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Low frequency ultrasonics for texture measurements in carrots (Daucus carota L.) in relation to water loss and storage
Postharvest Biology and Technology 14 (1998) 297 – 308
Low frequency ultrasonics for texture measurements in carrots (Daucus carota L.) in relation to water loss and storage M. Nielsen *, H.J. Martens, K. Kaack Department of Fruit, Vegetable and Food Science, Danish Institute of Agricultural Sciences, Kirstinebjerg6ej 6, DK-5792 Aarsle6, Denmark Received 17 March 1998; accepted 12 August 1998
Abstract The use of low frequency ultrasonics for evaluation of carrot texture was studied. Carrots were pretreated for 24 h in distilled water and either dried at 25°C for periods from 0 to 320 min or stored at 100 or 97% relative air humidity for 19 weeks. Uniaxial compression, microscopy, and analyses of density and dry matter provided results for evaluating relationships between texture and ultrasonic parameters. The loss of water from turgid carrot tissue did not cause significant change in compressive Young’s modulus (ECY) and fracture work (J). The results indicate that the preliminary loss of water caused a decrease in compressive ECY and J. At a water loss \ 5%, the results indicate an increase in compressive ECY and J. The loss of water in the drying experiment had only a small effect on the velocity (6) and attenuation (a) of ultrasound. The results indicate at the beginning of the drying period an increase in 6 and decrease in a. This was followed by a decrease in 6 and increase in a. ECY decreased, while strain (o) at failure increased during the storage period, especially after storage at low humidity. Studies with scanning electron microscopy and light microscopy comparing fresh and stored tissue indicated that low humidity induced shrinkage of the cell content, changes in wall structure and development of intercellular air spaces. The 6 of ultrasound decreased during storage at low humidity contemporary with changes in the mechanical and microstructural properties. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Carrot; Texture; Ultrasound; Compression; Microstructure
1. Introduction
* Corresponding author. Tel.: + 45 659 91766; fax + 45 659 91756; e-mail: [email protected]
Low frequency ultrasonics has been proposed as an objective analytical technique for non-destructive evaluation of maturity and quality, such as textural properties and dry matter content, in
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fruits and vegetables (Self et al., 1990; Cheng, 1993). The parameters commonly measured are velocity (6) and attenuation (a), which are characteristic for the material and can be related to its physical properties e.g. elastic constants, density, composition, and microstructure (McClemments, 1991). These physical properties are determined by biological properties, such as tissue turgor pressure, cell wall properties, cell to cell binding, cellular integrity and tissue anatomy. All these properties have a direct connection with quality (Self et al., 1992). Ultrasonic propagation parameters for the evaluation of quality during storage have been used for melon (Mizrach et al., 1994), avocado (Mizrach et al., 1994; Self et al., 1994), potato (Cheng, 1993) and apple (Upchurch et al., 1987). The 6 of ultrasound in avocado flesh and banana pulp appears to change simultaneously with a change in the volume fraction of intercellular air spaces (air content) (Self et al., 1990, 1994). In avocado, a positive correlation has been found between 6 and water content (Self et al., 1994). Investigating apples and potatoes, Self et al. (1992) found that the air content may have an effect on 6. It is also known that 6 measurements are sensitive to internal cellular arrangement and that the elastic modulus of tissue has a significant effect on 6 (Self et al., 1994). It has further been suggested that scattering of the wave from intercellular air spaces and modifications in the elastic properties of tissues could be two possible mechanisms by which the air content of fruits and vegetables affects 6 and a (Self et al., 1994). The moisture content of the tissue and cells is one of the most important components affecting the physical properties of vegetative materials. Phan et al. (1973) have shown that carrots stored in an atmosphere with a low RH of 75% at 1°C lost up to 50% of their initial fresh weight after 166 days in storage. This loss of moisture causes shrivelling, which begins at the skin, and gradually reaches interior tissues. When the loss amounts to 5% the eating quality is affected and when it amounts to 40 – 50%, the size of the carrots is drastically reduced (Phan et al., 1973). Reduction in quality through loss of firmness and apparent freshness, and also through suscep-
tibility of the tissue to damage is caused by the loss of water (Lin and Pitt, 1986). Being incompressible, the moisture content maintains texture in two different ways. The fluid cell content ensures that the volumes of all cells remain constant when a piece of the plant tissue is stretched (Nilson et al., 1958), and the fluid exerts a pressure on the cell walls (turgor pressure), causing them to be distended in a state of elastic stress (internal stress), even when no external forces act on the tissue (Nilson et al., 1958; Kokkoras, 1990; McGarry, 1993). It has been shown that the turgor pressure of cells affects tissue stiffness in potatoes (Nilson et al., 1958; Murase et al., 1980), and compressive failure stress and strain in apples (De Baerdemaeker et al., 1978; Pitt and Chen, 1983). Turgid tissue, tissue with a high cell turgor pressure, is brittle and crisp (Vincent, 1990). Measuring toughness by cutting, Atkins and Vincent (1984) found that carrots were much less tough when turgid than they were when flaccid. Toughness of carrot tissue is determined predominantly by the volume fraction and composition of the cell walls, the extent of adhesion between neighboring cells, and the turgor pressure (McGarry, 1995). At high turgor pressure, the amount of additional stress required to induce wall fracture is reduced (McGarry, 1995). Lin and Pitt (1986) showed that the turgor pressure influenced the mode of failure during constant strain rate loading of apple and potato tissue. Our objectives were to examine the changes in 6 and a of low frequency ultrasound transmission through the tissue of carrots with different air and water status and to examine how ultrasonic propagation parameters correlate with textural and structural changes during storage of carrots.
2. Materials and methods
2.1. Plant material Carrots (Daucus carota L. cv. Tamino) were grown in the experimental fields at Research Centre Aarslev for 5 months in 1994, harvested in October, and kept at 1–2°C. Prior to experiments
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carrots were cleaned and sorted according to size, 25 – 30 mm in diameter. Carrots with marks or bruises were discarded.
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were used for each combination of humidity and removal time from storage.
2.4. Sample preparation 2.2. Drying experiment For the drying experiment, the carrots were pretreated for 24 h in distilled water at 18°C to give the turgid weight, and placed in a heating cabinet at 25°C for 0, 20, 40, 80, 120, 160, 200, 240, and 320 min to give the final weight. Immediately after drying the carrots were placed in plastic bags at 18°C until the following day when they were analysed. This time was found to be sufficient to ensure moisture equilibration throughout the different tissues of individual roots. A few carrots with longitudinal splits were found after pretreatment in distilled water, and these were discarded. The experiment was performed in the period November 13 – 25, 1994. The nine treatments (drying periods) with two replicates were carried out in randomized order. Samples of 20 carrots were used for each combination of humidity and replication.
