JournalofFoodEngineering25 (1995) 167-178 Copyright 0 1995 Elsevier Science Limited Printed in Great Britain. All rights reserved

0260.8774/95/S9.50 0260-8774(94)00026-3

Air Dehydration of Strawberries: Effects of Blanching and Osmotic Pretreatments on the Kinetics of Moisture

Transport

C. A. Alvarez,” R. Aguerre,“” R. Gbmez, b S. Vidales, ad S. M. Alzamora”” & L. N. Gerschenson”’

“Tecnologia de Alimentos, Departamento de Industrias, “Bromatologfa, Departamento de Quimica Organica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos

Aires. Ciudad Universitaria, 1428 Buenos Aires, Argentina ‘Member of Consejo National de Investigaciones Cientificas y Tecnicas, ‘Fellow of

Universidad National de Lujin, Argentina

(Received 16 December 1993; revised version received 30 March 1994; accepted 9 May 1994)

ABSTRACT

The effect of blanching and glucose concentration before drying on the rate of moisture movement during the early stages of air dehydration of strawbern’es at 55°C was studied. It was found that the effective diffusion coeficient of water in strawberries (D=,$ was strongly affected by heat pretreatment, but glucose dipping after blanching caused no additional e,ffect. Electron microscopic studies of strawberry tissues were used to explain this behavior through a modification of cellular structure, disrup- tion of cell membranes of heat-treated fruit and subsequent cell wall alteration during the osmosis step.

INTRODUCTION

The strawberry is the principal berry grown commercially in Argentina and is largely intended for the fresh and frozen markets. In recent years, production has resulted in a surplus of fresh market fruit and the processing industry is looking for alternative, inexpensive, preservation methods.

Dried strawberries produced by conventional air drying (a low-cost preserva- tion process) are hard and have poor rehydration characteristics. Osmotic dehydration before freeze, vacuum or air drying has been suggested by a num- ber of authors in order to yield good quality fully dehydrated or intermediate

167

168 C. A. Alvarez et al.

moisture products of improved stability and requiring less drying time (Ponting et al., 1966; Hawkes & Flink, 1978; Flink, 1980; Levi et af., 1980; Kim & Toledo, 1987).

Many researchers have investigated the effect of osmotic dehydration prior to conventional drying on the rate of moisture transport. Vaccarezza et al. (1974) found that the moisture diffusivity of water in sugar beet root decreased as the sucrose content increased. Uddin and Hawlader (1990) reported similar water diffusion coefficients for fresh pineapples and those osmotically dehydrated in sucrose solutions. Flink (1980) reported similar dehydration rates for carrots slices with and without osmotic treatment when drying rates were expressed as a function of the original sample solids, concluding that uptake of solids in the osmosis process does not itself result in significantly lower drying rates, but he did not report Dcfr values for both types of carrot slices. Mazza ( 1983) observed that as the concentration of sucrose used for dipping carrot cubes was increased from 5 to 60%, the rate of moisture transport decreased. He attributed these lower rates of dehydration to (i) the depression of water vapour pressure in the product due to dissolved sugar, and (ii) the impairment of heat transfer and the lowering of the diffisivity of water vapour within the product, due to the crystal- lization of sucrose during the air drying process. But he did not calculate the effective moisture diffusion coefficients. Islam and Flink (1982) showed that the diffusion coefficients of water for the first falling rate period decreased in the order no osmosis > 60% sucrose > 45% sucrose/l 5% salt for potato slices dried at air temperatures of 52-68°C concluding that uptake of sugar and/or salt increased internal resistance to moisture movement.

However, when some of this literature is analyzed, it can be observed that some of these Deli values were obtained by applying Fick’s law of diffusion with boundary conditions of moisture transport controlled by the internal resistance, although the drying experiments were conducted under conditions of some external resistance.

