Jung Han. Innovations in Food Packaging

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Internal modified

atmospheres

coated fresh fruits

and vegetables:

understanding relative

humidity effects*

Luis Cisneros-Zevallos andJohn

Krochta

Introduction

............................................................

173

Theoreticalapproach

.....................................................

175

Results and discussion

....................................................

177

Conclusions

............................................................

182

References

..............................................................

183

Introduction

Coatings applied to the surface of fruits and vegetables can be formed from one or

more components, and are commonly called "waxes" (Hagenmeier and Shaw,

1992).

However, coating materials used are actually various mixtures of lipids, proteins, car-

bohydrates, plasticizers, surfactants, additives, and solvents like water and alcohols.

The different materials used and the properties they possess provide a wide range of

characteristics possible for the coating systems.

Although research in this area is extensive, much of the work has been empirical

(Banks

al.,

1993).

Commercial coatings are used mainly to prevent water loss

or impart gloss to hits and vegetables. However, coatings have not been widely

*This chapter

has

been reprinted from the authors' previous article, published in 2002 in the Journal of Food

Science

67(8),

2792-2797, with copyright permission from the IFT (Institute of Food Technologists).

Innovations in Food Packaging

ISBN: 0-12-3 11632-5

2005 Elsevier Ltd

All

rights of reproduction in

any

form reserved

174

Innovations in Food Packaging

implemented on a commercial scale for specific atmospheric modification, in part due

to the variation in the response of fruits to apparently similar coatings and to uncon-

trolled factors which influence the coating performance. Different factors that can

affect the performance of coating-hit systems are type of fruit, coating surface cov-

erage, coating thickness and permeability, and temperature.

Banks et al. (1 993) proposed a mathematical model to explain the interaction between

a fruit and a coating system. Through this study they tried to explain the variability of

results found in the literature. They described coatings performing either as a film

wrap or by blocking pores. The first mechanism is called the loosely adhering coating

(lac) model. In the lac model the coating is assumed to function like a film wrap cov-

ering the fruit skin, where coating and fruit resistances act in series (Ben-Yehoshua

and Cameron, 1989; Hagenmeier and Shaw, 1992). In this model, hit resistance is the

result of pore and cuticle resistance operating in parallel. The modified atmosphere

(MA)

generated in the coated fruit will depend on coating permeability and coating

thickness. The second mechanism is called the tightly adhering coating (tac) model.

The tac model assumed considers that the coating tightly covers pores and cuticle, so the

porelcoating and cuticlelcoating resistances are added in parallel. In this model, pore

blockage is more important than coating characteristics. Thus, MA will depend on the

differing proportions of pores blocked by the coating. Between the lac and tac models,

the real mechanism is still unknown.

One factor not considered throughout the literature is the relative humidity

(RH)

storage conditions. RH is known to affect the permeability values of hydrophilic films

(McHugh et al., 1993,1994; McHugh and Krochta, 1994a, 1994b). Usually fruits are

stored between 90 and 95% RH; however, values lower than these are common in

commercial practice and in research studies.

The transmission of molecules through polymer films, known as permeability, is a

basic function of the chemical structure and other factors such as polymer morphol-

ogy, including density, crystallinity, and polymer orientation. Higher densities, higher

degrees of crystallinity, and cross-linking will all decrease the permeability of a poly-

mer film. The type of solvent used in casting films and the drying rate will also influ-

ence the permeability coefficient (Pauly, 1989). Plasticizers are often added to increase

film flexibility; however, this will also increase film permeability (McHugh et al., 1994;

McHugh and Krochta, 1994a; Mate and Krochta, 1996). Water is the most effective

plasticizer in hydrophilic films, and the amount of moisture in the films is related to

the

of the environment through their moisture sorption isotherm behavior (Gontard

et al., 1996). Thus, permeability of hydrophilic films and films formed as coatings

increases at higher RH due to increasing moisture concentration within the film (Mark

et al., 1966; Elson et al., 1985; Rico-Pena and Torres, 1990; Hagenmeier and Shaw,

1991; McHugh and Krochta, 1994a). This increase in permeability is also related to a

decrease in the glass transition temperature of the film.

amorphous polymer matrix

may exist as a viscous glass or a liquid-like rubber state. The change from one state to

another will strongly depend on water content or other plasticizers. The glass-rubber

transition will affect molecular mobility of the system which can be observed

changes

in viscosity, difisivity or flexibility of the system (Roos and Karel, 199 1; Buera and

Karel, 1993; McHugh and Krochta, 1994a).

