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Effect of radio frequency heating on the sterilization and product quality of vacuum packaged Caixin
food and bioproducts processing 9 5 ( 2 0 1 5 ) 47–54
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Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp
Effect of radio frequency heating on the sterilization and product quality of vacuum packaged Caixin Qian Liu a , Min Zhang a,∗ , Baoguo Xu a , Zhongxiang Fang b , Dandan Zheng c a
State Key Laboratory of Food Science and Technology, School of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China b School of Public Health, Curtin Health Innovation Research Institute, International Institute of Agri-Food Security, Curtin University, Bentley, Western Australia 6102, Australia c Haitong Food Group Company, Zhejiang, Cixi 315300, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Radio frequency (RF) heating has the potential to be developed as a sterilization method
Received 16 February 2014
to control microbes of food products. In the present work, the effects of RF heating on the
Received in revised form 14 March
sterilization and product quality of vacuum packaged Caixin (Brassica campestris L.) were
2015
investigated. With the addition of salts and other food additives, the dielectric constant (ε )
Accepted 21 March 2015
and loss factor (ε ) of Caixin were increased, which was helpful to increase the temperature
Available online 28 March 2015
during RF heating. The results indicated that, coupled with a hot air drying of 60 ◦ C, RF
Keywords:
improve the microbial safety of Caixin whereas the physico-chemical and sensory qualities
Radio frequency heating
were not compromised.
Vacuum packaged Caixin
© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
heating at 6 kW, 27.12 MHz and 20 mm/20 min might be the most appropriate conditions to
Microbial safety Product quality Physico-chemical qualities Sensory qualities
1.
Introduction
Thermophilic or mesophilic bacteria are the main spoilage microorganisms of vacuum packaged Caixin (Brassica campestris L.), a green leafy vegetable and low acid food (Delin and Jia, 2008). Generally, there have two methods to improve the safety of the product: using commercial steam sterilization treatment (>105 ◦ C), or lowering its pH value to acid food level (pH <4.5) and pasteurizing at a temperature below 100 ◦ C. However, both the high temperature and acid condition may cause the degradation of chlorophyll which will adversely affect the sensory quality of the product, especially the natural green color (Gaur et al., 2006; James and Rickey, 1996). Therefore, there is an urgent need in the
∗
food industry to develop innovative sterilization technologies to improve the microbial safety while maintaining the quality of vacuum packaged green vegetables (Kim et al., 2012). Radio frequency (RF) heating is a technology of electromagnetic energy which is developed between electrodes at frequencies range from 1 to 300 MHz to generate heat in a dielectric material (Orsat et al., 2004). RF treatment is essentially a combination of electrical and thermal treatment (Uemura et al., 2012), and the synergistic effect has a great influence on energy efficiency (Geveke et al., 2009). Generally, an RF system is coupled with a hot air system to assist the heating up (Gao et al., 2011). Compared with conventional heating methods, the RF treatment has the advantages of
Corresponding author. Tel.: +86 510 85917089; fax: +86 510 85917089. E-mail address: [email protected] (M. Zhang). http://dx.doi.org/10.1016/j.fbp.2015.03.007 0960-3085/© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
48
food and bioproducts processing 9 5 ( 2 0 1 5 ) 47–54
saving process time and producing products with better qualities (Uemura et al., 2012), and therefore has the potential to be developed as a novel pasteurization method to control microbes in agricultural commodities (Gao et al., 2011). Conventionally, when a food is heating treated, the heat is transferred from the exterior of the food to the interior by convection or conduction (Kim et al., 2012). On the contrary, RF generates heat due to the friction of the rotational movement of the molecules and the space charge displacement in response to an alternating electric field, which offers a considerable speed advantage over conventional heating methods. Energy generated by RF heating is absorbed directly by the whole material (Orsat et al., 2001). In addition, RF heating does not require direct contact between the product and electrodes as RF waves will penetrate through the cardboard or plastic packaging (Marra et al., 2009). In a dielectric heating process, the dielectric materials have two basic characteristics that determine the amount of RF power required to raise their temperatures. The dielectric constant (ε ) represents stored energy when the material is exposed to an electric field and the loss factor (ε ) is dominated by ionic conductance that influences energy absorption and attenuation (Birla et al., 2008). The RF power absorption in any material is directly proportional to the loss factors, and different materials exposed to the same RF field would have a different heating result. From a microbial inactivation perspective, the benefit of RF heating comes from a potential selective killing effect on microorganisms or a targeted heating of the microorganisms other than a direct heating effect on the product (Orsat and Raghavan, 2005). The electric field strength, frequency, temperature and hold time (Geveke et al., 2009), as well as the dielectric properties, shape and size of samples, surrounding medium and relative distance (Birla et al., 2008) have significant effects on the results of inactivation. Orsat et al. (2001) reported that the quality of the RF treated carrot sticks was greater than either the control samples (chlorinated water) or hot-water-treated samples in terms of color, taste and microbial load. Geveke and Brunkhorst (2008) used radio frequency electric fields (RFEF) to pasteurize apple cider and the results showed that RF processing at an outlet temperature of 60 ◦ C reduced the population of Escherichia coli by 4.8 log, whereas conventional thermal processing at the same conditions had no effect. Radio frequency heating has also been conducted to pasteurize meat (Byrne et al., 2010), liquid egg white (Uemura et al., 2012), peanut butter cracker sandwiches (Ha et al., 2013), milk (Awuah et al., 2005), soybean milk (Uemura et al., 2010) and ‘Fuyu’ persimmons (Tiwari et al., 2008). However, there are few reports on utilizing RF heating on pasteurization of green leafy vegetables, especially its effect on the green color of the products. The objective of this study was to evaluate the potential of RF heating on pasteurization of vacuum packaged Caixin. The effects of RF treatment on the product microbial, physico-chemical and sensory qualities were assessed and discussed, which would be useful to apply this technology in green leaf vegetable processing.
2.
Materials and methods
2.1. Determination of dielectric properties of vacuum packaged Caixin Fresh Caixin was purchased from a local market in Wuxi city, China. The samples were washed, blanched in a water bath (95 ◦ C, 2 min) and cooled in ice water (about 0 ◦ C) for 5 min
to eliminate enzyme activity (Orsat et al., 2001). The samples were mixed with 5 g kg−1 (W/W) sodium alginate, 4 g kg−1 calcium chloride and 2 g kg−1 of potato starch to preserve the crisp texture, and 50 g kg−1 sodium chloride, 100 g kg−1 edible glycerol and 4 g kg−1 sodium carboxymethyl cellulose were added to protect the color. After these treatments, products were vacuum packed (50 g/package) in polyamide/cast polypropylene (PA/CPP) plastic bags and kept in a fridge (4–5 ◦ C) for further use. Dielectric constants (ε ) and loss factors (ε ) of the packaged Caixin samples were measured between 20 MHz and 40 MHz using a network analyzer (E5062A, Agilent Technologies, Santa Clara, CA, USA) (Koskiniemi et al., 2011). The measurement system was consisted of an open ended dielectric probe connected with an impedance analyzer which is interfaced with a personal computer. Prior to the measurement, the system was calibrated according to the standard procedure (Birla et al., 2008). The Caixin sample was homogenized to pulp with a JYL-C020 Waring blender (Joyoung Co. Ltd., Jinan, China) in order to maintain proper contact with the probe. In addition, the dielectric properties of the texture improvement agents (sodium alginate, calcium chloride, potato starch), color protecting agents (sodium chloride, edible glycerol and sodium carboxymethyl cellulose) and raw Caixin before the blanching treatment were also measured. Each measurement was repeated three times and results were reported as mean values.
2.2.
Radio frequency (RF) and hot air heating systems
A 6 kW, 27.12 MHz RF system (Strayfield International, Wokingham, UK) coupled with a hot air system was used to treat the Caixin samples (Fig. 1). The hot air flow was forced from the holes on the bottom electrode to the samples through perforated screens on the side and bottom walls of a plastic container, with a temperature of 60 ◦ C and speed of 3 m s−1 (Gao et al., 2010). An alternating electric field was developed between electrodes where the Caixin samples were placed. The gap between the two plate electrodes was adjusted to change RF power to the samples (Gao et al., 2011; Ukuku and Geveke, 2010). To determine the appropriate gap between the electrodes for RF treatments, three gaps, namely 20 mm, 30 mm and 60 mm, were tested (Gao et al., 2010). Treatments of 5 min, 10 min, 20 min and 30 min RF exposure were conducted to evaluate the efficacy of pasteurization on Caixin samples (Ha et al., 2013). Samples with the 60 ◦ C hot air heating but without RF treatment were used as control.
