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

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heightened perception of environmental issues, and reg-

ulations in these countries are being formulated at a

faster rate than in the history of the United States. As

printing ink becomes a global business, the successful

marketing of ink technology now requires attention to

an expanding body of worldwide regulations.

BIBLIOGRAPHY

R. W. Bassemir and A. J. Bean, ‘‘Inks’’ in The Wiley Encyclopedia

of Packaging, 2nd edition, pp. 511–514, Wiley, New York.

Cited Publications

1. R. W. Bassemir and A. J. Bean, ‘‘Inks’’ in The Kirk-Othmer

Encyclopedia of Chemical Technology, 5th edition, Wiley,

Hoboken, NJ, 2004.

2. B. Szabo, in D. Urban and K. Takamura, Polymer Dispersions

and Their Industrial Applications, Wiley-VCH, Weinheim,

2002, Chapter 5.

3. W. Herbst and K. Hunger, Industrial Organic Pigments, Wiley-

VCH, Weinheim, 2004.

General References

The Printing Ink Manual, 5th edition, Kluwer Academic, London,

1999.

C. Finley, Printing Paper and Inks, Van Nostrand Rheinhold,

New York, 1997.

R. Schwalm, UV Coatings, Elsevier, Amsterdam, 2007.

Ultraviolet/Electron Beam Curing Formulation for Printing Ink,

Coating and Paints, 3 Volumes, SITA Technology, London,

1991.

INTELLIGENT INKS IN PACKAGING

ANDREW MILLS

Department of Pure and

Applied Chemistry, University

of Strathclyde, Glasgow, UK

INTRODUCTION

The traditional roles fulﬁlled by packaging are protection,

containment, communication, and convenience, and the

ability to address these different roles has improved en-

ormously over the years (1). In packaging, the role of

communication is largely affected using printed inks, which

provide such diverse information as product and producer

names, ingredients, price, sell-by date, and bar code.

Although it is possible to print directly on plastics, as well

as paper, the latter is usually cheaper; thus, on primary

packages such information is usually conveyed via an

exterior self-adhesive label, comprising inert inks (i.e.,

oblivious to their environment) and providing information

that does not change during the lifetime of the package.

Nowadays, there is increasing interest across the food

packaging chain—from packager to wholesaler and from

retailer to end-user—for reactive, printable inks, that is,

intelligent, smart, or diagnostic inks that do respond to

their environment, for example, by exhibiting a change in

color or luminescence intensity to a change in the tem-

perature outside the package or the level of oxygen or

carbon dioxide inside and, in so doing, provide important

information, that is, intelligence (1–7). Such intelligent

inks are a key aspect to intelligent packaging (see Intel-

ligent Packaging), recently deﬁned as ‘‘capable of carrying

out intelligent functions, such as detecting, sensing, re-

cording, tracing, communicating and applying scientiﬁc

logic’’ (1).

Just like inert inks, intelligent inks are usually printed

on a surface that is applied to the package in the form of

sticky labels for ease of application and incorporation.

Because they are more than a normal label, such intelli-

gent labels are often called ‘‘smart package devices’’

(SPDs) (1). For internal applications, SPDs often have an

outer cover, usually a polymer membrane, to allow trans-

mission of the analyte under test (e.g., oxygen) to the

reactive ink in the label and prevent direct contact

between the package contents and printed ink. There

are a large number of patented SPDs that utlize intelli-

gent inks and cover a wide range of chemical and physical

processes. However, most of these are unlikely to have any

signiﬁcant commercial future, usually for reasons of cost,

especially when considered for use in the food and bev-

erage packaging industry, the largest of the packaging

industries. As a consequence, this limited overview will

illustrate the emerging area of intelligent inks in packag-

ing by focusing on the different intelligent inks and SPDs

which are now commercially available for use in the food

and beverage packaging industry.

TEMPERATURE

Inks that change color with temperature are thermochro-

mic (8, 9). Such inks are fairly commonplace, especially in

the novelty toy market and textiles, and have been used

for a variety of applications including: a safety feature for

baby spoons, cups or kettles (warning of a hot surface),

authentication devices on restaurant tickets, pre-paid

phone cards, and store coupons and a method to prevent

color copying of sensitive documents. In almost all these

cases, these thermochromic inks used exhibit a reversible

response, albeit with some hysteresis, and are indirect

systems—that is, ones in which the chromophoric mate-

rial reacts to changes in its environment brought about by

heat, involving composite thermochromic pigments.

The use of thermochromic inks is not unusual in

packaging and is a common feature of product labels and

advertising specialities. For example, they are currently

used as follows: to show when pancake syrup has been

microwave heated to the desired temperature (10); in the

lids of disposable coffee cups to indicate the coffee is still

hot and the lid is correctly placed (11); and on beer cans to

indicate when they are fully chilled (12). As an illustration

of the latter, the beer manufacturer, Grolsch UK, recently

featured a thermochromic ink on their cans so that the

consumer could identify when the beer had been sufﬁ-

ciently chilled, as illustrated by the pictures in Figure 1.

