EP0482281A1

EP0482281A1 - Controlled atmosphere package for fruit

Info

Publication number: EP0482281A1
Application number: EP91104141A
Authority: EP
Inventors: Michael J. Colucci; Charles L. Mott; Roland J. Weeks
Original assignee: DowBrands Inc
Current assignee: DowBrands Inc
Priority date: 1990-10-26
Filing date: 1991-03-18
Publication date: 1992-04-29
Also published as: CA2039019A1; JPH04215982A; MX9101323A

Abstract

A controlled atmosphere package (10) for maintaining the freshness and saleability of fruits and vegetables, including a lid (60) made from a selectively permeable, paper-based lidding stock.

Description

The present invention relates to controlled atmosphere packaging for delivering fruits and vegetables, particularly red-ripe fresh tomatoes, to the consumer.
Controlled atmosphere packages for various foodstuffs are well known in the art. Such packages may be rigid packages as in United States Patents No. 4,830,863 and 4,622,229, or nonrigid packages such as in Japanese Application 1984-223395 and United States Patents No. 4,079,152 and 4,897,274. Also known are packages which utilize a rigid enclosure to contain the fruit and a selectively-permeable, paper-based, nonrigid lidding stock to maintain a gaseous atmosphere of a desirable composition, such as described in United States Patent No. 4,830,863 to Jones.
A problem with both nonrigid controlled atmosphere packages and rigid packages with nonrigid lidding stock is that they are often comprised in substantial part of air-permeable thermoplastic films which are difficult to make and to work with and handle in the thin thicknesses required to obtain a sufficient degree of gas permeance.
One previously-suggested way of overcoming the processing and fabrication limitations of these thermoplastic films is to utilize a paper substrate and a thermoplastic layer in a laminate. Paper provides a measure of rigidity to the lidding material as well as a substrate for supporting selectively-permeable polymeric films and coatings.
While this lidding material gets around the processing and fabrication limitations of the films alone, other problems arise. One problem associated with this type of lidding material is in obtaining a substantially airtight seal between the lidding stock and a rigid enclosure. Means previously utilized to address this problem include the application of an adhesive between the lidding stock and the enclosure, and the coating of the inner surfaces of the enclosure with a heat-sensitive, tacky thermoplastic which can adhere to the lidding stock in areas where the enclosure and lidding stock come into contact. Applying the adhesive requires an additional process step, however, and coating the inner surfaces of the enclosure may frequently prove difficult due to the enclosure's shape.
Another problem arising out of the use of this type of selectively-permeable paper-based lidding stock is maintaining a desirable rate of permeance through the lidding stock. Though the paper substrate is typically freely porous with air freely permeating through it, the individual fibers comprising the paper substrate are typically substantially impermeable. At the interface between the substrate and the selectively-permeable polymeric coating or laminated film layer, the impermeable fibers effectively reduce the available surface area for permeance of gases through the selectively permeable coating or film. The reduction in available surface area at the interface undesirably reduces the rate of permeance of air through the selectively permeable coating or film, and thus through the lidding stock as a whole.
A controlled atmosphere package for fruits and vegetables is consequently needed which provides a lidding material that can be made and handled or processed like the paper-based laminates described above, but which does not have the drawbacks associated with these laminates and described above.
The present invention meets these needs by providing a controlled atmosphere package for maintaining the freshness and saleability of fruits and vegetables placed therein which comprises (a) a relatively rigid protective plastic enclosure having an opening therein, and (b) an air-permeable lid situated about and in substantially air-tight sealment with the enclosure at the opening. The air-permeable lid in turn comprises (i) a printable porous substrate layer of a nonwoven, fibrous material, (ii) a first intermediate layer of a porous non-fibrous material, and (iii) an inner layer of an air-permeable material. The first intermediate layer is situated between the substrate layer and the inner layer, substantially preventing contact between the substrate layer and the inner layer.
The present invention will be better understood upon reviewing the following specification together with the drawings, wherein:

Figure 1 is an isometric view of a controlled atmosphere package of the present invention with tomatoes therein;
Figure 2 is a greatly enlarged, fragmentary, cross-sectional view of the package lid in Fig. 1;
Figure 3 is an exploded isometric view of the package of Fig. 1; and
Figure 4 is a top view of a tomato.