2.3. Temperature distributions after radio frequency (RF) heating The temperature distributions of vacuum packaged Caixin after RF treatments were examined by an infrared thermal imaging camera (IRI 4010 Multi-Purpose Imager, IRISYS, UK). The thermal image showing the temperature distributions reflects the heating uniformity. Each measurement was taken about 3 s (Wang et al., 2008). The highest and lowest temperatures in the images were labeled by the software of the infrared thermal imaging camera. In addition, the sample temperatures at five different positions in the container of the RF system were measured using an infrared thermometer (ST18, Raytek Company, USA).
food and bioproducts processing 9 5 ( 2 0 1 5 ) 47–54
49
Fig. 1 – Schematic diagram of the radio frequency (27.12 MHz, 6 kW) heating system. 1. Bottom electrode; 2. Upper electrode; 3. Control panel; 4. View point (a and b); 5. Florescent tube light; 6. Heater (a and b) and air blower; 7. Keys for the RF system and doors; 8. Air outlet; 9. Door for the drying chamber; 10. Conveyor belt; 11. Hot air distribution chamber.
2.4.
Electrical conductivity measurements
calculated from chlorophyll a (Ca , mg/L) and chlorophyll b (Cb , mg/L) values with the following equations:
The control and RF treated Caixin samples were homogenized to pulp, and the electrical conductivity was measured by a Conductivity Meter (DS-11AT, Precision Instrument Co. Ltd., Shanghai, China). The values were recorded as the mean of triplicate measurements.
2.5.
2.6.
(2)
Cb = 24.96A649 − 7.32A665
(3)
C = Ca + Cb
(4)
Color measurements
Color properties were a significant factor to evaluate the quality of green leafy vegetables (Akbudak and Akbudak, 2013). The color properties of Caixin were determined using a Minolta CR 400 colorimeter (Konica-Minolta, Osaka, Japan). Calibration was performed using a standard white tile. L* (value on the white/black axis), a* (value on the red/green axis) and b* (value on the yellow/blue axis) of each sample were measured 10 times. Since the major color of leafy vegetable is green, ‘−a*’ values were considered as the visual parameter to describe the green color change (Yin et al., 2007). The chrominance C* is known as saturation: C∗ =
Ca = 13.95A665 − 6.88A649
a∗2 + b∗2
(1)
Chlorophyll content
The extraction of chlorophyll was a modification of the method of James and Rickey (1996). Briefly, sample of 50 g was homogenized to pulp and 0.2 g sample was mixed with 15 mL 80% aqueous acetone (v/v). When the color of the mixture solution turned white, it was filtered into a 25 mL brown bottle using a qualitative filter paper, and then adjusted the volume to 25 mL with the 80% acetone. The absorbance of the filtered solutions were measured at 665 and 649 nm with an UV 2600 spectrophotometer (Techcomp Instrument Co. Ltd., Shanghai, China). The chlorophyll concentration (C, mg/L) was
The chlorophyll content (mg/g) =
C × extraction volume × dilution factor . sample weight
(5)
2.7. Gas chromatography–mass spectrometry (GC–MS) analysis The volatiles of Caixin samples were isolated by solid phase micro-extraction (SPME), which is widely used to collect and concentrate the aroma fraction of fruit, vegetables, beverages, etc. (Wang et al., 2007). Caixin was homogenized to pulp and 6 g was placed into a 15 mL headspace vial. The sample was then extracted on a 75 m Carboxen/Polydimethyl siloxane (CAR/PDMS, Supelco, USA) fiber column at 50 ◦ C for 30 min and then eluted at 250 ◦ C for 3 min. The GC–MS analyses of the volatile compounds of Caixin samples were carried out by a Trace MS (Finnigan, USA). The GC conditions were as follows (Torres et al., 2010): capillary column: DB-WAX (30 m × 0.25 mm, 0.25 m; Supelco, USA); carrier gas: helium with a flow rate of 0.8 mL/min; injection temperature: 250 ◦ C; the oven temperature was programmed from an initial temperature of 40 ◦ C (4 min hold), increased to 80 ◦ C at 6 ◦ C/min, then to 230 ◦ C at 10 ◦ C/min, and held isothermal for 7 min. The MS was operated with ionization of 70 eV, emission current of 200 mA, ion source temp of 200 ◦ C, scan range of 33–350 m/z and detector voltage of 350 V.
50
food and bioproducts processing 9 5 ( 2 0 1 5 ) 47–54
Table 1 – Dielectric properties of materials used in the RF treatment of vacuum packaged Caixin. Materials
Dielectric constants (ε )
Deionized water Sodium alginate (5 g kg−1 ) Calcium chloride (4 g kg−1 ) Potato starch (2 g kg−1 ) Sodium carboxymethyl cellulose (4 g kg−1 ) Edible glycerol (100 g kg−1 ) Sodium chloride (50 g kg−1 , 100-fold dilute) Raw Caixin Vacuum packaged Caixin (100-fold dilute)
79.2 86.5 87.5 77.8 77.7 79.9 83.6 77.6 83.1
± ± ± ± ± ± ± ± ±
0.7e 2.1bc 1.8b 1.7e 2.9e 0.4e 1.8cd 3.1e 0.8d
Loss factors (ε ) 0.2 101.2 859.4 −1.5 75.8 −1.9 66.4 718.9 43.3
± ± ± ± ± ± ± ± ±
0.4g 4.7d 4.5b 0.5g 2.9e 0.4g 1.8e 19.9c 4.1f
The instrument has the limitations for NaCl and Caixin; samples were measured between 20 MHz and 40 MHz; results are mean ± standard deviation; values with different superscript letters in the same column are significantly different (P < 0.05).
The identification of the volatile compounds was based on the GC retention times and mass spectra (Torres et al., 2010). All compounds were identified by comparison to NIST library (including Wiley and Mainlib) spectral data bank. Only compounds whose similarity is more than 800 (the maximum similarity is 1000) were reported (Wang et al., 2007).
2.8.
Standard plate count (SPC)
Standard plate count was conducted to determine the level of bacteria in Caixin samples after RF treatment as described by Chinese National Standard (GB 4789.2-2010). About 25 g Caixin samples were taken with sterile scissors and nippers, diluted in 225 mL physiological saline solution (PS, 8.7 g kg−1 NaCl, pH 7.0) and homogenized in a homogenizer (10,000 rpm) for 1 min to achieve a 10-fold dilution. One milliliter was then put into 9 mL PS solution to get 100-fold dilution. Similarly, 1000-fold dilution was prepared. One milliliter of each different dilution was pipetted into a sterile Petri dish. To each Petri dish about 15 mL of nutrient agar (46 ◦ C) was added, and after solidification they were put into an incubator (36 ± 1 ◦ C) to culture for 48 ± 2 h and the total bacteria colonies were counted. The real SPC was the observed value multiplied by dilution, and the results were expressed as log10 CFU/g. Both the control and RF treated samples were determined at a 1-day interval from the day of treatment. Determinations were replicated three times for each sample.
2.10.
3.
Results and discussion
3.1. Dielectric properties and electrical conductivity of Caixin
Sensory evaluation
Sensory evaluation was carried out to assess the specific organoleptic attribute (color, texture, flavor, taste, and overall acceptability) variation over RF-treated Caixin samples. The panelists were consisted of a taste panel of 10 members who had previous experience of purchasing and/or consuming this product, with the age range of 23–27 and gender ratio of 1:1 (Emamifar et al., 2010). The scores were obtained by grading the sensory attributes using the following 9-point hedonic scales: 9 = extremely good, 8 = very good, 7 = good, 6 = below good/above fair, 5 = fair, 4 = below fair/above poor, 3 = poor, 2 = very poor, 1 = extremely poor. All the prepared samples coded with three-digit random numbers were presented in a random order to the panelists (Ha et al., 2013).
2.9.
tests using SPSS 15.0.1 (SPSS Inc., Chicago, IL, USA). Average values were considered significantly different when P < 0.05.
Statistics
Differences among average values were estimated using analysis of variance (ANOVA) with the application of Duncan’s
The dielectric properties of the raw Caixin, texture improvement agents (sodium alginate, calcium chloride and potato starch), color protecting agents (sodium chloride, edible glycerol and sodium carboxymethyl cellulose) and vacuum packaged Caixin after 27.12 MHz RF treatment are summarized in Table 1. The dielectric constant (ε ) and the loss factor (ε ) of vacuum packaged Caixin were both too high (9999) to be measured accurately. Therefore, it was diluted with deionized water to 100-fold and the ε and ε were 83.14 ± 0.81 and 43.27 ± 4.10 respectively. The ε and ε of raw Caixin were 77.61 ± 3.10 and 718.91 ± 19.94 respectively, which were significantly lower (P < 0.01) than that of vacuum packaged Caixin. Table 1 suggested that the high dielectric constant (ε ) and loss factor (ε ) of vacuum packaged Caixin might come from the salt and other agents added in the package. Similar results were also observed by Marra et al. (2009) and Birla et al. (2008) and they suggested that it is mainly the addition of salts that have increased the ionic conductivity and the loss factor of materials. The high ε and ε of vacuum packaged Caixin might be helpful to the temperature increment during the RF process. Fig. 2 shows that the electrical conductivity of vacuum packaged Caixin after RF heating also increased significantly. This might be because the RF treatment promoted the elution of soluble components and ions from the vegetable samples.