598 INTELLIGENT INKS IN PACKAGING

The key component of most thermochromic inks used in

packaging are polymer microcapsules, 1–10 mm in dia-

meter, containing three interacting chemicals responsible

for the overall temperature-dependent color in an ink,

which also comprise a resin and a solvent. The ﬁrst of the

three components in the microcapsules is a leuco (i.e.,

colorless) dye, such as a ﬂuoran, triphenyl methane

lactone, and spirolactone. The ring-closed forms of these

dyes are colorless—thus their ‘‘leuco’’ epithet, from the

Greek ‘‘leukos’’ meaning white. In contrast, the ring-

opened forms of these dyes are highly colored and gener-

ated by protonating the leuco form using a proton donor.

The protonation of the leuco dye, Crystal Violet Lactone, to

create its highly colored, protonated form is illustrated in

Figure 2. Not surprisingly, the second microcapsule com-

ponent is a weak acid that acts as a solvent and proton

donor to the leuco dye (i.e., a ‘‘color developer’’) such as

bisphenol A, alkyl p-hydroxybenzoate, and derivatives of

1,2,3-triazole and 4-hydroxycoumarin. The third compo-

nent is a polar co-solvent, such as lauryl alcohol or

butyl stearate, with a low melting point that controls

the temperature at which the color change will take place

(8, 9). The dye is usually protonated (i.e., highly colored)

when this solvent is a solid, but colorless, (i.e., in its leuco

form) when it is melted. Interestingly, although many

combinations of dye-developer-solvent are known, the

exact mechanism by which such thermochromic dyes

operate is still the subject of some debate (9); however, a

popular explanation is that the color of the solid form of

the material is due to an ion-pair interaction between the

dye and the developer—an interaction that breaks up

when the solvent melts, thereupon returning the dye to

its protonated, leuco form.

There is a temptation to include in this section details

of the most advanced of all the SPDs, in terms of commer-

cialization and product integration, namely time–tem-

perature indicators (TTIs, also see Time Temperature

Indicators), that is, devices that integrate over time the

exposure of the package to temperature (13). However,

most TTIs do not utilize a reactive, printable ink,

but rather are diffusion/melting point-based with a

mechanical activation element—for example, the break-

ing of a seal separating one reservoir from another

(13–15). There are some chemical-reaction-based TTIs,

such as FreshCheckt indicator (TempTime Corp.) based

on a lattice-controlled, solid-state reaction, proceeding

through a 1,4-addition polymerization route, of colorless

diacetylenic monomers, such as that shown in Figure 3

(14, 16, 17). These compounds polymerize thermally to

give very dark, conjugated polymers. The kinetics of the

polymerization depends upon the type of substitution and

thus provides a route by which TTIs with different

NMe

COOH

−H

Figure 2. Reaction scheme illustrating the

protonation of the well-known thermochromic

dye Crystal Violet Lactone. The lactone form of

this dye (left-hand-side structure) is colorless,

whereas its protonated, ring-opened form is

bright blue.

(a)

(b)

Figure 1. Photographs of a can of Grolscht beer (a) before

and (b) after correct chilling, the ink in the label becomes

lighter.

INTELLIGENT INKS IN PACKAGING 599

temperature sensitivities can be produced. In a typical

indicator the polymerization takes place in the center of a

bull’s-eye pattern, with the outer part of the bull’s-eye

acting as reference color ring. FreshCheckt indicators are

used by some supermarkets, such as Trader Joe’s in the

United States and Monoprix in France, on selected refri-

gerated products. Another, similar TTI is the irreversible

thermochromic colorless (to deep magenta) ink, developed

for use as a TTI bar code by SIRA Technologies as part of

their Food Sentinel Systemt (18). Details of this latter ink

are sparse, but, like the FreshCheckt TTI, the Food

Sentinel Systemt indicators appear to be full history

TTIs that are temperature-only activated (i.e., no mechan-

ical activation) and so require deep-freeze storage prior to

use—a feature that, although clearly an essential for all

temperature-only-activated TTIs, is costly and a major

limitation to any global product implementation.

The Cryolog company has recently developed an irre-

versible, novel, temperature-activated, printable TTI label

that utilizes food-grade microorganisms to simulate the

real deterioration of food products—called Traceo

tech-

nology (19). The growth of these microorganisms parallels

that of the microorganisms naturally present in the food

and produces a pH change in the indicator which is

revealed using a pH-sensitive dye and/or a pH-sensitive

obscuring agent, such as casein. This initially colorless,

transparent Traceo

technology is used to cover barcodes,

turning pink and concealing the barcode if the product is

no longer fresh due to improper temperature storage. As

with FreshCheckt and Food Sentinelt, the Traceo

TTIs

are full-history, temperature-only-activated indicators, re-

quiring deep-freeze storage prior to use.

