Yam, Kit L. (ed.). The Wiley encyclopedia of packaging technology

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OFF-ODORS PACKAGING

GREGORY V. PACE

Sun Chemical Corporation, and

Department of Food Science

Graduate Program,

Rutgers University

THOMAS G. HARTMAN

School of Environmental and

Biological Sciences

Department of Food Science,

Rutgers University

INTRODUCTION

Off-odor and off-ﬂavor compounds that are responsible for

the sensory response are detected by the taste and olfac-

tory systems. The challenge for analytical instrumenta-

tion is to detect and identify those compounds most

responsible for the off-odor or off-ﬂavor. This task is

further complicated by the fact that the compounds with

the highest sensory response have very low odor thresh-

olds. That is, they can be present at part per million (ppm)

or part per billion (ppb) levels. The analytical methodology

must be able to separate, identify, and quantitate these

off-odor compounds at very low levels in the package.

Since the 1970s, there have been excellent advances in

the instrumentation, experimentation, interpretation,

and application of measurement techniques, that have

increased the separation and lowered the detection limit

of compounds that have low odor thresholds and high

human sensory response. Much of the off-odor references

for investigation are derived from the ﬂavor and aroma

chemistry ﬁeld, in which the analytical methodology has

been centered on isolation and identiﬁcation of the volatile

compounds known to give the organoleptic human re-

sponse. Because of their volatility, these compounds are

predominantly analyzed by some form of gas chromato-

graphy (GC). Capillary GC can be tailored to give the

separation and sensitivity needed for the analysis of off-

odor compounds. It is easily amenable to various sample

introduction techniques, and speciﬁc, universal, or con-

ﬁrmatory detectors.

ANALYTICAL INSTRUMENTATION FOR DETERMINING

THE CHEMICAL COMPOSTION OFF-ODOR

COMPOUNDS

Gas chromatography (GC) is the separation of a mixture of

compounds that are put into the vapor state, carried

through a chemically coated column, selectively separated

based on a physical or chemical property, and identiﬁed

with a speciﬁc detector. GC was developed using a packed

metal column of about 2 m, with a stationary support that

was coated with a nonvolatile liquid phase. It is now

predominately performed with high-resolution GC

(HRGC), using capillary columns ranging from 15 to

100 m long. The type of GC detectors most often employed

in aroma and ﬂavor analyses are; ﬂame ionization detector

(FID); mass spectrometry—full scan (or total ion) mode

MS-TIC; mass spectrometry in selective ion monitoring

mode (MS-SIM); GC—olfactometry (GC-O). The selection

of the detector depends on the complexity of the volatile

mixture being separated, the properties of the compounds

in the mixture, and the levels at which those analytes of

sensory response interest exist.

The methodology chosen for off-odor and off-ﬂavor com-

pound isolation is a critical step in ensuring that the entire

mixtures that cause the sensory response are analyzed.

Solid-phase extractions (SPE) and SP-micro-extraction

(SPME), headspace (HS) extractions, thermal desorption

(TD), and purge and trap (P&T) are common techniques

employed to facilitate the extraction of the compounds and

then analyzed by the GC instrumental system.

Gas Chromatography—Olfactory (GC-O)

Gas chromatography—olfactometry (GC-O) is a true hy-

phenated analytical and sensory technique. It is widely

applied to conﬁrm the actual olfactometric property of a

suspected off-odor, off-ﬂavor, or off-aroma compound. The

isolated mixture is injected into a GC, the compounds

in the mixtures are separated, and the efﬂuent vapors

from the column are usually split to an instrumental

detector and a sniff port (Figure 1). The sniff port is

conditioned with humidiﬁed air to protect the nasal

passages of the analyst (or panelist) and the vapor

compounds separated from the mixture by the column

elute out of the sniff port and are detected by the human

nose. An electronic signal is generated by the analyst

Figure 1. Gas Chromatograph equipped with a sniff port.

839

upon detection of the odor. A descriptor is also recorded

for each compound. The ﬁnal olfactogram is recon-

structed by averaging the individual olfactograms of the

replicate samples, identifying the odor-active compounds

in the odorant areas (1). GC-O was successfully used in

conjunction with triangle tests to determine the effects of

oxidation of critical ﬂavor active compounds during wine

processing (4).

Gas Chromatography–Flame Ionization Detector (FID)

GC-FID is the most widely used GC system in many

different environments from production quality control

to fragrance research. There is an abundance of published

literature and application notes using this technique in a

wide variety of ﬁelds where the analysis of volatile com-

pounds is important (e.g., environmental, petroleum, in-

dustrial solvents, food packaging, ﬂavors and fragrances).

The identiﬁcation of a compound is based on the retention

time on a particular column, compared to a known stan-

dard. They can also be compared to a series of n-parafﬁn

hydrocarbons, which determines their retention index

(RI). This comparative retention index of a compound

allows comparison with different instrument conﬁgura-

tions to published data, as was done in the wine research

of Le Fur et al. (1). In the wine industry, GC-FID is used in

conjunction with GC-MS to quantitate the chemical com-

position of the volatile aroma compounds in foods (2, 3).

The GC-FID is also commonly used in-line concurrently

with GC-O, where the column efﬂuent is split to the FID

and sniff port at the same time (5). These techniques used

in the food and wine industry have been adapted to off-

odor analysis by the authors of this article.

Gas Chromatography–Mass Spectrometry (GC-MS)

The improvement of the mass spectrometer since the 1980s

has made it readily available to a wide variety of indus-

tries. It is the most appropriate analytical tool to enable

the detection, identiﬁcation, and quantitation of ﬂavor

aroma active compounds with odor thresholds in the very

low ppb range. The utility of the MS is in its generation of a

third dimension of the mass spectral identiﬁcation to the

chromatographic separation of column retention time in

the column and response at the detector. There are many

examples in the literature search for the utilization of GC-

MS in the conﬁrmation and identiﬁcation of sensory sig-

niﬁcant compounds (1–8). The ability to generate a mass

spectrum that is consistent and predictable to the chemical

structure has led to large (W500,000 compounds) commer-

cial databases of compounds that can be used to search and

identify unknowns. The speciﬁc GC–mass spectrum of the

individual off-odor and off-ﬂavor compounds allows for

their detection, identiﬁcation, and quantitation at ppm or

very low ppb levels (2).

