Evaluation of microbial inactivation and physicochemical properties of pressurized tomato juice during refrigerated storage

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LWT 41 (2008) 367–375 www.elsevier.com/locate/lwt

Evaluation of microbial inactivation and physicochemical properties of pressurized tomato juice during refrigerated storage Kuo-Chiang Hsua,, Fa-Jui Tanb, Hsin-Yi Chia a

Department of Health Diet and Restaurant Management, Chung Shan Medical University, No. 110, Sec. 1, Chien-Kuo N. Rd., Taichung 402, Taiwan, ROC Department of Animal Science, National Pingtung University of Science and Technology, No. 1, Sheuh Fu Rd, Neipu Hsiang, Pingtung 912, Taiwan, ROC

b

Received 26 January 2007; received in revised form 28 March 2007; accepted 31 March 2007

Abstract Effects of high pressure processing (300–500 MPa/25 1C/10 min) on microbial inactivation and processing qualities of tomato juices during refrigerated storage at 4 1C for 28 days were investigated to compare with those of conventionally thermal processing. Conventionally, thermal processing almost inactivated all the microorganisms and pectolytic enzymes and produced microbially and consistency stable tomato juices; however, they also reduced the color, extractable carotenoids and lycopene and vitamin C compared with fresh juice. During storage, all the pressure processing could improve the extractable carotenoids and lycopene contents compared with fresh juice, and they also retained more vitamin C contents than thermal processing. Although 300- and 400-MPa processing could retain a/b values of tomato juices as fresh juice during storage for 21 and 28 days, 500-MPa processing could improve the color of juices even after storage. Syneresis occurred in the 300- and 400-MPa processing juices by storing for 7 and 14 days; however, viscosity stable juice was produced by 500-MPa processing. Moreover, 400- and 500-MPa processing significantly inactivated microorganisms and the juices were microbially stable during storage. This study demonstrated that 500-MPa processing would be an alternative for conventionally thermal processing for tomato juice with improvement of some processing quality attributes. r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. Keywords: Tomato juice; Hydrostatic pressure; Lycopene; Color; Microorganism

1. Introduction The consumer demand for minimally processed products of high quality has increased remarkably last years. Preferences shift towards fresh, healthy and rich flavor ready-to-eat foods with enhanced shelf life. Besides microbial safety, important quality aspects of such tomato products are color, flavor, and consistency (Hayes, Smith, & Morris, 1998). In tomato products, an important reaction is the degradation of the red pigment lycopene during thermal process, resulting in changes of the color (Rodrigo, van Loey, & Hendrickx, 2007). Consistency of tomato products refers to their viscosity and the ability of their solid portion to remain in suspension throughout the Corresponding author. Tel.:+886 4 24730022X11836; fax: +886 4 23248188. E-mail address: [email protected] (K.-C. Hsu).

shelf-life of the product. The consistency of tomato products is strongly affected by the composition of the pectins. Controlling the breakdown or retention of the pectins, and the enzymes that lead to changes in the pectins, is thus of great importance during processing (Hayes et al., 1998). Two enzymes, pectin methylesterase (PME) and polygalacturonase (PG), are involved in the breakdown of pectins. The action of PME makes the pectin susceptible to further degradation by PG because this enzyme acts only on segments of the pectin chain that have been demethylated by PME. The degradation of the pectin chains reduces the viscosity of the juice. Two different processes are commonly used in the production of tomato products. In the hot-break process, tomatoes are rapidly heated to 90–95 1C immediately after homogenization. This process is believed to inactivate enzymes rapidly, particularly those involved in pectin degradation, and gives a product with high viscosity. In the cold-break process, the

0023-6438/$30.00 r 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2007.03.030