The above TTIs are compatible with label technology but

not with direct printing as reactive inks. However, more

promising is OnVut, recently promoted by Ciba Specialty

Chemicals and FreshPoint as a printable, irreversible ther-

mochromic ink that is UV-activated and thus able to act as a

TTI (20). The key ingredient is a colorless, valence isomer-

ization photochromic, such as a spirobenzopyran, that is

rendered dark blue (indicating formation of a meta-stable

state) upon exposure to UV light and remains in this form

when held at temperatures r11C, but reverts thermally to

its colorless, more stable form in about 7 days when held at

251C. Thus, exposure to temperatures higher than those

prescribed accelerates the lightening process of the color;

and when the activated color attains the same shade as the

reference, the product is deemed to have reached the end of

its shelf life. Evaluation of the status of the indicator is

made by reference to printed colors within the label. The

OnVut label is currently printed as simple labels using

water-based ﬂexo inks, but the slight, negative feature of

this label appears to be its continued sensitivity to UV light

once activated (since the latter reactivates the pigments).

This is addressed in the label by a UV ﬁlter ﬂap, which

needs to be placed over the label, after UV activation,to

protect it from recharging by ambient UV light. An online

labeling device has been made for this technology to allow

the basic indicator to be UV-activated and then protected

from re-activation by the overprinting of a UV ﬁlter.

Although initially developed for use by the meat and dairy

industries, plans are in place for OnVut indicators for fresh

salads, chilled prepared dishes, and juices and to serve the

medical industry, for blood, vaccines, and medication (20).

LIGHT

Dyes that change color upon exposure to light—that is,

photochromics (8)—are sometimes used to protect pro-

ducts from overexposure to light, especially UV light.

Because their main role is to respond to the environment

in such a way as to protect the product, rather than

provide information, they are best considered as an ex-

ample of active, rather than intelligent, packaging (21)

and thus will not be detailed here.

OXYGEN

At ﬁrst it may not be clear why the detection of oxygen is

important, especially in food packaging. However, oxygen

is the main cause of most food spoilage, since its presence

allows a myriad of aerobic food-spoiling microorganisms to

grow and thrive. Oxygen also spoils many foods through

enzyme-catalyzed reactions, as in the browning of fruit

and vegetables, destruction of ascorbic acid, and the

oxidation of a wide range of ﬂavors. An increasing popular

method of packaging food is modiﬁed atmosphere packa-

ging (MAP, see also Modiﬁed Atmosphere Packaging), a

process in which the atmosphere within the food package

is ﬂushed with an inert gas, such as nitrogen or carbon

dioxide, reducing the oxygen content to typically 0.5–2%

(7, 13, 22, 23). Unlike nitrogen, carbon dioxide is an active

gas in food packaging, since it affects most food-spoiling

microbes, reducing signiﬁcantly their growth, even if some

oxygen is present (22). Both carbon dioxide and nitrogen

slow the rates of respiring foods, such as fruits and

vegetables, and retard oxygen-based spoilage. As a con-

sequence, MAP is very effective at extending the shelf

lives of many foods, typically by a factor of 3–4, and is

widely used in food packaging; indeed, it is estimated that

52 billion food packages were MAPed in 2007 (23). An

intelligence ink for oxygen has great potential with regard

to MAP, since it would provide the packager with a method

R =

Pol

mer

Monomer

Figure 3. Polymerization of a diactetylenic monomer (colorless)

that leads to a dark product; this chemistry underpins the

FreshCheckt indicator (14).

600 INTELLIGENT INKS IN PACKAGING

of ensuring 100% quality assurance, help the retailer

identify packages of compromised integrity (e.g., damaged

in transit or subjected to tampering), and reassure the

consumer that the quality of the contents inside has not

been compromised by damaged packaging.

Intelligent ink oxygen indicators fall into two cate-

gories—luminescence or colorimetric—and most are lumi-

nescence-based, comprising a lumophore, L, dissolved in a

solvent along with a solvent-soluble resin, such as a

ﬂuoro-silicone polymer. The luminescence associated

with the electronically excited lumophore, L

, is quenched

irreversibly by molecular oxygen, that is,



þ O

!

ðO

L þO



ð1Þ

where k

) is the bimolecular rate constant for the

quenching process.

Although many different lumophores have been tested,

one of the most popular is tris(4,7-diphenyl-1,10-phenan-

throline) ruthenium (II) perchlorate, that is, [Ru(dpp)

]

(ClO

)

, where dpp is the complexing ligand, 4,7-diphenyl-

1,10-phenanthroline; [Ru(dpp)

](ClO

)

is a highly lumi-

nescent complex (probability of luminescence D0.3) with a

long-lived excited state (B5.3 ms), that is readily quenched

by oxygen (k

) D 2.5 10

mol

1

) (24, 25).

