Postharvest Biology and Technology 44 (2007) 63–70 | Wetting | Surface Tension

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  Postharvest Biology and Technology 44 (2007) 63–70 Optimization of edible coating compositionto retard strawberry fruit senescence Clara Ribeiro a , b , ∗ , Ant´onio A. Vicente a , Jos´e A. Teixeira a , Cˆandida Miranda b a Centro de Engenharia Biol´ ogica, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal b  Research and Development Department, Frulact, S.A., Rua do Outeiro, 589, Gemunde, 4475-150 Maia, Portugal Received 26 June 2006; accepted 21 November 2006 Abstract The ability of polysaccharide-based (starch, carrageenan and chitosan) coatings to extend the shelf-life of strawberry fruit ( Fragariaananassa ) were studied, mainly for industrial applications. The coatings and strawberries were characterized in terms of their physicalproperties (superficial properties, wettability, oxygen permeability) in order to optimize coating composition. The optimized coatings werethen applied to the fruit both in the laboratory and in the field and their effects on relevant quality parameters assessed. The superficial tensionof the strawberry was 28.94mN/m, and its polar and dispersive components were 5.95 and 22.99mN/m, respectively. The critical superficialtension of the strawberry, obtained from a Zisman plot, was 18.84mN/m. For each polysaccharide-based coating the best wettability wasobtained for compositions: 2% starch and 2% sorbitol; 0.3% carrageenan, 0.75% glycerol and 0.02% Tween 80; 1% chitosan and 0.1% Tween80. The oxygen permeability of carrageenan films was approximately 40% of that obtained with starch films. The addition of calcium tothe starch film-forming solution produced an increase in the film thickness; nevertheless no significant differences in oxygen permeabilitywere obtained between films with and without calcium. The effects of application of these coatings to fresh strawberries were assessed bydetermining color change, firmness, weight loss, soluble solids and microbiological growth over 6 days. No significant colour differenceswere found, and the minimum firmness loss was obtained in strawberries coated with carrageenan and calcium chloride. The minimum lossof mass was obtained for fruit with chitosan and carrageenan coatings both with calcium chloride. The addition of 1% di-hydrated calciumchloride to the coatings reduced the microbial growth rate on the fruit. The minimum rate of microbial growth was obtained for strawberriescoated with chitosan and calcium chloride. The industrial application of calcium-enriched carrageenan coating on fresh strawberries resultedin a decrease in firmness loss when compared to non-coated fruit.© 2006 Elsevier B.V. All rights reserved. Keywords: Edible coatings; Strawberry shelf-life; Oxygen permeability; Wettability of edible coatings 1. Introduction Research on edible coatings and films has been intense inrecent years. Attempts to diminish crop losses and maintainthe quality of fresh fruit for a longer period is a priority forall the producers. This is true both for fruit being directlysold to the consumer and for further processing. The devel-opment of coatings from polysaccharides has brought an ∗ Correspondingauthorat:CentrodeEngenhariaBiol´ogica,Universidadedo Minho, Campus de Gualtar, 4710-057 Braga, Portugal.Tel.: +351 229 287 910; fax: +351 229 287 919.  E-mail address: Ribeiro). increase in new types of coatings for extending the shelf-lifeof fruit and vegetables because of the selective permeabili-ties of these polymers to O 2 and CO 2 . Polysaccharide basedcoatings can be used to modify the internal atmosphereof the fruit and thus retard senescence (Nisperos-Carriedo,1994).Even though some edible coatings have been successfullyapplied to fresh products, other applications adversely affectquality.Modificationoftheinternalatmospherebytheuseof edible coatings can increase disorders associated with highCO 2 or low O 2 concentrations (Ben-Yehoshua, 1969)There- fore it is only natural that the control of gas permeability of the films should be a priority in their development. 