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homogenized tomatoes are heated only to around 60 1C. This is believed to have benefits in the production of products such as juice. The lower temperature reduces the amount of thermal abuse of the product, giving a greater retention of color and flavor components and reducing production of undesirable compounds. The lower temperature also does not entirely inactivate the enzymes PME and PG and allows these enzymes to break down of some of the pectins reducing the viscosity of the juice (Luh & Daouf, 1971). For microbial safety and extending shelf-life of tomato juices, conventional pasteurization or sterilization is required, however, concomitant quality losses in terms of flavor, color, sensory and nutritive qualities are also occurred (Goodman, Fawcett, & Barringer, 2002; Youssef & Rahman, 1982). Effect of thermal processing on the processing qualities of tomato juices was widely investigated (Sa´nchez-Moreno, Plaza, de Ancos, & Cano, 2006b; Sieso & Crouzet, 1977). The volatile components and vitamin C of canned tomato juice decreased by treated at 100 1C for 10 min (Sieso & Crouzet, 1977; Youssef & Rahman, 1982). The color of tomato juice degraded more rapidly with increasing temperature; therefore, one of the advantages of cold break over hot break is that the final product had more natural color (Goodman et al., 2002; Sa´nchez-Moreno et al., 2006b). However, heating causes an increase in the overall antioxidant potential of the tomato juice coincide with the positive effect of the temperature on the extractability of lycopene (Anese, Manzocco, Nicoli, & Lerici, 1999; Anese, Falcone, Fogliano, Nicoli, & Massini, 2002; Sa´nchez-Moreno et al., 2006b). Besides, lycopene in tomato is relative resistant to degradation, whereas other antioxidants (ascorbic acid, tocopherol and b-carotene) decrease as a function of thermal processing (Abushita, Daood, & Biacs, 2000). The consumers demand safe, fresh and minimally processed foods, therefore, a non-thermal food processing such as hydrostatic pressure has developed (Knorr, 1993; Popper & Knorr, 1990). Hydrostatic pressure processing is a technology with the objective to process and/or preserve foods by inactivation of vegetative microorganisms (Hoover, Metrick, Papineau, Farkas, & Knorr, 1989) and quality-related enzymes (Weemaes, Ludikhuyze, Van de Broeck, & Hendrickx, 1998). Due to the ability to maintain the quality attributes of the fresh material because mainly non-covalent bonds are affected by pressure, high pressure processing can be an alternative for heat treatment in the context of food preservation (Cheftel, 1992). Some researchers have demonstrated that high pressure processing improved viscosity and color properties in comparison with their conventional heat-processed counterparts (Porretta, Birzi, Ghizzoni, & Vicini, 1995). PG in tomatobased products could be totally inactivated at some pressure/temperature combinations: 550 MPa/20 1C (Fachin et al., 2003); 500 MPa/60 1C (Crelier, Robert, & Juillerat, 1999) and 800 MPa/25 1C (Shook, Shellhammer, & Schwartz, 2001). Tomato PME, a heat-labile enzyme at

ambient pressure, was dramatically stabilized against thermal denaturation at pressures above atmosphere and up to 500–600 MPa, and was completely inactivated at 800 MPa/70 1C over 20 min (Crelier, Robert, Claude, & Juillerat, 2001). Relatively mild hydrostatic pressure treatments (500 MPa/25 1C/3 min) resulted in a stable refrigerated tomato juice product (Porretta et al., 1995), as vegetative microorganisms were inactivated and outgrowth of remaining Bacillus spores was prevented by the low pH. A hydrostatic pressure treatment (700 MPa/90 1C/30 s) reduced Bacillus stearothermophilus spore contamination level in inoculated meatballs in tomato puree with at least 4.5 log units (Krebbers et al., 2003). Surprisingly, few researchers have reported that the effect of high-pressure treatments on microbial inactivation and processing qualities of tomato juices during storage in comparison with those of commercial traditional pasteurized tomato juices. According to our previous study, hydrostatic pressure treatments (300–500 MPa) at ambient temperature (25 1C) could be useful in processing tomato juice in considering the inactivation of the enzymes and maintenance or improvement of viscosity, color, extractable carotenoids and radical-scavenging capacity. Therefore, the main objective of this research was to investigate the effects of high-pressure treatments at 25 1C on the processing qualities of tomato juices during storage at 4 1C within 4 weeks. 2. Materials and methods 2.1. Tomatoes and juice preparation Red daydream tomatoes were purchased from Yenshui Farmer’s Association in Tainan, Taiwan. Daydream tomatoes were stored at 7 1C before treatments within 7 days. Washed tomatoes (daydream, 200 g), equilibrated at room temperature, were homogenized twice by a Warringblender with the high speed for 10 s and sieving (0.8 mm holes) to remove pieces of skin and any seeds. The tomato juice (adjusting pH to 4.5 with 10 mol/L NaOH) was obtained as a control in this study and used for the following processing immediately. 2.2. Conventionally thermal processing Tomato juice (150 g) was poured into double polyethylene bags (250  360 mm, thickness: 50 mm, Medisch Labo Service, Menen, Belgium) and vacuum-sealed followed by hot-break (92 1C; 2 min) and subsequently pasteurized at 98 1C for 15 min. Afterward, the tomato juice was immediately cooled in ice water for 2 min. 2.3. Hydrostatic pressure processing Tomato juice was filled into polyethylene pouches (10 cm  13 cm, capacity 200 mL), which were heat-sealed after removal of the air and subjected to 300–500 MPa at