The luminescence of a commercial luminescence-based

oxygen indicator, the O

xyDott, comprising [Ru(dpp)

]

(ClO

)

in a silicone polymer dot (5-mm diameter, 0.2 mm

thick) with a glass (or nowadays acetate) backing support,

is illustrated in Figure 4. The O

xyDott is attached inside

the package under test using typically an RTV silicone

rubber adhesive. It is possible to assess the level of oxygen

surrounding such a luminescence oxygen indicator (i.e.,

) by measuring its luminescence intensity. However,

such measurements require signiﬁcant calibration and a

very stable excitation light source and detector. As a

consequence, with such indicators it is generally preferred

to determine %O

via the measurement of luminescence

lifetime, t—that is, the time taken for the luminescence to

decay to 1/e of its initial value—although the equipment

costs associated with this technique are signiﬁcant. This is

certainly how the O

xyDott is used, and Figure 5 illus-

trates a typical set of luminescence decay curves for the

xyDott as a function of increasing oxygen partial pres-

sure (26). From these results it can be seen that t decreases

with increasing %O

, and further work shows that 1/t is

proportional to %O

; thus these indicators can be used for

quantitative analysis of O

. Although the major current,

commercial oxygen sensor system used in food packaging,

OxySenset, uses lifetime measurements and lumines-

cence-based oxygen indicators, this approach is still too

expensive, in terms of equipment and indicators, for in-

corporation in all MAPed packages and so its use is limited

to mainly package research (24, 26).

Luminescence is often hard to discern and requires

analytical equipment to assess the degree of quenching.

This represents a major disadvantage of luminescence-

based intelligent inks. In contrast, colorimetric indicators

can be assessed, semi-quantitatively at least, by eye; as a

result, there is much more interest in intelligent inks

based on color. A commercial, colorimetric ink that re-

sponds to oxygen has recently been reported and com-

prises the following: a UV-only absorbing semiconductor

photosensitizer, TiO

; a redox indicator, methylene blue

(MB); a mild reducing agent, glycerol; and a polymer

encapsulating material, hydroxyethyl cellulose (27, 28).

Figure 4. An O

xyDott in the presence

(left-hand-side image, higher level of lumi-

nescence intensity) and absence (right-hand-

side image) of oxygen.

0.2

0.4

0.6

0.8

0 2 4 6 8 10 12 14

t (

μs)

Relative Intensity

Figure 5. Luminescence decay proﬁles, produced using a

pulsed excitation light source, for the oxygen-quenchable

lumophore [Ru(dpp)

]

in an O

xyDott indicator under

different levels of oxygen (i.e., from slowest to fastest

decay: 0%, 2.1%, 6.3%, 12.6%, and 21.0%, respectively).

INTELLIGENT INKS IN PACKAGING 601

All of the indicator components are soluble, or easily

dispersed, in water and can be used to create a water-

based, oxygen-intelligent ink that can be printed on a

variety of common substrates used in food packaging.

The above MB/TiO

/glycerol oxygen indicator is acti-

vated with UV-light; and Figure 6 illustrates a typical

indicator ﬁlm, made from such an ink, before (blue-colored)

and after (colorless) UV activation. Before UV activation,

the ink is inert, but upon activation it acts as an oxygen

indicator, returning to its original color upon exposure to

air, or remaining colorless in the absence of oxygen. The

UV-activated oxygen indicator works via the simple me-

chanism illustrated in Figure 7. In brief, upon UV excita-

tion of the nanocrystalline, ﬁnely dispersed titania (TiO

)

pigment powder particles encapsulated in the polymer ﬁlm,

electron–hole pairs are generated. The photogenerated

holes are able to oxidize readily and irreversibly the

glycerol present, allowing the photogenerated electrons to

accumulate on the semiconductor particles, TiO



). These

electrons reduce the highly colored redox-indicator (MB) to

its usually bleached form (LMB). This latter species is

readily oxidized back to MB by oxygen, thereby regaining

its original color. However, if no oxygen is present, the ﬁlm

remains bleached. The Finnish-based label manufacturer,

UPM-Raﬂatac, has recently launched an oxygen-indicating

label based on this technology (29).

