Jung Han. Innovations in Food Packaging

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Sensory quality of foods associated with edible film and coating systems and shelf-life extension

435

shelf life and improve food quality. Edible films and coatings can also carry functional

ingredients, such as antioxidants, antimicrobials, nutrients, and flavors, to further

enhance food stability, quality, functionality, and safety (Krochta

al.,

1994;

Krochta

and De Mulder-Johnston,

1997;

Debeaufort

al.,

1998)

(Table

24.1).

The concept of

using edible films or coatings to extend the shelf life of foods and protect them from

harmful environinental effects has been emphasized

recent years. Increased con-

sciousness of environmental conservation and protection, the need for higher quality

foods, the demand for new food processing and storage technologies, and the scien-

tific discovery of the functionality of new materials have renewed researchers' and

industrial interests in edible films and coatings.

.jS

The success of an edible film or coating in extending the shelf life and enhancing

the quality of food strongly depends on its barrier properties to moisture, oxygen, and

carbon dioxide, which in

turn

depends on the chemical composition and structure of

the film-forming polymer and the conditions of storage. For example, due to the

hydrophilic nature of most film- and coating-forming materials, some edible films and

coatings are not satisfactory for controlling the moisture loss of high-moisture foods.

Also, one principle disadvantage of using edible coating on fresh produce is the poten-

tial development of off-flavors if the inhibition of

and

C02

exchange results in

anaerobic respiration.

addition, undesirable sensory qualities may develop on some

coated products, where non-uniform, white, and sticky surfaces become very unatbrac-

tive to consumers. This chapter focuses on the application of edible films and coatings

on fresh and minimally processed produce, illustrates the major quality attributes of edi-

ble

films

and coatings, discusses the applications of edible coatings for improving qual-

ity and extending the shelf life of fresh fruits and vegetables through case studies, gives

Table 24.1

Applications

and

functions

edible coatings

and

films

Foods

Film

mawrials Functionality

Poultry, meat, fish,

Gelatin; carraghenan and alginate Mold prevention;

02,

lipid

seafood coatings; whey protein; collagen, and moisture barrier; antioxidant

casein, cellulose derivatives

carrier; abuse protection;

texture improvement

Fruits, nuts, grains,

vegetables

Cheese, icecream,

Yogurt

Confections

Elaborate and

heterogeneous foods

(e.g. paste, puree,

cake, icecream cones)

Wheat gluten, whey protein, corn

zein, waxes, cellulose derivatives,

pectins

Gelatin, seaweed, pectinate

Corn zein, milk and whey

proteins, wax, MC

Mixture of stearic-palmitic acids

and HPMC; MC and palmitic

acid coating

02,

lipid, and moisture barrier;

antioxidant carrier; stickiness

prevention; salt binding

Fruit separation; elimination of

dripping; moisture barrier

02,

lipid and moisture barrier;

antioxidant carrier

Moisture barrier

Source: summarized information from Kester and Fennema (1 986), Cennadios and Weller (1990),

Krochta

etal.

(1994), McHugh and Krochta (1994), Krochta and De Mulder-Johnston (1997), and

Debeaufort

et a/.

998).

436

Innovations in Food Packaging

recommendations

insights regarding

of sensory test methods

and finally provides

Sensory quality attributes associated with

edible films and coatings

Theoretically, the use of edible films as protective coatings for food can help to retain

or improve product quality by:

Forming an efficient barrier to prevent moisture loss, thus controlling dehydration

Delaying the ripening process through their selective permeability to gases

Controlling the migration of water-soluble solutes to retain the natural color,

pigments and nutrients

Incorporating coloring, flavors, or other functional ingredients to enhance quality.

