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Comparison of thermal, ultraviolet-c, and high pressure treatments on quality parameters of watermelon juice
Food Chemistry 126 (2011) 254–260
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Comparison of thermal, ultraviolet-c, and high pressure treatments on quality parameters of watermelon juice Chao Zhang a, Bernhard Trierweiler b, Wu Li a, Peter Butz b, Yong Xu a, Corinna E. Rüfer b, Yue Ma a, Xiaoyan Zhao a,⇑ a b
Vegetable Research Center, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100097, China Max Rubner-Institute, Federal Research Institute of Nutrition and Food, Karlsruhe 76131, Germany
a r t i c l e
i n f o
Article history: Received 10 August 2010 Received in revised form 27 September 2010 Accepted 2 November 2010
Keywords: Watermelon juice Ultraviolet-c High pressure Browning Lycopene Dynamic viscosity
a b s t r a c t The effect of thermal, ultraviolet-c, and high pressure treatments on colour, browning degree, dynamic viscosity, and lycopene content of watermelon juice was evaluated based on its pectin methylesterase residual level. Each treatment had a different impact on parameters studied. Ultraviolet-c treatments were rapid and effective to inactivate the pectin methylesterase of the watermelon juice compared to the thermal and high pressure treatments in the same time and temperature. High pressure treatments at 600 MPa kept the colour of the treated watermelon juice close to an untreated one, and that at 600–900 MPa held the browning degree and dynamic viscosity of the treated watermelon juice comparable to an untreated one. Moreover, the high pressure treatment had a slight impact on the alltrans-lycopene, total cis-lycopene, and total lycopene concentration of the watermelon juice compared to the other treatments. In summary, the high pressure treatment showed the lowest changes in colour, dynamic viscosity, browning degree, and lycopene content of the treated watermelon juice amongst the three treatments. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Fresh watermelon juice is more and more favoured worldwide (Aguiló-Aguayo, Soliva-Fortuny, & Martín-Belloso, 2010). However, traditional thermal treatments lead to colour and dynamic viscosity changes of watermelon juice, which are mainly catalysed by its intrinsic polyphenol oxidase and pectin methylesterase (PME), respectively (Rodrigo et al., 2006). Lycopene loss has also been reported during the processing and storage of watermelon (Perkins-Veazie & Collins, 2004). Non-thermal technologies which could avoid colour and dynamic viscosity changes and lycopene loss of watermelon juice are an option for the processing of the watermelon juice. Ultraviolet-c (UV-C) and high pressure treatments, as nonthermal technologies, have been studied widely in processing of fruit juice. UV-C treatments, an electromagnetic spectrum from 200 to 280 nm, have been used to keep qualities of fresh-cut watermelon (Artés-Hernández, Robles, Gómez, Tomás-Callejas, & Artés, 2010), and inactivate PME in tomato (Barka, Kalantari,
⇑ Corresponding author. Address: Vegetable Research Center, Beijing Academy of Agriculture and Forestry Sciences, Banjing, Haidian District, Beijing 100097, China. Tel.: +86 10 51503053; fax: +86 10 88446286. E-mail address: [email protected] (X. Zhao). 0308-8146/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2010.11.013
Makhlouf, & Arul, 2000), strawberry (Pombo, Dotto, Martínez, & Civello, 2009), and apple (Manzocco, Dri, & Quart, 2009). High pressure treatments have been used not only to inactivate intrinsic enzymes of fruit juice (Fang, Jiang, & Zhang, 2008; Guiavarc’h, Segovia, Hendrickx, & Van Loey, 2005; Lacroix, Fliss, & Makhlouf, 2005), but also to keep their fresh texture and nutritional qualities (Doblado, Frias, & Vidal-Valverde, 2007; Laboissiere et al., 2007; Lambert, Demazeau, Largeteau, & Bouvier, 1999; Polydera, Stoforos, & Taoukis, 2005a, 2005b). Moreover, efficiencies of the UV-C and high pressure treatment had been compared with the traditional thermal treatments, respectively, in several studies (Barka et al., 2000; Doblado et al., 2007; Pan, Vicente, Martínez, Chaves, & Civello, 2004; Polydera et al., 2005a, 2005b). However, up to date comparison of the UV-C and high pressure treatment on quality parameters of the watermelon juice has not been carried out. Therefore, the aim of this study was to compare the effect of the thermal, UV-C, and high pressure treatments on quality parameters of the watermelon juice. Specifically, the colour, browning degree, dynamic viscosity, and lycopene content of the watermelon juice subjected to the thermal, UV-C, or high pressure treatment were generally compared. Our results would not only give a general evaluation of the thermal, UV-C, and high pressure treatments on quality parameters of the watermelon juice, but also give an instruction for productions of the watermelon juice.
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2. Materials and methods
2.3. Determination of PME activity
2.1. Preparation of watermelon juice
PME activity of the sample was measured at pH 7.5 and 23 °C according to the method proposed by (Kimball, 1991), which was based on carboxyl group titration. An aliquot of 0.20 ml of the sample was mixed with 20 ml of 1% pectin (Sigma, USA) containing 0.1 M NaCl and incubated at 23 °C. An aliquot of 0.025 ml of 0.1 N NaOH was added to the solution when its pH was adjusted to pH 7.5 by 0.1 N NaOH (TitroLine Easy, Schott, Mainz, Germany). The time for the solution returning to pH 7.5 was measured. PME activity (A) expressed in pectin methylesterase units was calculated by Eq. (1).
Watermelons (Citrullus lanatus Thunb.) were obtained from Real market located in Karlsruhe, which were imported from Brazil in September 2009 (Chiquita, Cincinnati, USA). Five watermelons were washed, peeled, mixed, and ground at a high speed for 1 min by a Rotor-Blender GT 800 (Spangenberg International BV, Mijdrecht, The Netherlands).
2.2. Thermal, UV-C, and high pressure treatments
A¼ The nomenclature of the thermal, UV-C, and high pressure treatment is listed in Table 1. The control was untreated watermelon juice. Specifically, the thermal treatment was carried out at an atmospheric pressure. Aliquots of 150 ml of the watermelon juice were packaged in a polyvinylchloride film and subjected to a water bath at 60 °C for 5, 20, 40, and 60 min with gentle stirring, respectively. After the thermal treatment, each juice was cooled down in an ice-water bath for further analysis. The UV-C treatment was carried out by a UV reactor (UVivatecÒ Lab, Bayer Technology Services GmbH (BTS), Leverkusen, Germany) at 23 °C. The essential part of the reactor is a helically wound TeflonÒ tubing wrapped around a quartz glass tube containing an UV lamp (a 9 W UV-C low-pressure mercury lamp at 254 nm). The watermelon juice passed through the coiled tubing (volume: 24 ml) at flow rate of 8.4 l/h, and circulated 3, 6, 9, and 12 cycles for UV-C dose of 2421, 4843, 7264, and 9685 J/l (Franz, Specht, Cho, Graef, & Stahl, 2009). After each treatment, the juice was cooled down quickly in an ice-water bath for further analysis. High pressure treatments were done in a hydraulic press U101 (Polish Academy of Sciences, Warsaw, Poland). Aliquots of 10 ml of the watermelon juice were pressurised at 300, 600, and 900 MPa for 5, 20, 40, and 60 min at 60 °C, respectively. The pressure transmitting medium was a mixture 7:3 of petroleum ether (boiling point 80–100 °C) and hydraulic oil. The temperature was controlled by a thermostat (Polystat, Huber, Germany) coupled to the sample chamber. After each treatment, the watermelon juice was cooled down quickly in an ice-water bath for further analysis.
C NaOH V NaOH 1 ¼ 400 V sample t V sample t
ð1Þ
where A is PME activity, CNaOH is the concentration of NaOH (=0.1 mol/l), VNaOH is the volume of NaOH used (=0.025 ml), Vsample is the volume of sample used (=0.20 ml), and t is the time needed for pH to return to 7.5 after the addition of NaOH (min).The PME residual level was calculated by Eq. (2).