Blanching of the vegetable tissue as a pre-drying treatment is usually carried out to prevent off flavors and color changes resulting from enzymatic reactions, and to decrease the initial microorganism load. Blanching is considered in the literature to affect the rate of vegetable drying in many ways. Saravacos and Charm (1962) found that steam blanching of potatoes had no significant effect on drying rate when compared to the control sample dried at 66°C. Vaccarezza and Chirife ( 1975) found that water blanching increased the drying rate of sugar beet due to the loss of soluble solids and the destruction of the semipermeability of the cell membranes. However, Rostein and Cornish (1978) reported that, at least for apples, the drying phenomenon was not controlled by the cellular permeability. Alzamora and C&rife ( 1980) found that blanching, either steam or water, decreased the rate of drying of potato and they ascribed this effect to changes caused by gelatinization of the starch. In avocado, however, these authors found that steam blanching had only a little effect on the drying rate. Mazza (1983) had shown that blanching increased significantly the drying rate of carrot cubes.

From the literature results it can be concluded that the influence of blanching and osmotic dehydration. on drying behavior differs widely, as the tissue properties change from one foodstuff to another. The objective of this study was to assess the effect of blanching and glucose dipping pretreatments on the

Air dehydration of strawberries 169

kinetics of moisture movement during air dehydration of strawberries. This investigation was aimed at acquiring a better understanding of the complex nature of moisture movement during the first falling rate period of drying.

MATERIALS AND METHODS

Materials

Strawberries of the Ptijaro variety ( = 86% w/w moisture content, wet basis) were washed, decapped by hand and selected to obtain samples of uniform size and maturity. They were kept in a refrigerator until use. Only in one case, were strawberries of Tioga Leico variety ( = 86% w/w moisture content, wet basis) used.

The volume of each strawberry ( V) was measured using a pycnometer (with water as the fluid) and the equivalent spherical radius (R,) was then computed from the formula for the volume of a sphere ( I/= 4nR2/3) using the strawberry volume. Measurements of volume and transverse cross-sections were performed before and after the air drying step.

In this work, strawberries were assumed to be ellipsoids having three charac- teristic diameters (2r,, = 2rM,12R,). The maximum length and width of the drawing, on a paper, of the silhouette of strawberries was identified, respect- ively, as 2R, and 2r,,,. No significant differences were observed between those computed from the formula of the volume of an ellipsoid ( VP = 4/3 m$,,R,) and the measured volume ( V).

Pretreatments

Steam blanching: the fruit was exposed to steam for 3 min at atmospheric pressure, and then cooled in water at 15°C. Osmotic treatment: strawberries were immersed into 35% w/w or 51% w/w cerelose (glucose monohydrate, food grade, Refinerias de Maiz S.A.) aqueous solutions for 48 h at 25°C. After that period, fruits were drained, held for two days in an hermetically closed container and then air dried.

Laboratory dryer

The drying equipment consisted of a centrifugal fan which blew the air through a heating section and then upwards through a vertical duct. The vertical duct had a flow-smoothing section of small glass spheres. A metal container with a mesh base and lid served as the drying chamber and was mounted on the outlet of the duct. A high constant air velocity (about 15 m/s) was used in all experi- ments in order to eliminate external resistances to the moisture loss (Vaccarezza et al., 1974). The inlet air dry bulb temperature (55.0 + 0*2”C) was regulated by an electronic proportional controller; wet and dry bulb thermometers were fitted in the drying chamber. Relative humidity of the drying air ( = 14%) was calculated from the wet bulb depression. A single layer of strawberries of similar volume (about four fruits) was used for each drying experiment, the progress of which was followed by weighing the sample periodically in a precision balance

170 C. A. Alvarez et al.

( f 0*0005 g). For this purpose, the drying container was removed, rapidly weighed and replaced in the drier. A very good reproducibility between pairs of drying runs performed under identical conditions was found.

Moisture determination

The moisture content of strawberries was determined gravimetrically using a vacuum oven at 70°C over magnesium perchlorate desiccant. The determina- tion proceeded until constant weight was achieved.

Glucose analysis

Glucose was determined according to the Shaffer-Somogyi Micro Method (AOAC, 1980). All reagents used in this technique were of analytical grade and were obtained from Merck Quimica Argentina.