Hence the sensory qualities of foods are strongly associated with the applications of

edible films and coatings. Meanwhile, many active compounds used in the manufac-

ture of edible films and coatings, including edible polymers, plasticizers, and other

active agents, may impact the sensory attributes of wrapped or coated products, since

most active agents have their own characteristic flavor and color, and the interactions

among the compounds may generate unique flavors. Since functional edible films and

coatings are edible portions of the packaged or coated foods, it is expected that all

components of the edible films and coatings should not interfere with the organoleptic

characteristics of the food product. Generally, tasteless edible films and coatings are

desirable to minimize any taste interference. Fortunately, the concentration of most

active compounds used in edible films and coatings is usually very low, and hence their

taste effects may be negligible. When high concentrations of natural active agents are

added to edible films and coatings, the film and coating layers may possess a strong

flavor of the incorporated active agents. This phenomenon becomes more significant

when plant and herb essential oilslextracts or phenolic flavors are added to edible film

layers. Unfortunately, few studies have examined the sensory qualities of edible films

and coating layers as well as warped or coated food products. The following sections

will illustrate some sensory attributes of foods associated with the applications of edible

films and coatings.

Appearance

Many quality attributes contribute to the appearance of foods, including surface dehy-

dration, whitening, waxiness, shininesslglossiness, and discoloration (e.g. enzymatic

browning). The use of edible coatings may significantly impact the surface appear-

ance of products. For example, by reducing moisture loss, edible coatings can control

surface dehydration and discoloration. By selecting the right material, a coating may

Sensory quality of foods associated with edible film and coating systems and shelf-life extension

437

enhance the surface shininess of food. The

barrier function of edible coatings can

also prevent enzymatic browning discoloration in some fresh-cut fruits and vegeta-

bles. Hence an appropriately designed coating has the potential to retain or enhance

surface appearance of food during storage.

The delay of whitening on the surface of fresh carrots and the control of darkness

on fresh strawberries by the use of edible coatings has been demonstrated in several

studies. White discoloration on the surface of mini-peeled carrots is the result of

reversible surface dehydration and irreversible formation of lignin, which affect their

storage quality and shelf life (Bolin and Huxsoll, 1991; Howard and

Griffin,

1993;

Cisneros-Zevallos

al., 1995). Edible coatings controlled white surface discoloration

and enhanced orange color intensity by acting as a moisture barrier or surface moistur-

izer (Avena-Bustillos

al., 1993a, 1993b; Howard and Dewi, 1995, 1996; Cisneros-

Zevallos

al., 1995; Li and Barth, 1998). The same results were demonstrated with

sensory evaluation, where baby carrots coated with xanthan gum received less white

surface discoloration and greater orange color intensity ratings

than

those of uncoated

ones, hence the coating resulted in products with the best color attributes @lei

al.,

2001) (Table 24.2).

Enzymatic browning is a major factor contributing to quality deterioration in some

fruits and vegetables, especially in fresh-cut produce. Using edible coatings to reduce

transfer into the surface of the product andlor to carry anti-browning agents into

the coating formulation can prevent enzymatic browning.

addition, some coating

materials, such as whey protein, can act as

scavengers (McHugh and Senesi, 2000;

Lee

al., 2003). The benefit of using edible coatings to prevent enzymatic browning

has become more attractive and important along with increased market demands for

fresh and minimally processed fruits and vegetables.

Table

24.2

Effects of edible coatings on sensory attributes of baby carrots during storage

Time Treatment* White Orange Crispness Sweetness Bitterness

Fresh

Slipperiness

discoloration intensity aroma flavor

1st week

1 3.83= 4.54' 5.8Sa 5.13= 1 .92" 5.76= 5.7" 1.91'

2 1 .68bC 6.4~"~ 5.92" 4.~2~~ 1 .99a 4.83ab 5.2Sa 5.14~

3 2.48b 6.03~ 6.06" 4.48ab 2.23" 4.43b 4.8gab 6.Sa

4 0.92' 7.2 7a 6.1 4.03~ 2.98= 3.83b 3.8gb 7.57a

2nd week

1 6.1

3.82d 5.67" 6.1 6' 1.71b 4.7gab 4.82ab 1.1 2d

2 2.36b 6.4gab 5.42" 5.31a 1 .93ab 5.68a 5.71a 3.10'

3 2.2gb 6.19~ 5.61" 4.48ab 1 .9sab 5.41ab 4.66ab ~.27~

4 1 .4Oc 7.4Ia 6.01a 4.41

2.83a 4.3gb 3.91b 6.76a

Source: adapted from Mei

etal.

