Scholarly article on topic 'Microwave flow and conventional heating effects on the physicochemical properties, bioactive compounds and enzymatic activity of tomato puree'

Microwave flow and conventional heating effects on the physicochemical properties, bioactive compounds and enzymatic activity of tomato puree Academic research paper on "Agriculture, forestry, and fisheries"

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Academic research paper on topic "Microwave flow and conventional heating effects on the physicochemical properties, bioactive compounds and enzymatic activity of tomato puree"

Microwave flow and conventional heating effects on the physicochemical properties, bioactive compounds and enzymatic

activity of tomato puree

Mitra Arjmandia,b,c, Mariano Otónc, Francisco Artésb,c, Francisco Artés-Hernándezb,c,

Perla A. Gómezc and Encarna Aguayob,c* *e-mail:

aCollege of Agriculture and Natural Resources. University of Tehran. Iran. bPostharvest and Refrigeration Group - Universidad Politécnica de Cartagena (UPCT). Paseo Alfonso XIII, 48. 30203 Cartagena, Murcia, Spain.

cInstitute of Plant Biotechnology. UPCT. Campus Muralla del Mar, 30202 Cartagena, Murcia, Spain.


BACKGROUND: Thermal processing causes a number of undesirable changes in physicochemical and bioactive properties of tomato products. Microwave (MW) technology is an emergent thermal industrial process that offers a rapid and uniform heating, high energy efficiency, and high overall quality of the final product. The main quality changes of tomato puree after a pasteurization at 96 ± 2 °C for 35 s, provided by a semi industrial continuous microwave oven (MWP) under different doses (low power/long time to high power/short time) or by conventional method (CP) were studied.

RESULTS: The results showed that all heat treatments reduced color quality, total antioxidant capacity and vitamin C, with a greater reduction in CP than in MWP. On the other hand, use of a MWP, in particular, high power/short time (1900

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/jsfa.7824

W/180 s, 2700 W/160 s and 3150 W/150 s) enhanced the viscosity, lycopene extraction and decreased the enzyme residual activity better than with CP samples. For tomato puree, polygalacturonase was the more thermos resistant enzyme, and could be used as an indicator of pasteurization efficiency.

CONCLUSION: MWP was an excellent pasteurization technique that provided tomato puree with improved nutritional quality, reducing process times compared to the standard pasteurization process.

Keywords: Carotenoids; Viscosity; Vitamin C; Thermal treatment; Smoothie.


Tomato (Lycopersicon esculentum L.) is widely grown around the world and becoming increasingly popular, both fresh and processed. Tomato and tomato products have very high levels of bioactive compounds such as carotenoids, especially lycopene, followed by P-carotene1. Dietary intake of tomatoes and tomato products containing lycopene, has been shown to reduce the risk of prostate cancer.2 Processed products contain more lycopene than fresh foods because thermal treatment causes transformation of the trans isomers in cis form.3

All tomato products are usually prepared by thermal processing for inactivating natural degrading enzymes and microorganisms that may cause unwanted modification during their storage.4 But this processing causes a number of undesirable changes in physicochemical properties of products and must be applied without compromising food safety, nutritional quality and shelf life.5'6 MW technology is an emergent thermal industrial process to achieve this purpose. It enhances microbial destruction and help to maintain the product quality.7 In comparison with conventional heating methods, the industrial MW oven offers a rapid and relatively uniform heating, high energy efficiency, reduced space utilization, precise process control, fast start-up, shutdown conditions and high overall quality and safety of the final product. 6,8 Several studies have assessed the safety as well as nutrient loss associated with MW cooking, and antioxidant activity of strawberry and kiwifruit apuree.9,10 Additionally, high-power/short-time MW processes reduced the adverse thermal degradation in food quality while ensuring food safety because of the nutrient characteristics of product being more sensitive to time than to temperature.11 In this way, the aim of the present work was to investigate quality parameter changes such as vitamin C, lycopene, P-carotene, total phenolics content (TPC), and total antioxidant capacity (TAC), as well as

color parameters, viscosity and enzymatic activity of a tomato puree after CP and MWP using different doses (powers and times) of processing.