A controlled atmosphere, controlled humidity package 10 of the present invention is illustrated in Fig. 1 with tomatoes therein. Package 10 comprises a relatively rigid protective plastic enclosure 15, a packet 22 of a humectant material, and a non-rigid lid 60. Enclosure 15 comprises a tray 20 and an insert 30. Package 10 is seen in an exploded view in Figure 3.
Tray 20 includes a chamber 25 which holds one or more tomatoes 12. Preferably, tray 20 is large enough to accommodate two tomatoes. The size of tray 20 and chamber 25 are determined by the size of the tomatoes or other fruit or vegetable to be held therein. The tomatoes 12 to be packaged in the package 10 typically weigh from 180 to 230 grams each, each have a diameter of about 3 inches (7.6 cm.), and each have a height of about 2 1/2 inches (6.4 cm.). For tomatoes of this size range, a suitable package is 6 to 8 inches (15.2 to 20.3 cm.) in length, from 3 to 5 1/2 inches (7.6 to 14.0 cm.) in width, and from 2 1/2 to 3 1/2 inches (6.4 to 8.9 cm.) in height.
Tray 20 defines an opening 33 which is bounded by a tray lip 24. The lip 24 is preferably substantially flat and preferably generally parallel with the bottom of tray 20. Tray 20 also defines inwardly-disposed ledge portions 36, 37, and 38 which receive peripheral portions of insert 30. Insert 30 is sized with respect to tray 20 such that peripheral portions of insert 30 rest upon and are supported by the ledges 36, 37, and 38. Tray 20 also includes the bottom cradles 28 which receive and contact surfaces of the tomatoes 12. A trough 23 defined in the bottom of the tray 20 opens toward the chamber 25 and insert 30, and holds the packet 22 of humectant material.
The bottom cradles 28 are formed by various cradle surfaces 27 defined by tray 20. These surfaces 27 are positioned to embrace and support the tomatoes 12 from the sides and from underneath, but with minimal surface contact to avoid bruising the tomatoes 12 and to provide for the free flow of gases and water vapor in the chamber 25 around the tomatoes 12. By providing avenues for the free flow of gases and water vapor, the package 10 avoids creating pockets of water vapor which might condense and lead to the formation of mold, and achieves a uniform gaseous composition and relative humidity within the chamber 25.
The package 10 defines means for holding and maintaining tomatoes 12 in a substantially fixed position with respect to the remainder of package 10 regardless of the orientation of package 10. In the embodiment of Figure 1, package 10 maintains tomatoes 12 in a substantially fixed position by providing various surfaces located above, below, and lateral to tomatoes 12 in the form of a tray 20, the cradles 28, and the insert 30.
Tomatoes 12, a representative specimen of which is seen in Figure 4, preferably rest upside down within tray 20 and package 10 with their stem scars 14 toward the bottom of package 10 and toward the packet 22. The stem scars 14 of the tomatoes 12 are preferably situated in proximity to the packet 22, and more preferably are situated within 1 inch of packet 22. Since most of the water vapor transpiring from a tomato is emitted through the stem scar, proximity of the stem scar to a packet of humectant material allows the water vapor from the stem scar to be effectively absorbed rather than accumulating to form liquid water on the surface of the tomato in the area of the stem scar. Such a buildup of liquid water in the package 10 is undesirable because it can accelerate mold formation.
The insert 30 performs several functions in package 10 including functioning as an internal lid for the tray 20, preventing contact between the tomatoes 12 and lid 60, protecting the tomatoes 12 from possible physical damage associated with stacking packages 10 upon one another in shipping, storage or display of the packages 10, and helping to keep the tomatoes 12 in a substantially fixed position within a package 10.
The insert 30 protects tomatoes 12 from possible physical damage in stacking two or more of the packages 10 by being supported on ledges 36, 37 and 38 when the package 10 is in an upright position, and by being sufficiently rigid to carry the load of successive stacked packages 10 without that load being transmitted through to the tomatoes 12 in the package 10 of which an insert 30 forms a part. The insert 30 is preferably not sealed to lip 24, tray 20, or any other component of package 10.
Insert 30 helps keep the tomatoes 12 in a substantially fixed position within package 10 by means of end caps 32 defined in insert 30. End caps 32 contact and cradle the surfaces of and the bottom ends of tomatoes 12 in the same manner generally as do the cradles 28 with respect to the top ends of tomatoes 12. End caps 32 thus function with cradles 28 and surfaces of the tray 20 lateral to tomatoes 12 to hold and maintain tomatoes 12 in a substantially fixed position with respect to the remainder of package 10, regardless of the orientation of package 10. It is understood, however, that the lid 60 may be utilized with the cradles 28 and the surfaces of the tray 20 lateral to the tomatoes 12 in lieu of the insert 30 to help maintain the tomatoes 12 in their substantially fixed positions.
Insert 30 and tray 20 are generally designed, sized and fit together so that one or more openings 34 are defined by the combination of the two, whereby gases may permeate between chamber 25 and the ambient environment outside package 10 through lid 60. These openings 34 are defined by the fit of insert 30 and tray 20 in the four corners of package 10, as seen clearly in Figure 1. Optionally, equivalent openings can be fashioned or defined wholly within the insert without regard to the fit between the insert and the tray. The openings 34 in either construction are preferably positioned so that the lid 60 cannot draw down across the openings 34.
