Aaron L. Brody

Containment of foods and beverages within packages for purposes of protection through distribution is a commendable primary objective. But if the consumer cannot retrieve the contents when and where needed or desired, protection becomes a moot issue.

Some of the many types of closures that are being used to provide convenience to the consumer. Top row, from left: soup can with easy-open steel end; twist cap for aluminum bottles; peelable membrane closures for metal cans; Silgan Dot-Top vacuum can; dual-open (pull tab or can opener) full-panel easy-open can. Center row, from left: peelable membrane plus plastic overcap for reclosure; dry-beverage canister with easy-open measuring cap; slide reclosure on shredded-cheese pouch; peel-off heat-seal flexible closure with reclosure feature. Bottom row, from left: condiment bottle with dispensing closure; mayonnaise bottle with easy-open dispensing feature; flip-top standup tube for spreading peanut butter; water bottle with easy twist-opening reclosure; gable-top paperboard composite carton with screw-top closure.For the first millennia of food packaging, packaging was either so flimsy and fragile that opening was a trivial act, or was so strong that the contents were not accessible without the work of several powerful individuals aided by major tools. Some of us are old enough to recollect (or reminisce through movies set during the 1930s and ’40s) manual can openers (“church keys”) to penetrate the ends of metal beverage cans, pliers to unscrew bottle caps, pries to remove crowns from bottles, corkscrews to pull corks from wine bottles (still in use today), and scissors to enter paperboard folding cartons. Senior packaging engineers recall vividly the torrents of consumer complaints about the difficulty of opening food packages.

Among the objectives of effective food and beverage packaging is ease of opening, with an axiomatic corollary, ease of access to or dispensing of contents. In today’s environment of consumer convenience, reclosure and subsequent reopening and accessibility to contents are nearly as important. With the diversity of commercial and developmental packages today and in the future, achievement of these “marketing”-oriented objectives is often challenging. On the other hand, convenience packaging has been demonstrated repeatedly to markedly enhance consumer acceptance and sales of products.

Historically, container opening was not a primary engineering packaging structural design objective: recall the first steel cans with their soldered closures or their later mechanically double-seamed ends. Attack with a sturdy implement such as knife or bayonet was required. Some might recollect the concept of a third-component narrow neck for metal cans closed with crowns that could be opened by prying up to break the internal pressure.

With the development of aluminum cans during the post–World War II period, carbonated beverage and beer sales increased smartly. Not until the introduction of the scored teardrop opening on aluminum ends in the 1960s did sales of these beverages begin to soar. The development required metallurgy and precisely engineered scoring, coupled with attachment by a mechanical rivet plus coating repair to overcome the damage wrought by stressing the aluminum. Easy removal of the wedge was certainly a major breakthrough, especially when it was demonstrated that beer in aluminum could not be distinguished from beer in traditional glass.

However effective the easy-open pull-off tab was, engineers did not reckon with the power of the environmentalists. Removed aluminum tabs were a blight on the landscape, were sharp knives, and could be picked at by animals who would be injured or die from attempting to “eat” the discarded tabs. These highly vocal issues led to movements for stay-on tabs, push-in buttons, hinged tabs, and many other alternatives to avoid the real or perceived problem. The current retained tab—whose sanitary attributes have been the subject of considerable debate—might be regarded as the forerunner of an entire generation of easy-open/easy-access package features that clearly contributed to exponential increases in sales.

And for the relatively few all-aluminum cans for foods such as puddings, full-panel pull-off ends were introduced, with some perceived disastrous results. Children, the target markets for aseptic unit-portion pudding cans, liked the product so much that they loved to lick the residual pudding off the interior and often cut their tongues. The ultimate solution, after many bloody mouths, was a double fold on the edge that functioned as a smooth rim. This misadventure by can makers constituted one driving force for the development of aseptically packaged barrier plastic cups with heat-sealed, peel-off, flexible, coated or aluminum-foil-lamination closures, the current standard.

Easy-Open Metal Cans
Successes with aluminum easy-open ends sparked the concept of easy-open steel food cans. The relatively simple notion of attaching aluminum ends to steel bodies was stymied by bimetallic reactions that are harmful to the food contents. Scoring steel ends in a manner similar to that of aluminum was a noble goal, but not surmountable with 1970s–’80s metallurgical engineering. Precision offset steel end scoring did not emerge as a viable technology until the late 1990s, with the most visible manifestation being the recent conversion of Campbell’s Soup cans after 150 years of flat ends.

