Oxygen Scavengers Offer Safety and Savings
- Published: November 01, 2001, By Dr. Boh C. Tsai and George M. Schroeder, George O. Schroeder Assoc.
Protecting food from oxygen is key to preserving shelf life and quality. Exposure to oxygen causes, or in some cases indirectly leads to, a host of problems that quickly result in food spoilage. These problems include microbiological growth (such as mold and aerobic bacteria); chemical changes (such as rancidity, nutritional loss, and color change); and physiological changes (such as respiration).
Until recently, protecting food from oxygen was the sole province of passive barrier packaging materials. Originally limited to materials such as metal and glass, and then expanded over the past few decades to include polymers like polyvinylidene chloride (PVDC), polyamides, and ethylene vinyl alcohol (EVOH), passive barrier materials act like walls — physically separating oxygen from the food inside the package.
Developing Active Materials
In the last few years, however, active barrier materials have been developed that can be incorporated directly into packaging constructions. These materials, often called oxygen scavengers or oxygen absorbers, chemically bind with oxygen to, in effect, capture it before it can cause damage to the food.
Importantly, scavengers bind not only with oxygen penetrating through the packaging (those molecules that breach the wall) but also with any oxygen trapped in the headspace of the package or within the product itself — sources of oxygen that passive barriers are not designed to handle.
While oxygen scavengers are widely acknowledged as effective tools for protecting food, there is a common perception among packagers that scavengers are too expensive to be used in most packaging applications. It is true that, for the most part, oxygen scavenging compounds are more expensive on a per-pound basis than their conventional passive barrier counterparts. However, as the following calculations show, rather than raising the cost of packaging, active barrier materials actually generate clear cost savings when used appropriately with passive barrier materials. This cost savings can be illustrated by examining the incremental cost increase patterns of both passive and active barriers and by exploring the cost/performance threshold of active barrier use.
Incremental Cost Increase Patterns
The incremental cost increase pattern for dry EVOH (0.05 cc mil/100 in.2 day atm permeability), a commonly used passive barrier material, is shown in Table 1. In this table the cost of increasing the amount of EVOH to protect products with higher oxygen sensitivity is described.
The table compares three hypothetical products: a food with low oxygen sensitivity (an acceptable maximum oxygen concentration of 28.5 ppm1 in the food after 180 days, meaning that permittable package oxygen permeation is 20 cc/100 in.2, see Column A); a food with moderate oxygen sensitivity (an acceptable maximum oxygen concentration of only 7.1 ppm in the food after 180 days, meaning that permittable package oxygen permeation is 5 cc/100 in.2, see Column C); and a food with high oxygen sensitivity (an acceptable maximum oxygen concentration of just 1.4 ppm in the food after 180 days, meaning that permittable package oxygen permeation is 1 cc/100 in.2, see Column E).
For the purposes of this discussion, the critical information is in Column B and Column D. Column B shows the cost of the additional EVOH needed to protect a moderately oxygen-sensitive product as opposed to the EVOH needed to protect a food with low oxygen sensitivity. This upcharge is 0.31 cents/100 in.2 (0.41 cents/100 in.2 in Column C minus 0.10 cents/100 in.2 in Column A). This cost, while important, is rather insignificant when compared to the cost of moving from a moderately sensitive food to a highly sensitive food.
As seen in Column D, this incremental cost increase is 1.64 cents/100 in.2 (2.05 cents/100 in.2 in Column E minus 0.41 cents/100 in.2 in Column C), or roughly five times the cost of the previous move. This pattern of exponential increase of barrier cost is an intrinsic characteristic of passive barrier polymers.
The Opposite Side
On the other hand, as shown in Table 2, the pattern of incremental cost increase for an active barrier oxygen scavenger is exactly opposite to that of passive barrier resins.
The table uses a hypothetical oxygen scavenger material with a 10 cc of oxygen per gram absorption capacity and costing $8.00/lb. This gives a very conservative performance figure of 5.7 cc of oxygen absorbed for each cent worth of scavenger.
With this performance, the incremental cost increase of moving from protecting a low oxygen-sensitive food (Column A) to a moderate oxygen-sensitive food (Column C) is 2.63 cents/100 in.2 (15 cc of oxygen divided by an absorption capacity of 5.7 cc/cent). This is much higher than the incremental cost increase of moving from a moderate oxygen-sensitive food (Column C) to a highly oxygen-sensitive food (Column E).
