Biopolymer coatings and fruit surfaces: Physicochemical interactions and coating performance

Different factors which may influence internal modified atmospheres of biopolymer-coated fresh fruits and vegetables were investigated. These factors were divided into three groups: intrinsic fruit characteristics, coating thickness and coating permeability.; Knowledge of fruit and vegetable surface tension values and roughness effects can provide reference targets for coating treatment formulations when complete wetting is necessary. Different techniques were used based on contact angle measurements. Results indicated that surface tensions depended on type of fruit, cultivar and possibly maturity stage. Calculated surface tensions were comprised mainly of dispersive (non-polar) components. Surface tensions in most cases were lower than 30 dynes cm{dollar}sp{lcub}-1{rcub}.{dollar}; Studies of different properties of coating solutions were conducted to develop a better understanding of the relationship between these factors and coating thickness. Fuji apples were coated with hydroxypropyl methylcellulose (HPMC) by dipping. Results indicated that coating thickness varied with viscosity, concentration, density and draining time of the HPMC solution. Results agreed with the theoretical approach for flat-plate dip-coating in low-capillary-number Newtonian liquids.; A steady-state mathematical model was used as a tool to investigate the effects of relative humidity (RH) on coated fruits, since RH affects the permeability of biopolymer films. As RH decreases, internal oxygen decreases and internal carbon dioxide increases. This effect becomes more pronounced as film thickness increases and for fruits with higher respiration rates. At a certain RH, anaerobic conditions may be generated inside the fruit and expressed as a large increase in internal carbon dioxide and respiration.; Whey protein isolate (WPI) edible films have proven to be excellent gas barriers WPI-coated apples showed that coating performance was a function of plasticizer level and the RH of the environment. WPI coatings lowered internal oxygen and increased carbon dioxide with decreasing RH conditions. Control fruits were not affected by RH. At a critical RH, anaerobic respiration was induced in coated fruits due to low internal oxygen concentration, produced by low oxygen permeability of the coating. Controlling coating thickness and film permeability will allow attainment of the appropriate oxygen and carbon dioxide levels for the fruit, avoiding anaerobic metabolism.