(2002).

*Treatment:

control;

xanthan gum alone;

xanthan gum/vitamin

xanthan gum/calcium.

a-d

Mean values with different superscript letters

the same column differ

0.05).

Sensory scale:

none,

intense.

438

Innovations in Food Packaging

The color, shininess, and transparency of edible films and coating layers vary sig-

nificantly depending on the chemical composition and structure of the polymer used,

which in

turn

impact the surface appearance of wrapped or coated foods. For example,

shellac-based coatings are commonly used in the fresh citrus fruit industry to increase

shine and marketability. Although imparting shine, the lack of durability of shellac-

based coatings reduces their use when commodities must be stored and shipped for

prolonged periods of time. If shellac-coated fruits experience too many changes in

temperature and humidity, causing them to "sweat", the shellac coatings may whiten,

causing the fruit to become unmarketable. This is especially true of fruits moved from

cold storage to a humid environment, and exposed to warm and

dry conditions such

as during store displays. The color stability of whey protein isolate (WPI) coatings is

also an issue, as milk proteins can undergo Millard reactions during storage, causing

yellowing (McHugh and Krochta, 1994; Miller et al., 1998). The rate of brown pig-

ment formation in whey powders has been shown to increase as storage temperature

and water activity increase from 25 to 45OC and 0.33 to 0.6, respectively (Labuza and

Saltmarch, 198

1).

Trezz and Krochta (2000) compared the yellowing rates of commer-

cial coatings to those of whey protein coatings during storage at 23O, 40°, and 55°C at

75%

RH,

and found that WPI coatings had lower yellowing rates than whey protein

concentrate (WPC) and the same rates as shellac; hydroxypropyl methylcellulose

(HPMC)

coatings had the lowest yellowing rates, and zein coatings became less yellow

during storage. It was concluded that WPI coatings could be used as an alternate coat-

ing to shellac or HPMC when low color change is desired, and WPC coatings have

potential applications when color development is desired.

Texture

Firmness and crispness are very important textural quality criteria of fresh produce, and

may be lost quickly during postharvest storage due to moisture loss and postharvest

ripening. While an edible coating reduces moisture loss and delays the ripening process

of fresh produce, it is also expected to help maintain product texture quality.

addition,

an edible coating may improve the mechanical integrity or handling characteristics of

products. It has been well documented in many studies that a coating has the potential

to maintain the firmness of fresh fruits and vegetables in comparison with uncoated

samples (Sumnu and Bayindirli, 1995; Mei et al., 2002; Han et al., 2004).

Edible coatings can improve the integrity of foods, especially frozen products.

Chitosan-based coatings result in at least a 24% reduction in the drip loss of

frozen-thawed raspberries (Han et al., 2004). Thus, edible coatings formed on the sur-

face of the fruit can survive the freezing process and storage, help to hold the liquid,

and prevent migration of moisture from the hit to the environment during freezing

and thawing. Reduced drip loss, in

turn,

improves the texture quality of frozen-

thawed products. In the case of frozen-thawed raspberries, a chitosan-coating con-

taining calcium increased firmness by about 25% in comparison with uncoated fruits.

Adding calcium to a coating formulation provided additional benefit in maintaining

the integrity and texture quality of frozen-thawed berries (Han et al., 2004).

Sensory quality of foods associated with edible film and coating systems and shelf-life extension

439

Flavor, odor, taste, and other sensory attributes

Edible coatings can retard the production of ethylene and delay the ripening process

due to their selective permeability to

and Cozy thus preventing development of off-

flavor and off-odor during postharvest storage of fresh produce. However, inappropri-

ately designed coatings may create an anaerobic condition, leading to the development

of off-flavors and off-odors.

Plasticizers are the major component of edible films and coatings, and could affect

the flavor and taste of the films and coatings. Common edible plasticizers are glycerol,

sorbitol, and polyethylene glycol. Polyethylene glycol is tasteless. Glycerol and sor-

bit01 taste sweet, but the sweetness of glycerol in protein films is negligible, while the

sweetness of sorbitol is noticeable. Hence, a plasticizer sho

avoid an undesirable flavor and taste.

