ABSTRACT
The use of the pasteurization unit as a measure of the lethal effect of heating processes and overall quality of ready-to-eat Jasmine rice was determined in order to compare the conventional and microwave technology. Cooked Jasmine rice was packed in a polypropylene plastic cup and sealed with a lidding film, then subjected to a continuous microwave system (eight 800 W; 2450 MHz) and conventional steamer. The pasteurization unit, log reduction, microorganism count, color, instrumental textural property, and sensory attributes were investigated over a 30-day storage at 8°C. The pasteurization process time was reduced from 420 s under the conventional process to 216 s under the microwave process for a 5-log reduction of L. monocytogenes. The microwave heating showed greater effectiveness for shelf life, extending the product from 7 days (conventional) to 30 days. Cooked rice after heating by microwave was whiter and showed lower hardness than the conventional heated sample. Throughout storage, sensory attributes of the pasteurized product heated by microwave were acceptable by panelists. The addition of 15% trehalose or 1% soybean oil into the rice before cooking decreased the hardness of the cooked rice during storage. This study successfully demonstrated that the continuous microwave pasteurization process, compared to the conventional process, required less process time for ready-to-eat cooked Jasmine rice and extended the shelf life while providing better product quality.
Introduction
Jasmine rice is the most popular rice variety in Thailand due to its pleasant aroma, white color, soft texture, and good eating quality. In Thailand, Jasmine rice is considered a vital crop for domestic consumers and a primary export commodity for economic growth.[Citation1] Ready-to-eat (RTE) foods are a group of food products that are pre-cooked, packaged, and ready for consumption without additional preparation or cooking. The demand for RTE meals is very large and is increasing every year. In 2014, ready meals in Thailand achieved retail value growth of 13%, with sales of 164 million USD, and a retail volume growth of 10%, with sales of 35,000 tons.[Citation2] RTE foods need heat processing to destroy foodborne pathogens that cause illness and public health concerns. Listeria monocytogenes, a major pathogens of refrigerated RTE foods, is killed by thermal processing. However, any thermal processing of RTE foods may cause cross-contamination, limiting the shelf life of the product to only 5–7 days under chilled conditions. If the shelf life of the product could be increased, the production capacity could be increased, creating more revenue.[Citation3,Citation4]
In traditional thermal processing methods such as pasteurization, heat is transferred from the heating medium to the food interior via convection and then by conduction within solid foods or by convection within liquid foods.[Citation4] In RTE foods, the thermal conductivity of the packaging material, entrapped air and the food itself is very low, thus limiting the rate of heat transfer. The long pasteurization process leads to negative thermal degradation of the product.[Citation3]
Microwave (MW) heating has been widely recognized in the field of food processing. It has gained popularity due to several advantages such as high heating rates, short heating time, ease of operation, less thermal degradation, and superior product quality. The applications include drying, baking, thawing, tempering, pasteurizing, and sterilizing of foods. Because of the direct interaction between an electromagnetic field and food components, such as water and salt, molecular friction and excitation to generate heat rapidly within the food itself. The volumetric heat generated by MWs can overcome the slow rate of heat transfer and reduce by 3–5 times the total heating time from a conventional (CV) process, enhancing bacterial destruction and maintaining product quality.[Citation5,Citation6]
The hardness and firmness of RTE cooked rice gradually increase during storage, which has been attributed to starch retrogradation. Early studies showed that sugars such as trehalose (TH), glucose, fructose, and sucrose could increase the gelatinization temperature and prevent the retrogradation of starch.[Citation7] The accumulated lethality, expressed as the pasteurization unit (PU), is used to refer to the total lethal time required in the CV heat treatment of the product. It is offered as a tool for comparing the efficiency of CV heating with that of MW heating. The objective of this research was to determine the PU value and microorganism count of RTE cooked Jasmine rice after pasteurization by CV and MW heating. Changes in the color, texture, and sensory quality of products after heating treatment were studied. The effects of anti-retrogradation agents to the quality of pasteurized cooked Jasmine rice were also investigated.
