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Comparison of drying kinetics and texture effects of two calcium pretreatments before microwave-assisted dehydration of apple and potato. Lılia Ahrnй,* Frйdйric ...
International Journal of Food Science and Technology 2003, 38, 411–420

Comparison of drying kinetics and texture effects of two calcium pretreatments before microwave-assisted dehydration of apple and potato Lı´lia Ahrne´,* Fre´de´ric Prothon & Tomas Funebo SIK – The Swedish Institute for Food and Biotechnology Process and Environmental Engineering, PO Box 5401, SE-402 29 Gothenburg, Sweden (Received 8 February 2002; Accepted in revised form 30 June 2002)

Summary

The effects on drying rate and texture of treating two plant tissues with calcium, before drying in air with microwave assistance, were studied in this work. The two tissues, potato and apple cubes, which have different structures and composition, were pretreated by immersion in CaCl2 solutions at 20 or at 70 C before microwave-assisted air dehydration at 50, 60 and 70 C. The pretreatments with calcium influenced the strength of the plant tissue cell wall, producing products of varying hardness after rehydration. The effect of the two calcium pretreatments was quite different for apples and potatoes. For apples, calcium pretreatment at 20 C increased the hardness of rehydrated apples compared with untreated apples, but calcium pretreatment at 70 C had no effect on texture. For potatoes, both calcium pretreatments at 20 and at 70 C significantly increased the hardness of rehydrated potatoes. The water diffusivity during drying varied mainly because of the type of plant tissue, with secondary effects caused by the drying temperature and the type of calcium pretreatment.

Keywords

Blanching, calcium infiltration, diffusivity, drying rate.

Introduction

Consumer awareness of product quality requires that any drying processes and pretreatments are optimized in order to improve the perceived quality of dehydrated foods. Drying with microwave assistance has potential for producing better quality dried products while considerably reducing the duration of drying (Bouraoui et al., 1994). However, the quality of dried products depends not only on the drying process itself but also on the various steps preceding the drying process. Blanching of fruits and vegetables is a common pretreatment that precedes further techniques of preservation, such as freezing, canning and drying. The blanching temperatures commonly used in industry (90– 100 C) may lead to undesirable tissue softening (Bourne, 1987). However, there are a number of *Correspondent: Fax: +46 31 833 782; e-mail: [email protected]

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methods available to processors that avoid the excessive softening caused by heat processing. Low blanching temperatures and addition of calcium are examples (Karel et al., 1975). Bourne (1987) showed that carrots blanched at 74 C for 4 min retained a better texture during cooking at 100 C than carrots blanched at 100 C for 4 min. Similar results have been reported for canned green beans, cauliflower, tomatoes and potatoes. Few studies have been reported on the effect of blanching before drying. Mate´ et al. (1998) studied the effect of blanching on mechanical properties of dried potato slices. No reports have been found detailing the effect of blanching or other pretreatments before microwave drying of apples and potatoes. The ability of calcium, added to the blanching water, to inhibit softening has been extensively studied for several vegetables (Hughes et al., 1975; McFeeters & Fleming, 1989, 1990). Valle et al. (1998) studied the effect of blanching and calcium infusion on the texture and microstructure of osmotically dehydrated apples.

