Food Bioprocess Technol DOI 10.1007/s11947-016-1813-z

ORIGINAL PAPER

Infrared Heating as a Disinfestation Method Against Sitophilus oryzae and Its Effect on Textural and Cooking Properties of Milled Rice Wasan Duangkhamchan 1 & Adisak Phomphai 1 & Ruchuon Wanna 2 & Lamul Wiset 3 & Juckamas Laohavanich 3 & Frederik Ronsse 4 & Jan G. Pieters 4

Received: 22 April 2016 / Accepted: 13 October 2016 # Springer Science+Business Media New York 2016

Abstract Infrared (IR) heating method against rice weevils (Sitophilus oryzae) in an egg stage was investigated. A kinetic model was developed to describe insect mortality in a temperature range of 40–60 °C. Effects of IR heating temperature (50–60 °C) and exposure time (1–3 min) on insect mortality and quality attributes of the treated rice were evaluated. The optimized condition obtained by means of the response surface method was used to analyze rice quality before and after IR treatment with storage. The results showed that the 0.5thorder thermal death kinetic equation was the most suitable model, and the S. oryzae eggs had less heat tolerance than the adults and some other species. Mortality achieved 100 % after 2 min for all temperatures. Both IR heating parameters significantly affected the treated milled rice qualities. The minimal changes in rice quality before and after storage could be obtained using optimized temperature and exposure time of 53.6 °C and 1.2 min, respectively.

* Wasan Duangkhamchan [email protected]

1

Research Unit of Process and Product Development of Functional Foods, Department of Food Technology and Nutrition, Faculty of Technology, Mahasarakham University, Khamriang, Kantarawichai, Mahasarakham 44150, Thailand

2

Department of Agriculture, Faculty of Technology, Mahasarakham University, Khamriang, Kantarawichai, Mahasarakham 44150, Thailand

3

Postharvest Technology and Agricultural Machinery Research Unit, Faculty of Engineering, Mahasarakham University, Khamriang, Kantarawichai, Mahasarakham 44150, Thailand

4

Department of Biosystems Engineering, Faculty of Bioscience Engineering, Ghent University, Coupure links 653, Ghent 9000, Belgium

Keywords Rice weevils . Insect control . Thermal death kinetics . Response surface

Introduction Milled rice (Oryzae sativa L.) is one of the major staple foods in the world due to its large content of carbohydrates and is the most important export product in Thailand. During storage or any time prior to consumption, infestation of milled rice by rice weevils (Sitophilus oryzae) may occur, which results in quality losses and reduced market values (Zhou et al. 2015). Up till now, methyl bromide and phosphine fumigation have been commonly used as an insect control method for most stored products over the world due to their effectiveness and simple low-cost operation (Rajendran and Sriranjini 2008). However, this conventional chemical disinfestation method has led to adverse effects on human health and environment (Wang and Tang 2004). Therefore, alternative disinfestation methods have been investigated and developed to replace the chemical ones. Several studies have investigated alternative physical methods replacing the conventional chemical fumigations, including ionizing irradiation (Follett 2008; Carocho et al. 2014), controlled atmosphere (Neven and Rehfield-Ray 2006), cold storage (Aluja et al. 2010), low pressure (Jiao et al. 2013; Suhem et al. 2013), and heating (Hansen et al. 2005; Zhao et al. 2007a, b; Jian et al. 2015; Wang et al. 2013; Alfaifi et al. 2014; Khamis et al. 2011). Among these attempts, heat treatments have been widely studied as an efficient and safe method. However, conventional thermal treatments, such as hot air (Li et al. 2011), water (Armstrong and Follet 2007), and steam heating (Samtani et al. 2012) still have disadvantages such as low heating efficiency, which may result in long treatment time at high medium temperatures and

