Apparent diffusion coefficient determination of sodium hydroxide through potato skin and flesh under...

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Apparent Diffusion Coefficient Determination of Sodium Hydroxide Through Potato Skin and Flesh under Different

Temperatures and Concentration Conditions

M. S. Chavez,“ J. A. Luna” & R. L. Garrote”

“Institute de Desarrollo Tecnol6gico para la lndustria Quimica (INTEC), Consejo National de Investigaciones Cienfificas y TCcnicas, Universidad National del Litoral, Giicmcs

34SO-(3000), Santa Fe. Argentina “lnstitutu dc Technologia de Alimentos (FIQ-UNL). Ciudad Univcrsitaria. C.C.No.

42%(3000), Santa Fe. Argentina

(Received 6 April 19%: revised 29 March 19%; accepted I April 19%)

ABSTRACT

The aims of the present study were to analyze the most important chemical reactions between sodium hydroxide and potato constituents involved in potato chemical peeling, and to measure the apparent diffsivity of NaOH in potato skin and flesh separately, as a function of temperature and NaOH concentration, selected according to potato chemical peeling process. Chemical reaction of the suberin of the potato skin with NaOH was proposed as the most important one to promote chemical peeling. Whereas starch hydrolysis, middle lamella dissolution and cell wall disruptions were proposed as the principal chemical reactions in flesh. Experiments to determine NaOH apparent diffusivities in skin and flesh were performed following a cell difj%sion method at 25, 40, 50, 60 and 70°C temperatures and concentrations of 4, 12 and 20 g NaOHllOO g of solution. Finally, correlation equations were determined to describe apparent diffusivity dependence on temperature and concentration. Copyright 0 1996 Elsevier Science Limited

NOMENCLATURE

A,, c: D;, Ea L

Correlation parameter Concentration (g NaOH/lOO 5 solution) NaOH apparent diffusivity (m is) Activation energy (cal/mole) Thickness of the sample (m)

377

378 M. S. Chavez et al.

m Correlation parameter

;4 Correlation parameter Parameter (mole/cm2)

R Gas law constant 1.9872 Cal/g-mole K T Temperature (“C) t Time (s) u Correlation parameter V Tank volume of the dilute solution (cm”)

INTRODUCTION

Potato chemical peeling with sodium hydroxide (NaOH) solution is a process involv- ing diffusion and chemical reaction, NaOH diffuses and reacts with cell walls and binding substances, removing the skin from the vegetable flesh (Floros et al., 1987; Floras & Chinnan, 1988). Sodium hydroxide penetration is desired to finish off as soon as the flesh is reached, yet flesh damage results in loss of the final product quality. This situation can not be obtained in the process, and NaOH also diffuses into the flesh (Reeve, 1976). Thus, solute mass transport through the skin as well as through flesh must be considered in the potato chemical peeling study.

Diffusion in foodstuffs is characterized by an apparent diffusion coefficient, which is commonly measured, and it contains solute mass transport information inside the solid. This coefficient corrects the assumption of one dimensional mass transfer supposition when it is assumed (Stahl & Loncin, 1979; Loncin, 1980) and adds structure characteristics of porous materials (Schwartzberg et al., 1982; Geurts et al., 1974; Turhan et al., 1992). During potato chemical peeling, chemical reaction information is also included in the apparent diffusion coefficient, since it is not experimentally simple to distinguish the diffusion effect from the chemical reaction effect.

There is very little published information as regards simultaneous diffusion and chemical reaction during potato processing. Gekas et al. (1993) calculated apparent diffusivities based on Fickian diffusion model in the nutrient retention study during potato blanching. Stahl & Loncin (1979) determined cyclohexanol apparent diffusivity in potato flesh using an experimental concentration profile. However, chemical reaction was not taken into account. Floras & Chinnan (1989) developed a cell diffusion method for measuring NaOH apparent diffusivity in fruits and vegetable skin. The cell method was used to determine NaOH apparent diffusivity in tomato and pimiento pepper skin. The author concluded that NaOH reacted with cell wall constituents and the middle lamella of the vegetable, slowing down the diffusion flux of NaOH, and consequently lower NaOH apparent diffusivity values were observed. On the other hand, Floros et al. (1987) observed that lye chemical action increased in pimiento pepper mesocarp (flesh). Thus, less diffusive flux resistance through potato flesh occurred and higher diffusivity values were expected.

The objectives of the present work were to analyze NaOH chemical reactions in potato that affects the peeling process directly and to measure the NaOH apparent diffusion coefficient in potato skin and flesh. Experiments were carried out following Djelveh’s et al. (1989) cell diffusion method. They were performed at temperatures and concentrations used in industrial chemical peeling of potato. Finally, experi-

mental data were correlated to obtain equations to characterize apparent diffusivities as a function of temperature and concentration.