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25 1C for 10 min. Afterward, the tomato juice was immediately cooled in ice water for 2 min. 2.4. High-pressure equipment A high-pressure apparatus (CIP UNIT, Mitsubishi Heavy Industries Ltd., Japan) with an oil-pressure generator and a compressing vessel, in which the internal portion (diameter: 50 mm; height: 120 mm) was a flatbottomed cylindrical shape, was employed. The vessel temperatures during pressure treatments were controlled at 25 1C by a circulator. 2.5. Refrigerated storage of tomato juice Fifty milliliter processed tomato juice were poured into a 100 mL sterilized transparent glass vial wrapped with aluminum foil. Each vial was placed in a refrigerated container, and the temperature was controlled at 4 1C with a storage period of 28 days. 2.6. Microbiological analysis Total viable count (cfu/mL) was determined by using a Plate Count Agar (Difco, Kansas City, USA) after incubation at 25 1C for 48 h. Enterobacteria was determined with Violet Red Bile Glucose Agar (Difco, Kansas City, USA) after incubation at 25 1C for 24 h. Lactic acid bacteria was determined with Rogosa SL Agar (Difco, Kansas City, USA) after incubation at 25 1C for 72 h. Yeasts and molds were determined with Malt Extract Agar (Difco, Kansas City, USA) acidified at pH 4.0 after incubation at 25 1C for 72 h. 2.7. Color Hunter L, a, and b of tomato juice were measured by a HunterLab colorimeter (Color Meter ZE-2000, Nippon Denshoku Co., Japan). The red-yellow ratio (a/b) was reported to indicate the redness of tomato juice (Min & Zhang, 2003). 2.8. Carotenoids and lycopene (Lin & Chen, 2003, 2005) 2.8.1. HPLC analysis of carotenoids A 8 g juice sample was mixed with 40 mL of ethanol– hexane (4:3, v/v) and 0.2 g magnesium carbonate. The solution was shaken in a shaker at 140 rev./min for 30 min, after which the upper layer was collected in a flask. The lower layer was further extracted with 32 ml ethanol– hexane (4:3, v/v) and shaken for 30 min. Again, the upper layer was collected in the same flask. The lower layer was repeatedly extracted with 15 mL hexane and shaken for 20 min, followed by addition of 5 mL hexane and the solution was homogenized by a polytron (PT-3000, KINEMATICA AG, Switzerland) at 12,000 rpm for 5 min. The mixture was filtered through Whatman No.1,

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and the filtrates were combined and poured into the same flask. Then, 150 mL distilled water and 100 mL 10 g/100 g NaCl solution were added to the filtrate for partition, and the upper phase was also collected. The lower layer was again extracted with 20 mL hexane. All the filtrates were pooled and evaporated to dryness under vacuum. The residue was dissolved in 1 mL methylene chloride and filtered through a 0.2 mm membrane filter for HPLC [Model L-5000 LC equipped with a Model L-4000 spectrophotometer and a Model D-2000 chromato-integrator (Hitachi Ltd., Japan)] analysis with a YMC C30 column (250  4.6 mm E.D, 5 mm particle; Tokyo, Japan). The injection volume was 20 mL. A gradient mobile phase of 1-butanol/acetonitrile (30:70, v/v) (A) and methylene chloride (B) was used: 99 g/100 g A and 1 g/100 g B initially, increased to 4 g/100 g B in 20 min, 10 g/100 g B in 50 min and returned to 1 g/100 g B in 55 min. The detection wavelength was 476 nm and the flow rate was 2.0 mL/min. 2.8.2. Identification and quantification of carotenoids The identification of all-trans carotenoids was carried out by comparing the retention times and absorption spectra with added standards. In addition, the cis isomers of carotenoids were identified based on absorption spectrum characteristics and Q ratios as described in the literature (Lin & Chen, 2003). Because of an absence of suitable internal standard, six concentrations of 1, 2, 4, 8, 10 and 20 mg/mL were used to prepare the standard curve of all-trans-lutein. Likewise, six concentrations of 3, 6, 18, 36, 48 and 60 mg/mL were used to prepare the standard curve of all-trans-b-carotene. For all-trans-lycopene, two standard curves were prepared since a great different in concentration was found between all-trans and cis form of lycopene in tomato juice. Six concentrations, 7, 70, 140, 280, 560 and 700 mg/mL were prepared for one curve, whereas 2, 4, 8, 10, 20 and 40 mg/mL were prepared for the other. Each cis isomer of carotenoids was quantified based on the standard curve of all-trans carotenoids because of similar in extinction coefficient (Lee & Chen, 2002). Duplicate analyzes were performed and the mean value was determined. Total carotenoids content was expressed as microgram of lutein and its cis isomers plus microgram of lycopene and its cis isomers plus microgram of bcarotene and its cis isomers. Results were calculated as microgram of the corresponding carotenoids per gram of tomato juice. 2.9. Viscosity The viscosity of tomato juice was studied using a Brookfield viscometer, springle #4 at 10 rpm, 25 1C and only the 10th round readings were recorded (mPa s)(Oke, Ahn, Schofield, & Paliyath, 2005). The analyzes were performed in duplicate for each processing condition, and each sample was measured in duplicate, resulting in 5 data points per condition.