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product quality degradation (Wang et al. 2001a, b). Dielectric heating methods, such as microwave, radio frequency (RF), and infrared heating have been recently used as alternative disinfestation methods for agricultural products due to their high heating efficiency (Yadav et al. 2014; Hou et al. 2016; Khamis et al. 2011). Recent studies revealed the successful application of microwave and RF heating methods for insect control as reported in the literature reviews of Yadav et al. (2014) and Hou et al. (2016). Information regarding these two effectively alternative disinfestations methods have briefly been reported in the following paragraphs. Microwave is an alternative to chemical methods for disinfesting grains. This method can provide not only a batch treatment, but it can also been operated continuously, allowing large amounts of product passing in a short period (Yadav et al. 2014). Zhao et al. (2007a) used an industrial continuous microwave oven, as an alternative to chemical fumigation, for killing adults and eggs of rice weevils (S. oryzae L.). This method was successfully employed to achieve 100 % insect mortality when the final rice temperature was higher than 55 °C. The results also revealed that eggs were more susceptible to temperature than adults. Even though the microwave method can be successfully employed for insect control, the resulting quality of treated rice has to be considered. Zhao et al. (2007b) further investigated rice quality as affected by microwave treatment. The results showed reduction in water content, free fatty acid content, and protein content, while the blue value (BV) of rice and the sensory quality of cooked rice increased as microwave energy consumption increased. Therefore, insect control with minimal changes in rice quality requires special attention when using the alternative microwave method. Radio frequency (RF) treatment has been one of the most promising insect control methods replacing the conventional fumigation. The feasibility of using RF energy for disinfesting milled rice without important product quality loss was investigated by Zhou et al. (2015). The results showed 100 % insect mortality when rice samples were exposed to RF radiation with intensity of 13.72 kW/kg for 4.3 min with corresponding rice temperatures ranging from 25 to 50 °C. Additionally, the results showed insignificant changes in rice quality in terms of moisture, protein, fat, starch, hardness, and color. Zhou and Wang (2016) further investigated the RF heating uniformity and validated the developed protocols for disinfesting not only milled rice, but also rough and brown rice. The results showed the success of the alternative RF method of disinfestation without affecting rice quality. However, even though the aforementioned physical disinfestation methods have been successfully employed replacing the chemical one, limitations with respect to energy efficiency and cost requirements still exist for small and medium enterprises. Consequently, many attempts have been made to use IR heating for disinfesting agricultural products due to its

advantage of simplicity of the required equipment and installation and low equipment cost (Laohavanich and Wongpichet 2009; Pan et al. 2011). The application of IR heating has been described as a sustainable, environmentally friendly technology for achieving simultaneous high drying rate, good milling and sensory quality, disinfestation, and disinfection of freshly harvested rough rice (Pan et al. 2008; Khir et al. 2011; Wang et al. 2014; Ding et al. 2015a, b). Pan et al. (2008) used a catalytic IR emitter to disinfest rough rice. The rice samples were IR heated for 60 s with corresponding rice temperature of 61.2 °C. After IR heating, followed by tempering and cooling, the tested insects were completely killed with high milling quality. These findings could be confirmed by Pan et al. (2011), investigating the effect of bed thickness on drying characteristics and rice quality. To design an efficient IR system for rough rice, Khir et al. (2011) studied the drying behavior as a function of moisture diffusivity. The results showed that higher moisture diffusivity coefficients gave rise to the higher drying rates in the IR drying method compared with the convective one. Besides drying and disinfesting rough rice, disinfection was also achieved by means of IR radiation. Wang et al. (2014) found that IR radiation with grain temperature of 60 °C resulted in a 7.2-log reduction in Aspergillus flavus. Since rice is normally stored in silo or warehouse for a prolonged period prior to meet the supply need, storage stability has to be taken into consideration. The previous works of Ding et al. (2015a, b; 2016) recommended the IR heating process as a promising technique that achieves not only high rice drying efficiency, but also improved storage stability for rough, white, and brown rice. The many works described above have proven that IR heating is a promising technique that achieves not only high rice drying efficiency and improved storage stability, but also efficient disinfestations and disinfection of rough rice. However, detailed research on using IR heating as a disinfestation method for milled rice has not yet been reported. Consequently, the objectives of this work were (1) to experimentally quantify the survival of S. oryzae eggs being exposed to IR heating under different conditions, (2) to develop thermal death kinetic models to describe and predict mortality in IR heating situations, and (3) to evaluate the milled rice quality attributes after IR heating. Finally, we expect to obtain an appropriate IR heating condition for disrupting the life cycle of S. oryzae with minimal changes in milled rice qualities.

Materials and Methods Preparation of Test Insects and Infested Milled Rice Milled rice (KDML105 variety) with moisture content of 13– 14 % (wet basis (wb)) was purchased from a local market in

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Maha Sarakham Province, Thailand. Rice samples were stored at −20 °C for more than 5 days to kill any pest eggs that might have been in the sample (Zhao et al. 2007a, b). Prior to the experiment, it was thawed at room temperature. In order to disrupt the life cycle of S. oryzae, the egg stage was chosen in this work. As modified from Zhao et al. (2007a, b), for each trial, an infestation loading ratio of 2:5 was provided by adding 60 male/female couples of actively moving insects in 150 g of insect-free rice sample. Plastic boxes containing insect-loaded rice samples were subsequently placed in a growth chamber with 27–30 °C and 75–76 % RH for 5 days to allow the insects to lay eggs. Infested milled rice without IR heating was considered as a control for mortality analysis. Infrared Heating System and Its Treatment Procedure