3.2.
Temperature distributions after RF treatments
Measured by the infrared thermometer, the initial temperature of Caixin samples was 25.8 ◦ C, which increased to 60 ◦ C after the RF heating treatment of 10–15 min combined with hot air. The heating rate was at an order of about 4 ◦ C/min and it took about 5 min for the samples to reach 45 ◦ C (data not shown). The temperature distributions of vacuum packaged Caixin after the RF treatments are shown in Fig. 3. The thermal images showing the temperature distributions could be used to rapidly determine the heating uniformity. The heating uniformity was better after a heating time of 20–30 min than after 5–10 min (Fig. 3), but no significant difference was detected between the treatment of 20 min and 30 min. Gao et al. (2011) suggested that the RF sample temperature could be maintained at a fairly constant value after a certain time,
food and bioproducts processing 9 5 ( 2 0 1 5 ) 47–54
Fig. 2 – Electrical conductivity (mean ± standard deviation) of Caixin (n = 3). as the absorbed RF power was balanced by the latent heat of water evaporation.
3.3. Effects of different RF treatments on the product quality of Caixin Color values of Caixin after RF treatment are summarized in Table 2. The L*, a* and b* values of RF treated Caixin were measured at random locations of the products. L* values had a slight declining trend after RF heating treatments, indicating a little darker of the samples after treatment. The a* and C* values of RF treated Caixin were not significantly different to those of non-treated samples except for the treatments of 30 mm/5 min, 60 mm/30 min and 60 mm/5 min. The −a* and C* values of Caixin with the treatments of 30 mm/5 min and 60 mm/5 min were significantly lower than others, whereas with the treatment of 60 mm/30 min were significantly higher. However, significant differences were not detected on the property of color among these Caixin samples by the sensory
51
evaluation. Similar results were also reported by Ha et al. (2013), where color values and sensory characteristics of the RF treated peanut butter and crackers were not significantly (P > 0.05) different from the control. The chlorophyll contents of Caixin samples were also measured, and the results showed that the chlorophyll contents of RF treated and untreated Caixin samples had no significant (P > 0.05) differences (Table 2), which suggested that the RF treatment have no negative effect on the green color of the product. To the best of our knowledge, this is the first report of RF treatment on the color quality and chlorophyll content of green leafy vegetables. The GC–MS analysis showed that the volatile compounds of RF treated and untreated Caixin samples had some differences. In the RF untreated Caixin (Table 3), 27 compounds were identified. Among these volatile compounds, hydrocarbons were the dominant (alkane 18.71%; aromatic hydrocarbon 27.44%), alcohols were the second (13.54%), and then were the acids (11.09%). Ester, aldehyde, ketone, phenol and nitrile were only existed in a small amount. However, 26 compounds were identified in the RF treated Caixin (30 mm/20 min), where hydrocarbons were also the dominant (alkane 13.22%; aromatic hydrocarbon 27.08%), but acids were the second (16.06%), and then was the alcohols (9.47%), and anhydride, aldehyde, ester and ketone were existed with a small amount (Table 3). The results indicated that aromatic hydrocarbon of oxime-, methoxy-phenyl- was the major volatile compound in RF treated (27.08%) and untreated samples (27.44%) and no significant differences were observed between the two samples. However, after RF treatment, the content of octanoic acid was significantly increased (untreated sample 8.08%, RF heating sample 13.48%). It was proposed that aldehydes (hexanal, 2-hexenal) are contributors to the green odor of vegetables (Huang et al., 2012). After RF treatment, the content of hexanal was significantly increased (untreated sample 0.34%, RF heating sample 0.67%) which implied that the RF treatment may have improved the green odor. However, because its relative content is very low, low relative content does not necessarily give low odor. In addition, compared with the control, anhydrides of acetic formic anhydride were appeared after
Fig. 3 – The temperature distributions of vacuum packaged Caixin after RF heating treatments (a1 , 20 mm/30 min; b1 , 20 mm/20 min; c1 , 20 mm/10 min; d1 , 20 mm/5 min; a2 , 30 mm/30 min; b2 , 30 mm/20 min; c2 , 30 mm/10 min; d2 , 30 mm/5 min; a3 , 60 mm/30 min; b3 , 60 mm/20 min; c3 , 60 mm/10 min; d3 , 60 mm/5 min).
52
food and bioproducts processing 9 5 ( 2 0 1 5 ) 47–54
Table 2 – Color parameters and chlorophyll content of Caixin after RF heating. Treatments Control 20 mm/30 min 30 mm/30 min 60 mm/30 min 20 mm/20 min 30 mm/20 min 60 mm/20 min 20 mm/10 min 30 mm/10 min 60 mm/10 min 20 mm/5 min 30 mm/5 min 60 mm/5 min
−a*
L* 33.7 29.7 29.1 30.8 28.0 31.0 29.8 30.2 29.5 32.2 30.4 30.1 32.8
± ± ± ± ± ± ± ± ± ± ± ± ±
a
0.4 1.2de 1.3de 1.6cd 0.5e 0.6bcd 0.6de 2.3cde 1.3de 1.9abc 0.5cd 0.5cde 0.6ab
8.6 8.6 9.0 10.0 8.6 8.1 8.4 8.8 8.4 8.8 8.4 8.0 7.4
± ± ± ± ± ± ± ± ± ± ± ± ±
C* bcd
0.2 0.3bcd 0.1b 0.3a 0.2bcd 0.2cd 0.4bcd 0.2bc 0.4bcd 0.4bc 0.7bcd 0.6de 0.1e
11.5 11.8 13.6 13.7 13.0 12.7 11.9 12.4 11.8 13.9 11.6 11.4 10.0
± ± ± ± ± ± ± ± ± ± ± ± ±
Chlorophyll (mg/g) bc
0.5 0.4bc 0.1a 0.7a 0.5ab 1.3abc 1.0bc 0.4abc 0.7bc 1.3a 0.8bc 1.4cd 0.3d
1.5 1.4 1.5 1.5 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.3 1.3
± ± ± ± ± ± ± ± ± ± ± ± ±
0.2a 0.1a 0.2a 0.2a 0.2a 0.1a 0.1a 0.2a 0.2a 0.2a 0.1a 0.1a 0.1a
Results are mean ± standard deviation; values with different superscript letters in the same column are significantly different (P < 0.05).
RF heating but phenol and nitriles were not detected. The analysis of volatile compounds indicated that RF treatment have some influence on the volatile compounds of Caixin samples, although the mechanisms are unknown. To date, no information is available about the volatile compounds of Caixin. Chinese cabbage (Brassicachinensis L.) is a close relative of Caixin (Brassica campestris L.). Yang et al. (2004) reported that in Chinese cabbage, saturated hydrocarbons were the dominant volatile compounds, aromatic hydrocarbons were second, and unsaturated hydrocarbons, aldehydes, alcohols, ketones, acids and heteroaromatic compounds were existed in a small amount. The relatively high content of aromatic hydrocarbon (methoxy-phenyl-) and octanoic acid in Caixin might account for the major odor differences to Chinese cabbage.
Moreover, the sensory evaluation by the panelists indicated that no significant differences were among the RF treated and untreated Caixin samples on the properties of color, texture, flavor, taste and overall acceptability (Table 4), suggesting RF treatment did not affect the overall quality of the vacuum packaged Caixin, although some differences were existed for the volatile compounds, as discussed above.
