variety parboiled rice (Oryzae sativa) were procured from the local market (Sahakari Bhandar, Mumbai, India). All required chemicals were procured from S.D. fine chemical and Hi-Media Mumbai, India. All chemicals and reagents were of analytical grade. 2.2. Methods 2.2.1. Plasma treatment of parboiled rice A bell jar type low pressure plasma reactor was connected to Radio-frequency (13.56 MHz) power supply. The electrode distance between two electrodes was kept constant (3 cm) in all experiments. Parboiled rice was uniformly spread on fine wire mesh kept on glass stand as shown in Fig. 1. Plasma treatment was given at power 30 W, 40 W & 50 W for 5, 10 and 15 min. In all treatments
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atmospheric air was used as gas for plasma generation and pressure was kept constant at 0.15 mbar. 2.2.2. Effect of plasma treatment on proximate composition of parboiled rice Moisture, fat, protein, and ash content was determined using procedures of (Chen et al., 2012) carbohydrate by difference. 2.2.3. Effect of plasma treatment on soaking properties of parboiled rice 2.2.3.1. Soaking time. Parboiled rice samples (5 g) were taken in a beaker and 50 ml of distilled water is added to it and kept at room temperature. At specific soaking time rice samples were taken onto a filter paper and blotted until it loses its shiny appearance. After blotting samples were weighed accurately and then soaking time was calculated as per (Chen et al., 2012). 2.2.3.2. Soaking loss. Parboiled rice samples (2 g) in 20 ml distilled water were soaked for minimum soaking time at room temperature. The soaked water was transferred to 50 ml beaker. The soaked rice was subjected to repeated washings with distilled water to extract the solids adhering to the surface of rice grains. The aliquot having leached solids was evaporated at 110 C in an oven until completely dried. The dried solids were weighed. 2.2.4. Effect of plasma treatment on cooking properties 2.2.4.1. Cooking time. Parboiled rice (2 g) samples were taken in a test tube and cooked in 20 ml distilled water in a boiling water bath. The cooking time was determined by removing a few kernels at different time intervals during cooking and pressing them between two glass plates until no white core was left as per procedure of Chen et al. (2012). 2.2.4.2. Water uptake of parboiled rice. Parboiled rice samples (2 g) were cooked in test tube containing 20 ml distilled water for a cooking time in a boiling water bath. After cooking excess water was drained off and contents were transferred on filter paper to remove surface moisture. The cooked samples were then weighed accurately and the water uptake ratio was calculated as per procedure of Singh, Kaur, Singh Sodhi, and Singh Sekhon (2005). 2.2.4.3. Cooking loss. Parboiled rice samples (2 g) in 20 ml distilled water were cooked for minimum cooking time in a boiling water bath. The gruels was transferred to 50 ml beaker. The cooked rice was subjected to repeated washings with distilled water to extract the solids adhering to the surface of rice grains. The aliquot having leached solids was evaporated at 110 C in an oven until completely dried. The dried solids were weighed and percent gruel solids were calculated as per Singh et al. (2005). 2.2.5. Effect of plasma treatment on texture profile analysis (TPA) of cooked rice The texture of rice after plasma treatment was analyzed using the TA-XT2i, Texture analyzer from Stable Microsystems (Surrey, England). The software used was the Texture expert exceeds software. A cylindrical probe P/36 R was used for measuring the texture. The force was measured in terms of compression (g). The instrument was calibrated with a 50 kg load cell. The test speed was 1 mm/s and the probe was allowed to compress 0.5 mm into the sample. Five replicate aliquots were tested on each sample and the recorded values from the resulting one compression cycle for each test were used for texture profile analysis (TPA), which includes attributes of hardness, stickiness and adhesiveness.
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Fig. 1. Schematic experimental setup for low pressure plasma system.
2.2.6. Effect of plasma treatment on color indices of parboiled rice Color of the plasma treated rice was measured using Hunter Lab Colorimeter model DP-9000 D25A (Hunter associates laboratory, Reston, VA, USA), The L*, a*, b* readings were recorded and white index (WI) was calculated. 2.2.7. Effect of plasma treatment on contact angle and surface energy of parboiled rice The effect of the plasma on the hydrophilicity of parboiled rice was determined by water contact angle measurements with an NRL -Hart Goniometre (Mountain Lakes, USA). A 2.5 ml A-100-00 Rame droplet of water was applied on the surface of rice kernel of both plasma and untreated samples. The evolution of the droplet shape was recorded each 10 s by a video camera, image analysis software was used to determine the contact angle evolution. Water drop height, width and surface energies were simultaneously measured. 2.2.8. Microstructure of rice surface Scanning Electron Microscope (SEM) was used to analyze the surface etching of rice grains. 2.2.9. Statistical analysis The results were statistically analyzed by one-way ANOVA using SPSS (IBM statistical analysis Version 19), and the significance among the samples was compared at p < 0.05 by the least significant difference post-hoc comparison, SPSS 19 version. All the results presented are the averages from three separate experiments.
