Peeling of tomatoes using novel infrared radiation heating technology

Innovative Food Science and Emerging Technologies 21 (2014) 123–130

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Peeling of tomatoes using novel infrared radiation heating technology Xuan Li a, Zhongli Pan a,b,⁎, Griffiths G. Atungulu a, Xia Zheng a,c, Delilah Wood b, Michael Delwiche a, Tara H. McHugh b a b c

Department of Biological and Agricultural Engineering, University of California, Davis, Davis, CA 95616, USA Western Regional Research Center, USDA Agricultural Research Service, Albany, CA 94710, USA College of Mechanical and Electrical Engineering, Shihezi University, Xinjiang, 832003, China

a r t i c l e

i n f o

Article history: Received 18 April 2013 Accepted 23 October 2013 Editor Proof Receive Date 21 November 2013 Keywords: Infrared radiation Tomato peeling Textural analysis Scanning electron microscopy Physical attributes Biomechanical properties

a b s t r a c t The effectiveness of using infrared (IR) dry-peeling as an alternative process for peeling tomatoes without lye and water was studied. Compared to conventional lye peeling, IR dry-peeling using 30 s to 75 s heating time resulted in lower peeling loss (8.3%–13.2% vs. 12.9%–15.8%), thinner thickness of peeled-off skin (0.39–0.91 mm vs. 0.38– 1.06 mm), and slightly firmer texture of peeled products (10.30–19.72 N vs. 9.42–13.73 N) while achieving a similar ease of peeling. IR heating increased the Young's Modulus of tomato peels and reduced the peel adhesiveness, indicating the tomato peels to loosen, become brittle, and crack more easily. Also, IR heating resulted in melting of cuticular membrane, collapse of several cellular layers, and severe degradation of cell wall structures, which in turn caused peel separation. These findings demonstrated the effectiveness of the novel IR dry-peeling process for tomatoes. Industrial relevance: Development of a sustainable and non-chemical peeling technique for food processing industry is urgent. Currently, industrialized peeling methods such as hot lye or steam peeling are water- and energy-intensive operation and result in a large amount of waste effluent. Disposal of these wastewater containing high salinity and organic solids poses negative environmental footprints. Tomato processors have long been interested in pursuing a sustainable and non7 chemical peeling alternative in order to minimize waste effluent containing high salinity and organic loads and reduce the negative environmental impacts associated with conventional hot lye peeling. The emerging infrared dry-peeling technique offers a novel approach to eliminate the usage of chemicals and water in the peeling process while maintaining high quality peeled products. The study explored several crucial and fundamental aspects of developing infrared radiation heating technology as a sustainable tomato peeling method. The findings of this research provide scientific evidence of the benefits of infrared dry-peeling in comparison to the conventional hot lye peeling and have been used for the development of a pilot scale tomato infrared dry-peeling system. Published by Elsevier Ltd.

1. Introduction Peeling is widely used in the food processing industry to produce premium quality canned fruits and vegetables. The conventional peeling process applies hot lye or steam for peel removal and is an energyand water-intensive operation. Particularly, the hot lye peeling using sodium hydroxide or potassium hydroxide solution results in a significant amount of peeling effluent discharges containing high salinity and organic solids. Disposal of the wastewater and threat to long-term water supply associated with lye peeling have become serious concerns to the tomato processors. To minimize the chemical contamination and negative environmental impacts, steam peeling has been adopted by food processors as an alternative peeling technique. However, steam ⁎ Corresponding author at: Department of Biological and Agricultural Engineering, University of California, Davis, Davis, CA 95616, USA. Tel.: +1 510 559 5861; fax: +1 510 559 5851. E-mail addresses: [email protected], [email protected] (Z. Pan). 1466-8564/$ – see front matter. Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.ifset.2013.10.011

peeling produces inferior products with deteriorated peeling appearance, high loss in firmness, and reduced peeling yields compared to conventional hot lye peeling. Therefore, there is an urgent need to develop sustainable and cost-effective peeling alternatives which can reduce water usage and wastewater generation while producing high quality peeled products without using lye and steam (Li et al., 2014; Pan, Li, Bingol, McHugh, & Atungulu, 2009; Rock, Yang, Goodrich-Schneider, & Feng, 2011). To address the critical challenges, different alternative peeling technologies have been considered and studied, such as enzymatic peeling, ohmic peeling, and ultrasonic peeling (Baker & Wicker, 1996; Li, 2012; Rock et al., 2011; Wongsa-Ngasri, 2004). However, industrialization of these technologies has been limited due to the high cost and low throughputs. Infrared (IR) radiation heating has a rapid surface heating characteristic and allows heating only a shallow layer of tomato surface while maintaining the edible flesh portion intact. The feasibility of IR radiation heating as an effective non-chemical method for tomato peel removal has been investigated through different peeling approaches,

