Peeling mechanism of tomato under infrared heating: Peel loosening and cracking

Journal of Food Engineering 128 (2014) 79–87

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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng

Peeling mechanism of tomato under infrared heating: Peel loosening and cracking Xuan Li a, Zhongli Pan a,b,⇑, Griffiths G. Atungulu a,1, Delilah Wood c, Tara McHugh b a

Department of Biological and Agricultural Engineering, University of California, Davis, Davis, CA 95616, USA Processed Food Research Unit, Western Regional Research Center, USDA Agricultural Research Service, Albany, CA 94710, USA c Bioproduct Chemistry and Engineering Research Unit, Western Regional Research Center, USDA Agricultural Research Service, Albany, CA 94710, USA b

a r t i c l e

i n f o

Article history: Received 18 July 2013 Received in revised form 15 December 2013 Accepted 22 December 2013 Available online 28 December 2013 Keywords: Infrared radiation Tomato peeling Puncture test Scanning electron microscopy Stress analysis Modeling

a b s t r a c t Critical behaviors of peeling tomatoes using infrared radiation heating are thermally induced peel loosening and subsequent cracking. Fundamental understanding of the two critical behaviors, peel loosening and cracking, remains unclear. This study aimed at investigating the mechanisms of peel separation for tomatoes subjected to a newly developed infrared dry-peeling process. Microstructural changes in tomato epidermal tissues under infrared heating were compared with those of fresh, hot lye and steam treated samples. Theoretical stress analyses coupled with the experimentally measured failure stress of tomato skin were combined to interpret the occurrence of peel cracking within a framework of elastic thin shell theory. With the use of light microscopy and scanning electron microscopy, it was observed that peel loosening due to infrared heating appeared to result from reorganization of extracellular cuticles, thermal expansion of cell walls, and collapse of several cellular layers, differing from samples heated by hot lye and steam. Crack behaviors of tomato skin were attributed to the rapid rate of infrared surface heating which caused the pressure build-up under the skin and strength decrease of the skin. In order to achieve a sufficient skin separation for effective peeling using infrared, promoting rapid and uniform heating on the tomato surface is essential. The findings gained from this study provide new insights for developing the sustainable infrared dry-peeling technology. Published by Elsevier Ltd.

1. Introduction Peeling is a particularly important unit operation in the production of canned fruits and vegetables. The process can affect the palatability and nutritive values of final canned products (Li, 2012). From a processing standpoint, the currently used lye and steam peeling methods are water and energy intensive, and pose serious salinity issues and wastewater disposal problems (Barringer, 2003; Masanet et al., 2007; Pan et al., 2009; Rock et al., 2011; Li et al., 2013). To address these challenges, a sustainable alternative of peeling tomatoes using infrared radiation heat without relying on water, steam, and chemicals has been developed. This peeling method is named as infrared dry-peeling (Pan et al., 2009). The infrared dry-peeling technology has been successfully tested both at the bench scale and pilot scale using tomatoes from multiple harvesting seasons. Currently, onsite demonstrations to compare the performance of the new method with conventional lye and

⇑ Corresponding author. Address: Processed Foods Research Unit, USDA-ARSWRRC, Albany, California 94710, USA. Tel.: +1 510 559 5861; fax: +1 510 559 5851. E-mail addresses: [email protected], [email protected] (Z. Pan). 1 Dr. Atungulu’s present affiliation is the Department of Food Science & Division of Agriculture, University of Arkansas, Fayetteville, AR 72704, USA. 0260-8774/$ - see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jfoodeng.2013.12.020

steam peeling methods are being conducted at various tomato processing plants in California. To further develop the technology and make it commercially applicable, clear elucidation of the mechanism underlying infrared dry-peeling of tomatoes is crucial. Although several experimental and modeling aspects have been addressed in our previous investigations (Pan et al., 2009; Li et al., 2011; Li, 2012; Wang et al., 2013), the thermally induced physical and biochemical changes of tomato peel, in particular the peel loosening and subsequent cracking phenomena, appear different from traditional wet-peeling methods and have not be fully understood. Study of the behavior of peel loosening and cracking should provide insight into the mechanism of dry-peeling of tomatoes using infrared. Limited studies have been conducted to determine the peeling mechanisms (Floros and Chinnan, 1988). Most previous research concentrated on prediction of peeling performance or optimization of various peeling processes mainly for the widely used lye and steam peeling (Barreiro et al., 1995; Das and Barringer, 2005; Milczarek and McCarthy, 2011; Garcia and Barrett, 2006a,b; Matthews and Bryan, 1969; Schlimme et al., 1984; Toker and Bayndrl, 2003; Wongsa-Ngasri, 2004). Possible mechanisms of steam and lye peeling of pimento pepper and tomato were proposed based on examination of the skin microstructural

