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Utilization of Microwaves in the Study of Reaction Kinetics in Liquid and Semisolid Media B. A. Welt,+J. A. Steet; C. H. Tong,'*+J. L. RossenJ and D. B. Lundt Department of Food Science and Center for Advanced Food Technology, Cook College, Rutgers University, P.O.Box 231, New Brunswick, New Jersey 08903
An apparatus, referred to as the microwave kinetics reactor (MWKR), incorporating a microwave-transparent pressure vessel with mechanical mixing together with a microwave temperature controller, was developed and tested. Experiments were performed to determine a safe and practical operating envelope for the MWKR. Temperatures measured at the center and at the top surface of varying viscosity solutions in the MWKR were used to determine the adequacy of mixing when the solutions were heated from room temperature to 120 "C and held for a prescribed period of time. The viscosities of the solutions were adjusted by adding (carboxymethy1)cellulose(CMC) to distilled water. A 2% CMC solution represented the upper viscosity limit to be heated homogeneously in the apparatus (approximately 2000 cP). T o illustrate the usefulness of this apparatus in obtaining kinetic parameters, thiamin hydrochloride degradation kinetics in pea puree were determined a t 100,120, and 130 "C and compared to those obtained from the literature. The MWKR offers an attractive, efficient, convenient, and accurate alternative for the study of reaction kinetics in viscous fluids and semisolids because of its short come-up time and mixing capability. Experimentally determined kinetic parameters compared very well to those obtained from the literature. As expected the kinetics was first-order, with an activation energy, Ea,of 27.0 kcal/mol.
Introduction The abundance of literature available on the effects of processing and environmental conditions on various food quality factors implies the importance of reaction kinetic data to the food industry. Process engineers rely on kinetic models to design processing equipment and materials, optimize processing and storage conditions, predict product shelf lives, and develop nutritional labels. Two methods have been used to determine the kinetic parameters of quality factors in foods, referred to as (1) steady-state and (2) un-steady-state methods. The steadystate approach is functionally simple and data reduction is straightforward. The un-steady-state approach is not desirable for fundamental kinetics research because it requires rigorous mathematical and/or numerical data reduction techniques. Such techniques require a certain amount of a priori knowledge of the kinetics studied; the reaction order must be known, and depending on the optimization scheme, solutions obtained for the reaction rate, k,and the activation energy, Ea, may not be unique. Researcherscommonly employ the steady-state method for the primary kinetic research of a quality factor, where reaction order, reaction rates (or D value), and activation energy (or z value) are sought (Hill and Grieger-Block, 1980; Mulley et al., 1975; Feliciotti and Esselen, 1957). Due to its complexity,use of the un-steady-state approach is limited to specific cases of thermal process design and optimization (Lenz and Lund, 1977a-c; Teixeira et al., 1975; Hayakawa, 1970; Ball and Olson, 1957). The steady-state approach is based on the assumption that the temperature of the sample is constant and uniform throughout the experiment. Ideally, the sample is in-
* Author to whom correspondence should be addressed.
t Department of t
Food Science. Center for Advanced Food Technology. 8756-7938/93/3009-0481$04.00/0
stantaneously and uniformly heated to the desired temperature, held at this temperature for the desired time, and then instantaneously and uniformly cooled to nondestructive temperatures at selected time intervals. Due to the driving force of conventionalheat transfer, however, the rate of heating/cooling depends on the temperature difference between the sample and the heating/cooling medium. As the sample approaches the desired temperature, the rate of heating/cooling decreases, resulting in an asymptotic approach to the desired temperature. Since the rate of most reactions increase exponentially with temperature, significant amounts of reactants may be consumed in this asymptotic region. Additionally, the existence of thermal gradients may cause uneven distributions of concentrations within a sample. In order to minimize come-up time and limit internal temperature gradients, researchers have been forced to reduce sample mass by using capillary tubes (0.8 mm i.d.) known as thermal death time (TDT) tubes (Lund, 1975). While TDT tubes are adequate for homogeneous liquid samples or bacterial suspensions, they suffer from the following disadvantages: (1)difficult preparation (Mikolajcik and Rajkowski, 1980); (2) sample-to-sample variations are possible; and (3) cannot be used when analytical methods require relatively large sample sizes. Alternatives to TDT tubes are (1)stainless steel or glass tubes with larger bore sizes (7-11 mm) and (2) TDT cans (63.5 mm i.d. X 9.5 mm height). Even when small-sized containers are used, however, the sample mass reaches the desired temperature only after a lag, and temperature gradients are unavoidable (Canjura et al., 1991; Mikolajcik and Rajkowski, 1980; Mulley et al., 1975). A need exists for the development of reliable steady-state methods that offer relatively short come-up times and uniform heating characteristics for sample sizes larger than the space TDT containers can offer.
