Whey Protein Coatings for Fresh Fruits and Relative Humidity Effects L. CISNEROS-ZEVALLOS AND J.M. KROCHTA

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1 JFS: Whey Protein Coatings for Fresh Fruits and Relative Humidity Effects L. CISNEROS-ZEVALLOS AND J.M. KROCHTA ABSTRACT: Whey protein isolate (WPI) films have proven to be excellent gas barriers in previous studies, making them potential coatings for fresh produce. WPI-coated apples and controls were stored at 20 C under RH s ranging from 54 to 92%. Results showed performance of WPI coatings depended on the environment RH. The internal oxygen was lowered, and carbon dioxide increased with decreasing RH conditions. RH did not affect control fruits. At low RH (about 70 to 80% RH), anaerobic respiration was induced in coated fruits due to low oxygen levels (about atm). Controlling thickness and film permeability will allow attainment of the appropriate oxygen and carbon dioxide levels for coated fruit. Keywords: WPI coatings, relative humidity, fruits and vegetables, modified atmospheres Introduction THE USE OF BIOPOLYMER COATINGS TO MODIFY THE INTERNAL atmospheres of fruits and vegetables has been extensively studied during the past 20 y. However, unpredictable results and diverse conclusions have been common in this research area. To obtain a more predictable performance, it is necessary to understand which factors may influence the response of coating systems. Internal modified atmosphere depends on factors such as coating permeability, film thickness, and film surface coverage. However, it is still not clear how these factors interact in relation to the internal modified atmospheres of coated produce. Fruit and vegetable skin structures such as lenticels, stem ends, calyxes, and cuticle and the presence of a coating barrier provides a complex challenge to achievement of understanding of the mechanism by which coatings achieve internal gas modifications. There are 2 proposals to explain internal gas modification based on gas transfer resistances offered by the skin and the coating barrier (Banks and others 1993). First, the loosely adhering coating (lac) model considers that the coating loosely covers the fruit, giving the opportunity for mixing of gases between the fruit surface and the coating. In this model, skin resistance (which considers the effective resistances of the pores and cuticle operating in parallel) and the coating resistance are in series. Second, the tightly adhering coating (tac) model considers that the coating adheres tightly to the edges of the pores in a way that there would not be an opportunity for mixing of gases between a fruit surface and the coating. In this model, coating resistances are in series with the respective pores and cuticle resistances, while the effective resistances of pores-and-coating and cuticle-and-coating are operating in parallel (Banks and others 1993). According to these models, gas film permeance, film thickness, or pore blockage may be important in internal gas modification. Thickness has been considered to influence the response by defining the coating thickness through which the gas permeant has to diffuse. Coating thickness has been indirectly related to coating gas resistance and internal gas modifications (Banks and others 1993; Ben-Yehoshua and Cameron 1988; Hagenmeier and Shaw 1992). Park and others (1994a, 1994b) related thickness to coating solution solid concentration. Coating thickness may be important in a lac model, but not necessarily in a tac model, according to Banks and others (1993). For the tac model, the percentage of pores blocked by the coating would have more importance in internal atmosphere modification. Whey Protein Isolate (WPI) can be used as a potential biopolymer for coating fresh fruits due to its excellent film gas barrier properties. WPI forms a water-soluble coating solution, which, after application to the surface and drying, forms an insoluble coating. This unique characteristic of WPI may be very attractive for the fruit and vegetable industry. However, it is necessary to identify and study which factors may influence the performance of WPI coatings. Like other hydrophilic biopolymer films, WPI film barrier properties depend on relative humidity (RH) of the environment (McHugh and Krochta 1994a, b; McHugh and others 1994, 1993). Plasticizers are added to film formulations to increase film flexibility; however, this will also increase film permeability (McHugh and Krochta 1994a; Mate and Krochta 1996; McHugh and others 1994). The most effective plasticizer in hydrophilic films is water, and the amount of moisture in the films is related