Static Headspace Gas Chromatography–Mass Spectrometry

(HS-GC-MS)

Static HS-GC-MS is used to obtain an overview of the

volatile aroma compound mixtures. It is the simplest of

the volatilization techniques, reproducing the natural

volatilization of the aroma and ﬂavor compounds in the

sample. A liquid sample is heated in a closed container,

and the volatile mixture of compounds present in the

headspace above the liquid is sampled and analyzed

by GC-MS. This technique is effectively used in food

packaging and wine proﬁling to compare acceptable and

unacceptable samples (i.e., off-odor and off-ﬂavor). Its

limitation is that the concentration of some off-odor or

off-ﬂavor compounds with very low odor thresholds are at

levels below the detection limit of this method (i.e., ppb).

The instrumentation available on the market allows for

automated sampling of up to 100 samples, facilitating

analysis of replicate samples to generate statistical sig-

niﬁcance of the results.

Purge and Trap Gas Chromatography–Mass Spectrometry

(P&T-GC-MS)

Purge and trap (P&T) is a dynamic headspace volatile

concentration technique. The volatiles are purged from

the sample and trapped on an absorbent material (i.e.,

Tenaxt). P&T is also used to prepare samples for con-

centrated GC-O (1) analysis. The trapped volatile com-

pounds are then desorbed into the GC-MS, and the

mixture is separated by the GC column and identiﬁed

using the MS. P&T has been mostly done off-line, where

the volatiles are trapped onto an absorbent material

(i.e., Tenax). The trap is then installed onto a desorption

apparatus, which sits over the injection port of the

GC-MS, and the volatiles are desorbed at high heat

(e.g., 3001C) and analyzed. P&T was used as a sample

preparation on many of the experiments performed in the

author’s laboratories.

Thermal Desorption–Gas Chromatography–Mass

Spectrometry (TD-GC-MS)

Thermal desorption (Figure 2) can be used to extract

trapped volatiles that have been captured onto a chemical

trap, as stated above in P&T-GC-MS. It has been used for

the identiﬁcation of off-ﬂavors from food packaging mate-

rials (9). One way it is used is by trapping the outgassed

volatiles P&T, which are then desorbed into the injection

port of the GC-MS or GC-O (1). Another approach used is

by direct thermal analysis (TA-GC-MS) of the plastic

polymer ﬁlm used for the food packaging, to release

trapped off-odor compounds from the polymer matrix

(11, 12). In that work by one of the authors, the cause of

an off-odor in a polypropylene food packaging made from

recycled resins was determined. Two compounds out of

175 chromatographic peaks were identiﬁed as the most

organoleptic signiﬁcant.

Electronic Gas Sensor Array (Electronic Nose)

e el

ectronic nose (e-nose) is a measurement device that

combines chemical sensing with electronic processing. Two

of the most popular designs of e-nose sensors used in ﬂavors

and aromas are based on a conductive organic polymer

sensor array and piezoelectric gas sensor. In the conductive

organic polymer sensor array, the polymers swell to differ-

ent degrees upon response to a volatile compound, changing

their resistance. The resistance patterns of the array of

840 OFF-ODORS PACKAGING

polymers are unique for each type of compound. The

pattern display resembles the cross ﬁber pattern theory of

taste, although the response mechanism is much different.

The signal pattern generated by the e-nose sensor are

downloaded to a chip (for handheld models), and also

analyzed by computer. To be successfully applied, it is

critical that the e-nose is ‘‘trained’’ with the compounds

and volatiles of interest. This sensor was used in food

quality control (6). It was shown that once the

e-nose is trained with the standard, it is successful in

detecting deviations and changes. The e-nose’s portability

make it an excellent in-line production screening tool,

capable of identifying foods that fall outside of the cali-

brated, acceptable range. Another type of e-nose made from

a piezoelectric gas sensor was applied to record and repro-

duce the odor of citrus to manufacture artiﬁcial citrus

ﬂavors (10). The array output pattern from the sensor

was compared to the ten essential citrus oil standards.

The multi-component mixtures were treated as single

components, allowing successful duplication of typical

orange ﬂavor. The e-nose can be trained to identify

if a food or package is different that the standard, but is

less capable to identify and quantitate an off-odor or off-

ﬂavor compound as well as GC-MS.

REFERENCES

1. Y. Le Fur, V. Mercurio, L. Moio, J. Blanquet, and J. M.

Meunier, A New Approach to Examine the Relationships

Between Sensory and Gas Chromatography–Olfactometry

Data Using Generalized Procrustes Analysis Applied to Six

French Chardonnay Wines. J. Agric. Food Chem. 51(2), 443–

452 (2003).

2. M. Aznar, R. Lopez, J. Cacho, and V Ferreira, Prediction of

Aged Red Wine Aroma Properties from Aroma Chemical

Composition. Partial Least Squares Regression Models. J.

Agric. Food Chem. 51(9), 2700–2707 (2003).

3. M. Bucking and H. Steinhart, Headspace GC and Sensory

Analysis Characterization of the Inﬂuence of Different

Milk Additives on the Flavor Release of Coffee Beverages.

J. Agric. Food Chem. 50(6), 1529–1534 (2002).

4. A. Escudero and P. Etievant, Effect of Antioxidants on the

Flavor Characteristics and the Gas Chromatography/Olfacto-

metry Proﬁles of Champagne Extracts. J. Agric. Food Chem.

47(8), 3303–3308 (1999).

5. C. Berger, N. Martin, S. Collin, L. Gijs, J. A. Khan, G.

Piraprez, H. E. Spinnler, and E. N. Vulfson, Combinatorial

Approach to Flavor Analysis. 2. Olfactory Investigation of a

Library of S-methyl thioesters and sensory evaluation of

Selected Components. J. Agric. Food Chem. 47(8), 3274–

3279 (1999).

6. R. Desrochers, P. Keane, S. Ellis, and K. Dowell, ‘‘Expanding

the Sensitivity of Conventional Analytical Techniques in

Quality Control Using Sensory Technology,’’ Food Qual.