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changes under different peeling conditions (Floros and Chinnan, 1988, 1990; Floros et al., 1987). In the steam peeling process, the main cause of skin separation is a combination of biochemical and physical changes due to the effects of high temperature steam. In lye peeling, chemical diffusion of hot lye solution into the tissue with subsequent dissolving of the cell wall materials is the primary cause of skin release. Light Microscopy (LM) and Scanning Electron Microscopy (SEM) have proved to be useful tools for observing the microstructural changes in skin morphology and anatomy occurring during lye and steam peeling of several vegetables, including tomatoes (Floros and Chinnan, 1990; Mohr, 1990). These microscopic techniques can be used to determine whether the loosened microstructure of infrared heated tomatoes is different from that resulting from lye and steam treatments. Both of the above mentioned peeling mechanisms may not directly apply to infrared dry-peeling because neither steam, water, nor chemicals are used. Instead, radiative thermal effects resulting in substantial changes in strength and biomechanical properties of tomato skin are presumably the main cause for infrared induced peel loosening and cracking. Several techniques have been attempted to experimentally determine skin strength and membrane biomechanical failure, including tensile, puncture, and bursting diaphragm methods (Calvin and Oyen, 2007; Haman and Burgess, 1986; Miles et al., 1969). The puncture-based method is a widely accepted approach for obtaining skin strength and failure stress. In this test, a force is applied uniformly on the skin membrane by using a smooth roundended probe or a uniform pressure loading so that the skin deforms in response to membrane biaxial tension. This technique enables detection of the increase in pressure on the skin membranes surrounding the fruit (Haman and Burgess, 1986; Henry and Allen, 1974). In light of former mechanical studies, the present study estimated the rupture stress of tomato skin during infrared peeling by determining the force–displacement relationship of the skin membrane. Because the tomato skin is much thinner than the overall fruit diameter, tomato skin is considered as a thin-walled shell (Considine and Brown, 1981; Henry and Allen, 1974). The stress on a thin spherical shell by an internal pressure loading under constant temperature can be estimated by using the membrane theory for spherical shells (Timoshenko et al., 1959; Upadhyaya et al., 1986, 1985). In this study, shell mechanics were applied for the analysis of the transient stress changes within the skin. The results were further analyzed to quantitatively evaluate the relationship between skin mechanical behavior and peel cracking susceptibility. The specific objectives of this study were to (1) compare the morphologies of epidermal cells of tomatoes subjected to infrared, lye, and steam treatments and fresh tomatoes; (2) use puncture test to determine tomato skin rupture stress after infrared heating; and (3) investigate the correlations between transient skin stress and increasing temperature during infrared heating by using an integrated approach of experimental measurements and theoretical analysis.

2. Materials and methods 2.1. Experimental setup and sample preparation Tomatoes of cultivars CXD179 and AB2 with uniform ripeness and size were subjected to infrared heating from two sides for 60 s. Tomatoes were collected at red-matured stage according to the USDA standard (i.e., USDA tomato classification 6) (Li et al., 2013). Only defect-free tomatoes at a size level ranging from 42 mm to 54 mm were used for peeling and subsequent measurements. During infrared heating, a tomato was rotated continuously