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including IR peeling, combined lye-IR peeling, enzyme peeling, and enzymatic pretreated IR peeling (Li et al., 2009). Among all approaches investigated, IR peeling yielded the best peeling results. The peeling performance and quality of IR peeled tomatoes were comparable to those from conventional hot lye peeling (Li et al., 2009; Pan et al., 2009). Because neither chemicals nor water was required during IR heating, it was named as IR dry-peeling (Li, 2012; Pan et al., 2009). In general, a sustainable peeling method should be to minimize product loss, quality change, water and energy consumption, and pollution loads (Setty, Vijayalakshimi, & Devi, 1993). Accordingly, different criteria have been developed to evaluate the peeling process for different purposes (Barrett, 2000; Garcia & Barrett, 2006a; Li et al., 2014; Milczarek, 2009). For example, the United States Food and Drug Administration (FDA) launched a specific regulation of tomato peelability, requiring that the amount of non-removed peel must be less than 0.015 cm2/g (FDA standard, 21CFR 155.190). Other commonly used criteria to evaluate peeling performance in literatures are peeling yield and peeling loss calculated based on the weight changes of tomatoes before and after peeling (Das & Barringer, 2005; Garcia & Barrett, 2006b; Schlimme, Corey, & Frey, 1984). It is worth mentioning that applying individual criteria in peeling evaluation has certain limitations. For instance, when only peelability is adopted to compare the efficacy of different peeling conditions, details about the ease of peel and peeling loss are not reflected in the index of peelability. In practice, commercial tomato processors are interested not only in the peelability and peeling yield, but also in the quality of the peeled fruit, particularly the firmness (Garcia, Watnik, & Barrett, 2006; Milczarek & McCarthy, 2011). Hence, a metric that can facilitate characterizations of different peeling performance and product quality is vital to precisely evaluate a peeling process. Relatively few works have been dedicated to comprehensive evaluations of the peeling process for fruits and vegetables. To fully develop IR heating into a sustainable dry-peeling method, in the present study we performed an in-depth characterization of tomatoes peeled using IR. Complete evaluation of IR dry-peeling for tomatoes was conducted from five perspectives, including an assessment of physical attributes of tomatoes from different cultivars, peeling performance, peeled product quality, biomechanical properties of skins, and anatomical and morphological features of skin tissue. Such information is critical for better understanding of the IR dry-peeling process and provides guidance for further scale-up towards industrial application. Our ultimate goal is to develop a new and sustainable peeling technology by using IR heating for tomatoes. The specific objectives of this study were to 1) compare the impact of IR and lye peeling on various metrics of peeling performance and product quality for tomatoes of two cultivars; and 2) characterize the biomechanical and anatomical features within skins and adjacent tissues of tomatoes subjected to IR heating.

Prior to peeling, tomatoes of each cultivar were randomly selected and evaluated for a number of physical attributes. For each fruit, tomato mass was measured with an electronic balance with 0.1 g sensitivity. Tomato dimensional attributes related to peelability were selected and measured according to Garcia and Barrett (2006a). As illustrated in Fig. 1, fruit height, maximum lateral diameter, height of maximum lateral diameter, shoulder height, and stem scar diameter were measured on whole tomatoes, while pericarp wall thickness and red layer thickness were measured on cut fruits at three locations: equator side, stem end, and blossom end. All dimensional measurements were performed using a Vernier digital caliper with an accuracy of 0.1 mm. During the 2009 season, 197 and 79 fruits were evaluated for cultivars AB2 and CXD179, respectively. Tomatoes with uniform shape and size were selected for peeling evaluations based on measurements of the physical attributes. They weighed between 60 and 110 g and possessed similar size (60 ± 6 mm in height and 49 ± 6 mm in width). The data of physical attributes of raw tomatoes were compared by using student t-test at a 0.05 probability level for the two cultivars. Correlations between dimensional attributes and peeling loss or peeled-off thickness were examined with Pearson correlation coefficient. 2.2. Tomato peeling procedure Prior to peeling, raw fruits were allowed to equilibrate to room temperature (23 °C) for at least 2 h, which ensured that all fruits were of the same initial temperature. For IR peeling, tomatoes were heated using double-sided IR (Fig. 2) with an optimal distance between two emitters of 90 ± 2 mm (Pan et al., 2009). To improve heating uniformity, the tomato was rotated continuously at a speed of 1 rpm by means of a motor driven turntable (Fig. 2). As a comparison, regular lye peeling conducted on a laboratory scale was used as a control. The procedures of lye peeling tomatoes were used as described by Pan et al. (2009). After IR and lye peeling, the loosened skins were manually removed for further evaluations. 2.3. Peeling performance