Pref. 13, 397–407 (2002).

7. T. Aishima, Correlating Sensory Attributes to Gas Chromato-

graphy–Mass Spectrometry Proﬁles and e-Nose Responses

Using Partial Least Squares Regression Analysis. J. Chroma-

togr. A 1054(1–2), 39–46 (2004).

8. S. J. Lee and A. C. Noble, ‘‘Characterization of Odor–Active

Compounds in Californian Chardonnay Wines Using GC–

Olfactometry and GC–Mass Spectrometry,’’ J. Agric. Food

Chem. 51(27), 8036–8044 (2003).

9. M. Huber, J. Ruiz, and F. Chastellain, ‘‘Off-Flavour Release

from Packaging Materials and Its Prevention: A Foods Com-

pany’s Approach,’’ Food Addit. Contam. 19(Suppl), 221–228

(2002).

10. M. Hirschfelder, A. Forster, S. Kuhne, J. Langbehn, W.

Junghanns, F. G. Pank, and D. Hanrieder, Using Multivariate

Statistics to Predict Sensory Quality of Marjoram from

Instrumental Data. Sensors and Actuators B: Chemical

69(3), 404–409 (2000).

11. T. Hartman, Determination of Off-Odors and Other Volatile

Organics in Food Packaging Films by Direct Thermal Analy-

sis-GC-MS, Scientiﬁc Instrument Services Application Note

1a, updated 1999, CAFT/New Jersey Agricultural Experi-

mental Station Publication # E-10570-1-90.

12. J. Manura, ‘‘Detection and Identiﬁcation of Volatile and Semi-

Volatile Organics in Synthetic Polymers Used in Food and

Pharmaceutical Packaging,’’ LC/GC 11(2), 140–146 (1993).

OXYGEN SCAVENGERS

STANISLAV E. SOLOVYOV

Multisorb Technologies, Inc.,

Buffalo, New York

Oxygen scavengers comprise the most commercially im-

portant subcategory in active packaging applications and

Figure 2. Thermal Desorption/Analysis System.

OXYGEN SCAVENGERS 841

are a part of a much broader category of absorbers or

sorbent materials for gases, vapors, liquids, and volatile

organic compounds. Depending on their physicochemical

nature, such absorbers can either reversibly or irreversi-

bly remove an undesirable gaseous or liquid species from

environments contained within a package. Common re-

versible absorption processes include physical adsorption

of gas molecules to solid surfaces, bulk dissolution, and

molecular diffusion of mobile species in solids and liquids.

Mobile species absorbers, whose action is irreversible at

the conditions of use, are generally referred to as scaven-

gers of such species. Irreversible absorption (scavenging)

generally proceeds via chemical reactions of the mobile

species with the components of a scavenger system to form

stable compounds. Oxygen scavenging processes include,

for example: oxygen chemisorption, catalyzed oxidation of

hydrogen gas to form water: catalyzed oxidation of ethy-

lenically unsaturated hydrocarbons and organic polymers

to aldehydes, ketones, and carboxylic acids; and oxidation

of reduced transition metals to form stable oxides.

Molecular oxygen (O

) is a highly reactive diatomic gas

that forms about 20.9% of the earth atmopshere (1) and is

necessary for sustaining organic lifeforms, but at the same

time it causes many types of food and beverage spoilage,

degradation of pharmaceutical formulations, and oxida-

tion and corrosion of metals, electronic components, and

various other products. Internal volume of any package is

necessarily limited and contains a limited amount of

oxygen; therefore it is possible to reduce or essentially

eliminate the oxygen present inside the package by using

oxygen scavengers. In food packaging, removal of oxygen

trapped inside the package allows us to control aerobic

microorganisms such as mold and many bacteria, slow

down enzymatic activity, and reduce detrimental color and

ﬂavor changes due to oxidative deterioration (2, 3). The

use of individually packaged oxygen scavenger formats,

which are inserted in a package but do not form a part of

food formulations, also allows food processors to reduce or

eliminate less desirable direct food additives such as

antioxidants and preservatives.

Individually packaged oxygen scavengers are produced

in the form of packets or sachets containing a sacriﬁcial

compound that is formulated to rapidly oxidize and

thereby eliminate free oxygen by chemically binding

with it before it reacts with the packaged product (4). In

addition to sachets, oxygen-scavenging internal labels

applied with pressure-sensitive adhesive backings and

oxygen-scavenging liners in bottle caps and crowns are

marketed. Oxygen-scavenging polymeric ﬁlms that ap-

peared in the early 1990s allow us to eliminate loose

sachets from the package and add an oxygen-scavenging

functionality to the entire packaging structure or its part.

The resulting hermetically sealed packages have a dual

function, simultaneously providing for oxygen removal

from the package interior and preventing ingress of atmo-

spheric oxygen through otherwise permeable container

walls by forming a reactive barrier to oxygen permeation.

Applications of reactive barrier technology include oxygen

scavenging plastic bottles for beer, juices, ketchup, and so

on, pharmaceutical blister packs, plastic lidstocks, and

thermoformed food trays.

The majority of all oxygen scavengers in commercial use

are based on the oxidation of iron powders, although

systems based on other oxidizable transition metals such

as copper, zinc, magnesium, manganese, aluminum, tita-

nium, and so on, have been proposed and patented. In

recent years, multiple patents have been issued, describing

organic oxygen scavenging chemistries with and without

use of metal-based oxidation catalysts, speciﬁcally for the

use as active barriers in plastic packaging applications. In

this case the low oxygen permeability of polymer matrix is

combined with high reactivity and substantial reactive

capacity of oxidizable substrate to create an efﬁcient and

long-lasting active barrier to oxygen permeation (5). Or-

ganic oxygen scavengers are often preferable due to their

transparency and compatibility with polymeric matrices

that make it possible to produce optically clear oxygen

barrier ﬁlms and structures. Several formulations have

been claimed as having zero effective oxygen transmission

rates through the reactive barrier ﬁlms for periods ranging

from 1 to 3 months up to 2 years, thus outperforming glass

packaging with essentially permeable seals and closures.

