Kinetics of Apple Polyphenol Diffusion in Solutions ... - ACS Publications

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Kinetics of Apple Polyphenol Diffusion in Solutions with Different Osmotic Strengths M. Kebe,†,‡,§ C. M. C. G. Renard,*,† G. N. G. Amani,§ and J.-F. Maingonnat† †

UMR408 Sécurité Qualité des Produits d’Origine Végétale (SQPOV), INRA, 84914 Avignon, France UMR408 Sécurité Qualité des Produits d’Origine Végétale (SQPOV), Avignon Université, 84000 Avignon, France § UFR Sciences et Technologies des Aliments, Nagui Abrogoua Université, 05 Abidjan, Ivory Coast ‡

ABSTRACT: Fruits contain polyphenols, widespread antioxidants beneficial for human health. Their mass transfer was studied during the leaching of apple slices immersed in mannitol solutions with ranging concentrations (0, 0.2, 0.4, and 0.6 M). The solution of Fick’s law for unsteady mass transfer in planar configuration was used to calculate apparent diffusivity (De). Polyphenols were quantified by high-performance liquid chromatography for each immersion time. Leaching from raw apple tissues occurred only when cell integrity was lost, here at a certain level of difference in osmotic pressure. Different diffusivity values were found in the two apple varieties. Values of De either decreased from 0.2 to 0.1 × 10−9 and 0.2 × 10−9 m2 s−1 for Golden Delicious and Granny Smith, respectively, or were not determined when the mannitol concentration increased from 0 to 0.6 M. The osmotic strength of the solution strongly impacted the leaching rate of polyphenols from apple cells. The structure of the polyphenols also affected De, with low values for procyanidins. KEYWORDS: Malus domestica Borkh., apparent diffusivity, mannitol, turgor, osmotic pressure

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INTRODUCTION Processing of fruits has become increasingly important. Various processes are applied that can be expected to affect the content, composition, or bioavailability of different solutes in derived products.1,2 Osmotic dehydration (OD) is widely used to remove water from pieces of fruit or vegetables by immersing the product in a relatively concentrated aqueous solution of sugar, salt, or both, with no phase change.3 The process, which is effective at ambient temperature and preserves the color, flavor, and texture of the foods from the effects of heat, is used as a pretreatment to improve the nutritional, sensory, and functional properties of foods.4 The influence of the main process variables, such as concentration and composition of osmotic solution, temperature, immersion time, pretreatments, agitation, nature of the food and its geometry, and solution/sample ratio, on the masstransfer mechanism and product quality have been studied extensively.5−7 Polyphenols are phytochemical compounds, present in almost all fruits and vegetables.8 Apple, a widespread fruit, represents the main fruit source of polyphenols in western countries.9 Several epidemiological studies have linked apple consumption to a reduced risk of cancer10 and cardiovascular diseases.11 Many studies have concerned the content and composition of phenolic compounds in different apple varieties. Several polyphenols have been identified in apples. Hydroxycinnamic acids, flavan-3-ols (including monomeric catechins and procyanidins), and dihydrochalcones can be found in all parts of the fruit.12−14 In studying factors modifying leaching from fruit, most investigators have considered sugars and water diffusion, in the context of osmotic diffusion, i.e., when confronted with solutions of higher osmotic pressure. Few have considered polyphenol mass-transfer mechanisms. Only Devic et al.3 © 2014 American Chemical Society

studied changes of phenolic compounds during OD processing. They found marked migration of hydroxycinnamic acids and monomeric catechins. Procyanidins were lost to a lesser degree. In OD, the main mechanism explaining polyphenol loss was leaching with water.3 Many processes (e.g., fresh-cut fruit and raw fruit salads)15 actually imply immersion of fruit in solutions of lower osmolarity than that of the fruit, either during preparation or storage, which may impact sensory and nutritional qualities.15 To our knowledge, no reports exist on the effects of osmotic pressure medium change on phenolic compound content when apple slices are immersed in solutions of low osmolarity. Osmotic pressure variation can be expected to induce some microstructural modifications that could lead to migration of phenolic compounds; the changes in phenolic compounds could depend upon the molecular weight or chemical structure. Here, we set out (i) to quantify the impact of immersion in solutions of low osmolarity and (ii) to establish whether, in these conditions, passive mechanisms linked to diffusion are relevant or whether cell integrity plays a role. Theoretical Considerations. Fick’s law, extensively adopted as a model for standard diffusion processes,16 is applied here for fruit tissue. Not all hypotheses for applying Fick’s second law are fulfilled notably because of tissue compartmentation by living cells and cell walls. However, the law gives a good phenomenological description.17 The description of the diffusion phenomenon given by Fick’s second law is Received: Revised: Accepted: Published: 9841