Insert 30 in this regard has spacers 35 projecting upward from the insert which assist in preventing drawdown of the lid 60 into the openings 34. By preventing this drawdown, a space between the insert 30 and lid 60 is maintained and is in communication via openings 34 with the remainder of the chamber 25. Whereas in the absence of this space all mass transfer across the lid 60 would be over the area of the openings 34 (the insert 30 being air-impermeable), the maintenance of a space between the lid 60 and the insert 30 permits the use of substantially the entire area of the lid 60 over chamber 25.
Insert 30 and tray 20 are preferably comprised of a relatively rigid thermoplastic material, and are most preferably comprised of the same material. Suitable thermoplastic materials include polystyrene, styrene- butadiene copolymers, blends of polystyrene with styrene-butadiene copolymers, polyvinyl chloride, polyethylene terephthalate, polyethylene terephthalateglycol, or a cellulosic material such as cellulose acetate propionate. A most preferred material is polyvinyl chloride. Preferably, the thermoplastic material comprising insert 30 and tray 20 is substantially transparent so that the tomatoes 12 within a package 10 may be visually inspected through such material by the consumer prior to purchase.
Insert 30 and tray 20 may be formed by any conventional plastic fabrication technique such as thermoforming or injection molding. Preferably, insert 30 and tray 20 are thermoformed, and have thicknesses of from 5 to 25 mils (0.127 mm to 0.635 mm).
Referring now particularly to Figure 2, sealing lid 60 comprises a substrate layer 50 of a porous, air-permeable material, a first intermediate layer 46 of a porous, air-permeable material adjacent the substrate layer, and an inner layer 40 of a selectively-permeable thermoplastic having a carbon dioxide to oxygen permeability ratio of greater than 4:1. Substrate layer 50 conventionally functions in lid 60 as a label for the package 10, and is preferably capable of receiving printed indicia on either side of the layer 50.
Lid 60 forms a selectively-permeable but otherwise substantially air-tight, water-tight seal over the top of insert 30 and tray 20. Lid 60 is preferably sealed to tray 20 at lip 24, and most preferably heat sealed thereto.
The porous material comprising substrate layer 50 is preferably nonwoven and fibrous in structure, and is further preferably cellulosic paper or paper-based. The type, density, permeability and thickness of the porous material in substrate layer 50 may be matched with the respiration requirements of the tomato. The porous material in the substrate layer 50 further preferably has an average distance between pores of 10 microns or more. The porous material, most preferably paper, has a density preferably between 20 and 100 lb. stock, and most preferably is 60 lb. stock. The paper material preferably has an oxygen permeability of more than 10,000 cc/100 sq. in.-atm.-day. Though the desirable range of the thickness of substrate layer 50 will vary depending upon the physical properties and characteristics of the paper material comprising it, the thickness of the layer 50 will be preferably between 1 mil (0.025 mm) and 15 mils (0.381 mm), more preferably between 1.5 mils (0.038 mm) and 5 mils (0.127 mm), and most preferably will be 3 mils (0.076 mm). Suitable porous materials which can be substituted for the cellulosic paper or paper-based materials include Tyvek^* (trademark of E. I. DuPont deNemours & Co.) nonwoven spun-bound polyethylene or synthetic papers.
The porous material comprising substrate layer 50 is preferably not selective to a substantial degree in terms of its permeability to the gases in air which play a part in the respiration of the particular fruit or vegetable in the package 10. The two most important such gases in controlled atmosphere packaging, carbon dioxide and oxygen, have relative permeabilities in substrate layer 50 in a carbon dioxide to oxygen ratio of preferably less than 4:1, more preferably less than 2:1, and still more preferably about 1:1.
Substrate layer 50 is preferably substantially free of contact with inner layer 40. Minimizing or eliminating contact between the layer 50 and layer 40 is believed to enhance permeation through lid 60 as compared to a construction wherein layer 50 contacts inner layer 40 to a greater degree. Without being bound by any particular theory, it is believed that the porous paper substrate material at the interface between the substrate layer and the inner layer effectively reduces the surface area available in the selectively-permeable inner layer for permeation therethrough.
Individual paper fibers which make up the porous substrate material are typically substantially oxygen-and carbon dioxide-impermeable with these gases permeating between the fibers rather than through the fibers. Thus, the air-impermeable (i.e., the oxygen- and carbon dioxide-impermeable) fibers reduce the surface area of the inner layer available for gaseous permeation to the extent that these fibers intersect with the layer 40 or otherwise occupy the area of the interface of layers 40 and 50. Contact between layers 50 and 40 is substantially reduced or eliminated by the presence of at least a first intermediate layer 46 therebetween.
First intermediate layer 46 is comprised of a porous material, and is positioned adjacent inner layer 40 and in between substrate layer 50 and layer 40. Sealing lid 60 further preferably comprises a second intermediate layer 48 situated between the first intermediate layer 46 and substrate layer 50.