3.4. Effect of RF treatment on the sterilization of vacuum packaged Caixin The total colony number of Caixin sample without RF heating treatment was 6.80 log10 CFU/g (Fig. 4). After RF heating of 5 min, the total colony number of Caixin were
Table 3 – Volatile compounds of Caixin samples with and without RF treatment. Retention time (min)
Hydrocarbons 10.44 19.92 21.04 Alcohols 16.16 Acids 16.09 21.03 22.75 24.35 25.35 Aldehydes 7.99 13.13 Esters 10.49 10.44 26.10 Nitrile 12.96 Ketones 13.53 Anhydride 16.08 Phenols 22.75 25.62
Main components
p-Trimethylsilyloxyphenyl-bis(trimethylsilyloxy)ethane Oxime-, methoxy-phenyl3-Butoxy-1,1,1,5,5,5-hexamethyl-3-(trimethylsiloxy) trisiloxane
Fraction (%) Control
RF
0.9 ± 0.2 27.4 ± 2.1 1.0 ± 0.3
– 27.1 ± 3.3 –
1-Heptanol
0.7 ± 0.2
Acetic acid Hexanoic acid Phosphonic acid, (p-hydroxyphenyl)Nonanoic acid n-Hexadecanoic acid
2.5 ± 0.3 – – – 0.5 ± 0.1
– 1.5 ± 0.5 0.4 ± 0.1 0.7 ± 0.1 –
Hexanal Octanal
0.3 ± 0.1 –
0.7 ± 0.1 0.5 ± 0.1
4-Hydroxymandelic acid, ethyl ester, di-TMS p-Trimethylsilyloxyphenyl-(trimethylsilyloxy)trimethylsilylacrylate 1,2-Benzenedicarboxylic acid, diisooctyl ester
– – 2.0 ± 0.3
0.4 ± 0.1 0.7 ± 0.1 –
3-Pentenenitrile
0.3 ± 0.1
–
–
2-Butanone, 3-hydroxy-
–
0.4 ± 0.1
Acetic formic anhydride
–
4.4 ± 0.8
Phenol Phenol, 2,4-bis(1,1-dimethylethyl)-
0.4 ± 0.1 0.4 ± 0.1
– –
The RF treatment condition is 30 mm/20 min; for each main component, the retention time and fraction are given; “–” means “not detected”.
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food and bioproducts processing 9 5 ( 2 0 1 5 ) 47–54
Table 4 – Sensory attributes of Caixin after RF treatment (at day zero). Treatments Control 20 mm/30 min 20 mm/20 min 20 mm/10 min 20 mm/5 min 30 mm/30 min 30 mm/20 min 30 mm/10 min 30 mm/5 min 60 mm/30 min 60 mm/20 min 60 mm/10 min 60 mm/5 min
Color 7.7 7.0 7.0 7.2 7.0 7.1 7.0 7.5 7.0 6.9 7.8 7.6 7.7
± ± ± ± ± ± ± ± ± ± ± ± ±
Texture a
1.4 1.7a 1.5a 1.7a 1.5a 1.3a 1.7a 1.3a 1.5a 1.4a 1.5a 2.0a 1.7a
8.1 8.0 8.5 8.6 8.3 8.7 8.1 8.4 7.9 7.9 8.4 8.5 8.5
± ± ± ± ± ± ± ± ± ± ± ± ±
Flavor a
1.2 1.3a 1.2a 1.3a 1.6a 1.3a 1.2a 1.4a 1.5a 1.3a 1.5a 1.6a 1.9a
7.6 7.9 7.9 7.5 7.5 7.0 7.0 7.7 7.5 7.7 7.3 7.9 7.5
± ± ± ± ± ± ± ± ± ± ± ± ±
Taste a
1.6 1.4a 1.2a 1.4a 1.2a 1.2a 1.5a 1.6a 1.7a 1.7a 1.7a 1.6a 1.4a
6.1 6.0 6.5 6.5 6.8 6.0 6.1 6.7 6.5 6.0 6.9 6.5 6.5
± ± ± ± ± ± ± ± ± ± ± ± ±
Overall a
1.5 1.4a 1.7a 1.6a 1.8a 1.5a 1.6a 1.3a 1.5a 1.8a 1.8a 1.7a 1.5a
7.5 7.0 7.9 7.6 7.5 7.0 7.3 7.1 7.5 7.6 7.0 7.8 7.0
± ± ± ± ± ± ± ± ± ± ± ± ±
1.8a 1.6a 1.3a 1.4a 1.5a 1.6a 1.8a 1.4a 1.4a 1.8a 1.3a 1.5a 1.8a
Results are mean ± standard deviation; values with different superscript letters in the same column are significantly different (P < 0.05).
loss factor (ε ) of Caixin might have come from the salt addition before packaging, and the high loss factor (ε ) would have promoted the temperature increase during RF treatment. The RF treatment of 20 mm/20 min reduced the total colony number of Caixin by 3–4 log, while the physico-chemical and sensory properties were almost the same to those of the RFuntreated samples, which suggested that RF treatment at the present study has the potential to increase the food safety of vacuum packaged Caixin while not compromising of its product quality.
Acknowledgments
Fig. 4 – The total colony number of Caixin. Results are mean ± standard deviation; values with different superscript letters (a–d) indicate significantly different (P < 0.05). 5.56 log10 CFU/g, 5.77 log10 CFU/g and 6.32 log10 CFU/g with the treatment gaps between the two electrodes of 20 mm, 30 mm and 60 mm respectively, whereas the corresponding data for the 10 min treatments were 4.65 log10 CFU/g, 4.72 log10 CFU/g and 6.47 log10 CFU/g respectively (Fig. 4). The results suggested that the RF treating time and the gaps between the two electrodes affected the sterilization effect of the Caixin samples. The gap between the two plate electrodes was adjusted to change RF power to the samples, and the changes of RF power can affect sterilization. However, compared with the control sample, only the treatments of 20 mm/20 min, 20 mm/30 min, 30 mm/20 min and 30 mm/20 min, had reduced the total colony number of Caixin by a significant of 3–4 log (Table 4). Because there were no significant differences among these four treatments, the treatment of 20 mm/20 min might be the most appropriate RF treatment, in terms of energy savings and the microbial safety of the Caixin products.
4.
Conclusion
Combined with hot air drying of 60 ◦ C, the effect of radio frequency heating at 6 kW, 27.12 MHz on the microbial, physico-chemical and sensory qualities of vacuum packaged Caixin was investigated. The high dielectric constant (ε ) and
This work was financially supported by the China 863 HI-TECH Research and Development Program (No. 2011AA100802) and Jiangsu Province R&D (BY2012063). The authors acknowledge the assistance of Haitong Food Group Company for their support of this study.
References Awuah, G.B., Ramaswamy, H.S., Economides, A., Mallikarjunan, K., 2005. Inactivation of Escherichia coli K-12 and Listeria innocua in milk using radio frequency (RF) heating. Innov. Food Sci. Emerg. Technol. 6 (4), 396–402. Akbudak, N., Akbudak, B., 2013. Effect of vacuum, microwave, and convective drying on selected parsley quality. Int. J. Food Prop. 16 (1), 205–215. Birla, S.L., Wang, S., Tang, J., Tiwari, G., 2008. Characterization of radio frequency heating of fresh fruits influenced by dielectric properties. J. Food Eng. 89 (4), 390–398. Byrne, B., Lyng, J.G., Dunne, G., Bolton, D.J., 2010. Radio frequency heating of comminuted meats – considerations in relation to microbial challenge studies. Food Control 21 (2), 125–131. Delin, L., Jia, L., 2008. The development trend of flexible packaging food. South North Bridge 6, 63–64 (in Chinese). Emamifar, A., Kadivar, M., Shahedi, M., Zad, S.S., 2010. Evaluation of nanocomposite packaging containing Ag and ZnO on shelf life of fresh orange juice. Innov. Food Sci. Emerg. Technol. 11 (4), 742–748. Gaur, S., Shivhare, U.S., Ahmed, J., 2006. Degradation of chlorophyll during processing of green vegetables: a review. Stewart Postharvest Rev. 2 (5), 1–8. Geveke, D.J., Brunkhorst, C., 2008. Radio frequency electric fields inactivation of Escherichia coli in apple cider. J. Food Eng. 85 (2), 215–221. Geveke, D.J., Gurtler, J., Zhang, H.Q., 2009. Inactivation of Lactobacillus plantarum in Apple Cider, using radio frequency electric fields. J. Food Prot. 72 (3), 656–661.
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Gao, M., Tang, J., Wang, Y., Powers, J., Wang, S., 2010. Almond quality as influenced by radio frequency heat treatments for disinfestation. Postharvest Biol. Technol. 58 (3), 225–231. Gao, M., Tang, J., Rojas, R.V., Wang, Y., Wang, S., 2011. Pasteurization process development for controlling Salmonella in in-shell almonds using radio frequency energy. J. Food Eng. 104 (2), 299–306. Huang, L.L., Zhang, M., Wang, L.P., Mujumdar, A.S., Sun, D.F., 2012. Influence of combination drying methods on composition, texture, aroma and microstructure of apple slices. LWT Food Sci. Technol. 47 (1), 183–188. Ha, J.W., Kim, S.Y., Ryu, S.R., Kang, D.H., 2013. Inactivation of Salmonella enterica serovar Typhimurium and Escherichia coli O157:H7 in peanut butter cracker sandwiches by radio-frequency heating. Food Microbiol. 34 (1), 145–150. James, W.H., Rickey, Y.Y., 1996. Discoloration of coleslaw is caused by chlorophyll degradation. J. Agric. Food Chem. 44 (2), 395–398. Koskiniemi, C.B., Truong, V.D., Simunovic, J., Mcfeeters, R.F., 2011. Improvement of heating uniformity in packaged acidified vegetables pasteurized with a 915 MHz continuous microwave system. J. Food Eng. 105 (1), 149–160. Kim, S.Y., Sagong, H.G., Choi, S.H., Ryu, S., Kang, D.H., 2012. Radio-frequency heating to inactivate Salmonella Typhimurium and Escherichia coli O157:H7 on black and red pepper spice. Int. J. Food Microbiol. 153 (1–2), 171–175. Marra, F., Zhang, L., Lyng, J.G., 2009. Radio frequency treatment of foods: review of recent advances. J. Food Eng. 91 (4), 497–508. Orsat, V., Gariepy, Y., Raghavan, G.S.V., Lyew, D., 2001. Radio-frequency treatment for ready-to-eat fresh carrots. Food Res. Int. 34, 527–536. Orsat, V., Bai, L., Raghavan, G.S.V., 2004. Radio-frequency heating of ham to enhance shelf-life in vacuum packaging. J. Food Process Eng. 27, 267–283.