PME residual level ¼
Asample 100% Acontrol
ð2Þ
where Asample is the PME activity of the sample after treatments, Acontrol is the PME activity of the control. 2.4. Determination of colour Colour assessment of the sample was conducted randomly in the reflectance mode for six times at 23 °C (Chromameter CR300, Minolta, Japan). Colour L*, a*, and b* value of the sample was measured and the total colour difference (DE) was calculated by Eq. (3).
DE ¼ ½ðL L0 Þ2 þ ða a0 Þ2 þ ðb b0 Þ2 1=2
ð3Þ
where DE is the total colour difference between a sample and the control, L is a lightness of a sample, L0 is a lightness of the control, a is a redness of a sample, a0 is a redness of a sample, b is a yellowness of the control, and b0 is a yellowness of the control. 2.5. Determination of browning degree
Table 1 Nomenclature of treatments. Treatments
Nomenclature
Time (min)
Pressure (MPa)
Temperature (°C)
UV-C dose (J/l)
Thermal treatment
T-05 T-20 T-40 T-60
5 20 40 60
0.1 0.1 0.1 0.1
60 60 60 60
0 0 0 0
High pressure treatment
H-05-300 H-20-300 H-40-300 H-60-300 H-05-600 H-20-600 H-40-600 H-60-600 H-05-900 H-20-900 H-40-900 H-60-900
5 20 40 60 5 20 40 60 5 20 40 60
300 300 300 300 600 600 600 600 900 900 900 900
60 60 60 60 60 60 60 60 60 60 60 60
0 0 0 0 0 0 0 0 0 0 0 0
U-03 U-06 U-09 U-12
3 6 9 12
0.1 0.1 0.1 0.1
23 23 23 23
2421 4843 7264 9685
UV-C treatment
Browning degree of the sample was evaluated using a spectrophotometric method described by Roig, Bello, Rivera, and Kennedy (1999). The sample was centrifuged with a Sigma 2K15 centrifuge (Sigma, Osterode, Germany) at 8603g at 4 °C for 20 min, and then passed through a 0.45 lm cellulose nitrate membrane. A browning degree value was determined by measuring the absorbance at 420 nm (Novaspec II, Pharmacia Biotech GmbH, Freiburg, Germany) at 23 °C with a 1 cm path length cell. 2.6. Determination of lycopene content Lycopene extraction and HPLC/DAD analysis of the sample were followed the previously described method (Stracke et al., 2009). Quantification of the sample was calculated by external calibration at k = 472 nm for all-trans-lycopene and total cis-isomers and at k = 450 nm for b-carotene and b-cryptoxanthin. Calibration curves for the carotenoids were constructed in the range of 0.0025– 25 mmol/l, in which the linearity of the response was given. The recovery for all carotenoids was greater than 95%. The CV of the method was below 5% (intra-assay). A typical chromatogram at k = 450 nm is shown in Fig. 1. The total cis-lycopene content is the sum of *-cis-lycopene (not identified), 15-cis-lycopene, 13-cislycopene, 9-cis-lycopene, and 5-cis-lycopene.
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Fig. 1. Representative HPLC chromatogram of the watermelon juice at k = 450 nm.
2.7. Determination of dynamic viscosity The dynamic viscosity of the sample was determined using a dynamic rheometer (Rheometer AR-550, TA Instruments, New Castle, Delaware, USA) with a conical end concentric cylinder (stator radius = 15.00 mm, rotor radius = 14.00 mm, immersed height = 42.00 mm, gap = 5920 lm). An aliquot of 19.6 ml of the sample was applied at each measurement at 25 ± 0.1 °C controlled by a circulating water system. Dynamic viscosities were the average of the flow curves at 1.2–5.0 min when the shear rate was constant. 2.8. Statistical analysis All data are expressed as the mean value ± standard deviation (n P 3). All statistical analyses are carried out with the Super ANOVA (Version 1.11, Abacus Concepts Inc., Berkeley, CA). One-way ANOVA and multiple comparisons (Fisher’s least-significant difference test) are used. The results are considered significant difference when the P value is lower than 0.05. 3. Results and discussion
of the PME residual level, were selected to compare the effect of the treatments on quality parameters of the watermelon juice. The PME residual level of T-05 and U-03 was statistically similar at the level of (100 ± 5)%, that of T-20, U-06, H-05-300, and H05-600 was statistically similar at the level of (75 ± 5)%, that of T-60, U-09, H-60-600, and H-20-900 was statistically similar at the level of (50 ± 5)%, and that of U-12, H-40-900, and H-60-900 was statistically similar at the level of (35 ± 5)%. Consequently, the following comparison was based on the four PME residual levels. The UV-C and high pressure treatments needed less time to reach the same PME residual level compared to the thermal treatment. For instants, U-06, H-05-300 and H-05-600 needed 5–6 min to reach the PME residual level of (75 ± 5)%, whilst T-20 needed 20 min. Being similar to our result, the high pressure treatments need less time to inactivate the PME of the white grape compared to the thermal treatments (Guiavarc’h et al., 2005). Moreover, a higher energy density of UV lamp or a higher flow rate of the UV-C treatment would further reduce the time to reach the same PME residual level. Hence, time saving was an advantage of the UV-C treatment.
3.1. Effect of the treatments on PME of the watermelon juice
3.2. Effect of the treatments on colour of the watermelon juice
The effects of the thermal, UV-C, and high pressure treatments on quality parameters of the watermelon juice are difficult to compare because those treatments could not be standardised to a same criterion, such as temperature, time, pressure, or transmitted energy. The quality parameters of the watermelon juice are mainly related to the catalysis of its intrinsic enzyme, such as PME and polyphenol oxidase (Aguiló-Aguayo et al., 2010). In a previous study, the effect of the thermal and high pressure treatments on quality parameters of orange juice was compared based on the PME residual level (Polydera et al., 2005a, 2005b). Moreover, the polyphenol oxidase activity was not detected in our watermelon juice. Consequently, the PME catalysis was the main factor influencing quality parameters of the watermelon juice, being similar to a recently result (Aguiló-Aguayo et al., 2010). Therefore, the PME residual level was selected as a criterion for the comparison of the thermal, UV-C, and high pressure treatments. The effect of the thermal, UV-C, and high pressure treatments on the PME residual level of the watermelon juice is shown in Fig. 2. Four levels, (100 ± 5)%, (75 ± 5)%, (50 ± 5)%, and (35 ± 5)%
The effect of the treatments on colour of the watermelon juice is shown in Table 2. All treatments leaded to a significant colour change because DE after each treatment was higher than 3.0. The ranking of DE was UV-C treatment > thermal treatment > high pressure treatment in each PME residual level. For instance, DE of U-06 was 6.76, being higher than that of T-20, H-05-300, and H-05-600 at the PME residual level of (50 ± 5)%. Furthermore, each treatment had a different influence on the colour of the watermelon juice. DE of the watermelon juice subjected to the thermal treatment increased with the treatment time, being similar to the previous results of strawberry (Pan et al., 2004) and pepper (Vicente et al., 2005). DE of the watermelon juice subjected to the UV-C treatment increased when its dose increased, being consistent to the results of tomato (Lingegowdaru, 2007). Being different to the thermal and UV-C treatments, a higher pressure (or a shorter time) of the high pressure treatment reduced the DE in each PME residual level, which was proved by comparisons of H05-300 and H-05-600, and H-40-900 and H-60-900, respectively. The similar results were also proved by the high pressure
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a
a
PME residual level (%)
100 b
b 75
b
b c
c
c c d
50
d
d
25
U-12
U-09
U-06
U-03
H-60-900
H-40-900
H-05-900
H-20-900
H-60-600
H-40-600
H-20-600
H-05-600
H-60-300
H-40-300
H-20-300
H-05-300
T-60
T-20 T-40
T-05
0
Fig. 2. Effect of treatments on the PME residual level. The PME residual level is calculated by Eq. (2). Data are means ± standard deviation (n P 3). The a, b, c, and d represent (100 ± 5)%, (75 ± 5)%, (50 ± 5)%, and (35 ± 5)% of the PME residual level of the watermelon juice, respectively, after each treatment.