CHEMICAL REACTION ANALYSIS DURING POTATO CHEMICAL PEELING

Chemical reactions in the potato skin

Potato is a thickened underground stem. Its skin is composed of russetted thick periderm, very similar to the bark of some trees, which is covered by a cuticle layer. The potato periderm is formed as a result of cell divisions in a cork cambium. The walls of the periderm cells become suberized, as it occurs in cork (Reeve, 1976). Suberized cells lie compactly together due to the action of suberin. preventing liquids and gases intakes.

First resistance imposed to NaOH diffusion is the cuticle layer, which is formed by waxes, cutin and pectins. The role of the cuticle is to control the interaction between the plant and other organisms in its environment. However, Floros et al. ( 1987) reported that NaOH solution solubilized epicuticular or surfaces waxes and the cuticle proper of the pimiento pepper skin. The same authors verified that penetration through the cuticle was also reported on other chemicals, such as herbicides, pesticides and nutrients. Kolattukudy (Stumpf & Corm, 1980) pointed out that cutin is susceptible to cleavage and isolation by chemical treatments. On the other hand, suberin could hardly be isolated because it was tightly linked with cell walls. Moreover, it was verified that suberin minimized water loss from plant tissues.

According to previous paragraphs, the suberin could be considered as the major barrier NaOH finds when trying to penetrate into the vegetable, since the cuticle could be cleaved more easily than the suberin. So, suberin breakdown was the most important contribution to potato peeling.

Brieskorn & Binnemann (1974) reported the chemical composition of potato peel suberin after alkaline hydrolysis (3% wt/wt KOH and temperature). The following reaction products were detected: 32.5% of w-Hydroxy C,,:, acid; 30.5% of C,s , dicarboxylic acid; 152% of C,J-C,,j acids and alcohol; 2.5% CzZ dicarhoxylic acids; n-alkanes and monobasic acids. Kolattukudy (Stumpf & Corm, 1980) adopted the previous composition and proposed a tentative model for the suberin structure (Fig. 1). The proposed structure was based on the methanesulfony chloride tech- nique, which provided the estcrified grade of the aliphatic monomer, together with the known composition of the aliphatic monomers of the suberin and the discovery that suberin drastically increased the diffusion resistance of suberized walls.

The suberin structure proposed by Kolattukudy (Stumpf & Corm, 1980), together with Brieskorn & Binnemann (1974) experimental results after potato suberin alkaline hydrolysis, gave rise to consider the saponification of the suberin ester groups as the most important chemical reaction in connection with potato chemical peeling.

Chemical reaction in the potato flesh

In the flesh, NaOH must react with the middle lamella, which is the cell binding substance, and cell wall constituents firstly and then react with potato starch. The middle lamella, located between cells walls, consisted of a peptic substance mainly


polygaracturonic acid. Cell walls were composed of polysaccharides (cellulose and hemicellulose) and glycoproteins. Floros et al. (1987) verified that lye solution (4 and 9%) solubilized the peptic and hemicellulosec polysaccharides during the study of pimiento pepper NaOH peeling. The same authors observed that cell layers of the mesocarp were dissolved and washed away, resulting in loss of edible fruit, proving that flesh cells offered less resistance to lye mass transport.

The main potato flesh component was potato starch, (65 - 80% of the dry weight of potato, Marcotte & Le Maguer, 1992). In this case, NaOH hydrolyze potato starch through a-elimination type reaction (Kirk et al., 1983). NaOH acted as a catalyst promoting the brake of the a-1-4 linkages. Dextrin of different molecular weights was obtained (Banks & Greenwood, 1975; Whistler & Paschall, 1965) but there was not NaOH consumption since it catalyzed the reaction.

0 0 O- C+&O-t-CH-CH OH

OCH,

Fig. 1. Suberin structure proposed by Kolattukudy (Stumpf & Corm, 1980).

D$/usion c$’ sodium hydroxide through potato 331

DIFFUSION ANALYSIS DURING POTATO CHEMICAL PEELING

Materials and methods

Preparation of skin and flesh samples Experiments were performed using Huincul potatoes, which were obtained from local markets. Flesh samples, cut in a cylinder shape, had 3.2 cm of diameter and 0.3 cm of thickness. Skin samples were obtained by removing flesh from it using a sharp razor carefully, until a thickness of 0.02 cm was achieved.

