Convective air drying of apples as affected by blanching and calcium impregnation

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1 Available online at Journal of Food Engineering 87 (2008) Convective air drying of apples as affected by blanching and calcium impregnation M. González-Fésler a, D. Salvatori b,c,p.gómez a, S.M. Alzamora a,c, * a Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, 1428 Ciudad Autónoma de Buenos Aires, Argentina b Departamento de Química, Facultad de Ingeniería, Universidad Nacional del Comahue, Buenos Aires 1400, 8300 Neuquén, Argentina c Consejo Nacional de Investigaciones Científicas y Técnicas de la República, Argentina Received 6 August 2007; received in revised form 6 December 2007; accepted 9 December 2007 Available online 23 December 2007 Abstract The effect of previous blanching and calcium impregnation at atmospheric pressure (AI) or in vacuum (VI) on the rate of moisture movement during the first falling rate period of air drying of apples at 60 C was studied. It was found that the effective diffusion coefficient of water (D ef ) calculated with Fick s second law was strongly affected by heat pretreatment. With the exception of non-blanched tissues subjected to VI or VI followed by AI for 1.5 h, calcium uptake during impregnation step appeared to modify the matrix resistance to water flux only when tissue was previously heated. Light microscopy studies of apple tissues allowed explaining the observed drying behaviour. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Calcium enrichment; Impregnation techniques; Apple; Air drying; Blanching 1. Introduction Vacuum impregnation (VI) and atmospheric impregnation (AI) have been recently received increasing attention as potential processes for the design of new enriched vegetable products (Ortiz et al., 2003; Anino et al., 2006; Fito et al., 2001a,b). These techniques basically consist in immersing the sample in an adequately formulated solution under specific conditions of pressure. When solutions of high water activity are used as impregnation medium, the incorporation of a certain compound can be achieved, without appreciably modifying the macroscopic integrity of the product. The use of blanching as a pretreatment is usually carried out to prevent off flavours and color * Corresponding author. Address: Departamento de Industrias, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, 1428 Ciudad Autónoma de Buenos Aires, Argentina. Tel./fax: address: alzamora@ciudad.com.ar (S.M. Alzamora). changes resulting from enzymatic reactions and decrease the initial microorganism load, although it has been infrequently included as an additional step to enhance mass transport in the tissue. It is known that blanching often produces profound structural alterations (swelling of cell walls, disruption of membranes, shrinkage of intercellular spaces, etc.), which could affect mass transport phenomena during the following steps (Alzamora et al., 2000; Nieto et al., 2001; Nieto, 2004). Therefore, important changes in physicochemical and structural properties can take place in vegetable tissues after blanching and/or impregnation soaking processes, which will affect food behaviour during the final preservation stage. The existent literature reveals that when air drying operation is selected as an alternative to lengthen the impregnated product shelf-life, the influence of these previous operations differs widely as the tissue properties change from one foodstuff to another (Alvarez et al., 1995; Nieto et al., 2001). An adequate control of blanching and impregnation treatments prior to drying may be used as a tool to improve /$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi: /j.jfoodeng

2 324 M. González-Fésler et al. / Journal of Food Engineering 87 (2008) Nomenclature A AI D ef L LM m m r R c D P area of the cell impregnation under atmospheric pressure effective moisture diffusivity (m 2 /s) slab thickness or cylinder length (cm) light microscopy local moisture content (kg H 2 O/kg dry mass) average moisture content (kg H 2 O/kg dry mass) distance along the radial axis (cm) cylinder radius (cm) cylinder diameter (cm) perimeter of the cell S v volume shrinkage S F shape factor for cells (S F =4pA/P 2 ) t time (s) VI vacuum impregnation z distance along the axial axis (cm) Subscripts 0 initial e equilibrium 1 time under vacuum 2 time under atmospheric