Fig. 1 A scheme of the IR heating apparatus

The pilot-scale infrared heating system consisted of an electrical IR emitter with adjustable intensity by tuning the temperature of the IR emitter. The IR emitter was mounted on top of an IR heating chamber (36.5 cm × 42.5 cm × 75 cm). The aluminized-steel chamber walls were equipped with a slide-up door allowing insertion and removal of a 27 cm × 37 cm heating tray placed under the IR emitter. IR heating procedures in this work were divided into three scenarios, as described in the following paragraphs. Based on temperatures with different heating methods summarized by Ben-Ialli et al. (2009), heating temperatures ranging from 40 to 60 °C were used in order to develop thermal death kinetic models of S. oryzae eggs. The IR emitter temperature remained constant at 500 °C. As a result that the rice temperature cannot remain constant at the desired temperature once the IR emitter temperature is kept constant at 500 °C, the desired temperatures were controlled manually by adjusting the distance between the emitter and the heating tray, resulting in a temperature fluctuation of ±2 °C. During the experiments, the grain surface temperature was measured using a type-K thermocouple of which the tip was placed approximately 2 mm (monolayer) above the heating tray, as shown in Fig. 1. Fifty grams of infested milled rice (infestation loading ratio of 2:5) was used for each trial. The infested rice heated by infrared radiation for each temperature-time combination was incubated in a plastic box under ambient conditions (27–30 °C, 75–76 % RH) for 45 days. The presence of an exit rupture or a hole in a rice grain was considered as an egg survival (Ben-Ialli et al. 2009). Each treatment was conducted in triplicate, and the averaged values were presented. In order to investigate the influences of IR heating parameters on milled rice qualities, 150 g of infested rice (corresponding to a monolayer) with the same infestation loading ratio of 2:5 was exposed to the IR radiation with variations of heating temperatures (50, 55, and 60 °C) and exposure times of 1, 2, and 3 min. Quality attributes of heated rice were

immediately evaluated and used for optimization of the IR heating treatment. Each treatment was conducted in triplicate, and the averaged values were presented. As it is vulnerable to insect attacks and easy to damage, milled rice quality after storage was also evaluated to verify the effective IR heating treatment. The infested rice with the same insect loading ratio was IR heated with the optimized heating conditions. The heated samples were stored in a plastic box under ambient environment for 4 months (27–30 °C, 75–76 % RH). Quality attributes after storage were compared to those obtained after IR heating and those of the unheated one (control). Thermal Death Kinetic Modeling of S. oryzae Eggs As commonly used (Yan et al. 2014), a temperature-time model was used to describe the thermal death kinetics of S. oryzae egg in this work. The equation of mean survival ratios as a function of exposure time for each temperature is as follows:   N  1−n d N N0 ð1Þ ¼ −k N0 dt Equation 1 was converted to logarithm forms for different reaction orders (Yan et al. 2014):   N ¼ −kt þ c ðn ¼ 1Þ ð2Þ ln N0  1−n N ¼ −kt þ c ðn≠1Þ ð3Þ N0 where N and N0 are the survival and initial numbers of S. oryzae eggs, respectively, t is the exposure time (min), k is the rate constant (/min), and n is the kinetic order of the reaction. All survival ratios as a function of IR heating

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exposure time for each temperature were fitted to the 0-, 0.5th-, 1st-, 1.5th-, and 2nd-order model. The best-fitting reaction order was selected based on the highest coefficients of determination (R2) over all treatment temperatures. The constants of k and c of each selected kinetic model were obtained from the regression equation. The activation energy (Ea, J/mol), which is the minimum energy needed to achieve the death of pests and reflects the sensitivity of the insect response to temperature changes, was calculated from the slope of the Arrhenius plot of log k versus (1/T) as follows (Hallman et al. 2005; Yan et al. 2014): logk ¼ logk 0 −

Ea 2:303RT

ð4Þ

where k0 is the reference thermal death rate constant (/min), R is the universal gas constant (8.314 J/mol/K), and T is the absolute temperature (K). In addition, kinetic death parameters, including D and z values, which is the time at a given temperature required to destroy 1 log cycle (90 %) of the insect, and the temperature range required to change the D value by a factor of 10, respectively, were used to determine the activation energy as expressed in the following equation (Armstrong et al. 2009): Ea ¼

2:303RT min T max z

ð5Þ

In Eq. (5), R is the universal gas constant (J/mol/K), Tmin and Tmax are the minimum and maximum temperature used in this work (K), respectively, and z is the negative inverse of the slope to the thermal-death-time curve (°C).

Quality Attributes of Milled Rice Before and After IR Heating Treatment with Storage Even though the alternative disinfestation methods have been successfully used for insect control (Yadav et al. 2014; Hou et al. 2016; Khamis et al. 2011), qualities of treated milled rice have to meet the demands of consumers. Therefore, quality attributes including physicochemical and cooking qualities and aroma before and after IR heating treatment with 4month storage were evaluated.

Physicochemical Qualities Moisture Content The moisture content (MC) of rice samples was determined according to the standard method of AOAC (1995). Five grams of the sample was dried at 103 °C in a hot air oven for 72 h, and weight difference was measured. The average value obtained from five replicates for each experiment was presented.