February 18, 2014 September 9, 2014 September 12, 2014 September 12, 2014 dx.doi.org/10.1021/jf503100d | J. Agric. Food Chem. 2014, 62, 9841−9847

Journal of Agricultural and Food Chemistry ∂C(x , t ) ∂ 2C(x , t ) = De ∂t ∂x 2

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Figure 1. Scheme of diffusion from slab (4a < L) (→, solute leaching direction).

∑ n=1

where M is the molarity, T is the temperature (K), and R = 0.008 314 kJ mol−1 K−1. To estimate the osmotic pressure (Ψop) of the cell contents in any solution, the osmotic pressure at incipient plasmolysis (Ψop0) was first determined. V0 is the sample volume at incipient plasmolysis, and V is the sample volume in the given solution. The turgor pressure (Pc) in a given solution was obtained from

1 (2n + 1)2

⎛ D (2n + 1) ⎞ exp⎜ − e π 2t ⎟ 2 2a ⎝ ⎠

(6)

Ψ = −MRT

film18 (Figure 1). The solution proposed by Crank19 for slab geometry is n →∞

MATERIALS AND METHODS

Plant Materials. Two varieties of dessert apple (Golden Delicious and Granny Smith), purchased in a local supermarket (Auchan, Avignon, France), were used for the experiments. Tissue from the parenchyma zone of the apple was chosen after manually removing the peel and core. Planar pieces 20 mm high were cut from the parenchyma tissue; four slices were obtained per fruit. To prevent any polyphenol oxidation, the apple slices were sprayed with a solution of 40 g/L sodium fluoride (NaF). A total of 10 fruits per cultivar were used for each leaching run, with different batches of 10 fruits replicated 3 times in leaching experiments. Determination of the Isotonic Point. Cylindrical samples of diameter 1 cm were cut with a core borer from the flesh side of the section, trimmed to a length of 4.70 cm using a double-bladed razor, and then soaked overnight in a mannitol solution (from 0 to 0.8 M). For each mannitol concentration, six apple cylinders coming from six different apples were used. To minimize cellular degradation of tissue strength during the experiment because of pH variation and following the method described by Lin and Pitt,21 the mannitol solutions were buffered at pH 7 with K2HPO4 (0.02 M) and KH2PO4 (0.02 M). Samples represented approximately 30% of solution volume. Before and after the soaking, weight and dimensions (diameter and height) of the samples were determined using an analytical balance (±0.01 g, CP124S, Sartorius, Germany) and digital calliper (Absolute Digimatic, Mitutoyo, U.K.). The apple tissue cylinders were carefully blotted with tissue paper to remove excess water before the measurements. The isotonic point was calculated by interpolation to zero weight gain or volume change from the weight gain or loss of tissue as a function of the mannitol concentration. The water potential (Ψ) in the cell after soaking was estimated from the molarity of the solution using the ideal gas law

(1)

To solve eq 1, the following boundary conditions were set: x = ±a, C = Co and −a < x < a, C = f(t), with sample thickness 2a. The geometry of apple slices can be reduced to a slab, i.e., a “thin film with negligible edge effects”. This means that diffusional mass transport through the edges of the film is negligible compared to that through the main surface of the

C(t ) − C∞ 8 = 2 Co − C∞ π

Article

Ψop = Ψop0(V 0/V )

(7)

Pc = Ψ − Ψop0

(8)

2

⎛ D ⎞ C(t ) − C∞ 8 = 2 exp⎜ − e2 π 2t ⎟ ⎝ 2a ⎠ Co − C∞ π

(2)