2.3. Storage experiment For the storage experiment, the carrots were placed in desiccators, of approximately 25 l, in a cold storage room at 1°C. To establish different humidity in the desiccators, air was forced, with a pump (WISA 302, 8.4 l min − 1), through 1500 ml distilled water or a solution of 600 g NH4Cl in 1500 ml distilled water. An air humidity of 79.5% is found over a saturated aqueous solution of NH4Cl within a closed space at 20°C. In a preliminary experiment, the humidity in the desiccators was measured by a relative humidity transmitter (General Eastern RH2). An air humidity of 97% was found in the desiccators when the air was forced through the saturated aqueous solution of NH4Cl at 1°C and an air humidity of 100% was found when the air was forced through distilled water at 1°C. The carrots were stored from November 28, 1994, and samples were taken after 0, 3, 7, 11, 15, and 19 weeks. Analyses were carried out after a shelf life period of 2 days in a cold storage room at 12°C. Samples of 20 carrots
For each treatment a cork borer was used to take a xylem parenchyma (‘core region’) cylinder of 15 mm in diameter from the 20 carrots, 10 mm from the top and parallel to the axis of the carrot. Five cylinders were used for measurement of respectively uniaxial compression, ultrasound 6 and a, dry matter content, and density.
2.5. Uniaxial compression Each of the five cylinders was cut by a mounted blade into three smaller cylinders, 10 mm in height, with parallel ends. The cylindrical specimens were immediately compressed between parallel plates of an Instron universal testing machine at a constant deformation rate of 20 mm min − 1 to beyond failure point, which was marked by a significant drop in force reading. Failure occurred as general cleavage of the tissue. ECY was defined as the slope of the loading curve at the point of its highest gradient before inflection. Strain (o) at failure was defined as the proportional deformation at fracture, and fracture work (J) as the area under the loading curve up to failure.
2.6. Water status, density and dry matter Weight loss was calculated from the difference between final weight and turgid weight. The density (g cm − 3) of the tissue was determined on each of five carrot cylinders using Archimedes’ principle. The content of dry matter (w/w%) was measured by weighing before and after freeze drying for 48 h at 10 − 6 mmHg and 20°C.
2.7. Microstructure In the storage experiment, a minimum of five tissue samples, originating from each combination of humidity, replication, and removal from storage, were used for microscopy. Xylem parenchyma from each cylinder was cut into small, oriented blocks (: 1×3× 5 mm) and fixed in 3%
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glutaraldehyde in a pH 6.8, 0.1 M phosphate buffer for 24 h using a mild vacuum for 30 min. For light microscopy, samples were washed, dehydrated through an ethanol series and infiltrated in Technovit resin (Kulzer). After embedding, sections of 3 mm were cut on a LEICA Supercut microtome, stained with Calcofluor white MR2, mounted in Fluoromount and observed in a Nikon Optiphot fluorescence microscope. For scanning electron microscopy, samples were dehydrated in acetone, critical point dried (Balzer CPD020), mounted on silver-painted stubs, and gold-sputtered (Polaron SC 7640 Sputter Coater). Observations were made on a JEOL JSM T-20 scanning electron microscope accelerating at 20 kV.
2.8. Velocity and attenuation of ultrasound Ultrasound waves were generated with a PUNDIT ultrasound generator (CNS Electronics Ltd., London) and 37 kHz transducers. The pulse generator operated at 1.2 kV with a pulse repetition frequency of 10 Hz. The time base synchronization pulse and the received waveform were displayed on a Tektronix 520 Digitizing Oscilloscope from which the time of flight of the ultrasound waves through the sample was determined (Self et al., 1992; Nielsen and Martens, 1997). The cylinders were placed between transducers coaxially with a 50 mm length of perspex of a slightly smaller diameter than the sections (Povey and McClements, 1988; Nielsen and Martens,
1997). The cylinders were serially shortened, from 40 to 20 mm, and the length of the cylinder, time of flight and amplitude at specific reference points on the pulse function were measured. This was repeated three times for each of the three lengths. Ultrasound 6 was determined from the slope of plots of sample length versus time of flight, and ultrasound a from the slope of plots of logarithmic decrement of the amplitudes versus sample length.
2.9. Statistics Data were subjected to analysis of variance (GLM) and means were separated using an F-test with significance defined at P5 0.05. LSD indicates the least significant difference at the 95% level.
3. Results
3.1. Drying experiment During pretreatment in distilled water for 24 h, the carrots absorbed water corresponding to 10.4% of initial fresh weight. As shown in Table 1, the weight loss increased during the entire drying period. The dry matter content increased slightly during the drying period, except for the abrupt drop at the beginning. No clear change was apparent for the density.
Table 1 Weight loss, dry matter content, and density after drying of carrots at 25°C Min
Weight loss (w/w%)
Dry matter content (w/w%)
Density (g cm−3)
0 20 40 80 120 160 200 240 320 LSD
— 0.71 1.15 2.93 3.83 4.94 7.13 8.56 12 0.97
10.47 9.33 9.47 9.78 10.3 9.65 10.75 10.91 11.43 0.73
1.021 1.0148 1.0128 1.0088 1.0212 1.0208 1.017 1.0201 1.0243 ns
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Table 2 Compressive Young’s modulus, fracture work, and velocity and attenuation of ultrasound in carrot cylinders, 15 mm in diameter, after drying at 25°C Min
Compressive Young’s modulus (kPa)
Fracture work (kN m−2)
Sound velocity (m s−1)
Sound attenuation (dB mm−1)
0 20 40 80 120 160 200 240 320 LSD
6962 6691 6997 6827 6802 6671 7787 7893 7835 ns
0.50 0.47 0.45 0.44 0.47 0.46 0.48 0.49 0.48 ns
396 431 527 437 399 365 429 409 406 134
1.23 1.26 1.09 1.20 1.21 1.37 1.33 1.40 1.46 n.s.
The results in Table 2 indicate that the preliminary loss of water caused a decrease in compressive ECY. Beyond 160 min, corresponding to a weight loss of :5%, compressive ECY seemed to increase. Fracture work showed a small decrease at the beginning of the drying period, followed by an increase to near the original value with further loss of moisture content (Table 2). The pulse transmitted through turgid tissue was received as a damped oscillation. Loss of moisture did not change the shape of the received pulse (data not shown) and had only a small effect on 6 and a (Table 2). A 6 of 396 m s − 1 was found in fully turgid carrots (drying time 0). The 6 increased to 527 m s − 1 after drying for 40 min, after which it returned to the original value, with only small changes during the remaining drying period. A attenuation of 1.25 db mm − 1 was found in fully turgid tissue (Table 2). A decrease at 40 min of drying was followed by an increase during the rest of the period (not significant).
