An ASABE Meeting Presentation Paper Number: 084053
Comparison between the continuous and intermittent heating methods for simultaneous infrared dryblanching and dehydration of apple slices
Yi Zhu, Food Scientist Frito Lay Inc., 7701 Legacy Dr., TX 75024
Zhongli Pan, Research Engineer / Associate Adjunct Professor Processed Food Research Unit, USDA-ARS-WRRC, 800 Buchanan St., Albany, CA 94710, USA,
[email protected] Processed Foods Research Unit, USDA-ARS-WRRC, 800 Buchanan St., Albany, CA 94710, USA,
[email protected]
Written for presentation at the 2008 ASABE Annual International Meeting Sponsored by ASABE Rhode Island Convention Center Providence, Rhode Island June 29 – July 2, 2008
The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of the American Society of Agricultural and Biological Engineers (ASABE), and its printing and distribution does not constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by ASABE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this work should state that it is from an ASABE meeting paper. EXAMPLE: Author's Last Name, Initials. 2008. Title of Presentation. ASABE Paper No. 08----. St. Joseph, Mich.: ASABE. For information about securing permission to reprint or reproduce a technical presentation, please contact ASABE at
[email protected] or 269-429-0300 (2950 Niles Road, St. Joseph, MI 49085-9659 USA).
Abstract. Simultaneous infrared dry-blanching and dehydration (SIRDBD) can be operated in two heating modes, continuous and intermittent heating. Under continuous heating, infrared radiation intensity was kept constant while the product temperature remained constant under intermittent heating in this study. Both heating modes showed success in blanching and partially dehydrating apple slices. The objective of this study was to compare the product quality under the two heating modes and provide recommendations of processing conditions for different manufacturing purposes. The three-factor factorial experimental design was used to investigate the effects of processing parameters, including apple slice thickness, processing time, and radiation intensity for continuous heating or slice surface temperature for intermittent heating, on final product quality in terms of surface color, moisture reduction, and polyphenol oxidase (PPO) and peroxidase (POD) activities. The results showed that the continuous heating mode resulted in a shorter processing time, lower moisture reduction, and similar overall surface color change than the intermittent heating mode to achieve about 90% POD inactivation. When prolonged heating was needed (e. g. more than a 1 log reduction of POD), the intermittent heating mode was more advantageous in the preservation of surface color than the continuous heating mode since no severe darkening occurred. To achieve a 1 log reduction of POD, the continuous heating mode resulted in moisture reductions in the range of 15.35% to 49.29% and overall surface color change (∆E) of 2.030 and 5.518, while the intermittent heating mode led to moisture reductions of 19.78% to 59.48% and ∆E of 2.274 and 5.5880. For quick blanching purposes, the recommended processing conditions are continuous heating under medium to high radiation intensities (4000-5000 W/m2) for 5 to 13 mm thick slices. If large moisture reduction is desired at the same time of blanching, intermittent heating at 75 ºC is suggested. Keywords. Infrared, blanching, dehydration, apple, continuous heating, intermittent heating
The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of the American Society of Agricultural and Biological Engineers (ASABE), and its printing and distribution does not constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by ASABE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this work should state that it is from an ASABE meeting paper. EXAMPLE: Author's Last Name, Initials. 2008. Title of Presentation. ASABE Paper No. 08----. St. Joseph, Mich.: ASABE. For information about securing permission to reprint or reproduce a technical presentation, please contact ASABE at
[email protected] or 269-429-0300 (2950 Niles Road, St. Joseph, MI 49085-9659 USA).
