Fruit Tissues
4.2 MATERIALS AND METHODS .1 E XPERIMENT
The experimental process has been well described by Alvarez et al. (1995) and Nieto et al. (1998, 2001). Briefly, apples (Malus pumila, Granny Smith cv.; ≅85–88% w/w moisture content, wet basis) and mangoes (Mangifera indica Linn, Keitt var.; ≅85–
88% w/w moisture content, wet basis) were hand peeled and cut into an infinite plate
shape (dimensions at the end of the pretreatment ≅4 × 4 × 0.4 cm). Strawberries (Pájaro or Tioga Leico var.; ≅86% w/w moisture content, wet basis) were washed, decapped by hand, and selected to obtain samples of uniform size and maturity.
For steam blanching, apple and mango plates were exposed to steam for 1 min at atmospheric pressure and then cooled in water at 5°C. Whole strawberries were exposed to steam for 3 min and cooled in similar conditions.
For osmotic dehydration at atmospheric pressure, samples were immersed into different glucose aqueous solutions with (mango and apple) or without (strawberry) forced convection at 25°C until the desired final water activity (aw) value was reached (≅3 h for mango and apple and 48 h for strawberry). For vacuum glucose impregnation, fruit samples were immersed in a 59.0% w/w glucose aqueous solution (aw= 0.84) at 25°C, and a pressure equal to 60 mmHg was applied to the system for 10 min. After the vacuum treatment, the system was placed at atmospheric pressure for 10 min; the final aw value reached by fruits was 0.97.
After osmosis, fruit samples were drained and dried at 60°C (mango and apple) or 55°C (strawberry) with air at high constant velocity (about 15 m/sec) to eliminate or minimize external resistance to moisture loss.
4.2.2 MOISTURE DIFFUSIVITY CALCULATION
The Deff value was obtained by applying Fick’s second law for species diffusion in a single phase, with boundary conditions of internal resistance controlling and uniform initial moisture content, integrated over the volume of the slab (apple, mango) or sphere (strawberry) (Luikov, 1968). The Deff values for spheres were affected by a shape factor to take into account that strawberries are ellipsoids (Becker, 1959; Aguerre et al., 1987).
Uniform internal fruit temperature was assumed due to the low Biot number for heat transfer usually found for conventional air drying of fruits (Alzamora et al., 1979). Negligible external heat transfer effects were also considered.
Assuming diffusion in an isotropic medium, constant diffusion coefficient, and isothermal process, and using the moisture concentration converted to moisture content on a dry basis, Fick’s second law for one-dimensional unsteady diffusion is given as:
(4.1) where x is the spatial coordinate, C is a constant (0 for infinite plate; 2 for sphere), and Deff is the effective moisture diffusivity.
For uniform moisture distribution (assuming no constant drying period takes place) and internal control to mass transfer, initial and boundary conditions are:
m(x, 0) = m0 at t = 0 (4.2)
Solutions for the different geometrical configurations can be expressed as:
(4.5) for a slab, where l0 is the half-thickness of the slab;
(4.6) for a sphere, where Re is the radius.
For long drying times, only the first term of Equations (4.5) and (4.6) is significant, and the values of Deff may be estimated from the linear relationship − me/m0− mevs. time on semilogarithmic coordinates.
Strawberries were assumed to be ellipsoids, having three characteristic diameters . For bodies having an ellipsoid shape, the Fourier number may be extended to (Aguerre et al., 1987; Becker, 1959):
(4.7)
where Vp is the volume and Sp is the surface of the body. Generally, the equivalent spherical radius (Re) is used as the characteristic length.
(4.8) where Re = 3(VS/SS), and SS and VS are the surface area and the volume of the sphere, respectively. If VS= VP= V, the relationship between F0 and is given by
(4.9)
whereΨ is defined as SS/Sp, and SS is the surface area of a sphere of volume equal to that of the fruit with surface area Sp, which is assumed to be an ellipsoid. The intrinsic diffusivity Deff is given by Ψ2 . Ψ is the shape factor of a solid, commonly known as the sphericity (Becker, 1959). The diffusion coefficient calculated from Equation (4.6) is , and it must be corrected by the factor Ψ2 when the product shape can be assumed as an ellipsoid:
Expressing the surface area of an ellipsoid as
(4.10)
and calculating the eccentricity, e, as (Aguerre et al., 1987):
(4.11) the shape factor results in:
(4.12)
4.3 RESULTS
4.3.1 EFFECT OF BLANCHING
Table 4.1 summarizes the main structural features that may be produced by fruit scalding (Ilker and Szczesniak, 1990; Alzamora et al., 1997). Depending on the preponderant phenomenon/a, drying rates of blanched tissues could be decreased or increased com-pared to the moisture transport rate for the fresh fruit.