3.2. Storage experiment The content of dry matter was higher in carrots stored at low humidity (Table 3). For the carrots stored at high humidity the content of dry matter decreased slightly through the storage period. This was not seen for carrots stored at low humidity. As expected, a pronounced rise in weight loss was seen during storage at low humidity. This weight loss was of course far beyond what would
be acceptable to consumers. At a weight loss of 20–30%, the carrots were clearly affected, showing shrivelling and a dry appearance. The density decreased during storage, especially for carrots stored at low humidity. ECY showed an increase from 0 to 3 weeks of storage at high humidity, and from 0 to 7 weeks of storage at low humidity. This was followed by a steady decrease to 6400 kPa during the remaining storage period at high humidity, and a steeper decrease to 6000 kPa at low humidity (Fig. 1). Compared with fresh carrots, o increased from 0.47 to 0.52 after storage of 3 weeks at both high and low humidity. At low humidity, o increased further to 0.6 at the end of the storage period. At high humidity o remained at 0.5–0.53 until the end of the storage period (Fig. 1). The micrographs of the fresh carrots show a compact tissue with turgid cells and few, small, intercellular spaces (Fig. 2A, B). Apart from some loss of adhesion between neighboring cells, only small differences could be observed between fresh carrots and carrots stored at high humidity for 19 weeks (Fig. 2C, D), whereas clear changes were observed during storage at low humidity for 19 weeks (Fig. 2E, F). This treatment gradually led to dehydration of the tissue, which was recognized by extensive development of intercellular spaces (Fig. 2F) and a collapsed cell shape (Fig. 2E, F). Structural changes were also apparent in the cellulosic component of the cell wall, and it appeared swollen and diffuse (Fig. 2F).
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Fig. 1. Compressive Young’s modulus and o at failure in carrot cylinders, 15 mm in diameter and 10 mm in height, during storage at high and low humidity.
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The pulse transmitted through the carrot cylinder was received as a damped oscillation. No clear change of the signal was seen with regard to storage time and humidity. Ultrasound 6 for carrots stored at low humidity showed a preliminary increase followed by a decrease during the storage period (Fig. 3). No clear change in 6, during the storage period, could be seen for carrots stored at high humidity. Ultrasound a during storage at high and low humidity showed a preliminary decreased after 3 weeks, followed by an increase to a maximum at the original value, after 11 weeks of storage (Fig. 3). No clear correlation was found between 6 and either density or compressive ECY (Fig. 4), but an indication of a positive relationship was seen. 4. Discussion The change in 6 and a of ultrasound during the drying and storage experiment is believed to be caused by different processes arising from change in microstructure and change in the contents of water and air in the tissue. Reduced 6 is expected with low water content (Self et al., 1994; Nielsen and Martens, 1997). This could be caused by reduction of the turgor pressure or by an increase in the air content of the tissue. The effect of air on 6 can be due to scattering of the ultrasound wave or modification of the elastic properties of the tissue (Self et al., 1992). A large air content is often, but not always, associated with a lower 6 (Self et al., 1992).
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Carrots pretreated in distilled water for 24 h became fully turgid. As a result of this, the internal stresses and strains in the tissue increased (Kokkoras, 1990). In general, tissue at high turgor pressure shows a high elastic modulus and a brittle or crisp texture. When the turgor is reduced the internal stresses are reduced (Kokkoras, 1990) and the tissue shows a lower modulus and becomes flaccid, but the strength of the tissue (ability to resist compression or tension) remains constant or increases (Vincent, 1990). As shown in Table 1, the amount of water removed during drying was of the same magnitude as that absorbed during pretreatment in distilled water (10.4%). According to Karmas (1980) it is presumed that the drying rate would be constant. In this investigation a small increase in drying rate was seen at a weight loss : 5%. This could be caused by morphological changes which make water transport in the tissue easier and thus enable faster evaporation. These changes, together with the small changes in density, could mean that at a weight loss of around (above) 5%, drying reduced the cell turgor pressure, the internal stresses and strains in the tissue, and induced small morphological changes by shrivelling but no development of internal air spaces. Increase in the dry matter content confirmed that some shrivelling took place. The preliminary loss of water from the turgid carrot tissue had only a small influence on compressive ECY and J. As the water is lost from the tissue, the turgor pressure is reduced and the cells
Table 3 Dry matter content, weight loss, and density during storage of carrots at high and low humidity Weeks
0 3 7 11 15 19 LSD
Dry matter (w/w%)
Density (g cm−3)
Weight loss (w/w%)
%RH 100
97
%RH 100
97
%RH 100
97
12.8 10.7 11.4 10.5 9.5 10.3 0.5
12.8 11.3 11.5 13.4 12.4 13.4 0.4
0 2.2 1.3 2.1 1.7 1.5 —
0 2.5 5.6 21.8 18.1 19.7 —
1.0329 1.0258 1.0257 1.0192 1.0181 1.0094 0.0139
1.0329 1.0292 1.0176 1.0101 0.9926 0.9943 0.0207
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Fig. 2. Cross sections of the xylem parenchyma scanning electron micrographs (bar = 100 mm). fluorescent brightener (bar = 50 mm). (A) and (B) humidity for 19 weeks. Parenchyma cell (P), cell
region of carrot affected by different storage treatments. Figs. A, C, and E are Figs. B, D, and F are light micrographs after staining with calcofluor white fresh; (C) and (D) stored at high humidity for 19 weeks; (E) and (F) stored at low wall (arrow), intercellular space (arrow head).
become less dilated and will be more closely packed. This would decrease the internal stresses in the tissue and make the cell walls more difficult to rupture. This could explain the increases in compressive ECY and increase J of the tissue at a loss of water \5%.
The loss of water in the drying experiment did not change 6 and a significantly. The results indicate that at the beginning of the drying period, 6 increased and a decreased. This could be expected with the initial change in water and air distribution in the tissue as the cells became less dilated.
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Fig. 3. Velocity and attenuation of ultrasound in carrot cylinders, 15 mm in diameter, during storage at high and low humidity.
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Fig. 4. Scatter plot of ultrasound velocity versus respectively density and compressive Young’s modulus in carrots stored at high and low humidity.