starch by glow discharge plasma. Significant difference in moisture values (p < 0.05) was found between and treated samples and untreated samples. Protein content of untreated control sample was found to be 5.97 and it was varied among treated samples from 6.03 to 6.18. Protein content was increased with increase in treatment time and applied power. Protein content decreased upto 6.08 g/100 g this might be due to disintegration of surface proteins and proteinaceous matters due to the impact of atomic oxygen playing the dominant role in degradation reactions (Deng et al., 2007). After plasma treatment significant difference (p < 0.05) in protein content was observed. Variation in fat content was found after plasma treatment this might be due to decrease in moisture content or oxidation of free radicals and intermediate compounds formed from oxidation of lipids which depends on different factors like power, time, type of gas (Lii et al., 2002a, 2002b). The hydroxyl (HO) and superoxide anion (O2) radicals that are generated by radiation could modify the molecular properties of the proteins and lipids causing oxidative modifications of the proteins and lipid peroxidation. Significant difference (p < 0.05) was seen in ash content of treated and untreated samples. Ash content was found least 0.44 for 50 W and 15 min sample. These types of changes are similar to the findings of Cho and Song (2000) by using irradiation. Carbohydrate content was found in the range 78.14e80.11. Decrease in carbohydrate content was due to depolymerization of starch major carbohydrate of rice by plasma immanent species. Thus a significant change in proximate composition was observed after plasma treatment.
3. Results and discussion
3.2. Effect of plasma on soaking properties of parboiled rice
3.1. Effect of plasma on proximate composition of parboiled rice
Soaking properties of parboiled rice before and after plasma treatment is shown in Fig. 2. It was found that the percentage water absorption significantly changed (p < 0.05) with increase in soaking time and power. Only a gradual increase was observed for untreated control sample with soaking time from 1 to 4 h but the increase was more in case of plasma treated samples. Significant difference (p < 0.05) was seen in varying treatment time and power. Sample treated for 15 min with Power 50 W showed higher water absorption for all variation soaking time. Fig. 2 shows that with respect to increase in treatment and soaking time the
The proximate composition of plasma treated rice samples is given in Table 1. Moisture content of rice was found to be decreased significantly from 15.57 (fresh weight) to 14.08 ± 0.03 this is due to application of vacuum system which sucks the surface moisture from the rice. With the application of plasma the moisture content was also found to be decreased from 13.04 to 12.73 this may be due to decomposition of water molecule which leads to the formation of oxygen radicals as reported by Zou et al. (2004) in modification of
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Table 1 Effect of plasma on proximate composition (g/100 g) of rice. Power
Time
Control Plasma treated 30
40
50
5 10 15 5 10 15 5 10 15
Moisture
Fat
Protein
Ash
Carbohydrates
14.08 ± 0.03f
0.47 ± 0.05ab
5.97 ± 0.15a
0.47 ± 0.005cd
80.11 ± 0.29f
13.04 12.97 12.90 12.0 12.90 12.83 12.91 12.81 12.73
± ± ± ± ± ± ± ± ±
0.01ef 0.32d 0.23c 0.15de 0.15c 0.11b 0.72c 0.15b 0.15a
0.50 0.52 0.51 0.52 0.52 0.50 0.50 0.48 0.47
± ± ± ± ± ± ± ± ±
0.06bc 0.05c 0.05c 0.11c 0.17c 0.15bc 0.15bc 0.01ab 0.01a
6.03 6.12 6.09 6.07 6.12 6.06 6.18 6.15 6.08
± ± ± ± ± ± ± ± ±
0.11b 0.18ee 0.13d 0.19c 0.20ee 0.08c 0.02g 0.11f 0.50cd
0.49 0.49 0.45 0.47 0.49 0.47 0.48 0.48 0.44
± ± ± ± ± ± ± ± ±
0.001e 0.001e 0.009b 0.001cd 0.002de 0.02bc 0.001cde 0.001cde 0.03a
79.51 79.92 79.87 81.37 80.87 81.94 79.02 78.02 78.14
± ± ± ± ± ± ± ± ±
0.31e 0.46cc 0.152bc 0.42d 0.23bc 0.10b 0.9b 0.19cc 0.15a
All the data are expressed as mean ± standard deviations. Means with the different superscript letters in a column differ significantly (P < 0.05).