2.1. Plant materials

Tomato (Lycopersicum esculentum Mill., Moneymaker cv.) were grown in greenhouse under Mediterranean climate (Mazarrón, Murcia, Spain). They were harvested according to commercial maturity stage, obtaining 4.73 ± 0.07 °Brix and 44.94 ± 0.19 h°. Fruits free from defects and with a similar visual appearance were blended with a commercial thermomix (Vorwerk Elektrowerke, Model TM 31-1, France) in order to obtain a puree.

2.2. Thermal treatment

600 mL of puree samples were heated in the above cited thermomix (conventional pasteurization, (CP). Alternatively, samples for each MW pasteurization (MPW) treatment were placed in 3 tempered and extra resistant MW glasses were used (Hostelvia, Vicrila, Leioa, Spain). These glass beakers, containing each one 200 mL of tomato puree was treated in an innovative semi-industrial prototype of continuous MW oven (Sairem Ibérica S.L. SI-MAQ0101, Barcelona, Spain) with a power control from 0 to 3,000 W (Fig. 1). Based on our preliminary studies several appropriate temperature/time combinations of MWP were selected with following conditions:

Low power/long time (390 W/848 s, 510 W/805 s, 770 W/460 s), medium power and time (980 W/848 s, 1,640 W/805 s, 1,700 W/230 s) and high power/short time (1,900 W/180 s, 2,700 W/160 s and 3,150 W/150 s). In both CP and MWP

processing the final temperature in all the treatments was 96 ± 2°C and they remained at this temperature for 35 s.

After both kinds of pasteurization, the samples were packaged aseptically into plastic tubes and rapidly cooled (5 °C) with an ice-water bath and then analyzed before (control) and after thermal treatments. For each heating method, the full experiment was conducted independently three times, each one constituting a repetition which was analysed.

2.3. Analysis and quality determination

Physical quality analysis

Color: The color of the samples was monitored by photo-colorimeter (Minolta CR-300, Ramsey, NJ, USA). Color was expressed as Hunter L*, a*, b*and hue angle (h° = tan-1b*/a*).

Viscosity: Viscous flow tests were determined in triplicate with a controlled shear rate/stress rheometer (AR G-2, TA Instruments, U.K) at 20 °C. Viscous flow tests were performed by using a shear rate range between 1 and 100 s-1.

2.3.2 Chemical quality attributes

Titratable acidity (TA), soluble solids content SSC and pH were analyzed as described by Aguayo et al.12

Total phenolic compounds (TPC) and total antioxidant activity (TAC) TPC was measured following by Swain and Hillis13 method using a Multiscan plate reader (Tecan Infinite M200, Mannedorf, Switzerland). TPC was expressed as mg chlorogenic acid equivalents (ChAE) kg-1 fresh weight (FW).

TAC was assessed using the Ferric Reducing Antioxidant Power (FRAP) technique14 with the same device as for TPC. Results were expressed as mg ascorbic acid equivalent (AAE) kg-1 (FW).

Total vitamin C

The ascorbic acid (AA) determination was performed as described by Falagan et al.15 10 mL of puree were mixed with 10 mL of a solution containing 45 g L-1 of metaphosphoric acid and 7.2 g L-1 of DTT (DL-1, 4-dithiotreitol). The mixture was centrifuged at 22,100 x g for 15 min at 4 °C (Eppendrof, AG 22331, Germany). The analysis of vitamin C was carried out by HPLC (Waters 2695, Detector UV-V 2687, Milford, USA). Detection was performed with an UV-visible spectrophotometer (Hewlet Packard, Model 8453, Columbia, USA) at 260 nm. Vitamin C was quantified through a calibration curve made with AA standards and results were expressed as mg (AA) kg-1 FW.


