Enzymatic modification of a model homogalacturonan with the thermally tolerant pectin methylesterase from Citrus: 1. Nanostructural characterization, enzyme mode of action, and effect of pH

Methyl ester distribution in pectin homogalacturonan has a major influence on functionality. Enzymatic engineering of the pectin nanostructure for tailoring functionality can expand the role of pectin as a food-formulating agent and the use of in situ modification in prepared foods. We report on the mode of action of a unique citrus thermally tolerant pectin methylesterase (TT-PME) and the nanostructural modifications that it produces. The enzyme was used to produce a controlled demethylesterification series from a model homogalacturonan. Oligogalacturonides released from the resulting demethylesterified blocks introduced by TT-PME using a limited endopolygalacturonase digestion were separated and quantified by high-pressure anion-exchange chromatography (HPAEC) coupled to an evaporative light-scattering detector (ELSD). The results were consistent with the predictions of a numerical simulation, which assumed a multiple-attack mechanism and a degree of processivity ～10, at both pH 4.5 and 7.5. The average demethylesterified block size (0.6-2.8 nm) and number of average-sized blocks per molecule (0.8-1.9) differed, depending upon pH of the enzyme treatment. The mode of action of this enzyme and consequent nanostructural modifications of pectin differ from a previously characterized citrus salt-independent pectin methylesterase (SI-PME).     

Monovalent salt-induced gelation of enzymatically deesterified pectin

Pectin gels were induced by monovalent salts (0.2 M) concurrently with deesterification of high methoxy pectin using a salt-independent orange pectin methylesterase (PME). Constant pH was maintained during deesterification and gelation. If salt or PME was absent, the pectin did not form a gel. The gel strength was influenced by both pH and species of monovalent cation. At pH 5.0, the pectin gel induced by KCl was significantly stronger than the NaCl-induced gel. In contrast, a much stronger gel was produced in the presence of NaCl as compared to KCl at pH 7.0. LiCl did not induce pectin gelation at either pH. Molecular weights of pectins increased from 1.38 x 10(5) to 2.26 x 10(5) during NaCl-induced gelation at pH 7. One proposal to explain these pectin molecular weight changes is a hypothetical PME transacylation mechanism. However, these pectin molecular weight changes can also be explained by metastable aggregation of the enzymatically deesterified low methoxy pectin. We postulate that gelation was induced by a slow deesterification of pectin under conditions that would normally salt out (precipitate) low methoxy pectin in the absence of PME.     

Separation and characterization of a salt-dependent pectin methylesterase from Citrus sinensis var. Valencia fruit tissue

A pectin methylesterase (PME) from sweet orange fruit rag tissue, which does not destabilize citrus juice cloud, has been characterized. It is a salt-dependent PME (type II) and exhibits optimal activity between 0.1 and 0.2 M NaCl at pH 7.5. The pH optimum shifted to a more alkaline range as the salt molarity decreased (pH 8.5-9.5 at 50 mM NaCl). It has an apparent molecular mass of 32.4 kDa as determined by gel filtration chromatography, an apparent molecular mass of 33.5 kDa as determined by denaturing electrophoresis, and a pI of 10.1 and exhibits a single activity band after isoelectric focusing (IEF). It has a K(m) of 0.0487 mg/mL and a V(max) of 4.2378 nkat/mg of protein on 59% DE citrus pectin. Deblocking the N-terminus revealed a partial peptide composed of SVTPNV. De-esterification of non-calcium-sensitive pectin by 6.5% increased the calcium-sensitive pectin ratio (CSPR) from 0.045 +/- 0.011 to 0.829 +/- 0.033 but had little, if any, effect on pectin molecular weight. These properties indicate this enzyme will be useful for studying the PME mode of action as it relates to juice cloud destabilization.     

Effect of intrinsic and extrinsic factors on the interaction of plant pectin methylesterase and its proteinaceous inhibitor from kiwi fruit

A proteinaceous pectin methylesterase inhibitor (PMEI) was isolated from kiwi fruit (Actinidia chinensiscv. Hayward) and purified by affinity chromatography on a cyanogen bromide (CNBr) Sepharose 4B-orange PME column. The optimal pH of banana PME activity was 7.0, whereas that for carrot and strawberry PME activity was 9.0. The optimal pH for the binding between kiwi fruit PMEI and these PMEs was 7.0. The kiwi fruit PMEI has a different affinity for PME depending on the plant source. The inhibition kinetics of kiwi fruit PMEI to banana and strawberry PME followed a noncompetitive type, whereas that to carrot PME followed a competitive type. The kiwi fruit PMEI was mixed with banana, carrot, and strawberry PME to obtain PMEI-PME complexes, which were then subjected to thermal (40-80 degrees C, atmospheric pressure) or high-pressure (10 degrees C, 100-600 MPa) treatment. Experimental data showed that the PMEI-PME complexes were easily dissociated by both thermal and high-pressure treatments.     