CARBON DIOXIDE

The popularity of CO

as a MAP gas has been an im-

portant driver in the development of an appropriate

intelligent ink to show up leaks in packages. The chem-

istry that is usually employed to sense CO

is based on its

ability to acidify water (30, 31), that is,

ðaqÞþH

O3H

þ HCO



ð2Þ

The detection of the protons released by carbonic acid,

, is easily achieved using a pH-indicating dye, such

as meta-cresol purple, which is blue when deprotonated

(MCP



) and yellow when acidiﬁed (MCPH). Thus, an

aqueous solution containing MCP



is blue in the absence

of CO

, but yellow when saturated with CO

. Reaction (2)

highlights the need for water in the formulation, but,

ideally, a solvent-based (i.e., largely hydrophobic), intelli-

gent ink for CO

is desired. This is achieved by coupling

the anion of the pH-indicating dye with a phase transfer

cation, Q

, such as the tetraoctyl ammonium cation, to

create a solvent-soluble ion pair, MCP



O, which, as

indicated by its molecular formula, usually retains a few

molecules of water. As a consequence, when incorporated

into an ink, the following reversible colorimetric process

can be effected (30, 32):

MCP



Blue

þCO

3 HCO



ðx 1ÞH

Yellow

MCPH

ð3Þ

The Trufﬂex GS carbon dioxide SPD, which has been

promoted by Cryovac-Sealed Air Ltd., is an example of

this technology (3).

FRESHNESS INDICATORS

Freshness indicators are designed to respond to chemicals

released by food as a result of spoilage; usually an

oxidative process is effected by bacteria, yeasts, and fungi,

which break down food carbohydrates, proteins, and fats

to a wide variety of low-molecular-weight molecules, such

as lactic and acetic acids, aldehydes, alcohols, sulfur-

containing species and nitrogen-containing molecules,

such as ammonia and amines. For example, when proteins

are bacterially decomposed, the products are amines that

are related to the original amino acids that make up the

protein. Thus, arginine is converted to putresceine, lysine,

to cadaverine while histadine is converted to histamine.

Putrescine, cadaverine, and histamine are volatile

amines, responsible for the smell of rotting protein, such

as meat and seafood. In much the same way that the

Figure 6. Photographs of a typical colori-

metric, oxygen-sensitive, TiO

/MB/glycerol/

HEC ink before (left-hand side) and after

(right-hand side) UV activation (3 min, 4 mW

2

). After UV activation the ink recovers

its initial blue color within 10 min in air, but

remains colorless in the absence of O

TiO

∗

TiO

−

)

Glycerol

Glyceric acid

LMB

Figure 7. Schematic illustration of the primary processes in-

volved in the functioning of the TiO

/MB/glycerol/HEC ink.

602 INTELLIGENT INKS IN PACKAGING

presence of CO

can be detected using an intelligent ink

based on pH-sensitive dyes that respond to its slightly

acidic nature, we note that intelligent inks, incorporating

pH-sensitive dyes, DH, can be created that respond to the

basic nature of volatile amines and ammonia, that is,

DH þRNH

Color A

3 D



þ RNH

Color B

ð4Þ

For example, the pH indicator dyes bromophenol blue,

bromocresol blue, and chlorphenol red have been used to

create a range of ammonia intelligent inks, comprising

dye, resin (poly(vinyl butyral)), plasticizer (tributyl phos-

phate), and solvent (methanol) (33). In the absence of

ammonia or volatile amines, such as putrescine, the ﬁlms

are yellow, but in their presence they are either blue or

purple. Figure 8 nicely illustrates the response of such a

typical ﬁlm coating a sample bottle when it is placed over a

sample bottle cap containing a few milligrams of putres-

ceine (a solid); within 20 min the initially yellow ﬁlm is

blue. This approach to sensing ammonia and volatile

amines has been adopted by COX technologies, with their

Fresh Tagt indicator which is attached to the outside of

the package as a small adhesive label and monitors the

freshness of seafood products inside, via a barb on the

backside of the tag that penetrates the packaging ﬁlm,

allowing any volatile amines to be wicked to the color-

based indicator outside, turning it from yellow to blue if

spoilage amines are present (2, 34). More recently,

Food Quality Sensor International Inc. (FQSI) has used

similar pH-sensing technology in their SensorQt, a label

that detects the freshness/spoilage level of packed meat

and poultry (35, 36). The SensorQt label, illustrated in

Figure 9, has an orange-colored center in the ‘‘Q’’ when

exposed to fresh meat, which turns dark green if the meat

is spoiled. The pH indicator dyes used in this sensor

appear to be anthocyanins, such as betanin or ﬂavonoid

dyes (36, 37). The SensorQt system is a stick-on label that

is used on the inside of packages (35).

As noted above, decaying foodstuffs, especially poultry

meat, can produce sulfur-containing compounds, such as

hydrogen sulﬁde and/or dimethyl disulﬁde. This feature

has been exploited by UPM-Raﬂatac with their volatile

sulphide indicator, comprising metallic silver and/or cop-

per particles, typically o10-mm diameter (pale pink),

which reacts to produce the dark gray/black colored metal

sulﬁde; for example,

2Ag þ H

S ! Ag

S þH

ð5Þ

The metal layer, 1–5 mm thick, can be screen or ﬂexogra-

phically printed on paper, board, or plastic, using an ink

containing 70–90% dry weight of metal particles. Films of

different sensitivity can be created because the sensitivity

of the indicator decreases with increasing metal ﬁlm

thickness. The indicator is placed inside the food package

and is promoted as a shelf-life indicator (38, 39).