at a speed of 1 rpm by means of a motor driven turntable to receive uniform heating. A custom-designed metal holder was used to place the tomato between the vertically aligned emitters (Li et al., 2013). The specific infrared heating setup and procedure are described in our previous publications (Pan et al., 2009; Li, 2012). During infrared heating, initial peel cracking was visually noted and the time was recorded by a stopwatch. Peel cracking was normally accompanied by a sudden sound due to skin rapture during infrared heating. After infrared heating, each treated tomato was sealed in a plastic bag to prevent further moisture loss, and it was allowed to cool to ambient temperature in the laboratory for about half an hour. Peels from these tomatoes were then used for microstructural studies and puncture tests that are described later. Light microscopy (LM) imaging was used to observe the layer separation in pericarp tissue of tomato treated by 60 s infrared heating. Pericarp cubes (approximately 1 cm3) of tomato with the skin attached were cut from the equatorial region of the tomato and prepared by fixation and critical point drying methods previously described (Li, 2012). Specimens were then viewed and photographed by using a Leica MZ16F stereoscope (Leica Microsystems, Wetzlar, Germany). Digital images were obtained with a QImaging Retiga 2000R FAST color camera (QImaging, Surrey, B.C., Canada). Representative images were presented. 2.2. Low temperature high resolution scanning electron microscopy Tomato pericarp tissue (approximately 3  3  4 mm) was obtained from the middle region of the tomato immediately after infrared heating. Each tissue specimen was trimmed to a wedge shape and mounted onto a copper sample holder with TissueTek adhesive (Sakura Finetek USA Inc., Torrance, Cal., USA) and then prepared for low temperature SEM that was conducted with an Alto 2500 cryo system (Gatan Inc., Pleasanton, Cal., USA). The sample holder was attached to the rod of a vacuum transfer device and plunged into super-chilled liquid nitrogen. The specimen was evacuated, pulled up into the vacuum transfer device, and transferred to a cryo preparation chamber. The specimen was fractured in the cryo preparation chamber at approximately 180 °C, warmed to 85 °C and held at that temperature for 15 min to remove excess surface water. The specimen was then cooled by shutting off the heater of the cryo system to less than 135 °C, sputter coated with gold palladium, and transferred to the cryo stage specimen chamber in the SEM. All samples were observed and photographed below 135 °C at 2.0 kV by means of a Hitachi S-4700 field emission SEM (Hitachi High-Technologies Corp., Tokyo, Japan). Digital images were collected at 2180  960 pixels and were viewed under the scanning electron microscope at 400 magnification for the outer surface of tomato skin and at 200 and 400 magnification for the cross-sectional images of tomato dermal system. Fresh tomatoes prepared by the same method were used as a control. To better understand infrared thermal effects on changing the tomato microstructure, a set of experiments was conducted to compare the differences of tomato samples treated with infrared, lye and steam heating for the same time period (60 s). Lye treated samples were prepared by using sodium hydroxide as described in Pan et al. (2009). Steam treated tomatoes were obtained by using saturated steam from boiling water under atmospheric pressure as described in Li (2012). A minimum of three replicates were obtained for each treatment method. 2.3. Measurement of skin rupture A small segment of skin membrane was carefully dissected from the peeled skin by using a cork borer with a diameter of

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6 Peak force

Force (N)

5 4 3 2 1

Rupture distance

B

0 0

A

1

2

3

4

5

Displacement (mm)

Fig. 1. Measurement of tomato skin rupture: (A) experimental setup for the puncture test; (B) a representative force–displacement diagram.

22 mm. As shown in Fig. 1A, the edges of the skin membrane were then clamped between two steel plates that had a central circular opening with a diameter of 12.8 mm. The sample was placed skin surface side down on this platform and centered over the opening. To provide adequate support to the sample, the diameter ratio of hole to probe was in the range of 1.5:1–3:1, as recommended by Bourne (2002). A force was applied onto the inner surface of the skin membrane using a Magness–Taylor type probe with a convex indenter. The probe measured 6.9 mm in diameter and had a small curvature so that the natural curvature of the tomato skin closely followed the curvature of the probe. Measurement of skin rupture resistance was carried out on a fruit texture analyzer TA-XT2i (Texture Technologies Corp., New York, N.Y., USA) equipped with a load cell of 19.6 N. The force–displacement relationship was recorded for each sample at a loading rate of 0.10 mm/s. To minimize variability, all samples were taken from approximately the same latitude around the periphery of the tomatoes. Each test was completed when the rupture of skin membrane occurred. The puncture test produces a biaxial state of stress on the skin membrane, which closely represents infrared heating conditions where internal pressure acts on the tomato inner skin. As shown in Fig. 1B, the rupture force is defined as the peak force, and the corresponding displacement was recorded as the rupture distance. After the test, skin thickness was measured at three locations on each dissected skin membrane by using a dial caliper with 0.001 mm accuracy. Average values of rupture force, rupture distance, and skin thickness were reported from 10 replicates and were used later for calculating the skin rupture stress. All puncture measurements were completed within approximately 2 h after infrared heating.