In such cases the essentially zero oxygen transmission

rates are achieved for reactive barrier systems character-

ized by a high degree of diffusion control of the overall

forward rate of oxygen scavenging reaction. Diffusion-

controlled reactions refer to the reactions of dissolved

oxygen with the scavenging agent immobilized in a solid

matrix, where the rate of oxygen removal by the reaction is

much faster than the rate of oxygen diffusion through the

solid matrix of the barrier layer (6).

Performance and packaging requirements to enclosed

oxygen savengers differ substantially from those for re-

active barrier applications. Enclosed scavenger formats

are designed for a controlled mode and rate of activation,

rate of oxygen absorption, and speciﬁc overall reactive

capacity to absorb oxygen. Scavenging chemistries that

are initially stable and inactive but can be activated on

demand are preferable because they reduce the possibility

of premature reaction and loss of scavenger activity before

application. Scavenging systems are generally designed to

be activated by external ﬁelds and penetrating agents

such as UV radiation (photoreducible organic compounds)

or moisture diffusion (oxidation of transition metals).

Scavengers that are immediately active upon their man-

ufacture are less desirable since they generally require

high levels of protection from environmental oxygen be-

fore use. To rapidly deoxygenate a package, an oxygen-

scavenging chemistry has to be readily accessible to both

activating agent and oxygen. Therefore the scavenger

packets are often made from microperforated or otherwise

highly permeable ﬁlm substrates. Numerous sizes and

formats of packaged oxygen scavengers with varying

oxygen absorption capacity and activated at different RH

levels have been developed for speciﬁc applications.

Oxygen-scavenging capacity of the packaged scavenger

unit as well as the mode and rate of its activation are

critical for packaging system design. The rate of oxygen

absorption by the scavenger, determined by its format and

chemistry, and overall oxygen transmission rate (OTR) of

the package wall determine the minimal attainable oxy-

gen concentration in the package headspace. OTR of the

842 OXYGEN SCAVENGERS

package, together with the overall reactive capacity of

the scavenger, control the overall duration of protection

from ingress of ambient oxygen. In well-designed oxygen-

scavenger/package systems, attaining and maintaining

100 ppm or less of residual headspace oxygen is consi-

dered to be the norm. At 100 ppm, effective control

of the detrimental effects of oxygen on foods has been

demonstrated (7). Oxygen scavengers are often used in

conjunction with modiﬁed atmosphere packaging (MAP)

techniques, such as vacuum packing and ﬂushing head-

space with inert gas before sealing, to reduce residual

oxygen amounts. The gas ﬂush reduces the scavenger

reactive capacity requirement per package and is a more

economic means of reducing the oxygen down to as little as

0.5%, but ﬂushing alone is not capable of reaching the

oxygen levels possible with the use of the scavenger (4).

The oxygen-scavenging technologies constitute a major

part of active packaging techniques and methods, and they

have been extensively patented worldwide, especially in

Japan. Commercial use of oxygen scavengers started in

1976 in Japan and in 1984 in the United States. UK-based

consulting and media concern Pira International Ltd.

estimated the global value of the total active packaging

market in 2005 to be worth more than US$1.5 billion and

has forecasted this market to be worth more than US$2.6

billion in 2010. The global market for packaged oxygen

scavengers was estimated to be 12 billion units in Japan,

500 million in the United States, and 300 million in

Western Europe in 2001. This market was forecast to

grow to 14.4 billion units in Japan, 4.5 billion units in

the United States, and 5.7 billion units in Western Europe

in 2007.

HISTORY

In 1869 Virgil Blanchard of Vermont obtained a U.S.

patent for ‘‘improvement in preserving fruits, meats, and

other substances’’ (8) by placing an insert with an alkaline

solution of pyrogallic acid into an air-tight vessel. Removal

of both headspace oxygen and oxygen leaked through the

seals was claimed via reaction of pyrogallic acid with

oxygen. In 1938 Henrik Tallgren of Finland was granted

a British patent for ‘‘keeping food in closed containers with

water carrier and oxidizable agents such as zinc dust, iron

powder, manganese dust, etc.’’ (9). F. Isherwood of the Low

Temperature Research Station at University of Cam-

bridge obtained a patent for ‘‘removing oxygen from a

container containing vacuum or gas packed food in which

a metal absorbs oxygen to form an oxide’’ in 1943 (10). This

early activity was followed by attempts to use powdered

metals to purify industrial gases and the development of a

system to scavenge oxygen by use of palladium catalysts

in a hydrogen-ﬂushed package by American National Can

in the 1960s. In 1969, Mr. Fujishima attempted to market

a hydrosulﬁte-based oxygen scavenger that reacted very

quickly, causing problems with lost capacity, and also had

a propensity to react with food volatiles to form noxious

sulfur-containing gases. None of these early developments

resulted in any signiﬁcant commercial activity. Mitsubishi

Gas Chemical Co. (MGC) of Tokyo, Japan has been issued

a patent for a ferrous-iron-based scavenger formulation in

1977 and was the ﬁrst company to start marketing of their

Ageless

brand sachets in 1976.

The early success of oxygen scavengers in Japan ap-

pears to be tied to the tradition of gift-giving and the

importance of ﬂawless presentation and quality (11),

especially in the hot and humid summer climate of Japan.

When the gift was a food item, the quality assurance

provided by an oxygen scavenger was a reasonable pre-

caution. Several confectionery items given as traditional

gifts also suffered from short shelf life due to mold growth.

For foods such as Japanese rice cake, the shelf life and

quality improvements afforded by a combination of a

barrier package and an oxygen scavenger allowed for a

longer distribution cycle and made Ageless

a commercial

success. Competitors followed in Japan and other Asian

countries, and today there are more than a dozen,

although MGC is still leading the industry. Metal-based

oxygen scavengers are not the only option, because many

organic scavenging systems have been proposed with the

goal to alleviate the possibility of food contamination by

metallic compounds, prevent setting off metal detectors,

and allow making microwaveable containers. Toppan

Printing Co. has marketed an ascorbic-acid-based oxygen

absorber since 1979. A catechol-based scavenger has been

marketed by Oji Kako Co. under the Tamotsut name.

Since early 1990s, many scavenger-based oxygen barrier

ﬁlms and structures have been developed with the goal of

preventing oxygen ingress into the sealed package.