percentage water absorption for 30 W, 40 W and 50 W was in the range 19e28%, 20e34.5% and 22e36.5% respectively. This trend is due the plasma immanent species like electrons, photons breaking the CeC & CeH bonds on solid surface and it disrupts the surface structure (fissures) and it can reduce the soaking time, and increase in water absorption. On the other hand soaking loss increased from 0.02 g/g to 0.07 g/ g for untreated to plasma treated as shown in Fig. 3. There is significant difference (p < 0.05) in soaking loss of treated samples.
Soaking solid loss was with respect to their soaking time. Greater amount of soaking loss was observed for 50 W 15 min sample. The increase in soaking loss might be due to leaching of particles through the surface fissures into the water. 3.3. Effect of plasma on cooking properties of parboiled rice Table 2 illustrates the cooking properties of parboiled rice. Cooking time was found to be reduced from 22.20 min to 12.36 min
Fig. 2. Soaking behavior of parboiled rice after plasma treatment time of A) 5 min, B) 10 min, C) 15 min at different power 30 W, 40 W and 50 W.
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Fig. 3. Soaking solid loss.
this could be because of the easy penetration of water into the rice grain through fissures compared to untreated sample similar observations are reported by chen et al. for plasma treated brown rice (Chen et al., 2012). However, fissures on plasma treatment and heat while cooking separates the chains, this might reduce the cooking time. Cooking time was found least for 50 W 15 min sample as the surface etching was more than other samples. Similar findings in cooking time of brown rice by gamma irradiation was observed by Sabularse, Liuzzo, Rao, and Grodner (1991) this may be attributed to starch fragmentation, opening up the kernel structure and degradation of other constituents resulting in increased water absorption by the rice kernel, thereby resulting in shorter cooking time. This type of advantage of reduction in cooking time can be used for the preparation of instant rice (Prasert & Suwannaporn, 2009). Lowpressure plasma and ultrasonic treatment belong to the physical method, and both can be used to effectively reduce the cooking time of rice (Chen, 2013). Significant difference (p < 0.05) in water uptake was found between treated and untreated samples. Water uptake ratio was found to be increased in treated samples which is in correlation with the reduction in cooking time (Mohapatra & Bal, 2006). Water uptake ratio was found highest 3.62 g/g for 50 W 15 min and least 3.12 g/g for 30 W 5 min. Untreated sample tend to have lower water uptake ratios than plasma treated samples. These results were attributed to possible starch and protein fragmentation which could have resulted in a greater number of water binding sites. There may also have been disruption of the protein matrix around the starch granule that serves as a physical barrier to water absorption. On the other hand cooking loss increased from 1.83% to 2.34% from untreated sample to plasma treated. There is significant difference (p < 0.05) in cooking loss of treated samples. Greater amount of cooking loss 2.34% was observed in 50 W 15 min sample. Table 2 Effect of plasma on cooking properties of parboiled rice. Power Control 30
40
50
Time
Cooking time (minutes)
5 10 15 5 10 15 5 10 15
22.20 18.65 17.35 17.02 16.42 16.25 15.43 14.19 13.22 12.36
± ± ± ± ± ± ± ± ± ±
011i 0.30h 0.10g 0.18f 0.20e 0.08e 0.76d 0.65c 0.13b 0.12a
Water uptake (g/g) 3.12 3.20 3.24 3.32 3.25 3.31 3.37 3.45 3.54 3.62
± ± ± ± ± ± ± ± ± ±
0.30a 0.20b 0.1c 0.02d 0.02c 0.05e 0.01f 0.30g 0.35h 0.20i
The increase in cooking loss might be due to disruption of the outer surface, leaching of low molecular weight compounds which easily facilitate particles on surface outside into the water during cooking process. This observation was found to be different from the plasma treated brown rice by Chen et al. (2012) in which they found a decrease in cooking loss in treated sample. 3.4. Effect of plasma on textural properties of cooked parboiled rice Parboiling process results in a less insect infested product and hardening of the grains, which makes them more resistant to breakage during milling. This in turn leads to an increased yield, giving an economic advantage to the process (Islam, Shimizu, & Kimura, 2004). From Table 3 it can be seen that the cold plasma affected the textural properties of parboiled rice. The hardness and adhesiveness are important parameters which are considered for the evaluation of cooked rice texture and consumer acceptability (Chen et al., 2012; Zhou, Robards, Helliwell, & Blanchard, 2007; Zou et al., 2004) reported that hardness and adhesiveness are the parameters which are related to the hydration process of starch granules. It is generally understood that cooked parboiled rice is harder and less sticky than raw cooked rice (Islam et al., 2001). Hardness depends upon the amylose content of rice samples. There is significant difference between the treated and untreated samples for hardness. Hardness of parboiled rice decreased for treated samples and the least was found for samples treated for 15 min at all power. Hardness values were found significantly different (p < 0.05) for different power. Hardness for untreated sample was 24.50 N and treated ranged between 20.30 N and 12.36 N, the decrease in hardness is due to decrease in amylose content of samples. Interestingly, stickiness of parboiled
Table 3 Effect of plasma on textural properties of cooked parboiled rice. Cooking loss (%) 1.83 2.06 2.22 2.3 2.26 2.32 2.44 2.38 2.52 2.34
± ± ± ± ± ± ± ± ± ±
0.35a 0.55b 0.26c 0.10de 0.20c 0.37d 0.40e 0.20f 0.25g 0.40h
All the data are expressed as mean ± standard deviations. Means with the different superscript letters in a column are differ significantly (P < 0.05).