Weight of 100 Kernels The hundred-kernel samples (W100) were randomly selected and weighed using a balance with accuracy of 0.01 mg. Ten replicates were carried out and their averaged values were presented. Whiteness Index A digital whiteness meter was used to evaluate the whiteness of rice based on the principle of reflective index of the sample surface. The whiter the sample is, the greater the light reflection is, and consequently, the generated electric current will be stronger, which causes a larger reading. Prior to analysis, this device was calibrated with a standard white plate obtaining the whiteness index of 87.1. Five replicates were carried out for each measurement. Fissuring As modified from Cnossen et al. (2003), the fissuring of milled rice treated by IR heating was determined by randomly sampling 100 kernels (approximately 2.5 g). The kernels were placed on a glass above a fluorescent bulb on which cracking of the sample could be inspected clearly. Visual inspection with a high-resolution digital camera was carried out. The magnified images were taken and visually inspected (Duangkhamchan and Siriamornpun 2015). The kernels with surface cracks and/or internal fissures characterized by Cnossen et al. (2003) were counted. Ten replicates were performed for each measurement, and the average value was presented. Cooking Properties Cooking Time To analyze the cooking properties, 2 g of the rice sample was cooked in 100 ml of boiling water (at 100 ± 1 °C). After 10 min, 10 kernels of cooked rice were sampled with 1-min time interval. The cooking time for each IR treatment was defined as the least amount of time required to fully cook the rice kernels, which was determined by a lack of core opaqueness when pressing three cooked kernels between two glass slides (Hsu et al. 2015). Solids Loss The method of Gujral and Kumar (2003) was modified to determine the %solids loss during cooking. One gram of rice grains were placed into a test tube containing 10 ml distilled water and then boiled until reaching the cooking time determined previously. The liquid obtained after boiling was further evaporated at 105 °C for 24 h. The resultant dry matter was weighed and then used for calculating the percentage of solids loss as follows: Solid loss ð%Þ ¼

wi −wo  100 ws

ð6Þ

where wi and wo are the weight before and after evaporation, respectively, and ws is the sample weight.

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Hardness and Stickiness Milled rice treated by IR heating was cooked and its texture properties were determined according to the updated International Standard method (ISO 11747:2012 E 2012). A texture analyzer (Stable Micro System, TA-XT2i, Surrey, UK) with a rice extrusion rig was used to determine the hardness and stickiness of cooked milled rice. After cooking for the time obtained previously, 23 g of cooked rice was immediately placed in the rig’s testing cell. It was pushed down by a plunger of similar cross-section to the cell, compressing the sample and extruding it through holes in the base extrusion plate at test and posttest speed of 1.6 and 10 mm/s, respectively. Hardness and stickiness were obtained from a forcetime curve using the Texture Expert software supplied with the instrument.

Determination of 2AP The methods of Wongpornchai et al. (2004) and Tulyathan et al. (2008) were applied to determine the 2-acetyl-1pyrroline (2AP) content in the rice samples. Briefly, rice powder ground by hand was screened using a 45-mesh sieve. The internal standard solution was prepared using 2,4,6-trimethylpyridine (99 % purity; Fluka Chemika) (TMP) with a concentration of 1000 mg/kg in isopropanol and subsequently spiked onto 5 g of rice powder. The sample was then sealed in a 27-ml vial fitted with a PTFE/silicone septum and secured by an aluminum cap. Prior to incubation in hot oil (140 °C) for 5 min, the sample vial was shaken and stored at room temperature (27 °C) for 30 min. A 1 cm 50/30 μm DVB/Carboxen™/ PDMS Stable Flex™ SPME fiber (Supelco, Bellefonte, PA) had been preconditioned in a GC injection port at 200 °C for 30 min, and subsequently used for extracting the volatiles of rice sample with a height of 1 cm above the sample surface after 10 min of adsorption, the volatiles onto the SPME fiber were desorbed into the splitless injection port of the GC-MS instrument. For the detailed GC-MS conditions, the reader is referred to Tulyathan et al. (2008). Table 1 Coefficients of determination (R2) from kinetic order (n) models for thermal mortality of Sitophilus oryzae eggs and constants of the most suitable model (i.e., n = 1) at five temperatures

Temperature (°C)

Response Surface Method and Sensitivity Analysis Response surface methodology (RSM) (Design Expert V. 7.1.5, Stat-Ease, Inc.) was used to describe the effects of all factors with respect to responses including insect mortality, MC, W100, WI, %fissuring, %solids loss, cooking time, hardness, stickiness, and 2AP of milled rice treated by IR heating. A full factorial experimental design was employed with two factors (temperature and heating time) and three levels for each factor. Among various polynomial equations with different orders, the suitable ones selected based on the highest value of determination coefficient (R2) were applied to describe the effects of factors on responses (Singh et al. 2006). Furthermore, the IR heating condition was optimized using the models obtained from the RSM associated with the desirability criteria, 100 % mortality, and targeting quality attributes of the untreated rice used as a control sample. The suitable condition was chosen based on the highest desirability value, ranging from 0 to 1, calculated from individual desirability of each response. In addition, sensitivity analysis associated with the parameter-response correlations obtained by the RSM was conducted. Sensitivity of operating parameters was described by means of the normalized sensitivity coefficient (K), which describes the relative change of a response (Ω) as a result of a relative change with 5 % perturbation of the input variable (ω) (Ronsse et al. 2009);   ΔΩ Ω  K¼  ð7Þ Δω ω