Experimental procedure NaOH apparent diffusivity, D;,, for potato skin and flesh was measured at different temperatures and concentrations, according to the experimental method proposed by Djelveh et al. (1989). The cell used consisted of two tanks of 350 cm3 each, separated by a wall where the sample was held. Due to samples characteristics, two types of sample holder had to be used in the cell (Fig. 2). After the sample was placed in the sample holder, water and NaOH solution were poured into the tanks simultaneously, this moment was considered as the starting time. NaOH concentration increased only by its diffusion through the sample, since [solution] tightness was obtained using O-rings and rubber seals. The solutions were stirred by two stainless steel rotation propellers. The cell was immersed in a thermostatic bath to keep the desired temperature. Temperature was controled within +O.l”C. At every selected time, 2 ml of dilute NaOH solution were taken from the cell, kept in closed tubes until the NaOH determination, which was performed by atomic absorption (Perknn- Elmer, 5000 Atomic Absorption Spectrophotometer). Temperatures and concentrations selected for the experiments were: 2.5, 40, 50, 60, 70°C and 20, 12, 4 g NaOHilOO g solution. NaOH solutions were titrated with HCl to measure the initial concentration. Flesh and skin experiments were performed by triplicate. Sodium potato tuber content was 32 ppm, approximately (Watt & Merril, 1975); a similar value was reported by Marcotte & Le Meguer (1992). Thus, an experiment was performed in order to know the Na contribution of the vegetable to the total amount of Na ion measured during NaOH apparent diffusivity determination. In this case, water was poured into both hemicells instead of NaOH solution. Both temperature (70°C) and time of experiments (2h for potato skin and lh for potato flesh) were selected in order to achieve the most favourable situations to promote Na ion diffusion flux.

Djelveh et al. (1989) proposed a mathematical model to describe the concentration profile in the food sample. The model solution was approximated to a linear one after sufficient experimental time had elapsed. The slopes of the NaOH concentration vs time curves were used to obtain the apparent diffusivity coefficients. Linear region was previously determined by a calibration curve.

RESULTS AND DISCUSSION

Results showed that Na ion content in the solution from the potato skin test was co.025 ppm and ~0.7 ppm in the potato flesh case. In both situations, potato tuber Na ion contribution was neglected in the NaOH apparent diffusivity determi- nations.


According to the proposed cell method, apparent diffusivities D, for potato skin and flesh were obtained from the following equation:

V(C-C,,) =PD,t-L2P/6 (1) where t is the experimental time; C is the NaOH concentration of the dilute solution at time t; Co is the NaOH concentration of the dilute solution at initial time (roughly zero, in this case); P is a parameter that depends on the flux diffusion

Fig. 2. Diffusion cell description: (1) fluid chambers, (2) sample holder, (3) Sample retainer. Diffusion cell dimensions: (H) 10.5 cm, (I) 75 cm, (F) 05 cm, (Kj’ 1.1 cm, (15) 16 cm, (d,) 3.2 cm, (dz) 2.6 cm, (d,) 1 cm. Diffusion cell assembly: (4,s) sealing O-rings, (5) flesh sample,

(6, 9, 12) washers, (7, 13) threaded bush, (10) rubber sealing washers, (11) skin sample.

D$fusion qf’sodium hydroxide through potato 3:13

surface, sample thickness, the equilibrium ratio between the solute concentration at the surface of the sample and in the solution of the tank and the initial NaOH concentrations in both tanks; L is the sample thickness; V is the tank volume of the dilute solution.

The left hand side of eqn (1) vs time curves were obtained from experimental data. P and D;, were estimated using the intercepts and the slopes of the straight lines. Flesh experiments at 60 and 70°C and 20 gNaOH/lOO solution, could not he carried out because the sample was completely destroyed due to NaOH action.

Experimental NaOH apparent diffusion coefficient values obtained in potato were in agreement with those reported in bibliography for similar cases (Table 1 and Table 2): Floros (1989) reported measured NaOH apparent diffusivity II:,

TABLE 1 Experimental and Theoretical NaOH Apparent Diffusivities in Potato Skin, D x 10” (ml/s)

--. NtrOH C‘oncentrution (8 NtrOHflOO ,g ,solrltion J

7’ C” t~.~/7~~rir~~c,rltrrl Tlworuticul Throwtictrl Expvimrntul Tlwwtic~crl Thtwwtictrl Experimentul Theowtkul Theoretlcrrl

I),, I),, l)<, I),, [),, I)<, I)<, I),, I),, qn (2) L’qll (3) eqn (-7) “‘,I, (3) “I” (2) “‘,” (_:I

5, 0. I78 f 0.03’)” 0.137 0. I94 0,298 *0~023 0.25 1 0.253 0.259 ~OCI9 0.436 0.31;

30 0.304 &0.02X 0.277 0.262 0.505 f wo2 I 0.499 0.4x I l~ls0+0~2hh 0.845 0.828 S(I ,I 0.402 0.325 0.541 ~0~018 0.7OS 0.697 I.187 I.328

60 0.3hO i 0~027 0.583 0.398 0~082~0~018 0.977 0.985 I.944 * 0.5 I I 1 h3.5 2.32’)

70 0.871 0.439 I.698 f 0.328 I.337 1.390 2.417 3.320 --.