2. Materials and methods

Comparison of peeling outcomes for IR and lye peeling was based on peeling performance and peeled product quality. Peeling performance which involved determination of peelability, ease of peeling, peeling loss, and peeled-off thickness was comprehensively evaluated using procedures described by Pan et al. (2009). In an attempt to minimize possible subjective bias, randomized double-blind experiments were conducted in scoring the ease of peeling. A higher value of the ease of peeling indicates that the skin is easier to remove. Peeled-off thickness was measured and studied to determine the differences in peeling mechanism between IR and lye peeling methods. Thickness of peeled skin was measured at three locations with a micrometer (model Central Tech 895, Microprecision Calibration Inc., Grass Valley, Cal., USA) with an accuracy of 0.001 mm, and the average value for each fruit was used in further statistical analysis.

2.1. Raw material characteristics

2.4. Product quality evaluation

Two processing tomato cultivars, AB2 and Campbell CXD 179, were acquired from local commercial fields (Campbell Seeds Co., Woodland, Cal., and ConAgra Foods Co., Woodland, Cal.) during the peak season of 2009. Random fruits were hand harvested at red maturity stage (i.e., USDA tomato classification 6) with an average total soluble solid content of 5.4 ± 0.2 °Brix and average firmness of 26.3 ± 6.5 N. Harvested tomatoes were sorted to exclude fruits with disease and visual defects, and stored at 10 °C and 80% relative humidity in a cooler according to the method of Kader (2002). Depending on the harvesting date and experimental schedule, different batches of harvested tomatoes were used for different sets of tests. All tomatoes were processed within four days after harvest.

Quality assessment of peeled tomatoes was quantified based on peeled tomato firmness that was determined by a flat-plate compression test (Cantwell, 2006) by using a texture analyzer (model TA-XT2i, Texture Technologies Corp., Scarsdale, N.Y., USA) equipped with a 19.62 N load cell. Tomatoes were placed horizontally on a three-point based stand (ASABE Standards, 2001; Slaughter, Crisosto, Hasey, & Thompson, 2006), and a 50 mm diameter probe with flat surface was used to compress the equator of the whole peeled tomato to a depth of 5 mm at 5 mm/s testing speed (Cantwell, 2006; Li et al., 2009). The average peak force of ten tomatoes is reported for each treatment. In addition to texture, the surface temperature of IR treated tomatoes was determined via a non-contact Infrared Thermometer (Oakton

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Fig. 1. Measurements of tomato dimensional attributes.

TempTestr IR, Lesman Instrument Co., Bensenville, Ill., USA) at the end of IR heating for each fruit at four locations: two sides, blossom end, and stem end. The average temperature of ten fruits is reported for different peeling times.

2.5. Biomechanical properties of tomato skins To determine the impact of IR heating on tomato skin, changes in biomechanical properties of tomato skin before and after IR heating were investigated using two mechanical tests, the skin adhesive test

Flow Meter

Pressure Gauge

Natural Gas Flow Divider Pressure Gauge

Natural Gas

2.5.1. Skin adhesive energy To explore the skin adhesive property, a tomato was placed vertically along its stem-blossom axial direction on the plate stand of the texture

Tomato

Natural Gas Pipelines

and the uni-axial tensile test of failure. The former test quantified the degree of peel loosening induced by IR heating, and the latter test reveals the mechanical behavior of tomato skin after IR heating. Comparisons were carried out on fresh tomatoes and the tomatoes subjected to 60 s IR heating for the two tomato cultivars. The average values of 10 fruits for each treatment are reported with outliers removed.

Catalyic IR Emitters

Online Control

Power Supply Watts Meter

Electrical Circuit P-9

x2

Turntable

u1 x1

Fig. 2. Experimental setup of IR dry-peeling system with continuous rotary heating.