Results and Discussion Thermal Death Kinetics Table 1 shows the coefficients of determination (R2) obtained from different kinetic order models at five IR heating temperatures for killing S. oryzae eggs. Based on the R2, the 0.5th-

(N/N0)1-n = −kt + c

R2

(n = 0.5)

40 45 50 55 60

0th

0.5th

1st

1.5th

2th

k

c

0.791 0.904 0.892 0.777 0.690

0.899 0.955 0.949 0.952 0.900

0.909 0.821 0.888 0.936 0.961

0.763 0.570 0.849 0.829 0.768

0.587 0.415 0.818 0.718 0.652

0.104 0.253 0.415 0.469 0.480

0.907 1.033 1.001 0.910 0.846

Food Bioprocess Technol Fig. 2 Insect population reduction from the 0.5th-order model in function of time for 40 °C (solid line), 45 °C (small dashed line), 50 °C (dashed line), 55 °C (dashed line with one dot), and 60 °C (dashed line with two dots)

order (n = 0.5) reaction model seemed to be the most suitable one having a high R2 (0.90–0.95) for all temperatures. The model parameters k and c calculated from these equations are also presented in this table. The kinetic rate constants were in a range of about 0.10–0.48 and increased with increasing IR heating temperature. It means that to achieve the same mortality of S. oryzae eggs, a higher temperature needed a shorter exposure time. This trend could also be observed for the adults (Yan et al. 2014). The suitable thermal death kinetic model (0.5th-order reaction) was subsequently used to plot the insect population reduction as a function of time for each temperature, as seen in Fig. 2. It could be seen from this figure that a slope of a curve was higher with increasing temperature meaning that population of S. oryzae in the egg stage reduced faster when temperature was higher. Figure 3 shows the thermal-death-time curve for S. oryzae egg described by the linear regression:

Fig. 3 Thermal mortality curve for egg S. oryzae at temperatures ranging in 40–60 °C

D = −0.1389T + 8.7333 (R2 = 0.8503), where D is the time (min) and T is the temperature (°C). The z value obtained from the negative inverse of the slope of the thermal-deathtime curve (Fig. 3) was 19.44 °C. Therefore, the activation energy calculated by Eq. (5) was 103 kJ/mol. In addition, the activation energy (Ea) calculated from the slope of the plot of log k against (1/T), as shown in Fig 4, was 64 kJ/ mol, which was lower than that obtained by Eq. (5). This value is lower than that of S. oryzae adults, 523 kJ/mol (Yan et al. 2014), indicating that the egg stage of S. oryzae has less heat tolerance than the adult one. This is consistent with the results obtained by Zhao et al. (2007a) and Wangspa et al. (2015) stating that the adult S. oryzae was the stage most tolerant to RF heating. Moreover, S. oryzae egg is more sensitive to heat than other species such as fruit fly (Anastrepha suspensa L.) and moth (Ephestia kuehniella) species, having Ea = 440–445 kJ/mol at 37–50 °C (Moss

Food Bioprocess Technol Fig. 4 Arrhenius plot of the rate constant (k (/min)) for the thermal death of S. oryzae eggs at different IR heating temperatures (T (K))

and Chan 1993), and having Ea = 102 kJ/mol at 54–75 °C (Ben-Ialli et al. 2009), respectively.

Quality Attributes of Milled Rice as Affected by IR Heating Treatment

Effects of IR Heating Treatment on Mortality of S. oryzae Eggs

The effective disinfestation method should completely eliminate pest while maintaining the rice quality. Therefore, quality attributes of milled rice after IR heating treatment were evaluated, as shown in Tables 3, 4, and 5. The rice qualities presented here were in terms of physicochemical properties (Table 3), cooking properties (Table 4), and aroma (Table 5).

Table 2 shows the mortality of S. oryzae eggs in milled rice IR-heated at three temperatures and exposure times. At all IR heating temperatures, the mortality of S. oryzae in the egg stage achieved 100 % (except at 50 °C, 99.9 %) after the infested milled rice was exposed to IR radiation for 2 min, shorter than the RF heating time (5 min) used for killing 99.9 % of the adult S. oryzae at 50 °C (Zhou and Wang 2016). This could be explained by differences in heating factors used such as the sample thickness and heating intensity. It is noteworthy that IR heating at the lowest temperature of 50 °C for 2 min could be effectively used to completely eliminate the rice weevils. Table 2 Mortality of S. oryzae eggs after IR heating at different treatment conditions Temperature (°C)

Exposure time (min)

Mortality (%)