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2.10. Vitamin C (Sa´nchez-Moreno, Plaza, de Ancos, & Cano, 2003) Ascorbic acid and total vitamin C (ascorbic acid+dehydroascorbic acid) were determined by HPLC. The procedure employed to determine total vitamin C was the reducing of dehydroascorbic acid to ascorbic acid, using DL-dithiothreitol as reductant reagent. Forty milliliter of tomato juice was homogenized with 25 mL of extraction solution (3 g/100 g meta-phosphoric acid+8 g/100 g acetic acid). The resulting mixture was centrifuged, filtered and adjusted up to 75 mL with distilled water. Samples were filtered through a 0.45-mm membrane filter (NY non-ste, Lida Manufacturing Co., Kenosha, Wis., USA) and duplicates of 20 mL for each extract were analyzed by HPLC. Results were expressed as milligram of ascorbic acid per 100 ml of tomato juice. An aliquot (0.5 mL) of the mixture was taken to react with 1.5 mL of a solution 20 mg/mL of DL-dithiothreitol for 2 h at room temperature and darkness. During this time, the reduction of dehydroascorbic acid to ascorbic acid took place. Samples were filtered through a 0.45-mm membrane filter and duplicates of 20 mL, for each extract, were analyzed by HPLC. Results were expressed as milligram of total vitamin C per 100 mL of tomato juice. Chromatographic analysis was performed on a model L-5000 LC equipped with a model L-4000 spectrophotometer for monitoring at 245 nm and a model D-2000 chromato-integrator (Hitachi Ltd., Katsuda, Japan). Separations were achieved on a reverse-phase Lichrospher 100RP-18 column (4.0  250 mm, 5-mm particle size, Merck & Co., Inc, Rahway, NJ). The solvent system used was an isocratic gradient of a 0.01 g/100 g solution of H2SO4, adjusted to pH 2.5–2.6. The flow rate was fixed at 1.0 mL/min. Calibration curves were built with a minimum of four concentration levels of ascorbic acid standard; the straight-line equations and their coefficients of correlation were calculated.

2.11. Statistical analysis Results were given as mean7standard deviation of six independent determinations. One-way analysis of variance (ANOVA) was used to compare the means. Differences were considered significant at Po0.05. All statistical analyzes were performed with Statgraphics Plus 2.1 (Statistical Graphics Corporation, Inc., Rockville, MD, USA). 3. Results and discussion 3.1. Microbiological analysis Effects of pressure processing on the microbiological properties of tomato juices during refrigerated storage are shown in Table 1. The control proved to be naturally contaminated by the microorganisms normally occurring in tomato juice, at the same levels as those found in this product. The thermal processing and pressure processing at 500 MPa both resulted in most efficient inactivation of all the microorganisms to a level below the detection limit in this study. These two products appeared stable during storage at 4 1C for at least 28 days. Total viable counts of tomato juices treated by 300 and 400 MPa decreased 0.9 and 1.5 log units, respectively; and after storage, they slightly elevated to 4.6 and 3.1 log cfu/mL. However, the 300-MPa pressurization could efficiently inactivate yeasts and molds, the both microorganisms still germinated or grew after storage. Some researchers have proved that pressure processing at 500 MPa/room temperature for 3 min already yielded a microbially stable tomato juice product regardless of pH values of 4.0–5.0 (Porretta et al., 1995). Hydrostatic pressure pasteurization at 700 MPa/ 20 1C for 2 min resulted in inactivation of natural flora to a level below the detection limit in tomato puree even after storage at 4 1C for 8 weeks, however, that up to 500 MPa/ 20 1C for 2 min decreased only 0.7 log units of natural flora

Table 1 Microbiological analysis of tomato juices treated by pressure processing after storage at 25 1C for 28 days Treatment

Storage time (days)

Log cfu/mL Total viable count

Entero-bacteria

Latic acid bacteria

Yeasts

Molds

Control

0

4.1

2.1

4.2

3.7

3.6

Thermal processing

0 28

o1.0 o1.0

N.D. N.D.

