Jung Han. Innovations in Food Packaging

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Introduction to modified atmosphere packaging 163

solution to this problem. This is usually accomplished by first creating a vacuum and

then incorporating the desired gas mixture in the package. Compared to the passive

method, active atmosphere modification is practically instantaneous and takes place at

the beginning of

storage.

The atmosphere should then remain practically unchanged,

provided that the proper barrier material is used and there is no leakage through pin-

holes or poor seals (Figure 10.2b).

The process of applying

vacuum can

considered

method of active atmosphere

modification (Floros, 1990). It is commonly used in packaging techniques such as can-

ning, or bottling in glass containers. The main purpose of

such

vacuum application is

to reduce the residual oxygen in the headspace of a

package,

which eventually retards

oxidative chemical reactions and aerobic microbial growth. When a vacuum is used

with

flexible

packages, the packaging material collapses around the product and prac-

tically eliminates the existence of the

headspace.

A vacuum

also ofl:en applied

stor-

age

rooms and transportation containers,

technique called hypobaric or low-pressure

storage. In this method, a slight vacuum is maintained in the storage room or con-

tainer, which reduces the partial pressure of oxygen and continuously removes undesir-

able gases such as ethylene. As a consequence, oxidative and physiological reactions

are retarded.

Finally, relatively recent technological innovations allow for in-package control of

a specific gas (oxygen, carbon dioxide or ethylene). Such "active" packaging systems

are designed to remove or add certain gases selectively. This is usually achieved by

using a substance that can bind (scavenger) or release (emitter) certain molecules as a

result of chemical or enzymatic reactions. The "active" substance can

placed in the

package (e.g. in a sachet), or in more sophisticated systems it can

incorporated into

the packaging material itself The method

referred

as active packaging. An exam-

ple of this technique is the addition of small sachets in cans of coffee. The sachets

contain a mixture of iron oxide and calcium hydroxide that binds oxygen and enables

the control of the package environment without gas-flushing or the application of a

vacuum (Floros, 1990).

Advantages and disadvantages of MAP

MAP offers many advantages to consumers and food producers. To the consumer, it

offers convenient, high-quality food products with an extended shelf life. It also

reduces and sometimes eliminates the need for chemical preservatives, leading to

more "natural" and "healthy"

products.

At the same

time,

producers also enjoy the ben-

efits of increased shelf

life.

By using MAP many products can be packaged centrally,

and their distribution cost is reduced because fewer deliveries over longer distances

become possible. Moreover, because of the extended shelf life, MAP allows trans-

portation of foods to remote destinations and increases product markets.

MAP also has several disadvantages. Usually, each MAP product needs a different

gas formulation. This requires the use of specialized and expensive equipment. At the

same time, production staff must receive special training. For most products, storage

164 Innovations in Food Packaging

temperature control is required and product safety must be established. Furthermore,

MAP causes larger package

volumes,

which leads to increased transportation and retail

display space needs. All the above add a noticeable cost, which must be paid by the

consumers. Finally, another disadvantage of

MAP

is that it loses all its benefits once

the consumer opens the package.

Effect of MAP on shelf life

The deterioration mechanisms vary widely among various

products.

Consequently, the

effect of MAP on the shelf life of different products varies and depends upon the prod-

uct. Some examples of modified atmosphere applications for different products are

given below in order to demonstrate the effect of MAP on shelf

life.

Table 10.1 shows

the recommended conditions to maximize the shelf life of various products.

Meats and poultry

The first commercial application of modified atmosphere preservation of meat was

developed in the 1930s. Based on scientific evidence, refrigerated beef carcasses were

transported by ship from New Zealand and Australia to the UK in a carbon dioxide-

enriched environment. However, MAP of retail-size meat packages was not introduced

until the

1950s,

in the

form of vacuum

packaging.

Much

later,

1981,

Marks

Spencer

introduced a wide range of

fi"esh

meats in the market, packaged in gas-flushed plastic

trays (Inns, 1987).

One of the main quality attributes of red meat, and an important indicator of prod-

uct "fi*eshness", is its color. The bright red color of fresh meat desired by consumers

is due to the presence of oxymyoglobin, a myoglobin pigment in its oxygenated form.

Over

time,

however, oxymyoglobin gradually changes to metmyoglobin, which has an

undesirable brownish

color.

Preservation of the desired bright red color can

achieved

by packaging the meat in oxygen-enriched atmospheres.

Meat is a highly perishable food, and the major factor that limits its shelf life is

microbial growth. It was noted earlier that carbon dioxide slows down the growth rate

microbes.