50 50 50 55 55 55 60 60 60

1 2 3 1 2 3 1 2 3

67.7 ± 1.1d 99.9 ± 0.2a 100.0 ± 0.0a 82.2 ± 1.2c 100.0 ± 0.0a 100.0 ± 0.0a 88.6 ± 0.9b 100.0 ± 0.0a 100.0 ± 0.0a

Different letters in the same column denote significant difference (p < 0.05)

Physicochemical Properties MC of milled rice at different IR heating conditions significantly changed, ranging from 11.55 to 13.94 % (wb), as seen in Table 3. At the same temperature, longer exposure time led to a decrease in MC due to longer water evaporation time. Similarly, MC of milled rice treated by IR heating tended to decrease with higher heating temperature at constant exposure time, especially for 3 min. As a result of the proposed disinfestation method being a heat treatment, the moisture contents of all treated rice samples were significantly lower than that of the untreated control. The most severe condition with the lowest MC (<12 % wb) could be found when milled rice IR was treated at 60 °C for 3 min. Table 3 also shows the mass of 100 rice grains (W100) at different IR heating conditions, ranging from 1.87 to 1.94 g. Due to loss of moisture previously explained, these results were consistent with the MC trend of the IR treated samples. W100 tended to decrease both with increasing temperature and time exposed to the IR radiation. Whiteness index (WI) is one of the key quality properties with respect to the commercial acceptability of rice. In Table 3, besides the control, the WI values obtained from the experiments with varying IR heating temperature and time are

Food Bioprocess Technol Table 3 Physical qualities of milled rice as affected by different IR temperatures and heating times

Temperature (°C)

Time (min)

MC % (wb)

W100 (g)

WI (−)

Fissuring (%)

50

1

13.84 ± 0.10b

1.94 ± 0.01b

46.81 ± 0.19d

1.17 ± 0.41e

50 50

2 3

13.60 ± 0.07c 12.73 ± 0.03d

1.92 ± 0.01b 1.91 ± 0.02bcd

48.07 ± 0.23c 48.10 ± 0.09c

4.33 ± 0.52d 6.67 ± 0.52bc

55

1

13.94 ± 0.06ab

1.91 ± 0.01cd

45.70 ± 0.25f

5.33 ± 0.52cd

55

2

13.27 ± 0.55bc

1.90 ± 0.01c

48.12 ± 0.08c

6.17 ± 0.41c

55 60

3 1

12.01 ± 0.04f 13.49 ± 0.07c

1.88 ± 0.01cde 1.90 ± 0.01cd

49.10 ± 0.45b 46.21 ± 0.03e

7.33 ± 0.52b 4.83 ± 0.41d

60

2

12.57 ± 0.11e

1.89 ± 0.01cde

48.50 ± 0.25b

6.50 ± 0.55bc

60 Control

3

11.55 ± 0.11g 13.98 ± 0.03a

1.87 ± 0.01e 1.96 ± 0.01a

49.92 ± 0.30a 45.54 ± 0.42f

17.67 ± 0.52a 0.00 ± 0.00f

Different letters in the same column denote significant difference (p < 0.05) MC moisture content, W100 weight of 100 kernels, WI whiteness index

also presented. The WI values were in a range of 45.70–49.92, which were higher than that of the control (45.54). This table shows an increasing trend with higher IR temperature and exposure time. The possible explanation could be the fact that the material properties in a rice kernel change when grain temperature surpasses the glass transition temperature, ∼50 °C for rice with MC of 12 % wb (Cnossen and Siebenmorgen 2000), at which the starch goes from a glassy into a rubbery state, and also typically occurs in rice drying (Cnossen and Siebenmorgen 2000; Cnossen et al. 2003). In addition, percentage of rice grain fissuring (%fissuring) as affected by IR heating treatment was also presented in Table 3. It could be observed from this table that a larger number of fissured kernels were found when increasing both IR heating parameters. Both surface and internal fissures were visually observed (data not shown). This could be due to the stress formation resulting from moisture gradients (Cnossen et al. 2003; Duangkhamchan and Siriamornpun 2015). At the highest IR temperature and the longest exposure time, the %fissure was highest (17.67 %), while the lowest value

Table 4 Cooking properties of milled rice as affected by different IR temperatures and heating times

(1.17 %) was found when IR heating at the lowest levels of both parameters.