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Post-harvest calcium chloride treatments have largely been utilized to control or reduce quality changes, such as fruit softening, and prolong the storage life of fruits and vegetables. In this case, the whole fruit is dipped in a calcium solution at ambient temperature over a given time. This treatment is often reported in the literature and is generally known as calcium infiltration. Contrary to blanching, calcium infiltration, as a pretreatment preceding other preservation techniques, has been seldom studied. It has been widely reported in the literature that calcium plays an important role in providing stability and mechanical strength to the cell walls (Knee & Bartley, 1981). Calcium not only has a major effect on cell wall structure and membrane integrity, but also plays a regulatory role in various processes that affect cell function. Poovaiah et al. (1988) discussed specifically the mechanism of calcium action in delaying fruit softening. However, the effect of exogenous calcium in improving the firmness of plant tissue is not always explained by chemical modifications in the cell wall (Fergunsson, 1984; Siddiqui & Bangerth, 1996). These authors suggested that extensive cross-linking with pectic polymers may restrict the access of hydrolytic enzymes to cell wall compounds, and therefore calcium has an indirect role in keeping the middle lamella intact. The effect of calcium combined with a mild heat pretreatment, such as blanching, in improving the texture of vegetables is not clearly understood. Ng & Waldron (1997) reported that the enhanced firmness of cooked carrot tissues during blanching results from the increase in the thermal stability of calcium-cross-linked pectic polysaccharides, which stabilize cell adhesion. Moreover, the heat treatment in itself can cause several chemical reactions in plant tissue, such as enzymatic degradation of pectic compounds and starch gelatinization, which will also influence the strength of the cell wall. In vegetables containing starch, such as potatoes, the role of starch in the texture changes is even less understood, and it has given rise to controversies about its function in potato texture (Hoff, 1972; Reeve, 1972; Shomer, 1995). The effect of blanching on effective moisture diffusivity (Deff ) of water during drying of fruits and vegetables depends largely on the type of product under study (Gekas & Lamberg, 1991;

Sahbaz et al., 2000; Nieto et al., 2001). No reports were found concerning the effect of calcium treatments on the loss of water during drying. The purpose of this work is to compare the effect of the two calcium pretreatments to control softening of fruits and vegetables: (a) calcium infiltration at 20 C for 14 h and (b) blanching in a calcium solution at 70 C for 2 min. These are potential treatments preceding microwave-assisted air dehydration. Microwave dehydration was tested at 50, 60 and 70 C, as in this range of temperature significant modifications of cell structure can occur. The effect of calcium treatments was studied on drying rate and texture of product after rehydration. The two calcium pretreatments were applied to both apples and potatoes to find out whether the effect of these two calcium pretreatments on the drying rate and texture is influenced differently by the structure and composition of the different tissues. Potatoes and apples have significant differences in chemical composition and cellular structure. The intercellular air space has been estimated to be 20–25% of the total volume in apple and about 1% in potato (Aguilera & Stanley, 1990). Potato has water content of 63.2–86.9, while starch accounts for 20% of its total weight. Apples, on the other hand, have water content of 80–90% and 0.6% starch. Both potato and apple contain about 0.8% pectin (Souci et al., 1981). Materials and methods

Materials Apples of the variety Golden delicious were obtained from a local market and stored at 4 C, 95% r.h. Potatoes of the Bintje variety were also obtained from a local market and stored at 7 C and 95% r.h. Apples or potatoes were removed from the refrigerator and equilibrated to room temperature before being cut into 1.3 · 1.3 · 1.3 cm cubes. Calcium pretreatments Before dehydration, 50 apple or potato cubes were either: (a) infiltrated with calcium by immersion in 500-mL deionized water containing 1% CaCl2 overnight (14 h) in a closed bottle kept at ambient

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Comparison of two calcium treatments L. Ahrne´ et al.