Characterization of a salt-independent pectin methylesterase purified from valencia orange peel

The pectin methylesterase (PME; EC 3.1.1.11) present in a commercial orange peel enzyme preparation was characterized to establish its identity among the multiple PME isozymes present in Valencia orange (Citrus sinensis L.) peel. We show the commercial enzyme corresponds to the major peak 2 PME previously separated by heparin-Sepharose chromatography (Cameron et al., J. Food Sci. 1998, 63, 253). Both PMEs have comparable elution profiles on cation-exchange and hydrophobic-interaction perfusion chromatography columns, molecular weights (ca. 34 kDa) and pI (pH 9.2), and biochemical properties, including a broad pH activity range and activity in the absence of added cations. An identical partial amino terminal peptide sequence was also obtained for the PMEs, which further demonstrated a structural identity with other plant PMEs. The biochemical and structural properties readily distinguish this Valencia orange PME from salt-dependent isozymes and further suggest that it is an ortholog to the salt-independent fruit-specific isozyme of tomato. This work provides a well-defined, enzymatically homogeneous, salt-independent (type 1) plant PME isozyme that is suitable for studying details of the enzyme's mode of action and for use in modifying methylester patterns for studying the structure-functional property relationships in pectin.     

Compositional changes in 'Bartlett' pear ( Pyrus communis L.) cell wall polysaccharides as affected by sunlight conditions

Preharvest conditions can have a great impact on fruit quality attributes and postharvest responses. Firmness is an important quality attribute in pear, and excessive softening increases susceptibility to bruising and decay, thus limiting fruit postharvest life. Textural characteristics of fruits are determined at least in part by cell wall structure and disassembly. Few studies have analyzed the influence of fruit preharvest environment in softening, cell wall composition, and degradation. In the current work 'Bartlett' pears grown either facing the sun (S) or in the shade (H) were harvested and stored for 13 days at 20 °C. An evaluation of fruit soluble solids, acidity, color, starch degradation, firmness, cell wall yield, pectin and matrix glycan solubilization, depolymerization, and monosaccharide composition was carried out. Sun-exposed pears showed more advanced color development and similar levels of starch degradation, sugars, and acids than shaded fruit. Sunlight-grown pears were at harvest firmer than shade-grown pears. Both fruit groups softened during storage at 20 °C, but even after ripening, sun-exposed pears remained firmer. Sunlight exposure did not have a great impact on pectin molecular weight. Instead, at harvest a higher proportion of water-solubilized uronic acids and alkali-solubilized neutral sugars and a larger mean molecular size of tightly bound glycans was found in sun-exposed pears. During ripening cell wall catabolism took place in both sun- and shade-grown pears, but pectin solubilization was clearly delayed in sun-exposed fruit. This was associated with decreased removal of RG I-arabinan side chains rather than with reduced depolymerization.     

果胶对脂类和类胡萝卜素消化利用影响研究进展

果胶已经被证实可以影响脂类的消化,脂溶性的类胡萝卜素在消化阶段需要被脂滴包裹才能进入小肠形成胶束,因此果胶对类胡萝卜素的消化利用也会存在潜在影响。该文综述了近年来果胶对脂类和类胡萝卜素消化利用影响研究进展,主要分为果胶对消化液黏度的影响、对消化酶的影响、与钙离子的相互作用、与胆盐的结合作用以及对脂滴的包裹作用这5个方面。该文为后续分析如何提高果蔬中类胡萝卜素生物利用度提供理论依据。     