Orsat, V., Raghavan, G.S.V., 2005. Radio-frequency processing. Emerg. Technol. Food Process 17, 446–468. Tiwari, G., Wang, S., Birla, S.L., Tang, J., 2008. Effect of water-assisted radio frequency heat treatment on the quality of ‘Fuyu’ persimmons. Biosyst. Eng. 100 (2), 227–234. Torres, C., Maroto, M.C.D., Gutierrez, I.H., Coello, M.S.P., 2010. Effect of freeze-drying and oven-drying on volatiles and phenolics composition of grape skin. Anal. Chim. Acta 660 (1–2), 177–182. Uemura, K., Takahashi, C., Kobayashi, I., 2010. Inactivation of Bacillus subtilis spores in soybean milk by radio-frequency flash heating. J. Food Eng. 100 (4), 622–626. Ukuku, D.O., Geveke, D.J., 2010. A combined treatment of UV-light and radio frequency electric field for the inactivation of Escherichia coli K-12 in apple juice. Int. J. Food Microbiol. 138 (1–2), 50–55. Uemura, K., Takahashi, G., Kobayashi, I., 2012. Inactivation of Lactobacillus brevis in liquid egg white by radio-frequency flash heating. Food Sci. Technol. Res. 18 (3), 357–362. Wang, J., Li, Y.Z., Chen, R.R., Bao, J.Y., Yang, G.M., 2007. Comparison of volatiles of banana powder dehydrated by vacuum belt drying, freeze-drying and air-drying. Food Chem. 104 (4), 1516–1521. Wang, S., Yue, J., Chen, B., Tang, J., 2008. Treatment design of radio frequency heating based on insect control and product quality. Postharvest Biol. Technol. 49 (3), 417–423. Yin, Y., Han, Y., Liu, J., 2007. A novel protecting method for visual green color in spinach puree treated by high intensity pulsed electric fields. J. Food Eng. 79 (4), 1256–1260. Yang, G., You, M.S., Wei, H., 2004. Component and content changes of volatiles from Chinese cabbaged damaged by Plutella xylostella. Chin. J. Appl. Ecol. 15 (11), 2157–2160.
Contents lists available at ScienceDirect
Food and Bioproducts Processing journal homepage: www.elsevier.com/locate/fbp
Effect of radio frequency heating on the sterilization and product quality of vacuum packaged Caixin Qian Liu a , Min Zhang a,∗ , Baoguo Xu a , Zhongxiang Fang b , Dandan Zheng c a
State Key Laboratory of Food Science and Technology, School of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China b School of Public Health, Curtin Health Innovation Research Institute, International Institute of Agri-Food Security, Curtin University, Bentley, Western Australia 6102, Australia c Haitong Food Group Company, Zhejiang, Cixi 315300, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Radio frequency (RF) heating has the potential to be developed as a sterilization method
Received 16 February 2014
to control microbes of food products. In the present work, the effects of RF heating on the
Received in revised form 14 March
sterilization and product quality of vacuum packaged Caixin (Brassica campestris L.) were
2015
investigated. With the addition of salts and other food additives, the dielectric constant (ε )
Accepted 21 March 2015
and loss factor (ε ) of Caixin were increased, which was helpful to increase the temperature
Available online 28 March 2015
during RF heating. The results indicated that, coupled with a hot air drying of 60 ◦ C, RF
Keywords:
improve the microbial safety of Caixin whereas the physico-chemical and sensory qualities
Radio frequency heating
were not compromised.
Vacuum packaged Caixin
© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
heating at 6 kW, 27.12 MHz and 20 mm/20 min might be the most appropriate conditions to
Microbial safety Product quality Physico-chemical qualities Sensory qualities
1.
Introduction
Thermophilic or mesophilic bacteria are the main spoilage microorganisms of vacuum packaged Caixin (Brassica campestris L.), a green leafy vegetable and low acid food (Delin and Jia, 2008). Generally, there have two methods to improve the safety of the product: using commercial steam sterilization treatment (>105 ◦ C), or lowering its pH value to acid food level (pH <4.5) and pasteurizing at a temperature below 100 ◦ C. However, both the high temperature and acid condition may cause the degradation of chlorophyll which will adversely affect the sensory quality of the product, especially the natural green color (Gaur et al., 2006; James and Rickey, 1996). Therefore, there is an urgent need in the
∗
food industry to develop innovative sterilization technologies to improve the microbial safety while maintaining the quality of vacuum packaged green vegetables (Kim et al., 2012). Radio frequency (RF) heating is a technology of electromagnetic energy which is developed between electrodes at frequencies range from 1 to 300 MHz to generate heat in a dielectric material (Orsat et al., 2004). RF treatment is essentially a combination of electrical and thermal treatment (Uemura et al., 2012), and the synergistic effect has a great influence on energy efficiency (Geveke et al., 2009). Generally, an RF system is coupled with a hot air system to assist the heating up (Gao et al., 2011). Compared with conventional heating methods, the RF treatment has the advantages of
Corresponding author. Tel.: +86 510 85917089; fax: +86 510 85917089. E-mail address: [email protected] (M. Zhang). http://dx.doi.org/10.1016/j.fbp.2015.03.007 0960-3085/© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
48
food and bioproducts processing 9 5 ( 2 0 1 5 ) 47–54
saving process time and producing products with better qualities (Uemura et al., 2012), and therefore has the potential to be developed as a novel pasteurization method to control microbes in agricultural commodities (Gao et al., 2011). Conventionally, when a food is heating treated, the heat is transferred from the exterior of the food to the interior by convection or conduction (Kim et al., 2012). On the contrary, RF generates heat due to the friction of the rotational movement of the molecules and the space charge displacement in response to an alternating electric field, which offers a considerable speed advantage over conventional heating methods. Energy generated by RF heating is absorbed directly by the whole material (Orsat et al., 2001). In addition, RF heating does not require direct contact between the product and electrodes as RF waves will penetrate through the cardboard or plastic packaging (Marra et al., 2009). In a dielectric heating process, the dielectric materials have two basic characteristics that determine the amount of RF power required to raise their temperatures. The dielectric constant (ε ) represents stored energy when the material is exposed to an electric field and the loss factor (ε ) is dominated by ionic conductance that influences energy absorption and attenuation (Birla et al., 2008). The RF power absorption in any material is directly proportional to the loss factors, and different materials exposed to the same RF field would have a different heating result. From a microbial inactivation perspective, the benefit of RF heating comes from a potential selective killing effect on microorganisms or a targeted heating of the microorganisms other than a direct heating effect on the product (Orsat and Raghavan, 2005). The electric field strength, frequency, temperature and hold time (Geveke et al., 2009), as well as the dielectric properties, shape and size of samples, surrounding medium and relative distance (Birla et al., 2008) have significant effects on the results of inactivation. Orsat et al. (2001) reported that the quality of the RF treated carrot sticks was greater than either the control samples (chlorinated water) or hot-water-treated samples in terms of color, taste and microbial load. Geveke and Brunkhorst (2008) used radio frequency electric fields (RFEF) to pasteurize apple cider and the results showed that RF processing at an outlet temperature of 60 ◦ C reduced the population of Escherichia coli by 4.8 log, whereas conventional thermal processing at the same conditions had no effect. Radio frequency heating has also been conducted to pasteurize meat (Byrne et al., 2010), liquid egg white (Uemura et al., 2012), peanut butter cracker sandwiches (Ha et al., 2013), milk (Awuah et al., 2005), soybean milk (Uemura et al., 2010) and ‘Fuyu’ persimmons (Tiwari et al., 2008). However, there are few reports on utilizing RF heating on pasteurization of green leafy vegetables, especially its effect on the green color of the products. The objective of this study was to evaluate the potential of RF heating on pasteurization of vacuum packaged Caixin. The effects of RF treatment on the product microbial, physico-chemical and sensory qualities were assessed and discussed, which would be useful to apply this technology in green leaf vegetable processing.