The photomicrographs in Figure 4.2 compare the structures of the fruits with and without blanching. All raw fruits (Figure 4.2A, C, F, G) exhibited parenchyma-tous cells containing intact membranes, cytoplasm, and organelles and had intact cell walls without degradation, disruption, or dissolution. The plasma and tonoplast membranes were closely associated with the cell wall, and the middle lamella was clearly seen cementing adjacent cells. On the contrary, in the three blanched fruits (Figure 4.2B, D, E, H, I), plasmalemma and tonoplast were disrupted, and numerous vesicles of the cytoplasm had been formed.
In blanched strawberry, the electronic density of the cell wall was much lower than that of fresh fruit walls (Figure 4.2B). The middle lamella practically disappeared, and microfibrils appeared distorted. Reduction in fruit volume was small (<10%) (Alvarez et al., 1995).
In mango, heated cell walls exhibited disorganized microfibrils and longitudinal striations with zones of high and low electron density (Figure 4.2I). Protoplasm appeared
TABLE 4.1
Major Reported Structural Phenomena Due to Blanching in Connection with Water Transport Rate During Drying of Fruit Tissues (from
Various Sources)
Disruption of cell membranes and increased permeability to water as well as loss of solubles to the surrounding medium and/or redistribution of solubles within the material during drying
Shrinkage of tissues due to contraction of some biopolymers and/or displacement of occluded air Loosening of the hemicellulose–cellulose and the pectin networks due to cleavage and solubilization
of polymers, increasing the cell wall porosity and decreasing cellular adhesion along the middle lamella
Crosslinking of pectic polysaccharides by Ca2+ and by ester linkages (facilitated by deesterification of pectin chains due to activation of pectin methylesterase)
Starch gelatinization and protein insolubilization
e= 1−(rm/Rm)2
FIGURE 4.2 Parenchyma tissue from strawberry, apple, and mango as affected by vapor blanch-ing. A,B: strawberry. A: fresh control. Cell wall with a nitid middle lamella and a very tightly packed pattern of microfibrils mainly toward the margin. B: blanched. Less densely stained wall. C–E: apple. C: fresh control. Intact membranes close to cell walls and conspicuous middle lamella. D, E: blanched. Disrupted membranes and densely stained wall with some striations; formation of vesicles. F–I: mango. F,G: fresh control. Polygonal cells closely aligned to one another, with grouped starch granules and protein–carbohydrate slime; cell walls with good electron density. H, I: blanched. Dense protoplasm due to gelatinization of starch and thermal denaturation of slime. Rounded cells, disorganized microfibrils and lon-gitudinal striations. A–E, G, I: TEM micrographs; F, H: LM micrographs. Scale: A, B, G, I:
0.5µm; C–E: 1 µm; F, H: 100 µm. cw: cell wall; p: plasmalemma; T: tonoplast; ML: middle lamella; s: starch granule; sl: slime; ve: vesicle.
dense probably due to gelatinization of starch and thermal denaturation of the protein–
carbohydrate slime typical of the fruit (Figure 4.2H). Blanching also produced a slight volume shrinkage (≅10%) and decreased intercell contact (Nieto et al., 2001).
Cell walls of blanched apple showed good optical density, only slightly lower than that of raw apples. Striations with zones of high and low electron density were observed but were less pronounced compared with those in heated mango. Separation of adjacent cells caused by dissolution of the middle lamella was not observed (micrographs not shown). Shrinkage of apples due to blanching was approximately 23%, a value very similar to the reported porosity of the fruit (air spaces comprise 20–30% of flesh tissue volume) (Nieto et al., 1998).
These alterations in the ultrastructure and microstructure of heated apple, mango, and strawberry tissues might explain, at least partially, the drying behavior (Figure 4.3).