Some coating materials may also generate undesirable o

due to the interactions among the components used for making films and coatings.

Citrus fruits and some apple varieties develop "off" flavors when coated with shellac.

Astringency, an undesirable mouthfeel associated with chitosan-based coatings, has

been a concern. When preparing chitosan-based edible coatings, acids (usually acetic

or lactic acid) are required to assist the solubility of chitosan in water-based solutions.

The pH of the solution is usually in the range of

3.9

to 4.2. In this pH range, the sen-

sation of bitterness and astringency develops, which has been a major limitation that

has made chitosan-coated foods less attractive on the market. The chemical reactions

promoting astringency in acids are still not well understood. One common hypothesis

is that acids precipitate the proteins in saliva or cause sufficient conformational

changes so that lubrication is lost when acids form complexes with salivary proteins or

mucopolysaccharides (Bate-Smith, 1954). The astringency associated with chitosan

could be the result of a rise in mine protonated groups when dissolved in an acid solu-

tion. The pKa of the amino group of glucosamine residue is about

6.3

(Muzzarelli,

Coating treatments* CON

LAE

Appearance likingX**

7.27a (1.29) 7.40a (1.33)

7.4Ia (1.28) 6.8~~ (1.46)

Overall liking*'

6.08'~ (1.93)

6.36a (1.73)

6.08~~ (1.77) 5.70~ (1.71)

Flavor liking (NS)

S.SS(2.26)

6.07(2.09)

5.74(2.11) 5.47(1.98)

Sweetness liking (NS)

5.33 (2.29) 5.75 (2.13)

5.34 (2.01) 5.00 (1.95)

Firmness liking (NS)

6.17 (1.90)

6.10 (2.07)

6.14 (1.75) 5.61 (1.96)

Source: adapted from Han

etal.

(2004),

using means of

consumer panelists.

Table

24.3

Consumer liking mean ratings of chitosan-coated strawberries after 1-day

storage at

2°C

and

89%

a-b

Different letter superscripts indicate significant differences at p

0.05

for each row separated

Tukey's HSD.

Nine-point intensity scale:

dislike extremely;

like extremely.

'~reatments: CON, control, uncoated samples;

AA,

acetic acid dissolved chitosan coating solution;

lactic acid dissolved chitosan coating solution;

ME,

lactic acid dissolved chitosan coating solution

containing

0.2%

vitamin

**,

***

indicates significance atp

0.05,

and p

0.001,

respectively.

440

Innovations

Food Packaging

1985); hence chitosan is polycationic at acidic pH value (Hwang and Damodaran,

1995; Fernander and Fox, 1997). Chitosan can selectively bind desired materials, such

as proteins (Kubota and Kihuchi, 1998). When binding with salivary proteins, the

afiinity of chitosan in acidic solutions might be increased. Rodriguez

al. (2003)

reported that at a higher pH of 4.6 to 6.3, less astringency is perceived.

recent study in the authors' laboratory has focused on improving the sensory qual-

ity of chitosan-based coatings, especially on overcoming astringency. It was found that

adjusting the pH of the coating solutions to above 4.0 can minimize astringency. Table

24.3 shows the results regarding consumer liking of five sensory attributes of straw-

berries subjected to different chitosan-coating treatments 1 day after coating. No dif-

ferences

.05) were identified in flavor, sweetness, and firmness liking among

coated and uncoated samples, or in samples coated with different coating formulations.

Edible coatings

improve the quality and

extend the shelf life

foods

case

studies

In general, an ideal coating should be safe, invisible, have no off-flavor and taste, be a

desirable moisture and gas barrier, and have a certain mechanical strength. Many fac-

tors determine the success of edible coatings in improving the quality and extending

the shelf life of foods. Among these factors, the chemical composition, structure,

methods used for forming films or coatings, storage conditions, and the properties of

the food itself (maturity stage, moisture and lipid content, etc.) are important factors.

When applying edible films and coatings, these factors have to be carefully considered.