六种果皮原料果胶的理化及凝胶特性比较

为了解不同品种水果的果皮(柚子皮、西番莲皮、脐橙皮、石榴皮、榴莲皮)以及向日葵盘所提取果胶的理化和质构特性,研究了不同原料果胶的得率、色泽、果胶酸含量、甲氧基含量、酯化度、黏度及质构特性,特别是采用高效液相色谱准确测定了各类果胶的分子量。结果表明:柚子皮、向日葵盘和脐橙皮果胶质量分数较高,分别为18.06%、14.61%和14.43%;西番莲皮果胶质量分数为8.76%;而石榴皮及榴莲皮果胶质量分数较低(均<3%)。从分子量看,石榴皮、脐橙皮果胶分子量较大(>1000kDa),向日葵盘果胶分子量最小(483kDa)。此外,几种果胶的溶胶均属低黏度值果胶(<25厘泊),且在pH值为7.0时黏度最大、在pH值为5.0时黏度最小。结合凝胶质构分析表明:石榴皮果胶分子量最大,凝胶强度最大,但为高甲氧基果胶,且得率较低;而向日葵盘果胶分子量最小,但得率较高,且为低甲基果胶,在非糖及含糖体系中均可形成性能优良的凝胶,因此是生产果胶的良好原料。该文为果胶的生产及应用提供参考。     

Enzymatic modification of pectin to increase its calcium sensitivity while preserving its molecular weight

A commercial high-methoxy citrus pectin was treated with a purified salt-independent pectin methylesterase (PME) isozyme isolated from Valencia orange peel to prepare a series of deesterified pectins. A series of alkali-deesterified pectins was also prepared at pH 10 under conditions permitting beta-elimination. Analysis of these pectins using high-performance size exclusion chromatography (HPSEC) with on-line multiangle laser light-scattering, differential viscometer, and refractive index (RI) detectors revealed no reduction in weight-average molecular weight (M(w); 150000) in the PME-treated pectin series, whereas a 16% reduction in intrinsic viscosity (IV) occurred below a degree of esterification (DE) of 47%. In contrast, alkali deesterification rapidly reduced both M(w) and IV to less than half of that observed for untreated pectin. PME treatment of a non-calcium-sensitive citrus pectin introduced calcium sensitivity with only a 6% reduction in the DE. Triad blocks of unesterified galacturonic acid were observed in (1)H nuclear magnetic resonance spectra of this calcium-sensitive pectin (CSP). These results demonstrate that the orange salt-independent PME isozyme utilizes a blockwise mode of action. This is the first report of the preparation of a CSP by PME treatment without significant loss of the pectin's M(w) due to depolymerization.     

Pectin methylesterase from kiwi and kaki fruits: purification, characterization, and role of pH in the enzyme regulation and interaction with the kiwi proteinaceous inhibitor

Pectin methylesterase was purified from kiwi (Actinidia chinensis) and kaki fruit (Diospyros kaki). The pH values of the fruit homogenates were 3.5 and 6.2, respectively. The kiwi enzyme is localized in the cell wall and has a neutral-alkaline pI, whereas the kaki enzyme is localized in the soluble fraction and has a neutral-acidic pI. The molecular weights of the kiwi and kaki enzymes were 50 and 37 kDa, respectively. The two enzymes showed a similar salt and pH dependence of activity, and a different pH dependence of the inhibition by the kiwi proteinaceous inhibitor.     

Partial purification and characterization of pectin methylesterase from acerola (Malpighia glabra L.)

The enzyme pectin methylesterase (PME) is present in acerola fruit and was partially purified by gel filtration on Sephadex G-100. The results of gel filtration showed different PME isoforms. The total PME (precipitated by 70% salt saturation) and one of these isoforms (fraction from Sephadex G-100 elution) that showed a molecular mass of 15.5 +/- 1.0 kDa were studied. The optimum pH values of both forms were 9.0. The total and the partially purified PME showed that PME specific activity increases with temperature. The total acerola PME retained 13.5% of its specific activity after 90 min of incubation at 98 degrees C. The partially purified acerola (PME isoform) showed 125.5% of its specific activity after 90 min of incubation at 98 degrees C. The K(m) values of the total PME and the partially purified PME isoform were 0.081 and 0.12 mg/mL, respectively. The V(max) values of the total PME and the partially purified PME were 2.92 and 6.21 micromol/min/mL/mg of protein, respectively.     