Thus, the normal stress in the skin membrane can be obtained as

rr ¼

Fp 2pRp t sinðhÞ

ð2Þ

Based on Eq. (2), skin rupture stress is expressed in terms of several measurable parameters (i.e., Fp, Rp, t, and h) and thus can be experimentally determined through a puncture test, which provides a measure of the skin strength changes before and after infrared heating. From the right angle ABC in Fig. 2B, the trigonometric identity can be used to determine the contact angle

a h ¼ arctan b

ð3Þ

where, ‘‘a’’ is the vertical distance of segment AB, mm; and ‘‘b’’ is the horizontal distance of segment AC, mm. Based on the geometry of a spherical cross section, parameter ‘‘a’’ can be expressed in terms of other geometric parameters D, d, and R, to give

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 a ¼ D  d ¼ D  R  R2  R2s

ð4Þ

where, D is the rupture distance, mm; d is the height of the convex tip end of the Magness–Taylor probe, mm; R represents the radius of the convex tip of the Magness–Taylor probe, mm; and Rs represents the radius of the hole of steel plate, mm. According to Fig. 2B, parameter ‘‘b’’ is the difference between the plate hole diameter and the probe diameter

b ¼ Rs  Rp

ð5Þ

3. Theoretical analysis 3.2. Estimation of stress in tomato skin membrane

3.1. Skin rupture stress In the puncture test, the skin membrane can be considered as a thin membrane under pressure. The stress within each skin specimen increased as the probe was pressed harder onto the skin membrane. Frictionless slip was assumed between the skin membrane and the round end probe. At the moment skin rupture occurred, the rupture force was equal to the peak puncture force (Fp). The equilibrium of forces in the vertical direction yields P Fp = Fy (Fig. 2A), and this relationship then leads to

F p ¼ 2pRp t rr sinðhÞ

ð1Þ

where, Fp is the total downward puncture force, N; Rp is the radius of puncture probe, mm; t is the thickness of skin membrane, mm; rr is the stress in skin membrane, MPa; h is the contact angle, degree.

During infrared heating, the stress in the skin increases as accumulation of internal pressure with the rising temperature. In order to estimate how much stress may arise within the loosened skin, tomato skin can be modeled as a thin membrane that is subjected to biaxial tensile stresses in two perpendicular directions, longitudinal, and circumferential, respectively (Timoshenko et al., 1959). When a tomato is considered as a spheroid, the biaxial membrane stresses in the skin are equal in the two perpendicular directions, according to the theory of thin-wall shell (Considine and Brown, 1981). Because the thickness of skin membrane is much less than the remaining radius of tomato fruit, the tomato can be considered as a thin-walled pressure vessel. Accordingly, the intensity of stress in a thin spherical membrane can be expressed as (Timoshenko et al., 1959)

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Fig. 2. Schematic diagram of the skin rupture test: (A) free body diagram when rupture occurs; (B) cross sectional view of the skin membrane being loaded by the Magness– Taylor probe.

r¼

pr 2d

ð6Þ

where, r is the membrane stress in tomato skin, MPa; p is the intensity of uniform internal pressure, MPa; r is the radius of skin membrane, mm; and d is the thickness of skin membrane, mm. In Eq. (6), three parameters (p, r, and d) can affect the membrane stress. The radius and thickness of the skin are descriptive of each individual tomato (Considine and Brown, 1981). For simplicity, this study explored an idealized case of great interest. It was assumed that the tomatoes were spherical and had uniform shape and size; the skin membrane had uniform thickness and physical properties; and the curvature of the tomato skin did not change abruptly. The equivalent radius of skin membrane (r) was justified to be that of a spheroid with the same surface area as a medium size tomato as defined in a previous publication (Li et al., 2011). The average thickness of skin membrane (d) can be obtained from our experimental measurements, which was 0.59 ± 0.038 mm in average for a 60 s infrared heating (Li, 2012). The overall moisture loss of tomatoes was found to be less than 2–3% during a 60 s infrared heating, and small bubbles were occasionally observed at the stem scar. It was concluded that limited moisture could pass through the tomato hydrophobic waxy surface due to the absence of stomata and non-liquefaction of the cuticular membrane covering the outside of the tomato. The model was simplified by assuming the skin membrane was impervious to moisture and was thus subjected to water vapor pressure under infrared heating. The total internal pressure on the skin membrane was almost entirely from the buildup of vapor pressure. The intercellular atmosphere of most horticultural commodities has been commonly assumed to be saturated (Ferrua and Singh, 2009a,b). The high moisture content of tomatoes (95% water) allows the reasonable assumption that the vapor pressure below the skin surface can be approximated by the saturated pressure of pure water using Antoine equation (Poling et al., 2001).