N.D. N.D.

N.D. N.D.

N.D. N.D.

300 MPa

0 28

3.2 4.6

o1.0 o1.0

o1.0 o1.0

o1.0 1.6

o1.0 1.6

400 MPa

0 28

2.6 3.1

N.D. N.D.

N.D. N.D.

N.D. N.D.

N.D. N.D.

500 MPa

0 28

o1.0 o1.0

N.D. N.D.

N.D. N.D.

N.D. N.D.

N.D. N.D.

N.D.: not determined.

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Table 2 The red-yellow ratio (a/b) of tomato juices treated by pressure processing as a function of storage time at 25 1C Storage time (days)

0 7 14 21 28

Control

3.6270.01c

Thermal processing

Pressure processing (MPa)

3.2170.02a 3.2070.02a 3.1870.03a 3.1970.02a 3.1770.03a

300

400

500

3.6670.03c 3.6370.01c 3.6270.03bc 3.5970.02bc 3.5570.05b

3.7270.01d 3.7070.02d 3.6670.04cd 3.6570.02c 3.6570.03c

3.8470.02e 3.7970.03e 3.82770.03e 3.7870.04e 3.8070.04e

Mean values7standard deviation (n ¼ 6) with different letters indicate significant differences (Po0.05).

(Krebbers et al., 2003). We suggested that the different effects of hydrostatic pressure processing on inactivation of microorganisms were mainly attributed by the various tomato-based products. In this study, pressure processing at 400 MPa and above is sufficient to produce microbially stable tomato juices for refrigerated storage at least for 28 days. 3.2. Color Effect of pressure processing on the red–yellow ratio, indicating the redness of tomato juices, during refrigerated storage is shown in Table 2. An a/b ratio of 1.90 or greater represents a first quality product in terms of color and an a/b ratio of less than 1.80 means that the tomato products may be unacceptable for inclusion in products where a bright red color is desired (Hayes et al., 1998). The a/b value of the control was 3.62 and appreciated more than that of conventionally thermal processing tomato juice of 3.21. The result in a low a/b value represented tomato juice of brown color due to the breakdown of lycopene and formation of Maillard reaction products by the intensive heat treatment (Krebbers et al., 2003; Shi & Le Maguer, 2000). High pressure processing beyond 400 MPa was appreciated more than control and the conventionally thermal processing on the colors of tomato juices. And a/b values of tomato juice increased up to 3.84 with pressure levels elevated to 500 MPa, and those were maintained after storing for 28 days. Results agree that an increase in the red color (a value; data not shown) of high-pressure treated tomato juice compared to thermal treatments, attributed to the better homogenization and brightening of the red color (Krebbers et al., 2003; Porretta et al., 1995). Although the 400-MPa processing increased the a/b value to 3.72, the values declined and were not significantly different from that of control after 21-day storage. Moreover, the 300-MPa processing only maintained the color for storage up to 21 days, and the a/b value was slightly lower than that of control probably due to the isomerization of lycopene. Isomerization converts all-trans isomers to cis-isomers, due to additional energy input, and results in an unstable, energy-rich situation (Shi & Le Maguer, 2000). Some all-trans-lycopene might transform to

Table 3 The viscosity of tomato juices treated by pressure processing as a function of storage time at 25 1C Storage time (days)

0 7 14 21 28

Control

1875b

Thermal processing

1624a 1608a 1615a 1597a 1601a

Pressure processing (MPa) 300

400

500

1856b N.D. N.D. N.D. N.D.

2020d 1976c N.D. N.D. N.D.