0925-5214/$ – see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.postharvbio.2006.11.015  64 C. Ribeiro et al. / Postharvest Biology and Technology 44 (2007) 63–70 The effectiveness of edible coatings for protection of fruitand vegetables depends primarily on controlling the wet-tability of the coating solutions, which affects the coatingthickness of the film (Park, 1999).Edible coating for- mulations must wet and spread uniformly on the fruit’ssurface and, upon drying, a coating that has adequate adhe-sion, cohesion, and durability to function properly (Krochtaand Mulder-Johnston, 1997)must be formed. Among otherfunctionalities, edible coatings can act as carriers for foodadditives such as antioxidants and antimicrobial agents ontothe surface of the food.The aim of this work was to study the ability of starch,carrageenan and chitosan based coatings to extend the shelf-lifeofstrawberryfruit.Thisstudywasdividedintotwoparts:in the first part coating composition was optimized and O 2 permeability of coating solutions was determined; in the sec-ond, the coatings were applied to the strawberries, both inthe laboratory and in the field, and the changes in the qualityparameters were followed during storage of the coated fruit. 2. Materials and methods 2.1. Materials All polysaccharides used in this study were food-grade.Specific materials included starch Crisp Coat 868 (NationalStarch,Germany),withapproximately30%amylosecontent,  -carrageenan DX5253 (FMC, Belgium), chitosan powderwith 90% deacetylation (Aqua Premier Co., Thailand), sor-bitol97%,polyethyleneglycolMW200andTween80(Acr¯osOrganics, Belgium), glycerol 87% (Panreac, Spain), NaOH99%,citricacidandcalciumchloride(Merck,Germany),HCl37% (Riedel deHa¨en, Germany). 2.2. Coating solutions Solutions with 2% (w/v) starch were gelatinized by heat-ingat90 ◦ C,thepHvaluewasadjustedto5.6with50%(w/v)citric acid solution and the solutions were equilibrated for10min. Sorbitol was added as a plasticizer at a concentrationof 2.0g solute  /L solution .Carrageenan solutions were prepared by dissolving 0.3%(w/v) carrageenan in distilled water, and heating at 80 ◦ C for10min; the pH value was adjusted to 5.6 with 50% (w/v)citric acid solution. Glycerol was used as a plasticizer, at aconcentration of 0.75% (w/v). Tween 80 was added to thesolutionasasurfactantwithvariousconcentrations(between0.01 and 0.1% (w/v)).ChitosansolutionswerepreparedaccordingtoElGhaouthet al. (1991).Chitosan solutions were made by dispersing 1gof chitosan in 80mL of distilled water to which 2.5mL of 10N HCl was added to dissolve the chitosan. The pH valuewas adjusted to 5.6 with 1N NaOH and 0.1mL of Tween80 was added as a surfactant. The solution was made up to100mL. 2.3. Wettability Both contact angle ( θ ) and liquid–vapor surface tension( γ  LV )weredeterminedwithafacecontactanglemeter(OCA20, Dataphysics, Germany). The surface tension of the coat-ing solution was measured by the pendant drop methodusingtheLaplace–Youngapproximation(SongandSpringer,1996).Samples of the coating solution where taken with a500  L syringe (Hamilton, Switzerland) in order to deter-minethedropshape,usingcomputer-aidedimageprocessing.The diameter of the needle (0.72 ± 0.01mm), necessary for γ  LV determination, was obtained with a digital micrometer(Mitutoyo, US). The contact angle at the strawberry sur-face was measured by the sessile drop method (Newman andKwok, 1999),in which a droplet of the tested liquid wasplaced on a horizontal surface and observed with a face con-tact angle meter. To avoid changes on the strawberry surface,measurements were made in less than 60s.Twenty replicates of both the contact angle at the straw-berry surface and of the liquid–vapour surface tensionmeasurements were performed at (19 ± 1) ◦ C.Estimation of the critical surface tension ( γ  C ) of straw-berrysurfacewasobtainedbyextrapolationfromtheZismanplot(Zisman,1964).Thisplothasbeenobtainedusingwater, formamide, bromonaphthalene and toluene. 2.4. Oxygen permeability The method used in was based on that of theASTMD3985-02 (2002).A film made from the same material aseach of the coatings was sealed between two superimposedchambers, each one having two channels. In the lower cham-ber, a controlled stream of pure O 2 flowed, thus maintainingconstant the O 2 pressure in this compartment ( ≈ 1atm). Inthe upper compartment a N 2 stream (5–15mL/min) acted asa carrier of the permeated O 2 . The entrance flows of the twochambers were linked to a pressure gauge to guarantee thatthere is no pressure gradient between the two chambers.The O 2 concentration at the upper chamber was deter-mined using an O 2 sensor at the outlet of that chamber,therefore allowing the calculation of the amount of O 2 cross-ing the film. At the end of each assay the film was carefullyremoved for determination of its thickness. For this analysisten measurements at distinct points on the film were madeusing a digital micrometer (Mitutoyo, USA). 