The quality and shelf life of many food products is enhanced by enclosing them in packaging that modifies or controls the atmosphere surrounding the product. Increased quality and longer shelf life result in fresher products for the consumer, less waste from spoiled produce, better inventory control, and appreciable overall savings for the food industry at both the retail and wholesale levels.
Modified atmosphere packaging (MAP) and controlled atmosphere packaging (CAP) are often used interchangeably in the industry, and much confusion exists on their exact meanings. Both refer to methods to control the atmosphere in the package. In the processed foods area, MAP is considered a static method for controlling the atmosphere whereby an initial charge of a specific gas composition, e.g. 30% CO2 and 70% N2, is introduced into a barrier container before sealing.
The oxygen transmission rate (OTR) of a film is expressed as cc O2/m2-day-atmosphere, where one atmosphere is 101325 kg/ms2. Generally, a barrier container is one that has an OTR of <70 cc/m2-day-atm. The units describing the flow of a particular gas through a film are “flux”, expressed as cc/day-atm.
For fresh produce, the primary means to extend quality and shelf life is temperature control. However, more than 50 years of evidence from industry practices on bulk storage of fresh fruits and vegetables in refrigerated controlled atmosphere storage rooms has shown that atmosphere control can contribute greatly to quality retention and shelf life. The use of MAP/CAP for fresh produce was a natural progression once packaging technology had advanced to include the production of non-barrier (often referred to in the industry as “breathable”) materials.
The goal in fresh fruit and vegetable packaging is to use MAP/CAP to preserve produce quality by reducing the aerobic respiration rate but avoiding anaerobic processes that lead to adverse changes in texture, flavor, and aroma, as well as an increased public health concern. Aerobic respiration can be defined by the following equation:(CH2O)n+nO2→nCO2+nH2O+heatwhere O2 from the air is used to metabolize carbohydrate ((CH2O)n) reserves and in the process, CO2, and H2O are produced and heat is generated. For each respiring item, there is an optimum O2 and CO2 level that will reduce its respiration rate and thereby, slow aging and degradative processes. Different fresh produce items have different respiration rates and different optimum atmospheres for extending quality and shelf life.
The concept of passive MAP became common with the development of packaging materials with OTRs of 1085 to 7000 cc/m2-day-atm for fresh-cut salads. In passive MAP, the produce is sealed in packages made from these low barrier materials and allowed to establish its own atmosphere over time through produce respiration processes. Sometimes the package is gas-flushed with N2 or a combination of CO2 and N2, or O2, CO2, and N2 before sealing to rapidly establish the desired gas composition inside the package. Alternately, a portion of the air may be removed from the pack, either by deflation or evacuation, before the package is sealed, to facilitate rapid establishment of the desired gas content.
In CAP, the atmosphere in the package is controlled at well-defined levels throughout storage. CAP can take many forms, and may even involve enclosing gas absorber packets inside processed food barrier packages. For example, CO2 absorber sachets may be sealed inside coffee containers to absorb and control the level of CO2 that continues to be generated by the ground coffee. Sachets containing iron oxides are enclosed in barrier packages of fresh refrigerated pasta to absorb low levels of O2 entering the package through the plastic material.
CAP of fresh produce is just a more controlled version of MAP. It involves a precise matching of packaging material gas transmission rates with the respiration rates of the produce. For example, many fresh-cut salad packages use passive MAP as described herein. If the packages are temperature-abused (stored at 6–10° C. or higher), O2 levels diminish to less than 1%, and CO2 levels can exceed 20%. If these temperature-abused packages are then placed back into recommended 3–4° C. storage, the packaging material gas transmission rates may not be high enough to establish an aerobic atmosphere (<20% CO2, >1–2% O2) so fermentation reactions cause off-odors, off-flavors, and slimy product. If the salad was in a CAP package, the O2 levels would decrease and CO2 levels increase with temperature abuse, but would be re-established to desired levels within a short time after the product is returned to 4° C. storage temperatures.
Today, films made from polymer blends, coextrusions, and laminate materials with OTRs of 1085 to 14,000 cc/100 m2-day-atm are being used for packaging various weights of low respiring produce items like lettuce and cabbage. These OTRs, however, are much too low to preserve the fresh quality of high respiring produce like broccoli, mushrooms, and asparagus. In addition, existing packaging material OTRs for bulk quantities (>1 kg) of some low respiring produce are not high enough to prevent sensory quality changes during storage. Therefore, several approaches have been patented describing methods to produce packaging materials to accommodate the higher respiration rate requirements and higher weights of a wide variety of fresh produce items.
U.S. Pat. No. 4,842,875, U.S. Pat. No. 4,923,703, U.S. Pat. No. 4,910,032, U.S. Pat. No. 4,879,078, and U.S. Pat. No. 4,923,650 describe the use of a breathable microporous patch placed over an opening in an essentially impermeable fresh produce container to control the flow of oxygen and carbon dioxide into and out of the container during storage. Although this method works effectively, the breathable patch must be produced by normal plastic extrusion and orientation processes, whereby, a highly filled, molten plastic is extruded onto a chill roll and oriented in the machine direction using a series of rolls that decrease the thickness of the web. During orientation, micropores are created in the film at the site of the filler particles. Next, the microporous film must be converted into pressure sensitive adhesive patches or heat-seal coated patches using narrow web printing presses that apply a pattern of adhesive over the microporous web and die-cut the film into individual patches on a roll. These processes make the cost of each patch too expensive for the wide spread use of this technology in the marketplace. In addition, the food packer has to apply the adhesive-coated breathable patch over a hole made in the primary packaging material (bag or lidding film) during the food packaging operation. To do this, the packer must purchase hole-punching and label application equipment to install on each packaging equipment line. These extra steps not only increase packaging equipment costs, but also greatly reduce packaging speeds, increase packaging material waste, and therefore, increase total packaging costs.