pressure mass transfer and to design preservation technologies for obtaining dehydrated enriched vegetable products (Fito et al., 2001a). The role of structure in vegetable dehydration appears evident to understand transport mechanisms. If food structure is also considered as a process variable, a better understanding of the dehydration kinetics and the effect of the pretreatments can be achieved (Nieto et al., 2001). The objective of this study was to analyze (1) the effect of calcium impregnation treatments at atmospheric pressure or under vacuum, with and without previous blanching, on the air drying kinetics of apple during the first falling rate period; and (2) the role of cell structure modifications and calcium composition on the moisture transport rate. 2. Materials and methods 2.1. Sample preparation and impregnation medium Fresh apples (Malus pumila, Granny Smith var.; a w ffi 0.98; Brix; ph 3.3 ± 0.1) were peeled, cut into cylinders (1.5 cm in diameter and 2 cm in length) and immediately immersed into the impregnation medium with forced convection at room temperature. Aqueous solutions containing 10.9% (w/w) glucose with calcium salts (5266 lg/g Ca 2+ ) were used as impregnation medium. A mixture of Ca 2+ lactate and Ca 2+ gluconate was chosen as Ca 2+ source because of its relatively high solubility at room temperature and the neutral taste imparted to the fruit (Anino et al., 2006). Potassium sorbate (1500 lg/g) was also added to the solution and the ph was adjusted to 3.5 by addition of citric acid to inhibit and/or retard microbial growth. The impregnation medium was isotonic regarding the content of apple native soluble solids to avoid water transfer mechanisms Pretreatments Different treatments were applied to apple samples before drying: calcium impregnation, blanching or a combination of blanching followed by calcium impregnation. For the incorporation of calcium to apple matrix two impregnation techniques were carried out by immersing the samples in the impregnation medium at room temperature under specific conditions of pressure: vacuum impregnation (VI) or atmospheric impregnation (AI). Fruit to solution ratio was 1:16 w/w. For VI, a vacuum pressure of 6,666 Pa was applied to the solution containing the apple cylinders for a time t 1 (15 min) and then atmospheric pressure was restored (t 2 0). AI was conducted under conditions of internal control to mass transfer and fruit samples were taken out of the solutions at different periods of time t 2 (1.5 h; 6 h). For blanching process prior to impregnation, samples were immersed in saturated vapour (2 min) at atmospheric pressure and then cooled in water at 1 2 C (5 min). These experimental conditions were selected according to previous results obtained in our laboratory (Anino et al., 2006; Salvatori et al., 2007). Under vacuum, short periods of time (t 1 = 5 15 min) were enough to reach the maximum absorption capacity of calcium, when the process ended immediately after atmospheric pressure was restored (t 2 = 0). On the contrary, during AI, plant cellular structure acts as a semi-permeable membrane, and the component is transferred from the concentrated solution to the cell by a process usually considered as diffusion driven. Although in this case longer treatment times (t 2 ) are required, a great solute final concentration can be achieved. Experiments were made in duplicate. For each experimental condition, four samples obtained from different apples were used. Two of them were identified, calcium impregnated (with and without blanching), measured, and analyzed for volume changes and cell tissue structure features (light microscopy). Other two samples were pretreated in the same way and then subjected to air drying Shrinkage determination Sample shrinkage was determined after the pretreatments. Measurements of height and diameter of apple cylinders were performed before and immediately following