2.
Materials and methods
2.1. Determination of dielectric properties of vacuum packaged Caixin Fresh Caixin was purchased from a local market in Wuxi city, China. The samples were washed, blanched in a water bath (95 ◦ C, 2 min) and cooled in ice water (about 0 ◦ C) for 5 min
to eliminate enzyme activity (Orsat et al., 2001). The samples were mixed with 5 g kg−1 (W/W) sodium alginate, 4 g kg−1 calcium chloride and 2 g kg−1 of potato starch to preserve the crisp texture, and 50 g kg−1 sodium chloride, 100 g kg−1 edible glycerol and 4 g kg−1 sodium carboxymethyl cellulose were added to protect the color. After these treatments, products were vacuum packed (50 g/package) in polyamide/cast polypropylene (PA/CPP) plastic bags and kept in a fridge (4–5 ◦ C) for further use. Dielectric constants (ε ) and loss factors (ε ) of the packaged Caixin samples were measured between 20 MHz and 40 MHz using a network analyzer (E5062A, Agilent Technologies, Santa Clara, CA, USA) (Koskiniemi et al., 2011). The measurement system was consisted of an open ended dielectric probe connected with an impedance analyzer which is interfaced with a personal computer. Prior to the measurement, the system was calibrated according to the standard procedure (Birla et al., 2008). The Caixin sample was homogenized to pulp with a JYL-C020 Waring blender (Joyoung Co. Ltd., Jinan, China) in order to maintain proper contact with the probe. In addition, the dielectric properties of the texture improvement agents (sodium alginate, calcium chloride, potato starch), color protecting agents (sodium chloride, edible glycerol and sodium carboxymethyl cellulose) and raw Caixin before the blanching treatment were also measured. Each measurement was repeated three times and results were reported as mean values.
2.2.
Radio frequency (RF) and hot air heating systems
A 6 kW, 27.12 MHz RF system (Strayfield International, Wokingham, UK) coupled with a hot air system was used to treat the Caixin samples (Fig. 1). The hot air flow was forced from the holes on the bottom electrode to the samples through perforated screens on the side and bottom walls of a plastic container, with a temperature of 60 ◦ C and speed of 3 m s−1 (Gao et al., 2010). An alternating electric field was developed between electrodes where the Caixin samples were placed. The gap between the two plate electrodes was adjusted to change RF power to the samples (Gao et al., 2011; Ukuku and Geveke, 2010). To determine the appropriate gap between the electrodes for RF treatments, three gaps, namely 20 mm, 30 mm and 60 mm, were tested (Gao et al., 2010). Treatments of 5 min, 10 min, 20 min and 30 min RF exposure were conducted to evaluate the efficacy of pasteurization on Caixin samples (Ha et al., 2013). Samples with the 60 ◦ C hot air heating but without RF treatment were used as control.
2.3. Temperature distributions after radio frequency (RF) heating The temperature distributions of vacuum packaged Caixin after RF treatments were examined by an infrared thermal imaging camera (IRI 4010 Multi-Purpose Imager, IRISYS, UK). The thermal image showing the temperature distributions reflects the heating uniformity. Each measurement was taken about 3 s (Wang et al., 2008). The highest and lowest temperatures in the images were labeled by the software of the infrared thermal imaging camera. In addition, the sample temperatures at five different positions in the container of the RF system were measured using an infrared thermometer (ST18, Raytek Company, USA).
food and bioproducts processing 9 5 ( 2 0 1 5 ) 47–54
49
Fig. 1 – Schematic diagram of the radio frequency (27.12 MHz, 6 kW) heating system. 1. Bottom electrode; 2. Upper electrode; 3. Control panel; 4. View point (a and b); 5. Florescent tube light; 6. Heater (a and b) and air blower; 7. Keys for the RF system and doors; 8. Air outlet; 9. Door for the drying chamber; 10. Conveyor belt; 11. Hot air distribution chamber.
2.4.
Electrical conductivity measurements
calculated from chlorophyll a (Ca , mg/L) and chlorophyll b (Cb , mg/L) values with the following equations:
The control and RF treated Caixin samples were homogenized to pulp, and the electrical conductivity was measured by a Conductivity Meter (DS-11AT, Precision Instrument Co. Ltd., Shanghai, China). The values were recorded as the mean of triplicate measurements.
2.5.
2.6.
(2)
Cb = 24.96A649 − 7.32A665
(3)
C = Ca + Cb
(4)
Color measurements
Color properties were a significant factor to evaluate the quality of green leafy vegetables (Akbudak and Akbudak, 2013). The color properties of Caixin were determined using a Minolta CR 400 colorimeter (Konica-Minolta, Osaka, Japan). Calibration was performed using a standard white tile. L* (value on the white/black axis), a* (value on the red/green axis) and b* (value on the yellow/blue axis) of each sample were measured 10 times. Since the major color of leafy vegetable is green, ‘−a*’ values were considered as the visual parameter to describe the green color change (Yin et al., 2007). The chrominance C* is known as saturation: C∗ =
Ca = 13.95A665 − 6.88A649
a∗2 + b∗2
(1)
Chlorophyll content
The extraction of chlorophyll was a modification of the method of James and Rickey (1996). Briefly, sample of 50 g was homogenized to pulp and 0.2 g sample was mixed with 15 mL 80% aqueous acetone (v/v). When the color of the mixture solution turned white, it was filtered into a 25 mL brown bottle using a qualitative filter paper, and then adjusted the volume to 25 mL with the 80% acetone. The absorbance of the filtered solutions were measured at 665 and 649 nm with an UV 2600 spectrophotometer (Techcomp Instrument Co. Ltd., Shanghai, China). The chlorophyll concentration (C, mg/L) was
The chlorophyll content (mg/g) =
C × extraction volume × dilution factor . sample weight
(5)
2.7. Gas chromatography–mass spectrometry (GC–MS) analysis The volatiles of Caixin samples were isolated by solid phase micro-extraction (SPME), which is widely used to collect and concentrate the aroma fraction of fruit, vegetables, beverages, etc. (Wang et al., 2007). Caixin was homogenized to pulp and 6 g was placed into a 15 mL headspace vial. The sample was then extracted on a 75 m Carboxen/Polydimethyl siloxane (CAR/PDMS, Supelco, USA) fiber column at 50 ◦ C for 30 min and then eluted at 250 ◦ C for 3 min. The GC–MS analyses of the volatile compounds of Caixin samples were carried out by a Trace MS (Finnigan, USA). The GC conditions were as follows (Torres et al., 2010): capillary column: DB-WAX (30 m × 0.25 mm, 0.25 m; Supelco, USA); carrier gas: helium with a flow rate of 0.8 mL/min; injection temperature: 250 ◦ C; the oven temperature was programmed from an initial temperature of 40 ◦ C (4 min hold), increased to 80 ◦ C at 6 ◦ C/min, then to 230 ◦ C at 10 ◦ C/min, and held isothermal for 7 min. The MS was operated with ionization of 70 eV, emission current of 200 mA, ion source temp of 200 ◦ C, scan range of 33–350 m/z and detector voltage of 350 V.
50
food and bioproducts processing 9 5 ( 2 0 1 5 ) 47–54
Table 1 – Dielectric properties of materials used in the RF treatment of vacuum packaged Caixin. Materials
Dielectric constants (ε )
Deionized water Sodium alginate (5 g kg−1 ) Calcium chloride (4 g kg−1 ) Potato starch (2 g kg−1 ) Sodium carboxymethyl cellulose (4 g kg−1 ) Edible glycerol (100 g kg−1 ) Sodium chloride (50 g kg−1 , 100-fold dilute) Raw Caixin Vacuum packaged Caixin (100-fold dilute)
79.2 86.5 87.5 77.8 77.7 79.9 83.6 77.6 83.1
± ± ± ± ± ± ± ± ±
0.7e 2.1bc 1.8b 1.7e 2.9e 0.4e 1.8cd 3.1e 0.8d
Loss factors (ε ) 0.2 101.2 859.4 −1.5 75.8 −1.9 66.4 718.9 43.3
± ± ± ± ± ± ± ± ±
0.4g 4.7d 4.5b 0.5g 2.9e 0.4g 1.8e 19.9c 4.1f
The instrument has the limitations for NaCl and Caixin; samples were measured between 20 MHz and 40 MHz; results are mean ± standard deviation; values with different superscript letters in the same column are significantly different (P < 0.05).
The identification of the volatile compounds was based on the GC retention times and mass spectra (Torres et al., 2010). All compounds were identified by comparison to NIST library (including Wiley and Mainlib) spectral data bank. Only compounds whose similarity is more than 800 (the maximum similarity is 1000) were reported (Wang et al., 2007).
