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Diffusion and Modeling of Natamycin and Potassium Sorbate in Whey Protein Isolate Films for Application to Cheddar Cheese

By, 
L.R. FRANSSEN, T.R. Rumsey and J.M. Krochta
Department of Food Science and Technology
 University of California, Davis, CA 95616

 As surface-applied preservatives migrate into cheese, mold spoilage occurs on the cheese surface. By incorporating preservatives in whey protein isolate (WPI) films, the preservative migration can be slowed.  Thus, WPI films formed as coatings on cheese surfaces can be used for microbial protection of cheese. By determining the preservative diffusion in films compared to cheese, a model can be developed to predict cheese shelf-life.

 Our objective was to determine the diffusion coefficients of natamycin and potassium sorbate in WPI films and in cheddar cheese and to develop a mathematical model of a preservative-containing WPI coated cheese.

 WPI films with WPI:glycerol (1:1- 15:1) and 0.3% natamycin or 1.6% potassium sorbate were cast. Preservative diffusion in the films was measured by immersing the films in a stirred 20% glycerol solution (Aw= 0.94) at 23C and measuring preservative concentration as a function of time in the solution using UV spectroscopy. Diffusion coefficients were calculated using Fick’s second law as applied to a plane sheet. Potassium sorbate diffusion in cheddar cheese was determined by immersing cheese in a potasssium-sorbate-containing solution, as well as coating cheese with potassium-sorbate-containing WPI film coating.  Preservative concentration in the cheese was determined by slicing the cheese and extracting the preservative in each slice using AOAC method 974.10 for sorbic acid in dairy products. The preservative diffusion coefficient in cheddar cheese was then determined from Fick's Law as applied to a semi-infinite sheet.

 Natamycin diffusion coefficients in WPI films were lower than potassium sorbate diffusion coefficients. The different diffusion coefficients are most likely due to large differences in the preservative structures. Film glycerol concentration had a smaller effect on natamycin diffusion than potassium sorbate. Diffusion coefficients of potassium sorbate in WPI films were significantly slower than that in cheddar cheese, indicating the potential of WPI coatings on cheese from extending shelf-life.

 Using the data from this study, a mathematical model of a coated cheese can be developed to predict shelf-life and, thus, help design the most effective preservative-containing WPI coating.

Potassium sorbate diffusion in WPI films exhibited Fickian behavior. The diffusion coefficients in WPI films were found to be larger than those of corn zein and wheat gluten films reported in the literature. The diffusion coefficients in WPI films increased with increasing BW content, opposite to the effect of lipid in corn zein and wheat gluten film.

The results indicate that WPI films without lipid have slower potassium sorbate diffusion and therefore would be better antimicrobial films because of their ability to retard potassium sorbate migration. WPI films, which are hydrophilic, showed increased diffusion coefficients upon addition of BW. This indicates that the permeability of potassium sorbate through the more hydrophobic and non-polymeric beeswax is greater than diffusivity through the plasticized whey protein film matrix.

For more information on Dr. Krochta’s work and the Food Science and Technology Department at UC Davis, please visit Dr. Krochta's Web site - http://foodscience.ucdavis.edu/fst/faculty/KrochtaWeb/index.htm.

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