to the RH of the environment through their moisture sorption isotherms (Gontard and others 1996). Permeability of hydrophilic films will increase with higher moisture content and higher RH (McHugh and Krochta 1994a; Rico-Pena and Torres 1990; Elson and others 1985; Hagenmeir and Shaw 1991; Mark and others 1966). The increase in permeability is related to a decrease of the glass transition temperature of the film. This temperature defines the transition from a viscous glass to a more liquid-like rubbery state in the amorphous polymer matrix. This change from one state to another is strongly associated with the water content or other plasticizers. The glass-rubber transition will affect molecular mobility of the system, which can be observed as changes in viscosity, diffusivity, or flexibility of the system (Roos and Karel 1991; Buera and Karel 1993; McHugh and Krochta 1994a). Most of the work on RH effects has been on film permeabilities. Exponential dependence of O 2 permeability on RH has been reported for methylcellulose-palmitic acid films (Rico-Pena and Torres 1990), shellac coatings (Hagenmeier and Shaw 1991), fruit coating waxes (Hagenmeier and Shaw 1992), amylose starch films (Mark and others 1966), hydroxypropylated starch films (Roth and Mehtretler 1967) and wheat gluten films (Mujica-Paz and Gontard 1997; Gontard and others 1996). Whey protein films have a similar 176 JOURNAL OF FOOD SCIENCE Vol. 68, Nr. 1, Institute of Food Technologists Further reproduction prohibited without permission

2 of O 2 and aroma permeability dependence on RH conditions (McHugh and Krochta 1994a; Miller and Krochta 1998, 1997; Miller and others 1998). There are a few reports of RH effects on coated produce (Elson and others 1985; Meheriuk and Lau 1988). Recently, we modeled the possible effects of RH on coated fruits. We proposed that RH may have a large effect on the internal modified atmospheres of fruits coated with hydrophilic materials. From that study we concluded that factors such as the film (ratio of CO 2 to O 2 permeability) values, film permeability and thickness, and the fruit maximum respiration rate will influence the RH response (Cisneros-Zevallos and Krochta 2002). The objectives of this study were to (1) evaluate WPI as a coating system for fresh produce, and (2) show the influence of RH on the performance of hydrophilic coatings on fruits. Materials and Methods Fruit material Fuji apples used for this study were harvested at commercial maturity stage from the experimental orchard of the Pomology Dept., UC-Davis. All fruits were carefully hand-harvested by the stems, thus avoiding touching the surface, then stored at 2.5 C until use. The average weight was kg, and average respiration was CO 2 cm 3 /kg h. WPI coating solution and coating procedure BiPRO whey protein isolate (WPI) used for coating was obtained from Davisco Foods International (Le Suer, Minn., U.S.A.). A WPI coating solution was prepared by heating a 10% WPI solution at 90 C for 30 min, followed by cooling to room temperature and degassing under vacuum. Glycerol (Gly) was added as a plasticizer at levels of 15 or 40% glycerol based on total solids. Each WPI-Gly aqueous solution was adjusted again to obtain a 10% solid concentration (w/w). Span 20 (sorbitan monolaurate) (Sigma Chemical Co., St. Louis, Mo., U.S.A.) was added at 0.5% of the WPI-Gly solution to aid wetting. Surface tension was determined using a Fisher Surface Tensiomat model 21 tensiometer (Fisher Scientific Co., Pittsburgh, Pa., U.S.A.) based on the DuNouy-ring method, and results are reported at 20 C and expresed as dynes cm 1. Viscosities of WPI solutions were measured using a Rotovisco RV-20 from Haake Mess-Tecknik GmbHu. Co. (Karlsruhe, Germany) and results are reported at room temperature as centipoise. Fuji apples were held in open trays at 20 C and 50% RH for 24 h to equilibrate. Fruits were thoroughly washed without touching the surface by running water (de-ionized water) to eliminate any visible contaminants and then allowed to dry at room temperature. Coating application was done by holding apples by the stem end, brushing until coating solution was running off. After the coating process, apples were allowed to drain for 2 and a half min then dried at 20 C and 50% RH using a fan with air velocity of 180 m min 1. Air velocity and RH were measured with a Solomat hygrometer (Solomat Corp., Stamford, Conn., U.S.A.). Relative humidity studies Controlling RH inside chambers. RH studies were performed by storing coated and uncoated apple fruits inside chambers that were connected to an air flow-through system at room temperature. The chambers