0 1993 American Chemlcal Society and American Institute of Chemical Engineers
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482 m
fi
I I LINE SOURCE
TEMP PROBE (TOP SUFACEI
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Figure 1. Schematic diagram of the microwave kinetics reactor
apparatus (MWKR).
Assuming that no, or an insignificant,thermal gradient exists in the sample at the end of the come-up time, degradation during come-up may be negated by using the concentration after come-up as the initial concentration, CO (Arabshahi and Lund, 1988). Destruction during cooling is not as critical, because the driving force for heat transfer is the greatest at the onset of cooling, which allows a rapid reduction in the reaction rate to relatively insignificant levels. The unique property of microwavesto rapidly heat lossy materials without the constraint of a temperature differential makes this form of heat transfer attractive in significantlyreducing come-up times while handling larger sample sizes. Welt et al. (1992) identified four problems associated with traditional attempts to use microwave energy in kinetics research: (1)lack of on-linetemperature measurement; (2) uneven heating due to microwave field distributions and the physical and electrical nature of the heated object: (3) lack of temperature control; and (4) evaporative losses from the sample during heating. Welt et al. (1993) introduced an apparatus which successfully utilized microwave energy for steady-state kinetics research. However, it was limited to low-viscosity reaction media and temperatures below 90 "C. The purpose of this work was three-fold. The first purpose was to develop a steady-state kinetics apparatus that successfullyutilizes the rapid heating characteristics of microwave energy for samples of significant viscosity, at temperatures sufficiently greater than 100 "C. The second purpose was to assess the performance of the apparatus in order to determine its operating capabilities and limitations. The third purpose was to illustrate the use of the apparatus by studying thiamin degradation kinetics in pea puree at 110, 120, and 130 "C.
Materials and Methods Apparatus. The apparatus consisted of two major components a microwave oven with a feedback temperature control system and the microwave kinetics reactor (MWKR). A schematic diagram of the apparatus is shown in Figure 1. The temperature feedback control system is described by Tong et al. (1992). However, a larger cavity oven rated at 900 W (Model MQS1403W, Quasar Co., Elk Grove, IL) was modified and used in this study. The operation of the feedback control system is demonstrated by Welt et al. (1993). As a review, a fiber optic temperature sensing unit (Model 755, Luxtron Corp., Santa Clara, CA)
G
Figure 2. Features of the microwave kinetics reactor pressure vessel.