Thus, atmospheres high in carbon dioxide should be used to suppress

microbes and extend shelf

life.

Consequently, MAP of meat in a

gas

mixture with high

amounts of both oxygen and carbon dioxide is ideal for preserving the product's quality

and extending its shelf

life.

The recommended atmosphere compositions for beef and

pork are shown in Table 10.1.

the

case of poultry, the presence of oxygen is not

necessary.

Sometimes, such

the case of turkey meat, the presence of oxygen may even

harmfiil because it causes

off-flavours (Inns, 1987). The shelf life of poultry products under MAP increases as

the concentration of carbon dioxide in

the

atmosphere

increases.

This

probably due to

the suppression of microbial growth. However, it has been observed that using carbon

dioxide in the gas mixture beyond a certain point results in product discoloration and

has negative effects on shelf

life.

Introduction to modified atmosphere packaging 165

Table 10.1 Recommended MAP conditions of various f^roducts

Commodity

Baked

products:

Bread (sliced)

Croissants

English muffins

Pitta bread

Pizza (crust)

Cheeses:

Hard cheeses

Soft cheeses

Fish:

Lean

Oily and smoked

Fruit:

Apple (whole)

Apple (sliced)

Avocado

Banana

Kiwifruit

Mango

Pineapple

Strawberry

Meats:

Beef

Pork

Poultry

Processed

foods:

Dried foods

Low-moisture foods

Fats and oils

Vegetables:

Asparagus

Broccoli

Cabbage

Lettuce (head)

Lettuce (shredded)

Mushrooms

Spinach

Tomatoes (mature)

Temperature range (°C) O2 (%*) C02(%*) Source

Ambient

1-4

0-2

0-5

5-13

12-15

0-5

10-15

0-5

-1-2

Ambient

0-5

12-20

2-3

10-12

2-5

60-80

1-2

3-5

2-5

1-2

3-5

100

20-40

1-5

8-11

3-10

2-5

15-20

20-40

25-35

0-100

5-10

5-7

10-12

10-15

10-20

Smith

etal.,

1990

Smith

etal.,

1990

Smith

ettf/.,

1990

Smith

etal.,

1990

Smith

etal.,

1990

Subramanian, 1998

Garthwaite, 1992

Kader,

1986

Lay, 1997

Kader,

1986

Kader,

1986

Kader,

1986

Kader,

1986

Kader,

1986

Kader,

1986

Blakistone, 1998

Floros, 1990

Blakistone, 1998

Floros, 1990

Kader,

1986

Kader,

1986

Kader,

1986

Kader,

1986

Price,

1992

Kader,

1986

Kader,

1986

Kader,

1986

* Volume or mole percentages; the remainder is nitrogen.

Fish and shellfish

The shelf life offish products packaged under modified atmospheres can be extended

significantly when the atmosphere is high in CO2 and the temperature is kept below

2°C.

This is mainly due to the inhibiting effect of carbon dioxide on the growth of

Pseudomonas

spp. and other psychrotrophic spoilage micro-organisms. In the case of

shellfish (crustaceans),

MAP

provides the additional benefit of reducing the formation

of black spots on the shells (Garthwaite, 1992).

166 Innovations in Food Packaging

The composition of the atmosphere used for

MAP

offish depends on the fat content

of the product. In general, an atmosphere of

30%

O2, 40% CO2, and 30%

is used

for lean

fish.

The elevated amount of oxygen contributes to the shelf-life extension of

marine

fish

through the reduction of trimethylamineoxide (TMAO), an osmo-regulator,

to trimethylamine (TMA) (Devlieghere et al., 2002). TMA is the major component

responsible for the undesired "fishy" odor. In the case of oily and smoked fish, how-

ever, the exclusion of

is recommended to avoid oxidative reactions and rancidity

development. Gas mixtures consisting of

60%

CO2 and

40%

are often used.

A negative effect of the application of modified atmospheres in

fish

packaging is the

increase of drip inside the package. This is due to the nature of the

fish

muscle, which

absorbs a great amount of carbon dioxide as dissolved

CO2

(Inns,

1987).

As a

result,

the

pH of the muscle is reduced, with a subsequent reduction in the water-holding capacity

of the muscle

proteins.

This

problem

can be

easily solved

with the

addition of an absorb-

ing pad (usually made out of cellulose) beneath the product.

The dissolution of carbon dioxide

in the

fish

muscle lowers the internal pressure of the

package, and may even cause package collapse. The incorporation of nitrogen into the

gas mixture of the package may solve this problem.