For strawberry, blanching would enhance Deff value due to:
1. The elimination of the cell membrane’s resistance to water diffusion 2. A decrease in the resistance of the cell walls to water flux
In apple, the effect of blanching on Deff could be ascribed to the increase in density of apples due to air exhaustion and the slight contraction of tissues, all phenomena predominating over the slight modification of cell wall resistance to water flux and/or the decreased transmembrane transport resistance due to membrane disruption.
The decrease of Deff in mango could be attributed to:
1. The slight increase in density due to the cell collapse
2. The alteration of cellular contents (gelatinization of starch and denatur-ation of slimes)
Both of these phenomena are more noticeable than the low ultrastructural dam-age of the cell wall and the disruption of membranes that would facilitate water flow.
FIGURE 4.3 Experimental Deff values for the first falling rate period of air drying of raw and blanched fruits (drying temperature: 60°C for mango and apple; 55°C for Pájaro var. strawberrry).
control; steam blanched
0.00E+00 5.00E-06 1.00E-05 1.50E-05 2.00E-05 2.50E-05
Deff(cm2/s)
apple strawberry mango
4.3.2 EFFECT OF OSMOTIC TREATMENT
Drying rate can be also affected by various structural modifications produced in the fruit tissue by osmotic treatments (Table 4.2) (Alzamora et al., 1997).
A general aspect of the drying curves of mango and apple as affected by glucose dipping at atmospheric pressure or in vacuum to reach aw 0.97 is seen in Figure 4.4.
FIGURE 4.4 Effect of blanching and glucose osmotic dehydration on drying curves of (a) apple and (b) mango at 60°C.
control (raw) ; x blanched; * osmotically dehydrated to aw 0.97 (atmospheric);
osmotically dehydrated to aw 0.97 (in vacuum).
0 1 2 3 4 5 6 7 8 9 10
0 100 200 300 400 500 600
t / lo
2(min / cm2) m (g H2O / g DM)
0 1 2 3 4 5 6
0 100 200 300 400 500 600
t / lo2
(min / cm2) m (g H2O / g DM)
(a)
(b)
_
Osmotic treatment appreciably reduced the moisture content of the fruits. No con-stant drying period was observed at any of the conditions studied. For both fruits, experimental Deffvalues decreased as the glucose concentration of the impregnation solution increased, that is, as aw of the slabs decreased (Figure 4.5).
When glucose impregnation was performed under vacuum, the Deffvalue was similar to that of the atmospheric treated slab with the same aw (0.97) for mango, but vacuum impregnated apple exhibited a Deffvalue much lower than that corresponding to apple impregnated at atmospheric pressure with aw 0.97 and similar to the Deffvalue of the atmospheric impregnated fruit with aw0.93.
Shrinkage of mango and apple tissues before drying depended on the pretreatment (Figure 4.5). The percentage volume reduction shown by atmospheric osmotically dehydrated mango or apple to aw 0.97 was similar to that exhibited by blanched tissues (≅10% and ≅23%, respectively), while treatments in glucose solutions with aw0.95 or 0.93 resulted in a more severe volume change (≅40–43%). Vacuum treated samples showed a greater shrinkage compared with the atmospheric impregnated fruit with the same aw(19 vs. 10% for mango and 37 vs. 24% for apple).
Sugar composition of mango and apple tissues was significantly influenced by the osmotic process (Table 4.3). Sugar exchange during osmosis (glucose gain and loss of
“natural” sugars, fructose and glucose) resulted in modified sugar profiles of the final products. Glucose content of mango and apple tissues impregnated at atmospheric pressure increased with the increasing glucose concentration of the solution. Vacuum glucose impregnated mango showed a lower uptake of glucose and a lower loss of
“natural” sugars than the slabs impregnated at atmospheric pressure with aw0.97, while the glucose content of the vacuum treated apple was 35% greater than that of atmo-spheric impregnated slabs with the same aw.