Carotenoids were measured according to the method of Nagata and Yamashita16 with the slight modifications. 5 mL of smoothie were mixed with 20 mL acetone-hexane (4:6). Two phases separated and the upper phase was taken for lycopene and ß-carotene measurements at 663, 645, 505 and 453 nm in a UV-visible spectrophotometer (Hewlet Packard, Model: 8453, Columbia, EEUU). Lycopene and ß-carotene were calculated according to the following equations:

Lycopene = -0.0458 A663 + 0.204 A645 + 0.372 A505 - 0.0806 A453 ß-carotene = 0.216 A663 - 1.22 A654 - 0.304 A505 + 0.452 A453 Results were expressed as mg lycopene or ß-carotene kg-1 FW.

Peroxidase (POD)

POD activity was measured using the method described by Elez-Martinez et al.17 0.009 mL enzyme extract, 0.243 mL of phosphate buffer 0.05 M, 0.018 mL of phenildiamina (10 g kg-1), and 0.009 mL of H2O2 (15 g kg-1) in a 96-well polystyrene flat-bottom plate. The absorbance was measured at 509 nm for 10 min at 25 °C by using the multiscan plate reader cited above.

Pectin methylesterase (PME)

PME activity was determined according to Ratneret et al.18 with slight modifications. A 2.5 mL simple of puree was homogenized with 10 mL of sodium chloride 0.2 M. After filtering the homogenate by cheesecloth, 2.5 mL of it was mixed with 15 mL pectin (10 g L-1). This solution was adjusted to pH 7.0 with 1N NaOH and the pH was kept at 7.0 during 10 min using 0.01 N NaOH. One PME U can be expressed as the amount of enzyme that produces 1 nmol of acid per minute at pH 7.0 and 22 °C.

Polygalacturonase (PG)

PG activity was measured according to Aguilo-Aguayo et al.4 with slight modifications. 2 mL of sample was homogenized two times in 15 and 10 mL of cold acetone for 30 and 15 min, respectively. The supernatant was again decanted and replaced with 5 ml of tris hydroxylmethyl aminomethane buffer (0.2 M), pH 7.0, including 0.5 g L-1 of sodium metabisulfite, 10 g L-1 PVPP and 1M NaCl. The extraction was carried out during 2 h in an orbital shaker (Stuart, Staffordshire, UK) at 200 x g in darkness inside a polystyrene box with ice at 4 °C. The homogenate was centrifuged at 20,000 x g for 15 min at 4 °C. The supernatant was used as enzyme extract. The PG activity was

quantified according to Gross.19 The substrate was constituted of 0.6 mL of a solution containing 4 g L-1 (w/v) polygalacturonic acid in 0.05 M sodium acetate buffer (pH 4.5) and the reaction was carried out by adding 0.15 mL of enzyme extract, followed by incubation at 37 °C for 10 min with shaker of 30 rpm. The reaction was stopped with 2 mL of 10 mM borate buffer at pH 9 and 0.4 mL of 10 g L-1 (w/v) cyanoacetamide. The mixture was put in a boiling water bath (100 °C) for 10 min and then chilled by ice. 200 ^L of extraction was put in a 96-well polystyrene flat-bottom plate the well. The absorbance of samples was measured. The absorbance was read at 276 nm using the same device as for POD at 22 °C. The quantity of reducing groups formed was determined using a calibration curve made with D-galacturonic acid and the enzyme activity was expressed as mM of galacturonic acid released per min. One unit (U) of PG activity was defined as the amount of enzyme that yielded 1 mM of reducing groups per min.

For all analysis, each of the three replicates was analyzed by triplicate. 2.4. Statistical analysis

Data were analyzed in a randomized design with three replicates per treatment. Data were subjected to one-way analysis of variance (p< 0.05) using Statgraphic Plus 5.1, Manugistic Inc, Rockville, MD, USA). Mean values were compared by multiple range least significant difference test to identify significant differences among treatments. A Pearson's correlation analysis was performed to corroborate relationships between specific parameters.