In the United States, oxygen scavengers were ﬁrst used

in 1984 when General Foods began to ship ground roast

coffee with their ‘‘Fresh Lock,’’ an oxygen scavenger for-

mulation supplied by MGC. Multiform Desiccants Inc.

(currently Multisorb Technologies, Inc.) of Buffalo, New

York entered the U.S. market with their Freshpax

brand

oxygen scavengers in 1988 and an initial application for

‘‘Meal, Ready-to-Eat’’ (MRE) rations as developed by the

U.S. Army Natick Research, Development and Engineering

Command. The Aquanautics Corp. introduced ‘‘SmartMix

and SmartCup’’ technology for compounding into packa-

ging such as bottle-cap liners in the late 1980s (12). In

addition, CMB Technology (France) introduced their Ox-

bar

oxygen barrier system, which used a cobalt salt

catalyst to accelerate oxidation of MXD-6 nylon blended

with PET (13). The original Oxbar

did not become a

commercial technology; however, its later reincarnations

as MonOxbar

and optically clear DiamondClear

tech-

nology, currently a part of Constar International (USA)

portfolio, have gained signiﬁcant publicity and resulted in

several commercial applications.

A wide acceptance of oxygen scavengers by the Japa-

nese food industry has not yet been repeated in other

markets. Food is more expensive in Japan than in the

United States; thus the cost of an oxygen scavenger versus

the value of preservation afforded a greater incremental

packaging expense in the Japanese market. The U.S. food

industry has had other reservations, especially regarding

the liability of loose scavenging sachets and potential

food contamination by their contents. The acceptance is

growing in the United States and Europe as the technol-

ogy has matured and adapted to speciﬁc applications that

OXYGEN SCAVENGERS 843

make economic sense in the U.S. and EU markets. Nota-

bly, food stores at Walmart supercenters in the United

States started switching from high-oxygen packaging for

case-ready meats to low-oxygen master packs with en-

closed oxygen scavengers in late 2007 to improve preser-

vation and shelf life of red meats such as beef steaks and

ground beef.

SPOILAGE CONTROL WITH SCAVENGERS

Preservation of foods and other products with the use of

an oxygen scavenger will be improved only when an

oxygen-related spoilage is the primary pathway of qual-

ity deterioration. The economic signiﬁcance of the var-

ious spoilage types attributable to oxygen also varies and

should be factored into any decision to employ a scaven-

ger. In situations where control of oxygen is warranted,

the observable advantages include greatly reduced losses

and returns due to spoilage, improvement in consumer

conﬁdence and relations, increased distribution radius

due to a longer shelf life with correspondingly increased

sales expectations, and improved plant scheduling due

to longer product runs taking advantage of the longer

shelf life.

Primary use of an oxygen scavenger is to control

aerobic microorganisms. While both MAP and vacuum

packaging have the capability to aid in control of aerobic

bacteria and mold, it has been reported that mold colonies

may grow in even 0.4% residual oxygen in as little as 10

days (7). Both vacuum and MAP techniques may not be

able to effectively deliver or maintain such low initial

residual oxygen concentrations, although an appropriate

concentration of carbon dioxide may also effectively sup-

press mold growth. However, Powers and Berkowitz

(14) noted that an oxygen absorber prevented mold

growth in specially formulated and packaged bread for

up to 3 years, and oxygen scavengers are also reported to

control mold growth on crusty rolls for at least 60 days

(15). In commercial practice, use of an oxygen scavenger to

control mold is a major use in Japan but has been of minor

importance elsewhere in the world. Use of barrier packa-

ging for baked goods appears to be more prevalent in

Japan than in the United States, and oxygen scavenger

use makes sense only where a suitable barrier to oxygen

permeation is employed. In the United States, the MRE,

beef jerky, and fresh pasta industries have adopted pack-

aged oxygen scavengers to control mold growth on their

products. Permeation of oxygen into the package over

time, which leads to mold growth, is the major concern

that can be addressed with either enclosed scavengers or

reactive barrier packaging.

Another oxygen related mode of food spoilage is

through oxidation of various pigments in the food itself

(16). This generally takes the form of a darkening

of color—for example, due to enzymatic browning reac-

tions (17). While color change does not in and of itself

make the food product inedible, consumers appear to

perceive that such darkening is an effective indicator

of other spoilage problems that may have occurred simul-

taneously. Therefore, such pigment darkening may

effectively end the shelf life of the packaged product.

The smoked/processed meat industry is especially aware

of the problems of discoloration and consumer reluctance

to purchase packages exhibiting a color darker than

other packages in the same retail case. Oxygen scaven-

gers, more than any other packaging method, excel at

reducing headspace oxygen to such low levels that pig-

ment oxidation does not occur for extended periods of time

(16, 18). Color preservation is often cited as a reason for

using oxygen scavengers in the United States and

Europe.

Another mode of oxygen related deterioration of foods is

through oxidation of fats and oils in the product (7, 16, 19).

This process is commonly referred to as the rancidity of

lipids and yields organoleptic changes in them that are

usually considered undesirable even though rancidity is

commonly accepted in some food products such as fried ﬁsh

and nutmeats. The byproducts of this type of oxidation may

include peroxides, aldehydes, ketones, and carboxylic acids.

The presence and undesirable odor of many aldehydes and

ketones is detected by humans at levels well below 1 ppm,

and this fact presents a major problem for food processors.

Use of an oxygen scavenger has been shown to positively

control this type of oxidation (20) because peroxide values

have remained essentially unchanged over time in

packages protected by the scavenger. Conversely, reduction

of headspace oxygen to the low ppm range tends to preserve

vitamin and nutrient content of foods.

Another example of a type of food spoilage, related to

oxygen in the package headspace, is insect infestation.

Many grains and ﬂour products suffer losses due to weevils

and the like, and it has been shown that these can be

controlled by use of an oxygen scavenger (21). While an

infestation may have been suppressed during the storage

of the bulk grain, either a later hatch or a reinfestation

may occur during packaging or distribution. Insect eggs,

larvae, or pupae may be present, but all need some amount

of oxygen to survive. Elimination of the headspace oxygen

will eventually kill all forms of the infestation and prevent

further damage, although it may take as long as 12 days to

kill the eggs of some of the more resilient insects. There

have been limited uses of this concept in the United States

for bird seed and preservation of museum artifacts, but it is

an accepted practice in Japan.