Internal modified atmospheres of coated fresh fruits and vegetables

175

Reports of RH effects on coated fi-uits are few. Elson et al. (1985) reported how

affected the

and

C02

permeability characteristics of a hydrophilic coating

(Nutri-save) and the resulting coating effect on apples. Meheriuk and Lau (1988)

related the loss of normal ripening in coated pear fruits stored at low RHs to a reduc-

tion in coating permeation. Most of the work of RH effects has been on film perme-

abilities. For example, Hagenmeier and Shaw (1991) showed that

permeability of

shellac coatings was dependent on

RH.

Coatings cast from ethanol, compared to water,

had lower permeability values and were less dependent on

RH.

Similar

dependency

of gas permeability for ht coating waxes was reported (Hagenmeier and Shaw, 1992).

It was stated that this RH dependence is in large part due to the polar components used to

raise pH and solubilize the polymer. Rico-Pena and Torres (1 990) found an exponential

dependence of

permeability with RH for an edible

methylcellulose-palmitic

acid

film. Whey protein films have a similar dependence of

and aroma permeability on

RH conditions (McHugh and Krochta, 1994a; Miller and Krochta, 1997, 1998; Miller

and others, 1998). RH dependence on gas permeabilities for amylose starch films (Mark

al., 1966) and hydroxypropylated starch films (Roth and Mehltretter, 1967) were also

reported. More recently, Gontard and others (1 996) and Mujica-Paz and Gontard (1997)

showed the

and

C02

permeability dependence on RH for wheat gluten films.

The objective of this study was to use steady-state mathematical models to understand

the influence of relative humidity (RH) on the performance of hydrophilic films formed

as coatings on fruits, and to explain in part the conflictive results found in the literature.

Theoretical approach

Fruit-coating model system

The fruit system model is analyzed as a Modified Atmosphere (MA) model system.

The

model is described as a function of the internal atmospheres. In this study, we

used the lac model and the tac model for the specific case of complete surface cover-

age and gas exchange mainly through pores. Gas exchange depends on coating per-

meability and thickness for achieving internal MA. The assumptions considered for

the model are steady-state condition, constant temperature, no effect of

C02

on respi-

ration rate, and no internal atmosphere composition gradients within the fruit. The

model consists of equations that describe hit respiration rate, diffusion through skin

and coating (Banks et al., 1993), and the coating permeability dependence on RH.

Equations of the fruit model system are those proposed by Banks et al. (1993) for

apple fruits. However, the approach can be extended to other fruits as well:

(a) Fruit

oxygen

uptake (rO,, cm3 kgp1 spl)

where

is the maximum rate of

consumption (cm3kgp1s-I);

is the

Michaelis-Menten constant (atm) andpo2i is the internal oxygen partial pressure

176

Innovations in

Food

Packaging

(atm) (Banks et al., 1993).

is the equivalent to the substrate concentration that

yields half maximal rate of O2 consumption and indicates the affinity of the

enzyme system to the substrate.

(b)

Respiratory C02production (aerobic and anaerobic), (rc02, cm3 kg-' s-l)

where RQm is the respiratory quotient when O2 is unlimited

l), constant

0.007, and constant b

At steady state, ro2

JO2, where Jo2 is the diffusive

flux

between internal

and external atmospheres per unit weight of fruit. According to Fick's law:

where is the fruit surface area (cm3),

poze

is the external oxygen concentra-

tion (atm),

is the fruit weight (kg) and

@:'

is the O2 diffusion resistance

between the fruit's internal and external atmospheres

atrn cm-'). Resistance

values will include skin andlor coating thickness.

Combining equations (1 1.1) and (1 1.3) and solving f0rp0~~:

(d) Internal carbon dioxide partial pressure, obtained from equations (11.2) and

1.3), is:

Here, is the internal carbon dioxide partial pressure (atm),

R$%

is the

resistance to C02 diffusion between the fruit's internal and external atmospheres

(s atm cm-

I).

(e) The lower oxygen limit

This is defined as thepozi under which fermentation is initiated, being product spe-

cific. LOL is determined when rco,/ro2

1.1

this study, where RQ is the

respiration quotient. The LOL is calculated as 0.012 atm for the fruit model system.

(f) For the lac model, the coating resistance

(Rcoaf)

and fruit resistance @fiui")re

added in series.

Internal modified atmospheres of coated fresh fruits and vegetables

177

Thus:

A similar relationship describes the

tac

model in the case of complete surface coverage

and gas exchange mainly through pores. In this latter case, Rfiuit

RpO"'.