Layer 46 is made of a porous material or materials providing a carbon dioxide to oxygen permeability ratio of less than 4:1, preferably less than 2:1, and more preferably 1:1. The materials in layer 46 are preferably substantially non-fibrous and preferably are not paper-based. The porous materials for layer 46 preferably have an average distance between pores of 1 micron or less. Useful porous materials for layer 46 include minerals such clays, calcium carbonate, talc, graphite and the like. The minerals are preferably in a particulate form. Other suitable porous materials for first intermediate layer 46 include open-celled polystyrene and polyethylene foams. A preferred material is clay with a binder. First intermediate layer 46 is most preferably a coating of a mixture of 92 parts clay, 8 parts polystyrene, and 16 parts styrene/butadiene copolymer as a binder.
The layer 48 in lid 60 is a thin layer of a tacky, air-permeable material which acts a binder or adherent for a first intermediate layer 46 and a substrate layer 50. The air-permeable materials for the layer 48 include coatings formed from aqueous solutions of polysaccharides such as starches, alginates, and vegetable gums. Layer 48 is an external sizing preferably comprised of a layer of primarily starch.
The layers 46 and 48 may take the form of coatings, thin sheets, or films laminated to substrate layer 50 or of coatings applied thereto by blade coating or some other industrial coating process. Preferably, layers 46 and 48 are formed by blade coating of layer 50 with aqueous solutions or mixtures of the porous material and binders. Solutions and methods useful in this regard are known, and include those described for example in United States Patent Nos. 2,790,735, 4,109,056, 4,183,766, 4,187,219, 4,241,142, 4,328,284, 4,423,118, 4,567,099, 4,436,789 and 4,837,087.
Layers 46 and 48 are preferably as thin as possible to minimize losses in the overall permeability of lid 60 as a result of their inclusion, but are thick enough to ensure that the substrate layer 50 and inner layer 40 are substantially free of contact. Layer 48 is preferably discontinuous and thin enough to not substantially affect the overall oxygen and carbon dioxide selectivity of lid 60. The actual thicknesses of the layers 46 and 48, however, may in practice be dictated by the thickness of the layers in commercially available clay-coated papers, since it will normally be advantageous to use such papers.
Inner layer 40 is comprised of a selectively-permeable thermoplastic characterized by a carbon dioxide to oxygen permeability ratio of greater than 4:1, preferably more than 5:1, and still more preferably from 6:1 to as high as 8:1. Materials exhibiting such a desired selectivity include ethylene/vinyl acetate copolymers, styrene-butadiene copolymers, and polyethylene homopolymers such as the low density and linear low density polyethylenes. A most preferred material is an ethylene/vinyl acetate copolymer containing about 18 weight percent vinyl acetate, such as is currently sold under the mark ULTRATHENE by the USI Division of Quantum Chemical. The thermoplastic material comprising inner layer 40 is, additionally, preferably heat sealable to lip 24 of tray 20 so that a substantially air-tight seal can be provided between the layer 40 and the lip 24.
The thickness of selectively-permeable inner layer 40 is preferably optimized so that the desired gas selectivity is obtained, but so that the rates of ingress and egress of oxygen and carbon dioxide, respectively, into and from the chamber 25 are matched to the respiratory behavior of the fruit or vegetable packaged therein and to the concentrations of oxygen and carbon dioxide which are desired in the chamber 25. Inner layer 40 is configured to provide a dry oxygen permeance of preferably 500 to 2500, more preferably 900 to 1500, and most preferably 1100 cc/100 sq. in.-day-atmosphere. Inner layer 40 is configured to provide a dry carbon dioxide permeance of 2000 to 15,000, more preferably 4500 to 12,000, and most preferably 8000 cc/100 sq. in.-day-atmosphere. The thickness of inner layer 40 will vary depending upon the characteristics of the material comprising it, but will normally be between 0.4 mils (0.01 mm) and 2.0 mils (0.051 mm), preferably between 0.4 mils and 1.0 mil (0.025 mm), and more preferably will be about 0.6 mils (0.015 mm).
Lid 60 further preferably comprises first and second top layers 52 and 54 of a porous material, to provide a surface which is more easily printed upon than the substrate layer 50. These layers 52 and 54 preferably take the form of coatings of clay or starch, particularly, on substrate layer 50 as in layers 46 and 48. Layers 52 and 54 preferably have the same composition as layers 48 and 46, respectively. Layer 52 is preferably coated on layer 50 in the form of an aqueous solution, and layer 54 is in turn blade coated on layer 52 in the form of an aqueous mixture.
Lid 60 is most advantageously formed by coating the selectively-permeable material of layer 40 upon a suitable commercially-available clay-coated paper. Suitable commercially-available clay-coated papers include Westvaco 60# C1 S, Wetstrength 60# C1 S, Mead 60# C1 S, Simpson 60# C2S, Consolidated Paper 40# C2S, and James River 26# C1 S. A most preferred clay-coated paper is Westvaco Sterling Web Gloss BW, 60 lb. C2S paper.
Proper selection of a selectively-permeable material for the layer 40 with the desired level of carbon dioxide permeability allows the present package to maintain the desired steady state levels of C0₂ within the chamber, without requiring a C0₂ absorber in the chamber. This is not to say that such an absorber could not also be used, however, if desired.