In this instance, the cost increase is only 0.70 cents/100 in.2 (4 cc of oxygen divided by an absorption capacity of 5.7 cc/cent) or less than one-third of the previous incremental cost. This pattern of linear decrease of barrier cost is an intrinsic characteristic of active barrier materials.
Cost/Performance Threshold Calculations
The exponential characteristic of the incremental cost increase of passive barrier materials and the linear characteristic of the incremental cost decrease of oxygen scavenger materials points to the synergistic cost benefit of using oxygen scavengers in combination with passive barrier plastics.
This synergistic effect naturally would vary according to the cost and performance of each material, creating a threshold of cost/performance for every combination.
The threshold would depend on a number of factors: maximum permissible oxygen permeation (oxygen sensitivity of food); oxygen permeability, density, and cost of the passive barrier; and the oxygen absorption capacity and cost of the oxygen scavenger. Once these factors are defined, the threshold can be identified clearly.
A useful method for identifying and quantifying this cost/performance threshold is to compare the amount of barrier performance generated by using a passive barrier combined with an oxygen scavenger versus the barrier performance produced by simply employing a thicker layer of passive barrier material.
If the cost of these two options is kept the same, the performance advantages of either approach can be quantified by comparing the amount of oxygen ingress reduction.
Two Options
As an example, assume that the starting point of this inquiry is a flexible construction containing a PVDC layer that is one-third of a mil thick (0.33 mils). Two options exist to boost performance: 1) increasing the thickness of the PVDC barrier, say to 0.50 mils; or 2) combining the 0.33 mils of PVDC with a specified amount of oxygen scavenger.
Analyzing these two options will answer a fundamental question: For the same cost, which approach is more effective at reducing oxygen ingress: the additional passive barrier material or the oxygen scavenger?
The amount of oxygen ingress reduction can be calculated for each approach, and a difference between the two can be generated. If this difference is positive (indicating a greater reduction), it will show that for the same cost, better performance is generated by using a combination of an oxygen scavenger and 0.33 mils of PVDC. As a corollary, it also shows that for the same performance, less scavenger can be used, thereby saving material costs.
If the difference is negative, the converse is true: It is better to use 0.5 mils of PVDC and eliminate the scavenger. The same calculations can be made for other gauges of passive barriers versus passive/active combinations.2
Three test cases comparing an active/passive approach to a passive-only option are provided in Table 3 (see p. 59). These cases compare one scavenger over two different gauges and two scavengers at the same gauge. Each of these cases is based on PVDC having an oxygen permeability of 0.10 cc mil/100 in.2 day atm, a density of 1.70 g/cc, and a cost of $1.55/lb or $0.00951/100 in.2/mil thickness. In these test cases, the product being packaged is assumed to have a high sensitivity to oxygen that requires that total oxygen permeability be kept below 1 cc/100 in.2 for the 180-day shelf life. As with the incremental cost increase pattern example, the scavenger for the first two cases (here identified as Scavenger A) is assumed to absorb 5.7 cc of oxygen for every cent worth of scavenging material used.
Test Case 1
Test Case 1 was 0.50 mils of PVDC versus 0.33 mils of PVDC and Scavenger A.
The cost of 0.167 mils of PVDC (the incremental amount of PVDC used to increase the barrier layer from 0.33 to 0.50 mils) is 0.1585 cents (0.167 mils × 0.951 cents/mil = 0.1585 cents). The amount of oxygen reduction resulting from the increase in thickness of 0.167 mils is 1 cc (3 cc - 2 cc = 1 cc).
On the other hand, the amount of oxygen reduction resulting from 0.1585 cents of oxygen scavenger is 0.90 cc (0.1585 cents × 5.7 cc/cent = 0.90 cc). As a result, the difference between the two performances is 0.10 cc (0.90 cc - 1 cc = -0.10 cc).
The conclusion is clear: It is more beneficial to use 0.5 mils of PVDC than 0.33 mils of PVDC and Oxygen Scavenger A.
Test Case 2
Test Case 2 studied 1.0 mil of PVDC versus 0.50 mils of PVDC and Scavenger A.