Table 2 Effect of treatments on colour of watermelon juice. PME residual level
Treatments
L*A
a* a,B
b*
DE
a
a
(100 ± 5)%
Control T-05 U-03
62.54 ± 0.24 57.43 ± 0.32b 60.76 ± 0.37c
6.36 ± 0.08 8.38 ± 0.38b 7.71 ± 0.15c
15.7 ± 0.35 17.87 ± 0.61b 10.10 ± 0.25c
0 5.87 6.02
(75 ± 5)%
T-20 U-06 H-05-300 H-05-600
56.91 ± 0.19b 63.91 ± 0.30d 58.36 ± 0.21b 60.24 ± 0.30c
6.88 ± 0.05a 6.65 ± 0.15a 8.52 ± 0.16b 6.84 ± 0.14a
17.67 ± 0.27b 9.10 ± 0.24d 14.58 ± 0.28e 11.28 ± 0.13f
5.95 6.76 5.97 4.99
(50 ± 5)%
T-60 U-09 H-60-600 H-20-900
55.59 ± 0.08e 64.14 ± 0.23d 61.79 ± 0.09c 62.29 ± 0.34a
6.86 ± 0.09a 5.77 ± 0.15d 6.33 ± 0.12a 5.66 ± 0.10d
16.05 ± 0.57b 8.39 ± 0.20g 9.75 ± 0.21cd 10.58 ± 0.10c
6.94 7.51 5.99 5.17
(35 ± 5)%
U-12 H-40-900 H-60-900
66.06 ± 0.07f 62.76 ± 0.41a 60.75 ± 0.06c
5.30 ± 0.06d 5.71 ± 0.06d 5.62 ± 0.04d
8.80 ± 0.17d 8.33 ± 0.24g 7.89 ± 0.18g
7.84 7.40 8.01
A L*, a*, and b* represent the lightness, redness, and yellowness of the sample, respectively. DE shows the total colour difference between the sample and the control which is calculated by Eq. (3). B Data are means ± standard deviation (n P 3). Means with different superscript letters in the same row represent a significant difference (P < 0.05).
treatments of the strawberry juice (Rodrigo, van Loey, & Hendrickx, 2007). However, two different results were reported that the colour of tomato puree (Rodrigo et al., 2007) and avocado puree (Lopez-Malo, Palou, Barbosa-Canovas, Welti-Chanes, & Swanson, 1998) keep constants after high pressure treatments. These differences maybe resulted from their different lycopene ingredients or solution pH. Colour a* is related to redness of the watermelon juice, which is its main colour. The thermal treatments of T-20 and T-60 kept its a* similar to that of the control in the PME residual level of (75 ± 5)% and (50 ± 5)%. Colour a* of the watermelon juice subjected to the UV-C treatment decreased when the dose of UV-C increased, being different to the previous results that UV-C treatments do not influence the redness of pepper (Vicente et al., 2005). This difference may result from the texture and microstructure varieties of watermelon juice and pepper, because these varieties cause changes in the nature and the extent of internally scattered light and the distribution of surface reflectance (Oey, Lille, Van Loey, & Hendrickx, 2008). Interestingly, colour a* of the watermelon juice subjected to the high pressure treatment at 600 MPa was similar to that of the control, whilst that subjected to the high pressure treatment at 300 and 900 MPa was statistically different to that of the control.
Therefore, the high pressure treatment at 600 MPa was effective to keep the colour of the treated watermelon juice as the control compared to the thermal and UV-C treatments in the same PME residual level. 3.3. Effect of the treatments on browning degree of the watermelon juice The effect of the treatments on the browning degree of the watermelon juice is shown in Fig. 3. The browning degree of the watermelon juice subjected to the high pressure treatment was lower than that subjected to the thermal and UV-C treatments in each PME residual level. Moreover, the high pressure treatments of H-60-600, H-20-900, H-40-900, and H-60-900 significantly decreased the browning degree of the treated watermelon juice. Consequently, the high pressure treatment with the pressure higher than 600 MPa was effective to avoid the browning of the treated watermelon juice, being consistent to the results that the high pressure treatments at 600 and 900 MPa reduce the browning rate of the grape juice (Castellari, Matricardi, Arfelli, Carpi, & Galassi, 2000). Each treatment had a different effect on the browning degree of the watermelon juice. The browning degree of the watermelon
C. Zhang et al. / Food Chemistry 126 (2011) 254–260
Treatments
258
H-60-900 H-40-900 U-12
a
H-20-900 H-60-600 U-09 T-60
a a
a
c
c e b
H-05-600 H-05-300 U-06 T-20
c b b
U-03 T-05
b b
Control 0.00
d
0.05
0.10
0.15
0.20
Browning degree Fig. 3. Effect of treatments on the browning degree of the watermelon juice Data are means ± standard deviation (n P 3). Means with different letter represent a significant difference (P < 0.05).
juice increased with an increase of time and UV-C dose of the thermal and UV-C treatments, respectively. The browning degree of the watermelon juice subjected to the high pressure treatment, decreased when the pressure increased. Similarly, minimal browning is found when potato and apple are subjected to the high pressure treatment at 800 MPa (Gomes & Ledward, 1996). 3.4. Effect of the treatments on the dynamic viscosity of the watermelon juice The effect of the treatments on the dynamic viscosity of the watermelon juice is shown in Table 3. The dynamic viscosity of the watermelon juice subjected to the thermal and high pressure treatment was statistically similar to that of the control at the PME residual level of (50 ± 5)% and (35 ± 5)%, whilst that subjected to the UV-C treatment was significantly different. Hence, the thermal and high pressure treatments were effective to keep the dynamic viscosity of the treated watermelon juice as the control
compared to the UV-C treatment in the same PME residual level. This phenomenon may result from the fact that the UV-C treatments not only inactivate PME, but also inactivate the polygalacturonase, cellulase, xylanase, b-D-galactosidase, and protease which mainly respond to the dynamic viscosity of the juice (Barka et al., 2000). The dynamic viscosity of the watermelon juice subjected to the high pressure treatment at 900 MPa was similar to that of the control, whilst that at 300 MPa was statistically different. Hence, the high pressure treatment at 900 MPa helped to stabilise the dynamic viscosity of the watermelon juice, being similar to the results about the effect of the high pressure treatment on the orange juice (Polydera et al., 2005a, 2005b). Being different, the dynamic viscosity of mango pulp increases after the high pressure treatment at 100 or 200 MPa (20 °C for 15 or 30 min), whilst a reduction in dynamic viscosity is observed after the high pressure treatment at 300 and 400 MPa (20 °C for 15 or 30 min) (Ahmed, Ramaswamy, & Hiremath, 2005).
Table 3 Effect of the treatments on the dynamic viscosity and lycopene concentration of the watermelon juice.