Power Control 30
40
50
Time
Hardness (N)
5 10 15 5 10 15 5 10 15
24.50 20.30 18.64 16.21 19.72 17.35 15.43 16.42 14.19 12.36
± ± ± ± ± ± ± ± ± ±
1.35i 1.05h 0.81f 0.92d 0.64g 0.45e 0.15c 0.82d 0.25b 01.6a
Stickiness (N mm) 0.41 0.47 0.48 0.54 0.48 0.50 0.58 0.50 0.52 0.65
± ± ± ± ± ± ± ± ± ±
0.01a 0.05b 0.04b 0.03e 0.01bc 0.03bc 0.04f 0.09bc 0.04cd 0.04g
Cohesiveness 0.045 0.064 0.082 0.101 0.079 0.065 0.164 0.098 0.134 0.205
± ± ± ± ± ± ± ± ± ±
0.05a 0.04b 0.04c 0.01d 0.05c 0.04b 0.08f 0.04d 0.02e 0.08g
All the data are expressed as mean ± standard deviations. Means with the different superscript letters in a column are differ significantly (P < 0.05).
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Table 4 Effect of plasma on color of cooked parboiled rice. Power Control 30
40
50
Time
L*
5 10 15 5 10 15 5 10 15
76.68 77.52 78.32 79.92 78.58 79.63 79.83 80.58 81.28 81.35
a* ± ± ± ± ± ± ± ± ± ±
0.12a 0.01b 0.04c 0.05e 0.08c 0.10d 0.06de 0.05e 0.04f 0.09g
0.54 0.62 0.66 0.70 0.75 0.80 0.83 0.86 0.89 0.93
b* ± ± ± ± ± ± ± ± ± ±
0.04a 0.02b 0.05b 0.02c 0.25d 0.04e 0.02f 0.4g 0.28h 0.12i
15.25 16.02 16.42 17.17 16.89 17.17 17.37 16.86 17.24 17.13
Whiteness index ± ± ± ± ± ± ± ± ± ±
0.07a 0.02b 0.08c 0.05d 0.05cd 0.04d 0.65e 0.48cd 0.25d 0.48d
72.13 72.38 72.79 73.57 72.71 73.34 73.36 74.26 74.53 74.65
± ± ± ± ± ± ± ± ± ±
0.52a 0.02b 0.04bc 0.56d 0.04bc 0.02d 0.04d 0.08e 0.08f 0.09g
All the data are expressed as mean ± standard deviations. Means with the different superscript letters in a column are differ significantly (P < 0.05).
Fig. 4. Contact angle of control (untreated) and plasma treated samples A) 5 min, B) 10 min, C) 15 min of treatment time at different power 30, 40, 50 W respectively.