3 M. González-Fésler et al. / Journal of Food Engineering 87 (2008) the pretreatment with a Teclock dial micrometer model SM-124 (Japan) (±0.01 cm) and volume sample was calculated from the outside geometric dimensions. Volume shrinkage (S v ) was calculated as [(Initial volume Volume after pretreatment)/initial volume] Moisture determination The moisture content of apples was determined gravimetrically using a vacuum oven at 70 C over magnesium perchlorate desiccant. The determination proceeded until constant weight was achieved Microscopic observations For light microscopy (LM) of fresh, impregnated and blanched samples, cubes (ffi3 mm 3 ) taken from the internal zone of the sample were fixed in glutaraldehyde solution (3 g/100 g) and then in 0.1 M potassium phosphate buffer (ph = 7.4) overnight at room temperature. Cubes were then rinsed three times with distilled water, postfixed in OsO 4 solution (1.5 g/100 g) at room temperature and dehydrated in a graded acetone series prior to be embedded in low viscosity Spurr resin. Sections (1 2 lm thick) of the Spurrembedded tissue were cut on a Sorvall MT2-B Ultracut microtome and stained with toluidine blue (1 g/100 g) and basic fuchsin (1 g/100 g) solutions and examined in a Zeiss Axioskop 2 microscope (Zeiss, Oberkochen, Germany). MatLab v. 7.0 (The MathWorks, Massachusetts, USA) was used to improve the image and to calculate the shape factor for cells (S F =4p A/P 2, where A is the area and P is the perimeter of the cell). Analysis was performed on cells on the average Air drying The drying equipment consisted of a centrifugal fan which blew the air through a heating section with six 2 kw electric resistances and then upwards through a vertical duct at the end of which the sample was suspended from a metallic frame. The vertical duct had a flowsmoothing section of small glass spheres. The air velocity was measured by a Wallac GGA23S anemometer (Finland) (±0.1 m/s) placed in the air duct. The drier was operated at constant air velocity (15 m/s) in all experiments. It was experimentally determined that an increase in air velocity did not influence the drying rate, and hence drying was controlled by the internal resistance to mass transfer. The inlet air dry bulb temperature was regulated by an electronic proportional controller. Wet and dry bulb thermometers were fitted in the drying chamber. All drying curves were conducted at 60 ± 1 C and the corresponding relative humidity values were 9 10%. The progress of drying was followed by weighing the sample periodically in a precision balance (± g). A very good reproducibly between pairs of drying curves performed under identical conditions was found. A pair of experiments was carried out to measure the temperature evolution of raw apples cylinders during drying. The measurement was performed by inserting very thin Cu Co thermocouples in the centre and near the surface of the apple cylinder Mathematical modeling To analyze drying process, mass transfer was estimated by application of Fick s second law of diffusion and the microscopic mass balance. When the sample is considered isotropic with regard mass transfer, this combination for a cylinder of radius R c and length 2L can be written as follows (Crank, 1975; Simal et al., 1998): þ om ot ¼ D o 2 m ef or þ 1 om o2 m ð1þ 2 r or oz 2 To solve this differential equation, the simplifying hypotheses used in most drying works were assumed (McMinn and Magee, 1996): 1. The initial moisture content was uniform throughout the solid. 2. The effective moisture diffusivity was constant throughout the solid. 3. The solid shape remained constant during the period considered (no shrinkage or deformation occurred). 4. The external resistance to heat and mass transfer was negligible, (i.e., the surface of the solid was at equilibrium with the surrounding air for the time considered). 5. The internal fruit temperature was uniform. With the following initial and boundary conditions, t ¼ r 6 R c and 0 6 z 6 L m ¼ m 0 t > 0 r ¼ R c and 0 6 z 6 L m ¼ m e om t > 0 r ¼ 0 and 0 6 z 6 L or ¼ 0 and considering a finite cylinder as the intersection of an infinite slab and an infinite cylinder, the solution of Fick s law for diffusion can be expressed as the product of the solutions of these two geometries (Crank, 1975). Integrating over the volume of the cylinder, results! m m e ¼ 8 X 1 1 m 0 m e p 2 n¼0 ð2n þ 1Þ exp D efð2n þ 1Þ 2 p 2 t 2 L 2 X1 4 exp b2 n D eft ð2þ n¼1 b 2 R 2 n c where m is the average moisture content (on a dry basis); m e is the equilibrium moisture content (on a dry basis) at the air dry bulb temperature; m 0 is the initial moisture content (on a dry basis); L is the slab thickness or cylinder length corresponding to m 0 ; R c is the cylinder radius corresponding to m 0 ; D ef is the effective moisture diffusivity; b n is the Bessel function roots (b 1 = ); t is the time.