 Dp  pv ¼ exp 16:3872 

 3885:70  103 230:170 þ Tðd; tÞ

ð7Þ

where Dp represents the pressure difference between skin membrane, MPa; pv is the vapor pressure, MPa, T is the temperature under a skin membrane with a thickness of d, °C, and t stands for the heating time, s. The estimated vapor pressure beneath the skin is modeled as a function of temperature. The range of temperature increase during heating depended on heating time and specific region, but it was

typically 23 °C to 100 °C in our test conditions. Normally, peel cracking occurred at a location that was directly exposed to the infrared emitter, where the strongest irradiation occurred. Based on the results from the heat transfer model we developed (Li and Pan, 2013a,b), this location was identified as in the region nearest the emitter. The skin membrane thickness (d) can be experimentally determined from the peeled skin thickness. For simplicity, the d value was assumed to be uniform, and the average value of peeled skin thickness measured from more than ten peeled tomatoes was used to represent d. Once the location of cracking and skin membrane thickness were identified, the predicted temperature history at that location beneath the skin was obtained from the constructed heat transfer model and used for calculating the internal pressure. Relationship between internal pressure and membrane stress given in Eq. (6) can be used to determine transient membranes stress within the tomato skin during infrared heating. Note that because the skin thickness values are very small (less than 1 mm), the temperature gradient within the skin membrane was assumed negligible in the study. This assumption simplified the theoretical calculation. As a case study, Eqs. (6) and (7) were evaluated for a typical peeling condition: 60 s infrared heating of a medium size tomato.

4. Results 4.1. Microstructure changes of tomato skin Fig. 3 shows the microstructure changes on tomato outermost surface after various treatments. The outermost tomato skin features a very thin hydrophobic waxy cuticular membrane (Domínguez et al., 2011). On the fresh tomato surface (Fig. 3A), clearly defined contours of cell wall structures can be observed. After infrared heating, the contour and overall shape of epidermal cells become difficult to discern and a knoblike protuberance arises from each cell surface (Fig. 3B). For the lye treated samples (Fig. 3C), although the knoblike protuberances are formed, contours of epidermal cells are more readily visible than those for infrared treated samples (Fig. 3A). This observation confirms previous reports that the action of lye can increase cell visibility due to the dissolution of waxes (Floros et al., 1987). It is suspected that the cell protuberance is caused by phase transition of the waxy cuticular membrane, and melting and re-distribution of cuticular wax results in less visible contours of cell walls due to distortion by cuticular wax. It is interesting that tomato skin treated by steam

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A

B

clearly defined contours

concave surfaces

100 m

C

knoblike protuberance

100 m

less visible cell contours

D

least damaged surface

increased visibility of cell contours 100 m

100 m

Fig. 3. Scanning electron microscopic images of tomato outermost surface: (A) fresh control skin; (B) infrared heated skin; (C) hot lye heated skin; (D) steam heated skin.

was less damaged than the skin of lye and infrared treated samples (Fig. 3D). This observation supports the remark from Garcia and Barrett (2006b) that lye peeling of tomatoes is more efficient than steam peeling. It is probable that steam causes less damage to the epidermal layers than the hot lye solution causes within the same timeframe. Therefore, in order to achieve a sufficient degree of peel loosening, pressurized steam and a longer exposure to heat are typically adopted by commercial steam peeling operations. From the heat transfer perspective, infrared radiative heating has a higher heat delivery rate and capability compared to convective heating of using steam. Within a very short duration (60 s), rapid radiative heating of infrared with a limited penetration depth (<1 mm) created a ‘‘heat shock’’ at tomato surface, causing thermal damage of several layers of tomato epidermal cells and adjacent flesh tissues. This evidences the effectiveness of infrared heating in promoting disruption of tomato epidermal layers towards peel loosening. Cross-sectional images of the outer pericarp tissues of the fresh, infrared-, lye-, and steam-heated tomatoes are presented in Fig. 4. Fig. 4A shows that the fresh tomato dermal system consists of a

