Progress 04/01/99 to 03/31/04
Outputs
Threshold leak sizes and leak rates were calculated for a number of liquid food products exhibiting a wide range of surface tension and viscosity values. From this data, it can be seen that mathematically, under typical pressure differentials generated in food packages (34.5 kPa), a leak will not start through a 2um defect. The calculated leak rates were compared to calculated evaporation rates of pure water. The evaporation rate exceeds the leak rate at lower sized microholes (2 and 5 um diameter) under typical pressure differentials found in food packages. If the liquid, typically aqueous in food products, is evaporating off faster than the leak itself, then there will be solids left behind that could effectively plug the leak. The critical leak size is the size micro-defect that allows microbial penetration into the package. The critical leak size of air-filled defects was found to be 7 um at all pressures tested. This size is considerably important to food
packagers because this is when sterility of the package is lost. Previous leak studies have shown that the critical leak size for liquid-filled defects coincide with the threshold leak size and pressure. In this study, air-filled defects were examined. A bioaerosol exposure chamber was used to test micro-defects, nickel microtubes of known diameters 2, 5, 7, 10, 20, and 50 um hydraulic diameters, against pressure differentials of 0, -6.9, -13.8, and -34.5 kPa. Using the threshold leak pressure equation, the threshold pressures for various products were calculated for each microtube size. Threshold leak size at a pressure greater than 6.9 kPa, (or less than minus 6.9 kPa) is approximately 7 um for liquids of low surface tension (dyes, oil). The threshold leak size increases to 10 um for mid-range surface tension products (dairy, beer, orange juice), then increases to a value between 10 um and 20 um for high surface tension products (juices, water). The threshold leak pressures were
calculated from density and surface tension measurements of a wide range of liquid food products and standard dyes used in dye penetration testing of package integrity. The threshold leak pressure equation can only be applied to liquid food products that have been filtered, as particulate matter in the liquid will clog even a 50 um defect. Considering typical pressures generated within a food package (34.5 kPa), a defect with a 2 um hydraulic diameter will never reach the threshold pressure for most filtered liquid food products. Even if the threshold pressure were breached, the leak rate would be slower than the evaporation rate, resulting in no net leakage. Also, since most filtered liquid food products are not pure water, if the evaporation rate is faster than the leak rate, then soluble solids may also be left behind, thus effectively resealing the leak. The critical leak size of an air-filled defect was experimentally determined, by bioaerosol exposure, to be approximately 7 um
for pressure differentials ranging from equilibrium (0 kPa) to -34.5 kPa. This is slightly different than the previous results of a liquid-filled defect, in which the critical leak size coincided with the threshold leak size.
Impacts
Package integrity testing has been the focus of significant interest to the flexible and semi-rigid packaging industry. It would be ideal for the industry to reach the same levels of sterility maintenance assurance that has been reached for cans. More companies are moving towards a 100 percent on-line nondestructive leak test that assures all containers leaving the facility to be sterile. Leak testing in this manner ultimately decreases the overall costs by reducing the number of reworked or recalled products. In order for an on-line leak tester to be efficient, the minimum leak size that is critical to microbial ingress must be scientifically established. The critical leak size is the size micro-defect that allows microbial penetration into the package. The critical leak size of air-filled defects was found to be 7 microns at all pressures tested. This size is considerably important to food packagers because this is when sterility of the package is lost.
Publications
- Suloff, E.C., J.E. Marcy, D.R. Brooks and B.J. Love. 2003. Preparation of polymeric stationary phase for use in inverse gas chromatography. Polymer Plastics Technology and Engineering. 42 (5) 957-967.
- Suloff, E.C., J.E. Marcy, B.A. Blakistone, S.E. Duncan, T.E. Long and S.F. O'Keefe. 2003. Sorption behavior of selected aldehyde-scavenging agents in poly(ethylene terephthalate) blends. J. Food Sci. 68 (6) 2028-2033.
- Marcy, J.E. 2004. Packaging R&D advances to enhance the quality of aseptic shelf-stable dairy beverages. Aseptipack 2004. Proceedings International Conference on Aseptic Processing and Packaging, Atlanta, GA. March 17, 2004.
- Marcy, J.E. 2003. Potential food packaging technologies for a space mission. An advanced food system for long duration space flight symposium. Annual Meeting of Institute of Food Technologists. Chicago, IL, July 13, 2003.