A B C
PME residual level
Treatments
Dynamic viscosity (mPa S)
All-trans-lycopene concentration (nmol/l)
Total cis-lycopene concentration (nmol/l)A
Total lycopene concentration (nmol/l)B
(100 ± 5)%
Control T-05 U-03
1.88 ± 0.20a,C 2.63 ± 0.22c 1.87 ± 0.19a
27.8 ± 3.1a 25.2 ± 1.9a 25.5 ± 4.1a
4.81 ± 0.78a 5.41 ± 1.50a 4.52 ± 0.15a
32.6 ± 2.6a 30.6 ± 2.9a 30.0 ± 4.2a
(75 ± 5)%
T-20 U-06 H-05-300 H-05-600
2.22 ± 0.37b 2.04 ± 0.34a 2.34 ± 0.09b 2.18 ± 0.27b
22.9 ± 1.8b 21.4 ± 3.2b 25.1 ± 2.4a 24.8 ± 0.6a
5.18 ± 1.90a 4.27 ± 0.26a 5.13 ± 1.12a 4.83 ± 0.15a
28.1 ± 3.2b 25.7 ± 3.3b 30.3 ± 3.1a 29.7 ± 0.7ab
(50 ± 5)%
T-60 U-09 H-60-600 H-20-900
1.96 ± 0.27a 2.19 ± 0.54b 1.91 ± 0.04a 2.01 ± 0.12a
16.3 ± 4.1c 16.6 ± 3.6c 20.5 ± 3.6b 19.8 ± 2.3b
4.76 ± 1.46a 3.66 ± 0.16b 3.27 ± 0.42b 4.00 ± 0.98b
21.0 ± 5.1c 20.2 ± 3.7c 23.7 ± 0.3c 22.8 ± 2.0c
(35 ± 5)%
U-12 H-40-900 H-60-900
2.21 ± 0.45b 1.97 ± 0.16a 2.03 ± 0.59a
12.6 ± 2.6d 18.6 ± 0.1c 17.0 ± 3.9c
3.56 ± 0.54b 3.86 ± 0.16b 3.70 ± 0.93b
16.1 ± 3.1d 22.4 ± 0.2c 21.2 ± 1.5c
Total cis-lycopene content is the sum of *-cis-lycopene (not identified), 15-cis-lycopene, 13-cis-lycopene, 9-cis-lycopene, and 5-cis-lycopene. Total lycopene is the sum of the all-trans-lycopene and cis-lycopene. Data are means ± standard deviation (n P 3). Means with different superscript letters in the same row represent a significant difference (P < 0.05).
C. Zhang et al. / Food Chemistry 126 (2011) 254–260
3.5. Effect of the treatments on the lycopene content of the watermelon juice The effects of the treatments on all-trans-lycopene, total cis-lycopene, and total lycopene concentration of the watermelon juice are listed in Table 3. The all-trans-lycopene concentration of the treated watermelon juice decreased when the PME residual level decreased. Moreover, the all-trans-lycopene concentration of the watermelon juice subjected to the high pressure treatment was significantly higher than that subjected to the thermal and UV-C treatments in the PME residual levels of (75 ± 5)%, (50 ± 5)%, and (35 ± 5)%. One might speculate that the high pressure treatments increase the extraction yield of the lycopene as described for tomato puree (FernandezGarcia, Butz, & Tauscher, 2001). cis-Lycopene has been proved to be more bioavailable than trans-lycopene in vitro and in vivo (Boileau, Merchen, Wasson, Atkinson, & Erdman, 1999; Edwards et al., 2003). The total cis-lycopene concentration of the watermelon juice after each treatment was similar to that of the control in the PME residual level of (100 ± 5)% and (75 ± 5)%, whilst it was statistically different to that of the control in the PME residual level of (50 ± 5)%, and (35 ± 5)% except for T-60. Hence, the total cis-lycopene concentration of the treated watermelon juice decreased with a loss of the PME residual level. The effect of the treatments on the total lycopene concentration of the watermelon juice was similar to that on the all-translycopene concentration in each PME residual level. Remarkably, the total lycopene concentration of the treated watermelon juice was statistically similar after the high pressure treatment at 300 MPa (H-05-300) and 600 MPa (H-05-600). Being different to our results, Qiu, Jiang, Wang, and Gao (2006) reported that the total lycopene content is maximum at 400 MPa and shows a significant loss at 500 and 600 MPa. On the other hand, a few results showed that the total lycopene content is in linear correlation with the colour a* of the watermelon juice (Lewinsohn et al., 2005; Perkins-Veazie & Collins, 2004; Tadmor et al., 2005). However, effect of the high pressure treatment on the total lycopene content and colour a* of the watermelon juice was different. Hence, the colour a* of the treated watermelon juice was speculated to be the combination of the browning and lycopene colour. Take together, the high pressure treatment was effective to hold the all-trans-lycopene, total cis-lycopene, and total lycopene concentration of the treated watermelon juice as the control compared to the thermal and UV-C treatments. 4. Conclusion The effect of the UV-C and high pressure treatments on colour, browning degree, dynamic viscosity, and lycopene content of the watermelon juice was compared with the thermal treatment based on its PME residual level. The UV-C treatment was a rapid and effective method to reach a certain PME residual level. High pressure treatments at 600 MPa kept the colour of the treated watermelon juice close to an untreated one, and that at 600–900 MPa held the browning degree and dynamic viscosity of the watermelon juice comparable to an untreated one. Moreover, the high pressure treatment was the only treatment which had only a slight impact on the total lycopene concentration. In summary, the high pressure treatment showed the lowest changes in colour, dynamic viscosity, browning degree, and lycopene content of the treated watermelon juice amongst the three treatments.
259
Acknowledgement The authors are grateful to the Modern Agricultural Industry R&D System (Cucumis melon & watermelon) by Ministry of Agriculture for financial support of this work. References Aguiló-Aguayo, I., Soliva-Fortuny, R., & Martín-Belloso, O. (2010). Colour and viscosity of watermelon juice treated by high-intensity pulsed electric fields or heat. Innovative Food Science & Emerging Technologies, 11, 299–305. Ahmed, J., Ramaswamy, H. S., & Hiremath, N. (2005). The effect of high pressure treatment on rheological characteristics and colour of mango pulp. International Journal of Food Science and Technology, 40, 885–895. Artés-Hernández, F., Robles, A. P., Gómez, A. P., Tomás-Callejas, A., & Artés, F. (2010). Low UV-C illumination for keeping overall quality of fresh-cut watermelon. Postharvest Biology and Technology, 55, 114–120. Barka, E. A., Kalantari, S., Makhlouf, J., & Arul, J. (2000). 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J., Perkins-Veazie, P., et al. (2003). Consumption of watermelon juice increases plasma concentrations of lycopene and ß-carotene in humans. Journal of Nutrition, 133, 1043–1050. Fang, L., Jiang, B., & Zhang, T. (2008). Effect of combined high pressure and thermal treatment on kiwifruit peroxidase. Food Chemistry, 109, 802–807. FernandezGarcia, A., Butz, P., & Tauscher, B. (2001). Effects of high-pressure processing on carotenoid extractability, antioxidant activity, glucose diffusion, and water binding of tomato puree (Lycopersicon esculentum Mill.). Journal of Food Science, 66, 1033–1038. Franz, C. M. A. P., Specht, I., Cho, G.-S., Graef, V., & Stahl, M. R. (2009). UV-Cinactivation of microorganisms in naturally cloudy apple juice using novel inactivation equipment based on Dean vortex technology. Food Control, 20, 1103–1107. Gomes, M. R. A., & Ledward, D. A. (1996). Effect of high-pressure treatment on the activity of some polyphenol oxidases. Food Chemistry, 56, 1–5. 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Food Chemistry, 67, 7–16. Lewinsohn, E., Sitrit, Y., Bar, E., Azulay, Y., Ibdah, M., Meir, A., et al. (2005). Not just colours-carotenoid degradation as a link between pigmentation and aroma in tomato and watermelon fruit. Trends in Food Science & Technology, 16, 407–415. Lingegowdaru, J. (2007). Effect of UV-C hormesis on quality attributes of tomatoes during post treatment handling. In Deparment of Bioresource Engineering, Master Thesis, Macdonald: McGill University, p. 100. Lopez-Malo, A., Palou, E., Barbosa-Canovas, G. V., Welti-Chanes, J., & Swanson, B. G. (1998). Polyphenol oxidase activity and colour changes during storage of high hydrostatic pressure treated avocado puree. Food Research International, 31, 549–556. Manzocco, L., Dri, A., & Quart, B. (2009). Inactivation of pectic lyases by light exposure in model systems and fresh-cut apple. Innovative Food Science & Emerging Technologies, 10, 500–505. Oey, I., Lille, M., Van Loey, A., & Hendrickx, M. (2008). Effect of high-pressure processing on colour, texture and flavour of fruit- and vegetable-based food products: A review. Trends in Food Science & Technology, 19, 320–328. Pan, J., Vicente, R. A., Martínez, A. G., Chaves, R. A., & Civello, M. P. (2004). Combined use of UV-C irradiation and heat treatment to improve postharvest life of strawberry fruit. Journal of the Science of Food and Agriculture, 84, 1831–1838. Perkins-Veazie, P., & Collins, J. K. (2004). Flesh quality and lycopene stability of fresh-cut watermelon. Postharvest Biology and Technology, 31, 159–166.