cuticle layer, one-cell thick layer of epidermal cells, and a two to four cell thick layer of thick-walled hypodermal cells. Adjacent to the hypodermal layer, cells become larger and tend to be round shaped (Fig. 4A). These round cells are parenchymatous cells and represent the edible flesh portion of tomato. In contrast to the fresh control, the infrared treated sample shows thermal expansion of cell walls and separation of the cytoplasm from cell membrane (Fig. 4B). These anatomical differences indicate that the thermal effect of infrared heating dramatically disrupts the tomato outermost skin layer and adjacent pericarp cells. Unlike in the infrared heated sample, cell wall expansion is not observed in lye treated tomatoes (Fig. 4C). Instead, cytoplasm separation and enlarged intercellular spaces are observed, which indicate severe degradation of pectin substances in the middle lamella by lye that has diffused into the pericarp. Thermal expansion of cell walls in steam treated samples was insignificant due to inefficient delivery and transfer of heat by steam treatment as compared to infrared treatment (Fig. 4D). To provide more detailed views of the cytoplasm inside cell walls, higher magnification cryo-SEM images were obtained, as

cuticle layer epidermal cell thermal expansion of cell walls

hypodermal cell parenchymatous cell

200µm

A

200µm

B

enlarged intercellular spaces

200µm

C

200µm

D

Fig. 4. Scanning electron microscopic images of cross-sectional images of tomato dermal system: (A) fresh control; (B) infrared heated skin; (C) hot lye heated skin; (D) steam heated skin.

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shown in Fig. 5. Even with this higher magnification, it is impossible to distinguish cell walls, middle lamella, and cytoplasm for the infrared treated tomato tissue (Fig. 5B), which is in contrast to the fresh tomato tissue (Fig. 5A). Loss of cell integrity indicated infrared heating caused severe thermal damage to certain cell layers, and resulted in mechanical failure of those cells and possible subsequent layer separation. Compared to infrared treated samples, lye treated tomato tissue exhibited thickened cell walls and icy crystals in the cytoplasm (Fig. 5C). These results are attributed to the diffusion and penetration of lye solution into the interior of tomato cells. Because no chemical or water was used in the infrared peeling process, such changes did not occur in infrared treated samples (Fig. 5B). Radiation heat transfer is the dominant cause of skin loosening during the infrared dry-peeling process. In theory, infrared irradiation first impinges on the skin of tomato, then the heat penetrates inside the tissue through conductive heat transfer. Thermal energy causes a sudden temperature increase in cell walls and inside cell fluids, and decreases the local resistance or adhesion of cells. The tissue damage is the result of a combination of vaporization of cell fluid, breakdown of pectin substances in the middle lamella, and degradation of cell wall polysaccharides. The resulting biochemical reactions and enzyme inactivation depend on the heating duration, penetration depth, and radiation intensity. From macroscopic appearance, it was inferred that heat treatment causes degradation of certain skin boundaries. To validate this speculation, a magnified image was obtained with LM; it confirmed the layer separation (Fig. 6). The average measured thickness of the separated layer obtained at multiple locations was 0.69 ± 0.09 mm, which was in good agreement with measured peeled skin thickness (0.59 ± 0.35 mm) after 60 s infrared heating. In contrast to the infrared and lye treated tissues, cytoplasmic content in steam treated samples were less damaged, which might be because the relatively low heat delivery capability of the steam used in this study (Fig. 5D). Similar to results obtained with infrared heating, the heat and diffusive mass transfer produced by steam treatment resulted in biophysical changes that led to loss of cell rigidity and reduced turgor pressure (Floros and Chinnan, 1988). The observed layer separation can be correlated with peel loosening and the ease of peel removal. The contributions of cell structure characteristics of tomato to the ease of peeling have been summarized previously by Mohr (1990). The microstructural