2.8.
Standard plate count (SPC)
Standard plate count was conducted to determine the level of bacteria in Caixin samples after RF treatment as described by Chinese National Standard (GB 4789.2-2010). About 25 g Caixin samples were taken with sterile scissors and nippers, diluted in 225 mL physiological saline solution (PS, 8.7 g kg−1 NaCl, pH 7.0) and homogenized in a homogenizer (10,000 rpm) for 1 min to achieve a 10-fold dilution. One milliliter was then put into 9 mL PS solution to get 100-fold dilution. Similarly, 1000-fold dilution was prepared. One milliliter of each different dilution was pipetted into a sterile Petri dish. To each Petri dish about 15 mL of nutrient agar (46 ◦ C) was added, and after solidification they were put into an incubator (36 ± 1 ◦ C) to culture for 48 ± 2 h and the total bacteria colonies were counted. The real SPC was the observed value multiplied by dilution, and the results were expressed as log10 CFU/g. Both the control and RF treated samples were determined at a 1-day interval from the day of treatment. Determinations were replicated three times for each sample.
2.10.
3.
Results and discussion
3.1. Dielectric properties and electrical conductivity of Caixin
Sensory evaluation
Sensory evaluation was carried out to assess the specific organoleptic attribute (color, texture, flavor, taste, and overall acceptability) variation over RF-treated Caixin samples. The panelists were consisted of a taste panel of 10 members who had previous experience of purchasing and/or consuming this product, with the age range of 23–27 and gender ratio of 1:1 (Emamifar et al., 2010). The scores were obtained by grading the sensory attributes using the following 9-point hedonic scales: 9 = extremely good, 8 = very good, 7 = good, 6 = below good/above fair, 5 = fair, 4 = below fair/above poor, 3 = poor, 2 = very poor, 1 = extremely poor. All the prepared samples coded with three-digit random numbers were presented in a random order to the panelists (Ha et al., 2013).
2.9.
tests using SPSS 15.0.1 (SPSS Inc., Chicago, IL, USA). Average values were considered significantly different when P < 0.05.
Statistics
Differences among average values were estimated using analysis of variance (ANOVA) with the application of Duncan’s
The dielectric properties of the raw Caixin, texture improvement agents (sodium alginate, calcium chloride and potato starch), color protecting agents (sodium chloride, edible glycerol and sodium carboxymethyl cellulose) and vacuum packaged Caixin after 27.12 MHz RF treatment are summarized in Table 1. The dielectric constant (ε ) and the loss factor (ε ) of vacuum packaged Caixin were both too high (9999) to be measured accurately. Therefore, it was diluted with deionized water to 100-fold and the ε and ε were 83.14 ± 0.81 and 43.27 ± 4.10 respectively. The ε and ε of raw Caixin were 77.61 ± 3.10 and 718.91 ± 19.94 respectively, which were significantly lower (P < 0.01) than that of vacuum packaged Caixin. Table 1 suggested that the high dielectric constant (ε ) and loss factor (ε ) of vacuum packaged Caixin might come from the salt and other agents added in the package. Similar results were also observed by Marra et al. (2009) and Birla et al. (2008) and they suggested that it is mainly the addition of salts that have increased the ionic conductivity and the loss factor of materials. The high ε and ε of vacuum packaged Caixin might be helpful to the temperature increment during the RF process. Fig. 2 shows that the electrical conductivity of vacuum packaged Caixin after RF heating also increased significantly. This might be because the RF treatment promoted the elution of soluble components and ions from the vegetable samples.
3.2.
Temperature distributions after RF treatments
Measured by the infrared thermometer, the initial temperature of Caixin samples was 25.8 ◦ C, which increased to 60 ◦ C after the RF heating treatment of 10–15 min combined with hot air. The heating rate was at an order of about 4 ◦ C/min and it took about 5 min for the samples to reach 45 ◦ C (data not shown). The temperature distributions of vacuum packaged Caixin after the RF treatments are shown in Fig. 3. The thermal images showing the temperature distributions could be used to rapidly determine the heating uniformity. The heating uniformity was better after a heating time of 20–30 min than after 5–10 min (Fig. 3), but no significant difference was detected between the treatment of 20 min and 30 min. Gao et al. (2011) suggested that the RF sample temperature could be maintained at a fairly constant value after a certain time,
food and bioproducts processing 9 5 ( 2 0 1 5 ) 47–54
Fig. 2 – Electrical conductivity (mean ± standard deviation) of Caixin (n = 3). as the absorbed RF power was balanced by the latent heat of water evaporation.
3.3. Effects of different RF treatments on the product quality of Caixin Color values of Caixin after RF treatment are summarized in Table 2. The L*, a* and b* values of RF treated Caixin were measured at random locations of the products. L* values had a slight declining trend after RF heating treatments, indicating a little darker of the samples after treatment. The a* and C* values of RF treated Caixin were not significantly different to those of non-treated samples except for the treatments of 30 mm/5 min, 60 mm/30 min and 60 mm/5 min. The −a* and C* values of Caixin with the treatments of 30 mm/5 min and 60 mm/5 min were significantly lower than others, whereas with the treatment of 60 mm/30 min were significantly higher. However, significant differences were not detected on the property of color among these Caixin samples by the sensory
51
evaluation. Similar results were also reported by Ha et al. (2013), where color values and sensory characteristics of the RF treated peanut butter and crackers were not significantly (P > 0.05) different from the control. The chlorophyll contents of Caixin samples were also measured, and the results showed that the chlorophyll contents of RF treated and untreated Caixin samples had no significant (P > 0.05) differences (Table 2), which suggested that the RF treatment have no negative effect on the green color of the product. To the best of our knowledge, this is the first report of RF treatment on the color quality and chlorophyll content of green leafy vegetables. The GC–MS analysis showed that the volatile compounds of RF treated and untreated Caixin samples had some differences. In the RF untreated Caixin (Table 3), 27 compounds were identified. Among these volatile compounds, hydrocarbons were the dominant (alkane 18.71%; aromatic hydrocarbon 27.44%), alcohols were the second (13.54%), and then were the acids (11.09%). Ester, aldehyde, ketone, phenol and nitrile were only existed in a small amount. However, 26 compounds were identified in the RF treated Caixin (30 mm/20 min), where hydrocarbons were also the dominant (alkane 13.22%; aromatic hydrocarbon 27.08%), but acids were the second (16.06%), and then was the alcohols (9.47%), and anhydride, aldehyde, ester and ketone were existed with a small amount (Table 3). The results indicated that aromatic hydrocarbon of oxime-, methoxy-phenyl- was the major volatile compound in RF treated (27.08%) and untreated samples (27.44%) and no significant differences were observed between the two samples. However, after RF treatment, the content of octanoic acid was significantly increased (untreated sample 8.08%, RF heating sample 13.48%). It was proposed that aldehydes (hexanal, 2-hexenal) are contributors to the green odor of vegetables (Huang et al., 2012). After RF treatment, the content of hexanal was significantly increased (untreated sample 0.34%, RF heating sample 0.67%) which implied that the RF treatment may have improved the green odor. However, because its relative content is very low, low relative content does not necessarily give low odor. In addition, compared with the control, anhydrides of acetic formic anhydride were appeared after
Fig. 3 – The temperature distributions of vacuum packaged Caixin after RF heating treatments (a1 , 20 mm/30 min; b1 , 20 mm/20 min; c1 , 20 mm/10 min; d1 , 20 mm/5 min; a2 , 30 mm/30 min; b2 , 30 mm/20 min; c2 , 30 mm/10 min; d2 , 30 mm/5 min; a3 , 60 mm/30 min; b3 , 60 mm/20 min; c3 , 60 mm/10 min; d3 , 60 mm/5 min).
52
food and bioproducts processing 9 5 ( 2 0 1 5 ) 47–54
Table 2 – Color parameters and chlorophyll content of Caixin after RF heating. Treatments Control 20 mm/30 min 30 mm/30 min 60 mm/30 min 20 mm/20 min 30 mm/20 min 60 mm/20 min 20 mm/10 min 30 mm/10 min 60 mm/10 min 20 mm/5 min 30 mm/5 min 60 mm/5 min
−a*
L* 33.7 29.7 29.1 30.8 28.0 31.0 29.8 30.2 29.5 32.2 30.4 30.1 32.8
± ± ± ± ± ± ± ± ± ± ± ± ±
a
0.4 1.2de 1.3de 1.6cd 0.5e 0.6bcd 0.6de 2.3cde 1.3de 1.9abc 0.5cd 0.5cde 0.6ab
8.6 8.6 9.0 10.0 8.6 8.1 8.4 8.8 8.4 8.8 8.4 8.0 7.4
± ± ± ± ± ± ± ± ± ± ± ± ±
C* bcd
0.2 0.3bcd 0.1b 0.3a 0.2bcd 0.2cd 0.4bcd 0.2bc 0.4bcd 0.4bc 0.7bcd 0.6de 0.1e
11.5 11.8 13.6 13.7 13.0 12.7 11.9 12.4 11.8 13.9 11.6 11.4 10.0
± ± ± ± ± ± ± ± ± ± ± ± ±
Chlorophyll (mg/g) bc
0.5 0.4bc 0.1a 0.7a 0.5ab 1.3abc 1.0bc 0.4abc 0.7bc 1.3a 0.8bc 1.4cd 0.3d
1.5 1.4 1.5 1.5 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.3 1.3
± ± ± ± ± ± ± ± ± ± ± ± ±
0.2a 0.1a 0.2a 0.2a 0.2a 0.1a 0.1a 0.2a 0.2a 0.2a 0.1a 0.1a 0.1a
Results are mean ± standard deviation; values with different superscript letters in the same column are significantly different (P < 0.05).
