Vol. 68, Nr. 2, 2003—JOURNAL OF FOOD SCIENCE 503 © 2003 Institute of Food Technologists Further reproduction prohibited without permission Food Engineering and Physical Properties Dependence of Coating Thickness on Viscosity of Coating Solution Applied to Fruits and Vegetables by Dipping Method L. CISNEROS-ZEVALLOS AND J.M. KROCHTA ABSTRACT: Hydroxypropyl methylcellulose solutions were used as coating systems in this study and solution concen- trations, viscosity, densities, and surface tensions were characterized. Fuji apples were coated by dipping and stored 4 d at room temperature, after which the internal oxygen and carbon dioxide were measured. Results indicated that coating thickness varied with viscosity, concentration, density, and draining time of the biopolymer solution. Coating thickness relates to the square root of viscosity and the inverse square root of draining time, which agrees with the theoretical approach for flat plate dip-coating in low-capillary-number Newtonian liquids. These results indicate the possibility of controlling coating thickness and internal gas composition based on coating solution properties. Keywords: coating thickness, viscosity, fruits, vegetables, modified atmosphere Introduction T HE USE OF BIOPOLYMER COATINGS TO MODIFY INTERNAL ATMO- spheres of fruits and vegetables has been studied extensively during the past 20 y. However, research in this area has been mostly empirical, providing unpredictable results and diverse conclusions. Internal atmosphere modification seems to depend on elements such as coating permeability, thickness and surface coverage by the film. However, little is known of the factors that may influence these elements. Thus, we believe that knowledge of the physical–chemical properties of the coating solutions and coating interactions with the surface of different commodities will allow a more predictable coat- ing performance (Cisneros-Zevallos and others 1994a). Film thickness is considered to influence coating response by defining the distance through which the gas permeant has to dif- fuse (Ben-Yehoshua 1967; Meheriuk and Lau 1988; Hagenmeier and Shaw 1992; Hagenmeier and Baker 1994; Park and others 1994a). The internal gas modification in coated fruits has been in- directly related to coating thickness through coating gas resistance parameters (Trout and others 1953; Banks and others 1993; Hagen- meier and Baker 1993), and through the solid concentration of coat- ing solutions (Park and others 1994a, 1994b). Studies on plate dip-coating have shown that the thickness of deposited liquid film coatings depends on the coating solu- tion properties such as density, viscosity and surface tension, as well as surface withdrawal speed from the coating solution. Different theoretical approaches have been used to estimate the film thickness from these properties (Levich 1962; Groenveld 1970; Scriven 1988; Derjaguin 1993). It has been reported that thickness, h, relates to withdrawal speed, U, by h ~ U 1/2 ; and under certain conditions h ~ U 2/3 . For viscosity, , the relation is h~  1/2 or h~  2/3 . The following equations have been proposed to describe the thickness of coatings: h = 0.944Ca 1/6 (U/g) 1/2 (1) and h = K(U/g) 1/2 (2) Eq. 1, proposed by Landau and Levich (1942), is used in the case where thickness is defined by a 3-sided force competition between the viscosity, surface tension and gravity. Eq. 1 holds for low capil- lary numbers (Ca < 10 –3 ). Eq. 2 defines thickness as a competition between viscous and gravity forces, with surface tension forces being overshadowed by the first 2. Eq. 2 was derived by solving the flow equation for a liquid film of constant thickness, in the case of Newtonian fluids where the capillary number is <10 -3 . However, it has also been used for high capillary numbers (Ca > 1). In both cas- es, the proportionality constant K can be experimentally deter- mined (Groenveld 1970; Scriven 1988; Guglielmi and Zenezini 1990; Guglielmi and others 1992). The same approach can be applied to problems of liquid coat- ing plate withdrawal and drainage. A plate that is pulled out of the bath in a very short time and then left to drain is considered a drain- ing plate. The transformation of withdrawal speed (U) to draining time is given as (Groenveld 1970; Toledo 1991): U = z/t (3) The objective of our study was to apply the physical principles of the dip coating process to apple fruits and to define the relation- ship between the coating thickness, the properties of coating solu- tion and the internal gas modification of fruits. Material and Methods Fruit material Fuji apples used in this study were harvested at commercial maturity stage from the experimental orchard of the Pomology Dept., Univ. California, Davis (Davis, Calif., U.S.A.). All fruits were carefully hand harvested leaving stems attached to the fruit and avoiding touching the surface, then stored at 2.5 °C until use. Coating solutions Hydroxypropyl methylcellulose (HPMC) (Methocel, Dow Chem- ical Co., Midland, Mich., U.S.A.) aqueous solutions were used for JFS: Food Engineering and Physical Properties