2255e 2244e 2238e 2233e 2225e

Mean values (n ¼ 6) with different letters indicate significant differences (Po0.05).

cis-isomers during high pressure processing (Qiu, Jiang, Wang, & Gao, 2006). 3.3. Viscosity The consistency of tomato juices expressed in viscosity and level of syneresis (serum separation) is important for the quality of tomato juices. The viscosity of fresh tomato juice (control) was 1875 MPa s and shown in Table 3. The juice by conventionally thermal processing had a significantly lower viscosity (1624 MPa s) than control and that by 300-MPa processing (1856 MPa s). Moreover, the viscosity value of the juice by 500-MPa processing increased by about 20% and was the highest in this study, and that by 400-MPa processing also increased by 8%. During storage at 4 1C for up to 28 days, the viscosity values of the juices by conventionally thermal and 500-MPa processing remained almost same; however, those by 400-MPa processing significantly decreased with elongation time (Pp0.05) and the ‘‘syneresis’’ phenomenon occurred in tomato juices by 300- and 400-MPa processing after storage for 7 and 14 days, respectively. Although a very high break temperature (107 1C for 45 s) appears to increase consistency by pectolytic enzyme inactivation, more soluble pectin leached from cell walls and cell structures more highly disrupted, prolonged heating at high temperature can lead to a pectin denaturation and a reduction in consistency (Fito, Clemente, & Sanz, 1983; Goodman et al., 2002; Hayes et al.,

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1998). High-pressure treatments at ambient temperature resulted in a more jelly-like, homogenous structure of the tomato puree due to protein-tissue coagulation and compacting compared to thermal treatments (Krebbers et al., 2003; Porretta et al., 1995; Verlent, Hendrickx, Rovere, Moldenaers, & van Loey, 2006). In addition, highpressure treatments could improve the viscosity attributed by PG inactivation and enhancement of water binding by pectins (Fernandez Garcia, Butz, & Tauscher, 2001; Krebbers et al., 2003), and the increase in viscosity due to pressure is attributed to an increase in the linearity of cell walls and volumes of particles due to rupture of the cellular envelope (Hayes et al., 1998). Some researchers showed the results that the highest loss in consistency of the tomato homogenate after combined pressure–temperature treatment was found at 300 MPa at all temperatures tested (30–70 1C) compared with those at the other pressure levels from 100 to 500 MPa (Verlent, Hendrickx, Rovere, Moldenaers, & van Loey, 2006). They observed that tomato PME was very active in presence of tomato PG at pressure up to 300 MPa. PME creates a good substrate for PG, which also has a sufficient high activity at 300 MPa. In our previous study, PG activities were reduced by 90% after 500-MPa processing at 25 1C, however, those decreased only by 13–23% after 300- and 400-MPa processing. Moreover, PME were also activated up to 48% after pressure processing between 300 and 500 MPa. The results in the loss of viscosity of tomato juices during storage showed that 300 and 400 MPa could not inactivate both PME and PG which degraded pectins (Thakur, Singh, & Nelson, 1996). However, the viscosity of tomato juices by conventionally thermal and 500-MPa processing remained of higher values compared with control even after storage for 28 days attributed to the complete inactivation of PG. Rovere et al.(1997) also showed a decrease in syneresis, positively correlated with pressure, temperature and treatment time. The results showed that 500 MPa would be the pressure level to produce a tomato juice product with improved and stable consistency.

3.4. Carotenoids and lycopene The total carotenoids and lycopene contents of control are 212.8 and 145.6 mg/g, respectively (Table 4), which are higher than those of tomato (Tau-Tai Lan T93) juices for about 20% probably due to different cultivars (Lin & Chen, 2003). In addition to the all-trans-lycopene, di-cislycopene (I), (II), 5-cis-, 9-cis-, 13-cis- and 15-cis-lycopene were determined in this study (data not shown). After conventionally thermal processing, the total carotenoids and lycopene contents of tomato juices insignificantly decreased by about 1%. Some researchers have demonstrated that heating tomato juice for 7 min at 90 1C and 100 1C resulted in a 1.1% and 1.7% decrease in lycopene content, respectively (Miki & Akatsu, 1970); at higher temperature the lycopene content decreased even more, and losses of 17.1% were observed at 130 1C. However, the conventionally thermal processing tomato juice in this study was sealed in polyethylene pouches without air and then subjected to pasteurization at 98 1C for 15 min, the oxidative degradation of lycopene should be inhibited in this situation. After all the high pressure processing in this study, total carotenoids and lycopene contents increased up to 62% and 60%, respectively. It has been reported that high-pressure treatment (500 MPa/20 1C/2 min) increased the lycopene content of tomato puree compared with the raw puree (Krebbers et al., 2003). In addition, some researchers have shown increases in extractable carotenoids and lycopene as a result of high-pressure treatment (400 MPa/25 1C/15 min) of tomato puree (Sa´nchezMoreno, Plaza, de Ancos, & Cano, 2006a). The enhancing effect on extraction of carotenoids and lycopene by pressure processing is attributed by affecting the membranes in vegetable cells (Shi & Le Maguer, 2000), whereas carotenoids are tightly bound to macromolecules, in particular to protein and membrane lipids, and high pressure processing is known to affect macromolecular structures such as proteins and polymer carbohydrates (Butz & Tauscher, 2002).