The cost of 0.50 mils of PVDC (the incremental amount of PVDC used to increase the barrier layer from 0.50 to 1.0 mils) is 0.4755 cents (0.50 × 0.951 cents/mil = 0.4755 cents). The amount of oxygen reduction resulted from an additional 0.50 mils of PVDC is 1 cc (2 cc - 1 cc = 1 cc). The amount of oxygen reduction resulting from 0.4755 cents of oxygen scavenger is 2.71 cc (0.4755 cents × 5.7 cc/cent = 2.71 cc).
As a result, the difference between the two performances is 1.71 cc (2.71 cc - 1 cc = 1.71 cc).
In this case a different conclusion is evident: It is more beneficial to use 0.5 mils of PVDC and Oxygen Scavenger A than one mil of PVDC.
Test Case 3
Test Case 3 looked at 0.50 mils of PVDC versus 0.33 mils of PVDC and Oxygen Scavenger B.
The same calculations can be made with a different scavenger (Oxygen Scavenger B) offering 10 cc/g absorption capacity, but priced at $6/lb. The performance of this oxygen scavenger is slightly better than Oxygen Scavenger A: 7.6 cc of oxygen absorbed for every cent in material cost.
Just as in the first test case, the cost of 0.167 mils of PVDC (the incremental amount of PVDC used to increase the barrier layer from 0.33 to 0.50 mils) is 0.1585 cents (0.167 mils × 0.951 cents/mil = 0.1585 cents). The amount of oxygen reduction resulting from the increase in thickness of 0.167 mils is also the same at 1 cc (3 cc - 2 cc = 1 cc). However, the amount of oxygen reduction resulting from 0.1585 cents of Oxygen Scavenger B is 1.20 cc (0.1585 cents × 7.6 cc/cent = 1.20 cc). As a result, the difference between the two performances is 0.20 cc (1.20 cc - 1 cc = 0.20 cc).
Unlike in the first test case, in this instance the slightly improved performance of Oxygen Scavenger B changes the balance, making it more beneficial to use 0.33 mils of PVDC and Oxygen Scavenger B than 0.5 mils of PVDC.
Conclusions
These three test cases illustrate three important points. First, they demonstrate how the cost/benefit threshold can be calculated for scavenger use. Second, they illustrate how even small changes in the capacity and cost of the active barrier component can radically alter the cost/benefit threshold. (Here, a small change in cost moves the threshold from 0.50 mils of PVDC to 0.33 mils of PVDC and an oxygen scavenger.)
And finally, directly stemming from these two points, these calculations clearly indicate how the use of oxygen scavengers can lower packaging costs.
1 Calculating the barrier needed to meet oxygen concentration limits requires four pieces of information: the package's surface area; the mass (weight) of the product being packaged; the target shelf life; and the oxygen sensitivity of the food expressed in the maximum amount of oxygen exposure the food can tolerate and retain its quality. For the purposes of these cost calculations, it will be assumed that the surface area of the package is 100 sq in.; the product inside the package weighs 100 g; and that the target shelf life is 180 days. Costs calculations will be made for a number of product sensitivities, ranging from total permeation into the package of as little as 1 cc/100 in.2 to as much as 20 cc/100 in.2.
2 For example, 1 mil of passive barrier material can be compared against 0.5 mils of passive barrier and a scavenger, 2 mils of passive barrier material can be compared against 1 mil and a scavenger, 3 mils of passive barrier material can be compared against 2 mils and a scavenger, and so on.
Dr. Boh C. Tsai is technical director of George O. Schroeder Assoc. Inc., Appleton, WI. He is an expert on active packaging, and in particular, oxygen scavenging materials. Dr. Tsai invented the award-winning Amosorb® 2000 oxygen scavenger technology while at Amoco Chemical Co./BP Amoco and managed Amosorb business development. Dr. Tsai joined George O. Schroeder Assoc. Inc. as technical director last year. Among his awards was the 1997 Isker Award for his work with oxygen scavenging technology.
George M. Schroeder is managing director of George O. Schroeder Assoc. Inc. and has been a consultant in packaging for 17 years, advising materials suppliers and packaging converters around the world. As an attorney, he apprises clients of new developments in packaging-related legal areas such as intellectual property, food safety, and environmental concerns. Both Dr. Tsai and Mr. Schroeder can be reached at 920/739-0396.
The views and opinions expressed in Technical Reports are those of the author(s), not those of the editors of PFFC. Please address comments to author(s).