Other techniques for closing metal cans to effect relative ease of opening appeared throughout the late 20th century. 3M’s flexible tape technology has been applied to teardrop-shaped openings on steel ends of hot-filled fruit beverage cans. Heat-resistant pressure-sensitive adhesives ensured hermetic sealing. The adhesives employed do not permit reclosure after opening, predating the 1980s mandate/desire for tamper evidence/tamper resistance.

Double seaming metal ends with die-cut openings covered with flexible lamination materials to can bodies combined reliable mechanical hermetic seaming with easy-open peel. The cost of this “dual” type of can closure has perhaps limited its widespread commercialization, although it is hardly uncommon in Europe.

Silgan® Containers Corp. (www.silgancontainers.com) double seamed an injection-molded barrier-plastic end to a two-piece steel or aluminum end. By scoring the perimeter, the bulk of the plastic end could be easily removed, leaving no sharp edges on either the open can or the end. This structure, Silgan’s Polyseal, was commercial for unit-portion-size fruit cans, but appears to have been withdrawn.

Developed originally by Metalgrafica Rojek in Brazil and imported into the United States by Silgan is Dot Top™, a concept that received the 2004 Can of the Year award of The Canmaker magazine. The top lid is a lined skirted steel device fitting the flange of the can body. Vacuum within the can draws the end tight and hermetically seals it. At the center of the end is a die-cut opening sealed closed by a barrier-plastic dimple. To open the can, the consumer peels back the plastic to release the internal vacuum, thus eliminating the pressure differential between the inside and the outside. The can end may then be lifted off and even replaced mechanically after removing some of the contents. The reclosure is not hermetic but serves as a cover.

Silgan asserts that the Dot Top is capable of being applied for both hot-filled high-acid and retorted low-acid contents. The first commercial application in the U.S. is by Hirzel Canning for a line of pizza and dipping sauces.

Snap-On Overcaps
Meanwhile, some food packagers the contents of whose products were not necessarily removed all at once recognized that reclosure was a desirable attribute. While not aiming at easy open (or were they blocked by internal pressure problems of carbon dioxide–emitting ground beans?), coffee roasters leapfrogged this issue to introduce plastic snap-over caps to reclose the can after can-opener opening. Hardly a perfect reseal, the feature was consumer friendly and was subsequently imitated on a host of other cans, as well as paperboard composite canisters and plastic cups, tubs, bowls, trays, etc. The primary closure for the plastic and paperboard composites was usually a heat-sealed peelable flexible material, with a secondary overcap that was initially and mainly a simple mechanism to contain the contents.

In later iterations during the 1980s, the snap cap was given much greater functionality. Plastic-barrier bucket-type cans for particulate foods intended for reheating in microwave ovens were engineered for maximum penetration of microwave energy. The surface-to-mass ratio, however, was suboptimum, so food packaging technologists added a supplementary non–microwave heating technique: steam retained by the secondary overcap. Incident microwave energy penetrated the food mass and generated steam that would normally be expelled into the cavity. By containing the steam within the plastic overcap, the heat of condensation was applied to heat the product. To minimize the possibility of snap cap blowoff due to steam pressure, holes in the plastic released a small portion of the steam.

This perforated overcap concept is now almost standard for closures on plastic trays for foods intended for reheating in microwave ovens, whether the foods are chilled or ambient-temperature shelf stable. Obviously, the overcap must be fabricated from a plastic that is capable of resisting steam temperatures. Furthermore, the fit of overcap to the tray, cup, bowl, etc., should permit ease of placement and analogous ease of removal. Generally, thermoformed overcaps have less precise dimensions and so are not used, with injection-molded polyethylene being the preferred structure.

Even more basic to package functionality for the multilayer barrier-plastic bucket can than the overcap is the primary closure. A full-panel easy-open aluminum end is seamed to the coextruded polypropylene/ethylene vinyl alcohol body flange. Conventional metal double seaming used almost universally to achieve hermetic closure had to be modified to obviate the real and potential problem of cracking the plastic as a result of the mechanical stresses applied during and after the seaming operations. The bucket shape was not initially a marketing demand but rather a means to increase the surface-to-mass ratio to facilitate entry of microwave energy. Thus, a relatively large-diameter opening was engineered into the can, leading to a large-circumference aluminum seam and scoring for later opening.