The detection of volatile decomposition gases is the

most likely and practical approach to the assessment of

food quality/freshness on a global basis and very compa-

tible to printing technology. However, these are effectively

indirect indicators of the presence of spoilage bacteria or

pathogens; and the possibly more attractive direct mea-

sure of such species, using immunochemical indicators,

Figure 8. Typical bromophenol blue in

poly(vinyl butyral)/tributyl phosphate ﬁlm-

coated sample bottle inverted over a sample

bottle cap containing putrescine. The images

were recorded after 0-, 10-, and 20-min ex-

posure to the vapor above the sample bottle

cap (33).

Figure 9. SensorQt indicator label from FQSI, before (left-hand

side) and after (right-hand side) exposure to amine containing

spoilage gases from meat (35). (Photographs courtesy of FQSI.)

INTELLIGENT INKS IN PACKAGING 603

has been reported in recent years—for example, by Food

Sentinel Systemt (SIRA) and Toxin Guardt (Toxin Alert)

(40–42). These indicators require direct contact with food

and the use of colored or ﬂuorescent labeled antibodies.

The combination of tough food contact regulations with

regard to direct food contact and a likely high cost of

production represents a major barrier to the widespread

implementation of these indicators in food packaging.

Finally, the RipeSenset label, developed in New Zea-

land, appears to address the problem of knowing

when fruit is ripe and was identiﬁed by TIMEt magazine

as one of its 36 greatest inventions for 2004 (43). As shown

in Figure 10, the indicator is red colored for unripe (crisp)

fruit, but yellow when the fruit is very ripe (juicy); the

indicator responds to the volatile gases emitted by many

fruits as they ripen—not ethylene, but probably aldehydes

and ketones. Developed initially for pears, and currently

sold in New Zealand, Australia, USA, Canada, Mexico,

Belgium, and France, work is in progress to develop

similar indicators for: kiwifruit, melon, mango, avocado,

and stone fruit, such as cherries and peaches. An ethy-

lene-based ripeness sticker, RediRipet, has been reported

by researchers at the University of Arizona. It has yet to

be released commercially, but it is estimated that each

sticker will cost less than 1 cent (44, 45).

CONCLUSIONS

The number of commercial intelligent inks is still rela-

tively small, but their diagnostic range is impressive and

is a reﬂection of the signiﬁcant and growing number of

patents and articles published on intelligent inks and

sensors. There is still some way to go before these indica-

tors are a global feature of packaging, but few can doubt

that that time will come, given increasing demand from

the packaging industry customers for better, more secure

packaging, a need that can be addressed via the informa-

tion provided by intelligent inks.

It is encouraging to note that EU legislation is now in

place to allow intelligent and active packaging in Europe

(46). EU regulations No. 1935/2004 and 2023/2006 author-

ize market placement of packaging which acts intelli-

gently when in contact with foodstuffs, either by

providing information on the quality (freshness) of the

product or preserving it for longer by introducing favor-

able modiﬁcations. However, this same legislation also

stresses that such active or intelligent packaging must not

mislead the consumer. Thus, indicators that give wrong

information concerning the food that could mislead the

consumer are not allowed. This quite reasonable condition

may be a factor in the slow take up of the technology by

many brand names, given the possibility that most in-

dicators could be misinterpreted by consumers, even

though such indicators provide greater customer reassur-

ance (e.g., via tamper evidence, sterilization indicators,

freshness indicators), prevent illness (time–temperature

indicators), reduce costs (through less waste), and en-

hances brand differentiation (47).

With respect to the general initial reaction and subse-

quent perception of the consumer with respect to intelli-

gent indicators, much can be learned from the few

established intelligent indicators that exist, such as Mono-

prix in France, who has been using TTIs for more than 10

years. Initially, leaﬂets and posters were used to raise

awareness and educate the consumer, but now most are

comfortable and familiar with ‘‘La Puce Fraicheur.’’ Cur-

rent surveys show that French consumers now perceive

the TTI as complementary to the ‘‘use by’’ date and that

the TTI is a more reliable guide than the ‘‘use by’’ date and

are prepared to pay a little more for the reassurance of

freshness and quality (48). Most surveys of intelligent

indicators show the consumer to be a keen advocate, given

sufﬁcient education and an associated modest increment

in price, provided the indicator concerned is reliable, clear,

and easy to interpret. With the gradual removal of legisla-

tion barriers and growing customer interest in informa-

tion and reassurance with regard to food, the conditions

appear promising for a major breakthrough in the use of

intelligent inks in the food packaging industry.