“ C’onfidence interval for 5% significance level.

” Not determined.

TABLE 2 Experimental and Theoretical NaOH Apparent Diffusivities in Potato Flesh. D x IO” (m’!s)

NtrOH C‘onc’mtrution (g NtrOHilOO g .solutior~)

2s 0~81.5~0~120” 0.734 0.776 0.923 *@I95 O.Y24 O.Y27 1.079 * 0~105 I.163 I.117

JO 0~929 *o, 129 1.071 I.067 1.417*0.275 1.337 1.344 I.552 i wo73 1.64’) I.671

50 ,I I.320 I.468 IsM+0~14S I.640 I .hS7 2.330 _t 0.2Xf, 2.015 3.171

ho 1 .hYY + lb200 1 h39 I .530 I .x33 * 0.145 2.01 I 2.040 2.490 2.745 _ 70 2.040 2.211 7.418i_O.258 2.425 2.4YO - 3.039 3.512

(’ Confidence interval for 5%’ significance level

” Not determined.


= 05.5 x 10.” m*/s (C = 8% wt/wt NaOH; T = 72°C) in pimiento pepper skin and, Da = 0.12 x lo-” m2/s (C = 12% wt/wt NaOH; T = 65) in tomato skin. NaOH apparent diffusion coefficients in potato skin were greater than those mentioned previously. These results were expected since fruit skins were covered by surface waxes, absent in potato skin, which caused a high resistance to mass transfer. Any other difference was related to the pH. Those fruits presented a low pH (~4.5) whereas pH potato was 6, approximately. Therefore, part of the NaOH intake was employed to neutralize fruit’s natural acidity (Floros, 1989). Stahl & Loncin (1979) obtained cyclohexanol D, = 6.1 x 10-t”

apparent diffusion coefficient in potato flesh, and D, = 2.0 x lo-“’ m’/s for different flesh water contents

(C = 1% wt/wt cyclohexanol; T = 20°C). Considering that cyclohexanol molecules were bigger than NaOH molecules and that NaOH concentrations were higher, it was expected that NaOH had less difficulty to moving inside potato flesh, resulting in higher values of NaOH diffusion coefficient in flesh represent the situation correctly. Theoretical NaOH apparent diffusion coefficient in flesh at 40°C was 1.47 x 10-O m*/s. Calculations were based on the infinite dilution electrolyte theory (Perry, 1973), potato porosity (0.86) and tortuosity factor (1.23) (Califano & Cal- velo, 1983). Experimental solute effective diffusion coefficient in flesh, at that temperature, varied from 0.929 x lo-” to 1.552 x lo-” m2/s in the concentration range of 4-20%, showing to be close to the theoretical one.

Experimental NaOH apparent diffusivity values in skin were lower than those obtained in flesh. This behavior can be explained taking into account potato skin and flesh function and their cell structure. The skin, formed by tightly packed cells, has the function of preventing the tuber from the penetration of any substance into the tuber (Vaughan, 1979). Whereas, flesh was in charge of the energy supply for the vegetable life. Therefore, mesocarp cell walls were thinner and smaller amounts of peptic substances were observed (Floros et al., 1987). So, greater diffusion mass transport resistance was expected in skin.

Chemical reaction contributed to NaOH diffusion transport opening porous structure of both media. According to this, NaOH apparent diffusion coefficient could not be measured without including NaOH consumption by chemical reaction; however, published information gave rise to assume that this coefficient, in potato skin, could be measured separately from NaOH consumption by chemical reaction. McFarland & Thomson (1972) experimentally verified that shrinking core model was appropriate to describe potato (NaOH) chemical peeling during the first few minutes when sodium hydroxide was still in the potato skin; this meant that the chemical reaction could be located at the outer edge of the NaOH diffusion transport region. On the other hand and due to potato flesh composition, the most important NaOH consumption by chemical reaction was expected to occur during starch hydrolysis, meanwhile, the middle lamella and cell wall polysaccharides dissolution probably consumed negligible diffusing NaOH. However, starch hydrolysis did not actually consume NaOH since it catalyzed the reaction. So, chemical reactions were assumed to precede solute diffusive transport leaving the skin with a different porous structure, in which the NaOH apparent diffusive coefficient was experimentally determined. Whereas, NaOH consumption by chemical reaction in flesh was expected to be negligible.