had small fans that insured that the air inside the jars was well mixed, giving a uniform RH through out the whole jar equal to the outlet air RH. The RH inside the jars was adjusted using a mass balance including the amount of water in the inflow and outflow air and the amount of water loss from the fruits. The amount of water in the inflow and outflow air was determined by their RHs, while water loss of the fruit was determined by using Fick s law. The moisture mass balance was defined as: amount water in inflow air amount water loss from fruits amount water in outflow air Thus: where r is inflow or outflow air (l/h), P s is saturation water vapor pressure (atm), RH i is relative humidity of inflow air (%), P t is standard pressure (1 atm), n is number of fruits, A is average fruit area (cm 2 ), RH f is relative humidity inside fruit (100%), RH j is relative humidity inside jar and of outflow air (%), and R H is fruit skin resistance to water vapor transmission (s atm/cm). 20 Solving for r: Factor A can be calculated by using the relationship A 581W 0.685, where W is the fruit weight (kg) (Clayton and others 1995). According to Equation 2, a selected RH can be established inside the jar for a given number of fruits. We can determine the required inflow air if we know the RH of the inflow air, the fruit area and the fruit skin resistance to water vapor transmission. WPI barrier properties and storage time. Treatments included coating apples with WPI solutions containing 15% glycerol (WPI 15), 40% glycerol (WPI 40) and noncoated apples used as controls. The WPI 15 and WPI 40 coated fruits were stored at 50 and 90% RH, respectively, reflecting 2 extreme cases: high barrier film at low %RH, and low barrier films at high %RH. Respiration and internal gas measurements were performed daily (for 4 d) using 4 fruits as replicates each time. A total of 72 fruits were used. WPI coated apples stored at different RHs. Control and WPI- 15 coated fruits were exposed to 5 different RHs, ranging from 54 to 92% RH. These apples were stored for 2 d at room temperature. Internal gas measurements were performed using 4 fruits as replicates. A total of 80 fruits were used. Internal modified atmosphere and gas exchange Internal gas composition. Stored noncoated and coated apples were sampled for internal oxygen, carbon dioxide, and ethylene after holding fruits at 20 C inside the controlled RH chambers. Gas sampling was performed with a 5 ml syringe having a side hole needle. Apples were sampled under water to avoid external gas contamination. The needle was introduced through the blossom end until it reached the internal cavity. Once the gas was withdrawn (about 2 ml), the needle was removed and a rubber stopper was used to seal the syringe under water. Afterwards, the syringes were removed from the water. The gas samples contained in the sealed syringes were transferred to 1-mL syringes by introducing the needles through the rubber stopper. The end of the 5-mL syringe was pushed, exerting pressure and thus filling the 1-mL syringe with the gas sample. By using this positive pressure, contamination was avoided. A PIR-2000 infrared CO 2 gas analyzer (IRGA) (Horiba Instruments, Irvine, Calif., U.S.A.) and a Model S-3A electrochemical O 2 analyzer (Applied Electrochemistry, Inc., Sunnyvale, Calif., U.S.A.) were used for gas measurements. These were positioned in tandem to obtain readings with only 1 injection (Saltveit and Strike (1) (2) Vol. 68, Nr. 1, 2003 JOURNAL OF FOOD SCIENCE 177

3 1989). Ethylene concentrations were analyzed using a gas chromatograph with a flame ionization detector. Respiration and ethylene production. The flow air rates necessary to maintain RHs 80% ranged from 10 to 30 L/h. Thus, it was not possible to measure gas exchange rates under those conditions. As an alternative we used a closed system. Every day fruits were taken out from the controlled RH jars and each fruit was transferred to a 0.7-L jar and its gas production measured. Afterwards, the fruits were returned to their respective flow-through chambers. Carbon dioxide and ethylene accumulation in the closed jars was monitored over a period of time and production rate was calculated. It was assumed that short periods inside the closed system (about 70 to 90 min) would not change the coating permeability due to possible RH changes in the closed jar. Coating resistance to gas transmission. Total resistance for O 2 (R O 2 total ) and CO 2 (R CO 2 total ) gas transfer of coated and noncoated fruits was calculated from steady-state transmission rate values, through a rearrangement of Fick s law (Banks and others1993): R total A ( p)/ J W (3) where J is steady-state transmission rate per unit weight or respiration (cm 3 /kg s), A is fruit area (cm 2 ), W is fruit weight (kg), is permeant partial pressure difference (atm), and R total is total resistance to gas transfer (s atm/cm). The mechanism of internal modified atmosphere (MA) assumed in this study is based on the loosely adhering coating (lac) model (Banks and others 1993). In the lac model, the coating covers pores and cuticle as a film wrap. Thus, the total resistance is the addition in series of the fruit and coating resistances (Ben-Yehoshua and Cameron 1988; Hagenmeier and Shaw 1992): R total R fruit R coat (4) where R fruit is fruit skin resistance to gas transfer and R coat is coating resistance to gas transfer. Coating resistance relates to coating thickness (H) and coating permeability (P) as R coat H/P. The ratio of CO 2 to O 2 coating permeability is a constant, defined as coat R O2 coat /R CO2 coat. In general, this approach could also satisfy a tac model in the specific case of complete surface coverage and mainly gas exchange through pores. The MA generated in the fruit will depend on coating permeability and thickness. having an apparent viscosity of about 100cp (10 s 1 shear rate) at room temperature. Thus, our studies with WPI solutions were done without surfactants WPI barrier properties and storage time WPI coated apples and internal modified atmosphere. WPIcoated and noncoated Fuji apples stored 96 h under 50 or 90% RH showed about 0.6 to 1.8% weight loss with an average R total H20 of atm s/cm. The R H 20 total and R H 20 fruit were similar, indicating that R coat H20 was negligible due to the poor water vapor barrier properties of WPI-coatings (Eq. 4). WPI-coated Fuji apples had an initial sharp decrease in p O2i and an increase in p CO2 i with time compared to controls. Steady-state conditions were apparently reached after 2 d of storage. Uncoated fruits had fairly constant p O2i and p CO during the whole period of 2i storage (Figure 1 and 2). WPI-15 coated fruits reached lower p O 2i and higher p CO2i values compared to WPI-40 coated fruits (Figure 1 and 2). This difference may be attributed to the higher barrier properties of the WPI-15 films under the RH conditions studied. According to McHugh and Krochta (1994a), WPI films with lower Gly content will have lower O 2 permeability. WPI-15 coated fruits had p O2i and p CO 2i similar to the p CO2i and p O2i, respectively, of noncoated fruits (Figure 1). The p O2i and p CO2i of WPI-40 coated fruits never reached the p CO 2i and p O2i values of the controls, suggesting the difference in barrier properties of both WPI films (Figure 2). Average calculated R total O2 was not different (p 0.05) between both WPI treatments. However, WPI-15 coated apples had larger R total CO2 values (p 0.05) compared to WPI-40 coated apples, indicating that the former coatings are less permeable to CO 2 (Table 1). The results indicate that WPI-15 and WPI-40 coatings increased the apple O 2 skin resistance by about 4.3 and 3.3 times, respectively, and the apple CO 2 skin resistance by about 4 and 1.8 times, respectively. The R O 2 total /R CO 2 total ratio ( total ) is larger for WPI-40 compared to WPI-15 coated apples, indicating that CO 2 permeation is larger than O 2 permeation for the former films (Table 1). The increase in total is due to an increase in coat. This increase in coat values for Statistics Statview 4.0 was used for statistical analysis (Abacus Concepts, Berkeley, Calif., U.S.A.). Analysis of variance and Fisher PLSD multiple-comparison tests were performed. Results and Discussions WPI film formation on fruit surface WPI in solution is surface active, reducing the surface tension of water from about 72 to 51 dynes cm 1. Surface tension was further reduced to 30 dynes cm 1 by adding surfactant Span 20. Spontaneous wetting can be achieved by reducing the surface tension of the coating solution to a value lower or similar to the solid substrate s surface tension (Zisman 1964). Fuji apples have a surface tension of about 26 dynes cm 1 (Cisneros-Zevallos and Krochta 2003), thus, wetting would increase with surfactant addition. However, we obtained complete wetting when applying the WPI coatings without surfactants. Most likely, viscous forces played an important role in coating film formation on fruits by delaying dewetting. A WPI solution behaves as a pseudo-plastic system, with a 10% WPI solution 178 JOURNAL OF FOOD SCIENCE Vol. 68, Nr. 1, 2003 Figure 1 p O2i and p CO2i for WPI-15 coated and noncoated Fuji apples stored for 4 d at 20 C and 50% RH. Error bars indicate 1-sided standard deviation.