Addition of pectin and soy soluble polysaccharide affects the particle size distribution of casein suspensions prepared from acidified skim milk

Pectins are negatively charged polysaccharides employed as stabilizers in acidified milk dispersions, where caseins aggregate because of the low pH and serum separation needs to be prevented. The objective of this research was to study the effect of charge on the stabilizing functionality of the polysaccharide in acid milk drinks. Unstandardized pectins with various charges (as degree of esterification, DE) as well as soybean soluble polysaccharide (SSPS) were tested for their stabilizing behavior as a function of pH and concentration. Skim milk was acidified by glucono-delta-lactone and then homogenized in the presence of polysaccharide at different pH values (in the range from 4.2 to 3.0). Measurements of particle size distribution demonstrated that pectins with a DE of 71.4, 68.6, and 67.4 stabilized milk at pH > 4.0. Pectins with a lower DE (63.9%) needed a higher concentration (0.4%) at the same pH to show a monomodal distribution of particle sizes. Pectins with lower DE (<50%) did not stabilize the dispersions. Although this difference in behavior was attributed mainly to the pectin charge, the efficiency in stabilizing the casein dispersion decreased with decreasing pectin size. For example, the high methoxyl pectin (HMP) with 63.9 DE was smaller in size than the HMPs with a higher charge. Pectins showed a pH-dependent stabilization effect, as at pH < 4.0 the dispersions contained aggregates. When SSPS was used to stabilize acid milk, at pH < 4.0, it showed a better stabilization behavior than HMP. When SSPS and pectin were used in combination, the particle size distribution of the acid milk dispersion was pH-dependent, and results were similar to those for samples containing pectin alone. This suggested that in the mixture, pectin dominated the behavior over SSPS, even when an excess of SSPS was added to the dispersions before homogenization.     

Changes in molecular weight of transacylated pectin catalyzed by tomato and citrus pectinesterases as determined by gel permeation chromatography

The changes in molecular masses of pectin in 0.5% pectin-pectinesterase (PE) mixtures (2 units/mL) incubated at various temperatures, pH values, and NaCl levels for 30 min were observed by a Toyopearl TSK HW-65 (F) gel permeation chromatography. The molecular mass of pectin was remarkably increased from 103 to 266 kDa when the incubation temperature of pectin-tomato PE was increased from 25 to 45 degrees C. A further increase in molecular mass was observed when a pectin-citrus PE mixture was incubated at 65 degrees C. The values of pH and NaCl levels were also crucial to the transacylation activity of PEs. Reaction at pH 7.5 with tomato PE and citrus PE remarkably expanded the molecular mass of pectin to 410 and 670 kDa, respectively. The NaCl level of 0.3-0.5 and 0.3 M was favorable for the transacylation reaction of tomato PE and citrus PE, respectively. Only high methoxylpectin was the suitable substrate for PE to conduct the transacylation reaction.     

Cultivar-dependent cell wall modification of strawberry fruit under NaCl salinity stress

Strawberry cultivars differ in their sensitivity to NaCl; fruits of cv. Elsanta suffer from softening, whereas those of cv. Korona retain their firmness. The mean fruit fresh weight is reduced in cv. Elsanta up to 46% and in cv. Korona up to 26%. Cell walls of fruits grown under 0, 40, or 80 mmol/L NaCl were extracted and analyzed. In fruits of cv. Korona, the content of the alcohol-insoluble residue remained comparatively stable as salt levels increased but was reduced in cv. Elsanta. The water-soluble pectin fraction was not affected in cv. Korona, but the content of low methoxy pectinates increased significantly, indicative of the generation of calcium and magnesium bridges that stabilize pectin polysaccharides of cell walls. In cv. Elsanta, the content of water-soluble pectin rose, indicating pectin solubilization. For both cultivars, the significant negative correlation of fruit Cl(-) contents with the contents of NaOH-soluble pectinates, when expressed per fruit fresh mass, indicated that covalently bound pectic substances were degraded. Especially the response of cv. Elsanta is in line with the general observation that severe osmotic stress results in slower cell expansion and weaker cell walls.     

Nonenzymatic degradation of citrus pectin and pectate during prolonged heating: effects of pH, temperature, and degree of methyl esterification

The underlying mechanisms governing nonenzymatic pectin and pectate degradation during thermal treatment have not yet been fully elucidated. This study determined the extent of nonenzymatic degradation due to beta-elimination, acid hydrolysis, and demethylation during prolonged heating of citrus pectins and its influence on physicochemical properties. Solutions of citrus pectins, buffered from pH 4.0 to 8.5, were heated at 75, 85, 95, and 110 degrees C for 0-300 min. Evolution of methanol and formation of reducing groups and unsaturated uronides were monitored during heating. Molecular weight and viscosity changes were determined through size exclusion chromatography and capillary viscometry, respectively. Results showed that at pH 4.5, the activation energies of acid hydrolysis, beta-elimination, and demethylation are 95, 136, and 98 kJ/mol, respectively. This means that at this pH, acid hydrolysis occurs more rapidly than beta-elimination. Furthermore, the rate of acid hydrolysis is diminished by higher levels of methyl esterification. Also, citrus pectin (93% esterified) degrades primarily via beta-elimination even under acidic conditions. Acid hydrolysis and beta-elimination caused significant reduction in relative viscosity and molecular weight.     