RF heating but phenol and nitriles were not detected. The analysis of volatile compounds indicated that RF treatment have some influence on the volatile compounds of Caixin samples, although the mechanisms are unknown. To date, no information is available about the volatile compounds of Caixin. Chinese cabbage (Brassicachinensis L.) is a close relative of Caixin (Brassica campestris L.). Yang et al. (2004) reported that in Chinese cabbage, saturated hydrocarbons were the dominant volatile compounds, aromatic hydrocarbons were second, and unsaturated hydrocarbons, aldehydes, alcohols, ketones, acids and heteroaromatic compounds were existed in a small amount. The relatively high content of aromatic hydrocarbon (methoxy-phenyl-) and octanoic acid in Caixin might account for the major odor differences to Chinese cabbage.
Moreover, the sensory evaluation by the panelists indicated that no significant differences were among the RF treated and untreated Caixin samples on the properties of color, texture, flavor, taste and overall acceptability (Table 4), suggesting RF treatment did not affect the overall quality of the vacuum packaged Caixin, although some differences were existed for the volatile compounds, as discussed above.
3.4. Effect of RF treatment on the sterilization of vacuum packaged Caixin The total colony number of Caixin sample without RF heating treatment was 6.80 log10 CFU/g (Fig. 4). After RF heating of 5 min, the total colony number of Caixin were
Table 3 – Volatile compounds of Caixin samples with and without RF treatment. Retention time (min)
Hydrocarbons 10.44 19.92 21.04 Alcohols 16.16 Acids 16.09 21.03 22.75 24.35 25.35 Aldehydes 7.99 13.13 Esters 10.49 10.44 26.10 Nitrile 12.96 Ketones 13.53 Anhydride 16.08 Phenols 22.75 25.62
Main components
p-Trimethylsilyloxyphenyl-bis(trimethylsilyloxy)ethane Oxime-, methoxy-phenyl3-Butoxy-1,1,1,5,5,5-hexamethyl-3-(trimethylsiloxy) trisiloxane
Fraction (%) Control
RF
0.9 ± 0.2 27.4 ± 2.1 1.0 ± 0.3
– 27.1 ± 3.3 –
1-Heptanol
0.7 ± 0.2
Acetic acid Hexanoic acid Phosphonic acid, (p-hydroxyphenyl)Nonanoic acid n-Hexadecanoic acid
2.5 ± 0.3 – – – 0.5 ± 0.1
– 1.5 ± 0.5 0.4 ± 0.1 0.7 ± 0.1 –
Hexanal Octanal
0.3 ± 0.1 –
0.7 ± 0.1 0.5 ± 0.1
4-Hydroxymandelic acid, ethyl ester, di-TMS p-Trimethylsilyloxyphenyl-(trimethylsilyloxy)trimethylsilylacrylate 1,2-Benzenedicarboxylic acid, diisooctyl ester
– – 2.0 ± 0.3
0.4 ± 0.1 0.7 ± 0.1 –
3-Pentenenitrile
0.3 ± 0.1
–
–
2-Butanone, 3-hydroxy-
–
0.4 ± 0.1
Acetic formic anhydride
–
4.4 ± 0.8
Phenol Phenol, 2,4-bis(1,1-dimethylethyl)-
0.4 ± 0.1 0.4 ± 0.1
– –
The RF treatment condition is 30 mm/20 min; for each main component, the retention time and fraction are given; “–” means “not detected”.
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food and bioproducts processing 9 5 ( 2 0 1 5 ) 47–54
Table 4 – Sensory attributes of Caixin after RF treatment (at day zero). Treatments Control 20 mm/30 min 20 mm/20 min 20 mm/10 min 20 mm/5 min 30 mm/30 min 30 mm/20 min 30 mm/10 min 30 mm/5 min 60 mm/30 min 60 mm/20 min 60 mm/10 min 60 mm/5 min
Color 7.7 7.0 7.0 7.2 7.0 7.1 7.0 7.5 7.0 6.9 7.8 7.6 7.7
± ± ± ± ± ± ± ± ± ± ± ± ±
Texture a
1.4 1.7a 1.5a 1.7a 1.5a 1.3a 1.7a 1.3a 1.5a 1.4a 1.5a 2.0a 1.7a
8.1 8.0 8.5 8.6 8.3 8.7 8.1 8.4 7.9 7.9 8.4 8.5 8.5
± ± ± ± ± ± ± ± ± ± ± ± ±
Flavor a
1.2 1.3a 1.2a 1.3a 1.6a 1.3a 1.2a 1.4a 1.5a 1.3a 1.5a 1.6a 1.9a
7.6 7.9 7.9 7.5 7.5 7.0 7.0 7.7 7.5 7.7 7.3 7.9 7.5
± ± ± ± ± ± ± ± ± ± ± ± ±
Taste a
1.6 1.4a 1.2a 1.4a 1.2a 1.2a 1.5a 1.6a 1.7a 1.7a 1.7a 1.6a 1.4a
6.1 6.0 6.5 6.5 6.8 6.0 6.1 6.7 6.5 6.0 6.9 6.5 6.5
± ± ± ± ± ± ± ± ± ± ± ± ±
Overall a
1.5 1.4a 1.7a 1.6a 1.8a 1.5a 1.6a 1.3a 1.5a 1.8a 1.8a 1.7a 1.5a
7.5 7.0 7.9 7.6 7.5 7.0 7.3 7.1 7.5 7.6 7.0 7.8 7.0
± ± ± ± ± ± ± ± ± ± ± ± ±
1.8a 1.6a 1.3a 1.4a 1.5a 1.6a 1.8a 1.4a 1.4a 1.8a 1.3a 1.5a 1.8a
Results are mean ± standard deviation; values with different superscript letters in the same column are significantly different (P < 0.05).
loss factor (ε ) of Caixin might have come from the salt addition before packaging, and the high loss factor (ε ) would have promoted the temperature increase during RF treatment. The RF treatment of 20 mm/20 min reduced the total colony number of Caixin by 3–4 log, while the physico-chemical and sensory properties were almost the same to those of the RFuntreated samples, which suggested that RF treatment at the present study has the potential to increase the food safety of vacuum packaged Caixin while not compromising of its product quality.
Acknowledgments
Fig. 4 – The total colony number of Caixin. Results are mean ± standard deviation; values with different superscript letters (a–d) indicate significantly different (P < 0.05). 5.56 log10 CFU/g, 5.77 log10 CFU/g and 6.32 log10 CFU/g with the treatment gaps between the two electrodes of 20 mm, 30 mm and 60 mm respectively, whereas the corresponding data for the 10 min treatments were 4.65 log10 CFU/g, 4.72 log10 CFU/g and 6.47 log10 CFU/g respectively (Fig. 4). The results suggested that the RF treating time and the gaps between the two electrodes affected the sterilization effect of the Caixin samples. The gap between the two plate electrodes was adjusted to change RF power to the samples, and the changes of RF power can affect sterilization. However, compared with the control sample, only the treatments of 20 mm/20 min, 20 mm/30 min, 30 mm/20 min and 30 mm/20 min, had reduced the total colony number of Caixin by a significant of 3–4 log (Table 4). Because there were no significant differences among these four treatments, the treatment of 20 mm/20 min might be the most appropriate RF treatment, in terms of energy savings and the microbial safety of the Caixin products.
4.
Conclusion
Combined with hot air drying of 60 ◦ C, the effect of radio frequency heating at 6 kW, 27.12 MHz on the microbial, physico-chemical and sensory qualities of vacuum packaged Caixin was investigated. The high dielectric constant (ε ) and
This work was financially supported by the China 863 HI-TECH Research and Development Program (No. 2011AA100802) and Jiangsu Province R&D (BY2012063). The authors acknowledge the assistance of Haitong Food Group Company for their support of this study.
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