4 Table 1 Total O 2 and CO 2 resistances for WPI coated and noncoated apples stored at 20 C. R total values were calculated at steady-state conditions ($ d 2). Treatment *R total O2 *R total CO2 (atm s/ cm) (atm s/ cm) b total b coat Control a a 0.96 WPI b b WPI b c *Average resistance values with different letters in a column indicate a significant difference at Values of R total were calculated using Equation 3. WPI-40 coatings could be due to the larger amount of glycerol in the film and the higher RH conditions they were exposed to. The use of p CO 2i vs p O2i plots may be useful as tools for coating design (Banks and others 1997; Cisneros-Zevallos and Krochta 2002a). These plots may show the target windows or combinations of internal gas partial pressures appropriate for storage conditions. The plots may also give information about the lower oxygen limit above which fruits can be stored safely, avoiding anaerobic conditions. If we plot p CO 2i vs p O2i graphs for both WPI-coated fruits under steady state conditions ( 2 d), we observe 2 distinct curves (Figure 3). For WPI-15 coated fruits, as p O decreases there is an increase in 2i p CO2i until a certain p O2i is reached, under which there is a sharp increase in p CO 2i due to anaerobic respiration. This p O2i is known as the lower oxygen limit (LOL). WPI-40 coated fruits did not reach this LOL value. These 2 curves will be the result of different WPI film composition reflected by different coat values (Table 1). The difference in behavior between WPI coatings would give the opportunity to tailor films that can avoid the LOL and the detrimental effects of anaerobic conditions when fruits are coated. Source of variation. The combination of p CO 2i and p O2i values obtained for each coated fruit (Figure 3) will depend on R total according to Equation 3. The RHs used were constant and should not affect coating permeability and R coat. Thus, it was expected to have constant R total s and very small variations in the combinations of p CO 2i and p O2i for each coated fruit. However, we observed variation of points along each curve (Figure 3). Most likely variations in film thickness, which affected R coat, and variation in fruit skin resistance (R fruit ) changed R total of coated fruits along both curves. The contribution of coating resistance variation ( R coat ) and fruit skin resistance variation ( R fruit ) to total resistance variation ( R total ) can be quantified analyzing the upper and lower values for gas partial pressures in each curve (from Equation 4): R total R fruit R coat (5) The contribution of R fruit is expressed as R fruit / R total, and the contribution of R coat is R coat / R total. For example, if we use upper and lower p CO values obtained from the curve corresponding to 2i WPI-15 coated fruits (Figure 3) and measured respiration values (J) for the selected fruits, we can determine R total and R fruit (Figure 4). From Figure 4 and Equation 5, we calculated that skin resistance variation ( R fruit ) and thickness variation ( R coat ) contributed by about 38 and 62% of the variation of R total, respectively. Similarly for WPI-40 coated fruits, the contribution of skin resistance and thickness to the variation of R total was about 32 and 68%, respectively. Our results suggest that fruits skin resistance and thickness variations will have large effects in the final internal MA. Most likely, natural skin thickness variations, different fruit maturity stage, fruit size, initial amount of coating load and coating drying rate are some of the factors among others that may affect variations in skin resistance to gas transfer and coating thickness. Production and internal Ethylene. Ethylene efflux for both WPI coated fruits sharply decreased with time compared to uncoated fruits which remained fairly constant (Figure 5). The decrease was larger for the WPI-15, compared to the WPI-40 coated fruits. This may at first suggest a reduction in ethylene production. However, there was an initial buildup of internal ethylene for both WPI-coated fruits that decreased with time, compared to uncoated fruits that remained constant (Figure 6). The buildup rate of internal C 2 H 4 was similar for both WPI-coated fruits. However, the larger increase and later decrease of internal C 2 H 4 for WPI-40 coated fruits suggest that C 2 H 4 were still synthesized inside the fruits due to a combination of higher p O 2i and lower p CO2i. For WPI-15 coated fruits, the lower p O2i Figure 2 p O2i and p CO2i for WPI-40 coated and noncoated Fuji apples stored for 4 d at 20 C and 90% RH. Error bars indicate 1-sided standard deviation. Figure 3 p CO2i vs p O2i plots for WPI-15 and WPI-40 coated and noncoated Fuji apples stored at 20 C and 50 or 90%RH. Dotted arrow indicates LOL value. Vol. 68, Nr. 1, 2003 JOURNAL OF FOOD SCIENCE 179