temperature (20 C); or (b) blanched in deionized water containing 1% CaCl2 at 70 C for 2 min. After blanching, apple or potato cubes were cooled under running tap water prior to drying. Dehydration Samples were dehydrated using microwave-assisted air dehydration at 50, 60 and 70 C in a specially designed hot air and microwave oven as described by Funebo & Ohlsson (1998). The centre temperature of one untreated apple cube measured with fibre optic probes (Luxtron 790, Luxtron Co., USA) was used to control the microwave power level in order to keep the temperature of the cubes at 50, 60 or 70 C (Funebo, 2000). The air temperature was kept at the same temperature as the cubes and the air velocity was 2 m s)1. A total of 50 cubes for each pretreatment were dried simultaneously for a maximum of 5 h. Samples (five cubes) were removed from the dryer every hour to measure the water content. The drying time was determined as the time required to obtain a product with 0.1 kg water per kg. Water content The water content of the potato and apple cubes was determined by drying for up to 15 h in a 70 C vacuum oven at 100 mmHg until constant weight (AOAC, 1995). Two samples were used in each analysis. Rehydration Samples were rehydrated by immersion in sterile water at 20 C for 14 h. Texture measurement The texture was quantified by measuring the penetration force (expressed in grams) needed to puncture the plants tissue using a puncture test (Thompson et al., 1982). The maximum force value is related to hardness or firmness of the plant tissue. Measurements were performed in a Texture Analyser (model TA-XTA from Stable Microsystems, England) at a constant speed of 0.1 m s)1 using a puncture probe 2.5 mm in diameter. The cubes were cut into two halves  2003 Blackwell Publishing Ltd

and the texture measured by puncturing the surface of each half. Results and discussion

The two calcium treatments, which were either infiltration at 20 C for 14 h or blanching in a calcium solution at 70 C for 2 min, cause the same calcium uptake by the plant tissue. In this way, it is possible to assess the firming effect of calcium with and without a thermal effect. The blanching process (70 C, 2 min) was chosen after preliminary experiments, in which plant tissue was blanched at 70 C from 1 to 10 min (Ahrne´, 1999). Using the diffusivity data published by Andersson (1994) for uptake of calcium chloride by potato tissue at 20 and 70 C, the infiltration time required at 20 C to achieve the same calcium uptake as 70 C for 2 min was about 14 h. Thus, in both treatments about 10% (w/w) of calcium was taken up by the plant tissue. Apple tissue showed no significant changes in the water content after pretreatment. However, a slight decrease in water content was observed for blanched potato samples, probably because of the gelatinization of starch on the surface layers of potato cubes. Starch gelatinization in potato starts at temperatures around 67 C (Pravisani et al., 1985). Effect of calcium pretreatments on texture of apple and potato Table 1 shows the maximum force required to puncture the potatoes and apples after the calcium pretreatments. To evaluate the effect of the pretreatment on the texture of plant tissue, the PIT (Pretreatment effect on the Initial Texture) parameter, as defined by Moreira et al. (1994), was calculated as follows: PIT ¼ ln

FPT FR

ð1Þ

where FR is the texture property of raw tissue and FPT the texture property of the tissue after the pretreatment. The values obtained are plotted in Fig. 1. This parameter quantifies the softening (if it is negative) or the hardening (if it is positive) of the raw material caused by blanching and calcium infiltration. Figure 1 shows that calcium

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Table 1 Maximum force required to puncture apple and potato untreated, blanched in 1% CaCl2 at 70 C for 2 min or infiltrated with 1% CaCl2 for 14 h at 20 C before drying Apple

Potato

Sample

Max. force (g)

Error 95%

t-test (95%)*

Max. force (g)

Error 95%

t-test (95%)*

Untreated Blanched in 1% CaCl2 Infiltrated with 1% CaCl2

265 204 340

17 19 33

a b c

969 823 942

24 21 22

a b a

*The same letter means that results are not significantly different with 95% significance level, and different letters means that results are significantly different. Bold values show the harder samples obtained.

0.40 0.30 0.20

PIT parameter

414

Apple Potato

0.10 0.00 –0.10

Blanching

Infiltration

–0.20 –0.30 –0.40

Figure 1 Pre-treatment effect on the initial texture (PIT) parameter (eqn 1) calculated for apple and potato blanched in 1% CaCl2 at 70 C for 2 min or infiltrated with 1% CaCl2 for 14 h at 20 C. Negative values indicate softening and positive values hardening compared with untreated product. Error bars represent confidence intervals (P < 0.05).