changes noted in the images from this study confirmed that earlier report (Mohr, 1990). For example, tomato cell size increased from smaller dermal cells towards the inner larger pericarp cells (Fig. 4). This abrupt cell size gradient coupled with thermal expansion resulting from infrared heating contributes to the tendency for layer separation. Water vapor resulting from infrared heating builds up at the interface where steep cell size transition exists, and enlarges the intercellular space. Thus, thermal effects presumably cause mechanical failure of the cells. 4.2. Transient membrane stress and peel cracking Theoretical calculations were performed for a typical case of 60 s infrared heating of a medium size tomato with a spherical radius of 28 mm. Fig. 7A illustrates the predicted temperature profile and the rise of internal vapor pressure at a place 0.6 mm under the skin with infrared irradiation of 5000 W/m2 during infrared heating. The temperature profile was obtained from a computer simulation study of tomatoes subjected to the infrared dry-peeling process, and readers interested in more details are referred to Li and Pan (2013a,b). Within the 60 s infrared heating time, temperature increases markedly from 23 °C to higher than 80 °C. Due to the increasing temperature, the internal vapor pressure gradually builds up (Fig. 7A), and eventually causes mechanical failure of cells and skin layer separation or loosening. The effects of temperature and pressure on the resulting changes in skin’s intrinsic strength are presented in Fig. 7B. Theoretically predicted skin membrane stress increases exponentially with temperature. Analyzing all of the measured skin rupture data using Eq. (2) yields an average rupture stress of 1.18 MPa after 60 s infrared heating. The mean experimentally determined skin rupture stress and its corresponding 95% confidence intervals are also graphed in Fig. 7B. At about 80 °C the predicted skin membrane stress value was virtually identical to the mean experimental value of skin rupture stress. Therefore, the results suggest that peel cracking may begin when the skin membrane stress exceeds the rupture stress of tomato skin. The 95% confidence interval ranging from 0.93 to 1.44 MPa estimates the overall uncertainty in the experimentally measured mean value of skin rupture stress. The predicted skin membrane stress intersected with the lower confidence limit value at about 75 °C but did not intersect with the upper limit value (Fig. 7B). This result indicates that occurrences of peel cracking may vary at

loss of cell integrity

layer separations

100 m

A

100 m

C

100 m

B

thicken cell walls icy crystals

100 m

D

Fig. 5. Scanning electron microscopic images of cross-sectional high magnification images of tomato exocarp tissue: (A) fresh control; (B) infrared heated skin; (C) hot lye heated skin; (D) steam heated skin.

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A

B thickness

layer separation

1 mm

1 mm

Temperature (oC)

0.06 80

0.05

Temperature Internal vapor pressure

60

0.04 0.03

40

0.02 20

0.01

A

0 0

10

20

30

40

50

0.00

60

Pressure Difference (MPa)

Fig. 6. Cross-sectional light microscopic image of outer portion of tomato pericarp tissue: (A) tomato skin after infrared heating; (B) fresh tomato skin.

5. Discussion and conclusion

Time (s)

Stress (MPa)

1.6 1.2 0.8 0.4

Mean of estimated rupture stress 95% confidence interval

B

0.0 20

30

40

50

60

70

80

90

Temperature (oC)

20

120

15

90

10

60

5

30

0

Cumulative Frequency (%)

Fig. 7. Temperature, internal pressure, and stress changes during infrared heating: (A) temperature and pressure profiles during infrared heating; (B) membrane stress changes with temperature.

Counts

75 s of heating. Second, the distribution of the cracking time, as shown in Fig. 8, reveals that 70% of cracks occurred within 60 s infrared of heating and a majority of the cracking happened after heating for 50–60 s. After 50–60 s heating, the temperature 0.6 mm under the tomato skin was between 76 °C and 82 °C (Fig. 7A), which is consistent with the temperature range we found in our previous Dynamic Mechanical Analysis.

0 35 - 40 40 - 45 45 - 50 50 - 55 55 - 60 60 - 65 65 - 70 70 - 75

Time Interval (s) Fig. 8. Frequency distribution of the recorded initial cracking time.

higher than certain temperature levels (80 °C in the studied case) but do not always happen under the selected condition, which is because the skin membrane stress is less than the rupture stress. The measured cracking time supported this argument. First, 90% of the tomatoes in all the infrared peeling tests cracked within