Retort Pouches and Trays
Achieving hermetic seals on retort trays has been a major challenge, especially when subsequent easy opening is also desired. Mechanical seaming supplemented by fusion heat sealing with plastic is one of the several alternatives for coated aluminum-foil structures at the outset. When properly made, the seals were indeed tight and required actual cutting of the aluminum foil to access the contents, counter to the convenience aspect advantage of retort trays. Later techniques included fusion heat sealing of closure to coated body flange without the seaming. The result during the 1980s was a seal that could not be peeled but rather required an implement to break the material or the bond. Numerous structures were proposed and introduced during that period, with some still in commercial use. Among the more intriguing was a mechanism that married, i.e., sealed, amorphous polypropylene in the closure interior to crystalline polypropylene in the body flange so that when opening stress was applied the interface fractured and opening was effected.

Before the development of notch cutting (starting the opening by cutting through the entire structure but only in the heat-seal area), laser cutting of one layer of flexible laminations could weaken the structure so that when the seal is stressed, a relatively easy opening could be achieved. Laser cutting through one or more layers of laminations remains one of the interesting methods of achieving easy-open heat seals. Both notch and laser cutting are now being combined with reclosure mechanisms in many flexible pouch and tray packaging applications.

Flexible Package Easy Open and Reclosure
The second half of the 20th century was witness to the development of packaged cured or processed meats and cheeses, usually under reduced-oxygen internal environments. The earliest of the barrier flexible packages for bacon, ham, bologna, salami, and Swiss cheese were tightly heat-sealed structures fabricated from flexible barrier laminations. Perimeters were, and are, wide heat-seal areas, and opening was literally attack with scissors. Reclosure was, for practical purposes, not possible.

Although the packages were far more convenient than the earlier delivery systems, they were not the ultimate in convenience. Thus began searches for alternatives that met the first criterion of low oxygen protection coupled with ease of opening and reclosure. Among the first of these mechanisms was the use of cohesive adhesive which functioned as a gas barrier and which could also be peeled apart and resealed. Cohesive adhesives bond only to themselves, usually without heat. If the adhesive is not contaminated by fat from the product, consumers can reclose the packages several times. Even the seemingly ubiquitous Velcro™ (by whatever nomenclature it is known generically) was proposed as a reclosure feature not long ago.

• Zippers. Although originally developed more than 40 years ago, zipper closure/reclosure features did not become prominent commercially until about 10 years ago with their application to shredded-cheese pouches. Zippers are devices that involve two interlocking portions, one a male member on one face of a flexible pouch that fits snugly into a female slot in the other face of the flexible material. The zipper is not a closure by itself, but rather complements the basic heat-seal closure of a flexible pouch or formed tray with flexible lidding closure. The consumer opens the primary closure in the usual fashion, easy-open tear or with an implement. After removing part of the contents, the two faces of the zipper are pressed together to reclose the package.

Deemed too expensive for use on flexible pouches because of the amount of material added and the issue of application to the base flexible structure, about 10 years ago cheesemaker Sargento nevertheless incorporated a zipper fastener into barrier pouches of shredded cheese. This bold move to permit opening of inert-gas-filled shredded-cheese pouches and subsequent reclosure—with relatively little barrier—permitted much more widespread use of shredded cheese and fostered an entirely new market for the product category, as well as for zipper and related closures generally.

Initially, the preformed zippers were heat sealed along the long dimension of the pouches, the direction the pouches were made on vertical form/fill/seal machines. This feature proved somewhat awkward for consumers, so applicators to apply the zippers across the short dimension were developed. At least one developer offered to extrude the profile directly onto the pouch without going to a preform, a task that proved to be somewhat challenging. Some zippers were intended to fit completely across the cross dimension, others fitted only partway across, and yet others were engineered to fit inside the pouch. Numerous preformed zipper designs were developed and commercialized, each for a specific application, e.g., some for trays with rigid bases and flexible lidding, some for flexible trays with flexible lidding, etc.

The applications from prominent suppliers such as Presto (now part of Alcoa ), and Zip-Pak (part of ITW), and others have blossomed across almost all standup flexible pouches for cookies, crackers, and candies, many processed-meat thermoformed trays, some specialty snacks, etc., until the zipper appears to be ubiquitous on food packages.