Light microscopy observations of atmospheric glucose impregnated apples to reach aw0.97 showed original arrangements of cells rather well maintained (micrographs not shown). Cells looked rounded but well defined, with plasmalemma separated from the wall. Examinations with transmission electron microscopy (TEM) indicated electronic
TABLE 4.2
Major Reported Structural Phenomena Due to Osmotic Dehydration in Connection with Water Transport Rate during Drying of Fruit Tissues (Various Sources)
Collapse of the tissue due to water removal
Loss/redistribution of native sugars and solutes due to decompartmentation and/or passive transport through membranes; uptake of osmotic solute(s)
Alteration of cell wall ultrastructure due to (a) leakage of soluble pectins to the osmotic medium that reduces the cohesiveness of the polysaccharide matrix and/or (b) specific interactions among solute(s) and cell wall components that modify to different degrees the strength of the walls as well as the porosity of the pectin matrix
Starch gelatinization
Stabilization of three-dimensional structure of proteins Plasmolysis and/or disruption of tonoplast and plasmalemma
dense cell walls and a network of microfibrils and a pectic matrix very similar to those of the fresh fruit (Figure 4.6C). When impregnation was made in vacuum, more rounded cells were observed in light microscopy (LM) (micrographs not shown), with some spaces between cells, but the degree of cell-to-cell contact did not decrease. Cell walls appeared with a very pronounced staining, exhibiting a very densely packed fibrillar material (Figure 4.6D).
Original arrangements of cells were also maintained in atmospheric glucose impregnated mango tissues with aw0.97 or 0.93. Tonoplast and plasmalemma appeared FIGURE 4.5 Experimental Deff values and volume shrinkage (Sv, %) for osmotically dehy-drated apple, mango, and strawberry (Tioga Leico, var.).
control; osmotically dehydrated, atmospheric pressure, aw 0.97;
osmotically dehydrated, vacuum, aw 0.97; osmotically dehydrated, atmospheric pressure, aw 0.95; osmotically dehydrated, atmospheric pressure, aw 0.93
0.00E+00 5.00E-06 1.00E-05 1.50E-05 2.00E-05 2.50E-05
Deff(cm2 /s)
apple strawberry mango
0 10 20 30 40 50
Sv,%
apple mango
intact, and cellular contents were plasmolyzed; this effect was very pronounced at aw 0.93. Due to plasmolysis, starch granules were localized near the center of the cells (micrographs not shown). As seen in TEM (Figure 4.6A), fibrillar material appeared darkly stained and tightly packed, and the middle lamella, either cementing adjacent cells or lining the walls at the intercellular spaces (micrographs not shown), exhibited high electron density.
Mango tissues impregnated under vacuum (aw 0.97) also showed a cell assembly similar to that of the control and cytoplasm plasmolysis. In some zones, disrupted membranes were observed. Ultrastructure observations indicated densely stained cell walls with clear reticulate pattern and a nitid middle lamella (Figure 4.6B).
On the contrary, the optical density of cell walls of osmotically dehydrated strawberries was lower than the optical density of fresh strawberry cell wall. The middle lamella partially disappeared, and distortion of miofibrils occurred (micro-graphs not shown), indicating degradation of the hemicellulose polysaccharides present in the cell wall, breakdown of pectins, and changes in the “crystallinity” of cellulose.
The many changes in ultrastructure, microstructure, and composition induced by predrying osmosis could affect drying behavior in many ways. In mango and apple, cell walls of glucose-impregnated tissues were not modified (hence, neither were the wall transport properties of the wall) to a great extent compared with the control. But shrinkage and glucose uptake during the impregnation step would increase the overall water transport resistance. The magnitude of the increase would be dependent on the degree of cell collapse and sugar incorporation.
In strawberry, glucose infusion would cause no effect on drying rate due to two counter-blanching effects: increased water transport resistance due to solute uptake
TABLE 4.3
Sugar Content (% W/W) of Glucose Impregnated Mango and Apple Slabs
Mango Apple
Pretreatment Fructose Glucose Sucrose Fructose Glucose Sucrose At Atmospheric Pressure
Control 3.6 ± 0.1 2.3 ± 0.1 4.3 ± 0.1 5.6 ± 0.1 4.1 ± 0.1 2.6 ± 0.1 Immersed in aqueous glucose solution of
aw 0.97 Vacuum impregnated with glucose solution of
aw 0.84 (final fruit aw= 0.97)
2.9± 0.1 6.9 ± 0.1 3.8 ± 0.2 7.0 ± 0.1 16.5 ± 0.3 1.5 ± 0.2
and the reduction of the cell wall resistance due to degradation/solubilization of polysaccharides of the cell wall.
4.4 CONCLUSIONS
There is a close relation between drying kinetics and structural modifications under-gone by fruits under blanching and osmotic dehydration pretreatments. Resistance of fruit tissues to water flux could be decreased by disruption of membranes and degradation/solubilization of the cell wall matrix, while drying rate would be adversely influenced by cell collapse, solute(s) gain, starch gelatinization, and protein denaturation.