Mass-Transfer Experiment. Mannitol was used as the osmotic agent. A solid/liquid ratio of 1:20 was used under magnetic stirring. Experiments were run at a constant temperature (20 °C) in stainlesssteel vessels, each containing a different concentration of osmotic solution, namely, 0.2, 0.4, and 0.6 M. An additional vessel of distilled water was used as a control. Apple pieces were retrieved for biochemical analysis after 1, 2, 4, and 6 h. Matrix mass changes were monitored to determine water gain, after each immersion time during soaking experiments. The measurements were repeated 3 times, with a different batch of apple each time. Water gain percentage (Wg)

(3)

where C is the average solute concentration at time t of sampling, Co is the initial uniform concentration, and C∞ is the solute concentration after time t. It is generally assumed that, for a Fourier number (−De/ 4a2π2) greater than 0.1,20 only the first term in the series becomes significant. When this condition is attained, the effective diffusivity can be calculated by plotting ln(Y) against time (t). Y is the dimensionless concentration (eq 5). Linear behavior must be observed in accordance with D 8 ln Y = ln 2 − e2 π 2t (4) 2a π

Wg (%) =

C(t ) − C∞ Co − C∞

(9)

where Mo is the matrix weight (g) before the experiment and M(t) is the matrix weight (g) during the experiment at a time t. Biochemical Analyses. Biochemical analyses were performed on powdered product after freeze-drying. All of the samples were freezedried in a USIFROID SMH 15 freeze-dryer (Usifroid, France) for 48− 72 h. Solvents and Chemicals. Methanol, acetonitrile, and acetone of chromatographic quality were provided by Fisher Scientific (Fair Lawn, NJ), and toluene-α-thiol was provided by Sigma-Aldrich (Deisenhofen, Germany). Deionized water was obtained with a

with

Y=

M(t ) − Mo × 100 Mo

(5)

For each set of conditions, the adjustable parameters of each model for decreases in sugar and polyphenols were determined. 9842

dx.doi.org/10.1021/jf503100d | J. Agric. Food Chem. 2014, 62, 9841−9847

Journal of Agricultural and Food Chemistry

Article

Milli-Q water system (Millipore, Bedford, MA). (+)-Catechin, (−)-epicatechin, caffeoylquinic acid, mannitol, glucose, and fructose were also provided by Sigma-Aldrich. Phloretin and p-coumaric acid were from Extrasynthese (Lyon, France). Phloridzin was from Fluka (Buchs, Switzerland). Quantification of Sugar. Fruit parenchyma freeze-dried powder (5 g) was added to 20 mL of distilled water and homogenized with a polytron homogenizer for 1 min. The homogenates were centrifuged at 9000g for 10 min at 4 °C. The supernatant was collected to analyze sugars by a colorimetric enzymatic method as described by Bureau et al.12 Quantification of Polyphenols by High-Performance Liquid Chromatography (HPLC) after Thioacidolysis. Polyphenols were quantified by HPLC after thioacidolysis as described by Bureau et al.12 The average degree of polymerization (DPn) was measured by calculating the molar ratio of all of the flavan-3-ol units (thioether adducts plus terminal units) to (−)-epicatechin and (+)-catechin corresponding to terminal units. A HPLC system equipped with a diode array detector (SPD-M20A, Schimadzu, Inc., Kyoto, Japan) and a Lichro 250 × 4 mm column, Lichrospher 100 RP-18.5 μm (25 cm × 4.6 mm), were used. Quantification was performed by the external standard method. Results were expressed in milligrams per kilogram of fresh weight (FW). Losses of phenolic compounds (MC) were calculated, using eq 10 and expressed as percentage (w/w) of the initial concentration

MC (%) =

DM(0)C(0) − DM(t )C(t ) × 100 DM(0)C(0)

Figure 2. Parenchyma tissue length changes in various mannitol solutions (three biological replicates). Two apple varieties were used (□, Golden Delicious; ◇, Granny Smith). Isotonic points were determined by linear regression. Pooled SD values were 0.026 for Golden Delicious and 0.039 for Granny Smith, respectively.