When the product to be protected is not a food product,

oxygen scavengers can also provide signiﬁcant beneﬁts.

Some pharmaceutical formulations are easily degraded by

oxygen: In particular, vitamin C, vitamin E, and b-car-

otene employed as antioxidants are commonly packaged

with an oxygen scavenger. Many newer hormone replace-

ment therapies and similar drugs are unstable in the the

presence of oxygen and will not meet common pharma-

ceutical shelf-life requirements without reactive oxygen

barrier packaging. Closely related to the food industry,

pet food and pet snacks are often protected with an oxygen

scavenger

as rancidity and

mold appear to be a problem

for pets. Protection of metals from corrosion in demanding

applications such as electronics often requires the use of

oxygen scavengers. In the meantime, the food industry

remains the dominant worldwide consumer of oxygen

scavengers.

844 OXYGEN SCAVENGERS

CHEMISTRY

Applications of scavenging technology are complex pri-

marily because of different chemistries involved, their

activation mechanisms, the endpoints of oxidation, the

reaction byproducts, and the interactions between the

scavenger and the product. The iron-based oxygen sca-

vengers are the most prevalent and they are examined

ﬁrst.

The iron technology is based on rapid oxidation of iron

powders to prevent headspace oxygen from reacting with

foodstuff components or to reduce oxygen permeation

rates through container walls. Reduced iron powders

(zero-valent iron Fe

) are highly reactive and possess a

higher capacity to react with oxygen than partially oxi-

dized ferrous iron (Fe

) compounds. Large exposed sur-

face area of iron powders per unit weight, which increases

with reduction in mean particle size and especially when

using highly porous ‘‘sponge’’ iron morphologies, promotes

simultaneous oxidation at multiple reactive sites, result-

ing in faster rates of oxygen scavenging. Presence of

electrolytes and an aqueous interface to facilitate electron

transfer during oxidation is known to accelerate oxidation

reactions of transition metals. This is exempliﬁed by rapid

rusting of ferrous metals and steels exposed to road salt

and salted seawater, in contrast with slow rates of corro-

sion in the dry climates. Ferrous and ferric iron oxides and

hydroxides produced by oxidative reactions are basic. As a

result, acidic environments promote oxygen scavenging by

shifting equilibrium to iron oxides. At low pH values,

oxidation of reduced iron proceeds in humid environments

even in the absence of oxygen, by deoxygenizing water

molecules and resulting in hydrogen gas formation.

Since low pH values and aqueous interfaces require the

presence of water, water vapor diffusion and variation in

relative humidity of the environment can be used to

control activation of oxygen scavenging capacity of iron

powders. The speciﬁc formulations are designed to trigger

the activation at desired RH levels, control the reaction

rate, achieve the full oxidative potential of the iron

powder, maintain compatibility with various products,

and ensure food safety. The reaction formula for complete

oxidation of reduced iron to eventually form ferric oxide

trihydrate complex (commonly known as rust) is

4Fe

þ 3O

þ 6H

O ! 4FeðOHÞ

! 2ðFe

 3H

OÞ

Many transition metals are recognized as being sufﬁ-

ciently oxidative to become candidates for an oxygen-

scavenger formulation; however, iron does offer unique

advantages that have driven the industry to general use

of this medium. Iron has a relatively high afﬁnity to

combine with oxygen on a per unit weight basis com-

pared to most alternatives. Complete oxidation of 1 g of

reduced iron removes 300 cm

of oxygen at STP condi-

tions. Elemental candidates that exceed it have draw-

backs such as the odor problems with the use of sulfur

and the propensity of aluminum to form an almost

impermeable oxidized skin layer that limits further

oxidation. Iron rust, on the other hand, ﬂakes off as it

forms and it is highly permeable to further duffusion of

water and oxygen. Of great importance is the food safety

of iron powders, which are recognized as nutrients

important for healthy blood and commonly used as food

enrichments in milled ﬂour, breads, breakfast cereals,

and baby foods. The same cannot be said about copper,

zinc, aluminum, and many other transition metals. The

abundance and relatively low cost of producing elemental

iron are also important, especially in comparison to

choices such as palladium and platinum metals and

catalysts based on them. Then there is the ready ability

to manipulate reactive capability of iron powder formula-

tions to adapt to a wide variety of applications for both

the rate of oxygen scavenging and the activation by the

moisture in a package headspace. One of the few draw-

backs is that the oxidation of iron is a temperature-

dependent reaction and normally slows dramatically as

the temperature approaches freezing.

Electrolytes and acidifying agents in a scavenging

formulation are commonly supplied by metallic salts

(22). The practical considerations of food safety have

dictated that common table salt (NaCl) is often the best

choice for the electrolyte, while various metal and organic

salts of strong acids are often used to increase the acidity

of the formulation. Another important factor in oxygen

scavenging chemistry is the desired control of activation

or triggering the scavenging reaction on demand in every

application. When the packaged product does not have a

required water activity to fully activate an iron-based

scavenger and provide the desired reaction rate, premois-

turized silicas, zeolites, and activated carbons are used as

self-contained water carriers in various proprietary for-

mulations to provide a reliable activation.

There are two broad categories of iron-based scavenger

formats, commonly described as ‘‘moisture-dependent’’

type and ‘‘self-reaction’’ type. For the moisture-dependent

chemistries, either no moisture or insufﬁcient moisture is

present in the sachet or label formulation and the chem-

istry must have the capability to absorb moisture from the

headspace of the package. A signiﬁcant advantage of these

formulations is that they are stable in ambient environ-

ments until sufﬁcient moisture has been absorbed to allow

reaction to occur. The package into which they are inserted

must provide a moisture source, which limits the use of

these formulations to food products with relatively high

water activity. For the self-reaction type, the necessary

moisture is added right into the sachet on a suitable

carrier. These carriers must carry a high percentage by

weight of moisture and must bind the moisture strongly

enough so that the formulation powder remains free-

ﬂowing, yet they must release the moisture as needed for

the reaction. This type of formulation has to be protected

from environmental exposure because it will start scaven-

ging oxygen immediately and can exhaust its capacity if

exposed for extended periods. An additional disadvantage

is that the preloaded moisture is subject to desorption and

equilibration in the package over time, which means that

with extremely dry products, the moisture will leave the

sachet. The loss of preloaded moisture means that the

oxygen scavenger will eventually stop working even

though it may still contain unoxidized iron.