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Polydera, A. C., Stoforos, N. G., & Taoukis, P. S. (2005a). Effect of high hydrostatic pressure treatment on post processing antioxidant activity of fresh Navel orange juice. Food Chemistry, 91, 495–503. Polydera, A. C., Stoforos, N. G., & Taoukis, P. S. (2005b). Quality degradation kinetics of pasteurised and high pressure processed fresh Navel orange juice: Nutritional parameters and shelf life. Innovative Food Science & Emerging Technologies, 6, 1–9. Pombo, A. M., Dotto, C. M., Martínez, A. G., & Civello, M. P. (2009). UV-C irradiation delays strawberry fruit softening and modifies the expression of genes involved in cell wall degradation. Postharvest Biology and Technology, 51, 141–148. Qiu, W. F., Jiang, H. H., Wang, H. F., & Gao, Y. L. (2006). Effect of high hydrostatic pressure on lycopene stability. Food Chemistry, 97, 516–523. Rodrigo, D., Cortes, C., Clynen, E., Schoofs, L., Van Loey, A., & Hendrickx, M. (2006). Thermal and high-pressure stability of purified polygalacturonase and pectin methylesterase from four different tomato processing varieties. Food Research International, 39, 440–448.
Rodrigo, D., van Loey, A., & Hendrickx, M. (2007). Combined thermal and high pressure colour degradation of tomato puree and strawberry juice. Journal of Food Engineering, 79, 553–560. Roig, M. G., Bello, J. F., Rivera, Z. S., & Kennedy, J. F. (1999). Studies on the occurrence of non-enzymatic browning during storage of citrus juice. Food Research International, 32, 609–619. Stracke, A. B., Rufer, E. C., Bub, A., Briviba, K., Seifert, S., & Kunz, C. (2009). Bioavailability and nutritional effects of carotenoids from organically and conventionally produced carrots in healthy men. British Journal of Nutrition, 101, 1664–1672. Tadmor, Y., King, S., Levi, A., Davis, A., Meir, A., Wasserman, B., et al. (2005). Comparative fruit colouration in watermelon and tomato. Food Research International, 38, 837–841. Vicente, R. A., Pineda, C., Lemoine, L., Civello, M. P., Martinez, A. G., & Chaves, R. A. (2005). UV-C treatments reduce decay, retain quality and alleviate chilling injury in pepper. Postharvest Biology and Technology, 35, 69–78.
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Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Comparison of thermal, ultraviolet-c, and high pressure treatments on quality parameters of watermelon juice Chao Zhang a, Bernhard Trierweiler b, Wu Li a, Peter Butz b, Yong Xu a, Corinna E. Rüfer b, Yue Ma a, Xiaoyan Zhao a,⇑ a b
Vegetable Research Center, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100097, China Max Rubner-Institute, Federal Research Institute of Nutrition and Food, Karlsruhe 76131, Germany
a r t i c l e
i n f o
Article history: Received 10 August 2010 Received in revised form 27 September 2010 Accepted 2 November 2010
Keywords: Watermelon juice Ultraviolet-c High pressure Browning Lycopene Dynamic viscosity
a b s t r a c t The effect of thermal, ultraviolet-c, and high pressure treatments on colour, browning degree, dynamic viscosity, and lycopene content of watermelon juice was evaluated based on its pectin methylesterase residual level. Each treatment had a different impact on parameters studied. Ultraviolet-c treatments were rapid and effective to inactivate the pectin methylesterase of the watermelon juice compared to the thermal and high pressure treatments in the same time and temperature. High pressure treatments at 600 MPa kept the colour of the treated watermelon juice close to an untreated one, and that at 600–900 MPa held the browning degree and dynamic viscosity of the treated watermelon juice comparable to an untreated one. Moreover, the high pressure treatment had a slight impact on the alltrans-lycopene, total cis-lycopene, and total lycopene concentration of the watermelon juice compared to the other treatments. In summary, the high pressure treatment showed the lowest changes in colour, dynamic viscosity, browning degree, and lycopene content of the treated watermelon juice amongst the three treatments. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Fresh watermelon juice is more and more favoured worldwide (Aguiló-Aguayo, Soliva-Fortuny, & Martín-Belloso, 2010). However, traditional thermal treatments lead to colour and dynamic viscosity changes of watermelon juice, which are mainly catalysed by its intrinsic polyphenol oxidase and pectin methylesterase (PME), respectively (Rodrigo et al., 2006). Lycopene loss has also been reported during the processing and storage of watermelon (Perkins-Veazie & Collins, 2004). Non-thermal technologies which could avoid colour and dynamic viscosity changes and lycopene loss of watermelon juice are an option for the processing of the watermelon juice. Ultraviolet-c (UV-C) and high pressure treatments, as nonthermal technologies, have been studied widely in processing of fruit juice. UV-C treatments, an electromagnetic spectrum from 200 to 280 nm, have been used to keep qualities of fresh-cut watermelon (Artés-Hernández, Robles, Gómez, Tomás-Callejas, & Artés, 2010), and inactivate PME in tomato (Barka, Kalantari,
⇑ Corresponding author. Address: Vegetable Research Center, Beijing Academy of Agriculture and Forestry Sciences, Banjing, Haidian District, Beijing 100097, China. Tel.: +86 10 51503053; fax: +86 10 88446286. E-mail address: [email protected] (X. Zhao). 0308-8146/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2010.11.013
Makhlouf, & Arul, 2000), strawberry (Pombo, Dotto, Martínez, & Civello, 2009), and apple (Manzocco, Dri, & Quart, 2009). High pressure treatments have been used not only to inactivate intrinsic enzymes of fruit juice (Fang, Jiang, & Zhang, 2008; Guiavarc’h, Segovia, Hendrickx, & Van Loey, 2005; Lacroix, Fliss, & Makhlouf, 2005), but also to keep their fresh texture and nutritional qualities (Doblado, Frias, & Vidal-Valverde, 2007; Laboissiere et al., 2007; Lambert, Demazeau, Largeteau, & Bouvier, 1999; Polydera, Stoforos, & Taoukis, 2005a, 2005b). Moreover, efficiencies of the UV-C and high pressure treatment had been compared with the traditional thermal treatments, respectively, in several studies (Barka et al., 2000; Doblado et al., 2007; Pan, Vicente, Martínez, Chaves, & Civello, 2004; Polydera et al., 2005a, 2005b). However, up to date comparison of the UV-C and high pressure treatment on quality parameters of the watermelon juice has not been carried out. Therefore, the aim of this study was to compare the effect of the thermal, UV-C, and high pressure treatments on quality parameters of the watermelon juice. Specifically, the colour, browning degree, dynamic viscosity, and lycopene content of the watermelon juice subjected to the thermal, UV-C, or high pressure treatment were generally compared. Our results would not only give a general evaluation of the thermal, UV-C, and high pressure treatments on quality parameters of the watermelon juice, but also give an instruction for productions of the watermelon juice.