Cooking Properties Cooking properties are normally used as a criterion for consumer acceptability. Table 4 shows cooking time, %solids loss, hardness, and stickiness of milled rice as affected by IR heating treatment. With variations of IR heating temperature and exposure time, cooking time did not change significantly at low temperatures (50–55 °C), while it slightly decreased with longer exposure time at 60 °C. An obvious change was found regarding %solids loss, ranging from 2.83 to 5.12 %; the higher the temperature, the higher the %solids loss. In addition, considering the effect of exposure time at the same temperature, loss of solids in milled rice tended to increase with treatment time. Both the decreases in cooking time and the increases in %solid loss at higher IR heating temperatures may be due to the higher fraction of fissured kernels. As a

Temperature (°C)

Time (min)

Cooking Time (min)

Solid loss (%)

Hardness (N)

Stickiness (N)

50 50 50 55 55 55 60 60 60 Control

1 2 3 1 2 3 1 2 3

16.67 ± 0.58ab 16.67 ± 0.58ab 16.00 ± 0.00b 16.33 ± 0.58bc 16.67 ± 0.58ab 16.33 ± 0.58bc 16.00 ± 0.00b 15.33 ± 0.58bc 15.00 ± 0.00c 17.00 ± 0.00a

2.86 ± 0.08g 3.06 ± 0.08f 3.24 ± 0.11ef 2.82 ± 0.07gh 3.45 ± 0.17e 4.67 ± 0.12b 4.03 ± 0.01c 3.93 ± 0.06d 5.12 ± 0.06a 2.64 ± 0.12h

41.47 ± 0.29b 35.23 ± 0.27e 30.13 ± 0.56g 39.14 ± 0.35c 33.11 ± 0.11f 27.58 ± 0.31h 39.21 ± 0.26c 36.65 ± 0.14d 27.55 ± 0.29h 45.29 ± 0.16a

3.34 ± 0.14g 3.91 ± 0.04e 4.48 ± 0.11d 4.46 ± 0.08d 4.57 ± 0.24cd 4.90 ± 0.16c 3.72 ± 0.05f 4.92 ± 0.24c 5.53 ± 0.14b 6.04 ± 0.26a

Different letters in the same column denote significant difference (p < 0.05)

Food Bioprocess Technol Table 5 The 2-acetyl-1-pyrroline (2AP) content of milled rice as affected by different IR temperatures and heating times

Table 6 Coefficient of determination (R2) obtained from linear (L) and quadratic (Q) estimates from the response surface model

Temperature (°C)

Time (min)

2AP (ppm)

Responses

50

1

0.1647 ± 0.0025b

50

2

0.1494 ± 0.0005d

50 55

3 1

0.1296 ± 0.0009e 0.1565 ± 0.0012c

55

2

0.1540 ± 0.0029c

55 60

3 1

0.1492 ± 0.0015d 0.1294 ± 0.0001e

60 60

2 3

0.1067 ± 0.0008f 0.1066 ± 0.0014f

control

0.1714 ± 0.0016a

Coefficient of determination (R2) Linear estimation

Quadratic estimation

Moisture content

0.89

0.95

Weight of 100 kernels Cooking time

0.79 0.46

0.82 0.60

Solid loss

0.82

0.88

Whiteness index Fissure

0.80 0.71

0.96 0.82

Hardness Stickiness

0.93 0.77

0.97 0.85

2AP

0.65

0.93

Different letters in the same column denote significant difference (p < 0.05)

Insect mortality

0.66

0.96

result of the higher amount of fissuring, more solids could be leached by water during cooking. Texture properties of cooked rice play an important role in consumer preference. Hardness and stickiness of cooked rice treated at different IR heating conditions are presented and compared to those of the control, as shown in Table 4. It can be observed from this table that IR heating treatment significantly affected both hardness and stickiness. Hardness values decreased with higher temperature and longer exposure time, while stickiness increased. This could be associated with the increased hydration of starch granules resulting from rice fissures, which is consistent with the results reported by Zhao et al. (2007b)). Although low IR heating temperature seemed to be a suitable condition due to promising hardness (41.47 N) compared to that of the control rice (45.29 N), it gave the least sticky cooked rice (3.34 N). Therefore, both texture properties should be taken into consideration when selecting the suitable IR heating condition.

The 2AP concentration

Fig. 5 Sensitivity of operating parameters (time and temperature) represented as the normalized sensitivity coefficient (K) for all responses

In addition to its reputation in physical qualities and cooking properties, rice variety KDML105 or jasmine rice has been well known as the most popular aromatic Thai rice. Therefore, its aroma, determined as the 2AP content of milled rice as affected by different IR temperatures and heating times, was also presented here, as shown in Table 5. The 2AP concentrations of milled rice treated by the IR heating method were in a range of 0.11–0.16 ppm, which was lower than that of untreated samples. The effect of IR heating treatment on the

Fig. 6 Desirability contour plot of IR heating temperature (A: Temp (°C)) and exposure time (B: Time (min))

Food Bioprocess Technol Table 7 Comparison of quality attributes of milled rice before and after IR heating using the optimized conditions (53.6 °C and 1.2 min) with 4-month storage

Quality

0 month

4 months

IR treated

Untreated

IR treated

Untreated

Insect mortality (%)

84.08 ± 1.3A

0.00 ± 0.0B

89.17 ± 2.8a

0.00 ± 0.02b

MC (%wb) W100 (g) WI (-) Fissure (%)