The present package provides a carefully controlled environment at steady state of preferably 70 to 90 percent relative humidity, and more preferably of from 70 to 78 percent relative humidity. The steady state oxygen concentration within the chamber is preferably 2 to 10 percent by volume and more preferably from 3 to 5 percent by volume. The carbon dioxide concentration within the chamber at steady state is preferably less than 5 percent by volume, and more preferably is from 2 to less than 4 percent by volume. The below ambient levels of oxygen reduce the respiration rate and thus the rate of ripening of the tomatoes. Carbon dioxide levels in excess of 5 volume percent may make the tomato susceptible to cellular damage, but some amount of carbon dioxide enhances the viability and longevity of the tomatoes in the package. The relative humidity and the oxygen and C0₂ concentrations provided by a given package may vary somewhat for different types of tomatoes, because tomatoes differ in their respiration and/or transpiration rates. Thus, the preferred and most preferred ranges for relative humidity and oxygen and C0₂ levels may also vary somewhat according to the type of tomato to be packaged in a particular package.
Steady state in the permeation of oxygen and carbon dioxide through the lid 60 generally occurs after the tomatoes have been packaged for about two days. When the tomatoes are first packaged, the composition of the gases within the chamber is the same as in the environment where the tomatoes were packaged. As the tomatoes respire, the level of oxygen decreases and the level of carbon dioxide increases within the chamber. After about two days, the tomatoes have respired to such an extent that the oxygen content within the chamber has decreased to from 2 to 10 percent by volume and the carbon dioxide content has increased to a level not exceeding 6 volume percent. Steady state may also be accomplished by gas flushing when the package is packed and sealed.
As has already been indicated, a lower than ambient oxygen content within the chamber slows the respiration rate of the tomatoes and the ripening process. The slowing of the ripening process lengthens the duration of the desirable red ripe stage. The red ripe stage can be extended by maintaining the desirable levels of oxygen and carbon dioxide in the chamber for preferably 7 to 14 days, and more preferably for up to 21 days, after which point the quality of the tomatoes can be expected to decline. If the package is maintained at below ambient temperatures such as 50-60 degrees Fahrenheit (10 deg. C. to 15.6 deg. C.), however, the tomato may be maintained in the red ripe stage for still longer periods because of the lower respiration rate of the tomatoes.
The steady state levels of oxygen and carbon dioxide within the chamber are determined by the permeability of the selectively-permeable layer 40 for oxygen and for carbon dioxide. These permeabilities are determined by the characteristics of the coated layer 40 and by the partial pressure gradients for each of oxygen and carbon dioxide across the coating. The greater the partial pressure gradient between the chamber and the ambient environment outside the package for either gas, the greater the flow across the lid 60.
At steady state, the flow of oxygen into the package will about equal the rate of oxygen consumption by respiration of the tomatoes, and the flow of carbon dioxide out of the package will about equal the rate of carbon dioxide generation by respiration of the tomatoes.
Packet 22 is preferably held in a substantially fixed, spaced position with respect to the tomatoes 12 regardless of the orientation of the package as a whole. The size of the packet 22 and the amount of humectant material needed to achieve the desired relative humidities in the package are determined by the number and size of tomatoes 12 and their water vapor transpiration rates. Preferably there is enough material to absorb the transpired water vapor from the tomatoes 12 for 7 to 14 days, and more preferably for up to 21 days.
The humectant material contained within packet 22 preferably maintains the desired relative humidity of package 10 at a generally constant level. Packet 22 preferably comprises a pouch of water vapor-permeable, substantially liquid water-impermeable material and a humectant material contained within the pouch. Suitable pouch materials include cellulose acetate and Tyvek^* (trademark of the E. I. DuPont de Nemours & Co.) nonwoven polyethylene fabric. A preferred packet is seen in United States Patent No. 3,990,872.
Suitable humectant materials include conventional humectant or desiccant materials such as chloride salts, sulphate salts, silica gel, or activated alumina. Preferred humectant materials exhibit only negligible absorption of water vapor up until 70 percent relative humidity is reached in the package. Preferred humectant materials maintain levels of relative humidity in the package of preferably 70 to 90 percent and more preferably from 70 to 78 percent at room temperature, and preferably further maintain these levels over a temperature range of from 50 to 90 ° (10°C to 32.2 C) in the chamber. Preferred humectant materials include sodium chloride, mannitol, and sorbitol. The most preferred material is sodium chloride due to its low cost, low toxicity, familiarity to consumers, and excellent performance.
Relative humidities of between 70 and 90 percent are preferred because levels below 70 percent may result in dehydration of the tomatoes and levels in excess of 90 percent may result in increased mold growth and liquid water buildup. A relative humidity of 70 to 78 percent is most preferred.