A relatively recent development is zippers that may be applied to hot-filled and retorted pouches. All such zippers are heat resistant and external to the principal heat seal that ensures a hermetic seal. Obviously meeting regulatory requirements since they are not the primary closure, retortable zippers have proven to be a major benefit for the emergent retort pouch business featuring packages of products that are not necessarily consumed all at once. Among the suppliers claiming retortable zippers are Presto Products Co. (www.prestoproducts.com) with its Fresh-Lock® Retort Zipper, Zip-Pak (www.zippak.com) with its Zip-Pak® Retort™ Zipper , and Pyramid Flexible Packaging (www.pyramidflexiblepackaging.com), with its Zip’n Store™ reclosable retort pouch.

• Slides. Hardly to be overlooked is the emergence of slide reclosures which represent to consumers an easier and more positive means of reclosing the pouch after intial opening. Among the intriguing feats has been to incorporate the slide inside the pouch to retain the internal inert gas. Obviously, once the pouch has been opened, the interior has been exposed to air.

Dispensing Devices for Bottles and Jars
Arriving in a torrent since the mid-1990s have been the multitudes of easy-open/easy-dispense bottle and jar closures that have virtually revolutionized the delivery of fluid foods such as catsup, mustard, mayonnaise, water, and many other products. What is amazing about this development is the fact that such closures have been used for many personal-care products such as shampoos, conditioners, body lotions, etc., for many years.

Injection-molded polypropylene flip-open/flip-close, push/pull, etc., types of devices are almost ubiquitous among consumers for bathroom use in wet environments and to obviate excess use and leakage after each individual use. Application of injection molding leads to the ability to precisely fit each element of the device to foster ease of one-hand opening and closing plus provide interference fits that totally close the opening even when some product is in the shoulder of the opening. Although injection molds are expensive, many for food are adapted from personal-care devices that fit on polyethylene and polyester bottles, so the capital costs are spread over large numbers for a variety of products.

These closures have sparked the movement toward “inverted” bottles for catsup, mayonnaise, and other condiments. Long used in Europe and a very logical means to provide consumers with convenient dispensing, inverted plastic bottles ensure that the contents are present at the opening for immediate dispensing.

A more recent application for fluid foods has been the incorporation of silicone closures. Uncovered, these devices contain openings that are expanded by the application of pressure on the bottle body that is transmitted through the fluid contents to the semi-rigid silicone, thus expanding the orifice and forcing the fluid through the opening. Upon releasing the pressure on the package wall, the silicone device reverts to its original closed position. Used by consumers for shampoos and conditioners, silicone devices represent a major new means of dispensing fluid foods from plastic bottles.

And no discussion on package closures is complete without reverent reference to the wonderful world of water. Who, a decade ago, could have imagined the multi-billion-dollar market for personal-size bottled water, appearing almost universally—in classrooms, offices, automobile beverage carriers, bicycle holders, belt holsters, and many other venues. Some contend that the water itself is the key element of this dramatic success, while others cite the polyester bottle, but I tend to attribute much of the sales volume to the closure. Single-handed dispensing while jogging on a trail or a treadmill, driving a vehicle, or taking notes in class appears to be a universal attribute—convenience of opening and dispensing and instant leakproof reclosure are fundamental to the popularity of bottled water today.

Closures Drive Acceptance and Demand
Where do we go from here? Among the lessons learned from this altogether too brief overview of closures and reclosures is that such features drive consumer acceptance and demand. Furthermore, too much time has elapsed between the technical development and the real commercial implementation. Too many immediate-cost guardians have failed to envision the spectacular increases in consumer sales when easy open/easy reclose is applied to a basic package. The costs of screw caps on gable-top juice cartons, inverted condiment bottles, and zippers on shredded-cheese pouches literally exploded industries far beyond any marketing research expectations.

And if you seek a means to satisfy consumers with the convenience of access to contents, look outside the box—into the realms of personal care, medical, and even over-the-counter drug packaging, where perhaps the costs are somewhat less critical than reliable functionality. There is where the future of food packaging lies—not in complex science and technology but in relatively simple product-access systems, so many of which have proven to be major drivers of consumer demand.

Contributing Editor
President and CEO,
Packaging/Brody, Inc., Duluth, Ga.
[email protected]

In This Article

  1. Food Processing & Packaging