Permeability dependence on

The coating system used in this study was plasticized, edible whey protein film

(McHugh and Krochta, 1994a) with an exponential O2 permeability dependence on

(25"C), defined in this study as:

where

RH is relative humidity

(%),

PO2 is O2 film permeability (cc p,mlm2 d kPa) and

and

are constants. For example, values of

for WPI-sorbitol films (WPIISorb, ratio

60140) and WPI-glycerol films (WPIlGly, ratio 70130) are 0.026 and 1.208, respec-

tively. Values of

for WPIISorb and WPIIGly films are 0.049 and 0.037, respectively.

Coating resistance to O2

(e;t,

s atm cm-l) is defined as:

where H is coating thickness (cm) and n is the conversion factor (n

1.1727

10-

m2 d kPaIpm cm s atm) used to obtain P02 values in cm21atm s.

A general assumption for the coating system is that it equilibrates completely with the

of the external atmosphere (no moisture concentration gradient with the film) at a

constant temperature (25°C). It is also assumed that the surrounding air is well mixed;

therefore, the bulk air

is similar to the air

at the filmlair interface. The film

thick-

nesses used were 0.75, 1.3 and 2.6 pm. The ratio between C02 and O2 film permeability

(Pco,/Po,), also known as

film value, was assumed to be 2,4 or 10. It was also assumed

that the fruit skin resistance (Rfiuit) to gas exchange was not affected by changes in

RH.

Assumed values for the different variables in this study are as follows: A

160 cm2,

160 g,

0.02 atm,

0.21 atm,

5996 atm slcm,

~~9;

6535

atm slcm, V, values used were 5,20 and 30 cm3/kg h (from Banks

al.,

1993).

By using this steady-state approach we can study the effects of

onp~~~, pc02i,

ro2, rco2 and RQ of coated fruits if we know values for V,,

K,,

A, W,

Po2 and RfiU".

Results and discussion

and coating thickness effects on internal gas

pressure of coated fruits

Oxygen permeability of whey protein film coatings increases with RH in an exponen-

tial form according to equation (1 1.7) (Figure 1 1.1). The same tendency is shown for

C02 permeability for an assumed

value of 4.

178

Innovations in Food Packaging

Figurr

11.1

Exponential relationship of O2 and C02 permeability with relative humidity

RH) for

a plasticized whey protein film (60% whey protein/40% sorbitol) at 20°C.

Figure

11.2

Predicted

and

pco,,

of WPI/Sorb coated fruits stored under different RH conditions and

for different coating thicknzss. Film

4 and film coating thicknesses are 0,0.75,

1.3

and 2.6 Fm. Fruit

V,,,

is 20cm3/kg h at 20°C.

0.2

When a hydrophilic film is formed as a coating on a fruit, the model predicts

pozi

andpco,i change

the external

decreases (Figure

1.2).

Internal oxygen is reduced

and carbon dioxide increasingly accumulated within the hit as

decreases. Thus,

storing hits at a given

(e.g.

65-70%

RH)

can deplete thepOzi inside the hit and

induce a large

pcoZi

increase. This effect will be influenced by coating thickness. For

Increasing

II I

thickness

Ill

-1-1-

I-

Ill

0.1

\\ \

50 55 60 65 70 75 80 85 90 95 100

Internal modified atmospheres of coated fresh fruits and vegetables

179

a given RH, the

pozi

within the coated fruit is lowered and

pcoZi

is increased with an

increase in thickness. As a result of this, fruits coated with thick films may reach large

pcozi at higher RH compared to fruits coated with thinner films.

Gas exchange will show similar tendency, with a decrease in ro2 and rco2 when RH

decreases. The explanation is that as

decreases, film O2 and C02 permeance

decreases, thereby lowering the

pozi

levels within the fruit. Increasing film thickness

will reduce even more the O2 and C02 permeances and lower even more the pozi

within the fruit. When a lower oxygen limit (LOL) is reached, anaerobic respiration

will dominate, inducing large rco2 production and accumulation of large

pcozi

within

the fruit at a "critical" RH. Thus, anaerobic conditions can be reached at different RHs

depending on coating film thickness.

Although fruit respiration has not been studied as a function of film thickness,

investigations have been based on the amount of coating material applied or coating

solution concentration (BenYehoshua

al., 1970; Banks

al., 1993). Film thickness

has been related to coating solution concentration (Park

al., 1994a, 1994b) and will

probably be influenced by physicochemical properties of the coating solution.

