Aaron L. Brody

In contemporary food economics, a three-year shelf life is passé—except for military and space feeding applications and except that if you achieve a three-year shelf life, at the end of three months, or 12 months, the quality can be superior to foods whose developers declare a 12-month shelf life.

PACKAGINGAlthough shelf life is a sometimes elusive concept related to consumer happiness with the food product, technically it may be dictated by microbiology or enzymology—or, if these two are controlled, biochemistry. And most, but not all, of the biochemical changes are oxidations. This leads me to my premise that if oxygen is controlled, so also is biochemical shelf life, mostly.

But, claim the suppliers of barrier package materials and packaging equipment, reduced oxygen is easily reached applying their offerings. They, not unanimously, believe that mechanically drawing a partial vacuum; generating a vacuum and compensating with an inert gas (read nitrogen); or flowing or counterflowing nitrogen or carbon dioxide into the package headspace reduces always-reactive oxygen sufficiently to significantly retard lipid, protein, staling, flavor, color, and probably other oxidative reactions. To which those who measure such phenomena respond: since when does the presence of 1,000–10,000 parts per million of oxygen in the gaseous environment eliminate reactions that are self-propagating?

Which food or beverage products are themselves treated to eliminate or reduce dissolved or occluded air, or both? Obviously, efforts are made to purge some products such as beer or even peanut butter, but the results are usually imperfect. Most food and beverage products subject to biochemical oxidation suffer from oxidative deterioration: canned fruit and vegetables, salty snacks, cereals, bakery goods, fruit beverages, tomato sauces, ready-to-heat-and-eat entrees, and even peanut butter. Among the few products that do not necessarily exhibit severe oxidative deterioration during distribution is beer, mainly because overt efforts are made to purge the product and its headspace and to employ oxygen-barrier packaging. Virtually all other food and beverage products undergo adverse oxidative flavor and color changes because the initial reaction is triggered by small quantities of oxygen and perpetuated by either ubiquitous small oxygen residuals or by auto-oxidation.

Of the major food preservation mechanisms, reducing temperature to near freezing can certainly retard any of the oxidative/auto-oxidative reactions. Reducing the oxygen content can delay the onset and slow the rate of adverse oxidative reactions, but generally not by any linear rate. Rather, each individual reaction complex—and all such food oxidations are not at all simple—must be studied and quantified for its kinetics related to the presence of oxygen. Although package headspace oxygen reduction coupled with barrier packaging can slow most oxidative reactions, the resulting rates are still finite—and faster than experienced with temperature reduction. The food/beverage quality benefit returns from conventional oxygen removal to 0.1–2% headspace are usually interesting but always imperfect.

Some food scientists/technologists would argue that the slight-to-modest oxidations generated in commercial package headspaces are desirable because consumers prefer these flavors and colors to the pristine and fresh. Legend holds that the “fresh” flavor of orange, apple, and tomato juices; potato chips; and even beer are not as acceptable as products with slight off-flavors. To which we might respond, freshly squeezed orange juice is much preferred over not-from-concentrate, which is greatly preferred over reconstituted; and a lot of effort is expended to reduce the oxygen in beer to the parts-per-billion level. And wine is often purged with argon, a sweep gas capable of reducing oxygen by an order of magnitude over nitrogen.

A recent search of the peer-review literature revealed a paucity of evidence on the quantitative effects of oxygen on the ubiquitous oxidative reactions. Many food scientists and technologists merely acknowledge that commercial in-package oxygen reduction has merit, but is qualitatively imperfect. Data hardly exist in the published literature on the quantitative correlation of product changes with oxygen concentration. Today, we have no models capable of accurately predicting product changes as a function of oxygen in the headspace or being transmitted into the package.

Why, then, are food packaging technologists struggling to improve oxygen barrier by using new polymers, coatings, coextrusion, and coinjection and by incorporating oxygen scavengers into in-package sachets, package material structures, and closure liners? One answer is that, based on experience, they perceive, believe, or hope that these actions will result in improved quality retention. Indeed, commercial experience with beer, fruit beverages, and both processed and dried meats suggests that headspace oxygen reduction leads to quality closer to fresh—but is not quantified. Although better, the products generally exhibit measurable change from that intended by the developers or that leaving the production line and entering the distribution channel.

What would be the benefit of delivering to the consumer product that is exactly as intended by the product development technologists and the producers? Would target consumers be able to perceive the quality, or, to be semantically precise, the absence of quality deterioration? And if the flavor, color, and mouthfeel quality could be consciously detected, would the target consumers care enough to purchase the very best? 

Classical Arthur D. Little, Inc., flavor profiling of food products—successfully used over years and decades for such products as Heinz ketchup, Budweiser beer, and others—indicated that common sensory characteristics associated with freshness did indeed constitute a major reason for continuing consumer satisfaction with the products. Without question, very few commercially successful food and beverage products lack the fully blended, identifiable sensory markers of the good product. And the very concepts of minimal and nonthermal processing are aimed explicitly at closer-to-fresh. 

Having declared with only anecdotal evidence that flavor quality is valuable to food processors/marketers, it is necessary to carry the concept further in two related directions: prove it quantitatively, and, more important, implement it on a commercial basis. 

The first—quantitative evidence—belongs to the academic community, which must comprehend the need at the outset, design the experiments, execute the testing—hardly an easy task—gather the data that prove the hypothesis, substantiate them, and correlate them to consumer response. One key to the research is the same as the basis for commercial application—removal of oxygen and retardation of non-oxidative reactions that contribute to adverse deteriorative reactions. 

If, as this hypothesis suggests, oxygen is the principal causative agent, then its removal is the basis for achieving the proposed quality benefit. If the oxygen is to be eliminated, the action must begin with the product and carry through in processing, packaging, and in the package. This means that oxygen-barrier package materials and structures are not the answer—and, in fact, good oxygen barriers may trap reactants and reaction products and accelerate sensory deterioration. Analogously, oxygen scavengers may not be the answer, since they may not react rapidly enough to stop initiation of auto-oxidation or have enough capacity to remove all of the oxygen. 

To achieve the ultimate objective of markedly reducing or arresting measurable oxidations, oxygen removal begins with the product. Ingredients should be treated with inert gas to remove occluded and dissolved oxygen. Mixing should be performed under inert gas. All product transfers should be under a blanket of inert gas. And certainly, packaging operations should be in an oxygen-free environment. 

Package structures should be fabricated from materials that ultimately function as oxygen barriers. Package elements such as seals, closures, and scores should be constructed to eliminate passages to air. Having engineered the package to function as a “near-perfect” oxygen barrier in the distribution channel, oxygen scavengers should be incorporated to complement the effect. 

And we should somehow be able to measure the oxygen—not exactly an easy task. 

Is this oxygen control easy to accomplish? Of course not. Is “near-perfect” oxygen control impossible to accomplish? Certainly not. 

If any of us expect to deliver the benefits being cited for minimally or nonthermally processed foods, we cannot avoid the indispensable arresting of distribution deterioration. And, if any of us propose to achieve “shelf life,” we must go far beyond oxygen-barrier package materials and think process, structures, and packaging operations. 

And, while we are addressing the dominating challenge of oxygen, we must begin to understand and cope with the non-oxidative deteriorations. 

Oh, what a mighty web we weave when first we enter the realm of quality retention and shelf life! 

For further information on near-zero oxygen, contact Louise Wicker at the Dept. of Food Science and Technology, University of Georgia, Athens, GA 30602-7610 ([email protected]). 

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Contributing Editor
President and CEO, Packaging/Brody, Inc.
Duluth, Ga.