Introduction In the past decades, more and more attention was paid to public health in respect to diet, especially to the consumption of fruits and vegetables. United States Department of America (USDA) executed campaigns to urge people eat at least five servings of fruits and vegetables in daily basis. However, fresh produces are not always accessible due to their harvest seasonality and short shelf life. On the other hand, processed fruits and vegetables are more shelf stable with less enzyme and microbial issues than the fresh produces. In many cases, the quality of the processed products needs to be as much comparable to the fresh produce as possible. Many processing technologies could be able to extend the shelf lives of fruits and vegetables, but the commonly used ones in current food industry are blanching and dehydration. Blanching is often used to inactivate enzymes by using steam or hot water. The dehydration process leads to lower product water activity by using heated air. The conventional blanching and drying methods have many drawbacks, such as low energy efficiency, long processing time, quality deterioration and additional cost for waste water treatment (Bomben, 1977; Vanlaanen, 2003). Simultaneous infrared dry-blanching and dehydration (SIRDBD) method has recently been developed for producing blanched and partially dehydrated fruits and vegetables with high product quality (Pan & McHugh, 2004). This processing method utilizes infrared (IR) energy to blanch and remove certain amount of moisture simultaneously. Therefore, the blanching and dehydration is combined into one-step-process, which is much simpler and more energy- and process efficient than the conventional two-step-process (Pan & McHugh, 2004). The generation of catalytic IR (CIR) energy is a unique process including the catalysis of natural or propane gas by platinum catalyst pad built in CIR emitter. The exothermic reaction produces water, carbon dioxide, and medium- and far-infrared energy with wavelength from 3 to 6 µm. Since this energy matches well with three of the absorption peaks of water, it is possible to achieve rapid heating and drying of high moisture foods such as fruits and vegetables (Gabel, Pan, Amaratunga, Harris, & Thompson, 2006; Pan & McHugh, 2004). SIRDBD can be operated in two heating modes, continuous and intermittent heating. During continuous heating, the radiation intensity is maintained constant since the supply of natural gas to the CIR emitter is continuous. For quick heating-up and moisture removal or enzyme inactivation, continuous heating is advantageous since it delivers high energy to the surface of the product relatively fast. However, when prolonged heating is required, continuous heating can cause severe surface discoloration. The intermittent heating has been suggested to solve the problem of limited penetration of FIR and the application of FIR on thick materials (Sandu, 1986). Intermittent heating is achieved by keeping product temperature constant through turning on and off natural gas supply in this study. The advantages of intermittent radiation are energy savings and improved product quality, since the desired processing temperature can be maintained (Chua & Chou, 2003). We have conducted a systematic experimental study for investigating the processing and quality characteristics of apple slices under SIRDBD in both continuous and intermittent heating modes (Zhu, 2008). The purpose of this study was to compare the process characteristics of continuous heating and intermittent heating of SIRDBD and the product quality of apple slices resulted from the two heating modes.
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Materials and Methods Sample preparation Apple was used as model fruit in this study due to its significant role in western diet. Apples (Malus domesticus Borkh. Var. Golden Delicious) were obtained from a local orchard (Apple Hill, Camino, CA). Freshly harvested apples were stored at 0 ºC ± 1 ºC with relative humidity of 90-95%. Before experiments, apples were removed from chilled storage and tempered to room temperature naturally. Only the medium sized apples (about 80 mm in diameter) were sliced along the vertical axis through the fruit into slices of 5, 9, and 13 mm thick using an HP Hobart industrial food slicer (Troy, OH). A cookie cutter (August Thomsen Corp, Glen Cove, NY) was used to cut the slices into slabs with diameter of 50 mm. In order to create the one dimensional heat transfer condition, two pieces of stainless steel covers (78×78×1 mm), each with a hole in the center with diameter of 42 mm, were placed on the top and the bottom of the slices. The sliced apple slab was subjected to heat treatment immediately after cutting to avoid surface browning in the air. After heat treatment, another cookie cutter with diameter of 36.5 mm was used to remove the central part of the slab for determination of residual enzyme activity and surface color.