measured the temperature of the sample and fed this information to an IBM/AT-compatible computer through an RS-232 cable. A custom computer program instructed the computer to determine whether the sample temperature was above a user-supplied, upper temperature limit or below a user-supplied, lower temperature limit about the desired set-point temperature. If the sample temperature was below the lower limit, the computer activated a relay inserted in the magnetron power supply circuit of the microwave oven, causing the sample to heat. If the sample temperature was above the upper limit, the computer deactivated the magnetron relay, allowing the sample to cool. The controller's response, or gain, was varied by adjusting the microwave power on a continuously adjustable variable transformer connected in the magnetron power supply loop. Welt et al. (1993) showed that the feedback temperature control system was capable of maintaining a desired set-point temperature to within a0.5 OC. It has been learned that control to within i0.3 "C is possible with careful adjustment of controller gain and selection of appropriate high and low set-point limits. The MWKR is a rugged vessel constructed from a poly(ether imide) polymer available under the registered trade name ULTEM 1000 (GE Plastics, Pittsfield, MA). This material was chosen for its high mechanical strength over a wide range of temperatures. As described below, the MWKR utilizes both dynamic and static O-rings to provide a completely sealed reaction cavity. The O-ring material chosen was a fluorocarbon elastomer available under the registered trade name, Viton (Small Parts, Inc., Miami Lakes, FL). The vessel was designed according to the following criteria: (1)capabilityto provide uniform heating over a wide sample viscosity range; (2) provide a short and uniform come-up time for a relatively large sample size: (3)withstand high temperatures and pressures; (4) provide a convenient and efficient sampling method; and (5) transparent to microwave energy. The MWKR (see Figure 2) consists of six major components: (1)reactor cavity; (2) reactor lid with sealed drive to mixing shaft coupling; (3) mixing shaft with impeller assemblies: (4) sampling port; (5) external pressure gauge with overpressure safety valve assembly; and (6) external DC motor for mixing. The reactor cavity (Figure 2A) has an internal volume of approximately 500 mL and is shaped as a regular, upright cylinder. The cavity has five significant features: (a) an O-ring groove on the top of the cylinder wall, which seals the cavity to the reactor lid (Figure 2B); (b) a guide hole
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at the center of the cavity floor, which fixes the bottom of the mixing shaft (Figure 2G);(c) male threading on the exterior wall to accommodate the screw cap (Figure 2C) which secures the reactor lid to the cavity; (d) a temperature probe penetration that allows measurement in the radial direction in the sample from approximately the cavity center to the cavity wall, depending on where the tip of the temperature probe is mounted; and (e) asampling port penetration located at the bottom of the vessel wall, just above the inner cavity floor. The reactor lid (Figure 2B) serves five essential functions: (a) to provide a pressure seal with the reactor cavity; (b) to provide a housing for the sealed drive to mixing shaft coupling; (c) to provide a penetration to the head space of the cavity in order to accommodate a pressure gauge and pressure relief valve; (d) to provide a temperature probe penetration from the top in the axial direction, such that measurements are possible at any depth in the sample from maximum depth to top surface, or even head space, depending on where the probe tip is mounted; and (e) to provide a penetration, incorporating a dynamic O-ring seal around the drive shaft to couple with the mixing shaft. The appearance of the reactor lid is that of a regular cylinder with a flanged bottom. The flange allows the screw cap to tighten the lid in place. The drive to mixing shaft coupling provides a freely rotating mechanical linkage between the drive shaft and mixing shaft. The coupling consists of three components: the actual connector (Figure 2D), an O-ring sealing plate (Figure 2E),and acomponent to secure the coupling inside the reactor lid (Figure 2F). During assembly, the O-rings and connector are generously lubricated with an inert grease,such as a siliconevacuum grease. Once the coupling is secured inside the lid, it need only be disassembled for wear inspection, O-ring replacement, or regreasing. The mixing shaft extends from the bottom of the lid to the guide hole in the bottom of the cavity. The shaft accommodatesvarious impellerelements (Figure21), which are held in place by set screws. The impeller elements used depend on the mixing requirements of each particular experiment. The sampling port was designed to allow semisolid material (such as vegetable and/or meat purees) to flow under the pressure gradient generated between the reactor cavity and ambient pressure. The port consists of three main components: (a) a Teflon "T"-type fitting; (b) a custom ULTEM lo00 screw-on plunger; and (c) a sampling vial equipped with a septum and screw cap (Supelco, Inc., Bellefonte, PA). The plunger was designed so that, as it is unscrewed from the Teflon fitting, the plunger is drawn beyond the boundary of the 90" side (outlet) of the "T", allowing the contents of the vessel to flow into a sample vial. A small hole was drilled in the vial's septum, so that it fits snugly over a short piece of Teflon tubing attached to the outlet via a compression fitting. When a sufficient sample is drawn, the plunger is screwed back into place, stopping the flow out of the vessel and returning the remaining material in the "T" back into the reactor cavity. The external pressure gauge and overpressure relief valve were attached to the MWKR by a sturdy Vd-in. Teflon tube attached to the reactor lid by a Teflon compression fitting (Figures 1and 2). The tubing passed through a hole drilled in the microwave oven wall. The tubing, gauge, and valve were mounted to a custom aluminum block with a common internal passage (Figure 1).