The effects of thermal treatment on tomato puree color are shown in Table 1. L* values decreased after any thermal treatment. The lowest L* reduction was obtained using high MW power/short time dose of processing. This reduction was only of 5.6% using 3,150 W/150 s compared to unheated samples. In this same trend, h° increased in samples thermally treated being higher in CP than in MWP treated under high power/short time, indicating a changing of color from red to orange. Lower h° is preferred as the best color properties in tomato.20 Results in this experiment showed that the use of MWP was able to keep the tomato puree color better than CP. The results agree with results obtained in orange juice21 and kiwi fruit puree10 treated by MW. The main red colored tomato pigment is trans lycopene and smaller amounts of cis-isomers (yellow colored pigment in tomato) and other carotenoids. In this case, Pearson correlation coefficient showed a positive correlation between the amount of lycopene and redness (a*) of treated samples (0.758). Thermal processing leads to isomerization of lycopene from trans to cis-form3 and since the redness of tomato depends on the level of trans-lycopene22 therefore, severe thermal treatment leads to decreasing of the redness.


This is an important quality attribute to determine the overall quality of processed tomato products which is influenced by the presence of pectin and inactivation of PME and PG after thermal treatment.23,24 From a rheological point of view, tomato puree can be considered as a weak gel23 and its viscosity is not stable and influenced by changing the degree of shear rate. The effect of MWP and CP on tomato puree viscosity (shown in low shear rate) is presented in Fig. 2. There was an increase (p<0.05) in the viscosity of the samples when pasteurized by both methods compared to unheated puree. The

MWP, in particular, high power combined with low time, provided the higher viscosity compared to CP and unheated samples. For low shear rates, the viscosity value ranged from 81.73 to 53.54 Pa.s for MWP puree compared to 43.85 Pa.s CP and 21.33 Pa.s for CP and unheated samples, respectively. This viscosity decreased for higher shear rates and reached 4 and 2.5 Pa.s in all treatments, at a shear rate of 100 s-1. Due to disruption of the samples treated cell wall during thermal treatment, the soluble pectin could be increased. In the current research and several other studies, an increase in viscosity of tomato products was found with increasing pectin content.25,26 On the other hand, different inactivation levels of PME and PG during pasteurization of puree as well as varietal characteristics and the maturity stage of fruits at processing have an influence on the viscosity.27 According to our results, the reduction of PME and PG activity by both thermal treatment methods lead to increased viscosity.

SSC, pH and TA

The SSC range between 4.73 and 5.27 °Brix, pH values 4.11 to 4.26 and TA had a mean of 0.37% in unheated and heated samples without significant differences (data not shown). The literature reported that temperature and treatment time had no effect on pH and °Brix of CP orange juice.28

The initial TPC in fresh tomato puree was 424 ChAE mg kg-1 (Table 2). The range from 268 to 523 mg kg-1 reported for different tomato juices.29 In this work, after any heat treatment the TPC was in the range between 430.6 and 441.2 ChAE mg kg-1 without significant difference between unheated or heated treatments. Similarly to our results it was reported a non-significant enhancement of TPC after CP at 90 °C for 30 or 60 s in

tomato juice30. Since POD is involved in the oxidative degradation of phenolic TPC31, inactivation of POD avoids degradation of TPC during thermal processing. Also the slight TPC enhancement could be attributed to the disruption of cell wall during heating, therefore making phenolics more accessible for extraction.5

The TAC was influenced by type of heating and decreased significantly (p<0.05) compared to unheated samples Table 2. As previously reported, TAC in MW treated tomatoes or tomato paste32 or watermelon juice33 is strongly decreased by heating. In the current work initial TAC in unheated samples was 725.2 mg AAE kg-1. The highest TAC degradation (around 28%) was found in the CP treated puree, whereas this level was only 6% in MWP samples treated by highest power/short time of processing. These results showed that at similar temperature (96 ± 2 °C) MWP maintained a better TAC than CP. In the same way, Stratakos et al.34 reported the TAC, in heated tomato juice was higher for MWP compared to the CP at 85 °C. TAC depends on the extract and the intensity of the heating applied to tomato samples.35 In our results, the highest power (1,900 to 3,150 W) combined with shorter duration (180 to 150 s) maintained the TAC better than low power (390 to 770 W) combined with higher duration (848 to 460 s). Arslan et al.36 found that MW drying at 700 W offered a lower TAC destruction than MW at 200 W. In summary, when comparing the efficacy of the MWP versus CP, advantage for keeping the TAC of tomato puree was found, in particular, using highest power and lowest time.

