Fruits and vegetables continue maturation and senescence after harvest. At this stage, they are deprived of their normal source of water, mineral and other nutrients, which would normally be available to them from other parts of the plant, and are dependent upon their reserves for water and other minerals. The rate at which the produce loses water or other respirable minerals determines its shelf life. In other words, the higher the respiration and transpiration rates the lower will be the shelf life of the produce. Hence, if the respiration and transpiration of the produce can be decreased, the shelf life of the produce can be increased.
Controlled atmosphere packaging (CAP) and modified atmosphere packaging (MAP) are packaging systems that increase the shelf life of most produce by reducing the respiration of the produce. CAP is an intentional modification of the internal gaseous atmosphere of packaging and maintenance of that atmosphere at a specified condition throughout the cycle, regardless of the temperature or other environmental variations. The operating costs for CAP are very high, and it is suitable for fruits and vegetables that have a long shelf life. In MAP, only the initial internal conditions of the package are established. The atmosphere inside the package is altered due to respiration by the produce and permeation of gases and vapors through the plastic film. Thus, in designing MAP for produce, the plastic film is selected to match the respiration rate of the produce at the anticipated temperature of storage. The produce consumes oxygen during respiration and if this is not replaced by permeation through the packaging material, the atmosphere inside the package will become anaerobic, and the produce will respire anaerobically. When the fruit respires anaerobically, the glycolytic pathway replaces the Krebs cycle as the main source of energy needed by the plant tissues. Pyruvic acid is decarboxylated to form ethanol, which results in the development of off-flavors and tissue breakdown. Thus, high permeability to oxygen is essential for maintaining the minimum level of oxygen inside the package to prevent the produce from respiring anaerobically.
MAP of fruits and vegetables in plastic films is well suited for products like apples, peaches, tomatoes, etc., i.e. products that have low to medium respiration rates, but is not suitable for products like mushrooms, broccoli, leeks, etc., i.e. products that have very high rates of respiration. Even the most permeable films currently available for packaging have oxygen permeability of only 18,000-25,000 cc mxe2x88x922 dayxe2x88x921 at 23-25xc2x0 C., which are still insufficiently permeable, and result in over modification of the pack atmosphere.
In the past few years, many manufacturers have developed micro-perforated films. The diameter of the micro-perforations range from 40 to 200 microns. Gas transport through a micro-perforated film is the sum of the gas transport through the polymeric film and the micro-perforation. The gas transport through the plastic film is generally negligible, however, compared to the gas transport through the micro-perforations. These films offer very high transmission of oxygen due to the gas exchange through the micro-perforations. By altering the size and the thickness of the perforations, different permeabilities can be obtained. Micro-perforated films show a tremendous potential for extending the shelf life of high-respiring produce. Many researchers have used these films with success to extend the shelf life of various products. They have also developed mathematical models to predict gas exchange through the micro-perforations, but to date experimental methods to determine permeation rates have not been developed.
The American Society for Testing Materials (ASTM) provides three methods for measuring the oxygen transmission through barrier plastic films (i.e. films without perforations). The methods are: (i) manometric method (ASTM D 1434), (ii) volume method (ASTM D1434) and (iii) coulometric sensor method (ASTM D3985). The first two methods use absolute pressure differences. When measuring the permeability of films with micro-perforations, care should be taken that there is no absolute pressure difference across the film because in the presence of absolute pressure difference there will be viscous flow along with diffusive flow. So, the pressure method and volume method, which uses absolute pressure difference, cannot be used in the case of micro-perforated films. The coulometric sensor method, which does not have absolute pressure difference, has a potential for being used for micro-perforated films. In the case of perforated films, the pressure drop across the film in this method has to be minimized to zero to prevent viscous flow and short circuiting of the carrier gas through the perforations. Present methods are not adopting this technique to minimize the pressure drop. Another drawback with this method is that currently available systems utilizing this method have a maximum range of 155, 000 cc mxe2x88x922 dayxe2x88x921, but the range of permeabilities of micro-perforated films exceeds that range. Also, the use of this system for measurement of permeability of micro-perforated films does not appear in the literature. So, there is a present need for a setup for measuring the permeability of micro-perforated films.
Micro-perforated films are increasingly used in modified atmosphere packaging (MAP) of fruits and vegetables with high respiration rates. In the design of MAP for fruits and vegetables, the knowledge of film permeability is essential. The goal of the present invention is to provide a novel setup to measure the permeability of micro-perforated films to oxygen, and to use the results obtained to evaluate the different mathematical models available for predicting the gas transport through micro-perforated films.
Static and flow through methods were used to study the oxygen transmission rate (OTR) through perforations. The static method simulated the real package situation, but was very time consuming. On the other hand, the flow through method was relatively simple and took less time to give results. It however gave higher values than those obtained by the static method. The OTR data obtained from the static setup was correlated with the OTR data of flow-through by a regression equation.
Published models for predicting gas exchange through micro-perforations were evaluated using experimental data obtained using the static method for six different films. The model proposed by Fishman et al. (1996)(where effective length of diffusion=thickness of the film+radius of the perforation) had a very good agreement with the results obtained through experimentation. The model presented by Fishman et al. was mathematically shown to explain the diffusion phenomena through micro-perforations.
One preferred embodiment of the present invention is a rapid flow-based method to measure experimentally the oxygen permeability of micro-perforated films, comprising providing a diffusion cell having first and second compartments separated by a test film having micro-perforations, each compartment having an inlet and an outlet for gas flushing, sweeping the test film with a measured flow of pure nitrogen gas in the first compartment and with a measured flow of oxygen in the second compartment, while maintaining precise localized equal pressures on both side of the micro-perforations, determining the volume fraction concentration of oxygen at the outlet of the first compartment; and computing the oxygen transmission rate (OTR) across the test film in cc daysxe2x88x921 utilizing the formula
xe2x80x83OTRflow=1440fx
where f is the oxygen flow rate in cc minxe2x88x921 and x is the volume fraction oxygen concentration at the outlet of the first compartment.
Another preferred embodiment of the present invention is a static-based method to measure experimentally the oxygen permeability of micro-perforated films, comprising providing a diffusion cell having a nitrogen gas receiving compartment of volume V separated from the atmosphere by a test film, the compartment having an inlet and an outlet for gas flushing, purging the oxygen from the nitrogen gas receiving compartment with nitrogen gas, measuring the volume fraction concentration of oxygen (co) within the nitrogen receiving compartment at time 0 to account for possible oxygen leakage through the micro-perforations, determining the volume fraction concentration of oxygen (c) within the purged nitrogen gas receiving compartment after the passage of time t, and computing the oxygen transmission rate (OTR) across the test film in cc dayxe2x88x921 from the slope of the line generated by the equation                               ln          ⁢                      xe2x80x83                    ⁢                      (                                          Δ                ⁢                                  xe2x80x83                                ⁢                c                                            Δ                ⁢                                  xe2x80x83                                ⁢                                  c                  o                                                      )                          =                              -                          OTR              V                                ⁢                      xe2x80x83                    ⁢          t                                    (        2        )            
where co is the initial oxygen concentration within the nitrogen gas receiving compartment.
It is a principal object of the present invention to provide a rapid method to experimentally measure the permeability of micro-perforated films.
It is a further object of the present invention to evaluate based on the experimental data the different mathematical models available for predicting the gas transport through micro-perforations.