A Stedfast laboratory stirrer (Model 14-505-1,Fisher Scientific, Springfield, NJ) was attached to the drive shaft
483
in order to rotate the coupling and mixing shaft. The motor was mounted above the microwave oven and connected to the drive shaft (Figure 2H) through a hole drilled through the roof of the oven. Performance Assessment. The operating envelope of the MWKR was determined by the following four criteria: (1) time to reach set-point temperature; (2) temperature uniformity during come-up; (3)time required to equilibrate temperature once the hottest point reached the set-point temperature; and (4) ability to maintain temperature stability and uniformity a t the set-point temperature for a desired time period. The following variables were identified for the performance assessment: (1) sample viscosity; (2) sample size; (3)mixing speed; (4) impeller type; (5)initial temperature; (6)final temperature; (7)magnetron power during comeup (as indicated on the variable transformer); and (8) magnetron power after come-up. Three impeller types were constructed due to simplicity of fabrication and expected performance characteristics (Oldshue and Herbst, 1990): (a) anchor; (b) vertical paddles; and (c) paddles at a45" angle. Preliminarymixing experiments in a similarly shaped, clear plastic bottle using methylene blue dye in pea puree showed that a combination of the anchor and vertical paddle impellersprovided the best mixing characteristics at high viscosity;therefore, these impellers were used throughout the study. The sample size was fixed at the maximum working capacity of approximately 400 mL in order to assess a worst case sample size situation. For convenience, the initial temperature was fixed a t room temperature. The final temperature, or set-point temperature, was fixed at 120 "C. The temperature probes were mounted such that the radial probe measured the center temperature, while the axial probe provided the top surface temperature (Figures 1and 2). If a temperature gradient existed in the sample, it was felt that these locations represented the extremes of possibility. Magnetron power was handled in the same manner for each experiment. During come-up, magnetron power was set a t the maximum by setting the variable transformer to ita maximum of 120V. After come-up, magnetron power was reduced by using approximately 80 % of the maximum available voltage. Although this protocol was followed throughout the performance assessment, it was found during kinetics experiments that, when a temperature gradient developed as a result of the heating rate exceeding the mixing capacity of the impellers, a lower magnetron power setting could be used in exchange for a slightly longer come-up time. Sample viscosity and mixing speed were studied in the performance assessment of the MWKR. Sample viscosity was adjusted by preparing different concentrations of (carboxymethy1)cellulose (CMC) in distilled water. Concentrations of 0, 1,and 2% CMC by weight were studied. It was found that the viscosity of the CMC solution varied almost linearly with CMC concentration, such that the corresponding viscosities were approximately 1,1o00,and 2000 cP, respectively. The mixing speed was adjusted by setting the stirrer to three of the ten settings available on its dial. These three settings were arbitrarily labeled as slow = 3, medium = 5, and fast = 10. The rotation rates of these settings corresponded to approximately 90, 150, and 300 rpm, respectively. Kinetics Studies. To demonstrate the usefulness of the MWKR in determining reaction kinetic data in liquid and semisolid media, thiamin degradation kinetics in pea