Vitamin C

The vitamin C amount in MWP and CP purees is presented in Table 2. Vitamin C content in fresh tomato is between 80 and 163 mg kg-1 FW 37, depending on the cultivar, the cultivation conditions and ripening stage.38 The vitamin C was degraded by 40% in CP, whilst in MWP puree was only 10% (highest power/short time) to 28% (low power/long time) showing that vitamin C of puree was maintained better by MWP than by CP. Similar results were obtained for strawberry puree showing that degradation of vitamin C in MWP (90 °C for 7 or 10 s) samples was from 4 to 22%, while achieving 62% by CP.9 After MW treatment of potato39, spinach40 and apricot41 the total AA content decreased with increasing processing time at a constant temperature. Decreasing of vitamin C occurs just after heating because this vitamin is very sensitive to heat.42


The lycopene levels ranged from 15.94 in unheated to 20.07 mg kg-1 in MWP puree (Table 2), being slightly but significantly enhanced by both heating methods in particular samples treated under high power MWP doses compared to CP samples. Since it is the main carotenoid responsible for the intense redness of the tomato, its level is considered as a quality index.44 Heating leads to isomerization of lycopene from trans-form to cis-form and a more efficient extraction from the matrix by breaking down cell walls, therefore making lycopene more accessible.45 Temperature kinetics play an important role in lycopene bio-accessibility as rapid heating of tomato puree can lead to higher accessibility compared to a slow temperature increase.46 In this experiment, we can add that even if obtaining the same final temperature of pasteurization, the kinetic of MWP power doses is also very important and the combination of highest power/short time of processing improved the lycopene content. On the contrary, other authors found that the lycopene content was stable in tomatoes under different thermal treatments.47,23

For the cells that were not disrupted during the puree preparation, such as tomato skin cells, a longer heating time or higher temperature may be needed to disrupt the cell walls sufficiently to release all the lycopene from cells.48 These authors showed that long-time/low temperature and short-time/high temperature can have the same effects on the tomato matrix. Also, the lycopene remains relatively stable during food processing, except at high temperature or long heating time.43

The P-carotene content was also affected by heating method, and increased after pasteurization compared to unheated samples (Table 2). The raw tomato had the lowest P-carotene content (6.76 mg kg-1). This value increased to reach 7.37 and 9.60 mg kg-1in CP and MWP, respectively. It has been reported that there is an enhanced bioavailability of carotenoids after heat treatment in tomato49 and pumpkin50 when compared to fresh sample. As explained with lycopene content, heat treatment might improve P-carotene bioavailability by breaking down of the cellulose structure of the plant cell walls.51

Enzyme activity

POD is responsible for enzymatic browning and can lead to reduction in nutritive quality, color, and flavor in many plant foods, being a common indicator of enzyme inactivation because of its high thermal stability.4 Both thermal treatments reduced the POD activity in tomato puree (Table 3). In comparison to CP, highest MWP power combined with short time of processing induced a higher decrease of POD activity. A similar POD inactivation has been reported at 90 °C for 30 or 60 s in apple juice.52

PME and PG are the most important enzymes affecting the processed tomato quality playing an important role in the pectin degradation in the primary cell wall and middle lamella.53 PME also leads the pectin chain to be susceptible to more pectin

degradation by PG reducing the tomato viscosity.54 Consequently PME and PG inactivation is needed to avoid quality losses.