In package 10, the packet 22 is located in proximity to and preferably within one inch (2.54 cm.) of the stem scars 14 of tomatoes 12. This proximity to the stem scars 14 allows for substantial absorption of water vapor by the humectant material, and minimizes the formation of localized regions of excessive water vapor or liquid water resulting from condensation of this water vapor. Preferably, however, the packet 22 does not contact the stem scars 14 because such contact may operate to seal in water vapor in the area of the stem scars 14 and to obscure the consumer's view of the stem scar.
The present package preferably maintains a substantially uniform relative humidity and gaseous composition within all regions. A substantially uniform relative humidity means that the humidity varies by no more than 20 percent and preferably by less than 10 percent from any given region of the tomato- containing chamber to another. The package preferably defines a chamber in this regard which is structurally configured to provide sufficient avenues for mass transfer surrounding the tomato so that localized concentrations of water vapor or condensate do not develop to more than a negligible extent. Preferably, no region of the chamber is sealed off from other regions by contacting surfaces of the tomato or tomatoes and the package. Further preferably, no surface of the package is in direct contact with the stem scar to minimize localization of water vapor and/or condensate in this critical area. The proximity of the packet 22 to the stem scars 14 of the tomatoes 12 also aids in minimizing localization of water vapor and/or condensate, and thus in maintaining a substantially uniform relative humidity.
Insert 30 may be modified according to the present invention to provide a compartment for the humectant material as the functional equivalent of a packet 22 of humectant material in the trough 23. Such a compartment may define a microporous or other water vapor-permeable layer on the underside of the insert so that the humectant material will be in close proximity to but not contacting the stem scars 14 of the tomatoes 12. With such a compartment defined within the insert 30, tomatoes may be situated right side or stem scar up in the package. Alternately, the packet 22 of humectant material may be attached to or positioned beneath the underside of an insert 30. With the packet 22 positioned beneath the insert 30, the tomatoes also may be situated right side or stem scar side up.
The present package also defines means for maintaining the separation and lack of physical contact of the tomatoes regardless of the orientation of the package. In package 10, the separation and lack of physical contact of tomatoes 12 is maintained by the embracing of tomatoes 12 in separate positions by various surfaces located above, below, and lateral to tomatoes 16 in the form of tray 20, curved surfaces 27 of cradles 28 of tray 20, and insert 30.
The present package is configured to minimize contact between the surfaces of the tomato or tomatoes and the surfaces of the package, to minimize the possibility of bruising and to enhance gaseous circulation in the chamber. The rigid package is configured such that the tomato or tomatoes are held in substantially fixed positions in the package by contact between the surfaces of the tomatoes and the package at multiple points and regions around the tomato. Contact at multiple points and regions results in the tomato or tomatoes being held in a substantially fixed position in the package with, however, a minimal area of physical contact between the two. Preferably, less than about 20 percent of the total surface area of the tomato or tomatoes is in physical contact with the package regardless of the orientation of the package. Further preferably, any given point or region of contact comprises no more than about 10 percent of the total surface area of the tomato or tomatoes regardless of the orientation of the package.
It is understood that the present package may also be used with perishable fruits or vegetables other than tomatoes. Suitable fruits and vegetables include but are not limited to peaches, grapes, bananas, green beans, sweet corn, and apples.
It is also understood that a suitable controlled atmosphere package for maintaining the freshness and saleability of fruit may be formed partly or entirely of the lidding material described above. The package may for example take the form of a flexible container formed partly or entirely of this lidding material.
The package 10 is preferably assembled by thermoforming a tray 20 with trough 23. Packet 22 is placed in the trough 23 through the opening 33 in tray 20. The tomatoes 12 are then placed in the tray 20, followed by the thermoformed insert. The lid 60 is then adhered in substantially air-tight sealment to tray 20 about opening 33.
Adhering lid 60 to the tray 20 comprises a) contacting tray 20 with a lid 60 having an inner layer 40 comprised of a thermoplastic, such as an ethylene-vinyl acetate copolymer, which becomes tacky on exposure to temperatures of 80 ° C or greater, then b) applying heat to the lid 60 such that the inner layer 40 becomes tacky, and c) allowing the inner layer 40 to cool until the seal between lid 60 and tray 20 is formed. Preferably, heat is applied to the lid 60 by contacting the top surface of the lid 60 above the tray lip 24 with a heated platen (not shown).
After the lid 60 is adhered to the tray 20, excess portions of the lid 60 overlapping beyond lip 24 are preferably cut or trimmed away. The structure of lid 60 is in this regard easily cut or trimmed.
The examples below further illustrate the present invention.