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puree was studied. This food system was chosen because it represented the upper viscosity limit that the apparatus could uniformly heat, and its thiamin degradation kinetics has been studied extensively (Mulley et al., 1975;Feliciotti and Esselen, 1957). It is generally known that the degradation of thiamin in pea puree follows first-order reaction kinetics. Reactions were studied in duplicate at 110,120, and 130 "C. A large batch of frozen peas was purchased from a local supermarket. The peas were freeze-dried in a Stokes commercial freeze drier (F. J. Stokes Machine Co., Philadelphia, PA) and ground into a fine powder with a Fritzemill Model D comminuting machine (W. J. Fitzpatrick Co., Chicago, IL). The powder was sealed in home canning jars and stored at -20 "C until use. Approximately 500 mL of pea puree was prepared for each experiment. A 400-g quantity of distilled water was added to ca. 100 g of freeze-dried pea powder to prepare the puree. Thiamin hydrochloride (Fisher Scientific, Springfield,NJ) was added at a concentration of ca. 35 mg of thiamin hydrochloride to 100 g of puree. After preparation, the puree was placed in the refrigerator for several hours in order to ensure complete hydration of the pea powder. Samples taken during each run were first cooled in an ice bath and then covered with aluminum foil and stored in a refrigerator until assayed for thiamin concentration by HPLC. Approximately 5 g of sample was diluted with 20 g of HPLC-grade water (Fisher Scientific, Springfield, NJ). The dilution was well mixed and filtered fist through Whatman No. 42 filter paper (Fischer Scientific, Springfield, NJ) and then through a 0.22-pm filter (Fisher Scientific, Springfield, NJ) prior to injection into the HPLC. The thiamin assay was performed in accordance with the high-performance liquid chromatography procedure described by Arabshahi and Lund (1988). The HPLC system consisted of the following components, all manufactured by Waters Associates (Milford, MA): a C18 pBondaPak column, a Model U6K injector, a Model 510 pump, and a Model 484 tunable UV detector set at 254 nm. The HPLC system was interfaced to an IBM PS/2 computer by the Waters system interface module. The mobile phase was an isocratic solution prepared in the proportions of 50 mL of HPLC-grade methanol (Fisher Scientific, Springfield, NJ), 5 mL of glacial acetic acid (Fisher Scientific, Springfield, NJ), and 12.5 mL of buffered pentanesulfonic acid (Fisher Scientific, Springfield, NJ) brought to 100 mL with HPLC-grade water (Fisher Scientific, Springfield, NJ). Aliquots of 30 pL of diluted pea puree sample were injected into the reversedphase column. Thiamin eluted in approximately 8 min. Chromatographic data were analyzed by the Baseline 810 (WatersAssociates,Milford, MA) software package. Peak areas were recorded and compared to a thiamin concentration calibration curve. The calibration curve was generated by injecting known concentrations of solutions of pure thiamin hydrochloride in water and recording the resulting peak areas. Peak area versus thiamin concentration was linear throughout the concentration range studied.
Results and Discussion PerformanceAssessment. The MWKR worked well with low-viscositysamples at medium (150 rpm) to high (300 rpm) mixing rates. Figure 3 shows the difference between the top surface and center temperatures for water at the medium mixing setting (150 rpm). The figure shows
k
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-Top surface - Center
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200
300 400 500 660
700 800
900
Time (sec)
Figure 3. Temperatures at the top surface and center of 400 g of 0% CMC solution inthe M W K R during come-up and on/off control at 120 O C under the medium mixing setting (150 rpm).