2.5. Physicochemical properties of coated fruit  The laboratory assays were made with fresh strawber-ries ( Fragaria ananassa cv. Camarosa) purchased at thelocal market; the fruit were randomly distributed in sevenequal lots, weighing 5kg each. One of the lots was takenas the control while the remaining six were used for theexperiments with the three coatings, either in the absenceor presence of calcium. Sprays of the different coating solu-tions were applied to the lots, which were then stored at  C. Ribeiro et al. / Postharvest Biology and Technology 44 (2007) 63–70 65 controlled temperatures (0–5 ◦ C) with a relative humidity of 85–90%.Weight loss of fresh strawberries during storage was mea-sured by daily monitoring the weight changes of 20 fruit,randomly chosen (with a total approximate mass of 200g)from each of the 5kg lots.For the analysis of color and soluble solids content, 20randomly chosen fruit (with a total approximate mass of 200g) from each 5kg lot were homogenized with a highspeed mixer. Soluble solids were determined according tothe AOAC 932.12 standard method; the analyses were per-formedwitharefractometer(GPR12–70,IndexInstruments,UK). Three replicates were made for each measurement andthe soluble solids content was expressed in % of solublesolids. The color of the fruit was determined with a Minoltacolorimeter (CR 300; Minolta, Japan), recording CIE L * a * b values,being  L * (lightness), a * (redness)and b * (yellowness).The color measurements were performed as follows: threeportions of 25g were taken from every sample of homoge-nized fruit (each of those samples representing each of the5kg lots) and were placed on three black dishes with 5.5cmof diameter. On each dish (i.e., on every replicate) the colorwas measured in five different points: in the centre, and inthe vertices of an imaginary square of 5.5cm, centered in thecentre of the dish. These five values were averaged and thethree averages thus obtained for each sample were averagedagain,thecolorbeingexpressedaslightness(  L * )andchroma(( a 2 + b 2 ) 1/2 ).Forthelaboratoryassays,firmnesswasdeterminedusingaTexture Analyzer (TA-XT2, Stable Micro Systems, UK). AnOttawa cell with a holed extrusion plate ( φ =6.5mm) wasused with a compression load cell of 25kg. Each experimentwas conducted with 50g of strawberries (approximately fivestrawberries)atacompressionspeedof1.5mm/s.Fourrepli-cates were used for each determination. The firmness wasreported as peak force and expressed in newtons per gram of sample.The industrial assay was carried out in the producer’sfield (Huelva, Spain) with 800kg of fresh strawberries usingthe selected coating (carrageenan+calcium—see Section3).The coating was sprayed (with a 10L shoulder strap pres-sure sprayer) in every 5kg perforated tray in the assembled800kgpallet,andthenallowedtodrip.Thepalletofstrawber-ries was stored in a controlled temperature chamber (0–5 ◦ C)with a relative humidity of 85–90% and ventilation to reducethe overall temperature of the fruit. The coated and uncoatedfruit were taken through the normal chain of transportationfrom the producer to the processing plant (approximately 2days).For the industrial assays, the firmness was reported interms of internal and external firmness as determined by anInstron3342machine(USA)withaperforationneedle(3mmof diameter) using a 500N compression load cell. Perfora-tions of 3mm depth were made in each assay at a velocity of 30mm/min.Foreachfirmnessdetermination20strawberrieswere randomly chosen and, for each strawberry, the firm-ness was determined at four different points (both internaland external). The firmness is reported in terms of maximumload (kN). 2.6. Microbial assays The total microbial count was made according to thePortuguese standardNP 4405 (2002).The samples were collected in sterilized jars and homogenized in asepticconditions. One milliliter of each sample was transferredto each of two Petri dishes. To each inoculated dish,approximately 15mL of Plate Count Agar was added andcooled to 44–47 ◦ C. The samples were mixed immedi-ately after pouring by rotating the Petri dish sufficientlyto obtain evenly dispersed colonies after incubation. Aftercomplete solidification, the plates were inverted and incu-bated at 30 ± 1 ◦ C for 72 ± 3h. The count was expressed inCFU/g sample . 