*/*

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analyzer. A rectangular strip with 8 mm width was carved using a razor blade on the tomato surface along the direction from the stem scar to the blossom end. The detached skin was then carefully lifted up along the cut-line and then mounted onto a custom-designed probe carrier that was connected to the texture analyzer (ASTM Standards, 2004). To minimize the curvature effect and slipperiness, the tomato peel strip was aligned in parallel to the probe carrier and gripped using flat end clamps and tapes (Fig. 3A). The peel was then pulled upwards at a rate of 3 mm/min up to 50 mm height. This procedure produced a wedgeshape peel with a typical length of about 20 mm. The width and thickness of each peel were recorded at the position within the initial 1.5 mm pulling distance by means of a dial caliper. Because of the curved shape of tomatoes, the skin adhesive energy required to peel the skin off was only determined at the initial movement (i.e., the initial 1.5 mm displacement) so that the radius of curvature within a small distance became negligible (Fig. 3B). Before the tensile test, the surface temperature of tomatoes was cooled to room temperature, so that the thermal lag effect could be neglected.

2.5.2. Tensile test to failure Peel segments used for the tensile tests were cut from the equatorial region, parallel to the stem-blossom axis. Flat dumbbell-shaped testing

A

B

specimens formed by a die cutter (Fig. 4A) were prepared according to the standard of ASTM Standards (1999). The dissected specimen had a total length of 50 mm, a test length of 5.7 mm, and a width of about 4.25 mm when measured with a digital caliper. Each end of flat dumbbell-shaped specimen was mounted directly onto the upper and lower tensile clamps and gripped firmly by flat-faced clamps and tapes to avoid slipping during stretching. Before each test, alignment of the clamps was inspected to ensure that each peel specimen was placed on the same plane. During the test, the specimen was stretched under uniaxial tensile loading with a speed of 3 mm/s until failure. Fig. 4B shows the representative force-deformation plot for tomato peel under uniaxial tensile testing. The maximum force was identified as the rupture force. The slope from the initial force to maximum force together with measured dimensional information of each specimen was then calculated as the Young's Modulus.

2.6. Scanning electron microscopy To further understand the effect of IR heating or lye diffusion on the tissue of tomato skin and pericarp, fixative scanning electronic microscopy (SEM) was used to examine microstructural characteristics at the cellular level. Samples of fresh tomatoes and tomatoes heated by IR for 60 s were used. Pericarp cubes (approximately 1 cm3) of tomato fruits with the skin attached were cut from the equatorial region of the tomato fruit and placed directly into a fixative containing 5% glutaraldehyde, 2% formaldehyde, and 2% sucrose buffered to pH 5.5 in 0.1 M sodium cacodylate. The fixative and samples were placed under a slight vacuum to improve penetration of the fixative for approximately 5 min and then refrigerated at 4 °C overnight. The fixed samples were trimmed then placed in fresh fixative, and fixation was continued at 4 °C for an additional 12 h. The specimens were rinsed three times in the buffer, 20 min per exchange, then dehydrated in a graded series of

C

A

0.8 4

D

B Tensile Force (N)

Force (N)

0.6

0.4

0.2

3

2

1

0.0 0

5

10

15

20

25

30

Displacement (mm)

0 0

1

2

3

4

5

Deformation (mm) Fig. 3. Measurements of tomato peeling energy: (A) front view of peeling test; (B) side view of the peeling test; (C) the tear shapes obtained in the experiment; (D) a representative force-deformation diagram of peel specimen.

Fig. 4. Uniaxial tensile test to failure: (A) a picture of custom designed die cutter; (B) a representative force-deformation diagram of peel specimen.

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ethanol solutions of increasing concentration. After the chemical rinsing, the specimens were plunged into liquid nitrogen, fractured by means of a pre-chilled razor blade held in a vice, and then returned to 100% ethanol to thaw. The critical point method was used to dry the specimens by using a Tousimis 815-Autosamdri Critical Point Dryer (Tousimis, Rockville, Md., USA). The dried specimens were then mounted onto aluminum specimen stubs and coated with gold-palladium in a Desk II sputter coating unit (Denton Vacuum, Moorestown, N.J., USA). The specimens were viewed and photographed in a Hitachi S-4700 scanning electron microscope (Hitachi High-Technologies Corporation, Tokyo, Japan).

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of the physical attributes to peeling performance will be discussed in the next section. Thickness of the pericarp wall and the red layer ranged between 5.7 and 6.3 mm, and 1.8 and 1.9 mm, respectively. The results are consistent with similar measurements for other peeling cultivars grown in the same locations but different growing seasons (Garcia & Barrett, 2006a). In addition, other physical properties, including color, soluble solids, and pH, were monitored in an experiment station through the season of 2009. For reference, the reported values were 24.9 °Hue, 4.89 °Brix, and pH 4.37 for cv. AB2, and 24.5 °Hue, 5.39 °Brix, and pH 4.41 for cv. CXD179. Based on all the above information, it was inferred that phenotypic variations of the two tested cultivars are in the normal range.