The above experimental data and theoretical analysis elucidated the relationship between the increase in temperature and increase in skin stress. Quantifying the dynamic temperature-dependent stress developed in tomato skin is vital for accurate analysis of peel cracking. From an engineering point of view, the surface layer of tomato pericarp can be modeled as a porous media since it consists of solid cellular tissues with voids in between. The total stress in the dermal layer can be decompose into effective stress assigned on the cellular skeleton and pore pressure formed in the void space (Ho et al., 2013; Datta, 2007). Further in-depth study of stress generation could be coupled with transport phenomena in the porous tissue of tomato surface layer, including the diffusion of cytosolic fluid and its capillary effects. Prediction of the time and temperature at which peel cracking will occur is affected by several other factors in the practical infrared peeling process, such as variable tomato shape and size, skin thickness, and skin permeability. For simplicity, in this study the peeling mechanism was modeled using a spherical tomato with uniform temperature and thickness. For the actual elongated shape of processing-tomato, the horizontal stress in the hoop direction is supposed to be greater than the vertical stress in the meridional direction due to the non-axial symmetry (Considine and Brown, 1981; Timoshenko et al., 1959). According to the theory of thin-walled shell behavior, cracking of a prolate spheroid type of shape is most likely to occur in the longitudinal direction (stemblossom axis direction) at a 45° incline. This theoretical rupture pattern agrees well with the pattern of observed peel cracking. Hence, the effect of different tomato shapes should be given particular attention in further investigations. Skin permeability is another important factor to consider for a complete explanation of the infrared peeling mechanism. Small bubbles formed by water vapor were experimentally observed at the stem scar in the last stage of infrared heating. These bubbles indicate that the tomato skin still functions as a vapor barrier before rupture. If the skin was completely permeable, the skin would not be under any stress at all and bubbles would not form. It is proposed that although the skin is slightly permeable to vapor

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generated by infrared heating of leaking cytosolic fluids, an internal pressure under the skin membrane is generated when the rate of the pressure build-up is larger than the rate of vapor leakage through skin. Impermeable skin was assumed for the theoretical models in this study. There is a need to carry out further research to determine more accurately the vapor permeability of tomato skin. From this study, we learnt that when the internal vapor pressure reaches a high enough level, the skin cracks because of the pressure-generated stress and reduced skin strength caused by infrared radiation. In the design of a commercial infrared dry-peeling system, a vacuum chamber could be included to increase the pressure difference across the skin membrane and thus enhance the occurrence of skin cracking. This idea was implemented and tested in a devised pilot scale infrared dry-peeling system for tomatoes. Nearly all tomatoes cracked after a sequential infrared and vacuum treatments, which validated the vacuum effect (Pan et al., 2012). According to the theoretical model used in this study, peel cracking depends on pressure difference, the skin thickness and the indenter tip radius. In practice, biological factors, such as tomato maturity and cultivar characteristic also affects the occurrence of peel-loosening and cracking. It must note that the evolution of stress with pressure and temperature is a major but not the sole factor determining the skin separation. Responses of tomato skin to temperature increase through other multi-physical and biochemical phenomena may also contribute to the skin rupture and degradation in skin inner tissues. For example, thermal softening due to temperature increase at tomato surface may reduce the overall skin strength and lower the critical rupture stress of tomato skin, leading to an easier skin rupture. Various biochemical reactions along with the rupture of cell walls and changes in moisture and temperature occur at tomato dermal systems during infrared heating, contributing to the pronounced changes in skin’s dynamic biomechanical properties. The heterogeneity and anisotropy nature of biomechanical properties of fruit skin may play an important role in the formation of skin separation resulting from infrared heating, particularly for peel cracking. This speculation is true when extending the infrared dry-peeling technology to other fruits and vegetables with different and complex skin characteristics such as clingstone peach or Bartlett pear (Li et al., 2014). To optimize the peeling performance over different fruits and vegetables, the in-depth and comprehensive elucidation of infrared dry-peeling peeling mechanism is vital and warrants further study. In conclusion, the analysis presented in this study provides a fundamental understanding of tomato peel loosening and cracking phenomena that occurred during infrared heating. Thermally induced skin separation was identified from LM images. SEM images revealed that microstructure changes in tomato skin resulting from infrared heating differ from changes induced by other peeling methods using hot lye or steam. SEM images showed that infrared heating altered the organization of skin extracellular cuticles, caused expansion of cell walls, and damaged middle lamella. It was concluded that the mechanism underlying infrared drypeeling was fundamentally different from the mechanisms of conventional hot lye and steam peeling. By combining experimental data with a theoretical model based on a thin-walled shell, this study quantitatively analyzed the relationship between skin microstructure and the mechanical behavior of skin membrane. Infrared heating caused vaporization of escaped cytosolic fluid and thereby caused an increase of internal vapor pressure under the loosened skin membrane. This increase in internal vapor pressure increased stress within the skin membrane. Peel cracking occurred when skin membrane stress exceeded the critical rupture stress. For a typical infrared dry-peeling condition, it was determined that peel cracking may occur when skin temperature

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