Applications of selected edible coatings, based on coating material characteristics, for

improving the quality and extending the shelf life of fresh and minimally processed

fruits and vegetables are discussed in the following sections.

Whey

protein

Whey protein has excellent nutritional and functional properties, and the ability to

form films. Whey protein has been shown to produce transparent, bland, flexible,

water-based edible films with excellent oxygen and oil barriers, and is flavorless and

odorless (McHugh and Krochta, 1994). WPI coatings give a gloss comparable to

those of shellac and zinc coatings, and a higher gloss than HPMC coatings. Whey-

protein coatings have been utilized in coating peanuts and walnuts to increase their shelf

life, and in coating fresh-cut fruits and vegetables to control enzymatic browning.

Four different WPI/plasticizer formulations were compared to determine which

provided the most gloss and which was most stable with time when applied on choco-

lates (Lee

al., 2002a). The four plasticizers studied were glycerol, polyethylene gly-

col 400 (PEG 400), propylene glycol (PG), and sucrose, all in a

1 ratio with WPI.

WPUsucrose coatings provided the highest and most stable gloss. It was found that

with optimization, water-based WPUsucrose coatings could be an alternative source

of glaze to alcohol-based shellac coatings in the confectionery industry. In addition,

Sensory quality of foods associated with edible

film

and coating systems and shelf-life extension

441

consumer acceptance of WPI-coated chocolate-covered almonds was compared with

that for shellac-coated chocolate (Lee

al., 2002b). Four WPI coating formulations

were tested: two formulations were without lipid, and two were with lipid. The shellac

formulation consisted of 30% solids, of which 90% was shellac and 10% was propylene

glycol. A central location consumer test was carried out for attributes such as overall

degree of liking. Results indicate that water-based WPI-lipid coatings can be used as

an alternative glaze, with higher consumer acceptance than alcohol-based shellac.

Lipid oxidation is one of the leading causes of deterioration in peanuts. Oxygen

concentration plays an important role in oxidation. Oxygen uptake can be impeded by

specialized packaging systems or an edible coating, which in

turn

will decrease the

rate of lipid oxidation. WPI-based films were shown

be excellent O2 barriers at low to

intermediate relatively humidity (Mate and Krochta, 1996). WPI coatings were shown

significantly to reduce the oxidative rancidity of coated dry-roasted peanuts (Lee

al.,

2002c), thus extending their shelf life (Lee and Krochta, 2002). Descriptive sensory

analysis results revealed that the rancidity was significantly lower for whey protein-

coated peanuts than for uncoated ones (Lee

al., 2002~). This finding was also con-

firmed by static headspace gas chromatography (GC) analysis.

WPI coatings have also been used on fresh-cut fruits and vegetables to prevent

enzymatic browning (Le Tien

al., 2001; Lee

al., 2003; PCrez-Gago

al.,

2003a).

Color analysis of apple and potato slices coated with whey-protein solutions showed

that the coatings efficiently delayed browning by acting as oxygen barriers andlor

reactive oxidative species scavengers (Le Tien

al., 2001). Lactose present in com-

mercial whey can also account for the increased antioxidative activity. Moreover, the

addition of a polysaccharide-like carboxyrnethyl cellulose (CMC) to the formulations

can further improve the antioxidative potential of whey-protein films. Combinations

of WPC coating with antibrowning agents (ABA) were developed for extending the

shelf life of minimally processed (MP) Fuji apple slices packaged with Cryovac bags

and stored at 3OC (Lee

al., 2003). The coatings and ABA extended the shelf life of

apple slices by 2 weeks, maintaining their color and reducing the counts of mesophilic

and psychrotrophic micro-organisms. WPC reduced initial respiration rates by 20%.