4 326 M. González-Fésler et al. / Journal of Food Engineering 87 (2008) Eq. (2) can be expressed in terms of the Fourier number corresponding to an infinite slab (Fo s = D ef t4/l 2 ) and an infinite cylinder ðfo c ¼ D ef t=r 2 c Þ. Moreover, the Fo s number can be expressed as a function of number Fo c : 2 2R c Fo s ¼ Fo c ¼ Fo c a 2 ð3þ L where R c = cylinder radius and a = D/L. Eq. (2) can then be rewritten in terms of Fo c number as follows: m m e ¼ 8 X 1 m 0 m e p 2 p 2 L 2 þ b2 1 R 2 c X1 n¼0 n¼1 1 ð2n þ 1Þ 2 exp 4 b 2 n expð b 2 n Fo cþ! ð2n þ 1Þ 2 p 2 Fo c a 2 4 For values of Fo c number P0.1, it was demonstrated that the mathematical solution for the finite cylinder including only the first term of each infinite series in Eq. (4) represents 95% of the complete solution, so that terms with n > 1 could be neglected. The resulting linear equation is! m m e 32 log ¼ log 1 p 2 a 2 m 0 m e p 2 b 2 2:3 4 þ b2 1 Fo c ð5þ 1 From Eq. (5) and Fo c ¼ D ef t=r 2 c, the final model used is obtained:! m m e 32 log ¼ log p2 m 0 m e p 2 b 2 L þ b2 1 Def t ð6þ 2 R 2 2:3 1 c To establish the adequacy of Eq. (6) to experimental data, logðm=m 0 Þ was analyzed as a function of t and values of effective water diffusivity (D 2:3 ef) during the first falling rate period were determined by linear regression of data. D ef values were obtained using statistical software. Suppression of m e in Eq. (6) would not introduced significant errors since equilibrium moisture content values obtained from desorption isotherms of apple and other fruits for the same air temperature and relative humidity ranged between 0.02 and 0.08 g H 2 O/g dry mass (Nieto, 2004) Statistical analysis Statistical analyses were carried out using the STAT- GRAPHICS PLUS package (Washington, USA). Data of calcium concentration, sample shrinkage and D ef were expressed as mean ± standard error of the mean (mean ± SE). Differences between treatments were tested by one-way analysis of variance (ANOVA). When ANOVA showed statistical differences (P ), intragroup comparisons were established by applying multiple range tests. The significance of differences among D ef values was analyzed through a comparison between curves based on an analysis of variance (F-test of Snedecor, P ). ð4þ 3. Results and discussion 3.1. Air drying kinetics of blanched and/or calcium impregnated apples The rate at which drying is accomplished is governed by the rate at which heat and mass transport processes proceed. Analysis of moisture changes must then involve solution of coupled differential equations. However, apple temperature during drying can be considered uniform due to the low Biot number for heat transfer usually found for conventional air drying of fruits (Alzamora et al., 1979); so drying was considered to occur without gradient temperature into the solid. The study of temperature evolution throughout drying experiments carried out in this work revealed that the temperature in the centre of the cylinder of raw apple as well as in the surface increased rapidly at the beginning of drying (Fig. 1). Temperature near the surface reached the dry bulb temperature of air (60 C) at about 6 min of process, while temperature in the centre was tending towards this value from about 13 to 14 min. So the solid temperature could be considered as constant during drying process and equal to the dry bulb temperature of air (i.e., isothermal process). As a mode of example, some typical drying curves obtained for calcium impregnated apples, with and without previous blanching, are shown in Fig. 2, where average moisture content was expressed as a fraction of the initial water content of the fruit (m 0 ). A constant drying rate period was not detected in any of the experiments, which was in agreement with the drying behaviour reported in the literature for apples and other fruits and vegetables, such as carrots, potatoes, strawberries, green peas, mango, tomatoes, etc. (Nieto et al., 1998, 2001; Alvarez et al., 1995; Simal et al., 1998). T (ºC) t (min) Fig. 1. Experimental temperature evolution in the center (}) and near the surface (h) of raw apples during drying at 60 C.