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With further loss of water from the tissue, which reduced turgor pressure and the internal stress and strain in the tissue, the results indicate a decrease in 6 and an increase in a. In contrast to the drying experiment, the cells began to separate and intercellular spaces developed as water was removed during storage of carrots. This caused a clear decline in rigidity of the tissue measured as compressive ECY (Fig. 1). Decline in the rigidity of the tissue will additionally be affected by weakening of the cell walls and cell membranes during storage. The content of air and water in the tissue changes during storage of carrots at low humidity. Self et al. (1994) have shown that, during ripening of avocado fruit, the volume fraction of intercellular spaces (air content) was negatively correlated with the density of the flesh. The negative correlation between weight loss and density in this investigation could mean that the content of air in the tissue increased as water was lost. In the work by Self et al. (1994) the density was not dependent on water or dry matter content. This has also been shown in apples (Self et al., 1992) and seems to apply to carrots too (Tables 1 and 3). Self et al. (1992) describe this as a consequence of the very large density difference between cells and air, so that small changes in air content have a much greater effect on tissue density than similar changes in cellular composition. The more pronounced changes in mechanical properties seen after storage at low humidity were confirmed by the microstructural observations. After 19 weeks of storage the reduced humidity affected the texture by shrinkage of cell contents and development of intercellular spaces. The cellulosic polymers in the cell wall appeared swollen and diffuse, a phenomenon which is possibly due to alteration of the crystallinity of cellulose fibres (Sterling and Shimazu, 1961; Grote and Fromme, 1984) and may very well affect the texture. Storage at high humidity showed only minor differences in the microstructure compared with fresh carrots. This is in agreement with Davis and Gordon (1977) who found no effect on the morphological appearance, when viewed in SEM, of carrot phloem and xylem after 10 weeks of storage. When viewed by LM, however, carrots stored
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at high humidity showed some loss of turgidity (Fig. 2D). Ultrasound 6 at the beginning of storage was 405 m s − 1 (Fig. 3). For fresh carrots, Mizrach et al. (1989) found 6 of 341 m s − 1 at a frequency of 50 kHz and Self et al. (1992) found 569 m s − 1 at 37 kHz. The difference in 6 can be caused by the use of different cultivars, sample size, and measurement conditions. The decreasing 6 during storage of carrots (Fig. 3), especially at low humidity, occurred simultaneously with a decrease in compressive ECY (Fig. 1). The development of intercellular spaces and the increasing content of air in the tissue during storage at low humidity will affect the junction between cells (Self et al., 1990, 1992). This in combination with a general softening of the tissue will lead to a decreasing Young’s modulus (rigidity) of the tissue and thus contribute to a reduced 6. But no clear correlation was found between compressive ECY and 6 (Fig. 4). The reduced 6 at increasing weight loss is in agreement with the positive correlation between 6 and water content found for avocado flesh (Self et al., 1994). The development of intercellular air spaces could lead to a higher degree of scattering at certain frequencies of the ultrasonic wave, which likewise could affect the transmission of ultrasound through the tissue (Self et al., 1992). The density and 6 decreased during storage (Table 3 and Fig. 3). No clear correlation was found, but an indication of a direct relationship between density and 6 were seen (Fig. 4). This was also found by Self et al. (1992) in apples and potatoes. The relationship disagrees with the equation of 6 of wave propagation, from which the sound 6 is inversely related to the square root of the density, but could be caused by the dominant effect of air content and deformability modulus which masks the effect of density (Self et al., 1992). Ultrasound 6 and a through carrot tissue seem to be affected by change in texture. This influence is not only dependent on the change in mechanical parameters but also by changes in microstructure and content of water and air in the tissue. The use of ultrasound clearly has potential as a method for texture (quality) measurements in carrots but more research is needed to further ex-
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plain the relationship between the ultrasonic and mechanical parameters and to improve the measuring technique and signal analyses.
References Atkins, A.G., Vincent, J.F.V., 1984. An instrumented microtome for improved histological sections and the measurement of fracture toughness. J. Mater. Sci. Lett. 3, 310–312. Cheng, Y., 1993. Non-destructive quality evaluation of fruits and vegetables using ultrasound. Ph.D. thesis. Virginia Polytechnic Institute and State University, Blacksburg, Virginia. pp. 170. Davis, E.A., Gordon, J., 1977. Morphological comparison of two varieties of carrots during growth and storage: scanning electron microscopy. Home Econ. R. J. 6, 15–23. De Baerdemaeker, J., Segerlind, L.J., Murase, H., Merva, G.E., 1978. Water potential effect on tensile and compressive failure stresses of apple and potato tissue. ASAE paper No. 78-3057. Grote, M., Fromme, G.F., 1984. Electron microscopic investigations of the cell structure in fresh and processed vegetables (carrots and green bean pods). Food Microstruct. 3, 55– 64. Karmas, E., 1980. Techniques for measurement of moisture content of foods. Food Technol. April, 52–59. Kokkoras, I.F., 1990. The Effect of Temperature and Water Status on the Susceptibility of Carrot Roots to Damage. Ph.D. thesis. Cranfield Institute of Technology, Silsoe Campus, p. 224. Lin, T., Pitt, R.E., 1986. Rheology of apple and potato tissue as affected by cell turgor pressure. J. Text. Studies 17, 291 – 313. McClemments, D.J., 1991. Ultrasonic characterization of emulsions and suspensions. Adv. Colloid Interface Sci. 37, 33– 72. McGarry, A., 1993. Influence of water status on carrot (Daucus carota L.) fracture properties. J. Hort. Sci. 68, 431– 437. McGarry, A., 1995. Cellular basis of tissue toughness in carrot (Daucus carota L.) storage roots. Ann. Bot. 75, 157–163.
.
Mizrach, A., Galili, N., Rosenhouse, G., 1989. Determination of fruit and vegetable properties by ultrasonic excitation. Trans. ASAE 32, 2053 – 2058. Mizrach, A., Galili, N., Rosenhouse, G., 1994. Determining quality of fresh products by ultrasonic excitation. Food Technol. December, 68 – 71. Murase, H., Merva, G.E., Segerlind, L.J., 1980. Variation of Young’s modulus of potato as a function of water potential. Trans. ASAE 23, 794 – 796. Nielsen, M., Martens, H.J., 1997. Low frequency ultrasonics for texture measurements in cooked carrots (Daucus carota L.). J. Food Sci. 62, 1167 – 1175. Nilson, S.B., Hertz, C.H., Falk, S., 1958. On the relation between turgor pressure and tissue rigidty II. Physiol. Plant. 11, 818 – 837. Phan, C.T., Hsu, H., Sarkar, S.K., 1973. Physical and chemical changes occurring in the carrot root during storage. Can. J. Plant Sci. 53, 635 – 641. Pitt, R.E., Chen, H.L., 1983. Time dependent aspects of the strength and rheology of vegetative tissue. Trans. ASAE 26, 1275 – 1280. Povey, M.J.W., McClements, D.J., 1988. Ultrasonics in food engineering. Part I, Introduction and experimental methods. J. Food Engin. 8, 217 – 245. Self, G., Povey, M., Wainwright, H., 1990. Non-destructive methods of evaluating maturity, ripening and quality in tropical fruits. In: Proceeding from XXIII International Horticultural Congress (2422), p. 650. Self, G.K., Povey, M.J.W., Wainwright, H., 1992. What do ultrasound measurements in fruit and vegetables tell you? In: Povey, M.J.W., McClements D.J. (Eds.), Developments in Acoustics and Ultrasonics. Procter Department Food Science, The University of Leeds, Leeds, pp. 129 – 163. Self, G.K., Ordozgoiti, E., Povey, M.J.W., Wainwright, H., 1994. Ultrasonic evaluation of ripening avocado flesh. Postharvest Biol. Technol. 4, 111 – 116. Sterling, C., Shimazu, F., 1961. Cellulose crystallinity and the reconstitution of dehydrated carrots. J. Food Sci. 26, 479 – 484. Upchurch, B.L, Miles, G.E., Stroshine, R.L., Furgason, E.S., Emerson, F.H., 1987. Ultrasonic measurement for detecting apple bruises. Am. Soc. Agric. Eng. 30, 803 – 809. Vincent, J.F.V., 1990. Fracture properties of plants. Adv. Bot. Res. 17, 235 – 287.