Example 1

Lidding stocks useful in packages according to the present invention were tested conventionally for 0₂ and C0₂ permeance. Commercially-available two-sided, clay-coated papers were extrusion coated with EVA (ethylene/vinyl acetate copolymer) layers of varying thickness. The particular copolymer utilized in this Example was ULTRATHENE 662-156 ethylene-vinyl acetate copolymer (from USI Division of Quantum Chemical). The results of these tests are set forth in Table 1.

Example 2

Lidding stocks formed by extrusion coating of commercially-available clay-coated papers with a selectively-permeable EVA copolymer (EVA) were evaluated for their oxygen and carbon dioxide permeance. The clay-coated papers used in this Example were clay-coated on one side only. The EVA was then extrusion coated on the paper on the clay coating side in some instances or on the free paper side in other instances. The EVA copolymer utilized was the same as in Example 1. The test results are set forth in Table 2.
Both types of lidding stock demonstrated oxygen and carbon dioxide permeance. The lidding stocks with the EVA coating on the clay-coated side showed enhanced permeance of both oxygen and carbon dioxide in comparison to the lidding stocks with the EVA coating on the free paper side.

Example 3

A package constructed according to the present invention was tested for its performance in maintaining the saleability of red-ripe tomatoes.
Fifty such packages of two tomatoes each were prepared and stored at conditions simulating those encountered in actual distribution. For comparative purposes, a control of forty packages of two tomatoes each were stored in open trays at the same conditions.
The fifty packages tested were constructed substantially as shown in the Figures. The tray was comprised of polyvinyl chloride. The humectant packet was a Tyvek^* (trademark of E. I. DuPont deNemours & Co.) packet containing 15 grams of sodium chloride. The lidding stock comprised a paper substrate of Westvaco Sterling Web Gloss BW 60# coated on each side with a starch coating and further with a clay coating comprising about 92 parts clay, 8 parts plastic pigment, and 16 parts styrene/butadiene copolymer binder. The coated paper substrate was extrusion coated with a 0.6 mil-thick (0.015 mm) layer of ULTRATHENE 662-156 ethylene-vinyl acetate copolymer (from USI Division of Quantum Chemical). The resulting package lids had an oxygen permeance of 1080 cc/100 sq.in.-atm-day and a carbon dioxide permeance of 6000 cc/100 sq.in.-atm-day.
Tomatoes utilized in the tests were of the BHN variety grown in Baja, California, and weighed in the 170-230 gram range. The tomatoes were prepared for packaging by spray washing with water containing 150 ppm chlorine and 500 ppm sodium lauryl sulfate for 1 minute, and then rinsing with 150 ppm chlorine for 1 minute. The tomatoes were then allowed to dry overnight.
The packages were prepared by consecutively placing the humectant packet, the tomatoes, and the package insert in the tray. The lidding stock was then sealed to the tray by contacting the top surface of the lidding stock with a heated die matching the shape of the tray lip. Sealing conditions were 250 ° F (121 ° C) for 2 seconds at 20 psig.
Packages were stored in a controlled environment chamber at 11 ° C for 4 days and then at 21 ° C for the remainder of the test. The chamber of the package was maintained at 60% relative humidity for the entire test.
The tomatoes were evaluated after a certain number of days for quality and saleability using a 9 point visual rating scale. A "9" was excellent, "7" was considered good, "5" was fair, "3" was poor, and "1" was inedible. The end of the product's shelf life was defined as the point when 10% of the inventive or control packages, respectively, were rated below a 5. A package was considered below a 5 if either of the two tomatoes in it was below a 5. A 5 rating in turn was taken as corresponding to the point when decay becomes serious enough in a tomato that the decayed portions can no longer be easily cut away. Tomatoes are not likely to be used by a typical consumer at this point. Using this method, the shelf life of the packaged tomatoes in this test was 13 days and the shelf life of the control tomatoes was 5 days. The test results are fully set forth in Tables 3 and 4.