OXYGEN SCAVENGERS 845

It should be noted that since oxygen scavengers are

routinely used with MAP, there will be situations in which

the application dictates gas ﬂushing of the package head-

space with gas mixtures containing carbon dioxide. This is

an area for caution because many iron-based formulations

are unsuitable for use with carbon dioxide because they will

absorb it preferentially over oxygen. Many of these formu-

lations have a capacity for about ﬁve times as much CO

and will thus fail to remove enough oxygen to prevent

spoilage. Furthermore, these formulations can also reduce

the beneﬁcial effects of carbon dioxide by reducing its

intended concentration in the headspace. However, formu-

lations are available that both scavenge oxygen and release

carbon dioxide (CO

emitters), and one of these should be

used when gas ﬂushing with CO

is used.

Sulﬁtes, bisulﬁtes, and their analogues have been

suggested and used as oxygen scavengers for a long time

(23). Sulﬁtes are commonly added to bottled wines to

preserve their quality. Desirable oxygen scavenging action

is produced by oxidation of sulﬁtes to sulfates. The reac-

tive capacity of such scavengers is rather small to be used

as a reactive ﬁller in barrier structures, therefore, their

use was limited to sachets and liquid sulﬁte layers (24).

Use of iron-based oxygen scavengers can create pro-

blems with metal detection because there is enough iron in

most sachets and labels to set off a metal detector. This is

generally overcome by using metal detectors just before

the scavenger insertion to assert that no extraneous metal

is present and then another just after to assert that the

scavenger has been added. In situations where this has

not been possible, organic oxygen scavenging chemistries

can be successfully used. Organic scavengers are also

beneﬁcial for making polymeric barrier ﬁlms and struc-

tures due to often improved compatibility with matrix

resins and higher achieveable levels of dispersion com-

pared to dispersion of inorganic powders, generally inso-

luble in plastics.

Ascorbic acid and other ascorbates are known organic

antioxidants because they oxidize to dehydroascorbic acid

in the presence of transition metal catalysts, preferably

copper. Pillsbury Co. (now a part of General Mills, Inc.)

holds a patent to use ascorbic acid as a reducing agent. A

combination of ascorbates and sulﬁtes was used as an

oxygen scavenger embedded in plastic closure liners by

the Darex Container Products division of W. R. Grace &

Co. The technology has only achieved minor acceptance in

the United States, but it does have unique advantages and

disadvantages. Due to a lower capacity to scavenge oxy-

gen, it generally requires a larger sachet and weight of

reactant to accomplish the same oxygen removal as for the

iron-based technology. Carbon dioxide is also released as a

byproduct of oxidation.

Other proposed organic oxygen scavengers included

gallic acid, n-propyl gallate, butylated hydroxyanisoles

(BHA), butylated hydroxytoluene (BHT), and dihydro-

guaretic acid, which can serve as reactive barriers when

added to plastic packaging structures rather than being

used as food additives. Migration of these low-molecular-

weight chemicals as well as byproducts of their oxidation

into the product cannot be completely prevented. To over-

come this problem and increase the available activated

capacity of scavengers, more recent developments focused

on oxidizable polymers that can be used as barrier resins

or blended with common packaging polymers to create

reactive barriers to oxygen permeation. Ethylenic unsa-

turation in linear hydrocarbon polymer backbones and

pendant groups as well as in polyunsaturated fatty acids

is subject to auto-oxidation, especially in the presence of

transition-metal-based catalysts:

2CH ¼ CH2 þ O

! 2CH ¼ CðOOHÞ2

The resulting hydroperoxides can decompose to produce

free radicals, which can in turn combine by crosslinking,

can initiate free radical polymerization of other double

bonds if enough of them are present, or can cleave the

double bonds in the course of successive oxidation reac-

tions. Such reactions can be catalyzed by salts of transi-

tion metals (e.g., cobalt is most common). Rearrangement

and cleavage of the hydroperoxides to aldehydes and

ketones result in low-molecular-weight byproducts that

adversely affect odor and ﬂavor of packaged foods in very

low concentrations. These effects limited the use of un-

saturated hydrocarbons in food packaging. Oxygen

Scavenging Polymer (OSP) developed by Chevron-Phillips

Chemical Co. gets around this problem by including the

double bond in a pendant cyclohexene ring (25). This way

pendant carboxylic acid groups are formed on the polymer

backbone as a result of oxidation rather than free low-

molecular-weight byproducts.

Rapid rates of oxidation of certain polyamides in the

presence of transition metal catalysts have been exploited

to produce reactive oxygen-scavenging barriers. MXD-6

nylon with a cobalt salt catalyst has been the primary

system patented by MGC and is targeted for incorporation

into the inner layers of polymeric packaging structures.

Having a large reactive capacity and high reaction rates,

MXD-6 nylon-based scavengers can provide nearly zero

oxygen transmission rates through nylon barrier layers.

However, their use is limited by layer delamination issues,

the potential migration of low-molecular-weight reaction

byproducts into the packaged product, relatively high cost,

and a lack of regulatory approval for direct food contact.

Oxygen-scavenging barrier systems based on MXD-6

nylon and cobalt salts have experienced some limited

success when multiple producers started blending them

as minor additives to reduce migration and improve

barrier properties of PET and other packaging resins.

CSIRO Australia has introduced photosensitive dyes as

oxygen scavengers independent of transition metal cata-

lysts (26). Functionalized anthraquinone-based chemis-

tries can act as singlet oxygen acceptors after being

excited by UV light. Excited dye molecules sensitize

ground-state oxygen molecules that diffuse through a

polymer in a singlet state. Singlet O

molecules react

with the dye and are consumed. ZerO

is a CSIRO line

of UV-activated oxygen scavengers incorporated into re-

active barrier ﬁlms. Despite lower capacity than that

provided by metal-based scavengers, ZerO

scavengers

have advantages such as efﬁcient low-energy activation on

demand and their reactivity being independent of moist-

ure diffusion and transition metal based catalysts.