Determinations were performed in duplicate and the average was reported, the error being approximately 10%.

Preparation of sections for microscopical examination

Samples (c. 2 mm”) from the region between central and external zones of the fruit were fixed twice in 3% glutaraldehyde in 0.1 M potassium phosphate buffer (pH 7.4) overnight at room temperature. The slices were then rinsed three times with distilled water, postfixed in 0~0, at room temperature and dehydrated in a graded acetone series prior to being embedded in low viscosity Spurr resin.

For light microscopy (LM), sections (1-2 pm thick) of the Spurr-embedded tissue were cut on a Sorvall MT2-B Ultracut microtome and stained with aqueous 1% toluidine blue and 1% basic fuchsin solutions. For transmission electron microscopy (TEM), ultra-thin sections were stained with uranyl acetate/lead citrate and examined with a Siemens Elmiskop I TEM at 80 kV.

RESULTS AND DISCUSSION

The effect of steam blanching and glucose dipping on the drying curves of strawberries at 55°C is shown in Fig. 1. The average moisture content, fi, on a dry basis (DB), is plotted versus time, t, for untreated (control) and treated samples. No constant drying period was observed at any of the conditions studied. It can be observed that treatment with glucose removes up to 10 or 19% of the initial moisture content for strawberries that were soaked in 35% (w/w) or 51% (w/w) aqueous glucose solutions, respectively. The glucose content of these fruits was determined experimentally and had values of = 56% and 70% (w/w, dry basis) in each case.

Drying is a simultaneous heat and mass transfer process and correlations of moisture changes should involve solution of coupled differential equations. Vaccarezza (1974, 1975) showed that sugar beet air dried at a dry bulb tem- perature of 81”C, increased its temperature at a high rate at the beginning of drying, tending towards the dry bulb temperature of air. Alzamora et al. (1979)

1 8

171

0

Fig. 1. Effect of blanching and glucose immersion on drying curves of strawberry at 55°C: (m) control (raw); (A j blanched; (0) blanched and soaked in 35% w/w aqueous

glucose solution; (*) blanched and soaked in 5 1% w/w aqueous glucose solution.

observed the same trend for avocado dried at 57_8”C, potato dried at 68°C and apple dried at 71*C, concluding that the temperature of vegetables and fruits with high initial moisture content undergoing dehydration increases rapidly at the beginning of drying towards the air dry bulb temperature. Furthermore, those authors found that the internal vegetable temperature during drying may be considered uniform due to the low Biot number for heat transfer, Bi = hR /k (where h = heat transfer coefficient, R = characteristic dimension, i.e. radius for a sphere, and k= heat conduction coefficient), usually found for conventional air drying of foods. But to establish a simplified model, it was assumed that the process was isothermal. For these reasons, heat transfer effects were neglected and E‘ick’s second law was used for describing the rate of moisture movement during the first falling rate period of drying.

Although strawberries do not have a spherical shape, Luikov (1968) pointed OUT that the diffusion problem for any geometry can be reduced to the analytical solution corresponding to a sphere, by modifying The Fourier number, F,, = D&/R*, using a shape factor.

The expression of Fick’s law for diffusion out of a sphere, with boundary conditions of internal resistance controlling, integrated over the volume of the sphere is (Luikov, 1968)

fi=---_= m-me 2 B,exp[-,ufF,,] mu -4 R-l

0)

where

172 C. A. Alvarez et al.

t = time m,, = uniform initial moisture content, dry basis m, = equilibrium moisture content (at the air dry bulb temperature), dry basis R, = sphere radius

Defl = effective moisture diffusivity

For high values of F,, eqn ( 1) rapidly converges, and terms with n > 1 can be neglected with the result

k-m 6 $fL

m,-m, x (2)

As stated earlier, in this work strawberries were assumed to be ellipsoids, having three characteristic diameters (2r,, = 2rM,12R,,,). Aguerre et al. (1987) established that for bodies having an ellipsoid shape, the Fourier number may be extended to