apples, DM data means are plotted against time. The rates of matter loss were determined from the graph. Water entry into the sample was also monitored, as presented in Figure 3. Water gain resulted in a weight increase from 12 to 16% in apple matrix in 0 M mannitol solution (pure water). No data are available for other mannitol solutions. Similar results were reported by Linn and Pitt21 in hypotonic solution. Two simultaneous phenomena, DM loss and water entrance, took place during the leaching process. Their combined effect could explain DM losses according to immersion time. Apparent diffusivity and equilibrium matter content (C∞/Co) were therefore determined. For Golden Delicious, DM decreased at 0 and 0.2 M mannitol contents. No changes were observed for higher mannitol concentrations. For Granny Smith, a DM decrease was observed only at 0 M. The losses of DM were probably due to diffusion mostly of sugars in soaking solution. According to the isotonic point of each apple variety, the leaching occurred only for a difference of at least 0.4 M between cells and external solution. DM losses were closely linked to differences in osmotic pressure between fruit and external solution. Apparent diffusivity values were estimated from analytical solutions by Crank19 by fitting DM. Apparent diffusivity was then determined. Table 1 presents diffusivity values as a function of the mannitol concentration. DM losses occurred only from 0 to 0.2 M for Golden Delicious and 0 M for Granny Smith. The apparent diffusivity values were therefore determined. DM losses decreased with increasing the mannitol concentration. The decrease in turgor pressure between cells and aqueous solution seemed to limit DM losses. Effect on the Total Sugar Content. The majority of the DM in ripe apple is made up of simple sugars. Figure 4 shows the plots of the sugar concentration versus immersion time. For both apples, total sugar losses decreased as mannitol in the solution increased from 0 to 0.6 M. The total sugar retained in the matrix then went from 5.3 to 7.7 g/100 g of FW for Golden Delicious and from 6.7 to 9.5 g/100 g of FW for Granny Smith. The losses depended upon the mannitol concentration in the medium. As in previous results, sugar leaching in solution could also be due to an osmotic pressure difference. The experimental data were then fitted to estimate apparent diffusivity from eq 2, with only the first term of the series being retained. The results presented in Table 1 show that the decrease in apparent

(10)

where C(0) is the initial phenolic compound concentration in the sample (mg/kg of dry weight), C(t) is the phenolic compound concentration in the sample at time t (mg/kg of dry weight), DM(0) is the initial dry matter of the sample (g/100 g of FW), and DM(t) is the dry matter of the sample at time t (g/100 g of FW) Statistical Analysis. Data underwent statistical analysis using the analysis of variance (ANOVA) test with SPSS (version 7.0, IBM, Chicago IL) Windows 7 software. Means were compared using Duncan’s multiple range tests. Pooled standard deviations (SDs) were also estimated.22 Pooled SDs were calculated using the sum of individual variances weighted by the individual degrees of freedom of each series of replicates.23 A multiple regression analysis module was also used to determine raw material characteristics on the response to osmotic variation. To obtain the model parameters, the R (version 3.0.1, free license) nonlinear regression tool was used.

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RESULTS AND DISCUSSION Determination of the Isotonic Point. Figure 2 presents matrix length changes as a function of the mannitol concentration. Turgor and osmotic pressure values in soaking solution were calculated. Osmotic pressure increased and turgor pressure decreased as the mannitol concentration increased. A relationship between the solution osmotic pressure and cell turgor pressure was observed. The isotonic points of Granny Smith and Golden Delicious were determined by plotting the sample volume variation against the mannitol concentration. The point of equilibrium of Granny Smith was 0.43 M, and the point of equilibrium of Golden Delicious was 0.61 M. Therefore, the value of the isotonic point for Granny Smith was −1.15 MPa, and the value of the isotonic point for Golden Delicious was −1.49 MPa. Similar results were found by Lin and Pitt21 and Alamar et al.24 on apple slices in various mannitol solution concentrations. We can assume that variety and composition effects explain differences in isotonic point values. Effects on Dry Matter (DM) Changes. During the leaching process, the DM content at different immersion durations was experimentally obtained over a range of concentrations (0, 0.2, 0.4, and 0.6 M). In Figure 3, for both 9843



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Figure 3. DM changes in different osmotic solutions (◇, 0 M; □, 0.2 M; △, 0.4 M; and ○, 0.6 M) against time for two apple varieties: (a) Golden Delicious and (b) Granny Smith. Water gain (+) percentage against time in different osmotic solutions.