Coating thicknesses have been reported to be in the range of 1-5 pm (Trout

al.,

1953; Brusewitz and Singh, 1985; Elson

al., 1985; Hagenrneier and Shaw, 1992)

and from 4.5-13 pm (Park

al., 1994b). Our model predictions may explain in part

the variability observed in the past when studying coated fruit systems. Changes in

RH and variation in coating thickness can interact, inducing large effects in internal

of coated fruits.

and maximum respiration rate effects on internal gas

pressure of coated fruits

We considered a fruit model with three different Vm values (low

moderate

20,

and high

30cm3/kg h), which represent fruits with differing Vm values or fruits

stored at different temperatures. Coated fruits have a constant film thickness.

In this case the model predicts that coated fruits with higher Vm would yield lower

internal

pozi

decreases, compared to coated fruits with lower V,. The trend is

similar to the effect observed when film thickness decreases in coated fruits (Figure

1 1.2). Similarly, internal

pcoZi

will be larger for fruits with higher Vm values as

decreases. For all fruits there is a RH at which a large increase inpcoZi will occur. This

critical RH will be lower for fruits with lower Vm, and higher for fruits with higher V,.

Oxygen uptake, ro2, for coated fruits with differing Vm will also be affected by

in similar fashion.

We hypothesize that coated fruits with a large Vm will reach the LOL at higher RH

values compared to fruits with a lower V,. Even though film permeability declines with

decreasing RH,pozi, ro, and rco2 for fruits with lower V, remains fairly constant down

to lower RH values. This makes it possible to store fruits with low Vm over a wider range

of RH conditions. Fruits with a large Vm are severely affected by RH when coated

with hydrophilic films. Large decreases inpo2, ro, and rco2 are observed with small

drops in RH. This response can be explained because of the higher demand of

180

Innovations

Food Packaging

by the fruit and the decreasing

film

gas permeability with lower

RH.

Therefore, coated

fruits with a large V, can only be stored over a short range of

before anaerobic

conditions are reached.

Internal

C02

plots

influenced

RH,

thickness,

maximum respiration rates and film

values: their

importance in coating design

Carbon dioxide vs oxygen plots for internal gas composition of coated fruits can be

used in a similar way to the C02 vs O2 plots of external gas composition used for

modified atmosphere packaging design (Mannaperuma

al.,

1989). The latter plots

are used to target

external^^^^

andpc02e combinations or "windows" for storing fruits

under external modified atmospheres. For coated fruits, we use the po2i and pco2i

within the fruits, and the target "windows" would be the combinations of internal gases

appropriate for storage. These plots can also give us information on the internal

LOL

values, above which fkuits can be stored safely and under which anaerobic conditions

are generated.

By changing external RH conditions for a coated fruit, we can generate a

pco,i

poZi

plot. Each point of the curve corresponds to a different

condition. The posi-

tion of the curve will be influenced by the fruit's V,, producing curves closer to the

X-axis for those fruits which have lower V, (Figure 11.3).

Rpwa

11.3

Predicted

pcozi

plots generated

storing WPI/Sorb coated and uncoated fruits under

different

conditions and different V,,, values. Fruit V,,,values are

and

cm3/kg h. Film thickness is

1.3

pm and Film

Internal modified atmospheres of coated fresh fruits and vegetables

181

The ratio between O2 and C02 permeabilities (Pco2/Po2) also has an effect on these

plots. Film

values are generally in the range of 3-5 for coating materials (Hagenrneier

and Shaw, 1992). Coated fruits with larger film

values will yield curves closer to the

X-axis and have smaller slopes, compared to coated fruits with smaller film

values.

More recently, Gontard

al. (1996) have shown that

increases with increasing

depending on film type.

generated curve assuming non-constant

value will have

similar trend as those curves generated assuming constant

values (Figure 1 1.3).

The composition of the film coating will influence the moisture sorption properties

of the coating material, and may cause the dependence of film gas permeability and

values on

(Gontard

al., 1996). The trend of hydrophilic films is to have similar

moisture sorption properties and gas permeability dependence on

RH.

However, gas

permeabilities of non-hydrophilic films (e.g. pure lipid films) are not affected by

since their moisture sorption properties are negligible or independent of

(McHugh

al., 1993). For emulsified wax films (e.g. commercial waxes) the gas permeability

dependence on

will depend on

film

composition. For example, an emulsion

film

may

consist of lipid particles dispersed in a hydrophilic matrix, giving a gas permeability

dependence on

between that of a hydrophilic film and a pure lipid film. This gas

permeability dependence may also be influenced by lipid particle size, particle arrange-

ment within the matrix, and lipid concentration (Shellhammer and Krochta, 1997).