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Progress 10/01/02 to 09/30/03
Outputs
The mechanism by which loss of package sterility occurs has eluded researchers for many years. Much emphasis has historically been placed on contaminated cooling water for loss of sterility in metal cans, as well as leakers in the double seam, rough handling after processing during distribution, and high relative humidity during storage. As a by-product of such emphasis, package integrity tests such as immersion biochallenging were developed to simulate the conditions that metal cans were expected to tolerate during their life, through manufacturing to consumer use. Although the mechanism of microbial ingress into the package leading to post-process spoilage was not previously understood, precautions were implemented to reduce product loss as a result of loss of package sterility. For example, handling of hot metal cans and/or wet metal cans is discouraged until dry due to correlations made between poor package handling practices and spoilage . Documentation of such
relationships can be found in decades of research throughout the scientific literature. Recently, quantitative links were established between the presence of liquid product on the outside of a package as a result of traversing through a defect, and loss of package sterility. Keller et al. confirmed that liquid spoilage results when a liquid pathway is established through a defect. Investigation of contaminated packages consistently showed that microorganisms were able to penetrate packages by traversing through a defect within the package. Microorganisms are thought to penetrate packages via defects by exercising motility, and/or by transport via a liquid passing through the defect as a result of pressure differentials. The purpose of this study was to determine the significance of microorganism characteristics, such as size and motility, on loss of package sterility. The effects of microorganism size and motility, along with the imposed pressure required to initiate liquid flow on
the leak size critical to the sterility of a package were measured. Pseudomonas fragi Lacy-1052, Bacillus atrophaeus ATCC 49337, and Enterobacter aerogenes ATCC 29007 were employed to indicate loss of package sterility. One hundred twenty-six microtubes with interior diameters (I.D.s) of 5, 10, and 20 microns and 7 mm in length were used as the manufactured defects. Forty-two solid microtubes were used as a control. No significant differences were found between test organisms with respect to size and motility for loss of sterility as a result of microbial ingress into test cells with microtube IDs of 5, 10, and 20 microns (p more than 0.05). Interactions between the initiation of liquid flow as a result of applied threshold imposed pressures, and the sterility loss of test cells were significant (p less than 0.05).
Impacts
This research documents that bacterial motility and size are not significant factors in leaker spoilage. The presence of food or water in the leak is the controlling factor in migration of bacteria through package discontinuities. This information is needed to control the amount of spoiled food and to devise strategies for control of leaker spoilage.
Publications
- Keller, S.W., J.E. Marcy, C.R. Hackney, B.A. Blakistone, G. Lacy and H.W. Carter. 2003. Effect of microorganism characteristics on lead size critical to predicting package sterility. J. Food Prot. 66 (9): 1716-1719.
- Keller, S.W., J.E. Marcy, C.R. Hackney, B.A. Blakistone, G. Lacy and H.W. Carter. 2003. Application of fluid modeling to determine the threshold lead size for liquid foods. J. Food Prot 66 (7): 1260-1268.
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Progress 10/01/01 to 09/30/02
Outputs
Threshold leak sizes and leak rates were calculated for a number of liquid food products exhibiting a wide range of surface tension and viscosity values. From this data, one can see that mathematically, under typical pressure differentials generated in food packages (less than plus or minus 34.5 kPa), a leak will never start through a 2 micron defect. The calculated leak rates were compared to calculated evaporation rates. The evaporation rate exceeds the leak rate at lower sized microholes (2, and 5 micron diameter) under typical pressure differentials found in food packages. If the liquid, typically aqueous in food products, is evaporating off faster than the leak itself, then there will be solids left behind that could effectively plug the leak. The critical leak size is the size micro-defect that allows microbial penetration into the package. The critical leak size of air-filled defects was found to be 7 micron at all pressures tested. This size is considerably
important to food packagers because this is when sterility of the package is lost. Previous leak studies have shown that the critical leak size for liquid-filled defects coincide with the threshold leak size and pressure. If this is in fact true, then air-filled defects should exhibit a larger critical leak size than the liquid-filled defects. In this study, air-filled defects were examined. A bioaerosol exposure chamber was used to test micro-defects, nickel microtubes of known diameters 2, 5, 7, 10, 20, and 50 mm hydraulic diameters, against pressure differentials of 0, minus 6.9, minus 13.8, and minus 34.5 kPa.
Impacts
Testing of foods with a wide variety of surface tensions resulted in establishing 2 microns as the absolute threshold leak size for liquid foods. However, evaporation will almost certainly exceed flow in this threshold size and result in a plugged defect. Air filled leaks were less likely to cause loss of hermetic seals.
Publications
- Gibney, M. J. 2001. Predicting Package Defects: Quantification of Critical Leak Size. M.S. Thesis, Virginia Polytechnic Institute and State University.
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Progress 10/01/00 to 09/30/01
Outputs
Liquid surface tension, viscosity, and density were obtained for sixteen liquids. The imposed pressures (Po) required to initiate flow through microtubes of IDs 0, 2, 5, 7, 10, 20 or 50 micron, were measured using 63 test cells filled with safranin red dye, tryptic soy broth, and distilled water with surface tensions, of 18.69 mN/m, 44.09 mN/m, and 64.67 mN/m, respectively. Significant differences were found between observed threshold pressures for safranin red dye, tryptic soy broth, and distilled water (p less than 0.05). Liquids with small surface tensions, such as safranin red dye, required significantly lower threshold imposed pressures than liquids with large surface tensions, such as distilled water (p less than 0.05). An equation was developed to quantify the relationship between liquid surface tension, threshold imposed pressure, and defect size. Observed threshold pressures were not significantly different (p greater than 0.05) than those predicted by the
equation. Imposed pressures and vacuums generated within packages during random vibration and sweep resonance tests were measured for aseptic packages brick-style (250 ml), metal cans size 76.2-mm x 114.3-mm (425 ml), quart gable top packages (946 ml), one-half gallon gable top packages (1.89 L) and one-gallon milk jugs (4.25 L). Significant differences were found between packages for observed generated pressures during vibration testing (p less than 0.05). An equation to calculate the threshold size based on liquid surface tension and imposed pressure was established.