Bakery products

The main factor that limits the shelf life of bakery products

microbiological spoilage,

in particular mold growth, which is responsible for significant economic losses (Smith

and Simpson, 1996). Because of its bacteriostatic and fimgistatic properties, and its

inhibitory effect on insect growth, high carbon dioxide MAP can be used with bakery

products to eliminate growth of molds and subsequently

extend

the

product shelf

life.

However, the high solubility of carbon dioxide in water and fats may lower

the

pH of

the product and lead to some flavor changes. Another potential problem is that the

absorption of carbon dioxide by the product may result in package collapse. The latter

problem can be easily solved by using nitrogen as a

filler

gas. Nitrogen is also used in

MAP of certain bakery and snack products with low water activity in order to replace

oxygen and prevent rancidity problems (see Table 10.1).

An additional problem in

MAP

of some bakery products is that aerobic spoilage may

take place even when a very small amount of residual oxygen exists (e.g. trapped air

in the food, poor gas

flushing).

This problem has been addressed with the use of novel

MAP techniques such as active packaging, and in particular with the use of oxygen

absorbents. Oxygen absorbents eliminate all residual oxygen, and extend

the

mold-free

shelf life of certain products

by up

900%

as compared

traditional MAP techniques

(Smith

ah,

1990). More information on oxygen absorbents/scavengers

can be

found

in Chapter

of this book.

Fruits and vegetables

The live tissue of fresh

fi"uits

and vegetables respires and transpires. The respiration

rate varies greatly among different species, and depends heavily on temperature. As a

Introduction to modified atmosphere packaging 167

result, MAP of fresh produce requires

different approach as compared

other prod-

ucts.

The main goal of modified atmosphere applied to

fi-uits

and vegetables is to min-

imize the respiration rate of the product. This includes suppressing the production of

ethylene,

a gas

responsible for accelerating ripening and deterioration,

and

hastening the

onset of senescence in

fi-uits

and vegetables.

Reduced levels of oxygen and increased levels of carbon dioxide in the atmosphere

around fresh produce appear to have several positive effects. Among others, they pre-

serve product quality because they slow down respiration, decrease softening rates,

and improve chlorophyll and other pigment retention. In addition, as in other products,

elevated levels of carbon dioxide reduce the rate of microbial growth and spoilage.

On the other

hand,

the use of MAP techniques with fresh

produce,

especially fruits,

has a few potential hazards. The complete elimination of oxygen from the package

quickly results in anaerobic respiration, the production of ethylene, and, subsequently,

a fast and dramatic deterioration of the product quality. This is normally due to the

accumulation of acetaldehyde, ethanol, and organic acids, the development of off-

flavours, and, finally, the discoloration and the softening of the tissue. However, in

some cases

more important hazard is the potential growth of

Clostridium

botulinum,

which can grow under anaerobic conditions and may produce its deadly toxin.

Since the

mid-1990s,

the application of MAP to minimally processed

fi-uits

and

veg-

etables has grown dramatically, and it

has

now become

multi-billion dollar industry in

the USA

alone.

The treatments involved

preparing and handling minimally processed

fiiiits and vegetables result in fast deterioration and quality decline due to the dramatic

increase of physiological and biochemical activities in the tissue (Price and Floros,

1993).

These negative effects can

offset or significantly reduced by the use of MAP

and refrigeration. MAP extends the shelf

life,

maintains the high quality (particularly

the

fi-esh-like

characteristics), and improves the convenience of minimally processed

fruits and vegetables. As a result, MAP has contributed significantly to the increased

consumption of fresh fruits and vegetables by the average North American consumer.

Cheese

The quality of cheese deteriorates primarily because of surface mold growth and/or

lipid oxidation.

Both problems

can easily

addressed with

the

application of

vacuum

and the removal of

air.

This has been done successfiilly for many

years.

However, vac-

uum packaging of some cheeses has certain disadvantages - for example, the package

cannot be opened easily, and the vacuum packaging of soft cheeses or cheeses with a

crumbled texture may damage the product. The use of MAP overcomes these prob-

lems without sacrificing the benefit of extended shelf

life.

Hard cheeses like Cheddar are usually packaged under 100% carbon dioxide con-

ditions. In the case of soft cheeses,

mixture of

20^0%

carbon dioxide with 60-80%

nitrogen

preferred, to prevent package collapse. Solutions become more complicated

for MAP of mold-ripened cheeses or cheeses that produce carbon dioxide. In the first

case,

for example, the complete elimination of oxygen from the package would ulti-

mately result

in the

death of usefiil

fiingi.