2.7. Experimental design and statistical analysis 3.2. Peeling evaluation To investigate the effects of cultivars, peeling methods, and heating time on the peeling performance and peeled product quality, a split– split-plot design was employed for peeling evaluation. The two tomato cultivars (i.e., AB2 and CXD179) were set as the main plot and the two peeling methods (i.e., IR and lye peeling) were selected as the subplot. The four heating times (i.e., 30, 45, 60 and 75 s) were completely randomized within each peeling method, and served as the sub-sub plot. Ten replicates (blocks) were used at each time level. Reported values are the average of the ten replicates. The peeling outcomes, including peeling performance and peeled product quality, were evaluated through analysis of variance (ANOVA) followed by a Duncan's Multiple Range test at a significant level of 0.05. 3. Results 3.1. Physical attributes of tomatoes Table 1 shows the evaluated physical attributes for tomato cv. AB2 and cv. CXD179. The weight variations for each of the cultivars were large, similar to results noted in a previous report (Garcia & Barrett, 2006a). Therefore, in the following peeling studies, tomatoes were presorted based on weight and tomatoes ranging from about 60 to 100 g were selected to ensure uniformity. Results from studies of dimensional attributes suggest that the two studied peeling tomato cultivars have similar morphological features but subtle differences, especially in their blossom ends. The cv. CXD179 has a unique nipplelike tip while cv. AB2 has a flat tip. As expected, this dissimilarity in shape is reflected in the longer average length of cv. CXD179. Another difference in external shape is the height to maximum diameters between the two cultivars. In terms of the overall body shape, the ratios of height to maximum diameter revealed that both possess an elongated oval shape. Similar shoulder height was found in both whereas a larger stem scar diameter was found in cv. CXD179. As stated earlier, a large stem scar and a deep shoulder height can affect the peelability and the ease of peeling (Garcia & Barrett, 2006a). Possible correlations Table 1 Measured physical attributes of tomato cvs. AB2 and CXD179. Descriptions

Mass (g) Height (mm) Maximum lateral diameter (mm) Ratio of height to maximum lateral diameter (mm) Height of maximum diameter (mm) Shoulder height (mm) Stem scar diameter (mm) Pericarp thickness (mm) Red layer thickness (mm)

AB2 (197)

CXD179 (75)

Mean

Std.

Mean

Std.

78.7 58.5 49.0 1.2 38.4 1.7 7.0 6.3 1.8

20.6 5.9 5.4 0.1 5.1 0.6 1.2 0.8 0.3

79.6 61.1 47.8 1.3 32.3 1.8 7.9 5.7 1.9

16.9 5.3 5.5 0.1 4.7 0.6 1 1 0.6

Sig.

All tested tomatoes in the 2009 season met the 0.015 cm2/g FDA peelability requirement. Table 2 provides a comparison of peeling results for lye and IR treated tomatoes of cv. AB2. In general, as lye dipping time increased the ease of peeling and the peeling loss increased, whereas peeled tomato firmness decreased, which resulted from accumulated heat in tomatoes. The general trends are in agreement with the previous results (Li et al., 2009; Pan et al., 2009) and findings of other researchers (Garcia & Barrett, 2006b; Matthews & Bryan, 1969). The average peel thickness resulting from lye peeling ranged from 0.85 to 1.06 mm. As a comparison, when IR was used for peeling, the average peel thickness was reduced to a range between 0.39 and 0.91 mm. The corresponding mean peeling loss of IR treated tomatoes ranged from 8.3% to 13.2%, which was also significantly lower (p b 0.05) than that of lye treated tomatoes (from 13.2% to 15.8%). Notably, the peeling loss of IR treated tomatoes consisted of mainly the weight of the skin and the attached red layer tissues and, additionally, possibly a small amount of moisture loss during IR heating. To quantify the moisture loss, each IR peeled tomato was weighed before and after IR heating, and the calculated moisture loss was found to account for less than 2% of weight change during IR heating. This insignificant moisture loss is due to relatively short heating duration and presence of the impermeable cuticle membrane that covers the tomato outer surface. In addition, no significant correlations were found between peeling loss or peel thickness and tomato red layer thickness or pericarp thickness for either cultivar. Such findings were consistent with reports from other researchers (Garcia & Barrett, 2006a), even though the red layer was found to be partially or even fully removed during the tomato peeling process. Reproducibility of the peeling performance and product quality were related to the heterogeneity of tomato skins for different tomato cultivars. Cultivar differences need to be considered in developing the IR dry-peeling technology. For example, when IR heating time was 75 s, tomato cv. CXD 179 had an average peeling loss of 10.4% and skin thickness of 0.64 mm, both of which were higher than those of

Table 2 Comparison of peeling performance between IR and lye for tomato cv. AB2. Methods and conditions

* * * * *

Note: numbers in the brackets followed by tomato cultivars represent sample size. “Std.” stands for standard deviation. “Sig.” stands for significance. An asterisk star symbol indicates a significant difference for the measured physical attributes between two cultivars by student t-tests (α b 0.05).