Materials and methods
Preparation of in-packaged RTE cooked Jasmine rice
Thai Jasmine rice (Oryza sativa) was obtained from C.P. Intertrade Co., Ltd. and stored in airtight containers until used. One thousand grams of Jasmine rice was cooked in an electric rice cooker (Sharp, Model KS-ZT 18) for 30 min with a rice to water ratio of 1:1.3. One hundred and thirty grams of cooked rice were packed in polypropylene plastic containers (diameter 95 mm, height 45.3 mm, Toyo Seikan Co., Ltd.) and sealed with cast polypropylene lidding film by a hand sealer machine.
Continuous MW system and CV heating treatment
The in-packaged Jasmine cooked rice was heated in the continuous MW system and CV steamer tunnel conveyor belt heating treatment until the temperature of the sample reached more than 80°C, a temperature that has proved to be sufficient for pasteurizing most products.[Citation3] Nine packages were treated in each run. The industrial continuous MW system developed by PrimAsia Technology Co., Ltd. was used for the thermal treatment. This system consisted of eight 800 W magnetron, 2450 MHz MW generators. The dimensions of the stainless steel cavity were 800 × 4000 × 1700 mm (W × L × H). The MW treatments were performed at three heating times: 48 (MW1), 62 (MW2), and 79 (MW3) s. After the heat treatments, the samples were held in the cavity for a period equal to the heating time. Then they were immediately cooled down by chilled water at 0–4°C for 120 s when the temperature reached 50°C. For the CV heating treatment, the steamer tunnel conveyor belt developed by GEA Co., Ltd. was used as a heating medium for transferring heat to the product. The heating time of CV process was 180 s, with additional 120 s for holding in the cavity and 120 s for cooling down to a temperature of 50°C. Fiber optic sensors were used for measuring the temperatures at the middle of the sample-filled containers to obtain time-temperature profiles. After cooling down, samples were stored for 30 days at 8°C and analyzed for quality at 0, 7, 14, 21, and 30 days.
PU and log reduction calculation
The PU was calculated by Eq. (1) where T is the measured temperature at each treatment time (°C), Tref is the reference temperature (80°C), z has a value of 13.62°C for L. monocytogenes, and t is the heating time interval (min).
The log reduction, a 10-fold (one decimal) or 90% reduction in numbers of live microbes, was calculated by Eq. (2) where PU(Tref) is the PU at the reference temperature (Tref = 80°C; min) and D(Tref) is the time required to kill 90% of the live microorganisms at the reference temperature (Tref = 80°C; min).[Citation8]
Microbial analysis
The microorganisms counts in the pasteurized cooked rice during storage were investigated by Testing Laboratory of CPF (Thailand) Public Co., Ltd. The types of foodborne pathogens tested after heated treatment complied with the standards for pathogenic microorganisms in food that were announced by the Department of Medical Sciences, Ministry of Public Health, Thailand. The references methods were AOAC 990.12 (2012), AOAC 997.02 (2012), AOAC 2003.11 (2012), FDA BAM Online Chapter 16 (2001), and ISO 11290-1:1996/Amd 1:2004. The microbial counting of the samples treated by MW2 condition was compared to the samples treated by the CV heating treatment.
Whiteness measurement
The whiteness of pasteurized cooked rice was measured by the MiniScan XE plus (Hunter Associates Laboratory Inc, USA). The whiteness index (WI) was calculated by Eq. (3). The color parameter was based on the CIELAB system of L* (lightness), a* (redness), and b* (yellowness).[Citation1]
Instrumental texture analysis
The hardness, maximum compressive force during extrusion (N), of the pasteurized cooked rice was measured by the texture analyzer (TA-XT2i, Stable Micro System, England) using a spherical plate plunger of 35 mm diameter for compression of the sample. Fifteen grams of pasteurized cooked rice was placed inside the beaker of 250 mL and pressed with a 100 g force. The pre-test, test, and post-test speeds of plunger were set at 1.0, 1.0, and 10.0 mm/s, respectively. The compression distance was 50% strain.[Citation1]
Consumer preference
The consumer preference was performed by 30 untrained panelists. All sensory attributes were evaluated using a 9-point hedonic scale ranging from 1 = extremely dislike to 9 = extremely like. The panelists were required to compare their liking in terms of appearance, color, flavor, texture, and overall liking.