infiltration increased the hardness of both the apple and potato tissue compared with the untreated tissue, while blanching had the opposite effect. It has been reported in the literature that blanching at moderate temperatures increases the strength of fruits and vegetables by activation of pectin methylesterase (Bourne, 1987). This enzyme is responsible for the demethylation of cell wall pectins and this facilitates the cross-linking of adjacent pectic polymers by calcium in the cell wall, particularly in the middle lamella, to form a cell wall network of greater mechanical strength (Damarty et al., 1984). The maximum activity of pectin methylesterase in apples and potatoes is observed around 60 C, while 50% of maximum activity is still observed after 2 min at 70 C, and no activity is observed at 90 C (Castaldo et al., 1989; Andersson et al., 1994). In our experiments, blanching at 70 C for 2 min decreases the

hardness of either potato or apple tissue compared with raw tissue. The effect of this pretreatment on changing the hardness of dehydrated products will be described later in this article. The uptake of calcium at 20 C significantly increased the strength of apple tissue but not potato tissue. As no greater water content was observed in apples or potatoes infiltrated with calcium solution at 20 C, compared with untreated samples, the fact that more calcium solution had filled the air spaces of apples than the potato, respectively, 20 and 1% (Aguilera & Stanley, 1990), may not explain the differences observed. Calcium may have higher affinity for the starch than for the pectins in the cell wall. Therefore, the calcium taken up by the potato tissue may link preferentially to starch rather than to pectins. Thus, no increase in the strength of the cell wall was observed in potatoes. In potatoes, calcium is usually linked to starch and it is only released during gelatinization (Bartolome & Hoff, 1972). Drying kinetics Untreated, calcium-blanched and calcium-infiltrated apple and potato were dehydrated simultaneously by microwave energy and air at constant temperatures at one of 50, 60 or 70 C. These temperatures were selected in order to understand the effect of starch gelatinization and cell membrane denaturation on the loss of water during drying. Cell membrane denaturation starts somewhere around 55 C for apples and potatoes (Emch, 1980). Andersson (1994) verified that, at temperatures above 65 C, the cell membrane denaturation occurred so fast that it was difficult to calculate the diffusivity of sugars in potatoes. Starch gelatinization, starting around

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Comparison of two calcium treatments L. Ahrne´ et al.

67 C (Pravisani et al., 1985), decreases the value of water diffusivity by affecting the mobility of water. This is a greater magnitude of change than other structural changes caused by blanching (Marousis et al., 1991; Mate´ et al., 1998). Figure 2a shows a typical drying curve, in this case, obtained during drying at 60 C. The dimensionless concentration M* was calculated for all the samples. M* is defined as M  Me ð2Þ M0  Me where M is the average water content on a dry basis (g water per g dry solids), Me the water content on a dry basis at equilibrium (in this case 0.1 g per g dry matter) and M0 the water content on a dry basis before drying. Water loss during drying is faster in apples than potatoes, but no larger differences were observed due to the pretreatment. M ¼

Kg water per kg dry matter

9.0 AI PI

7.0

AB PB

ANT PNT

6.0 5.0 4.0 3.0 2.0 1.0 0.0 0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

(b)

–Ln (M*)

2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0

1.0

2.0

3.0

4.0

5.0

6.0

Drying time (h)

Figure 2 Moisture content vs. time (a) and natural logarithm of dimensionless water content (M*)(1/3) (b) in blanched (B), infiltrated (I) and untreated (NT) apple (A) and potato (P) cubes during microwave-assisted drying at 60 C.