13.74 ± 0.21B 1.91 ± 0.01B

13.98 ± 0.02A 1.96 ± 0.007A

13.47 ± 0.09b 1.90 ± 0.02b

13.95 ± 0.04a 1.94 ± 0.02a

46.51 ± 0.26A 3.58 ± 0.48A

45.54 ± 0.42B 0.00 ± 0.00B

45.76 ± 0.15a 5.30 ± 0.50a

44.49 ± 0.06b 2.67 ± 0.52b

Cooking time (min) Solid loss (%)

16.75 ± 0.57 NS 3.10 ± 0.12A

17.00 ± 0.00 NS 2.64 ± 0.12B

17.24 ± 0.58 ns 3.12 ± 0.09a

17.33 ± 0.58ns 2.24 ± 0.05b

Hardness (N)

38.73 ± 0.25B

45.29 ± 0.16A

40.71 ± 0.50b

46.30 ± 0.17a

Stickiness (N) 2AP (ppm)

4.05 ± 0.16B 0.16 ± 0.0018B

6.04 ± 0.25A 0.17 ± 0.0016A

1.49 ± 0.07 ns 0.12 ± 0.0041a

1.45 ± 0.12 ns 0.10 ± 0.0087b

Different letters in the same row denote significant difference (p < 0.05) with capital and small letters for 0- and 4month storage, respectively NS, ns not significant (p > 0.05)

concentration of 2AP in milled rice was significant, with lower 2AP concentrations for higher temperature and longer exposure time. This trend was consistent with the results obtained by Wongpornchai et al. (2004), indicating that a lower 2AP concentration was found when using higher temperature and longer storage period. Sensitivity Analysis and Optimization of IR Heating Parameters Figure 5 shows the normalized sensitivity coefficients of both IR heating temperature and exposure time on quality attributes of rice treated by IR heating. Positive and negative values denote a positive and negative relation between input and output change, respectively. Among operating parameters tested in this work, it could be concluded from this figure that IR heating temperature had a major impact on most qualities of milled rice, except for the hardness. The value of %fissure was most sensitive to overall IR heating treatment (i.e., temperature and time combined). Table 6 presents the coefficients of determination (R2) obtained for linear (L) and quadratic (Q) estimates from the response surface model. It could be seen from this table that the quadratic correlation could suitably describe all responses as affected by IR heating parameters based on the highest R2. So far, the results have revealed that even though the IR heating treatment could be employed as an alternative disinfestation method for S. oryzae in milled rice, the effective method could achieve the highest mortality of insect eggs with minimal change in rice quality. Therefore, the attributes of milled rice quality as affected by the proposed method had to be taken into account for optimizing the treatment. Figure 6 illustrates the contour plot of IR heating desirability based on the mortality of S. oryzae eggs and the targeted

quality attributes of the control. The highest value, considered as the suitable choice, was 0.714 corresponding to the IR heating temperature of 53.59 °C and 1.2 min exposure time, considered as the globally optimal IR heating condition. The changes in milled rice quality of untreated rice and rice treated under optimized IR heating conditions are presented in Table 7, including the quality attributes and the insect mortality obtained after a 4-month storage period. Due to the criteria for optimizing the IR heating condition (highest insect mortality with minimal quality changes), the insect mortalities were 84.08 ± 1.3 and 89.17 ± 2.8 % for infested milled rice treated by IR heating at 0 and 4 months, respectively. Discrepancy between these values may due to new generation of insect during storage. Compared to the untreated samples, slight decreases in MC, W100, hardness, stickiness, and 2AP concentration were observed, while WI, %fissure, and %solid loss slightly increased prior to storage. Cooking time did not significantly change after IR heating. The quality attributes of milled rice treated using the optimal conditions were also evaluated after storing for 4 months. The trends in quality change of the stored sample were similar to those observed before storage, except for the stickiness which was not significantly different compared to that of the untreated one.

Conclusions IR heating was used as an alternative disinfestation method for disrupting the life cycle of S. oryzae in milled rice. The 0.5thorder thermal death kinetics model suitably described and predicted the insect mortality in the egg stage. The insect mortality achieved 100 % after exposure to IR radiation for 2 min at all temperatures tested in the range of 50–60 °C. Both IR heating temperature and exposure time significantly

Food Bioprocess Technol

affected the mortality and quality attributes of heated rice. The former parameter both considered changes in physicochemical properties as well as changes in and cooking properties. Even though the minimal change in rice qualities before and after the 4-month storage could be obtained using the optimized IR heating condition (53.6 °C and 1.2 min for temperature and time, respectively), the quality attributes with respect to consumer acceptability should be taken into account for verifying the proposed disinfestation method for rice weevils (S. oryzae) in milled rice. Acknowledgments The authors thank the Faculty of Technology, Mahasarakham University, for financial support. Thanks are also given to the Research and Researcher for Industry (RRI) (No. MSD5710045). The authors also thank Asst. Prof. Dr. Perayot Kangkan for his advice on the 2-acetyl-1-pyrroline analysis and Seri Rungruenag Kit Rice Mill for financial and material support.

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