Stabilizing behavior of soy soluble polysaccharide or high methoxyl pectin in soy protein isolate emulsions at low pH

The stability of emulsions prepared with soy protein isolates was investigated as a function of pH in the presence of two negatively charged polysaccharides: high methoxyl pectin (HMP) and soy soluble polysaccharide (SSPS). Both polysaccharides are composed of a backbone which contains galacturonic acid but, when added to soy protein isolate-stabilized emulsions, SSPS showed a different behavior than that of HMP. At neutral pH and above a critical concentration of stabilizer (0.05%), HMP caused flocculation of the emulsion droplets via a depletion mechanism. On the other hand, the emulsions containing a similar amount of SSPS did not show creaming or flocculation. At acidic pH (<4.0) the addition of pectin caused extensive droplet aggregation, while no aggregation was observed with the addition of SSPS. The differences in the stabilization behavior between the two polysaccharides can be attributed to their differences in charge, neutral sugars side chains, and molecular weight.     

Citrus pectin: characterization and inhibitory effect on fibroblast growth factor-receptor interaction.

This study was undertaken to characterize the pectin from four citrus species and to determine their in vitro inhibitory activities on the binding of fibroblast growth factor (FGF) to the FGF receptor (FGFR). Pectin from various parts of lemon, grapefruit, tangerine, and orange were isolated and characterized. Tangerine had the highest pectin content among the four citrus species. Segment membrane contained as much as or more pectin than flavedo/albedo. Anhydrogalacturonic content was highest in pectin from segment membrane of tangerine and flavedo/albedo of grapefruit. Lemon pectin contained the highest methoxyl content (MC), and grapefruit contained the largest proportion of lower molecular weight (<10000 Da) pectin. Tangerine contained the highest neutral sugar in both flavedo/albedo and segment membrane. The interdependency of heparin on factor-receptor interaction provides a means for identifying new antagonists of growth factor activity and thus for treatment of various diseases. These results showed that pectin significantly inhibited the binding of FGF-1 to FGFR1 in the presence of 0.1 microg/mL heparin. The pectin from the segment membrane of lemon was the most potent inhibitor. The inhibition activity was significantly correlated with sugar content, MC, and size of pectin. Kinetic studies revealed a competitive nature of pectin inhibition with the heparin, a crucial component of the FGF signal transduction process. The observation that the heparin-dependent biological activity of FGF signal transduction is antagonized by citrus pectin should be further investigated for the use of these pectins as anti-growth factor agents for potential health benefits.     

Mode of de-esterification of alkaline and acidic pectin methyl esterases at different pH conditions

Highly esterified citrus pectin was de-esterified at pH 4.5 and 8.0 by a fungal pectin methyl esterase (PME) that was shown to have an acidic isoelectric pH (pI) and an acidic pH optimum and by a plant PME that was characterized by an alkaline pI and an alkaline pH optimum. Interchain and intrachain de-esterification patterns were studied by digestion of the pectin products with endo-polygalacturonase and subsequent analysis using size exclusion and anion-exchange chromatography. No effect of pH was observed on the de-esterification mode of either of the two enzymes. Acidic, fungal PME converted pectin according to a multiple-chain mechanism, with a limited degree of multiple attack at the intrachain level, both at pH 4.5 and at pH 8.0. A multiple-attack mechanism, with a high degree of multiple attack, was more appropriate to describe the action mode of alkaline, plant PME, both at pH 4.5 and at pH 8.0.     