5 and higher p CO2i may have reduced ethylene synthesis. For both treatments, the increase and later decrease in internal C 2 H 4 would be due to the interaction between C 2 H 4 synthesis and C 2 H 4 diffusion through the film. These results suggest that steady-state conditions were not reached for ethylene exchange. They also indicate that WPI coatings have large C 2 H 4 barrier properties when applied to fruit systems; however, the buildup of internal ethylene can be negative if detrimental physiological levels are reached and remain for long time. WPI coated apples stored at different RHs RH influenced the internal concentration values of p O 2i and p CO2i of WPI-15 coated fruits compared to uncoated fruits (Figure 7). As RH decreased, p O 2i decreased and p CO2i increased, while for noncoated fruits RH had no effect. The R O2 total and R CO2 total most likely decreased with increasing RH, while for noncoated fruits RH did not affect total gas resistance. Our results suggest that RH affects the permeability of the WPI coatings applied to fruits, by interfering with WPI polymer-polymer interactions due to higher moisture concentration within the film at higher RH (McHugh and Krochta 1994a, Mate and Krochta 1996). When plotting p CO2i against p O2i (Figure 8), we observed that all points, that represent individual fruits at different RHs, fall along the same curve. Control fruits at different RHs have higher p O2i and lower p CO 2i, compared to WPI coated fruits. As RH decreased, p O2i Figure 4 Calculated R CO2 total and respiration (J) for WPI-15 coated and noncoated fruits as a function of p O2i at steadystate conditions ($ 2 d). Values of R total and R fruit were obtained using Equation 5. Dotted arrow indicates LOL value. Figure 6 Internal ethylene concentration for WPI-15 and WPI-40 coated and noncoated Fuji apples stored at 20 C and 50 or 90%RH. Error bars indicate 1-sided standard deviation. Figure 5 Ethylene efflux for WPI-15 and WPI-40 coated and noncoated Fuji apples stored at 20 C and 50 or 90%RH. Error bars indicate 1-sided standard deviation. Figure 7 po2i and pco2i for WPI-15 coated and noncoated Fuji apples stored for 2 d at 20 C under different RH s ranging from 54 to 92%RH. Error bars indicate 1- sided standard deviation. 180 JOURNAL OF FOOD SCIENCE Vol. 68, Nr. 1, 2003

6 decreases and p CO2i increased for coated fruits. At a level of p O2i about atm (shown by arrow), there was a sharp increase in p CO2i. This oxygen level indicates a shift to anaerobic metabolism and corresponds to the LOL described before. Thus, anaerobic conditions were induced in WPI-coated fruits stored at about 70 to 80% RH (Figure 8). The coat may change with RH conditions for hydrophilic films (Gontard and others 1996). We did not determine total resistances or coat values. However, most likely for WPI-15 coated fruits coat changed with different RH conditions. In general, our results indicate that WPI coatings modify the internal gas composition of coated fruits following a lac model, where permeability and thickness play an important role. These results do not agree with a tac model for the case of incomplete pore coverage, where the number of blocked pores plays the main role in p O2i depression (Banks and others 1993). Conclusions WHEY PROTEIN ISOLATE (WPI) WAS TESTED AS A COATING SYSTEM FOR fresh fruits at 20 C. Results indicate that WPI coating films are good gas barriers that depend on the RH of the environment, affecting the coating resistance to oxygen and carbon dioxide. As RH decreased, WPI coating resistance to gas transfer increased. At low RH, oxygen decreased and carbon dioxide increased in coated fruits. At about70 to 80% RH, anaerobic metabolism was induced due to low oxygen levels. Results suggest that the mechanism of internal gas modification for coated fruits is based on coating thickness and permeability. The excellent gas barrier properties of WPI coatings indicate the potential for tailoring films with adequate film permeability and film thickness to achieve appropriate internal modified atmospheres. Figure 8 p CO2i vs p O2i plot for WPI-15 coated and noncoated Fuji apples stored for 2 d at 20 C under different RHs ranging from 54 to 92%RH. Dotted arrow