Preparation of in-packaged RTE cooked Jasmine rice with anti-retrogradation agents
One thousand grams of Thai Jasmine rice was cooked in an electric rice cooker (Sharp, Model KS-ZT 18) for 30 min with a rice to water ratio of 1:1.3. Water used to cook the rice was added with the anti-retrogradation agents by a percentage of rice as 5 and 15% sorbitol (SB), 5 and 15% TH and 1 and 3% of soybean oil (SO). One hundred and thirty grams of cooked rice were packed in polypropylene plastic containers and sealed with cast polypropylene lidding film by a hand sealer machine. After pasteurization by the MW system, samples were stored for 30 days at 8°C and analyzed for quality at 0, 7, 14, 21, and 30 days.
Statistical analysis
All experiments were done in triplicate. All data were analyzed using the SPSS of version 15. One-way analysis of variance (ANOVA) and Duncan’s multiple-range test at 95% confidence level were applied to evaluate the significance of the difference in quality attributes during the 30-day storage.
Results and discussion
Effect of MW heating on PU and suppression of microbial growth
The time-temperature profiles of RTE cooked rice during MW and CV pasteurization treatments are shown in . The temperature of the product under MW heating reached 80°C within 40 s, while that under CV heating reached 80°C after more than 120 s. In the CV heating method for RTE meals, the thermal energy transferred from the container surface to food interior is very low, which limits the rate of heat transfer. Then, a great deal of time is required for the pasteurization process.[Citation9] On the other hand, the MW heating generates volumetric heating, the thermal energy within the food, which can overcome the limitation imposed by the slow rate of heat transfer during the CV heating. This is attributed to the sharp reduction in the MW processing time required to raise the target temperature compared to the CV processes.[Citation10,Citation11] shows the process time, PU and log reduction for each pasteurization treatment. In order to achieve a 5-log reduction of L. monocytogenes that is recommended by the USFDA for pasteurized products, the CV process required 180 s for heating and 120 s for the holding time, compared to only 48 s for the heating and 48 s for the holding time in the MW process. The heating time of MW1 was about four times less than the CV process because of its volumetric heating. The PU and log reduction of MW2 was about six times higher than the CV process and that of MW3 about 15 times higher. The differences in the PU and log reduction between the MW and CV process contributed to the greater effectiveness of the MW in suppressing the growth of microorganisms during storage. It is generally believed that a short processing time produces better food quality than a long one. Our findings, in fact, were that the short process time of the MW led to considerably higher product quality than the long CV process.[Citation12]
Table 1. Pasteurization process time, PU, and log reduction of each treatment.
Figure 1. Time–temperature profiles of RTE rice during MW and CV pasteurization process.