Lye — 30 s Lye — 45 s Lye — 60 s Lye — 75 s IR — 30 s IR — 45 s IR — 60 s IR — 75 s

Ease of peeling

Peeling loss (%)

Peel thickness (mm)

Peeled firmness (N)

Mean

Std.

Mean

Std.

Mean

Std.

Mean

Std.

4.4 4.6 4.7 4.8 4.3 a 4.5 a 4.8 b 4.9 b

0.7 0.7 0.5 0.4 0.3 0.3 0.1 0.4

13.2 a 14.8 a,b 15.8 b 15.7 b 8.3 a 13.2 b 10.1 a 9.0 a

2.2 3.0 2.0 2.7 1.4 1.8 3.5 1.6

0.85 a,b 1.01 b 1.06 b 0.63 a 0.56 a 0.91 b 0.59 a 0.39 a

0.14 0.43 0.42 0.31 0.22 0.30 0.38 0.10

13.73 a 12.56 a 11.97 a 9.42 b 19.72 a 15.79 b 12.26 c 12.16 c

2.55 3.83 2.35 2.16 3.53 3.43 2.84 4.51

Note: mean separation was analyzed via Duncan's Multiple Range Test. Means with a different letter in each column are significantly different at the 0.05 level. “Std” stands for standard deviation. The same statistical analysis applied for tomato cv. CXD179 in Table 3.

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cv. AB2, whereas the average firmness of cv. CXD179 was slightly lower than that of cv. AB2 (Table 3). For both cultivars, the ANOVA analyses show that the IR heating time significantly affected peeling performance and peeled product quality (Tables 2 and 3). Compared to results for each time level of conventional lye peeling, IR dry-peeling showed a lower peeling loss, a thinner peel thickness, and a similar or slightly firmer texture. For the purpose of achieving a better ease of peeling (N4.5) with less heating time, IR heating for 60 s could be a reasonable selection for both cultivars under the studied heating configuration. Hence, the subsequent studies mainly considered the condition of 60 s IR heating.

250 Fresh tomato After 60s IR heating

Skin Peeling Energy (mJ)

128

200

150

100

50

3.3. Biomechanical properties of tomato skin related to peeling 0

Force-deformation curves were recorded during the adhesive tearing test. Based on the energy theory of fracture, the overall peeling energy represents three combined components, including surface energy of adhesion, potential energy due to peeling force, and elasticity energy due to the extension and bending of the skin film (Hamm, Reis, LeBlanc, Roman, & Cerda, 2008; Kendall, 1975, 1994). Consequently, the area under the force–displacement curve was interpreted as the peeling energy required for tearing off the tomato peel. Fig. 5 clearly shows that significant differences in skin peeling energy exist between IR heated samples and fresh control. Fresh tomatoes for both cultivars exhibited higher resistance to peel tearing. In contrast, less effort for peel removal was required for IR heated tomatoes due to thermally induced peel loosening. The required peeling energy for the two cultivars differed. Compared to peel tearing of fresh tomatoes, the peeling energy was lower by 40% for cv. AB2 and lower by 56% for cv. CXD179. Although variations are large due to the heterogeneous nature of biomaterial, sensitivity of such comparative measurements is still adequate to quantify the sample difference before and after IR heating. Fig. 6 shows that for both cultivars IR heating significantly increased Young's modulus. This increase indicates the skin was stiffer (i.e., more difficult to stretch) and more brittle (i.e., more likely to fracture or crack). The increase in skin stiffness due to IR heating was possibly caused by two mechanisms: (1) the vaporization of free water in tomato skin during IR heating and (2) thermally induced alteration of the matrix of polysaccharides in the epidermal cell walls. Note that an increase in Young's modulus of tomato peel has also been found throughout tomato growth stages (Bargel & Neinhuis, 2005; Bargel, Spatz, Speck, & Neinhuis, 2004), a finding attributed to the structural changes in the fibrous polymer network of cell walls during tomato maturation (Hetzroni, Vana, & Mizrach, 2011).