CIR blancher/dehydrator setup For each experiment, only one piece of apple slice was placed in the CIR infrared blancher/dehydrator in order to receive well-controlled and constant radiation intensity from the emitter (Figure 1). The CIR blancher/dehydrator was equipped with two CIR emitters powered with natural gas (Catalytic Infrared Drying Technologies LLC, KS). Each emitter had a stainless steel wave-guard attached to create a relatively uniform distribution of radiation intensity. The sample tray made by chicken wire was placed in between the two IR emitters in a position parallel to the emitter faces. A square area (4.5×4.5 cm) was removed from the center of the sample tray so that the apple slice with covering pieces could be held on the sample tray but the wire of the tray would not block the radiative heat transfer from the emitters to the surface of the apple slice. An automatic data acquisition and control system developed in our laboratory was used to control and record various operation parameters, such as flow rate of gas, emitter temperature, and product temperature. Infrared heating was conducted by using two heating modes, continuous heating and intermittent heating. During continuous heating, the radiation intensity was kept constant with a continuous supply of natural gas to the emitter. The continuous heating had the advantage of delivering high power to the surface of the product in a relatively short time for quick heating and moisture removal. On the other hand, with intermittent heating, the product surface temperature was kept at a constant value for the entire duration of processing. This was done by detecting the surface temperature of the product by inserting a thermocouple just beneath the surface of the apple slice. This thin T-type thermocouple (model HYP-0, Omega, CT) was connected to the data acquisition system to control the on and off time for the gas supply to the emitter.
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Catalytic IR Emitter
Natural Gas In Gas Flow Regulator Valve
Data Acquisition
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Figure 1. Schematic diagram for SIRDBD of apple slices.
Experimental Trials Two individual systematic three-factor factorial experimental designs were conducted to determine the effects of processing parameters on the product quality and process characteristics of SIRDBD in both continuous and intermittent heating modes (Zhu, 2008). For continuous heating, three investigated processing parameters were: radiation intensity (3000, 4000, and 5000 W/m2), slice thickness (5, 9 and 13 mm), and processing time (2-20 min). For intermittent heating, the studied processing variables included product surface temperature (70, 75, and 80 ºC), slice thickness (5, 9 and 13 mm), and processing time (2-40 min). All the experimental treatments of intermittent heating were conducted at radiation intensity of 4000 W/m2. Therefore, for comparison purpose, continuous heating experimented under 4000 W/m2 was compared to intermittent heating trials done at 75 ºC at various slice thicknesses. The researched final product quality included surface color, moisture reduction, and residual PPO and POD activities. For each combination of slice thickness and radiation intensity or surface temperature, the blanching and partial dehydration process was ceased when approximately 90% of POD was inactivated. Each experimental condition was repeated three times.
Quality measurements Surface and center temperatures The actual surface and center temperatures of the product were measured by using thin T-Type thermocouples with a response time of 0.5 sec (hypodermic needle probes model HYP-0, Omega, CT) and were recorded by an Omega HH147 RS-232 data logger thermometer (Omega, CT) with an interval of 5 sec. Five measurements were performed for each processing condition and the average value was reported. The insertion of the thermocouple to the center of the slice was aided by a thin “hole” created by mini drill press. A piece of apple slice was held by a small vise and drilled with a drill bit 0.53 mm in diameter (QTE North America, Inc, Rancho Cucamonga, CA) attached to the mini drill press (Model 204D, Philips and Hiss Company Inc., Hollywood, CA) through the thin dimension of the apple slice. The thermocouple was then inserted into the horizontal center of the apple slice to measure the center temperature during processing. To measure the product surface temperature, the thermocouple was inserted from the side to just beneath the surface of the apple slice.
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Moisture reduction The setup of the CIR blancher/dehydrator was modified slightly for the moisture reduction measurement. One piece of apple slice wrapped with aluminum tape on its side (in order to keep the one dimensional heating transfer condition) was placed on a stainless steel rack. The rack, made simply by two parallel thin wires, was positioned stably on a digital balance. The initial weight of the apple slice and the weight for every 30 sec during processing were recorded manually. The initial moisture content of raw apple samples was determined by vacuum oven method at 70°C for 24 hrs according to AOAC 37.1.12. The moisture content of apple slices at each interval of the process was determined by the measured moisture loss. The moisture reduction by the end of the process was defined as the ratio of evaporated water to the initial weight of moisture in the sample.
Color measurement The color of each apple slice was measured immediately before and after SIRDBD treatment by using Minolta colorimeter CR-200 (Osaka, Japan). Three readings, at angles of 120 degrees apart, were taken for each side of both surfaces of the apple slice. The colorimeter (illuminant D65, 2º observer) was calibrated against a standard ceramic white tile (Y=94.4, x=0.3159, y=0.3333). The overall color change was evaluated as:
∆E = ( L0 − L) 2 + (a0 − a ) 2 + (b0 − b) 2
(1)
where subscript “zero” refers to the color reading of fresh apple slices used as the reference, and L, a, and b values stand for whiteness or brightness/darkness, redness/greenness, and yellowness/blueness, respectively. A large ∆E value denotes greater color change from the reference material. To minimize the variability between different raw samples, the normalized L, a, and b values (obtained by dividing the parameters by the corresponding reference values) were also reported (Cruz, Vieira, & Silva, 2007).