Example 4

Lidding stocks useful in packages according to the present invention were tested for 0₂ and C0₂ permeance. Various papers and a non-paper porous material were extrusion coated with EVA (ethylene/vinyl acetate copolymer) layers at varying thicknesses over the latex coatings. The EVA copolymer utilized was ULTRATHENE 156 ethylene-vinyl acetate copolymer (USI Division, Quantum Chemical). Test results are set forth in Table 5.
While a particular embodiment of a package of the present invention has been specifically described and shown, it will be appreciated that a number of other embodiments are possible which are nevertheless still fairly within the scope of the present discovery.

Claims

1. A controlled atmosphere package which comprises:

(a) a relatively rigid protective plastic enclosure having an opening therein; and

(b) an air-permeable lid situated about and in substantially air-tight sealment with the enclosure at the opening, the air-permeable lid in turn including:

(i) a printable porous substrate layer of a cellulosic paper or paper-based material, a synthetic paper or nonwoven, spun-bound polyethylene;

(ii) a first intermediate layer of a porous non-fibrous material; and

(iii) an inner layer of a selectively permeable material, with the first intermediate layer being situated between the substrate layer and the inner layer and substantially preventing contact between the substrate layer and the inner layer.

2. A controlled atmosphere package as defined in Claim 1, wherein the protective plastic enclosure is substantially air-impermeable.

3. A controlled atmosphere package as defined in Claims 1 or 2, wherein the substrate layer has an oxygen permeability of more than 10,000 cc/100 in^2-day-atm., and a carbon dioxide to oxygen permeability ratio of less than 2:1.

4. A controlled atmosphere package as defined in Claim 3, wherein the substrate layer has a carbon dioxide to oxygen permeability ratio of about 1:1.

5. A controlled atmosphere package as defined in any of Claims 1-4, wherein the inner, selectively-permeable layer has a dry oxygen permeability of from 900 to 1500 cc/100 in^2-day-atm.

6. A controlled atmosphere package as defined in Claim 5, wherein the inner, selectively-permeable layer has a dry oxygen permeability of about 1100 cc/100 in^2-day-atm.

7. A controlled atmosphere package as defined in Claims 5 or 6, wherein the inner layer has a carbon dioxide to oxygen permeability ratio of more than 4:1.

8. A controlled atmosphere package as defined in Claim 7, wherein the inner layer has a carbon dioxide to oxygen permeability ratio of more than 5:1.

9. A controlled atmosphere package as defined in Claim 8, wherein the inner layer has a carbon dioxide to oxygen permeability ratio of from 6:1 to 8:1.

10. A controlled atmosphere package as defined in any of Claims 1-9, wherein the first intermediate layer is a coating of clay with a binder, of calcium carbonate, of talc or graphite, or is an open-celled polyethylene or polystyrene foam.

11. A controlled atmosphere package as defined in Claim 1, wherein the air-permeable lid consists of a clay-coated paper and a selectively permeable inner layer, the inner layer being in the form of a film or thin sheet laminated to a clay-coated side of the paper or in the form of a coating on a clay-coated side of the paper.

12. A controlled atmosphere package in the nature of a flexible container which is comprised in part or entirely of the material forming the air-permeable lid in the packages of any of Claims 1-11.