2.7. Statistical analysis SPSSsoftware(Version12.0,SPSSInc.,US)wasusedforall statistical analysis. Analysis of variance (ANOVA), T  -testandregressionanalysiswereappliedatasignificancelevelof 0.05. Values followed by the same letter are not significantlydifferent. 3. Results and discussion 3.1. Critical surface tension According toZisman (1964),in systems having a surface tensionlowerthan100mN/m(low-energysurfaces),thecon-tact angle formed by a drop of liquid on the solid surfacewill be a linear function of the surface tension of the liq-uid, γ  LV (were phase V is air saturated with the vapor of liquid, L). According toOwens and Wendt (1969),Rabel (1971)andKaelble (1970)the surface tension of the liquid can be separated according to the interactions between theirmolecules. Such interactions are of two types: polar and dis-persive.Givenapureliquid,forwhichthesurfacetensionandits polar ( γ  pL ) and dispersive ( γ  dL ) contributions are known, if  θ is the contact angle between the liquid and some solid, theinteraction can be described in terms of the reversible work of adhesion, W  a , as: W  a = W  da + W  pa = 2   γ  dS + γ  dL +   γ  pS + γ  pL  = γ  L (1 + cos θ ) (1)where γ  pS and γ  dS are the polar and dispersive contributions of the surface of the studied solid. Rearranging Eq.(1),yields: 1 + cos θ 2 γ  L   γ  dL =   γ  pS   γ  pL γ  dL +   γ  dS (2)  66 C. Ribeiro et al. / Postharvest Biology and Technology 44 (2007) 63–70 Table 1Surface tension components of the liquids used for characterization of thestrawberry surfaceCompound γ  L (mN/m) γ  dL (mN/m) γ  pL (mN/m)Water a 72.10 19.90 52.20Bromonaphthalene a 44.40 44.40 0.00Formamide a 56.90 23.50 33.40Toluene b 28.50 27.18 1.32 a Data adopted fromBusscher et al., 1984. b data adopted fromJanczuk and Bialopiotrowicz, 1989.Fig. 1. Linear regression (confidence level of 0.05 and n =80). Once experimental data are plotted in a graph of 1 + cos θ/ 2 γ  L /   γ  dL versus   γ  pL /γ  dL , it is possible to obtain thevalues γ  pS and γ  dS . It is also possible to define the cohesioncoefficient ( W  c ), or work of cohesion, by: W  c = 2 γ  LV (3)This is related to the reversible work required to separatetwo surfaces of unit area of a material with a determined γ  S .While the adhesion forces cause the liquid to spread on thesurface, the cohesion forces cause the liquid to contract. Thebalance between W  c and W  a gives the spreading coefficient, W  s , according to: W  s = W  a − W  c (4)The spreading coefficient, or wettability, represents theability of a given liquid to spread on a solid surface. Themaximum value that can be obtained for this coefficient iszero.The surface tension determined for the strawberry was28.94mN/m (therefore, it is a low-energy surface), its polarand dispersive components being 5.95 and 22.99mN/m,respectively. These values, based on the data fromTable 1, Fig. 3. Zisman plot for strawberry surface (confidence level of 0.05 and n =80). are reported inFig. 1according to Eq.(5). 1 + cos θ 2 γ  L   γ  dL = (2 . 1391 ± 0 . 1229)   γ  pL γ  dL + (4 . 7944 ± 0 . 0093); R = 0 . 9712(5)Thecorrelationvalue(  R =0.9712)suggestsastronglinearcorrelation between the dependent and independent vari-ables. The validity of the global model was tested by the F  -test, and the significance of the parameters of Eq.(5)wastested by the T  -test, both with a significance level of 0.05.Sincethesurfaceofthestrawberryisalowenergysurface,it is now possible to apply the Zisman plot. According toJohnson and Dettre (1993)this kind of surface interacts withliquidsprimarilybydispersionforces.Thisfactexplainswhydrops of polar liquids were not absorbed after a short period(Figs. 2 and 3). Again, the validity of the global model was tested by the F  -test, while the significance of the parametersof Eq.(6)was tested by the T  -test, both with a significancelevel of 0.05.Eq.(6)representsthelinearregressiontothedatainFig.3. cos θ = ( − 0 . 0175 ± 0 . 0008) γ  L + (1 . 3339 ± 0 . 0011); R = 0 . 9683 (6)The determination coefficient (  R 2 =0.94) indicates thatthere is a strong linear association between the cosine of thecontact angle ( θ ) and the surface tension of the tested liquid,94% of the variation of cos θ being explained by the model.In spite of the good correlation, the values obtained should Fig. 2. Contact angle of the tested liquids on strawberry surface: (a) water; (b) formamide; (c) bromonaphthalene; (d) toluene.
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