Ln =
L a b ; an = ; bn = a0 b0 L0
(2)
Residual enzyme activity Preparation of homogenates After each SIRDBD treatment, the apple slice was homogenized for determining residual enzyme activity. A modified method of Anthon and Barrett (2002) was used to prepare the apple homogenates for enzyme analysis. The apple slice was homogenized with chilled 0.1 M phosphate buffer (pH 6) in the ratio of 1:4 by blender (Waring Commercial, Torrington, CT) for 30 sec. The homogenate was filtered through a double layer cheesecloth, centrifuged at 3800g for 5 min, and placed in an ice-water bath before assayed (Anthon & Barrett, 2002).
Polyphenol oxidase activity The activity of PPO was assayed following the method of Espin et al. (1995). At room temperature, 0.8 mL of homogenate was mixed with 0.2 mL of 10% (w/v) Triton-100 to eliminate the oxidized phenolics and enhance the total PPO activity. Twenty microliters of the mixture were combined with 1.0 mL of a medium containing 25.0 mM DHCA (3, 4dihydroxyhydrocinnamic acid), 2.5 mM MBTH (3-methyl-2-benzothiazolinone hydrazone), and
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0.1 M acetate buffer (pH 4.5). The increase in absorbance at 500 nm for 1 min by Beckman spectrophotometer DU 7500 (Beckman Coulter, Inc., Fullerton, CA) was measured. Finally, the initial slope (usually best to use the interval between 10 and 30 sec) was calculated to determine PPO activity. One unit of PPO activity was defined as an increase of 0.001 unit of absorbance per min.
Peroxidase activity Activity of POD was measured according to Setti et al. (1998). A 20 µl of homogenate was incubated with 1.0 mL of a medium containing 25 mM guaiacol (in 0.1 M acetate buffer, pH 5.5), 2.5 mM MBTH, and 50 mM H2O2 at room temperature. Absorbance increase at 500 nm was monitored for up to 2 min with the slope of the linear portion of the curve used to determine activity. The initial slope (usually best to used the interval between 20 and 60 sec) was calculated to determine POD activity. One unit of POD activity will be defined as an increase of 0.001 unit of absorbance per min.
Protein assay The PPO and POD activities were reported in enzymatic unit per milligram of protein. The concentration of the protein in homogenates was determined by the standard method of Bradford reagent (Simga, Saint Louis, MI). Bovine serum albumin (BSA) was used as the standard. The residual enzyme activities were calculated as: Residual enzyme activity (%) =
At × 100 A0
(3)
where At is the remaining enzyme activity (unit/mg of protein) obtained after the SIRDBD treatment, and A0 (unit/mg of protein) is the total initial activity.
Results The comparison between continuous and intermittent heating modes was carried out on various slice thicknesses. The processing condition of radiation intensity at 4000 W/m2 for continuous heating was compared with a surface temperature at 75 ºC for intermittent heating. The center temperatures of the samples, residual PPO and POD activities, moisture reductions, and overall surface color changes for both heating modes are displayed in one graph for each slice thickness for easy comparison (Figures 2, 3, and 4).