In this experiment, MWP decreased the PME residual activity better than CP (Table 3). PME activity was significantly affected by the highest power/short time MWP treatment (12%). Similarly, in the current work, PME inactivation in orange juice was faster by MW heating than by CP.21,55 In most industrial uses the PME residual activity remaining below 10% guarantees the tomato quality and shelf-life.4

The PG is present in tomato as PG1 (thermo-stable form), inactivated at 90 °C, 5 min and PG2 (thermo-labile form), inactivated at 65 °C, 5 min.54,56 The PG activity decreased as a function of thermal treatment (Table 3). The major reduction (71%) was found in MWP at 3,150 W/150 s, whereas only 52 and 55%inactivation was reached at 390 W/848 s and CP, respectively. The high PG activity after all treatments could be attributed to the presence of PG1 and prolonged heating leads to complete inactivation of PG.57 Results in the present study indicated that MWP might improve PME and PG inactivation through high power and reduced processing time more than with CP. According to Pearson coefficient, there was a negative correlation between residual PG and PME enzyme activity and viscosity of treated smoothies (-0.895 and -0.876, respectively). This correlation showed that the viscosity strongly was influenced by the reduction in PG and PME enzyme activity4.

As reported in fruit purees and strawberry puree, the POD activity in tomato puree was better inactivated than PG and PME in a micro waved product.57 For this tomato puree, PG was the more thermo- resistant enzyme, and could be used as an indicator of pasteurization efficiency.


Physicochemical properties of tomato puree, especially color, were greatly influenced by heat treatments. MWP was able to preserve tomato puree redness, one of the major quality indicators, better than CP. Generally, MWP induced an enrichment of health-promoting compounds, leading to more retention of antioxidant capacity and vitamin C and enhancing lycopene content. PME and PG enzyme activities were highly decreased by MWP, in particular when high power/short time doses were used, resulting in a better viscosity. For all these reasons the semi-industrial continuous MW heating method studied, using high power combined with short processing time, could be recommended as an emergent pasteurization technique for maintaining quality of tomato puree.


This work was financially supported by MINECO-FEDER (AGL2013-48830-C2-1-R). REFERENCES

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Figure 1. Semi-industrial microwave oven, process diagram.

1.- Motor

2.- Magnetron

3.- Sample

4.- Fan

5.- Conveyor belt

6.- Control process

7.- Fiber optical temperature sensor

8.- Microwave chamber

5« d b

Ä Ä Ä v^l v^l &


Figure 2. Viscosity in unheated (control), conventional (CP) and microwave (MWP) pasteurized tomato puree. Different letters indicate significant differences among mean values (p<0.05).

Table 1. Color changes in unheated (control), conventional (CP) and microwave (MWP) pasteurized tomato puree

Treatments L* Hue angle

MWP Doses Untreated CP 39.60 ± 0.17a 33.00 ± 0.38f 44.94 ± 0.19d 48.44 ± 0.10a

£ o 390 W-848 s 510 W-805 s 35.36 ± 0.12e 35.80 ± 0.12de 48.70 ± 0.80a 47.45 ± 0.19ab

-J 770 W-460 s 35.84 ± 0.22de 47.03 ± 0.23abc

980W-340 s 36.38 ± 0.41cd 46.61 ± 0.50abc

Medium 1640 W-300 s 36.39 ± 0.37cd 46.18 ± 0.64abc

1700 W-230 s 36.82 ± 0.40bc 45.67 ± 0.86bc

High 1900 W-180 s 2700 W-160 s 3150 W-150 s 36.93 ± 0.14bc 37.08 ± 0.09bc 37.36 ± 0.25b 45.45 ± 0.27bc 45.12 ± 0.60bc 45.06 ± 0.19c

Values are mean ± standard error (n=3). Different letters in the same column indicate significant differences among mean values of different treatments (p< 0.05). "ns" means there are no significant differences. Low MWP: Microwave pasteurization at low power and long time. Medium MWP: Microwave pasteurization at medium power and medium time. High MWP: Microwave pasteurization at high power and short time.

Table 2. Total antioxidant capacity (TAC, mg AAE kg-1), total phenolic compound (TPC, ChAE mg kg-1), Vitamin C (mg kg-1), lycopene and P carotene (mg kg-1) in unheated (control), conventional (CP) and microwave pasteurized (MWP) tomato puree.