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2. Materials and methods
2.3. Determination of PME activity
2.1. Preparation of watermelon juice
PME activity of the sample was measured at pH 7.5 and 23 °C according to the method proposed by (Kimball, 1991), which was based on carboxyl group titration. An aliquot of 0.20 ml of the sample was mixed with 20 ml of 1% pectin (Sigma, USA) containing 0.1 M NaCl and incubated at 23 °C. An aliquot of 0.025 ml of 0.1 N NaOH was added to the solution when its pH was adjusted to pH 7.5 by 0.1 N NaOH (TitroLine Easy, Schott, Mainz, Germany). The time for the solution returning to pH 7.5 was measured. PME activity (A) expressed in pectin methylesterase units was calculated by Eq. (1).
Watermelons (Citrullus lanatus Thunb.) were obtained from Real market located in Karlsruhe, which were imported from Brazil in September 2009 (Chiquita, Cincinnati, USA). Five watermelons were washed, peeled, mixed, and ground at a high speed for 1 min by a Rotor-Blender GT 800 (Spangenberg International BV, Mijdrecht, The Netherlands).
2.2. Thermal, UV-C, and high pressure treatments
A¼ The nomenclature of the thermal, UV-C, and high pressure treatment is listed in Table 1. The control was untreated watermelon juice. Specifically, the thermal treatment was carried out at an atmospheric pressure. Aliquots of 150 ml of the watermelon juice were packaged in a polyvinylchloride film and subjected to a water bath at 60 °C for 5, 20, 40, and 60 min with gentle stirring, respectively. After the thermal treatment, each juice was cooled down in an ice-water bath for further analysis. The UV-C treatment was carried out by a UV reactor (UVivatecÒ Lab, Bayer Technology Services GmbH (BTS), Leverkusen, Germany) at 23 °C. The essential part of the reactor is a helically wound TeflonÒ tubing wrapped around a quartz glass tube containing an UV lamp (a 9 W UV-C low-pressure mercury lamp at 254 nm). The watermelon juice passed through the coiled tubing (volume: 24 ml) at flow rate of 8.4 l/h, and circulated 3, 6, 9, and 12 cycles for UV-C dose of 2421, 4843, 7264, and 9685 J/l (Franz, Specht, Cho, Graef, & Stahl, 2009). After each treatment, the juice was cooled down quickly in an ice-water bath for further analysis. High pressure treatments were done in a hydraulic press U101 (Polish Academy of Sciences, Warsaw, Poland). Aliquots of 10 ml of the watermelon juice were pressurised at 300, 600, and 900 MPa for 5, 20, 40, and 60 min at 60 °C, respectively. The pressure transmitting medium was a mixture 7:3 of petroleum ether (boiling point 80–100 °C) and hydraulic oil. The temperature was controlled by a thermostat (Polystat, Huber, Germany) coupled to the sample chamber. After each treatment, the watermelon juice was cooled down quickly in an ice-water bath for further analysis.
C NaOH V NaOH 1 ¼ 400 V sample t V sample t
ð1Þ
where A is PME activity, CNaOH is the concentration of NaOH (=0.1 mol/l), VNaOH is the volume of NaOH used (=0.025 ml), Vsample is the volume of sample used (=0.20 ml), and t is the time needed for pH to return to 7.5 after the addition of NaOH (min).The PME residual level was calculated by Eq. (2).
PME residual level ¼
Asample 100% Acontrol
ð2Þ
where Asample is the PME activity of the sample after treatments, Acontrol is the PME activity of the control. 2.4. Determination of colour Colour assessment of the sample was conducted randomly in the reflectance mode for six times at 23 °C (Chromameter CR300, Minolta, Japan). Colour L*, a*, and b* value of the sample was measured and the total colour difference (DE) was calculated by Eq. (3).
DE ¼ ½ðL L0 Þ2 þ ða a0 Þ2 þ ðb b0 Þ2 1=2
ð3Þ
where DE is the total colour difference between a sample and the control, L is a lightness of a sample, L0 is a lightness of the control, a is a redness of a sample, a0 is a redness of a sample, b is a yellowness of the control, and b0 is a yellowness of the control. 2.5. Determination of browning degree
Table 1 Nomenclature of treatments. Treatments
Nomenclature
Time (min)
Pressure (MPa)
Temperature (°C)
UV-C dose (J/l)
Thermal treatment
T-05 T-20 T-40 T-60
5 20 40 60
0.1 0.1 0.1 0.1
60 60 60 60
0 0 0 0
High pressure treatment
H-05-300 H-20-300 H-40-300 H-60-300 H-05-600 H-20-600 H-40-600 H-60-600 H-05-900 H-20-900 H-40-900 H-60-900
5 20 40 60 5 20 40 60 5 20 40 60
300 300 300 300 600 600 600 600 900 900 900 900
60 60 60 60 60 60 60 60 60 60 60 60
0 0 0 0 0 0 0 0 0 0 0 0
U-03 U-06 U-09 U-12
3 6 9 12
0.1 0.1 0.1 0.1
23 23 23 23
2421 4843 7264 9685
UV-C treatment
Browning degree of the sample was evaluated using a spectrophotometric method described by Roig, Bello, Rivera, and Kennedy (1999). The sample was centrifuged with a Sigma 2K15 centrifuge (Sigma, Osterode, Germany) at 8603g at 4 °C for 20 min, and then passed through a 0.45 lm cellulose nitrate membrane. A browning degree value was determined by measuring the absorbance at 420 nm (Novaspec II, Pharmacia Biotech GmbH, Freiburg, Germany) at 23 °C with a 1 cm path length cell. 2.6. Determination of lycopene content Lycopene extraction and HPLC/DAD analysis of the sample were followed the previously described method (Stracke et al., 2009). Quantification of the sample was calculated by external calibration at k = 472 nm for all-trans-lycopene and total cis-isomers and at k = 450 nm for b-carotene and b-cryptoxanthin. Calibration curves for the carotenoids were constructed in the range of 0.0025– 25 mmol/l, in which the linearity of the response was given. The recovery for all carotenoids was greater than 95%. The CV of the method was below 5% (intra-assay). A typical chromatogram at k = 450 nm is shown in Fig. 1. The total cis-lycopene content is the sum of *-cis-lycopene (not identified), 15-cis-lycopene, 13-cislycopene, 9-cis-lycopene, and 5-cis-lycopene.
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Fig. 1. Representative HPLC chromatogram of the watermelon juice at k = 450 nm.
2.7. Determination of dynamic viscosity The dynamic viscosity of the sample was determined using a dynamic rheometer (Rheometer AR-550, TA Instruments, New Castle, Delaware, USA) with a conical end concentric cylinder (stator radius = 15.00 mm, rotor radius = 14.00 mm, immersed height = 42.00 mm, gap = 5920 lm). An aliquot of 19.6 ml of the sample was applied at each measurement at 25 ± 0.1 °C controlled by a circulating water system. Dynamic viscosities were the average of the flow curves at 1.2–5.0 min when the shear rate was constant. 2.8. Statistical analysis All data are expressed as the mean value ± standard deviation (n P 3). All statistical analyses are carried out with the Super ANOVA (Version 1.11, Abacus Concepts Inc., Berkeley, CA). One-way ANOVA and multiple comparisons (Fisher’s least-significant difference test) are used. The results are considered significant difference when the P value is lower than 0.05. 3. Results and discussion
of the PME residual level, were selected to compare the effect of the treatments on quality parameters of the watermelon juice. The PME residual level of T-05 and U-03 was statistically similar at the level of (100 ± 5)%, that of T-20, U-06, H-05-300, and H05-600 was statistically similar at the level of (75 ± 5)%, that of T-60, U-09, H-60-600, and H-20-900 was statistically similar at the level of (50 ± 5)%, and that of U-12, H-40-900, and H-60-900 was statistically similar at the level of (35 ± 5)%. Consequently, the following comparison was based on the four PME residual levels. The UV-C and high pressure treatments needed less time to reach the same PME residual level compared to the thermal treatment. For instants, U-06, H-05-300 and H-05-600 needed 5–6 min to reach the PME residual level of (75 ± 5)%, whilst T-20 needed 20 min. Being similar to our result, the high pressure treatments need less time to inactivate the PME of the white grape compared to the thermal treatments (Guiavarc’h et al., 2005). Moreover, a higher energy density of UV lamp or a higher flow rate of the UV-C treatment would further reduce the time to reach the same PME residual level. Hence, time saving was an advantage of the UV-C treatment.