Table 1. Apparent Diffusivity with Equilibrium Content (C∞/Co) for DM and Sugars Leaching from Apple Slices in Mannitol Solutions (Three Biological Replicates) Granny Smith

Golden Delicious

DM

a

−9

total sugars

−9

total sugars

mannitol concentration (M)

De (×10 )

C∞/Co

De (×10 )

C∞/Co

De (×10 )

C∞/Co

De (×10−9)

C∞/Co

0 0.2 0.4 0.6

1.81 ± 0.39

0.65 0.91 0.96 0.93

2.48 ± 0.31

0.67 0.95 0.75 0.85

2.20 ± 0.19 0.73 ± 0.29

0.63 0.78 0.96 0.99

1.8 ± 0.43 1.31 ± 1.90

0.53 0.77 0.64 0.67

a

−9

DM

0.29 ± 0.38

Standard error.

Figure 4. Total sugar concentration changes in different osmotic solutions (◇, 0 M; □, 0.2 M; △, 0.4 M; and ○, 0.6 M) against time for two apple varieties: (a) Granny Smith and (b) Golden Delicious. Models of diffusion are represented by full lines.

Table 2. Concentration of Phenolic Compounds (mg/kg of FW) in Parenchyma Tissue of the Apple Cultivar Samplesa apple variety

CAT

EPI

PCA

DPn

CQA

pCQ

XPL

PLZ

PT

Golden Delicious Granny Smith

26 ± 25 20 ± 17

70 ± 37 58 ± 25

580 ± 280 760 ± 50

7.17 ± 0.01 6.52 ± 2.26

87 ± 13 56 ± 9

1.5 ± 0.8 10 ± 0.7

11 ± 4 7±1

12 ± 3 9±2

780 ± 30 910 ± 100

Data are expressed as the mean ± SD for three biological replicates. CAT, (+)-catechin; EPI, (−)-epicatechin; PRO, procyanidins; DPn, average degree of polymerization of procyanidins; CQA, cafeoylquinic acid; pCQ, para-coumarylquinic acid; XPL, phloretin xyloglucoside; PLZ, phloridzin; and PT, total polyphenols.

a

diffusivity was from 1.8 to 0.3 × 10−9 m2 s−1 for Golden Delicious and only 2.5 × 10−9 m2 s−1 for Granny Smith. No apparent diffusivity value was determined for the sugar content, which did not decrease significantly. The model did not apply. The values of De were found to depend upon the mannitol solution concentration, with no effect of the apple variety. Total sugars were decreased during the leaching process. However, the increase of the solution concentration by an osmotic agent, such as mannitol, could preserve total sugar losses.

This conclusion agrees with results obtained for the extraction of total sugar in similar conditions for apple in different concentrations of osmotic solution.25,26 The different behaviors of total sugar may reflect the mechanism of transport in the cell.25 As reported by Lin and Pitt21 in distilled water, cell rupture because of high turgor pressure caused cytolysis. According to the apple variety, an increase in the mannitol concentration in aqueous solution decreased total sugar losses. This may be linked to the extent of cytolysis or the slope of the concentration gradient. 9844



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Figure 5. Changes in phenolic compounds in different osmotic solutions (◇, 0 M; □, 0.2 M; △, 0.4 M; and ○, 0.6 M) according to time for two apple varieties: (a and b) Granny Smith and (c and d) Golden Delicious (three biological replicates). Models of diffusion are represented by full lines. PCA, procyanidins; CQA, caffeoylquinic acids.