不同柑橘类果胶分子的理化特性、结构及凝胶性能

为了解不同果胶的理化性质、结构与其功能特性的区别,拓展不同性能果胶的针对化应用,提高柑橘加工副产物的附加值。该研究采用酸提法从上饶柚子皮、赣南脐橙皮、南丰蜜桔皮、日本柚子皮、小青柑皮、柠檬皮6种柑橘类水果果皮渣中分别提取果胶,依次为HMP-1、HMP-2、HMP-3、HMP-4、HMP-5和HMP-6,并对其理化特性、结构及凝胶性能进行比较分析。结果表明：不同来源的6种柑橘果胶酯化度在67.20%～75.45%,均为高酯果胶（high methoxyl pectin, HMP）,具备阴离子多糖特性,在提取过程中均出现了不同程度的降解,理化特性和分子结构上存在一定差别。果胶的分子量、黏度、分子链结构等因素共同作用于果胶的凝胶性能,6种HMP均能形成具有一定强度的酸/糖凝胶,将水分截留在三维凝胶网络中,经历冻融循环过程持水性仍在96%以上,具备较好的稳定性。其中分子量较低的HMP-1和HMP-6凝胶强度最高,胶凝度均在200°以上,这与HMP-1的高同型半乳糖醛酸聚糖（HG）含量、较完整的分子结构,以及HMP-6较高的鼠李糖半乳糖醛酸聚糖-I（RG-I）含量及分支度有关,而分子量及黏度较高的HMP-5的弱凝胶强度则与其较大的分子链降解程度及低RG-I含量有关,表明HG和RG-I结构域对凝胶的形成均有促进作用。原子力显微镜图像也显示,HMP-1和HMP-6中出现较为明显的分子链交联。对不同柑橘类果胶的理化性质、结构与凝胶特性进行比较分析,为确定凝胶性能更加优异的提取原料、果胶的扩大化生产及针对性应用提供了理论参考。     

Studies on water transport through the sweet cherry fruit surface. 11. FeCl3 decreases water permeability of polar pathways

The effect of FeCl3 (10 mM) on osmotic water uptake into detached sweet cherry fruit (Prunus avium L.) and on the (3)H2O permeability (P(d)) of excised exocarp segments (ES) or enzymatically isolated cuticular membranes (CM) was investigated. ES or CM were mounted in an infinite dose diffusion system, where diffusion is monitored from a dilute donor solution through an interfacing ES or CM into a receiver solution under quasi steady-state conditions. In the absence of FeCl3, (3)H2O diffusion through stomatous ES was linear over time, indicating that P(d) was constant. Adding FeCl3 to the donor decreased P(d) by about 60%. P(d) remained at a decreased level when replacing the FeCl3 donor again by deionized water. The decrease in P(d) was positively and linearly related to the stomatal density of the ES. There was no effect of FeCl3 on the P(d) of astomatous sweet cherry fruit ES or CM regardless of the presence of wax (epicuticular or cuticular). FeCl3 decreased P(d) when added to the donor (-63%) or receiver (-16%), but there was no effect when it was added to donor and receiver solutions simultaneously. The decrease in P(d) depended on the pH of the receiver and the presence of citrate buffer. There was no effect of FeCl3 with citrate buffer as a receiver regardless of pH (range 2.0-6.0). When using nonbuffered receiver solutions with pH adjusted to pH 2.0, 3.0, 4.5, or 6.0, FeCl3 markedly decreased (3)H2O diffusion at pH > or = 3 but had no effect at pH 2.0. FeCl3 increased the energy of activation (E(a)) for (3)H2O diffusion (range 15-45 degrees C) through stomatous ES but had no significant effect in astomatous CM. The increase in E(a) by FeCl3 was positively related to stomatal density. FeCl3 decreased the P(d) for 2-(1-naphthyl)[1-(14)C]acetic acid (NAA) and 2,4-dichloro[U-(14)C]phenoxyacetic acid (2,4-D) in stomatous ES. The magnitude of the effect depended on the degree of dissociation and was larger for the dissociated acids (pH 6.2) than for the nondissociated acids (pH 2.2). Incubating whole fruit in isotonic solutions of selected osmotica resulted in significant water uptake that was inversely related to the molecular weight of the osmotica and was consistently lower for fruit treated with FeCl3. The FeCl3 induced decrease in water fluxes was larger for osmotica having a low molecular weight than for those with a higher molecular weight. Our data indicate that FeCl3 decreased the permeability of the stomatous sweet cherry exocarp to water and other polar substances by pH-dependent formation of precipitates that decrease transport along polar pathways. Decreasing the permeability of polar pathways by a precipitation reaction is a useful target in developing strategies against rain-induced fruit cracking.