that before the feedback control system was activated (at approximately 1 min on the time axis), the temperature probes indicated a uniform temperature at room temperature. Once the temperature controller was activated, the come-up time was approximately linear and required about 250 s. The sample temperature stabilized quickly and remained uniform at 120 "C for the remainder of the experiment. Figure 4 depicts an example of insufficient mixing. The 1% CMC solution (approximately lo00 CP at room temperature) was too viscous to achieve temperature uniformity at the slow mixing setting (90 rpm). Figure 4 shows that a temperature difference exists immediately after activation of the temperature controller. For safety reasons, the temperature controller software was designed to use the probe reading the highest temperature as feedback input. In this case, the top surface reached 120 "C before the center. The controller used the top surface temperature as the feedback input signaland began cycling the magnetron on and off. This occurred approximately 230 s after the controller was activated. The trend of temperature rise at the center is altered at this point, reflecting the reduced contribution of microwave heating, as a result of the variable transformer in the magnetron power loop being adjusted to 80% of maximum. For comparison, Figure 5 shows the result of sufficient mixing for the 1% CMC solution. In this case the maximum mixing setting (300rpm) was used. A maximum temperature difference of 5 "C occurred during come-up, which was eliminated almost immediately upon reaching the set-point temperature. No significant temperature difference developed throughout the remainder of the experiment. Figure 6 provides an example of a solution that represented the viscosity limit for the MWKR. As seen in Figure 4, a temperature difference existed immediately after the controller was activated. Figure 6 shows a maximum temperature difference of about 8 "C in the come-up curves. The top surface reached the set-point temperature beforethe center, which caused a trend change in the come-up curve at the center probe from steep to shallow,reflecting the reduced contribution of microwave heating as a result of the variable transformer in the magnetron power loop being adjusted to 80% ' of maximum. As previously mentioned, temperature gradients during come-up can be reduced and even eliminated by reducing the microwave power, and therefore the rate of heating, during the come-up phase. When kinetics research is being performed on a system with significant viscosity, it is advisable to perform several preliminary heating exper-
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485 120,
..
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500
600
700
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Time (sec) Figure 4. Temperatures at the top surface and center of 400 g of 1% CMC solution in the MWKR during come-up and on/off control at 120 "C under the slow mixing setting (90 rpm). I""
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Time (Seconds) Figure 7. Temperatures at the top surface and center of 400 g of pea puree in the MWKR during come-up and on/off control at 110 "C under the fast mixing rate (300rpm), compared to the temperature at the center of pea puree in a smallbore stainless steel tube heated in a 110 "C oil bath. shown that, as the conventionally heated sample approached the set point of 110 O C , the driving force for heat transfer was reduced, causing an asymptotic form of the heating curve. In contrast, the rate of temperature rise for the sample in the MWKR remained essentially constant throughout the come-up period, regardless of the proximity to the set-point temperature. Although not shown in Figure 7, temperature gradients from the tube surface to the center of the puree existed in the conventionallyheated sample throughout the come-uptime. A significant feature shown in Figure 7 is that approximately 75 % (200 of 260 s) of the come-up time for the conventionallyheated sample took place at temperatures above 90 "C (reaction rates for most food quality attributes become significant above 90 "C). In comparison, only about 25% (70 of 260 s) of the come-up time for the sample heated in the MWKR was spent above 90 "C. Less reactant would be consumed in the MWKR during the come-up phase than for a conventionally heated sample. Because of ita short come-up time and mixing capability, which completely eliminate temperature and concentration gradients, the MWKR is ideal for use in the study of reaction kinetics in viscous fluids and semisolids. A reaction studied in the MWKR fulfillsthe assumptions of a steady-state reaction when C, is measured at the time the sample reaches the desired temperature. It must be noted that this apparatus is an experimental device designed and intended for use at moderately elevated temperatures and pressures. Our laboratory has used the apparatus successfully at temperatures up to 130 "Cfor water-based media, which have vapor pressures of approximately 5.8 kPa (25 psig) at this temperature. The MWKR was pressure-tested dry at room temperature with compressed air to approximately 10.9 kPa (60 psig), prior to any high-temperature-pressure work. Adequate safety tests should be performed prior to using this apparatus at temperatures above 130 O C and 5.8 kPa (25 psig). Kinetics Studies. Normalized concentration curves for the degradation of thiamin hydrochloride in pea puree a t 110, 120, and 130 "C are shown in Figure 8. C, was measured from samples taken immediately after the sample reached the desired temperature. The linearity of these data on a semilog plot for two replicates confirms the expectation that the reaction is first-order. The rate constants, determined from the slopes of these curves, are tabulated in Table I. Regressions performed on the replicates a t each temperature produced r2 values in the range of 0.99.