3.1. Effect of the treatments on PME of the watermelon juice
3.2. Effect of the treatments on colour of the watermelon juice
The effects of the thermal, UV-C, and high pressure treatments on quality parameters of the watermelon juice are difficult to compare because those treatments could not be standardised to a same criterion, such as temperature, time, pressure, or transmitted energy. The quality parameters of the watermelon juice are mainly related to the catalysis of its intrinsic enzyme, such as PME and polyphenol oxidase (Aguiló-Aguayo et al., 2010). In a previous study, the effect of the thermal and high pressure treatments on quality parameters of orange juice was compared based on the PME residual level (Polydera et al., 2005a, 2005b). Moreover, the polyphenol oxidase activity was not detected in our watermelon juice. Consequently, the PME catalysis was the main factor influencing quality parameters of the watermelon juice, being similar to a recently result (Aguiló-Aguayo et al., 2010). Therefore, the PME residual level was selected as a criterion for the comparison of the thermal, UV-C, and high pressure treatments. The effect of the thermal, UV-C, and high pressure treatments on the PME residual level of the watermelon juice is shown in Fig. 2. Four levels, (100 ± 5)%, (75 ± 5)%, (50 ± 5)%, and (35 ± 5)%
The effect of the treatments on colour of the watermelon juice is shown in Table 2. All treatments leaded to a significant colour change because DE after each treatment was higher than 3.0. The ranking of DE was UV-C treatment > thermal treatment > high pressure treatment in each PME residual level. For instance, DE of U-06 was 6.76, being higher than that of T-20, H-05-300, and H-05-600 at the PME residual level of (50 ± 5)%. Furthermore, each treatment had a different influence on the colour of the watermelon juice. DE of the watermelon juice subjected to the thermal treatment increased with the treatment time, being similar to the previous results of strawberry (Pan et al., 2004) and pepper (Vicente et al., 2005). DE of the watermelon juice subjected to the UV-C treatment increased when its dose increased, being consistent to the results of tomato (Lingegowdaru, 2007). Being different to the thermal and UV-C treatments, a higher pressure (or a shorter time) of the high pressure treatment reduced the DE in each PME residual level, which was proved by comparisons of H05-300 and H-05-600, and H-40-900 and H-60-900, respectively. The similar results were also proved by the high pressure
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a
a
PME residual level (%)
100 b
b 75
b
b c
c
c c d
50
d
d
25
U-12
U-09
U-06
U-03
H-60-900
H-40-900
H-05-900
H-20-900
H-60-600
H-40-600
H-20-600
H-05-600
H-60-300
H-40-300
H-20-300
H-05-300
T-60
T-20 T-40
T-05
0
Fig. 2. Effect of treatments on the PME residual level. The PME residual level is calculated by Eq. (2). Data are means ± standard deviation (n P 3). The a, b, c, and d represent (100 ± 5)%, (75 ± 5)%, (50 ± 5)%, and (35 ± 5)% of the PME residual level of the watermelon juice, respectively, after each treatment.
Table 2 Effect of treatments on colour of watermelon juice. PME residual level
Treatments
L*A
a* a,B
b*
DE
a
a
(100 ± 5)%
Control T-05 U-03
62.54 ± 0.24 57.43 ± 0.32b 60.76 ± 0.37c
6.36 ± 0.08 8.38 ± 0.38b 7.71 ± 0.15c
15.7 ± 0.35 17.87 ± 0.61b 10.10 ± 0.25c
0 5.87 6.02
(75 ± 5)%
T-20 U-06 H-05-300 H-05-600
56.91 ± 0.19b 63.91 ± 0.30d 58.36 ± 0.21b 60.24 ± 0.30c
6.88 ± 0.05a 6.65 ± 0.15a 8.52 ± 0.16b 6.84 ± 0.14a
17.67 ± 0.27b 9.10 ± 0.24d 14.58 ± 0.28e 11.28 ± 0.13f
5.95 6.76 5.97 4.99
(50 ± 5)%
T-60 U-09 H-60-600 H-20-900
55.59 ± 0.08e 64.14 ± 0.23d 61.79 ± 0.09c 62.29 ± 0.34a
6.86 ± 0.09a 5.77 ± 0.15d 6.33 ± 0.12a 5.66 ± 0.10d
16.05 ± 0.57b 8.39 ± 0.20g 9.75 ± 0.21cd 10.58 ± 0.10c
6.94 7.51 5.99 5.17
(35 ± 5)%
U-12 H-40-900 H-60-900
66.06 ± 0.07f 62.76 ± 0.41a 60.75 ± 0.06c
5.30 ± 0.06d 5.71 ± 0.06d 5.62 ± 0.04d
8.80 ± 0.17d 8.33 ± 0.24g 7.89 ± 0.18g
7.84 7.40 8.01
A L*, a*, and b* represent the lightness, redness, and yellowness of the sample, respectively. DE shows the total colour difference between the sample and the control which is calculated by Eq. (3). B Data are means ± standard deviation (n P 3). Means with different superscript letters in the same row represent a significant difference (P < 0.05).
treatments of the strawberry juice (Rodrigo, van Loey, & Hendrickx, 2007). However, two different results were reported that the colour of tomato puree (Rodrigo et al., 2007) and avocado puree (Lopez-Malo, Palou, Barbosa-Canovas, Welti-Chanes, & Swanson, 1998) keep constants after high pressure treatments. These differences maybe resulted from their different lycopene ingredients or solution pH. Colour a* is related to redness of the watermelon juice, which is its main colour. The thermal treatments of T-20 and T-60 kept its a* similar to that of the control in the PME residual level of (75 ± 5)% and (50 ± 5)%. Colour a* of the watermelon juice subjected to the UV-C treatment decreased when the dose of UV-C increased, being different to the previous results that UV-C treatments do not influence the redness of pepper (Vicente et al., 2005). This difference may result from the texture and microstructure varieties of watermelon juice and pepper, because these varieties cause changes in the nature and the extent of internally scattered light and the distribution of surface reflectance (Oey, Lille, Van Loey, & Hendrickx, 2008). Interestingly, colour a* of the watermelon juice subjected to the high pressure treatment at 600 MPa was similar to that of the control, whilst that subjected to the high pressure treatment at 300 and 900 MPa was statistically different to that of the control.
Therefore, the high pressure treatment at 600 MPa was effective to keep the colour of the treated watermelon juice as the control compared to the thermal and UV-C treatments in the same PME residual level. 3.3. Effect of the treatments on browning degree of the watermelon juice The effect of the treatments on the browning degree of the watermelon juice is shown in Fig. 3. The browning degree of the watermelon juice subjected to the high pressure treatment was lower than that subjected to the thermal and UV-C treatments in each PME residual level. Moreover, the high pressure treatments of H-60-600, H-20-900, H-40-900, and H-60-900 significantly decreased the browning degree of the treated watermelon juice. Consequently, the high pressure treatment with the pressure higher than 600 MPa was effective to avoid the browning of the treated watermelon juice, being consistent to the results that the high pressure treatments at 600 and 900 MPa reduce the browning rate of the grape juice (Castellari, Matricardi, Arfelli, Carpi, & Galassi, 2000). Each treatment had a different effect on the browning degree of the watermelon juice. The browning degree of the watermelon
C. Zhang et al. / Food Chemistry 126 (2011) 254–260
Treatments
258
H-60-900 H-40-900 U-12
a
H-20-900 H-60-600 U-09 T-60
a a
a
c
c e b
H-05-600 H-05-300 U-06 T-20
c b b
U-03 T-05
b b
Control 0.00
d
0.05
0.10
0.15
0.20
Browning degree Fig. 3. Effect of treatments on the browning degree of the watermelon juice Data are means ± standard deviation (n P 3). Means with different letter represent a significant difference (P < 0.05).