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ð3Þ This equation can be simplified by taking the first term of the series solution:   M  Me 8 Dt ð4Þ ¼ 2 exp p2 2 L M0  Me p For a finite geometry, such as a cube the dimensionless water content is given by (Singh & Heldman, 1993):    M  Me 8 Dt 3  ¼ ¼ 2 exp p2 2 ð5Þ Mcube p L M0  Me

(a) 8.0

The constant rate period typical of air-drying was only observed during the first minute of microwave drying (Bouraoui et al., 1994). Thus, it can be considered that internal diffusion was the mechanism responsible for the loss of water during the dehydration process, and the water diffusivity (D) can be calculated from the experimental drying data using a solution of Fick’s second law. Assuming uniform initial water distribution and negligible external resistance, the solution proposed by Crank (1979) for an infinite slab was:   1 M  Me 8X 1 Dt exp ð2n þ 1Þ2 p2 2 ¼ 2 L M0  Me p n¼0 ð2n þ 1Þ

where D is the water diffusivity (m2 s)1), L the thickness of the cube (m) and t the drying time (s). The representation of (M*)(1/3) vs. time on a semilogarithmic graph allows the determination of the diffusivity (D) from the slope of the curves (Fig. 2b). For potato, a linear trend was observed for all pretreatments and at all temperatures tested during the first 5 h of drying. However, for apple, this linear trend was only observed during the first 3 h. The diffusivity values calculated for each product and pretreatment at different drying temperature are shown in Fig. 3. Diffusivities between 0.873 and 2.17 · 10)9 m2 s)1 were obtained for potato and 1.57 and 4.37 · 10)9 m2 s)1 for apple. Similar values for drying of apple and potato have been reported by Mittal (1999). In Fig. 3, it is interesting to note that the diffusivity of apples is similar to that of potatoes during dehydration at 50 C, but at 70 C it is twice that of potatoes. As expected, with the exception of blanching of potatoes, the pretreatments did not significantly affect the water diffusitivity. Blanching of potatoes decreased the water diffusivity compared with calcium infiltration and untreated

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–20.4

6.0 ANT PNT

5.0

AI PI

Apple

AB PB

–20.2 –20.0

4.0

Ln D

Diffusivity × 10−9(m2s−1)

3.0

–19.8 –19.6 NT

–19.4

I

2.0

–19.2

1.0

B

–19.0

0.0

–21.0

40

50

60

70

80

Potato –20.8

Drying temperature (°C)

–20.6 Ln D

Figure 3 Diffusivity of blanched (B), infiltrated (I) and untreated (NT) apples (A) and potatoes (P) during drying at 50, 60 and 70 C.

–20.4 –20.2

potatoes (Fig. 3). A similar decrease in water diffusivity, because of blanching, has been reported by Gekas & Lamberg (1991) and Mate´ et al. (1998). As mentioned in the introduction, the lower diffusivity value observed for blanched samples has been explained by the gelatinization of the starch during blanching, which reduces water mobility. This theory seems to be supported by comparing the diffusivity of apples and potatoes at 50 C, where starch is not gelatinized, and at 70 C, where most of starch is gelatinized. At 50 C similar values of diffusivity are observed for apples and potatoes, but at 70 C apples have considerably higher diffusivity values than potatoes (Fig. 3). The effect of the temperature on diffusivity can be described by an Arrhenius-type equation:   Ea ð6Þ D ¼ D0 exp  RT where D0 is the preexponential factor, Ea the activation energy, R the gas constant (8.314 J mol)1) and T absolute temperature (K). The graph of ln D vs. the reciprocal of the absolute temperature (1/T) (Fig. 4) showed that the diffusivity for water loss can be related by using this equation. The activation energy values were calculated from the slope of curves (Fig. 5). The activation energy values that were obtained compared well with reported activation energy for water diffusivity in apple (47.3–68.3 kJ mol)1) and potato (30–66 kJ mol)1) (Mittal, 1999). Apples are more sensitive to drying temperature than potatoes; i.e. increasing the drying

NT I B

–20.0 –19.8

0.0029

0.00295

0.003 0.00305 1/T (K)

0.0031

0.00315

Figure 4 Natural logarithm of the diffusion coefficient as a function of the reciprocal of the absolute temperature for blanched (B), infiltrated (I) and untreated (NT) apples and potatoes.