Table 4 Total carotenoids and lycopene contents (mg/g) of tomato juices treated by pressure processing as a function of storage time at 25 1C Storage time (days)

Control

Thermal processing

Pressure processing (MPa) 300

400

500

Carotenoids

0 7 14 21 28

212.876.5c

210.774.2c 185.673.8b 176.674.2a 174.772.9a 173.874.5a

280.974.5d 278.673.6d 277.472.5d 277.876.3d 275.874.0d

336.272.8e 335.873.3e 335.574.6e 335.672.8e 334.772.1e

344.773.4f 345.872.3f 343.671.0f 344.572.5f 343.271.8f

Lycopene

0 7 14 21 28

145.673.5c

144.372.8c 134.673.1b 128.772.4a 127.372.8a 126.773.0a

186.474.2d 188.471.2d 182.571.9d 184.773.5d 183.874.3d

214.073.3e 212.672.8e 210.776.2e 213.672.8e 211.773.2e

233.073.6f 233.574.8f 229.873.9f 231.874.2f 230.773.8f

Values are means7standard deviation, n ¼ 6. Different letters for carotenoids or lycopene during storage period indicate significant differences (Po0.05).

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Oxidation is the major cause of carotenoids (including lycopene) loss, and it depends on the carotenoids involved. Oxidation is stimulated by light, heat, metals, enzymes, and peroxides, and it is inhibited by antioxidants, such as tocopherols and ascorbic acid (Sa´nchez-Moreno et al., 2003). During storage for the first 14 days, total carotenoids and lycopene of the conventionally thermal processing tomato juice rapidly degraded by 16% and 12% compared with control (Table 4), and those degraded insignificantly during 14–28-day storage. Moreover, the contents of all-trans-lycopene decreased by 51% for the fist 14 days (data not shown) due to its coplanarity structure being very reactive to oxidation (Lin & Chen, 2005). On the other hand, those of pressure processing tomato juice slightly but insignificantly decreased. The total carotenoids and lycopene of the conventionally thermal processing tomato juice by storing at low temperature and in the dark further degraded probably due to the presence of residual oxygen in the juice (Lin & Chen, 2005). In this study, oxidation could be probably mediated by pressure processing because of the inactivation of peroxidase and the maintenance of vitamin C content. Previous studies have shown that peroxidase in orange juice was inactivated under 300 MPa/20 1C, and high pressure may lead to a release of other antioxidants found in tomato (some carotenoids, ascorbic acid, phenolic components) (Cano, Herna´ndez, & de Ancos, 1997; Qiu et al., 2006). Lin and Chen (2005) suggested that, in addition to isomerization, oxidative degradation is a major factor causing lycopene and other carotenoids loss during storage of thermal processing tomato juice. In this study, we demonstrated that pressure processing beyond 300 MPa could produce a carotenoids-stable tomato juice product with higher contents in lycopene compared with fresh tomato juice. 3.5. Vitamin C The ascorbic acid and total vitamin C contents in control are 18.6 and 21.0 mg/100 ml, respectively (Table 5). After

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thermal processing, the ascorbic acid and total vitamin C contents decreased to 38% and 42% compared with control. All the pressure processing tomato juices also showed a decrease in ascorbic acid and total vitamin C both of about 30%, with no statistically significant differences among them. Although Sa´nchez-Monreno et al. (2006a) revealed that ascorbic acid and total vitamin C contents of tomato puree applied to pasteurization at 90 1C for 1 min declined only by 34% and 26%, the temperature and time used as thermal processing in this study were higher and longer. This is in accordance with the large body of evidence supporting vitamin C destruction in tomato products with thermal treatments (Hayes et al., 1998; Nicoli, Anese, Parpinel, Franceschi, & Lerici, 1997). In addition, we also found depletion in the vitamin C content in the pressure processing tomato juice, which could be due to the long time treatment (10 min) and mild temperature (25 1C). Several studies in orange juice and tomato puree showed that the depletion of vitamin C after pressure treatment is dependent mainly on temperature intensity and treating time (Sa´nchez-Moreno et al., 2003, 2006b). The results showed that pressure processing could not preserve vitamin C in tomato juice, and the pressure levels between 300 and 500 MPa showed the same effect on degradation of vitamin C. During storage for 28 days, tomato juices by each pressure processing showed that ascorbic acid and total vitamin C contents almost remained constant (Table 5). Furthermore, the 500-MPa processing juice showed significantly greater values in ascorbic acid content during storage from 7 to 21 days and in total vitamin C content during storage at 7 days than the 300-MPa processing juice. However, at the end of the storage for 28 days, there were no significant differences of ascorbic acid and total vitamin C contents between all the pressure processing tomato juices in this study. Both ascorbic acid and total vitamin C of thermal processing tomato juice underwent further degradation by about 18% during the first 14-day storage probably due to the presence of residual oxygen in