Treatment TAC TPC Vitamin C Lycopene ß-carotene

MWP Untreated 725.2 ± 0.17a 424.0 ± 0.47 ns 100.0 ± 0.02a 15.94 ± 0.86e 6.76 ± 0.90ns

Doses CP 519.4 ± 0.25g 430.6 ± 0.02ns 59.8 ± 0.02g 17.19 ± 0.88de 7.37 ± 0.06ns

390 W-848 s 559.1 ± 0.40f 430.0 ± 0.36ns 72.4 ± 0.06f 17.66 ± 0.07cd 9.08 ± 0.04ns

£ 510 W-805 s 559.5 ± 0.25f 430.7 ± 0.35ns 72.4 ± 0.02f 17.98 ± 0.22cd 9.11 ± 0.63ns

-J 770 W-460 s 560.8 ± 0.43f 430.7 ± 0.37ns 73.5 ± 0.06f 17.99 ± 0.15cd 9.08 ± 0.53ns

980W-340 s 578.8 ± 0.44e 431.7 ± 0.37ns 77.4 ± 0.01e 18.05 ± 0.12cd 9.49 ± 0.50ns

E = "■3 1640 W-300 s 615.9 ± 0.23d 432.6 ± 0.40ns 87.0 ± 0.08d 18.15 ± 0.21bcd 9.54 ± 0.21ns

s 1700 W-230 s 619.6 ± 0.37d 433.1 ± 0.63ns 88.1 ± 0.03cd 18.47 ± 0.07bcd 9.56 ± 0.10ns

1900 W-180 s 663.1 ± 0.20c 440.3 ± 0.33ns 89.0 ± 0.05bc 19.17 ± 0.36abc 9.57 ± 0.34ns

JS 2700 W-160 s 682.6 ± 0.38b 441.1 ± 0.41ns 89.5 ± 0.04b 19.60 ± 0.07ab 9.56 ± 0.12ns

S 3150 W-150 s 682.7 ± 0.47b 441.2 ± 0.39ns 89.8 ± 0.06b 20.07 ± 0.12a 9.60 ± 0.14ns

Values are mean ± standard error (n=3). Different letters in the same column indicate significant differences among mean values of different treatments (p<0.05). "ns" means there are no significant differences. Low MWP: Microwave pasteurization at low power and long time. Medium MWP: Microwave pasteurization at medium power and medium time. High MWP: Microwave pasteurization at high power and short time.

Table 3. Residual activity (%RA) of peroxidase (POD), pectin methylesterase (PME), and polygalacturonase (PG) in unheated (control), conventional (CP) and microwave (MWP) pasteurized tomato puree.

Treatments POD PME PG

MWP Doses Untreated CP 100.00 ± 0.00a 15.99 ± 0.55b 100.00 ± 0.00a 19.57 ± 0.10b 100.00 ± 0.00a 55.77 ± 0.44b

£ o 390 W-848 s 510 W-805 s 15.90 ± 0.61b 15.80 ± 0.49b 16.97 ± 0.15c 16.48 ± 0.10c 52.05 ± 0.29c 49.43 ± 0.37d

-J 770 W-460 s 15.32 ± 0.29b 16.29 ± 0.13c 48.33 ± 0.60de

980W-340 s 14.81± 0.33b 16.16 ± 0.05c 47.97 ± 0.27e

Medium 1640 W-300 s 14.03 ± 0.78c 16.13 ± 0.06d 32.06 ± 0.56f

1700 W-230 s 13.66 ± 0.94c 16.02 ± 0.03de 32.84 ± 0.50f

High 1900 W-180 s 2700 W-160 s 12.95 ± 0.64cd 12.26 ± 0.52d 15.02 ± 0.22de 14.99 ± 0.07de 32.80 ± 0.52f 30.51 ± 0.56g

3150 W-150 s 11.72 ± 0.82e 14.65 ± 0.10e 29.22 ± 0.52g

Values are mean ± standard error (n=3). Different letters in the same column indicate significant differences among mean values of different treatments (p < 0.05). Low MWP: Microwave pasteurization at low power and long time. Medium MWP: Microwave pasteurization at medium power and medium time. High MWP: Microwave pasteurization at high power and short time.