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Fig. 5. Surface energies of plasma treated parboiled rice samples at different Power and treatment time.
rice increased slightly after treatment at higher power and time. For 50 W 15 min stickiness was value 0.65 significant difference in stickiness was seen for lower power and time. Leaching of starch components in different proportions of amylose and amylopectin is responsible for stickiness (Ong & Blanshard, 1995). It also reported that amount of amylose and short amylopectin are responsible for hardness and stickiness respectively. In the present observation the hardness and stickiness were found less for the sample treated for higher power and time which was found to be having higher cooking loss similar finding were observed by (Rewthong, Soponronnarit, Taechapairoj, Tungtrakul, & Prachayawarakorn, 2011) in cooking of rice using electric rice cooker and boiling method. Increase in treatment time and power of plasma increased the cohesiveness of parboiled rice this is due to leaching components and can be responsible for a decrease in hardness. 3.5. Effect of plasma on color and whiteness index of parboiled rice Table 4 shows the color and whiteness index of parboiled rice. The color was measured in terms of Hunter L (lightness ranging from 0 to 100 indicating black to white), a (þa; redness and a greenness) and b (þb; yellowness and b; blueness). The maximum value for lightness is 100 indicating white. Therefore increase in L value indicates an increase in whiteness. In the same way, an increase in the Hunter b value indicates that there is an increase in yellowness. The color of the rice samples (Hunter L, a, b and whiteness index) are given in table. Whiteness index was found to have significant difference between the treated and untreated samples. With increase in both time and power it was found that there is increase in whiteness index from 72.1 to 74.65. 3.6. Effect of plasma on contact angle and surface energy of parboiled rice Results of contact angle measurements of both untreated and plasma treated are shown in Fig. 4. The contact angle is defined as the angle formed by the intersection of liquidesolid interface (a water drop placed on the surface of a material) and liquidevapor
interface. Contact angle is measured as change in droplet size or contact angle with respect to time and it is related to the wettability of the material (Andrade, Simao, Thire, & Achete, 2005; Yuan & Lee, 2013). For untreated rice samples, the initial contact angles were high. On application of plasma significant difference (p < 0.05) in contact angles were observed. It was found that contact angle among the samples was decreased from 91.67 to 69.70 this may be due to more absorption of (Andrade et al., 2005) water resulting in decrease in contact angle. Sample treated for 50 W for 15 min was found to have least contact angle (69.70 ± 1.01) which was found to have high water uptake ratio and least cooking time whereas control sample with more contact angle was found to have more cooking time and least water uptake ration. The simultaneous action of absorption, spreading and evaporation may explain the decrease in drop height and in contact angle values as a function of time. Variations in drop width and drop height as a function of time for untreated and treated was also observed which resembles in change in contact angle. The decrease in contact angle was found in low pressure plasma treated samples similar results were observed for surface modification of maize starch films by low-pressure plasma by Andrade et al. (2005). Significant increase (p < 0.05) in drop width and height was observed. The fast decrease of contact angles visualized for treated rice samples may be attributed mainly to the rapid absorption of the drop. Surface energy is generally defined as amount of increase in free energy when the area of surface increases. From the Fig. 5 it can be observed that surface energy increased in low pressure plasma treated samples resulting in more surface area between solid interface rice and liquid droplet making surface more hydrophilic which resulted in more water uptake ratio and shorter cooking time. 50 W 15 min sample was found to have more surface energy (41.85 mJ/m2) with lower contact angle this shows that contact angle and surface energies are inversely related. 3.7. SEM micrographs SEM micrographs of parboiled rice before and after low pressure plasma treatment are shown in Fig. 6. After the treatment the surface of grain showed fissures and depression similar to the type of effect was seen in plasma treated brown rice by Chen et al. (2012) and this is called as ‘surface etching’. Plasma treatment changed the natural surface morphology of grains resulting changes in cooking and textural parameters, this similar change in morphology was observed by Sujka and Jamroz (2010) when starch granules are subjected to ultrasonic treatment. From the SEM graphs it is evident that 50 W 15 min causes more etching which resulted in less cooking time, contact angle and higher water uptake ratio and surface energy. 4. Conclusion From the results it can be concluded that there are significant change in the physico-chemical properties of cold plasma treated parboiled rice. Plasma treatment effectively reduced the cooking time but there is increase in water uptake ratio and cooking loss. Plasma treatment greatly affected the textural properties leading to reduction in hardness and stickiness of rice. Reduction in contact angles was seen with increase in plasma treatment. These results were also found evident with SEM micrographs. Basic information obtained from this study can be used to develop instant rice and other food products where cooking time is an important factor. It was determined that the utilization of cold plasma, a novel technology in whole grain processing, not only maintained the high nutritional values of parboiled rice, but also provided a better textural quality and reduction in cooking time.
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Fig. 6. SEM micrographs of parboiled rice A) control (untreated), B) plasma treatment at 30 W for 15 min, (C) plasma treatment at 40 W for 15 min, (D) plasma treatment at 50 W for 15 min.
Acknowledgement This research work was funded by Department of Science and Technology (DST) and Ministry of Food Processing Industries, Government of India.
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