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Low frequency ultrasonics for texture measurements in carrots (Daucus carota L.) in relation to water loss and storage M. Nielsen *, H.J. Martens, K. Kaack Department of Fruit, Vegetable and Food Science, Danish Institute of Agricultural Sciences, Kirstinebjerg6ej 6, DK-5792 Aarsle6, Denmark Received 17 March 1998; accepted 12 August 1998
Abstract The use of low frequency ultrasonics for evaluation of carrot texture was studied. Carrots were pretreated for 24 h in distilled water and either dried at 25°C for periods from 0 to 320 min or stored at 100 or 97% relative air humidity for 19 weeks. Uniaxial compression, microscopy, and analyses of density and dry matter provided results for evaluating relationships between texture and ultrasonic parameters. The loss of water from turgid carrot tissue did not cause significant change in compressive Young’s modulus (ECY) and fracture work (J). The results indicate that the preliminary loss of water caused a decrease in compressive ECY and J. At a water loss \ 5%, the results indicate an increase in compressive ECY and J. The loss of water in the drying experiment had only a small effect on the velocity (6) and attenuation (a) of ultrasound. The results indicate at the beginning of the drying period an increase in 6 and decrease in a. This was followed by a decrease in 6 and increase in a. ECY decreased, while strain (o) at failure increased during the storage period, especially after storage at low humidity. Studies with scanning electron microscopy and light microscopy comparing fresh and stored tissue indicated that low humidity induced shrinkage of the cell content, changes in wall structure and development of intercellular air spaces. The 6 of ultrasound decreased during storage at low humidity contemporary with changes in the mechanical and microstructural properties. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Carrot; Texture; Ultrasound; Compression; Microstructure
1. Introduction
* Corresponding author. Tel.: + 45 659 91766; fax + 45 659 91756; e-mail: [email protected]
Low frequency ultrasonics has been proposed as an objective analytical technique for non-destructive evaluation of maturity and quality, such as textural properties and dry matter content, in
0925-5214/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S0925-5214(98)00056-8
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fruits and vegetables (Self et al., 1990; Cheng, 1993). The parameters commonly measured are velocity (6) and attenuation (a), which are characteristic for the material and can be related to its physical properties e.g. elastic constants, density, composition, and microstructure (McClemments, 1991). These physical properties are determined by biological properties, such as tissue turgor pressure, cell wall properties, cell to cell binding, cellular integrity and tissue anatomy. All these properties have a direct connection with quality (Self et al., 1992). Ultrasonic propagation parameters for the evaluation of quality during storage have been used for melon (Mizrach et al., 1994), avocado (Mizrach et al., 1994; Self et al., 1994), potato (Cheng, 1993) and apple (Upchurch et al., 1987). The 6 of ultrasound in avocado flesh and banana pulp appears to change simultaneously with a change in the volume fraction of intercellular air spaces (air content) (Self et al., 1990, 1994). In avocado, a positive correlation has been found between 6 and water content (Self et al., 1994). Investigating apples and potatoes, Self et al. (1992) found that the air content may have an effect on 6. It is also known that 6 measurements are sensitive to internal cellular arrangement and that the elastic modulus of tissue has a significant effect on 6 (Self et al., 1994). It has further been suggested that scattering of the wave from intercellular air spaces and modifications in the elastic properties of tissues could be two possible mechanisms by which the air content of fruits and vegetables affects 6 and a (Self et al., 1994). The moisture content of the tissue and cells is one of the most important components affecting the physical properties of vegetative materials. Phan et al. (1973) have shown that carrots stored in an atmosphere with a low RH of 75% at 1°C lost up to 50% of their initial fresh weight after 166 days in storage. This loss of moisture causes shrivelling, which begins at the skin, and gradually reaches interior tissues. When the loss amounts to 5% the eating quality is affected and when it amounts to 40 – 50%, the size of the carrots is drastically reduced (Phan et al., 1973). Reduction in quality through loss of firmness and apparent freshness, and also through suscep-
tibility of the tissue to damage is caused by the loss of water (Lin and Pitt, 1986). Being incompressible, the moisture content maintains texture in two different ways. The fluid cell content ensures that the volumes of all cells remain constant when a piece of the plant tissue is stretched (Nilson et al., 1958), and the fluid exerts a pressure on the cell walls (turgor pressure), causing them to be distended in a state of elastic stress (internal stress), even when no external forces act on the tissue (Nilson et al., 1958; Kokkoras, 1990; McGarry, 1993). It has been shown that the turgor pressure of cells affects tissue stiffness in potatoes (Nilson et al., 1958; Murase et al., 1980), and compressive failure stress and strain in apples (De Baerdemaeker et al., 1978; Pitt and Chen, 1983). Turgid tissue, tissue with a high cell turgor pressure, is brittle and crisp (Vincent, 1990). Measuring toughness by cutting, Atkins and Vincent (1984) found that carrots were much less tough when turgid than they were when flaccid. Toughness of carrot tissue is determined predominantly by the volume fraction and composition of the cell walls, the extent of adhesion between neighboring cells, and the turgor pressure (McGarry, 1995). At high turgor pressure, the amount of additional stress required to induce wall fracture is reduced (McGarry, 1995). Lin and Pitt (1986) showed that the turgor pressure influenced the mode of failure during constant strain rate loading of apple and potato tissue. Our objectives were to examine the changes in 6 and a of low frequency ultrasound transmission through the tissue of carrots with different air and water status and to examine how ultrasonic propagation parameters correlate with textural and structural changes during storage of carrots.
2. Materials and methods
2.1. Plant material Carrots (Daucus carota L. cv. Tamino) were grown in the experimental fields at Research Centre Aarslev for 5 months in 1994, harvested in October, and kept at 1–2°C. Prior to experiments
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carrots were cleaned and sorted according to size, 25 – 30 mm in diameter. Carrots with marks or bruises were discarded.
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were used for each combination of humidity and removal time from storage.
2.4. Sample preparation 2.2. Drying experiment For the drying experiment, the carrots were pretreated for 24 h in distilled water at 18°C to give the turgid weight, and placed in a heating cabinet at 25°C for 0, 20, 40, 80, 120, 160, 200, 240, and 320 min to give the final weight. Immediately after drying the carrots were placed in plastic bags at 18°C until the following day when they were analysed. This time was found to be sufficient to ensure moisture equilibration throughout the different tissues of individual roots. A few carrots with longitudinal splits were found after pretreatment in distilled water, and these were discarded. The experiment was performed in the period November 13 – 25, 1994. The nine treatments (drying periods) with two replicates were carried out in randomized order. Samples of 20 carrots were used for each combination of humidity and replication.