60.0 Activation energy (kJ mol−1)

416

Apple Potato

50.0 40.0 30.0 20.0 10.0 0.0 No treatment

Blanching

Infiltration

)1 Figure 5 Activation energy values (kJ mol ) for the water diffusivity of untreated, blanched and infiltrated apples and potatoes during drying.

temperature of apples from 50 to 70 C reduces the drying time to a greater extent than for potatoes. Blanched apples clearly showed slightly lower activation energy compared to untreated or infiltrated potatoes, but blanched potatoes showed a higher thermal sensibility than infiltrated or untreated potatoes. Once again, the difference is probably because of the mechanisms of starch gelatinization during blanching and drying.

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Comparison of two calcium treatments L. Ahrne´ et al.

Texture of rehydrated samples

1.20

Table 2 Hardness of rehydrated apple and potato measured as maximum force (g) in a puncture test. The deviations are confidence interval (P < 0.05)

140 ± 27 151 ± 32 86 ± 14

AB PB

AI PI

0.80

0.40

0.00

–0.40

–0.80

–1.20 40

50

60

70

80

Drying temperature (˚C)

Figure 6 Dimensionless maximum force of rehydrated apples (A) and potatoes (P) that have been blanched (B), infiltrated (I) and untreated (NT) before drying at 50, 60 and 70 C.

(Evans & Haisman, 1982), which may explain the differences between blanched and infiltrated samples, and untreated samples. Finally, to assess the effect of the pretreatment on the final texture of the rehydrated material, the force values for apples and potatoes that had been blanched or infiltrated with calcium before drying were compared with untreated samples. For this purpose the PVP parameter (Pretreatment effect on the texture Variation due to Processing), as defined by Moreira et al. (1994), was used as follows:   F =FPT ð7Þ PVP ¼ ln FNT =FR where FPT is the texture property of the tissue after the pretreatment and FR the texture property of raw tissue; FNT and F defined the texture of the rehydrated material, respectively, for untreated and pretreated material. This parameter quantifies how the pretreatment will affect further changes during drying; when the parameter is zero, the

Apple Potato Drying temperature No No (°C) treatment Blanching Infiltration treatment 50 60 70

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ANT PNT

Ln (force/force before drying)

The quality characteristics of the rehydrated samples depended not only on the drying process but also on the pretreatments before drying. Table 2 shows the hardness of rehydrated apple and potato measured as the maximum force needed for puncture. Figure 6 shows a dimensionless force value, expressed as a logarithm (force rehydrated/force before drying), and quantifies the softening (if it is negative) or the hardening (if it is positive) of either the apples or the potatoes caused by the drying process. As expected, increasing the drying temperature from 50 to 70 C caused a softening of the rehydrated samples for all samples studied. Figure 6 shows that similar curves were obtained for potatoes infiltrated at 20 C and blanched at 70 C, where a hardening of potato tissue is observed during drying at 50 and 60 C, but not at 70 C. While untreated samples are more sensitive to temperature, at 60 C no hardening effect is observed. Apples, on the other hand, showed similar curves for blanched and untreated samples, where a significant softening of the tissue during drying was observed at all temperatures. However, calcium infiltration significantly improved the strength of the apple tissue during drying at any of the temperatures tested. Mild thermal pretreatment during blanching may cause damage to the cellular structure of apples. Consequently, no calcium could cross-link the pectin structure, and no improvement in the cell wall strength was observed at any of the drying temperature tested. However, in potatoes, the blanching pretreatment probably did not damage the structure of the cell wall, and therefore blanched and infiltrated samples had similar behaviour. Calcium chloride also decreases the gelatinization temperature of starch

145 ± 39 134 ± 18 85 ± 27

683 ± 110 445 ± 42 404 ± 72

Blanching Infiltration

1302 ± 92 1220 ± 65 990 ± 132 1190 ± 82 398 ± 109 841 ± 86

1487 ± 111 1298 ± 98 873 ± 124

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1.8

AI PI

Ln (force rehydrated/force before drying)