Characterization of the Polyphenol Composition in Fresh Fruit. The two dessert apple varieties were selected for their different phenolic contents and concentrations; the initial measurements are presented in Table 2. The main antioxidants in Golden Delicious and Granny Smith are polyphenols. Marked differences in total polyphenol content were found as a function of cultivar. Granny Smith contained more polyphenols than Golden Delicious. Our results were similar to those of previous studies.14 HPLC individual identification found polyphenols from three subclasses in the flesh of both apples: (1) flavan-3-ols (monomeric catechins and procyanidins), (2) hydroxycinnamic acids (chlorogenic acid and para-coumaroylquinic acid), and (3) dihydrochalcones (phloretin xyloglucoside and phloridzin). In agreement with previous studies,14 procyanidins were the main phenolic compounds, representing 74 and 84% of total polyphenols in Granny Smith and Golden Delicious, respectively. Their degree of polymerization (DPn) of 7.2 and 6.6 for Golden Delicious and Granny Smith, respectively, were also close to those reported earlier.14 Monomeric flavan-3-ols, accounted for 12 and 8% of total polyphenol in Golden Delicious and Granny Smith parenchyma, respectively. Caffeoylquinic acid, the main hydroxycinnamic acid, represented 11 and 6% of total polyphenols in Golden Delicious and Granny Smith parenchyma, respectively. Finally, dihydrochalcones represented less than 3% of the phenolic compounds in both apple varieties. Effects of Osmotic Solutions on Polyphenol Leaching. Because slices were pretreated by NaF, a potent inhibitor of polyphenol oxidase, no oxidation was expected to occur, and within the time scale of the experiment, auto-oxidation was not

significant. Losses of phenolic compounds from apple tissues in this experiment were therefore solely due to leaching. Osmolarity of mannitol solutions affected polyphenol leaching. Figure 5 presents the evolution of concentrations (relative to FW) of procyanidins and caffeoylquinic acid. Losses of phenolic compounds were estimated by the difference in their concentrations relative to FW of slices before and after leaching. Flavan-3-ol Subclass. For the Granny Smith variety, (+)-catechin decreased during the first 4 h of the treatment in distilled water and fell to 15% of the initial content. However, no (+)-catechin losses could be quantified in mannitol solutions, regardless of the mannitol concentrations. Losses of (−)-epicatechin were 47% in distilled water, with again no losses in the different mannitol solutions. Similar results were also found with losses of 31% procyanidins in distilled water and no losses in different mannitol solutions. In Golden Delicious, (+)-catechin decreased by 47% in distilled water but only very slightly in mannitol solutions. Similar results were obtained for (−)-epicatechin and procyanidins. Hydroxycinnamic Acid Subclass. In the Granny Smith variety, caffeoylquinic acid losses decreased with increasing the mannitol concentration. For p-coumaroylquinic acids, the opposite behavior was observed. For the Golden Delicious variety, caffeoylquinic acid losses were also observed. However, no losses of p-coumaroylquinic acids were observed. The results present few different trends as a function of apple cultivar. Dihydrochalcones. In the Golden Delicious variety, phloridzin losses were 30 and 35% in 0 and 0.2 M mannitol solutions, respectively. No losses were observed at higher 9845



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Table 3. Apparent Diffusivity of Phenolic Compounds from Apple Parenchymatous Tissues in Different Osmotic Solutions (Three Biological Replicates)a phenolic compound CAT

EPI

PCA

CQA

PT

mannitol concentration (M)

GD (×10−9, m2 s−1)

C∞/C0

GS (×10−9, m2 s−1)

C∞/C0

0 0.2 0.4 0.6 0 0.2 0.4 0.6 0 0.2 0.4 0.6 0 0.2 0.4 0.6 0 0.2 0.4 0.6

0.28 ± 0.04 0.20 ± 0.05 0.06 ± 0.04 nd 0.25 ± 0.06 0.17 ± 0.09 0.19 ± 0.04 0.04 ± 0.20 0.17 ± 0.15 0.18 ± 0.04 nd nd 0.21 ± 0.04 0.14 ± 0.05 nd nd 0.19 ± 0.12 0.16 ± 0.04 nd nd

0.66 0.70 0.70 0.81 0.60 0.66 0.69 0.81 0.72 0.69 0.79 0.62 0.60 0.77 0.75 0.85 0.62 0.69 0.79 0.62

0.07 ± 0.15 ndc nd nd 0.28 ± 0.16 nd nd nd 0.20 ± 0.03 0.01 ± 0.01 nd nd 0.37 ± 0.18 0.20 ± 0.19 0.21 ± 0.07 nd 0.20 ± 0.02 nd nd nd

0.87 nd nd nd 0.53 nd nd nd 0.69 nd nd 0.65 0.67 0.70 0.66 0.67 0.67 nd nd 0.85

b

a

CAT, (+)-catechin; EPI, (−)-epicatechin; CQA, caffeoylquinic acid; PT, total polyphenols; GD, Golden Delicious; GS, Granny Smith; and C/Co, equilibrium polyphenol content. bStandard error. cNot determined.