Curves similar to those in Figure 11.3 can also be constructed for coated fruits

stored at only one

RH.

For a coated fruit with a given

and constant film

value,

changes in film thickness or film permeability will give points on the same curve. For

example, thicker films will give points shifted towards the left, while films with a

larger permeability will give points shifted to the right. This indicates the possibility

of tailoring films with appropriate

values, thickness and permeability characteris-

tics, allowing us to reach specific combinations ofpozi andpcoZi or avoid detrimental

anaerobic conditions.

These

pc02i

POti

plots can be used as tools to design coatings which can target

specific combinations of O2 and C02 for coated fruits (Banks

al., 1997). Coating

design for fruits with differing

values would be possible by selecting coating

materials with appropriate film

values, permeabilities and thicknesses, and by con-

trolling

conditions.

Possible sources of variation in internal gas composition

One assumption in this model system is that the hydrophilic coating equilibrates com-

pletely with the

of the external atmosphere, with no moisture concentration gra-

dients within the

film.

Film permeability is a function of moisture concentration, which

is related to

of the surrounding air. Thus, film permeability would be constant

throughout the film with our assumption. This assumption may be valid if the fruit

skin resistance to water vapor is high and the

film

resistance to water vapor is low. In this

way the moisture concentration within the film would be similar to that of the coating

surface facing the atmosphere. However, fruits with low skin resistance to water vapor

may generate large gradients of moisture concentration within the coating. For example,

fruits with low skin resistance that are exposed to low

may lose moisture readily,

.,,

,nnovations

rood Packaging

exposing the inner fruitlcoating surface to large moisture concentration values and the

coatingfair interface to smaller moisture concentration values. If this occurs, then the

average coating permeability would be in between the permeability values for the con-

ditions at the frultlcoating and the coatinglair surfaces, depending on the moisture sorp-

tion isotherm of the biopolyrner. In the extreme case of fresh cut surfaces of fruits

(Baldwin

al., 1995), coatings facing the fruiucoating surface will most likely be

exposed to a water activity of =1

(==

100%

RH). This will indeed increase the overall

coating permeability and reduce its barrier properties. Thus, we propose that similar coat-

ings applied to different fruit surfaces may have quite different barrier performances.

It was also assumed that there is no boundary layer resistance to water vapor trans-

mission. The existence of a boundary layer would increase the moisture concentration

of the coating due to an increase in the surrounding water vapor pressure adjacent to the

fildair surface. This increase in film moisture concentration will increase the overall

coating gas permeability affecting the internal gas composition. The assumption of a

negligible boundary layer will hold if the air velocity of the surrounding atmosphere

is high enough to reduce it. In commercial storage conditions as well as in research,

this assumption is not always met.

Another source of variation is the

gradients experienced in commercial storage

facilities, as well as in research techniques used for controlling RH. In the latter, it is

common to adjust RH of the air inlet in flow-through systems when measuring respi-

ration of fruits in jars. However, large

gradients may still occur inside the jars,

depending on the speed and

of the air inlet, the amount of fruits in the jar and the

fruit water loss rate.

Other sources of variation to consider are temperature fluctuations and film thick-

ness variations. Changes in temperature may affect the fruit

V,,

the film gas perme-

ability and the external

conditions. Film thickness may vary

individual fruits,

changing the total gas resistance of the fruit.

In summary, the predicted trends obtained by varying RH conditions support the

idea that film coating permeability and coating thickness are important in modified

atmospheres of coated fruits, following either a lac model or a tac model for the specific

case of complete pore coverage. However, if incomplete pore blockage is achieved,

then film coating permeability and thickness may not be important in modifying the

atmosphere within the fruit (Banks

al., 1993). Thus, for the incomplete pore block-

age model, RH would not have a major effect in the

of coated fruits.

Conclusions

It is possible, when using the approach described in this paper, to observe and under-

stand the interaction of storage conditions (e.g. RH) and composition of film coatings

applied to a fruit surface. This analysis can explain, in part, the variability of results

observed when coatings are applied on a commercial scale and

research. The approach

supports the idea that film coating thickness and permeability play an important role

in internal gas modification of coated fruits. Therefore, it is necessary to understand