AB2

CXD179

Fig. 5. Comparison of peeling energy between IR heated tomatoes and the fresh control for cvs. AB2 and CXD179 (n = 10 tomatoes).

four layers of thick-walled hypodermal cells (Floros, Wetzstein, & Chinnan, 1987; Mohr, 1990). Beneath the exocarp tissue, parenchymal cells are larger and rounder. These parenchymal cells represent the edible flesh portion of tomato fruits. By comparing Fig. 7B and D, two surface changes are evident. First, micro-cracking or micro-damage is evident on the surface of an IR heated tomato in Fig. 7D (indicated by the arrows). The micro-cracking results from phase transition of waxy cuticular membranes due to heating (Floros & Chinnan, 1988, 1990). Second, the cuticular membrane is thinner and cell walls are less visible in the IR heated tomato (Fig. 7D) compared to the fresh control (Fig. 7B). Since IR radiation first impinges on the surface of the tomato and then penetrates to the inside, the structural changes, in particular for the outermost cuticular membranes, were probably due to biomechanical effects. The effect of IR thermal treatment on the outer portion of tomato pericarp tissues is shown in Fig. 8. Well defined cell shapes and intact cell walls were observed in the control fruit as shown in Fig. 8A. In contrast, cell wall collapse occurred at the interface of smaller dermal cells and larger parenchymal cells in Fig. 8B (pointed out by the arrows). This result is most likely due to the abrupt cell size change between exocarp and parenchyma and the low penetration depth of IR radiation. Since different IR intensities and heating times may be applied in an actual tomato dry-peeling process, the location and degree of cell layer collapse may vary. It was inferred that the thermal energy required to loosen the tomato skin should be adjusted to destroy the thin epidermal and hypodermal layers while damaging the inner pericarp tissue as little as

3.4. Microstructural characteristics of tomato skin subjected to IR heating

Table 3 Comparison of peeling performance between IR and lye peeling for tomato cv. CXD179. Peeling loss (%)

Peel thickness (mm)

Peeled firmness (N)

Methods and conditions

Ease of peeling Mean

Std.

Mean

Std.

Mean

Std.

Mean

Std.

Lye — 30 s Lye — 45 s Lye — 60 s Lye — 75 s IR — 30 s IR — 45 s IR — 60 s IR — 75 s

4.5 a 4.7 a,b 4.5 a 4.9 b 4.2 a 4.4 a 4.4 a 4.9 b

0.3 0.3 0.4 0.3 0.5 0.4 0.5 0.1

14.1 a 14.3 a 12.9 a 18.3 b 8.7 a 12.9 b 11.7 b,c 10.4 a,c

1.2 2.0 2.0 3.0 1.2 3.0 3.9 0.8

0.87 a 0.77 a 0.62 a,b 0.38 b 0.50 a 0.78 b 0.77 b 0.64 a,b

0.33 0.36 0.40 0.38 0.28 0.19 0.29 0.19

12.65 11.87 12.26 10.59 14.03 a 12.46 a,b 11.18 b 10.30 b

4.81 3.14 2.06 2.65 3.14 2.75 2.84 2.65

25 Fresh control IR treated tomatoes

Young's Modulus (MPa)

A cross-section of the outer pericarp tissue and the surface for both fresh and IR heated tomatoes are presented in Fig. 7. It can be seen in Fig. 7A that the tomato dermal system, also called the exocarp, consists of the cuticle, a single tabular form of the epidermis layer, and two to

20

15

10

5

0 AB2

CXD179

Fig. 6. Comparison of Young's modulus between IR heated peels and the fresh control of cvs. AB2 and CXD179 (n = 10 tomatoes).

X. Li et al. / Innovative Food Science and Emerging Technologies 21 (2014) 123–130

A

129

B

1mm

200µm

C

D

1mm

200µm

Fig. 7. Anatomical features of pericarp tissue and outer surface of tomatoes: (A) fresh tomato pericarp; (B) fresh tomato surface; (C) IR heated tomato pericarp; (D) IR heated tomato surface.

possible. This proposed objective provides a guide for an effective design of the IR dry-peeling system and the determination of related heating parameters. 4. Discussion Besides the measured quality parameters, appearance of the peeled tomato is another important quality indicator. The experiments from our feasibility tests demonstrated that the color of peeled products is relatively stable during short heating durations for both IR and lye peeling (Li et al., 2009; Pan et al., 2009). Similar conclusions were obtained from previous tomato lye peeling studies (Garcia & Barrett, 2006a; Schlimme et al., 1984). Therefore, evaluation of color changes of peeled tomato is not included in this study. In addition, the final product surface temperature correlated positively with the firmness of peeled tomatoes. As expected, longer exposure time to IR radiation can cause higher product temperature and softer product texture. Therefore, in order to achieve successful peel removal while minimizing product thermal softening, it is necessary to increase the IR heating rate while reducing the IR heating time. On the other hand, reduced IR heating time may result in less uniform heating on tomato surface, and thus