5 M. González-Fésler et al. / Journal of Food Engineering 87 (2008) m / m 0 m / m t (min) log (m / m 0 ) log (m / m 0 ) ( π 2 /L /Rc 2 )*t (min/cm 2 ) t (min) Fig. 2. Experimental drying curves of raw, blanched and/or calcium impregnated apples at 60 C. (A) Ca 2+ impregnated by AI (t 1 =0, t 2 = 6 h) with and without previous blanching; (B) Ca 2+ impregnated by VI (t 1 = 15 min, t 2 = 0) with and without previous blanching. raw; blanched; M Ca 2+ impregnated; s blanched and Ca 2+ impregnated. m = average moisture content. Drying curves were represented according to the Fick s diffusional model (Eq. (6)) in Fig. 3, where log ðm=m 0 Þ p was plotted vs 2 þ b2 L 2 1 t for non-blanched (Fig. 3A) as R 2 c 2:3 well as for blanched apples (Fig. 3B). A very good linear correlation (regression coefficients ranging , data not shown) was obtained after fitting Eq. (6) to experimental data in the first falling rate period. The calculated D ef values as well as the range of moisture for which Eq. (6) was valid are presented in Tables 1 and 2. The D ef value found for only blanched apples was significantly greater than those of raw apples (control) and apples with other pretreatments. Calcium incorporation did not affect drying behaviour of non-blanched samples as compared with control one, except when performed under vacuum or under vacuum followed by atmospheric impregnation for short periods of time t 2 (Table 1). Impregnation of blanched -2.5 ( π 2 /L /Rc 2 )*t (min/cm 2 ) Fig. 3. Experimental semi-logarithmic drying curves of apples subjected to calcium impregnation at 60 C. (A) without blanching; (B) with blanching. raw; blanched; AI(t 1 =0, t 2 = 1.5 h); AI (t 1 =0,t 2 = 6 h); s VI(t 1 = 15 min, t 2 = 0); h AI + VI (t 1 = 15 min, t 2 = 1.5 h); M AI + VI (t 1 = 15 min, t 2 = 6 h). m = average moisture content. apples at atmospheric pressure or under vacuum slightly decreased D ef values when compared to control (Table 2), especially after 6 h immersion at atmospheric pressure (AI) or when subjected to combined VI + AI treatment (t 1 = 15 min, t 2 = 6 h). Tables 1 and 2 also show the changes in sample volume due to pretreatments as well as the content of calcium determined in a previous study performed under the same experimental apple impregnation conditions (González- Fesler et al., 2004; Salvatori et al., 2007). These studies revealed that a combination of a slight thermal treatment followed by vacuum or atmospheric impregnation techniques led to an apple matrix with a high level of calcium. Calcium concentration increased from 7.44 lg/g (average value for fresh fruit) to 1100 lg/g after vacuum impregnation, improved by previous blanching (1600 lg/g). For non-blanched apples subjected to AI during 6 h, calcium content of apples was similar to that of non-blanched vacuum impregnated ones (1200 lg/g), but considerably increased when a previous blanching was applied (up to

6 328 M. González-Fésler et al. / Journal of Food Engineering 87 (2008) Table 1 Calcium concentration ([Ca 2+ ]) and % volume shrinkage (% S v ) of apples after calcium impregnation pretreatments and the corresponding effective moisture diffusion coefficient (D ef ) [Ca 2+ ] a (ppm) % S v D ef (m 2 /s) Control (raw fruit) 7.44 ± 0.10a ± a ( ) b Blanched 21 ± 4a 17 ± 3a,d ± 0.005b ( ) b Impregnated AI t 1 =0h,t 2 =1.5 h 565 ± 163b 8 ± 4b ± 0.002a ( ) b t 1 =0h,t 2 = 6 h 1220 ± 246c 13 ± 2c ± 0.002a ( ) b Impregnated VI t 1 = 15 min, t 2 = 0 h 1101 ± 139c 15 ± 2a,c ± 0.005c ( ) b VI + AI t 1 = 15 min, t 2 = 1.5 h 1208 ± 44c 15 ± 3a,c ± 0.002c ( ) b t 1 = 15 min, t 2 = 6 h 3016 ± 91d 19 ± 2d ± 0.005a ( ) b Means in the same column with the same letters were not significantly different (P < 0.05). a The values are expressed as total content of wet impregnated sample. b Values between brackets correspond to the range of moisture for which Eq. (6) was valid. Table 2 Calcium concentration ([Ca 2+ ]) and % volume shrinkage (S v ) of apples after blanching and calcium impregnation pretreatments and the corresponding effective moisture diffusion coefficient (D ef ) [Ca 2+ ] a (ppm) S v D ef (m 2 /s) Control (raw fruit) 7.44 ± 0.10a ± a ( ) b Blanched 21 ± 4a 17 ± 3a ± 0.005b ( ) b Blanched and impregnated AI t 1 =0h,t 2 = 1.5 h 2465 ± 210b 17 ± 2a ± 0.003c ( ) b t 1 =0h,t 2 = 6 h 4637 ± 74c 14 ± 2b ± 0.002e ( ) b Blanched and impregnated VI t 1 = 15 min, t 2 = 0 h 1623 ± 493d 26 ± 2c ± 0.007c ( ) b VI + AI t 1 = 15 min, t 2 = 1.5 h 2699 ± 162b 16 ± 2a,b ± 0.003d ( ) b t 1 = 15 min, t 2 = 6 h 3991 ± 532e 18 ± 3a ± 0.003e ( ) b Means in the same column with the same letters were not significantly different (P < 0.05). a The values are expressed as total content of wet impregnated sample. b Values between brackets correspond to the range of moisture for which Eq. (6) was valid lg/g). A combination of different t 1 and t 2 values (VI + AI) seemed to strongly affect the matrix response to calcium incorporation. For non-blanched apples, changing t 1 from 0 to 15 min led to higher values of calcium incorporation throughout t 2. However, when samples were blanched prior to impregnation, calcium uptake would appear to be similar to that obtained when there was not a previous vacuum stage ( lg/g after t 2 = 1.5 h and lg/g after t 2 = 6 h). These results would denote the role that cell membranes and walls have in calcium transport. As it was expected, only blanched samples showed an increase in initial moisture content (m 0 varied from 6.84 ± 0.01 to 8.52 ± 0.01), due to partial disruption of cells of apple cylinders, and the consequent absorption of water from the cooling medium used after blanching. Impregnation treatments did not affect moisture content (m ± 0.01 to 6.10 ± 0.01), since they were carried out by immersion in isotonic solutions regarding the content of apple native soluble solids. All pretreatments resulted in shrinkage of tissues. For non-blanched apples, the percentage volume reduction shown by 1.5 h atmospheric impregnated apples was small (8%). Atmospheric impregnation (AI) during 6 h and vacuum impregnation alone (VI) or combined impregnation (VI + AI) resulted in a greater tissue contraction (13 19%) of non-blanched apples. Blanching as only treatment also caused an important decrease in volume (17%). Atmospheric impregnation (AI) for 1.5 h and combined impregnation (VI + AI) of heated apples did not change this shrinkage value. But contraction was more severe for samples subjected to only a vacuum stage after blanching (26%). On the contrary, blanched samples subjected after to 6 h atmospheric impregnation exhibited a slight increase in volume as compared to blanched ones Structure features Optical microscopy studies were performed to evaluate the changes produced at cellular level by the different pre-

7 M. González-Fésler et al. / Journal of Food Engineering 87 (2008) treatments, and so to better understand the water transport during drying. The photomicrographs in Figs. 4 6 show cross-sections of the parenchyma tissue of apple after the different pretreatments. In fresh tissue, cells and intercellular spaces were loosely arranged in a net-like pattern which was inhomogeneous and anisotropic (Fig. 4A). Intercellular spaces exhibited various shapes and sizes. Cells, more or less regular in shape, appeared turgid with an apparent consistent cell wall structure. The shape factor (S F ) evidenced the regular shape of parenchyma cells: 48% of cells had S F between 0.6 and 0.8 and 15% of cells had S F greater than 0.8. The large amount of cell volume was occupied by the central vacuole, and the protoplasm, bounded by the plasmalemma and the tonoplast, was present as a thin layer lining the cell surface. A well defined middle lamella was cementing adjacent cells (Fig. 4B and C). Blanching of tissue resulted in breakage of membranes and in great damage in cell walls, which appeared with interruptions in many areas (Fig. 4D F). A redistribution of cell wall polysaccharides seemed to take place, showing aggregates of high density followed by rupture zones without material. A contraction of tissue was also observed (Fig. 4D), which confirmed the experimental shrinkage value shown in Table 1. Cells became slightly more irregular in shape, with about 43% of cells showing S F values lower than 0.6. In spite of the great tissue contraction (S v = 17%, as shown in Table 1), samples only blanched exhibited a drying rate higher than that of raw apple. Taking into account the three possible ways for the movement of water into and out of cells, that is, through plasmalemma membrane boundaries, via plasmodesmata and the cell wall pathway (Tyree, 1970; Nieto et al., 1998), the increase in D ef value due to blanching would be ascribed to the elimination of membrane resistances and to the continuous interruptions of cell wall, which would modify in a great extension the cell wall resistance to water flux. After Ca 2+ impregnation by AI (t 1 = 0 min, t 2 = 1.5 h), cellular arrangement was rather similar to that of the raw fruit (Fig. 5A). Number of cells with S F greater than 0.8 increased from 15% for fresh apple to 28% in sample impregnated for 1.5 h, while number of cells with most irregular shape (S F < 0.6) and cells with S F between 0.6 and 0.8 decreased to 32% and to 40%, respectively. Cell walls exhibited a higher staining when compared with those of fresh fruit, but with some discontinuities (Fig. 5A and B). Very reinforced walls and a slight shrinkage (8%) of the tissue would counteract the wall zones of low optical density, resulting in a resistance to water flux similar to that of the control. When impregnation time t 2 increased from 1.5 to 6 h, plasmalemma separated from the wall and showed disruptions in some areas (Fig. 5C D). Folding and a low optical density of cell walls were also observed (Fig. 5D). Although more important wall degradation seemed to take place, the D ef value did not change, probably due to the opposite effect of the slightly greater volume contraction (S v 13%). When non-heated apples were subjected to VI (t 1 = 15 min, t 2 = 0 min) (Fig. 5F G), the degree of cellto-cell contact seemed to increase. Cell walls appeared marked, with a global staining density rather similar to those of fresh fruit. As cell wall integrity maintained, tissue shrinkage (15%) would decrease drying rate. Combined VI + AI treatment (t 1 = 15 min, t 2 = 1.5 h) provoked Fig. 4. Light microscopy images of apple parenchyma tissue subjected to blanching. (A C) raw fruit; (D E) blanched fruit.