Effect of pasteurization on the microbiological characteristics during storage
It is clear that the MW is effective in destroying microorganisms and inactivating enzymes.[Citation13] The microbial inactivation kinetics of the MW is essentially the same as the inactivation kinetics of the CV thermal processing.[Citation11] Under the food standard regulations, Clostridium perfringens and L. monocytogenes in the 25 g samples must not be detected and are allowed to be detected in less than 500 CFU of yeasts and molds, 100 CFU of Staphylococcus aureus and 106 CFU of total plate counts (TPCs) per gram of RTE foods tested. Various microorganisms were detected in the samples heated by the CV process over the standard at 10 days; thus the shelf life of the product was 7 days. The current shelf life of the chilled meal product is generally limited to 5–7 days as a result of the contamination of the raw materials during production and/or from water, air or humans. The temperature of the product heated by the MW process reached to 106°C and was stable at a high temperature above 90°C for a long period. The high temperature and long time period (high PU and log reduction) of the MW process helped suppress microorganism growth during storage. Thus only TPCs were detected in the products during a 30-day storage, and the number of TPCs did not exceed the food pathogen standard. The results indicated that the product under MW processing had a shelf life of up to 30 days. A similar result has been reported by manufacturers of RTE meals in Europe, who found that products pasteurized by a MW can increase the shelf life to 25–90 days.[Citation14] Recently, the MW has been used to heat foods in commercial pasteurization and sterilization in order to enhance microbial destruction, extend shelf life, and produce superior product quality.[Citation10,Citation15]
Effect of pasteurization on the WI and hardness of cooked rice during storage
The changes in the WI and hardness of pasteurized cooked rice during storage are shown in . The values L*, a*, and b* were investigated (data not shown). However, there was no significant difference in color parameters between the heating treatments and during storage. The WI is another factor that influences the quality of cooked rice and consumer’s preferences. The WI of pasteurized cooked rice heated by the CV process was significantly lower than that of the MW process. The index decreased with increased heating time, mainly due to the Maillard type non-enzymatic browning reaction. In addition, some nutrients’ pigments leached out during the heating process, contributing to discoloration of the rice.[Citation16] There was no significant difference in the WI of the samples during the storage time. The color of the cooked rice did not change during storage at low temperatures.
Table 2. Micro-organisms counts during storage.
Table 3. Whiteness index and hardness of pasteurized cooked rice during storage.
The hardness of the pasteurized cooked rice heated by the MW process was not significantly different from the CV process during the first seven days. The high temperature and long heating time of the CV process may have cause retrogradation. This phenomenon led to the packing of amylose and amylopectin and the release of water, which, in turn resulted in slightly more hardness in the rice than occurred in the MW process. This result agrees with a previous report that the hardness of cooked rice increased with an increase in the heating time.[Citation17] In addition, increasing the storage time significantly increased the hardness. The results of various researches, in fact, revealed that increased storage time led to greater hardness and firmness of cooked rice and decreased the stickiness because of an increase in the starch retrogradation during storage. The contribution of starch retrogradation to texture changes during storage of cooked rice appears to be similar to the mechanism involved in the staling of baked cereal products during storage.[Citation17,Citation18]
Consumer preferences
Cooked rice texture has been shown to affect consumers’ preferences of rice consumed as a whole grain. The appearance, color, flavor, texture, and overall liking scores of the pasteurized RTE cooked rice were found to be not significantly different between the MW and CV process in the first 14 days storage, as the data in shows. The sensory scores of pasteurized RTE cooked rice under the MW process were significantly different in a 30-day storage duration; sensory scores decreased when storage duration increased. Consumers commented that cooked rice had a harder texture and less flavor when the shelf life of the product increased. This might be attributed to increasing degrees of starch retrogradation during storage resulting in increased rice hardness and decreased stickiness and adhesion in cooked rice.[Citation18] Products heated by the MW process kept for 14–30 days had significantly lower sensory scores than the CV process at 0 days, but consumers still accepted those products. This implies that MW heating did not adversely affect the overall sensory quality of the products. Similar results have been reported; products processed by the MW had superior quality, an attractive appearance, and high consumer acceptance than products heated by the CV process.[Citation10,Citation15]
Table 4. Sensory scores of pasteurized cooked rice during storage.
Effect of anti-retrogradation agents on the WI and hardness of cooked rice during storage
The changes in the WI and hardness of pasteurized cooked rice with anti-retrogradation agents during storage are shown in . The WI of all treatments tends to significantly decrease during the storage time. The changes in rice color during storage were observed by Maillard reaction, which usually depends on the storage temperature and time.[Citation19] Sugar gives support to occurred Maillard reaction in the cooked rice sample. If sugar is added, then the color of all samples is more yellowish than that of the control. SO might be coated on the surface of the cooked rice, which causes a reduction of the WI of the samples. During the first 14 days, the cooked rice with SB 15% had the highest WI, and the rice with SB 5% had a higher WI than other samples.