Coated apple slices maintained acceptable sensory scores for color, firmness, flavor,

and overall preference, even after 14 days; WPC containing 1 g/100ml each of ascor-

bic acid and calcium chloride was the optimal treatment. In another study, edible coat-

ings from WPI-beeswax (BW) with various total solids

(8%,

12%, 16%, 20%) and

BW content

(0%,

20%, 40%, 60%, dry bases) were applied on apple slices (P6rez-

Gago

al., 2003a). Coated apples had higher lightness and lower Brown Index (BI)

than uncoated apples, indicating that whey proteins exert an antibrowning effect. The

BI decreased as the solid content of the coating emulsion increased. Increasing BW

content decreased enzymatic browning. Coating application did not reduce weight

loss in fresh-cut apples, probably due to the product's high relative humidity.

Sem

perfres

hTM

SemperfreshTM, manufactured by Agricoat Industries Ltd (7B Northfield Farm, Great

Shefford, Berkshire, England) is a commercial coating formulated with sucrose esters

innovations

Food

Packaging

of fatty acids, mono- and di-glycerides, and the sodium salt of carboxymethylcellu-

lose. It forms an invisible coating which is odorless and tasteless, and creates a barrier

that is differentially permeable to oxygen and carbon dioxide. In this way, a modified

atmosphere of reduced oxygen with slightly raised carbon dioxide levels results.

SemperfreshTM coating has been applied successfully to retard ripening on bananas

(Banks, 1984), apples (Drake

al., 1987; Bauchot

al., 1995; Surnnu and Bayindirli,

1995), and mangoes (Dhalla and Hanson, 1988) in aqueous solution containing 1.5,

1.0, and 0.75% (wlv), respectively. Effectiveness of coatings for increasing the shelf

life and post storage life ofAmasya apples was studied (Sumnu and Bayindirli, 1995).

SemperfreshTM applied after harvesting maintained apple firmness for 25 days.

Increasing the coating concentration caused increased firmness values and extended

the shelf life by 10,25,30 and 35% at treatment levels of 5, 10, 15 and 20 gll, respec-

tively. SemperfreshTM coating reduced weight loss and loss of ascorbic acid from apples.

Application of SemperfreshTM was effective in the retention of soluble solids, in reduc-

ing ascorbic acid loss, and in the retention of color change. It is concluded that

g/l

of SemperfreshTM is needed for coating apples.

Application of Semperfreshm for extending the shelf life and improving the qual-

ity of fresh sweet cherries and tomatoes has also been studied (Yaman and Bayondrl,

2001,2002). It was demonstrated that Semperfreshm effectively reduced the weight

loss and increased the firmness, ascorbic acid content, titratable acidity, and skin color

of cherries during storage. However, soluble solid content and sugar content were not

affected by coating. SemperfreshTM increased the shelf life of cherries by 21% at

3°C and by 26% at O°C without perceptible losses in quality. By dipping fully

matured tomato fruits (cv. Pusa Early Dwarf) in a 0.244% SemperfreshTM solution

for 10 minutes, the shelf life of the tomatoes at 24-3 lS°C, 77-90%

was extended

by delaying ripening and decreased physiological weight loss.

Chitosan

Chitosan, a high molecular weight cationic polysaccharide and tasteless fiber obtained

by the deacetylation of chitin, might be an ideal preservative coating material for fresh

fruits and other food products because of its excellent film-forming and biochemical

properties (Ghaouth

al., 1991). Chitosan, a by-product of the seafood industry, was

approved by the United States Food and Drug Administration (USFDA) as a feed addi-

tive in 1983 (Knorr, 1986). Chitosan has been shown to inhibit the growth of several

fungi (Ghaouth

al., 199 1

;

Jiang and Li, 200 1) and to induce defense enzymes, such

as chitinase and P-1,3-glucanase (Zhang and Quantick, 1998). Although the FDA has

not approved the application of chitosan in food, sigdcant research has been conducted

to understand its safety and potential application in foods

(Arai

al., 1968; Hirano

al.,

1990). Chitosan-based coatings have been studied for their wide application in food,

including fresh berries (Ghaouth

al., 1991; Garcia

al., 1998; Zhang and Quantick,

1998), raw eggs (Bhale

al., 2003), fresh-cut Chinese water chestnuts (Pen and Jiang,

2003), and peaches (Li andYu, 2001).