2.3. Storage experiment For the storage experiment, the carrots were placed in desiccators, of approximately 25 l, in a cold storage room at 1°C. To establish different humidity in the desiccators, air was forced, with a pump (WISA 302, 8.4 l min − 1), through 1500 ml distilled water or a solution of 600 g NH4Cl in 1500 ml distilled water. An air humidity of 79.5% is found over a saturated aqueous solution of NH4Cl within a closed space at 20°C. In a preliminary experiment, the humidity in the desiccators was measured by a relative humidity transmitter (General Eastern RH2). An air humidity of 97% was found in the desiccators when the air was forced through the saturated aqueous solution of NH4Cl at 1°C and an air humidity of 100% was found when the air was forced through distilled water at 1°C. The carrots were stored from November 28, 1994, and samples were taken after 0, 3, 7, 11, 15, and 19 weeks. Analyses were carried out after a shelf life period of 2 days in a cold storage room at 12°C. Samples of 20 carrots
For each treatment a cork borer was used to take a xylem parenchyma (‘core region’) cylinder of 15 mm in diameter from the 20 carrots, 10 mm from the top and parallel to the axis of the carrot. Five cylinders were used for measurement of respectively uniaxial compression, ultrasound 6 and a, dry matter content, and density.
2.5. Uniaxial compression Each of the five cylinders was cut by a mounted blade into three smaller cylinders, 10 mm in height, with parallel ends. The cylindrical specimens were immediately compressed between parallel plates of an Instron universal testing machine at a constant deformation rate of 20 mm min − 1 to beyond failure point, which was marked by a significant drop in force reading. Failure occurred as general cleavage of the tissue. ECY was defined as the slope of the loading curve at the point of its highest gradient before inflection. Strain (o) at failure was defined as the proportional deformation at fracture, and fracture work (J) as the area under the loading curve up to failure.
2.6. Water status, density and dry matter Weight loss was calculated from the difference between final weight and turgid weight. The density (g cm − 3) of the tissue was determined on each of five carrot cylinders using Archimedes’ principle. The content of dry matter (w/w%) was measured by weighing before and after freeze drying for 48 h at 10 − 6 mmHg and 20°C.
2.7. Microstructure In the storage experiment, a minimum of five tissue samples, originating from each combination of humidity, replication, and removal from storage, were used for microscopy. Xylem parenchyma from each cylinder was cut into small, oriented blocks (: 1×3× 5 mm) and fixed in 3%
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glutaraldehyde in a pH 6.8, 0.1 M phosphate buffer for 24 h using a mild vacuum for 30 min. For light microscopy, samples were washed, dehydrated through an ethanol series and infiltrated in Technovit resin (Kulzer). After embedding, sections of 3 mm were cut on a LEICA Supercut microtome, stained with Calcofluor white MR2, mounted in Fluoromount and observed in a Nikon Optiphot fluorescence microscope. For scanning electron microscopy, samples were dehydrated in acetone, critical point dried (Balzer CPD020), mounted on silver-painted stubs, and gold-sputtered (Polaron SC 7640 Sputter Coater). Observations were made on a JEOL JSM T-20 scanning electron microscope accelerating at 20 kV.
2.8. Velocity and attenuation of ultrasound Ultrasound waves were generated with a PUNDIT ultrasound generator (CNS Electronics Ltd., London) and 37 kHz transducers. The pulse generator operated at 1.2 kV with a pulse repetition frequency of 10 Hz. The time base synchronization pulse and the received waveform were displayed on a Tektronix 520 Digitizing Oscilloscope from which the time of flight of the ultrasound waves through the sample was determined (Self et al., 1992; Nielsen and Martens, 1997). The cylinders were placed between transducers coaxially with a 50 mm length of perspex of a slightly smaller diameter than the sections (Povey and McClements, 1988; Nielsen and Martens,
1997). The cylinders were serially shortened, from 40 to 20 mm, and the length of the cylinder, time of flight and amplitude at specific reference points on the pulse function were measured. This was repeated three times for each of the three lengths. Ultrasound 6 was determined from the slope of plots of sample length versus time of flight, and ultrasound a from the slope of plots of logarithmic decrement of the amplitudes versus sample length.
2.9. Statistics Data were subjected to analysis of variance (GLM) and means were separated using an F-test with significance defined at P5 0.05. LSD indicates the least significant difference at the 95% level.
3. Results
3.1. Drying experiment During pretreatment in distilled water for 24 h, the carrots absorbed water corresponding to 10.4% of initial fresh weight. As shown in Table 1, the weight loss increased during the entire drying period. The dry matter content increased slightly during the drying period, except for the abrupt drop at the beginning. No clear change was apparent for the density.
Table 1 Weight loss, dry matter content, and density after drying of carrots at 25°C Min
Weight loss (w/w%)
Dry matter content (w/w%)
Density (g cm−3)
0 20 40 80 120 160 200 240 320 LSD
— 0.71 1.15 2.93 3.83 4.94 7.13 8.56 12 0.97
10.47 9.33 9.47 9.78 10.3 9.65 10.75 10.91 11.43 0.73
1.021 1.0148 1.0128 1.0088 1.0212 1.0208 1.017 1.0201 1.0243 ns
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Table 2 Compressive Young’s modulus, fracture work, and velocity and attenuation of ultrasound in carrot cylinders, 15 mm in diameter, after drying at 25°C Min
Compressive Young’s modulus (kPa)
Fracture work (kN m−2)
Sound velocity (m s−1)
Sound attenuation (dB mm−1)
0 20 40 80 120 160 200 240 320 LSD
6962 6691 6997 6827 6802 6671 7787 7893 7835 ns
0.50 0.47 0.45 0.44 0.47 0.46 0.48 0.49 0.48 ns
396 431 527 437 399 365 429 409 406 134
1.23 1.26 1.09 1.20 1.21 1.37 1.33 1.40 1.46 n.s.
The results in Table 2 indicate that the preliminary loss of water caused a decrease in compressive ECY. Beyond 160 min, corresponding to a weight loss of :5%, compressive ECY seemed to increase. Fracture work showed a small decrease at the beginning of the drying period, followed by an increase to near the original value with further loss of moisture content (Table 2). The pulse transmitted through turgid tissue was received as a damped oscillation. Loss of moisture did not change the shape of the received pulse (data not shown) and had only a small effect on 6 and a (Table 2). A 6 of 396 m s − 1 was found in fully turgid carrots (drying time 0). The 6 increased to 527 m s − 1 after drying for 40 min, after which it returned to the original value, with only small changes during the remaining drying period. A attenuation of 1.25 db mm − 1 was found in fully turgid tissue (Table 2). A decrease at 40 min of drying was followed by an increase during the rest of the period (not significant).