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Figure 2. Comparison of quality and processing characteristics between continuous heating (4000 W/m2) and intermittent heating (75 ºC) for the 5 mm slice
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Time (sec) 4000-9 center temp 4000-9 PPO 4000-9 POD 4000-9 M-R 4000-9 color change
75-9 center temp 75-9 PPO 75-9 POD 75-9 M-R 75-9 color change
Figure 3. Comparison of quality and processing characteristics between continuous heating (4000 W/m2) and intermittent heating (75 ºC) for the 9 mm slice
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Figure 4. Comparison of quality and processing characteristics between continuous heating (4000 W/m2) and intermittent heating (75 ºC) for the 13 mm slice
For the 5 mm slices, the center temperatures of the slices at the heat-up stage were very close for both heating modes. After about 5 min, the center temperature under the continuous heating mode increased rapidly while that under the intermittent heating mode was stabilized. PPO was inactivated quickly under both heating modes and the required processing times to achieve a similar residual PPO did not deviate from each other much. On the other hand, it took a much longer time for intermittent heating than continuous heating to achieve the same level of POD inactivation. The prolonged heating for achieving more than a 1 log reduction of POD under the intermittent heating mode caused a large moisture reduction and the significant increase of overall surface color change. For 9 and 13 mm slices, continuous heating showed an obvious advantage over intermittent heating as it needed much shorter processing time to inactivate POD. However, the long processing time of intermittent heating did not lead to a significant increase of overall surface color change, which may be due to the high final moisture content of the product. Therefore, continuous heating at medium radiation intensity was more advantageous than intermittent heating for blanching and partial dehydrating thin apple slices (5 mm thickness). On the other hand, both heating modes showed promise as an alternative processing method for producing dehydrofrozen apple slices with thickness of 9 and 13 mm. SIRDBD process can serve the purposes of blanching alone and simultaneous blanching and dehydration. For certain fruits and vegetables, moisture removal may not be desired during blanching. In this case, quick blanching and limiting the moisture reduction as much as possible was necessary. Therefore, continuous heating is beneficial for such application. On the contrary, for certain scenarios drying needs to be conducted after proper blanching (e. g. to produce dehydrofrozen products). In such a case, intermittent heating can be suggested since it
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does not causes severe surface darkening. An appropriate heating mode and appropriate processing conditions need to be chosen based on the processing purpose and the property of the materials.
Conclusion Based on the experimental results of SIRDBD with a continuous heating mode and an intermittent heating mode, it has been concluded that the continuous heating mode resulted in a shorter processing time, lower moisture reduction, and similar overall surface color change than the intermittent heating mode in order to achieve about 90% POD inactivation. When prolonged heating was needed (e. g. more than a 1 log reduction of POD), the intermittent heating mode was more advantageous in the preservation of surface color than the continuous heating mode since no severe darkening ever occurred. SIRDBD with both continuous and intermittent heating modes showed promise as an alternative blanching and partial dehydration method for processing fruits and vegetables while maintaining high product quality.
References Anthon, G. E., & Barrett, D. M. (2002). Kinetic parameters for the thermal inactivation of qualityrelated enzymes in carrots and potatoes. Journal of Agricultural and Food Chemistry, 50, 4119-4125. Bomben, J. L. (1977). Effluent generation, energy use and cost of blanching. Journal of food process engineering; Oct 1977, 1 (4); 329 341. Chua, K. J., & Chou, S. K. (2003). Low-cost drying methods for developing countries. Trends in food science and technology; 2003 Dec; 14(12): 519 528, 14(12), 519-528. Cruz, R. M. S., Vieira, M. C., & Silva, C. L. M. (2007). Modeling kinetics of watercress (Nasturtium officinale) colour changes due to heat and thermosonication treatments. Innovative food science and emerging technologies, 8, 244-253. Gabel, M., Pan, Z., Amaratunga, K. S. P., Harris, L., & Thompson, J. F. (2006). Catalytic infrared dehydration of onions. Journal of Food Science, 71(9), 351-358. Pan, Z., & McHugh, T. H. (2004). Novel infrared dry-blanching (IDB), infrared blanching, and infrared drying technologies for food processing, U.S. Patent Application. 20060034981. Filed 8/13/2004, published 2/16/2006. Sandu, C. (1986). Infrared radiative drying in food engineering: a process analysis. Biotechnology progress; Sept 1986; 2(3): 109 119, 2(3), 109-119. Vanlaanen, P. (2003). Freezing fruits and vegetables. Texas Agricultural Extension Service, L2215. Zhu, Y. (2008). Processing and Quality Characteristics of Apple Slices under Simultaneous Infrared Dry-blanching and Dehydration (SIRDBD). Unpublished Ph. D. Dissertation, University of California, Davis, Davis, CA.
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