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INTELLIGENT PACKAGING

KIT L. YAM

PAUL T. W. TAKHISTOV

Department of Food Science,

Rutgers University, New

Brunswick, New Jersey

JOSEPH W. MILTZ

Department of Biotechnology &

Food Engineering,

Technion-Israel Institute of

Technology, Haifa, Israel

INTRODUCTION

To understand intelligent packaging, it is helpful to review

the historical development of packaging. Traditionally,

the basic functions of packaging have been classiﬁed

into four categories (Figure 1), as follows: protection,

INTELLIGENT PACKAGING 605

communication, convenience, and containment (1, 2). The

package is used to protect the product against the external

environment, communicate with the consumer as a mar-

keting tool, provide the consumer with greater ease of use

and time-saving convenience, and contain products of

various sizes and shapes. Nevertheless, these functions

are not totally exclusive; for example, the communication

function of the package through warning labels and cook-

ing instructions can also help to enhance food protection

and convenience.

Although traditional packaging has contributed greatly

to the early development of the food-distribution systems,

it is no longer sufﬁcient because today’s society is becom-

ing increasingly complex. Innovative packaging with en-

hanced functions is constantly sought in response to the

consumer demand for minimally processed foods with

fewer preservatives, increased regulatory requirements,

market globalization, concern for food safety, and more

recently, the threat of food bioterrorism. How can the

existing functions of a mature, and sometimes taken for

granted, technology be enhanced? It probably requires

rethinking and shifting in the existing paradigm (3).

Active packaging and intelligent packaging are the results

of ‘‘thinking outside the box.’’

During the past two decades, the popularity of active

packaging (AP) has signiﬁed a major paradigm shift in

packaging; namely, the protection function of packag-

ing has been shifted from passive to active. Previously,

primary packaging materials were considered as ‘‘pas-

sive,’’ which means that they functioned only as an inert

barrier to protect the product against oxygen and moist-

ure. Recently, a host of new packaging materials

have been developed that provide ‘‘active’’ protection to

food products. AP has been deﬁned as a system in

which the product, the package, and the environ-

ment interact in a positive way to extend shelf life or to

achieve some characteristics that cannot be obtained

otherwise (4). It has also been deﬁned as a packag-

ing system that actively changes the condition of the

package to extend shelf life or improve food safety or

sensory properties, while maintaining the quality of the

food (5).

All AP technologies involve some physical, chemical,

or biological action for altering the interactions between

the package, the product, and the package headspace to

achieve a certain desired outcome (6, 7). Gas absorbing/

emitting packaging (5, 8) is a group of technologies that

uses packaging ﬁlms or sachets to absorb gases (such as

oxygen, water vapor, or ethylene) from the package

headspace or to emit gases (such as carbon dioxide or

ethanol) to the package headspace, so that a favorable

internal package environment and thus an extension in

shelf life are achieved. Controlled-release packaging is a

group of technologies that uses packaging materials as a

delivery system to release active compounds (such as

antimicrobials, antioxidants, enzymes, ﬂavors, and nu-

traceuticals) to protect against microbial spoilage and

enhance food quality. Most attention in this group has

been focused on antimicrobial packaging (9, 10) and

antioxidant packaging (11, 12). Recently, an additional

step forward was made by developing antimicrobial

packages that contain natural instead of synthetic addi-

tives (13). Smart ﬁlms are breathable ﬁlms that enable

the control of oxygen, water vapor, and carbon dioxide

permeation at rates beneﬁcial to modiﬁed atmosphere

packaging of fresh produce (14). Microwave susceptors

(15, 16) are metallized polyester-based structures that

interact with microwaves to provide crispness and

browning of foods during microwave heating.

Active Packaging is placed above the protection

function in the model of Figure 1. The placement

reﬂects the situation that most AP technologies are

aimed at and limited to enhancing the protection function

of the package. This is not to say that AP does not

provide other functions: For example, the microwave

susceptor is an active packaging technology that pro-

vides convenience and food quality. Why were these

technologies not simply described as a packaging system

with enhanced protection functions, instead of inventing

the term active packaging? An answer to this question is

that this term has inspired researchers to develop

innovative technologies by thinking packaging in a new

way.

In recent years, the terms intelligent packaging (IP)

and smart packaging (SP) have also begun to appear with

increasing frequency in conferences, journals, and maga-

zines. Unfortunately, clear and unequivocal deﬁnitions

are not yet available in the scientiﬁc/technical literature.

IP and SP are often used interchangeably at conferences

and symposiums. Brody et al. (7), however, deﬁned IP as a

packaging system that senses and communicates, and

they deﬁne SP as one that possesses the capabilities of

both AP and IP. Clarke (17) deﬁned smart packaging as

one which communicates and intelligent packaging as one

that includes logic capability. Rijk (18) deﬁned IP as one

that monitors the conditions of packaged foods to give

information about the quality of the food during transport

and storage. A major deﬁciency of these deﬁnitions is that

the authors simply stated them without providing careful

justiﬁcations.

