Edible coatings controlling preservative migration from surface to food bulk could inhibit surface microbial growth which is often the main cause of spoilage for many food products. In this project we focused our attention upon methylcellulose, hydroxypropyl methylcellulose, and chitosan as the structural component for such edible films. These films were generally transparent and effective at thicknesses in the order of 20 to 100 μm. We expect them to have little impact on the sensory properties of a food. Permeability cell measurements were used to evaluate the effect of coating composition. Further film characterization included film thickness and electron microscopy studies. To gain an understanding of the permeation process, the permeability tests were done at 5, 24, 32, and 40°C. Among these polysaccharide films, methylcellulose was the most promising diffusion barrier with a permeability constant of 3.4 and 1.4xl0⁻⁸ (mg/sec cm²)(cm)/(mg/ml) at 24 and 5°C, respectively. These barrier properties were enhanced by the incorporation of lipids into the film formulation. The permeability of sorbates in methylcellulose and hydroxypropyl methylcellulose emulsified with lauric, palmitic, stearic and arachidic acid was found to depend upon the polysaccharide, the fatty acid chain length, and the number of fatty acid double bonds. Potassium sorbate permeation increased in the following order lauric>palmitic>stearic>arachidic acid. The effect of the double bond type, i.e. cis vs. trans was also determined. The permeability rate of potassium sorbate increased in the order of oleic>elaidic>stearic acid. The effect of temperature on potassium sorbate permeability was analyzed using an Arrhenius activation energy model for the permeation process. Permeability determinations at four different temperatures showed excellent agreement with this model and suggest that the permeation process is diffusion controlled. Electron microscopy studies showed the absence of pores, channels or other defects which might be introduced during casting, drying, handling or permeability determination. This observation is consistent with our hypothesis that potassium sorbate permeation is diffusion controlled. Furthermore, our experimental data suggest that the diffusion is controlled by the properties of the solvent embedded in the film. Further studies are required to confirm this hypothesis. The effect of casting technique was examined by coating a pure polysaccharide film with a fatty acid mixture or bees wax and by laminating a fatty acid mixture or hydrogenated palm oil between two layers of pure polysaccharide films. Unfortunately, most of these films cracked easily and could not be tested in our permeability cell. On the other hand, hydroxypropyl methylcellulose films coated with bees wax showed exceedingly low potassium sorbate permeability values. These modifications of the polysaccharide film properties reduced the potassium permeability down to 10⁻⁹ to 10⁻¹¹ (mg/sec cm²)(cm)/(mg/ml) depending upon temperature, film composition and film casting technique. A simplified procedure previously published was used to evaluate surface microbial stability enhancement. With this information a food processor can select the appropriate film, application procedure and film thickness to achieve the desired shelf life under ambient or refrigerated storage conditions. / Graduation date: 1988

The use of edible coatings in combination with antimicrobial agents enhances the microbial stability of foods. In this study we evaluated the potassium sorbate and sorbic acid permeability of an edible methylcellulose (MC) - palmitic acid (PA) film with a MC:PA ratio of 3:1. Permeability cell measurements were used to evaluate the effect of pH and water activity (a [subscript w]) on the film permeation rate by sorbic acid and potassium sorbate. For films with a thickness of 55-66 pm, potassium sorbate permeability increased from 2.3x10⁻¹⁰ to 2.0x10⁻⁸ (mg/sec cm²)/(cm)/(mg/mL) as a [subscript w] increased from 0.65 to 0.80. Films were not stable at a [subscript w] levels above 0.80. The permeability of the film to sorbic acid decreased from 3.3x10⁻⁸ to 9.1x10⁻¹⁰ (mg/sec cm²) (cm)/(mg/mL) when pH in the permeability cell was increased from 3 to 7. This permeability decrease with pH could balance the loss of effectiveness of sorbates due to the lowering of the percentage of undissociated molecules at high pH. Another characterization of the MC-PA film was the evaluation of the effect of relative humidity (RH) on its oxygen transmission rate (OTR). In general, the film was highly permeable to oxygen (OTR values at 24°C, 660 to 1400 mL O₂ (STP)/m² 24h atm, average film thickness of 55 μm). Therefore, there is no risk to develop anaerobic conditions on food surfaces coated with the MC-PA film. There was no relative humidity effect on OTR in the 0-60% RH range; whereas, the OTR doubled when the RH was increased from 60 to 80%. These results agree well with the moisture uptake of the MC-PA film. Its moisture sorption isotherm shows a large moisture content increase at RH levels larger than 60%. The application of methylcellulose-based films as moisture permeability barriers in simulated sundae ice cream cones showed that a MC-PA film practically stopped moisture transfer from the ice cream to the sugar cone. Sugar cone crispness was retained for a period longer than three months when stored coated at -10°F and +10°F. The crispness of commercial sundae ice cream cones is retained for periods much less than three months. / Graduation date: 1990