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Rate constants from this work and from the literature for thiamin degradation in pea puree (Mulley et al., 1975, Feleciotti and Esselen, 1957) are shown in the Arrhenius plot (Figure 9). For this work the activation energy, E,, determined from the slope of the Arrhenius plot was 27.0 kcal/mol. Ea values determined from the reported data of Mulley et al. (1975) and Feliciotti and Esselen (1957) were 27.9 and 27.5 kcal/mol, respectively. Although no significant difference was found in all three E , values, Figure 9 shows that the rate constants determined by Mulley et al. (1975)and Feliciotti and Esselen (1957)differ by approximately 1.5-2.0 times. It is interesting to observe that the rate constants determined in this work fell very close to the average values reported by Mulley et al. (1975) and Feliciotti and Esselen (1957). Mulley et al. (1975) and Feliciotti and Esselen (1957) relied on small sample sizes in their work. Mulley et al. (1975) used a special device that handled only 20 pL of sample, while Feliciotti and Esselen (1957) used 2-mL samples in TDT cans. This suggeststhat the MWKR is a reliable and more convenient apparatus for performing reaction kinetics work, because it is easy to use and successfully satisfies the assumption of steady-state conditions. The reaction rate constant, k, for thiamin degradation in pea puree can be related to temperature through the Arrhenius equation: 13 576.3 In (k)= 29.936 - -
T
where k is the rate cosntant (min-l) and T is the temperature in kelvin. The numerical values in eq 1were obtained from a linear regression performed on the data from this work, shown in Figure 9. The slope of the curve is -EdR, where E , is the activation energy in cal/mol and R is the ideal gas law constant (1.987 cal/(mol-K)); the intercept is the preexponential factor. Thiamin concentration as a function of heating time and temperature can be calculated from the following equation: C = -kt In C O
where Co is the initial thiamin concentration, 12 is the temperature dependent reaction rate constant determined from eq 1, and C is the thiamin concentration at any time, t. Thiamin concentrations as a function of heating time at 110,120, and 130 "C as calculated by eqs 1 and 2 are also shown in Figure 8.
0.0025
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Time (Minutes) Figure 8. Normalized concentrationsof thiamin hydrochloride in pea puree as a function of time at 110, 120, and 130 O C .
1
This work
Figure 9. Arrhenius plots for thermal degradation of thiamin hydrochloride in pea puree. Table I. Reaction Rate Constants for Thermal Degradation of Thiamin Hydrochloride in Pea Purae Determined Using the MWKR Apparatus
temperature ( O C ) 110 120 130
rate constant (min-1) 0.004013 0.010 676 0.023 234
The MWKR has been utilized as a primary apparatus for determining the kinetic parameters of numerous food quality attributes, both in buffer systems and in semisolid model food systems in our laboratory. Although it is believed that microwaves do not induce nonthermal effects, this question needs to be investigated further since there is a large volume of literature that implies the existence of such effects. We have begun to compare the effects of microwave energy versus conventional heating methods on various microbiological systems, nutrients, various organicchemical reactions involving proteins and enzymes, and free radical formation using the MWKR apparatus and other conventional means.