In this article, we deﬁne intelligent packaging based

on the historical development of packaging functions

and a proposed conceptual framework. Speciﬁc exam-

ples are also presented to clarify the meaning of the

deﬁnition.

Protection

Containment

Communication

Convenience

Packaging

Functions

Figure 1. Model of packaging.

606 INTELLIGENT PACKAGING

DEFINITION OF INTELLIGENT PACKAGING

According to the American Heritage Dictionary, the word

‘‘intelligent’’ is deﬁned as ‘‘showing sound judgment and

rationality’’ and as ‘‘having certain data storage and

processing capabilities.’’ A prerequisite of making sound

decisions is effective communication—the ability to ac-

quire, store, process, and share information—and this is

where IP can make a signiﬁcant contribution.

We deﬁne IP as a packaging system based on the

science and technology that can carry out intelligent

functions (such as detecting, sensing, recording, commu-

nicating, and applying scientiﬁc logic) to facilitate decision

making and corrective actions to extend shelf life, enhance

safety, improve quality, provide information, and warn

about possible problems. We believe that the uniqueness

of IP is in its ability to communicate. Because the package

and the food move constantly together throughout the

supply chain cycle, the package is the food’s best compa-

nion and is in the best position to communicate the

conditions of the food. Accordingly, we place IP above the

communication function in the model of Figure 1, building

on the historical development of packaging functions. We

have also developed a conceptual framework in the next

section to delineate the anatomy of IP and provide more

depth to its deﬁnition.

As alluded to earlier, the purpose of introducing in-

telligent packaging is to inspire people to expand the

communication function of packaging in an innovative

and useful way. The purpose of introducing the conceptual

framework is to provide a roadmap to facilitate the

systematic study to advance understanding and applica-

tion in this ﬁeld. Note that the deﬁnitions of AP and IP are

not mutually exclusive: Some packaging systems may be

classiﬁed either as AP or IP or both, but this situation does

not detract the purpose of these terms. We have also been

careful not to introduce other terms such as ‘‘responsive

packaging’’ and ‘‘clever packaging,’’ because too many

terms tend to complicate the already confusing

terminology.

According to our deﬁnition, a package is ‘‘intelligent’’ if

it has the ability to sense the environment and commu-

nicate with humans: For example, an intelligent package

is one that can monitor the quality/safety condition of a

food product and provide early warning to the consumer or

food manufacturer. It is important to emphasize that IP is

a product delivery system that involves not only the

package but also the food product, the external environ-

ment, and other considerations. We believe that the

emergence of IP has signiﬁed another paradigm shift in

the concept of food packaging—shifting the package from

a mediocre communicator to an intelligent communicator.

CONCEPTUAL FRAMEWORK OF INTELLIGENT

PACKAGING

Intelligent packaging can play an important role in facil-

itating the ﬂows of both materials and information in the

food supply chain cycle. In Figure 2, the outer circles

represent the supply chain cycle from raw material

through manufacturing, packaging, distribution, product

use, and disposal. The package, in one form or another

(such as a pouch, container, or drum, pallet), is tradition-

ally used to facilitate the ﬂow of materials (represented by

the arrows in the ﬁgure) from one location to another, by

performing the basic functions of containment and protec-

tion of the product. Furthermore, the package can also

facilitate the ﬂow of information (represented by the

communication links between the inner circle and outer

circles), although this communication function has been

largely overlooked. The package can indeed be a highly

effective communicator—it can carry actual information

in the direction of material ﬂow (e.g., via truck, train, or

ship), and it can transmit information visually (e.g., via an

indicator) or electronically (e.g., via a barcode or the

Internet) throughout every phase of the supply chain

cycle.

A conceptual framework describing the ﬂow of informa-

tion in an IP system is illustrated in Figure 3. The system

consists of the following four components: smart package

devices, data layers, data processing, and information

highway (wire or wireless communication networks) in

the food supply chain. The smart package devices are

largely responsible for generating the concept of IP, be-

cause they impart the package with a new ability to

acquire, store, and transfer data. The data layers, data

processing, and information highway are collectively re-

ferred here as the decision support system.

As shown in Figure 3, the smart package devices and

the decision support system are designed to work together

to monitor changes in the internal and external environ-

ments of the food package and to communicate the condi-

tions of the food product, so that timely decisions can be

made and appropriate actions taken. From the quality and

safety viewpoint, the external environment can be divided

into the ambient, physical, and human environments (2),

which are factors important for determining shelf life.

Customer/

Consumer

Distribution

Product/

Package

Disposal/

Recovery

Raw

Material

Product

Manufacture

Intelligent

Packaging

Figure 2. Material ﬂow and information ﬂow in the food supply

chain.

INTELLIGENT PACKAGING 607