3.2. Storage experiment The content of dry matter was higher in carrots stored at low humidity (Table 3). For the carrots stored at high humidity the content of dry matter decreased slightly through the storage period. This was not seen for carrots stored at low humidity. As expected, a pronounced rise in weight loss was seen during storage at low humidity. This weight loss was of course far beyond what would
be acceptable to consumers. At a weight loss of 20–30%, the carrots were clearly affected, showing shrivelling and a dry appearance. The density decreased during storage, especially for carrots stored at low humidity. ECY showed an increase from 0 to 3 weeks of storage at high humidity, and from 0 to 7 weeks of storage at low humidity. This was followed by a steady decrease to 6400 kPa during the remaining storage period at high humidity, and a steeper decrease to 6000 kPa at low humidity (Fig. 1). Compared with fresh carrots, o increased from 0.47 to 0.52 after storage of 3 weeks at both high and low humidity. At low humidity, o increased further to 0.6 at the end of the storage period. At high humidity o remained at 0.5–0.53 until the end of the storage period (Fig. 1). The micrographs of the fresh carrots show a compact tissue with turgid cells and few, small, intercellular spaces (Fig. 2A, B). Apart from some loss of adhesion between neighboring cells, only small differences could be observed between fresh carrots and carrots stored at high humidity for 19 weeks (Fig. 2C, D), whereas clear changes were observed during storage at low humidity for 19 weeks (Fig. 2E, F). This treatment gradually led to dehydration of the tissue, which was recognized by extensive development of intercellular spaces (Fig. 2F) and a collapsed cell shape (Fig. 2E, F). Structural changes were also apparent in the cellulosic component of the cell wall, and it appeared swollen and diffuse (Fig. 2F).
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Fig. 1. Compressive Young’s modulus and o at failure in carrot cylinders, 15 mm in diameter and 10 mm in height, during storage at high and low humidity.
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The pulse transmitted through the carrot cylinder was received as a damped oscillation. No clear change of the signal was seen with regard to storage time and humidity. Ultrasound 6 for carrots stored at low humidity showed a preliminary increase followed by a decrease during the storage period (Fig. 3). No clear change in 6, during the storage period, could be seen for carrots stored at high humidity. Ultrasound a during storage at high and low humidity showed a preliminary decreased after 3 weeks, followed by an increase to a maximum at the original value, after 11 weeks of storage (Fig. 3). No clear correlation was found between 6 and either density or compressive ECY (Fig. 4), but an indication of a positive relationship was seen. 4. Discussion The change in 6 and a of ultrasound during the drying and storage experiment is believed to be caused by different processes arising from change in microstructure and change in the contents of water and air in the tissue. Reduced 6 is expected with low water content (Self et al., 1994; Nielsen and Martens, 1997). This could be caused by reduction of the turgor pressure or by an increase in the air content of the tissue. The effect of air on 6 can be due to scattering of the ultrasound wave or modification of the elastic properties of the tissue (Self et al., 1992). A large air content is often, but not always, associated with a lower 6 (Self et al., 1992).
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Carrots pretreated in distilled water for 24 h became fully turgid. As a result of this, the internal stresses and strains in the tissue increased (Kokkoras, 1990). In general, tissue at high turgor pressure shows a high elastic modulus and a brittle or crisp texture. When the turgor is reduced the internal stresses are reduced (Kokkoras, 1990) and the tissue shows a lower modulus and becomes flaccid, but the strength of the tissue (ability to resist compression or tension) remains constant or increases (Vincent, 1990). As shown in Table 1, the amount of water removed during drying was of the same magnitude as that absorbed during pretreatment in distilled water (10.4%). According to Karmas (1980) it is presumed that the drying rate would be constant. In this investigation a small increase in drying rate was seen at a weight loss : 5%. This could be caused by morphological changes which make water transport in the tissue easier and thus enable faster evaporation. These changes, together with the small changes in density, could mean that at a weight loss of around (above) 5%, drying reduced the cell turgor pressure, the internal stresses and strains in the tissue, and induced small morphological changes by shrivelling but no development of internal air spaces. Increase in the dry matter content confirmed that some shrivelling took place. The preliminary loss of water from the turgid carrot tissue had only a small influence on compressive ECY and J. As the water is lost from the tissue, the turgor pressure is reduced and the cells
Table 3 Dry matter content, weight loss, and density during storage of carrots at high and low humidity Weeks
0 3 7 11 15 19 LSD
Dry matter (w/w%)
Density (g cm−3)
Weight loss (w/w%)
%RH 100
97
%RH 100
97
%RH 100
97
12.8 10.7 11.4 10.5 9.5 10.3 0.5
12.8 11.3 11.5 13.4 12.4 13.4 0.4
0 2.2 1.3 2.1 1.7 1.5 —
0 2.5 5.6 21.8 18.1 19.7 —
1.0329 1.0258 1.0257 1.0192 1.0181 1.0094 0.0139
1.0329 1.0292 1.0176 1.0101 0.9926 0.9943 0.0207
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Fig. 2. Cross sections of the xylem parenchyma scanning electron micrographs (bar = 100 mm). fluorescent brightener (bar = 50 mm). (A) and (B) humidity for 19 weeks. Parenchyma cell (P), cell
region of carrot affected by different storage treatments. Figs. A, C, and E are Figs. B, D, and F are light micrographs after staining with calcofluor white fresh; (C) and (D) stored at high humidity for 19 weeks; (E) and (F) stored at low wall (arrow), intercellular space (arrow head).
become less dilated and will be more closely packed. This would decrease the internal stresses in the tissue and make the cell walls more difficult to rupture. This could explain the increases in compressive ECY and increase J of the tissue at a loss of water \5%.
The loss of water in the drying experiment did not change 6 and a significantly. The results indicate that at the beginning of the drying period, 6 increased and a decreased. This could be expected with the initial change in water and air distribution in the tissue as the cells became less dilated.
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Fig. 3. Velocity and attenuation of ultrasound in carrot cylinders, 15 mm in diameter, during storage at high and low humidity.
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Fig. 4. Scatter plot of ultrasound velocity versus respectively density and compressive Young’s modulus in carrots stored at high and low humidity.