Conclusions The MWKR apparatus offered an efficient and accurate method for performing steady-state reaction kinetics studies in viscous liquids and semisolids. The rate of thiamin degradation in pea puree, as determined using the unique MWKR apparatus, was consistent with data obtained from the literature. The performance of the MWKR apparatus was determined to be ideal for a broad range of kinetics research. For low-viscosity, aqueous type reaction media, a relatively mild mixing protocol was sufficient to achieve a uniform sample temperature. As expected, the mixing requirement became more severe as the viscosityof the reaction mediaincreased. In the stated configuration, a solution of approximately 2000 CP (2% CMC solution) represented a practical upper viscosity limit for the MWKR apparatus. Acknowledgment This is publication number D-10209-1-93 of the New Jersey Agricultural Experiment Station. This work was financially supported by the Center for Advanced Food Technology (CAFT),Rutgers University, through funding provided under contract by the Department of Defense/ Defense LogisticsAgency at Cameron Station, Alexandria, VA. B.A.W. was supported by a National Needs Fellowship sponsored by the United States Department of Agriculture. The authors also gratefully acknowledge the
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assistance of Peter and Chris Ralph for guidance in the fabrication of the pressure vessel.
Literature Cited Arabshahi, A.; Lund, D. B. Thiamin Stability in Intermediate Moisture Food. J. Food Sci. 1988,53 (l),199. Ball, C. 0.; Olson, F. C. W. Sterilization in Food Technology; McGraw-Hill: New York, 1957. Canjura, F. L.; Schwartz, S. J.; Nunes, R. V. Degradation Kinetics of Chlorophyll8 and Chlorophyllides. J. Food Sci. 1991,56 (6),1639. Feliciotti, E.; Esselen, W. B. Thermal Destruction Rates of Thiamin in Pureed Meats and Vegetables. Food Technol. 1967,2, 77. Hill, C. G., Jr.; Grieger-Block, R. A. Kinetic Data: Generation, Interpretation, and Use. Food Technol. 1980,2,56. Lenz, M. K.; Lund, D. B. The Lethality-Fourier Number Method: Experimental Verificationof a Model for Calculating Temperature Profiles and Lethality in Conduction-Heating Canned Foods. J. Food Sci. 1977a,42 (4), 989. Lenz, M. K.; Lund, D. B. The Lethality-Fourier Number Method: Experimental Verificationof a Model for Calculating Average Quality Factor Retention in Conduction-Heating Canned Foods. J. Food Sci. 1977b,42 (4), 997. Lenz, M. K.;Lund, D. B. The Lethality-Fourier Number Method: Confidence Intervals For Calculated Lethality and
Mass-Average Retention of Conduction-Heating Canned Foods. J. Food Sci. 1977c,42 (4), 1002. Lund, D. B.In Principles of Food Science; Fennema, 0.R., Ed.; Marcel Dekker, Inc.: New York, 1975;pp 31-92. Mikolajcik, E. M.; Rajkowski, K. T. Simple Technique to Determine Heat Resistance of Bacillus stearothermophilus Spores in Fluid Systems. J. Food R o t e c t . 1980,43(lo),799. Mulley, E. A.; Stumbo, C. R.; Hunting, W.M.Kinetics of Thiamin Degradation by Heat. J. Food Sci. 1975,40,986. Oldshue, J. Y.; Herbst, N. R. Fluid Mixing; Mixing Equipment Co., a Division of General Signal Corp.: Rochester, NY,1990. Teixeira, a. A.; Stumbo, C. R.; Zahradnik, J. W. Experimental Evaluation of Mathematical and Computer Models for Thermal Process Evaluation. J. Food Sci. 1975,40,653. Tong, C. H.; Lentz, R. R.; Lund, D. B. A Microwave Oven with Variable Continuous Power and a Feedback Temperature Controller. Biotechnol. h o g . 1993,in press. Welt, B. A.; Tong, C. H.; Lund, D. B. Effect of MicrowaveEnergy on Destruction of Sporesof Clostridiumspomgenes. Preaented at the 27th MicrowavePower Symposium of the International Microwave Power Institute, 1992. Welt, B. A,; Tong, C. H.; Rossen, J. L. An apparatus for Providing Constant and Homogeneous Temperatures in Low Viscosity Liquids During Microwave Heating. Microwave World 1993, 13 (2), 9. Accepted May 18,1993.