8 330 M. González-Fésler et al. / Journal of Food Engineering 87 (2008) Fig. 5. Light microscopy images of apple parenchyma tissue subjected to Ca 2+ impregnation. (A and B) Ca 2+ impregnated by AI (t 1 =0h,t 2 = 1.5 h); (C and D) Ca 2+ impregnated by AI (t 1 =0h, t 2 = 6 h); (E G) Ca 2+ impregnated by VI (t 1 = 15 min, t 2 = 0 h); (H and I) Ca 2+ impregnated by VI + AI (t 1 = 15 min, t 2 = 1.5 h); (J) Ca 2+ impregnated by VI + AI (t 1 = 15 min, t 2 = 6 h). neither an additional shrinking nor a great change in the optical density of walls as compared with only VI, although cells looked more rounded (the number of cells with S F greater than 0.8 increased to 53% while cells with S F lower than 0.6 account for about 25% of cells) (Fig. 5H and I). Thus AI for 1.5 h after VI caused no additional effect on water diffusivity. When t 2 increased to 6 h, the greater collapse (19% vs 15%) was overcame by the great damage in cell walls, as evidenced by a lower optical density. Probably during the prolonged immersion, degradation of polysaccharides and leaching of pectin and other wall soluble components occurred (Fig. 5J), decreasing wall resistance to water movement. With previous blanching, cell walls of calcium impregnated samples appeared overall less disrupted than in only blanched samples and with an optical density very similar to that of raw fruit (Fig. 6). This would indicate that Ca 2+ penetration improved wall structure of blanched tissues, increasing resistance to water flux. In general, as t 2 increased, cell walls appeared with a higher optical density, indicating a very densely packed fibrilar material (Fig. 6D and H). As calcium concentration increased (and so staining of wall materials), water mass transport decreased. After VI with previous blanching (Fig. 6E and F), samples showed an important swelling of cell walls. This structural modification, in spite of the great

9 M. González-Fésler et al. / Journal of Food Engineering 87 (2008) Fig. 6. Light microscopy images of apple parenchyma tissue subjected to blanching and Ca 2+ impregnation. (A and B) blanched and Ca 2+ impregnated by AI (t 1 =0h, t 2 = 1.5 h); (C and D) blanched and Ca 2+ impregnated by AI (t 1 =0h, t 2 = 6 h); (E and F) blanched and Ca 2+ impregnated by VI (t 1 = 15 min, t 2 = 0 h); (G and H) blanched and Ca 2+ impregnated by VI + AI (t 1 = 15 min, t 2 = 6 h). tissue contraction (26%), slightly increased D ef value, as compared with vacuum infused apples without previous heating. 4. Conclusions Effective diffusion coefficient was strongly affected by previous heating of apple as well as by tissue contraction. With the exception of non-blanched tissues subjected to VI or VI followed by AI for 1.5 h, calcium uptake during impregnation step appeared to modify the matrix resistance to water flux only when tissue was previously heated. Observation of alterations in the structure of the tissue after pretreatments allowed explaining the differences obtained in D ef values between fresh and treated samples. The D ef values calculated from Eq. (6) lumped together the volume changes of the fruit matrix and the structure integrity of cell walls and membranes.

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