AB PB

1.60 1.20 PVP value

0.80 0.40 0.00 40

50

60

70

80

Figure 7 Effect of pretreatments on texture of rehydrated apple and potato compared with no use of pretreatment. Positive vales of PVP (eqn 7) indicate that a harder product is obtained.

pretreatment did not affect the sensitivity of the vegetable tissue to the drying. If it was positive, then less softening (or more hardening) occurred during drying as a result of the changes promoted by the pretreatment, and the reverse was true if it was negative. In Fig. 7, this parameter is shown for drying at 50, 60 and 70 C. Compared with drying of untreated material, blanching and calcium infiltration improved the hardness of apples and potatoes. However, this effect was almost insignificant at 50 C for all samples except apples infiltrated with calcium. For drying at 70 C, a significant increase in hardening, compared with untreated samples, was shown for all samples except blanched apples. Relationship between texture and diffusivity The main source of water in plant tissue is the cell. In plant tissues, the loss of water during drying involves water transport through the cell membrane and cell wall. In an attempt to understand how the changes in cell wall, caused by calcium and perceived as texture changes, affected the diffusivity of water during drying, the changes in texture were plotted against the diffusivity for each product and pretreatment (Fig. 8). The trend lines showed an increase in diffusivity with the softening of samples. However, temperature increases both diffusivity and softening of plant tissue. Comparing the trend lines for apples and potatoes, it seems that the increase or decrease in cell wall strength did not influence the water diffusivity. Although the uptake of calcium during blanching decreased the hardness of both potato and apple

Potato

Harder sample Lower diffusivity

1.2

PNT PI PB

50°C 60°C

0.6

70°C

0 –0.6 Softer sample Higher diffusivity

–1.2 –1.8

Drying temperature (˚C)

0

1.8

Ln (force rehydrated/force before drying)

418

1.5

1

0.5

Harder sample Lower diffusivity

1.2

2

2.5

3

ANT PI AB

Apple 50°C 60°C

70°C

0.6

0

–0.6 –1.2 Softer sample Higher diffusivity

–1.8 0

1

2

3 9

4

5

2 -1

Diffusivity × 10 (m s )

Figure 8 Relationship between diffusivity and firmness, expressed by the ln (force dehydrated/force before drying) for calcium blanching (B), infiltrated (I) and untreated (NT) apples (A) and potatoes (P).

compared with raw tissue (Fig. 1), a decrease of diffusivity was observed for potatoes but not for apples. On the other hand, calcium infiltration significantly increased the strength of apple tissue but not potato tissue (Fig. 1); nevertheless, no significant differences were observed between calcium infiltration and untreated samples either for potatoes or apples. Conclusions

The loss of water is similar in apples and potatoes during drying at 50 C, but during drying at 70 C the loss of water is significantly slower in potatoes. For apples, the pretreatments did not influence the loss of water during drying, but blanching before drying of potatoes slightly decreased the water diffusivity values compared with the untreated samples. The strengthening of cell walls by calcium did not influence the water diffusivity, but gelatinization of starch retarded water diffusion. The application of exogenous calcium to increase the firmness of plant tissue produced

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Comparison of two calcium treatments L. Ahrne´ et al.

different results in apples and potatoes depending on whether or not calcium pretreatment is combined with mild thermal treatment. The application of calcium during blanching of apples did not improve the firmness of the rehydrated tissue, but the firmness of potatoes was noticeably increased. On the other hand, the uptake of calcium at ambient temperature significantly increased the texture of rehydrated potato similarly to blanched potato. It seems that the strengthening of the cell wall by calcium in potatoes and apples does not follow the same mechanisms. Further research is necessary to understand the effect of calcium on starch and hydrolytic enzymes involved in cell wall degradation and the effect of heat treatment on cell wall and membranes. Acknowledgments

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