On the

other

hand,

high oxygen concentrations

in the package would result in uncontrolled mold growth. Due to the large number of

168 Innovations in Food Packaging

cheese types and their wide range in composition and shelf

life,

the packaging of each

type of cheese should be considered separately (Subramaniam, 1998).

Other products

The list of products that utilize MAP techniques is not limited to the above examples.

Many other applications exist, including coffee, nuts, snacks, pasta, delicatessen

products, yoghurt, milk, and milk powders. In each case, extended shelf life is

achieved by the application of MAP principles.

Effect of MAP on micro-organisms - safety

issues

The extension of product shelf life by low oxygen atmospheres also results in the con-

trol of aerobic bacteria, which are mainly responsible for spoilage. Moreover, high

concentrations of carbon dioxide also extend the shelf life of MAP foods. The antimi-

crobial effect of carbon dioxide occurs at or near a 10% level, and increases with

higher concentrations. Using 20% carbon dioxide can control the growth of several

aerobes, including

Pseudomonas,

Acenatobacter

and Momxella, but higher concen-

trations in the ranges of

20^0%

usually give better

results.

However, very high con-

centrations of carbon dioxide may stimulate the growth of

the

anaerobe

Clostridium

botulinum.

Although the mechanism of carbon dioxide's antimicrobial activity is not

fully understood, it appears that CO2 extends the lag phase in the microbial growth

(Ray, 2001) in many

ways:

it penetrates the microbial cell wall and alters the cell per-

meability; it solubilizes inside the cells and produces carbonic acid (H2CO3), which

reduces the pH of the cell; and finally, it interferes with several enzymatic and bio-

chemical pathways inside the microbial cells, and thus reduces their growth

rate.

The

effect of MAP on each micro-organism and its rate of growth also depend on the food

itself,

the amount of oxygen dissolved or entrapped in the product, the available nutri-

ents,

and the presence of reducing components.

The potential for Clostridium botulinum (type E and non-proteolytic t3^e B)

growth in refiigerated

MAP

products is of major concern. Other food-borne pathogens

that belong to the non-sporeforming psychrotrophic micro-organism group can also

grow under modified atmospheres. This is especially true for facultative anaerobes,

including Listeria

monocytogenes

and

Yersinia

enterocolitica.

Moreover, if refriger-

ated products were subject to temperature abuse, mesophilic facultative microbes like

certain

Salmonella

strains and

Staphylococcus aureus

can also grow in MAP foods.

The inhibition of spoilage bacteria combined with the possible growth of psy-

chrotrophic pathogens makes the safety problems of MAP products more serious,

because the food may be unsafe for human consumption well before it becomes

organoleptically

unacceptable.

In order

improve the safety of

MAP

products and elim-

inate such hazards, additional measures are necessary. For example, the continuous

Introduction to nnodified atmosphere packaging 169

and tight control of the temperature is of paramount importance, not only to inhibit the

growth of mesophilic pathogens but also to maximize the antimicrobial effect of

carbon dioxide and minimize the growth rate of psychrotrophs. Another requirement

for improving the safety of MAP products is the use of high-quality raw materials and

their subsequent handling, according to strict guidelines for hygiene and sanitation.

Effect of MAP on nutritional quality

As mentioned above, MAP can significantly extend the shelf life of many products.

However, the effect of

MAP

on the nutritional quality of foods during storage is not

well understood. Recently, Devlieghere et al (2002) reviewed the subject, but

reported that only limited information is available. One of the well-known effects of

low oxygen MAP is the retardation of oxidative reactions (Farkas et al, 1997), which

in turn helps to maintain the nutritional quality of foods. According to Devlieghere

al.

(2002), no information is available regarding the effect of high oxygen MAP on

the nutritional quality of products such as meat and fish.

Regarding the effect of MAP on the nutritional quality of respiring foods, the

antioxidant constituents of fruits and vegetables are of special interest. Lee and Kader

(2000) reported that the loss of vitamin C after harvesting can be reduced by storing

apples and vegetables in atmospheres with reduced oxygen levels, and carbon dioxide

levels of

to 10%. Above this limit, and depending on the commodity, the loss of

vitamin C is accelerated. MAP of fruits and vegetables with either increased levels of

carbon dioxide or reduced levels of oxygen appears to prevent the loss of provitamin A

(Kader et al,

1989),

but it also inhibits the biosynthesis of carotenoids. Studies on the

retention of carotenoids in fresh-cut products under MAP did not show any loss

(Devlieghere et al, 2002). However, it appears that MAP has a negative effect on the

preservation of

anthocyanins.