Table 5 Vitamin C contents (mg/100 ml) of tomato juices treated by pressure processing as a function of storage time at 25 1C Storage time (days)

Control

Thermal processing

Pressure processing (MPa) 300

400

500

Ascorbic acid

0 7 14 21 28

18.670.8f

7.170.4c 6.570.3b 5.870.4a 5.670.5a 5.570.2a

13.670.7de 13.370.6d 13.270.4d 13.370.6d 13.170.5d

13.870.4de 13.770.5de 13.770.5de 13.470.3d 13.470.3d

14.270.2e 14.470.2e 14.070.2e 13.970.1e 13.770.4de

Total vitamin C

0 7 14 21 28

21.070.7f

8.870.6c 8.070.2b 7.270.5a 7.170.3a 7.070.4a

14.770.6de 14.470.4d 14.370.4d 14.570.3d 14.270.3d

14.970.2de 14.670.1de 14.570.2d 14.270.4d 14.170.5d

15.170.2e 15.170.1e 14.970.3de 14.870.4de 14.770.5de

Values are means7standard deviation, n ¼ 6. Different letters for ascorbic acid or total vitamin C during storage period indicate significant differences (Po0.05).

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the juice (Lin & Chen, 2005), and that insignificantly declined with the further storage period. Nienaber and Shellhammer (2001) studied the ascorbic acid content in orange juice treated at 800 MPa and 25 1C for 1 min. They stored the juice at different temperatures (4, 15, 26 and 37 1C) and observed, as expected, that the highest ascorbic acid retention occurred in orange juices stored at 4 1C and progressively diminished with the increase in the storage temperature. During further storage, ascorbic acid gradually decreased in all samples. The authors concluded that over 80% of ascorbic acid was retained after 3 months at 4 1C or after 2 months at 15 1C. In our case, even though there were some losses just after pressure processing for tomato juices compared to control, at the end of the shorter period of 28 days at 4 1C there were no losses in ascorbic acid and vitamin C. 4. Conclusion Based on microbial inactivation, color, viscosity, extraction of carotenoids and lycopene, vitamin C of tomato juice, high pressure processing can be an alternative for conventional pasteurization (thermal processing). In this study, thermal processing produced a microbial stable product but with losses in all the quality parameters during storage for 28 days compared with fresh juice (control). High pressure processing beyond 300 MPa could improve or retain color and extractable carotenoids and lycopene of tomato juices after storage compared with control. Syneresis occurred in the tomato juice processed by 300- and 400-MPa during storage for 7–14 days due to that PME and PG still being active. Although vitamin C degraded by all processing, high pressure processing could retain relatively higher vitamin C contents compared with thermal processing. We concluded that 500-MPa processing would be an alternative for thermal processing to produce microbial stable tomato juice products with improvement or preservation of quality attributes. Acknowledgment This study was financially supported by National Science Council, ROC, No. NSC 94-2313-B-040-005. References Abushita, A. A., Daood, H. G., & Biacs, P. A. (2000). Change in carotenoids and antioxidant vitamins in tomato as a function of varietal and technological factors. Journal of Agricultural and Food Chemistry, 48(6), 2075–2081. Anese, M., Falcone, P., Fogliano, V., Nicoli, M. C., & Massini, R. (2002). Effect of equivalent thermal treatments on the color and the antioxidant activity of tomato puree. Journal of Food Science, 67(9), 3442–3446. Anese, M., Manzocco, L., Nicoli, M. C., & Lerici, C. R. (1999). Antioxidant properties of tomato juice as affected by heating. Journal of the Science of Food and Agriculture, 79(5), 750–754. Butz, P., & Tauscher, B. (2002). Emerging technologies: chemical aspects. Food Research International, 35(3), 279–284.

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