Impacts
Current research indicates that surface tension of the food product greatly affects threshold leak size and loss of hermetic seal. An equation has been developed to predict threshold leak size for various food products and test solutions.
Publications
- Marcy, J.E.; Blakistone, B.A. and Keller, S. 2001. Application of fluid modeling to determine the threshold leak. Abstract IFT Annual Meeting, 81-1.
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Progress 10/01/99 to 09/29/00
Outputs
Assuring sterility maintenance continues to be a prominent concern for producers of aseptically packaged products. Such producers have aggressively embraced new technologies to manufacture flexible and semi-rigid packaging. Although many technologies have been developed to maintain package sterility, a problem that remains unresolved is the identification of the critical leak size which indicates container sterility is jeopardized. Onset of liquid flow through defect as a result of imposed pressures or vacuum was linked to the sterility loss of a package. 567 test cells with microtubes of 0, 2, 5, 7, 10, 20 or 50 microns were manufactured to simulate packages with defects. They were biochallenged via an aerosol concentration of 1,000,000 cells per cubic cm of Pseudomonas fragi Lacy-1052, under conditions of imposed pressure of vacuum of 20.7, 13.8, 6.9, 0, -6.9, -13.8, -20.7 kPa, respectively and temperatures of 4, 25, and 37 C. A statistically significant
relationship between loss of sterility due to microbial ingress in test cells and the initiation of liquid flow were found (p less than 0.05). Microbial ingress was not found in test cells with microtube internal diameters of 2 microns under any conditions. Leak sizes critical to sterility maintenance were based on the relationship between liquid surface tension and imposed pressure. Threshold leak sizes where the onset of liquid flow was initiated, and critical leak sizes at which loss of sterility occured, were not significantly different (p more than 0.05).
Impacts
Food processors currently do not have the ability to leak test flexible and semi-rigid containers of shelf-stable foods. Unlike rigid metal and glass containers, these containers do not hold a vacuum; therefore traditional methods are inadequate. This research has identified parameters which affect the minimum leak size and identifies methods to predict when a container will lose it's hermetic seal. This information is needed to develop new leak testing methods.
Publications
- Berends, C.L. and J.E. Marcy. 2000. Stability of aseptically packaged food as a function of oxidation initiated by a polymer contact surface. Packaging Technology and Science. (Accepted for Publication)
- Berends, C.L. and J.E. Marcy. 2000. Low density polyethylene initiated oxidation of d-limonene. Packing Technology and Science. (Accepted for Publication)
- Gibney, M. 2000. Predicting critical leak size: Quantification of threshold leaks. M.S. Thesis, Virginia Tech.
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Progress 10/01/98 to 09/30/99
Outputs
A mathematical model which describes fluid flow in microleaks is being used verified with actual microleak data. Microtubes in 1,2,5,10, 20, 50, and 100 microns have been obtained through a cooperative research agreement with the Department of Defense. These microtubes are being used to verify the Hagen-Poiseuille equation for fluid flow in microleaks. As a result of the fluid flow work, we are in now testing a separate model developed in our lab to describe the initiation of fluid flow in microtubes. It is suggested that the Hagen-Poiseuille equation is not valid for very low flow rates, therefore an equation is needed to describe the lower limit of the Hagen-Poiseuille equation. We are investigating the effect of surface tension on the initiation of fluid flow in microleaks. Previous research has shown that presence of fluid in a microleak is needed for bacterial contamination. Current methodology for leak detection involves using dyes of very low surface tension
for a dye penetration test. These dyes are being tested to determine the relationship between the ability of a dye to predict a leak and an actual food going through the same size leak.
Impacts
This project is only six months completed, but we are gathering data to indicate the effectiveness of current leak detection methods for aseptic packaging. Since aseptic containers are non-rigid containers they can not be vacuum tested in the traditional manner. Non-rigid food containers are becoming more common in the US so there is a need to understand their ability to provide a secure seal and a safe product.
Publications
- Keller, S., Marcy, J., and Blakistone, J. 1998. Determination of Leak Size Cricial to Package Serility Maintenance. Chapter 6, pp. 74-95. Food Processors Institute, Washington, DC.
- Marcy, J.E. 1998. Determination of Threshold Leaks in Food Packaging. Abstract. Emerging Technology Symposium, National Meeting of NFPA. Chicago, IL.
- Marcy, J.E. 1998. Advances in Package Testing Methods for Aseptic Packages. Abstract. Aseptic Workshop, North Carolina State University. Raleigh, NC.
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