Finally, the available information on the loss of gluco-

sinolates is very limited, and the effect of MAP is still unclear.

Combination of MAP with other technologies

Refrigeration

Many foods, including fresh

produce,

meats, and other minimally processed products,

are handled and marketed under refrigerated conditions. In these cases, MAP is used

synergistically to provide additional benefits and increase positive effects, such as

reduced

rates

of enzymatic, oxidative, and other undesirable chemical

reactions;

slower

respiration rates; delayed ripening; and suppressed microbial growth. The combina-

tion of high carbon dioxide atmospheres with refrigeration is of particular importance.

This is because the amount of dissolved carbon dioxide and

its

antimicrobial effective-

ness increase as temperature decreases (Gould and Jones, 1989).

170 Innovations in Food Packaging

Freezing

MAP of

frozen

products in the form of vacuum packaging is often

necessary.

Although

chemical reactions proceed very slowly under

freezing

temperatures, they may still cause

significant quality changes

over

time.

The oxidation of vitamins

and pigments

continues

to take place, and results in the loss of

color,

flavor

and nutritive value. Another com-

mon problem in

frozen

products, the so-called

freezer

bum, is due to water lost

from

the

surface of the product

the surrounding environment. All of the above problems can be

significantly reduced or completely eliminated

the application of vacuum packaging.

Irradiation

One of the big advantages of MAP is the extended shelf life it

provides,

while preserv-

ing the fresh state of the product. On the other

hand,

MAP brings some concerns about

the microbial safety of products. However, the combination of

MAP

with irradiation

treatments can minimize such safety concerns. Irradiation is a physical treatment that

exposes the food to small doses of ionizing radiation and results in the reduction of

microbial

loads.

Of particular importance is the fact that irradiation is not

heat treat-

ment and, just like MAP, it preserves the fresh like characteristics of products.

Furthermore, another advantage of irradiation

that it can

applied

pre-packaged

MAP products to reduce the risk of post-contamination.

Sometimes the combination of the two treatments leads to complications. The res-

piration rate of

fi*uits

and vegetables increases temporarily during and after the irradia-

tion treatment. This must be taken into account when designing the package. The film

permeability must

high enough

avoid

the

complete consumption of oxygen, which

may lead to anaerobic respiration, oflF-flavour development, and the production offer-

mentation by-products.

Furthermore, radiation treatments may negatively affect the packaging material

itself (Ozen and Floros, 2001). The package and its seal should both be able to with-

stand the radiation treatment, or the resulting defects may cause recontamination and

loss of the modified atmosphere. Research studies show that the negative effects of

irradiation are minimal at low doses (<10kGy), but some plastic materials are sus-

ceptible even at such low doses and therefore should not be used with irradiation. The

FDA has a list of approved packaging materials that can be used with different irradi-

ation doses.

Hurdle technology

In hurdle technology, an intelligent combination of various preservation techniques is

applied to the product. The "hurdles" are chosen carefully in order to maximize the

preserving effect while minimizing the impact on product quality. Usually, this is

achieved by combining processes with synergistic effects. MAP can act synergis-

tically with a number of

technologies,

such as refrigeration, freezing, canning, asep-

tic processing etc., resulting in a better quality product with longer shelf life and

increased stability.

Introduction to nnodified atmosphere packaging 171

Edible coatings

Edible coatings are extensively discussed in Chapter 15, and can be considered as a

special form of

MAP.

Their application on the surface of products limits gas exchange

and moisture transportation between the food and the surrounding environment (Kester

and Fennema, 1986). This results in the formation of an internal modified atmosphere;

however, in many cases this is not enough to provide the desired preserving effects,

and therefore an additional package is needed. However, the combination of edible

coatings with other packaging materials can lessen the overall need for packaging

materials and may reduce waste. Similarly, the combination of edible coatings with

traditional MAP can reduce the total packaging cost.

Biological control

Biological control of a certain micro-organism implies the use of antagonistic bacte-

ria, fungi, and/or yeast in order to retard or inhibit its growth. This alternative method

recently received significant attention due to the environmental and health concerns

that arise

firom

using chemicals to control micro-organisms. MAP alone retards micro-

bial growth, but its combination with biological methods may control the growth of

undesirable micro-organisms much better. This requires that the optimum growth con-

ditions of the antagonist are the same or similar to the recommended storage conditions

of the product. Further research is necessary to establish viable combinations of MAP

with biological control methods, and to test consumer perception and acceptance of

such products (Dock et aL, 1998).

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