An alternative to microporous patches for MAP/CAP of fresh fruits and vegetables is to microperforate polymeric packaging materials. Various methods can be used to microperforate packaging materials: cold or hot needle mechanical punches, electric spark and lasers. Mechanical punches are slow and often produce numerous large perforations (1 mm or larger) throughout the surface area of the packaging material, making it unlikely that the atmosphere inside the package will be modified below ambient air conditions (20.9% O2, 0.03% CO2). Equipment for spark perforation of packaging materials is not practical for most plastic converting operations, because the packaging material must be submerged in either an oil or a water bath while the electrical pulses are generated to microperforate the material. The most efficient and practical method for making microperforated packaging materials is using lasers.
UK Patent Application No. 2 200 618 A and European Patent Application No. 88301303.9 describe the use of a mechanical perforating method to make perforations with diameters of 0.25 mm to 60 mm in PVC films for produce packaging. Rods with pins embedded into the surface of the cylinder are used to punch holes in the film. For each produce item to be packaged, the rod/pin configuration is manually changed so that the number of perforation rows in the film, the distance apart of the rows, the pitch of the pins used to make the holes, and the size of the holes are adjusted to meet the specific requirements of the produce. The produce requirements are determined by laboratory testing produce packed in a variety of perforated films. The invention does not disclose any mathematical method to determine the appropriate size or number of perforations to use with different produce items. In addition, the hole sizes, 20 mm to 60 mm, which are claimed, would be too large to effectively control the atmosphere inside packages containing less than several kilograms of produce. Furthermore, the complicated perforation method would cause lost package production time due to equipment (perforation cylinder) change-overs for different perforation patterns. In addition, the invention cautions that the produce should be placed in the package so that the perforations are not occluded and care should be taken to prevent taping over the perforations in the film. Since the perforations are not registered in a small area on the package, but are placed throughout the main body of the plastic film, the likelihood is high that perforations will be occluded by the produce inside the package or by pressure sensitive adhesive labels applied on packages for marketing purposes. When holes are blocked, the principal route for gas transmission through the film is blocked which leads to anaerobic conditions and fermentative reactions. The result is poor sensory properties, reduced shelf life and possible microbiological safety concerns. Therefore, it is important that perforations be registered in a well-defined area of the package where the likelihood of their occlusion during pack-out, storage, transportation, and retail display is minimized.
U.S. Pat. No. 5,832,699, UK Patent Application 2 221 692 A, and European Patent Application 0 351 116 describe a method of packaging plant material using perforated polymer films having 10 to 1000 perforations per m2 (1550 in2) with mean diameters of 40 to 60 microns but not greater than 100 microns. The patents recommend the use of lasers for creating the perforations, but do not describe the equipment or processes necessary to accomplish this task. The patents describe the limits of the gas transmission rates of the perforated film: OTR no greater than 200,000 cc/m2-day-atm (12,903 cc O2/100 in2-day-atm), and MVTR no greater than 800 g/m2-day-atm (51.6 g/100 in2-day-atm). However, the OTR of a film does not define the total O2 Flux (cc O2/day-atm) needed by a fresh produce package to maintain a desired O2 and CO2 internal atmosphere based on the respiration rate of the specific produce item, the weight of the produce enclosed in the package, the surface area of the package, and the storage temperature. A 50-micron perforation has a very small surface area (1.96×10−9 m2) and a low O2 Flux (about 80 cc/day-atm) compared to its very high OTR (>200,000 cc O2/m2-day-atm). Therefore, one 50-micron perforation would exceed the OTR limit of this invention. Furthermore, fresh produce items such as fresh spinach are very susceptible to moisture that accumulates inside packages so produce weights greater than 0.5 kg requires 2–3 times more moisture vapor transmission than the upper limit described in this patent.
The above inventions do not address the issue of microperforation occlusion by produce inside the package when microperforations are placed throughout the length and width of the film. Since 20 to 100 micron holes cannot be readily seen with the naked eye, it is impossible to prevent occlusion of the microperforations either by the produce or by adhesive labels applied to the packages when microperforations are placed across and along the entire film. Finally, the size and location of the microperforations in the film also makes it impossible for the packaging user to quickly inspect the films for consistency of perforation size and number. These deficiencies have been roadblocks in the wide spread commercialization of films made according to this invention.
As indicated, the current practices of producing microperforated materials for modifying or controlling the atmosphere inside fresh produce packages are not satisfactory. There is a need for packaging in which the microperforations are registered in a small identifiable area that will not be blocked by adhesive labels or adjacent packages during package stacking or handling. The fresh produce should be placed in a product-specific package where the microperforation size, location, and number of microperforations are optimally selected to obtain the desired film gas transmission rates and gas flux for maintaining the quality of that specific produce item. In addition, a method is needed for accurately predicting the size and number of microperforations required by a particular weight of respiring produce at a specified temperature to maintain a pre-selected atmosphere inside the package during storage. And, there needs to be a cost-effective method of manufacturing microperforated packaging materials according to the requirements of the present invention.