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With further loss of water from the tissue, which reduced turgor pressure and the internal stress and strain in the tissue, the results indicate a decrease in 6 and an increase in a. In contrast to the drying experiment, the cells began to separate and intercellular spaces developed as water was removed during storage of carrots. This caused a clear decline in rigidity of the tissue measured as compressive ECY (Fig. 1). Decline in the rigidity of the tissue will additionally be affected by weakening of the cell walls and cell membranes during storage. The content of air and water in the tissue changes during storage of carrots at low humidity. Self et al. (1994) have shown that, during ripening of avocado fruit, the volume fraction of intercellular spaces (air content) was negatively correlated with the density of the flesh. The negative correlation between weight loss and density in this investigation could mean that the content of air in the tissue increased as water was lost. In the work by Self et al. (1994) the density was not dependent on water or dry matter content. This has also been shown in apples (Self et al., 1992) and seems to apply to carrots too (Tables 1 and 3). Self et al. (1992) describe this as a consequence of the very large density difference between cells and air, so that small changes in air content have a much greater effect on tissue density than similar changes in cellular composition. The more pronounced changes in mechanical properties seen after storage at low humidity were confirmed by the microstructural observations. After 19 weeks of storage the reduced humidity affected the texture by shrinkage of cell contents and development of intercellular spaces. The cellulosic polymers in the cell wall appeared swollen and diffuse, a phenomenon which is possibly due to alteration of the crystallinity of cellulose fibres (Sterling and Shimazu, 1961; Grote and Fromme, 1984) and may very well affect the texture. Storage at high humidity showed only minor differences in the microstructure compared with fresh carrots. This is in agreement with Davis and Gordon (1977) who found no effect on the morphological appearance, when viewed in SEM, of carrot phloem and xylem after 10 weeks of storage. When viewed by LM, however, carrots stored
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at high humidity showed some loss of turgidity (Fig. 2D). Ultrasound 6 at the beginning of storage was 405 m s − 1 (Fig. 3). For fresh carrots, Mizrach et al. (1989) found 6 of 341 m s − 1 at a frequency of 50 kHz and Self et al. (1992) found 569 m s − 1 at 37 kHz. The difference in 6 can be caused by the use of different cultivars, sample size, and measurement conditions. The decreasing 6 during storage of carrots (Fig. 3), especially at low humidity, occurred simultaneously with a decrease in compressive ECY (Fig. 1). The development of intercellular spaces and the increasing content of air in the tissue during storage at low humidity will affect the junction between cells (Self et al., 1990, 1992). This in combination with a general softening of the tissue will lead to a decreasing Young’s modulus (rigidity) of the tissue and thus contribute to a reduced 6. But no clear correlation was found between compressive ECY and 6 (Fig. 4). The reduced 6 at increasing weight loss is in agreement with the positive correlation between 6 and water content found for avocado flesh (Self et al., 1994). The development of intercellular air spaces could lead to a higher degree of scattering at certain frequencies of the ultrasonic wave, which likewise could affect the transmission of ultrasound through the tissue (Self et al., 1992). The density and 6 decreased during storage (Table 3 and Fig. 3). No clear correlation was found, but an indication of a direct relationship between density and 6 were seen (Fig. 4). This was also found by Self et al. (1992) in apples and potatoes. The relationship disagrees with the equation of 6 of wave propagation, from which the sound 6 is inversely related to the square root of the density, but could be caused by the dominant effect of air content and deformability modulus which masks the effect of density (Self et al., 1992). Ultrasound 6 and a through carrot tissue seem to be affected by change in texture. This influence is not only dependent on the change in mechanical parameters but also by changes in microstructure and content of water and air in the tissue. The use of ultrasound clearly has potential as a method for texture (quality) measurements in carrots but more research is needed to further ex-
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plain the relationship between the ultrasonic and mechanical parameters and to improve the measuring technique and signal analyses.
References Atkins, A.G., Vincent, J.F.V., 1984. An instrumented microtome for improved histological sections and the measurement of fracture toughness. J. Mater. Sci. Lett. 3, 310–312. Cheng, Y., 1993. Non-destructive quality evaluation of fruits and vegetables using ultrasound. Ph.D. thesis. Virginia Polytechnic Institute and State University, Blacksburg, Virginia. pp. 170. Davis, E.A., Gordon, J., 1977. Morphological comparison of two varieties of carrots during growth and storage: scanning electron microscopy. Home Econ. R. J. 6, 15–23. De Baerdemaeker, J., Segerlind, L.J., Murase, H., Merva, G.E., 1978. Water potential effect on tensile and compressive failure stresses of apple and potato tissue. ASAE paper No. 78-3057. Grote, M., Fromme, G.F., 1984. Electron microscopic investigations of the cell structure in fresh and processed vegetables (carrots and green bean pods). Food Microstruct. 3, 55– 64. Karmas, E., 1980. Techniques for measurement of moisture content of foods. Food Technol. April, 52–59. Kokkoras, I.F., 1990. The Effect of Temperature and Water Status on the Susceptibility of Carrot Roots to Damage. Ph.D. thesis. Cranfield Institute of Technology, Silsoe Campus, p. 224. Lin, T., Pitt, R.E., 1986. Rheology of apple and potato tissue as affected by cell turgor pressure. J. Text. Studies 17, 291 – 313. McClemments, D.J., 1991. Ultrasonic characterization of emulsions and suspensions. Adv. Colloid Interface Sci. 37, 33– 72. McGarry, A., 1993. Influence of water status on carrot (Daucus carota L.) fracture properties. J. Hort. Sci. 68, 431– 437. McGarry, A., 1995. Cellular basis of tissue toughness in carrot (Daucus carota L.) storage roots. Ann. Bot. 75, 157–163.
.
Mizrach, A., Galili, N., Rosenhouse, G., 1989. Determination of fruit and vegetable properties by ultrasonic excitation. Trans. ASAE 32, 2053 – 2058. Mizrach, A., Galili, N., Rosenhouse, G., 1994. Determining quality of fresh products by ultrasonic excitation. Food Technol. December, 68 – 71. Murase, H., Merva, G.E., Segerlind, L.J., 1980. Variation of Young’s modulus of potato as a function of water potential. Trans. ASAE 23, 794 – 796. Nielsen, M., Martens, H.J., 1997. Low frequency ultrasonics for texture measurements in cooked carrots (Daucus carota L.). J. Food Sci. 62, 1167 – 1175. Nilson, S.B., Hertz, C.H., Falk, S., 1958. On the relation between turgor pressure and tissue rigidty II. Physiol. Plant. 11, 818 – 837. Phan, C.T., Hsu, H., Sarkar, S.K., 1973. Physical and chemical changes occurring in the carrot root during storage. Can. J. Plant Sci. 53, 635 – 641. Pitt, R.E., Chen, H.L., 1983. Time dependent aspects of the strength and rheology of vegetative tissue. Trans. ASAE 26, 1275 – 1280. Povey, M.J.W., McClements, D.J., 1988. Ultrasonics in food engineering. Part I, Introduction and experimental methods. J. Food Engin. 8, 217 – 245. Self, G., Povey, M., Wainwright, H., 1990. Non-destructive methods of evaluating maturity, ripening and quality in tropical fruits. In: Proceeding from XXIII International Horticultural Congress (2422), p. 650. Self, G.K., Povey, M.J.W., Wainwright, H., 1992. What do ultrasound measurements in fruit and vegetables tell you? In: Povey, M.J.W., McClements D.J. (Eds.), Developments in Acoustics and Ultrasonics. Procter Department Food Science, The University of Leeds, Leeds, pp. 129 – 163. Self, G.K., Ordozgoiti, E., Povey, M.J.W., Wainwright, H., 1994. Ultrasonic evaluation of ripening avocado flesh. Postharvest Biol. Technol. 4, 111 – 116. Sterling, C., Shimazu, F., 1961. Cellulose crystallinity and the reconstitution of dehydrated carrots. J. Food Sci. 26, 479 – 484. Upchurch, B.L, Miles, G.E., Stroshine, R.L., Furgason, E.S., Emerson, F.H., 1987. Ultrasonic measurement for detecting apple bruises. Am. Soc. Agric. Eng. 30, 803 – 809. Vincent, J.F.V., 1990. Fracture properties of plants. Adv. Bot. Res. 17, 235 – 287.
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