juice increased with an increase of time and UV-C dose of the thermal and UV-C treatments, respectively. The browning degree of the watermelon juice subjected to the high pressure treatment, decreased when the pressure increased. Similarly, minimal browning is found when potato and apple are subjected to the high pressure treatment at 800 MPa (Gomes & Ledward, 1996). 3.4. Effect of the treatments on the dynamic viscosity of the watermelon juice The effect of the treatments on the dynamic viscosity of the watermelon juice is shown in Table 3. The dynamic viscosity of the watermelon juice subjected to the thermal and high pressure treatment was statistically similar to that of the control at the PME residual level of (50 ± 5)% and (35 ± 5)%, whilst that subjected to the UV-C treatment was significantly different. Hence, the thermal and high pressure treatments were effective to keep the dynamic viscosity of the treated watermelon juice as the control
compared to the UV-C treatment in the same PME residual level. This phenomenon may result from the fact that the UV-C treatments not only inactivate PME, but also inactivate the polygalacturonase, cellulase, xylanase, b-D-galactosidase, and protease which mainly respond to the dynamic viscosity of the juice (Barka et al., 2000). The dynamic viscosity of the watermelon juice subjected to the high pressure treatment at 900 MPa was similar to that of the control, whilst that at 300 MPa was statistically different. Hence, the high pressure treatment at 900 MPa helped to stabilise the dynamic viscosity of the watermelon juice, being similar to the results about the effect of the high pressure treatment on the orange juice (Polydera et al., 2005a, 2005b). Being different, the dynamic viscosity of mango pulp increases after the high pressure treatment at 100 or 200 MPa (20 °C for 15 or 30 min), whilst a reduction in dynamic viscosity is observed after the high pressure treatment at 300 and 400 MPa (20 °C for 15 or 30 min) (Ahmed, Ramaswamy, & Hiremath, 2005).
Table 3 Effect of the treatments on the dynamic viscosity and lycopene concentration of the watermelon juice.
A B C
PME residual level
Treatments
Dynamic viscosity (mPa S)
All-trans-lycopene concentration (nmol/l)
Total cis-lycopene concentration (nmol/l)A
Total lycopene concentration (nmol/l)B
(100 ± 5)%
Control T-05 U-03
1.88 ± 0.20a,C 2.63 ± 0.22c 1.87 ± 0.19a
27.8 ± 3.1a 25.2 ± 1.9a 25.5 ± 4.1a
4.81 ± 0.78a 5.41 ± 1.50a 4.52 ± 0.15a
32.6 ± 2.6a 30.6 ± 2.9a 30.0 ± 4.2a
(75 ± 5)%
T-20 U-06 H-05-300 H-05-600
2.22 ± 0.37b 2.04 ± 0.34a 2.34 ± 0.09b 2.18 ± 0.27b
22.9 ± 1.8b 21.4 ± 3.2b 25.1 ± 2.4a 24.8 ± 0.6a
5.18 ± 1.90a 4.27 ± 0.26a 5.13 ± 1.12a 4.83 ± 0.15a
28.1 ± 3.2b 25.7 ± 3.3b 30.3 ± 3.1a 29.7 ± 0.7ab
(50 ± 5)%
T-60 U-09 H-60-600 H-20-900
1.96 ± 0.27a 2.19 ± 0.54b 1.91 ± 0.04a 2.01 ± 0.12a
16.3 ± 4.1c 16.6 ± 3.6c 20.5 ± 3.6b 19.8 ± 2.3b
4.76 ± 1.46a 3.66 ± 0.16b 3.27 ± 0.42b 4.00 ± 0.98b
21.0 ± 5.1c 20.2 ± 3.7c 23.7 ± 0.3c 22.8 ± 2.0c
(35 ± 5)%
U-12 H-40-900 H-60-900
2.21 ± 0.45b 1.97 ± 0.16a 2.03 ± 0.59a
12.6 ± 2.6d 18.6 ± 0.1c 17.0 ± 3.9c
3.56 ± 0.54b 3.86 ± 0.16b 3.70 ± 0.93b
16.1 ± 3.1d 22.4 ± 0.2c 21.2 ± 1.5c
Total cis-lycopene content is the sum of *-cis-lycopene (not identified), 15-cis-lycopene, 13-cis-lycopene, 9-cis-lycopene, and 5-cis-lycopene. Total lycopene is the sum of the all-trans-lycopene and cis-lycopene. Data are means ± standard deviation (n P 3). Means with different superscript letters in the same row represent a significant difference (P < 0.05).
C. Zhang et al. / Food Chemistry 126 (2011) 254–260
3.5. Effect of the treatments on the lycopene content of the watermelon juice The effects of the treatments on all-trans-lycopene, total cis-lycopene, and total lycopene concentration of the watermelon juice are listed in Table 3. The all-trans-lycopene concentration of the treated watermelon juice decreased when the PME residual level decreased. Moreover, the all-trans-lycopene concentration of the watermelon juice subjected to the high pressure treatment was significantly higher than that subjected to the thermal and UV-C treatments in the PME residual levels of (75 ± 5)%, (50 ± 5)%, and (35 ± 5)%. One might speculate that the high pressure treatments increase the extraction yield of the lycopene as described for tomato puree (FernandezGarcia, Butz, & Tauscher, 2001). cis-Lycopene has been proved to be more bioavailable than trans-lycopene in vitro and in vivo (Boileau, Merchen, Wasson, Atkinson, & Erdman, 1999; Edwards et al., 2003). The total cis-lycopene concentration of the watermelon juice after each treatment was similar to that of the control in the PME residual level of (100 ± 5)% and (75 ± 5)%, whilst it was statistically different to that of the control in the PME residual level of (50 ± 5)%, and (35 ± 5)% except for T-60. Hence, the total cis-lycopene concentration of the treated watermelon juice decreased with a loss of the PME residual level. The effect of the treatments on the total lycopene concentration of the watermelon juice was similar to that on the all-translycopene concentration in each PME residual level. Remarkably, the total lycopene concentration of the treated watermelon juice was statistically similar after the high pressure treatment at 300 MPa (H-05-300) and 600 MPa (H-05-600). Being different to our results, Qiu, Jiang, Wang, and Gao (2006) reported that the total lycopene content is maximum at 400 MPa and shows a significant loss at 500 and 600 MPa. On the other hand, a few results showed that the total lycopene content is in linear correlation with the colour a* of the watermelon juice (Lewinsohn et al., 2005; Perkins-Veazie & Collins, 2004; Tadmor et al., 2005). However, effect of the high pressure treatment on the total lycopene content and colour a* of the watermelon juice was different. Hence, the colour a* of the treated watermelon juice was speculated to be the combination of the browning and lycopene colour. Take together, the high pressure treatment was effective to hold the all-trans-lycopene, total cis-lycopene, and total lycopene concentration of the treated watermelon juice as the control compared to the thermal and UV-C treatments. 4. Conclusion The effect of the UV-C and high pressure treatments on colour, browning degree, dynamic viscosity, and lycopene content of the watermelon juice was compared with the thermal treatment based on its PME residual level. The UV-C treatment was a rapid and effective method to reach a certain PME residual level. High pressure treatments at 600 MPa kept the colour of the treated watermelon juice close to an untreated one, and that at 600–900 MPa held the browning degree and dynamic viscosity of the watermelon juice comparable to an untreated one. Moreover, the high pressure treatment was the only treatment which had only a slight impact on the total lycopene concentration. In summary, the high pressure treatment showed the lowest changes in colour, dynamic viscosity, browning degree, and lycopene content of the treated watermelon juice amongst the three treatments.
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