Apparent diffusivity values decreased with increasing the mannitol solution concentration. Total phenolic values varied from 0.1 to 0.2 × 10−9 m2 s−1 for Golden Delicious and from 0.2 to 0.3 × 10−9 m2 s−1 for Granny Smith. Total phenolic compound apparent diffusivities depended upon the osmotic solution concentration and were lower than those observed for total sugars. Sugars present in cells seem to leach faster than polyphenols, possibly owing to their lower molecular weight but also their physiological role in the plant. Phenols must thus overcome the physiological barriers, mainly cell membranes, which provide dominant resistance for phenol release and influence mass transfer.32 Different cell states in osmotic solution as reported21,26 also seemed to affect polyphenol migration out of the matrix. Procyanidins presented lowest diffusivities, possibly owing to their high molecular weight33 and non-covalent interaction with cell wall materials.34 Sugar and polyphenol concentrations decreased during the immersion of apple slices in mannitol solution depending upon a certain level of the mannitol concentration. The changes were probably due to leaching into the soaking solution. The application of Fick’s second law allowed for the determination of apparent diffusivity. The link found between the decrease in solute loss and increase in the mannitol concentration could be associated with integrity of the cell structure. In all cases, the concentration of phenolic compounds changed according to the difference in osmotic pressure of the soaking solution. In raw fruit, leaching occurs if cell integrity is lost. Some of the difference in phenolic compound leaching was found to be a function of apple variety.

mannitol solutions. Phloretin xyloglucoside decreased by 15% in pure water, while no changes appeared in mannitol solutions. For the Granny Smith variety, phloridzin losses were 50% in pure water. No losses were observed in mannitol solutions. Phloretin xyloglucoside losses were 30% in 0 and 0.2 M mannitol solution, but in 0.4 and 0.6 M, losses fell to 20%. Monomeric catechins, hydroxycinnamic acids, and dihydrochalcones were either preserved or had limited losses in mannitol solution. The same trends were found for procyanidins. The decreases in the leaching of phenolic compounds in the presence of mannitol were due to physiological barriers in cells. Cell structural integrity thus affected polyphenol diffusion. Difference in osmotic pressure caused changes in cell structure integrity. The losses found here were considerably lower than those observed by Renard,27 who studied diffusion to water of thermally treated pear slices. Devic et al.,5 after 3 h in an optical density (OD) system, i.e., with very high sugar contents in the external solution, observed very limited diffusion of polyphenols. The differences between monomeric phenols (catechins, hydroxycinnamic acids, or dihydrochalcones) and procyanidins could be explained by their affinity with cell walls, as stated by Renard et al.28 and Le Bourvellec et al.29 A further indication for the involvement of this mechanism is an increase in the degree of polymerization for the retained procyanidins (up to 7.2 in Golden Delicious and 7.4 in Granny Smith after 6 h). Le Bourvellec et al.30 found that procyanidins were mainly bound to pectins. Watrelot et al.31 also found that stronger association was obtained with longer procyanidin molecules interacting with highly methylated pectins. Estimation of Apparent Diffusivity of Polyphenols. The experimental data were also fitted, and results are presented in Table 3. Apparent diffusivity values were deternmined when changes in polyphenol content were significant.

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*Telephone: +33-4-32-72-24-94. Fax: +33-4-32-72-24-92. Email: [email protected]. 9846



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The authors thank the Ministry of Higher Education and Research of Côte d’Ivoire for grants to Mouhamadou Kebe. The research leading to these results received funding from the European Community’s Seventh Framework Program (FP7/ 2007-2013) under Grant Agreement FP7-222-654 DREAM. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the SQPOV Laboratory (Fruit Team) for their assistance and technical expertise.

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Kinetics of Apple Polyphenol Diffusion in Solutions ... - ACS Publications

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