Table 5. Whiteness index and hardness of pasteurized cooked rice with anti-retrogradation agents during storage.
All of the cooked rice samples with sugar had a significantly lower hardness than the control sample because of the interactions between sugar and water molecules, which suppressed the retrogradation of starch. Sugar has the ability to compete with starch for water, so that the water activity and amount of water available for retrogradation were reduced. Moreover, the antiplasticising effects of the sugar–starch and sugar–water interactions led to the dilution of starch components available for retrogradation.[Citation7,Citation20] The hardness of the cooked rice with 5% of SB and TH was not significantly different from that of the rice with 15% of SB and TH during storage, except at 30 days. The contribution of sugar–water interactions in suppressing retrogradation of rice starch decreased with an increasing sugar concentration due to the other effects arising at high sugar concentrations.[Citation20] During the first 14 days, the cooked rice with TH had more hardness than the rice with SB, which had more the hardness than the rice with TH. Sugars with low molecular weights such as SB were less effective in the reduction of retrogradation than those with the higher molecular weights such as TH. The same results were reported elsewhere.[Citation7] It might be SB has lower ability to interact with water or starch molecules than trehalose, thus it has the lower capability for reducing water activity and antiplasticising action.[Citation20] The cooked rice with SO had significantly the lowest hardness among the samples. SO and emulsifying agents can prevent cohesiveness or aggregation in cooked rice; hence, they can decrease its hardness during storage at low temperature.[Citation21]
Effect of anti-retrogradation agents on the sensory scores of cooked rice during storage
The sensory scores of all samples decreased when the storage duration increased, as shown in . Samples with 5 and 15% of SB at 21–30 days had significantly lower the sensory scores than samples at 0–14 days. Samples with 5 and 15% of TH at 30 days had significantly lower sensory scores than samples at 0 days. Also, samples with 1 and 3% of SO at 7–30 days had significantly lower sensory scores than samples at 0 days. The consumers gave comments that the flavor of cooked rice decreased when the shelf life of sample increased. The sensory scores of samples with 3% of SO were the lowest among the samples because the sample appeared greasy and oily from the addition of SO. It might negatively influence to the acceptability of the consumers. The sensory scores of the control sample were significantly different from those of the samples with 15% TH and 1% SO. During storage, the samples with 15% TH and 1% SO had the highest sensory scores and these were significantly different from the control samples. Therefore, the addition of 15% TH and 1% SO could improve the qualities of pasteurized RTE cooked rice by the continuous MW system if the cooked rice is kept at 8°C during a 30-day storage period.
Table 6. Sensory scores of pasteurized cooked rice with anti-retrogradation agents during storage.
Conclusion
The MW heat-treated RTE Jasmine cooked rice reduced the heating time by half of the CV steamer method to accomplish the same log reduction. MW 2 was the best condition for pasteurized RTE cooked rice because this condition was considered to be the ultra-pasteurization process that used ultra-high temperature, short heating time and rapid cooling to create the best quality and extended shelf life of the product. Pasteurization by MW had great effectiveness in suppressing the growth of pathogenic microorganisms. The extension of the product’s shelf life increased from 7 to 30 days. MW pasteurized RTE cooked rice has the advantages of retaining color, texture and sensory quality throughout the shelf life. The additional 15% of TH and 1% of SO in the rice improved the texture and sensory scores of pasteurized RTE cooked rice throughout 30 days of storage. Therefore, the use of the industrial continuous MW system is an excellent alternative to the heating process by CV pasteurization.
Funding
This research was supported by grants from the Thailand Research Fund (TRF) and CPF (Thailand) Public Co., Ltd. under “Research and Researchers for Industry Program (RRI).”
Acknowledgments
This article was first presented in the Second International Conference on Food Properties (iCFP 2) held in Bangkok, Thailand on May 31 to June 2, 2016 and it received one of the best paper recognitions.
Additional information
Funding
References
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