A

cause over- or under-heating problems. Continuous rotary heating was used in this study to improve heating uniformity. For industrial production, achieving a desirable heating rate and uniformity could be implemented by other means, such as designing a curved-shape emitter to match the tomato geometry and increasing the emitter power density. This research has developed and demonstrated the concept of using IR heating as a sustainable and non-chemical approach for peeling tomatoes. Based on the results of this research, a prototype IR drypeeling system has been recently designed and built. In the scaled-up IR dry-peeling system, the primary function of IR heating is to loosen tomato peel from flesh through the rapid heat treatment. The loosened peels were subsequently released by a vacuum chamber and removed by mechanical pinch rollers. The developed IR dry-peeling technology yields high quality peeled tomatoes and greatly simplifies waste management of peeling residues due to the elimination of chemicals and water, which is a significant step towards a sustainable peeling process. Elimination of chemical contaminations and water usage allows huge savings in water and water-related energy consumption. Because no chemicals are used in peeling, produced peels offer the potential for recovery as a value-added byproduct, which merits our further investigations.

B

50µm

50µm

Fig. 8. Anatomical features of tomato pericarp tissue before and after IR heating: (A) dermal layers of fresh tomato; (B) dermal layers of IR heated tomato.

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5. Conclusion In this study, the effectiveness of IR dry-peeling of tomatoes was comprehensively investigated from multiple aspects, including physical attributes of tomatoes from different cultivars, peeling performance, peeled product quality, and biomechanical and anatomical features of IR peeled skins. The following conclusions were drawn: (1) Morphometric features, including the height of maximum lateral diameter, the ratio of height to maximum lateral diameter, the stem scar diameter, and the pericarp thickness, are important in distinguishing different tomato cultivars. However, these morphometric features may not significantly impact on peeling performance; (2) Compared to conventional lye peeling, IR dry-peeling using 30 to 75 s heating time showed a lower peeling loss (8.3%–13.2% vs. 12.9%–15.8%), thinner thickness of peeled-off skin (0.39– 0.91 mm vs. 0.38–1.06 mm), and slightly firmer texture of peeled products (10.30 to 19.72 N vs. 9.42–13.73 N) while achieving an ease of peeling higher than 4.0. For the cultivars studied, IR heating for 60 s is reasonable and can be expected to achieve an ease of peeling of 4.5 or higher; (3) Compared to control, IR heating reduced the energy to tear tomato peels by 42% for cv. AB2 and 56% for cv. CXD179. Reduced adhesiveness and increased peel brittleness after IR heating revealed the impacts of IR heating and built a foundation for more in-depth future analyses of changes in biomechanical properties of tomato peel; (4) IR heating resulted in melting of cuticular membrane, collapse of several cellular layers, and severe degradation of middle lamella and cell wall structures, which in turn caused skin separation. In order to achieve desired skin separation while preserving the inner flesh tissue, appropriate design of the IR heating system needs to provide for rapid and uniform heating. References ASABE Standards (2001). Compression test of food materials of convex shape. Vol. ASAE S368.4 DEC00 (pp. 580–587). St. Joseph, Michigan: American Society of Agricultural Engineering. ASTM Standards (1999). Standard test method for tensile properties of thin plastic sheeting. D882-12. Annual book of ASTM standards. Philadelphia, Pa: ASTM Intl. ASTM Standards (2004). Standard test method for peel or stripping strength of adhesive bonds. D903-98. Annual book of ASTM standards. Philadelphia, Pa: ASTM Intl. Baker, R. A., & Wicker, L. (1996). Current and potential applications of enzyme infusion in the food industry. Trends in Food Science & Technology, 7(9), 279–284. Bargel, H., & Neinhuis, C. (2005). Tomato (Lycopersicon esculentum Mill.) fruit growth and ripening as related to the biomechanical properties of fruit skin and isolated cuticle. Journal of Experimental Botany, 56(413), 1049–1060. Bargel, H., Spatz, H. C., Speck, T., & Neinhuis, C. (2004). Two-dimensional tension tests in plant biomechanics — Sweet cherry fruit skin as a model system. Plant Biology, 6(4), 432–439. Barrett, D.M. (2000). Tomato attributes and their correlation to peelability and product yield. VII International Symposium on the Processing Tomato (pp. 65–74). ISHS.

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