This hypothesis were verified by experimental results from the application of the diffusion cell method. Due to the fact that Djelveh’s et al. (1989) curves, obtained for gel systems, and the ones obtained for potato skin and flesh had similar shapes

Diffirsion of sodium hydroxide through potato

IO -

8-

0 5 10 15 20 25 30 35

time (min)

Fig. 3. Experimental points for potato skin using the diffusion cell methods.

(Figs 3 and 4) the reacting-diffusive system behavior might be considered analogous to a diffusive one. That is, NaOH chemical reactions will not interfere with solute diffusive transport after they have occurred.

Experimental diffusivities were correlated to the following equations:

(2)

-I

15 20 25 30 35

time (min)

Fig. 4. Experimental points for potato flesh using the diffusion cell methods.


TABLE 3 Correlation Parameters of eqns (2) and (3)

Equation (2) Equation (3)

Medium A,, (m’s_‘) m (K 70~‘)~ n (K) E,,/R (K) U (K-l 70~‘) W (%-‘) A, (m’s_‘)

Skin 2.34 x 1W” -22.327 4384.36 1062.28 2.470 x lo-’ 0.708 5.907 x lo- ” Flesh 2.30 x lo-’ -8.715 2448.11 1872.06 0.413 x lop9 0.101 3649 x lop7

“g NaOH/lOO g/g solution.

D, =A,e-““IKT+(UT-W)C‘ ‘I (3)

Equation (2) was a semiempirical kind of correlation equation considering Arrhe- nius type exponential temperature dependence and introducing the concentration variation. Equation (3) was proposed by Husain et al. (1972) for correlating humidity diffusivity during rice dehydration and was adopted here due to NaOH diffusivities similar behavior. A(,, m and iz correlation parameters of eqn (2) and, Al, -Es/R, U, W parameters of eqn (3) were estimated according to experimental data (Table 3). Theoretical apparent diffusivities of NaOH in skin and flesh of potato obtained from eqns (2) and (3) were summarized in Tables 1 and 2. Percentages of deviation for eqn (2) were 36.7 and 8.74% for skin and flesh, respectively, whereas percentages of deviation were 18.25 and 8.1% for eqn (3). Percentages of deviation were determined using the equation proposed by Heldman (1974).

According to percentages of deviation obtained, eqn (2) as well as eqn (3) reproduced NaOH diffusivity experimental values in flesh correctly. However, eqn (3) reproduced better solute diffusivity coefficients in skin due to their stronger nonlinear temperature and NaOH concentration dependence.

NaOH apparent diffusion coefficient increased their values not only because of solute concentration and temperature increase but due to the effect these variables had in the cell denaturation opening vegetal porous structure. Floros et al. (1987) verified that the degree of degradation and the loss of natural properties of pimiento pepper cell walls was proportional to the severity of the lye treatment applied, this effect was expected to be stressed with temperature. This might be the cause of the abrupt increment of the measured NaOH apparent diffusive coefficient (12%) in skin produced between 50 and 60°C. Because of potato starch presence inside the cell and the temperature range tested, another important cell denaturation must be mentioned here: the starch gelatinization process. This process was very sensitive with respect to temperature, once the gelatinization temperature was reached the gelatinization process became very fast, thus producing an observable discontinuity in starch properties. Lamberg & Hallstrom (1986) and Verlinden et al. (1995) proposed 66 and 67*5”C as the potato gelatinization temperature, respectively. First authors considered that gelatinization started at 55°C and ceased at 75°C. So, observed NaOH apparent diffusion coefficient discontinuity at 60°C (12%) might be produced by gelatinization process.

Diffusion of sodium hydroxide through potato 387

CONCLUSIONS

The cell method was used to evaluate NaOH apparent diffusivities in skin and flesh of potato. Diffusivities were correlated with temperature and concentration. Corre- lation equation validity range was 25570°C and 4-20 g NaOH/lOO g solution. According to experimental NaOH apparent diffusivity values and chemical reaction analysis, it can be deduced that chemical reactions contributed to solute diffusion transport by opening medium structure. Skin suberin chemical reaction with NaOH was proposed as the controling chemical reaction of the potato chemical peeling. In flesh, NaOH was proposed to catalyze starch hydrolysis and to promote other chemical reactions with flesh constituents.

ACKNOWLEDGEMENTS

We thank Mr Roberto R6 and Mr Ricardo A. Bertone for their technical assistance.

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