indicates LOL value. References Banks N, Dadzie B, Cleland D Reducing gas exchange of fruits with surface coatings. Postharv Biol Technol 3: Banks N Cutting J, Nicholson S Approaches to optimizing surface coatings for fruits New Zealand J Crop Hort Sci 25: Ben-Yohoshua S, Cameron A Exchange determination of water vapor carbon dioxide, oxygen, ethylene and other gases of fruits and vegetables. In: Linskens HF, Jackson JF, editors. Gases in plant and microbial cells. Mod Meth Plant Anal New ser 9: Buera P, Karel M Application of the WLF equation to describe the combined effects of moisture and temperature on nonenzymatic browning rates in food systems. J Food Proc Preserv 17: Cisneros-Zevallos L, Krochta J Internal modified atmospheres of coated fresh fruits and vegetables: understanding relative humidity effects. J Food Sci 67: Cisneros-Zevallos L, Krochta J Estimation of surface tension, roughness and wettability of fruits and vegetables from contact angle measurements. J Food Sci 67: Clayton M, Amos N, Banks N Estimation of apple fruit surface area. N.Z.J. Crop Hort Sci 23:345-9 Elson C, Hayes E, Lidster P Development of the differentially permeable fruit coating Nutri-SaveR for the modified atmosphere storage of fruit In: Blankenship SM, editor. Proceedings of the 4th National Controlled Atmosphere Research Conference. Dept. Horticultural Science, N.C.S. Univ., Raleigh, N.C. Horticultural Report 126: Gontard N, Thibault R, Cuq B, Guilbert S Influence of relative humidity and film composition on oxygen and carbon dioxide permeability of edible films. J Agric Food Chem 44: Hagenmeir R, Shaw P Permeability of shellac coatings to gases and water vapor. J Agric Food Chem 39: Hagenmeir R, Shaw P Gas permeability of fruit coating waxes. J Amer Soc Hort Sci 117: Mark A, Roth W, Mehltretter C, Rist C Oxygen Permeability of Amylomaize starch films. Food Technol 1(Jan.):75-7. Mate J, Krochta J Comparison of oxygen and water vapor permeabilities of whey protein isolate and b-lactoglobulin edible films. J Agric Food Chem 44: McHugh T, Avena-Bustillos R, Krochta J Hydrophilic edible films: modified procedure for water vapor permeability and explanation of thickness effects. J Food Sci 58: McHugh T, Aujard J, Krochta J Plasticized whey protein edible films: water vapor permeability properties. J Food Sci 59:416-9,423. McHugh T, Krochta J. 1994a. Sorbitol-vs glycerol-plasticized whey protein edible films: integrated oxygen permeability and tensile property evaluation. J Agric Food Chem 42: McHugh T, Krochta J. 1994b. Milk-protein based edible films and coatings. Food Technology 48: Meheriuk M, Lau O Effect of 2 polymeric coatings on fruit quality of Bartlett and d Anjou pears. J Amer Soc Hort Sci 113: Miller S, Krochta J Oxygen and aroma barrier properties of edible films: a review. Trends Food Sci Technol 8: Miller S, Krochta J Measuring aroma transport in polymer films. Transact ASAE 41: Miller S, Upadhyaya S, Krochta J Permeability of d-limonine in whey protein films. J Food Sci 63: Mujica-Paz H, Gontard N Oxygen and carbon dioxide permeability of wheat gluten film: Effect of relative humidity and temperature. J Agric Food Chem 45: Park H, Chinnan M, Shewfelt R. 1994a. Edible coating effects on storage life and quality of tomatoes. J Food Sci 59: Park H, Bunn J, Vergano P, Testin R. 1994b. Gas permeation and thickness of the sucrose polyester Semperfresh coatings on apples. J Food Proc Preserv18: Rico-Pena D, Torres A Oxygen transmission rate of an edible methyl cellulose-palmitic acid film. J Food Proc Eng 13: Roth W, Mehltretter C Some properties of hydroxypropylated amylomaize starch films. Food Technol 21: Roos Y, Karel M Applying state diagrams to food processing and development. Food Technol 45:66-71,107 Saltveit M, Strike T A rapid method for accurately measuring oxygen concentration in milliliter gas samples. Hort Sci 24: Zisman WA Relation of the equilibrium contact angle to liquid and solid constitution. In: Contact angle wettability and adhesion. Adv Chem ser 43:1-51. MS Submitted 8/14/01, Revised 10/29/02, Accepted 9/4/02, Received 10/10/02 We wish to acknowledge with gratitude funding through the California Dairy Research Foundation and Dairy Management, Inc. Author Cisneros-Zevallos is with the Dept. of Horticultural Sciences, Texas A&M Univ., College Station, TX Author Krochta is with the Dept. of Food Science and Technology, Univ. of California, Davis, CA Direct inquiries to author Cisneros-Zevallos ( lcisnero@tamu.edu). Vol. 68, Nr. 1, 2003 JOURNAL OF FOOD SCIENCE 181