(3)

where I/ is the volume and SP is the surface of the body. Generally, the equi- valent spherical radius (R,) is used as the characteristic length

where R, =3( Vs/.Ss) and S, and V, are the surface area and the volume of the sphere, respectively. If V, = VP = V, the relationship between F, and F b is given bY

(5)

where W is defined as SJS,, and S, is the surface area of a sphere of volume equal to that of the fruit with surface area S,, which is assumed to be an ellip- soid. The intrinsic diffusivity D,, is given by Y*D& Y is the shape factor of a solid, commonly known as the sphericity (Becker, 1959). It can be concluded that the diffusion coefficient calculated from eqn (1) is D& and that it must be corrected by the factor Y* when the product shape can be assumed as an ellipsoid.

Expressing the surface area of an ellipsoid as

rMRM s,=24+2x - ( i

sir- ‘e e

Air dehydration of strawberries

and calculating the eccentricity, e, as ( Aguerre et al., 1987)

173

e=Jl -(rM/RM) (7)

the shape factor results

To establish the adequacy of eqn (2) to the experimental data, the In(ti/m,) was analyzed as a function of time (t) using statistical software, and a very good linear correlation (regression coefficient 0.99) was obtained for m/m, lower than 0.70 (Fig. 2). The equilibrium moisture content obtained from a desorption isotherm (Vidales et al., 1993) was O;OS g H,O/g dry matter, showing that sup- pression of m, in the expression for M did not introduce significant errors.

As eqn (2) does not take into account the terms with n > 1, effective water diffusion coefficients found by applying experimental data to that equation were used to check the validity of neglecting the higher order terms in eqn (l), obtaining F, numbers high enough to validate the precited assumption.

The values of the effective diffusion coefficient (corrected by shape factor), Y2 and initial R, are shown in Table 1. The significance of differences among the obtained Deff values was analyzed through a ‘t’ parameter test (Moroney, 1965) with a 95% confidence level; the errors were calculated with the same confidence level.

As some shrinkage in volume was observed in drying, the D,, values were recalculated using the final equivalent sphere radius (Vaccarezza et al., 1974) and the results are also shown in Table 1.

In m/m0

,.,,--

-2-1 I, I I I I I, / / / / I I I,, I I / I / I, I I I, I, I/ /(I I(1 0 100 200 300 380

t(min)

Fig. 2. Comparison of experimental and predicted drying curves of strawberry at 55°C. Experimental: (m) control (raw); (A) blanched; (a) blanched and soaked in 35% w/w aqueous glucose solution; (*) blanched and soaked in 51% w/w aqueous glucose

solution; (- ) calculated.

TA

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Pret

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,.

Air dehydration of strawberries 175

From the results shown in Table 1, it can be concluded that all pretreatments increase the effective moisture diffusion coefficient, the values being similar for blanched and for blanched osmotically-dehydrated samples. The correction for the final sphere radius did not change this trend, probably due to the similar shrinkage occurring in both pretreatments.

To explain the drying behavior it is necessary to consider the possible path- ways for the transport of water to the outside. In the first pathway, water never leaves the cytoplasm (except to enter the vacuole) but passes from one cell to the next via cytoplasmic strands (plasmodesmata). In the second pathway, water alternatively enters and leaves successive cells along its pathway by passing through plasmalemma membrane boundaries (Tyree, 1970). To these pathways it must be added at least one more, that is, the cell wall. Indeed, Tyree (1970) had concluded that the cell wall may well be the preferred pathway for many substances, particular the smaller non-ionic species like water, and Molz and Ikenberry (1974) had pointed out that an increasing body of experimental evidence indicates that cell walls constitute an important pathway for water movement through plant tissues, constituting a significant portion of the water flux in the cell wall.