FIGURE 4.6 Ultrastructure (TEM) of parenchyma tissue of mango and apple as affected by osmotic dehydration. A, B: mango; C, D: apple. A: immersed in glucose solution of aw 0.97.
Densely stained wall and plasmolyzed cytoplasm; intermixed microfibrillar pattern; conspic-uous middle lamella. B: vacuum impregnated, aw 0.97. Interrupted membranes; visible middle lamella; moderate electronic density of the walls. C: immersed in glucose solution of aw 0.97.
Miofibrillar disorganization but moderately electron dense wall. D: vacuum impregnated, aw 0.97. Walls darkly stained; very densely packed fibrillar material; broken membranes with formation of vesicles. Scale: A, D: 500 nm; B, C: 1 µm.
NOMENCLATURE
l0 Half-thickness of the slab (cm) Re Sphere radius (cm)
F0 Fourier number DM Dry matter (g) aw Water activity
Average moisture content, dry basis (kg water/kg dry matter) mo Initial moisture content, dry basis (kg water/kg dry matter) m Moisture content, dry basis (kg water/kg dry matter)
me Moisture content in equilibrium with the surrounding temperature and humidity conditions, dry basis (kg water/kg dry matter)
Deff Effective moisture diffusivity in the solid (cm2/sec) x Characteristic dimension (cm)
Ψ Shape factor of a solid, adimensional
REFERENCES
Achanta, S. and Okos, M.R., Predicting the quality of dehydrated food and biopolymers:
research needs and opportunities, Drying Technol., 14, 1329–1368, 1996.
Aguerre, R.J., Gabitto, J.F., and Chirife, J., Shape factors for the analysis of diffusion in air drying of grains, Int. J. Food Sci. Technol., 22, 701–705, 1987.
Alzamora, S.M., Chirife, J., and Viollaz, P.E., A simplified model for predicting the temper-atures of foods during air dehydration, J. Food Technol., 14, 369–380, 1979.
Alzamora, S.M, Gerschenson, L.N., Vidales, S.L., and Nieto, A., Structural changes in min-imal processing of fruits: some effects of blanching and sugar impregnation, in Food Engineering 2000, Fito, P., Ortega-Rodríguez, E., and Barbosa-Cánovas, G.V., Eds, Chapman and Hall, New York, 1997.
Alvarez, C.A., Aguerre, R., Gómez, R., Vidales, S., Alzamora, S.M., and Gerschenson, L.N., Air dehydration of strawberries: effects of blanching and osmotic pretreatments on the kinetics of moisture transport, J. Food Eng., 25, 167–178, 1995.
Becker, H.A., A study of diffusion in solids of arbitrary shape, with application to the drying of the wheat kernel, J. Appl. Polym. Sci., 1, 212–216, 1959.
Ilker, R. and Szczesniak, A.S., Structural and chemical bases for texture of plant foodstuffs, J. Texture Stud., 21, 1–36, 1990.
Lewicki, P.P., Effect of pre-drying treatment, drying and rehydration on plant tissue properties:
a review, Int. J. Food Prop., 1, 1–22, 1998.
Luikov, A.V., Analytical Heat Diffusion Theory, Academic Press, London, 1968.
Molz, F.J. and Ikenberry, E., Water transport through plant cells and cell walls: theoretical development, Soil Sci. Soc. Am. Proc., 38, 699–704, 1974.
Nieto, A.B., Castro, M.A., and Alzamora, S.M., Kinetics of moisture transfer during air drying of blanched and/or osmotically dehydrated mango, J. Food Eng., 50(3), 175–185, 2001.
Nieto, A., Salvatori, D., Castro, M.A., and Alzamora, S.M., Air drying behavior of apples as affected by blanching and glucose impregnation, J. Food Eng., 36, 63–79, 1998.
Tyree, M.T., The symplast concept: a general theory of symplastic transport according to the thermodynamics of irreversible processes, J. Theor. Biol., 26, 181–214, 1970.
Zogzas, N.P., Maroulis, Z.B., and Marinos-Kouris, D., Moisture diffusivity data compilation in foodstuffs, Drying Technol., 14, 2225–2253, 1996.
m