846 OXYGEN SCAVENGERS

Biochemical reactions in living cells are accelerated by

biologically speciﬁc organic catalysts called enzymes. Cer-

tain enzymes can function as oxygen scavengers within

barrier structures. To act as scavengers, enzymes must be

ﬁrst immobilized within the polymer matrix. Although

many enzyme candidates can be thought of as oxygen

absorbers, few systems have been attempted in practice.

Oxygen removal by a combination of glucose oxidase and

catalase is a primary example (27). It should be noted that

oxygen-scavenging enzymes and coupled enzyme systems

cannot act without water and are very sensitive to

changes in pH, water activity, temperature, salt content,

and other factors. On the other hand, common thermo-

plastics such as polyethylene and polypropylene are good

substrates for enzyme immobilization. Stability of immo-

bilized enzymes in the ﬁlm is not fully analyzed. Other

food-grade enzymes can potentially be used for oxygen

scavenging: One example is ethanol oxidase, which oxi-

dizes ethanol to acetaldehyde. Bioka Ltd. of Finland

commercially produces enzyme-based scavenging sachets.

Some organometallic complexes known as ligands are

capable of trapping free oxygen in a manner similar to

that of blood hemoglobin (containing Fe

ligand in heme)

and other peptides with metal-containing prosthetic

groups. When immobilized on a solid support such as

silica, these ligands have been demonstrated to form a

highly reactive oxygen-scavenging medium (28). Such

systems anchored to an insoluble support can be dispersed

in polymer matrices as powders to form reactive barriers.

Advanced Oxygen Technologies, Inc. (New York, NY) has

developed a series of oxygen scavengers based on synthe-

tically produced ligands. Incorporation of ligands into

plastic ﬁlm structures has also been claimed.

APPLICATIONS

Individually packaged oxygen scavenger formats are now

found in multiple applications with packaged foods being

the primary use. Fresh refrigerated pasta is a major user

of oxygen scavengers to maintain quality during its dis-

tribution cycle. The smoked/processed-meat industry has

several segments that rely on oxygen scavengers to main-

tain color or prevent mold during distribution. Both pre-

sliced pepperoni in thermoformed packages and bacon bits

in glass and plastic jars use an oxygen scavenger for color

retention. In Great Britain and the United States, sliced

deli meats are often produced with very low levels of

preservatives and are thus susceptible to rapid discolora-

tion under retail lighting. These are now increasingly

packaged with oxygen scavengers to prevent discoloration

for up to one month. Beef jerky is commonly packaged

with an oxygen scavenger for reasons of both color reten-

tion and prevention of mold growth. Case-ready meat

master packs often include oxygen scavengers to extend

distribution time through deoxygenation. The individual

cases are then exposed to ambient oxygen before placing

them on the shelves to reoxygenate the meat and restore

its original red color. Roasted nuts are often protected

from rancidity by scavengers, especially where the pro-

duct will be used for export or for upscale packages. The

MRE military rations remain a major U.S. application that

is being adopted by other military organizations around

the world. Freeze-dried products often have an elevated

oxygen sensitivity, and items such as backpackers’ trail

foods are now packaged with scavengers. Pizza crusts are

also being packaged for some markets with a scavenger to

prevent mold during fresh distribution. Baked goods and

confections are often packaged with scavengers to prevent

mold. Oxygen-scavenging liners in juice and wine bottle

caps have been introduced with limited success in some

markets. Oxygen scavengers with simultaneous humidity

control (e.g., StabilOx

from Multisorb Technologies) are

used for protection of pharmaceuticals, dietary supple-

ments, diagnostic tests, and other medical supplies. Other

uses include prevention of metal corrosion in archeology,

art storage, and electronic components, as well as con-

servation of oxygen sensitive products such as rubber,

plastics, and greases.

Reactive oxygen barrier packaging is another rapidly

growing market. Beer, wine, and fruit juices in PET

bottles, ketchup in plastic containers, and oxygen-sensi-

tive pharmaceuticals and nutraceuticals in oxygen-

scavenging blister packs are some of the current and

emerging applications. Sulfur-containing components in

beer originating from hops are particularly subject to

oxidation and, when oxidized, adversely change beer taste

proﬁle. Reaction with 1–2 ppm of oxygen is often enough to

cause a beer taste unacceptable to many consumers.

Protection of beer in plastic packaging is a major applica-

tion of oxygen-scavenging barrier technology.

PRODUCTS

Sorted by application format, the following oxygen-

scavenging products have been offered by major

manufacturers:

Sachets and Labels. Ageless

sachets and labels (Mit-

subishi Gas Chemical Co., Japan), Freshmax

labels,

Freshpax

and StabilOx

sachets (Multisorb Technolo-

gies, USA), ATCO

sachets and labels (Standa Industrie,

France), O-Buster

sachets (Desiccare, USA), Bioka sa-

chets (Bioka Ltd., Finland), Freshilizer

sachets (Toppan

Printing Co., Japan), Keplont sachets (Keplon Co., Ltd.

Japan), Tamotsut sachets (Oji Kako, Japan), Oxyeater

sachets (Ueno Seiyaku, Japan), Vitalon

sachets (Toago-

sei Chemical Industry Co., Japan), Seculet (Oxysorbt in

the U.S.) sachets (Nippon Soda Co., Japan), Sansocutt

sachets (Finetec Co., Japan), and several other Japanese,

Korean, and Chinese sachet producers.

Closure Liners. Pureseal

(W. R. Grace and Co., USA),

DarExtend

(Darex Container Products, USA), Tri-

SO2RB

(Tri-Seal International, USA), Oxycap

bottle

crowns (Standa Industrie, France).

Oxygen Scavenging Plastic Packaging Materials. OS

2000

(Cryovac, USA), OSP

(Chevron-Phillips, USA),

Shelfplus

(Ciba Specialty Chemicals Corp., Switzer-

land), Oxyguard

(Toyo Seikan Kaisha Ltd., Japan),

OXYGEN SCAVENGERS 847