In order to evaluate the changes produced at the structural level by the different pretreatments, and so to better understand the mechanism of water transport during drying, electron microscopy studies were performed. The photomicrographs in Fig. 3 compare cross-sections of the intermediate portion (i.e. from the region between the surface and interior of the fruit) of the fruit from each pretreatment. In raw fruit (Figs 3(A), 3(B), 3(C)), parenchymatous cells contained intact membranes, cytoplasm and organelles, and had intact cell walls without degradation, disruption or dissolution and exhibiting a high optical density. The plasma and tonoplast membranes were closely associated with the cell wall. The middle lamella was clearly seen cementing adjacent cells. As the fruit was treated with steam (Figs 3(D), 3(E), 3(F)), the membranes (plasmalemma and tonoplast) of all the cells were completely disrupted and only vesicles of the cytoplasm remained. The optical density of cell walls was lower than the optical density of fresh strawberry cell walls. The middle lamella practically disappeared and distortion of microfibrils occurred, indicating degradation of the hemicellulosic polysaccharides present in the cell wall, breakdown of pectins in the middle lamella and changes in the ‘crystallinity’ of cellulose (Willis & Teixeira, 1988; McFeeters, 1985). The osmotically dehydrated tissue reveals severe ultra structural damage of the cell walls (Figs 3(G), 30-U h w ose optical density appeared very much reduced. Kanujoso and Luh ( 1967) remarked that the binding force between cells is largely dependent on the pectic material in the middle lamella of the cell wall, and that the higher concentration of hydronium ions present in the high acid fruits may accelerate the breakdown of the binding materials. The protopectin conversion to water- soluble pectin was suggested as being involved in the softening of canned peaches during processing and storage, as well as in the leaching of this water- soluble pectin to the syrup. Forni et al. (1986) observed a decrease in the amount of total pectic substances and particularly the residual protopectin, for apricots during the osmosis process and correlated these tendencies with the decrease in textural firmness observed during processing. According to the pre-cited authors, probably during the osmosis step, degradation of polysaccha- rides occurred as well as leaching of pectins and other wall soluble components.

176 C. A. Alvarez et al.

Fig. 3. Comparison of raw and pretreated strawberry tissue. A, B, C: control fruit; A: general aspect; B, C: details. D, E, l? steam blanched fruit; D: general aspect; E, F: details. G, H: steam blanched and glucose immersed fruit (glucose content: 70% w/w, dry basis). cw = Cell wall, T= tonoplast, ml = middle lamella, c = cytoplasm, P= plasmalemma,

Va = vacuole, v = vesicle.

Air dehydration of stra wberries 177

These alterations in the ultrastructure of strawberry tissue might explain, at least partially, the drying behavior. Blanching enhances effective moisture diffusivity due to: (a) the elimination of the cell membranes resistance to water diffusion (the membrane integrity is destroyed by heat treatment), and (b) a decrease in the resistance of the cell walls to water flux. When blanched straw- berries had been partially dehydrated in glucose solutions, it is to be expected that compositional changes occurring due to osmosis (uptake of sugar and loss of water) increase internal resistance to moisture movement. But, as can be seen from Table 1, immersion in glucose solutions after blanching causes no addi- tional effect on water diffusivity. This is probably the result of two counter- balancing effects: (a) solute uptake that increases water transport resistance and (b) a significant reduction of the cell wall’s resistance due to degradation. When assaying another variety of strawberries (Tioga Leico) in the drying experience, the authors obtained an effective diffisivity coefficient ( Deff) of (5.76 * 1.35) X 10eh cm*/s for raw fruit and of (5.43 + O-65) x 10e6 cm*/s, when the fruit was immersed in a 51% w/w cerelose solution without previous blanching. These results showed no significant differences at the 95% con- fidence level, demonstrating the contribution of the two pre-cited counter- balancing effects to results obtained in this work for osmosed strawberries. However, more research must be done in this area to clarify the observed tendencies.

ACKNOWLEDGEMENTS

The authors acknowledge the financial support of the Secretaria de Ciencia y Tkcnica (Programa National de Tecnologia de Alimentos), Consejo National de Investigaciones Cientificas y T&nicas, Universidad de Buenos Aires and OEA. They also thank Merck Quimica, Argentina, for supplying some of the chemicals.

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