Chitosan coatings have improved the quality and storability of strawberries and

raspberries (Ghaouth

al., 1991; Garcia

al., 1998; Zhang and Quantick, 1998).

Sensory quality offoods associated with edible film and coating systems and shelf-life extension

443

A chitosan coating significantly reduced the decay of strawberries and raspberries

stored at 13OC and induced a significant increase of chitinase and P-1,3-glucanase

activities of the berries as compared to uncoated berries (Zhang and Quantick, 1998).

Chitosan coating proved almost as effective as the fungicide TBZ (thiabendazole)

in controlling the decay of berries stored at 13OC caused by

Botrytis cinerea

and

Rhizopus

sp. Chitosan coating had the beneficial effects of retaining firmness,

titratable acidity, vitamin C content, and anthocyanin content of strawberries and

raspberries stored at 4OC. Control of decay by chitosan coating was also demon-

strated with peaches (Li and Yu, 2001). The 5-10mg chitosanlml treatment showed

the potential to protect peaches from brown rot by prolonging the incubation period

of spores, reducing the rot area, and reducing the incidence of lesions. This protective

effect appeared to be related to chitosan induction of the defense response, as

well as an antifungal activity. Chitosan semipermeable coatings decelerated aging

by reducing respiration rate and ethylene production, reducing malondialdehyde

formation, stimulating superoxide dismutase activity, and maintaining membrane

integrity.

In a study of chitosan coating applied to fresh-cut Chinese water chestnuts (cv.

Guilin) stored at a low temperature, it was found that the coatings retarded the devel-

opment of browning in a dose-dependent fashion by reducing the activities of PAL,

PPO, and POD, and reducing phenol levels (Pen and Jiang, 2003). Sensory quality

was maintained longer in coated slices, and levels of total soluble solids, acidity, and

ascorbic acid were better retained, also in a dose-dependent manner. Overgrowth by

spoilage micro-organisms was delayed by the coatings.

A recent study evaluated the internal and sensory quality of eggs coated with chitosan

during

weeks of storage at 25OC (Bhale

al.,

2003). Three chitosans with high

(HMW, 1 100 KDa), medium

(MMW,

746 KDa), and low (LMW, 470 KDa) molecular

weights were used to prepare coating solutions. Coating with LMW chitosan was

more effective in preventing weight loss than with MMW and HMW chitosans (Pen

and Jiang, 2003). The Haugh unit and yolk index values indicated that the albumen

and yolk quality of coated eggs can be preserved for up to

weeks at 25OC, which is

at least 3 weeks longer than was observed for non-coated eggs. Consumers could not

differentiate between the external quality of coated eggs and that of non-coated eggs.

Overall acceptability of the coated eggs was not different from that of the control and

commercial eggs.

Lipid-HPMC composite coatings

Polysaccharides and proteins are good film-forming materials, but make poor mois-

ture barriers. Lipids, on the other hand, provide a better moisture barrier, but present

low mechanical integrity and require solvents or high temperatures for casting. Each

group of coating materials has certain advantages and disadvantages. For this reason,

many formulations are composite coatings of both groups. For example, there are some

studies regarding the development and application of lipid-HPMC composite coatings

for fresh hits and vegetables (PQez-Gago

al.,

2002,2003a, 2003b, 2004~).

444

Innovations in Food Packaging

HPMC-lipid composite coatings were developed and applied on mandarin oranges

and plums (PCrez-Gago

al., 2003b, 2003~). The coatings consisted of beeswax,

carnauba wax, or shellac, at two lipid contents (20% and 60%). It was found that the

weight loss of coated fruits decreased significantly as the lipid content increased.

Beeswax (60%) emulsions were associated with the lowest weight loss. Coated fruits

had higher internal C02, lower internal 02, and higher ethanol contents than uncoated

fruit. At 20% lipid content, internal

was lower and ethanol content was higher than

at 60% lipid content, which could be related to the low oxygen permeability of

hydroxypropyl methylcellulose and the higher viscosity of these emulsions, which

could have affected the final coating thickness. For plums, however, no differences in

weight loss were observed between uncoated and 20% lipid-coated plums, indicating

that the natural waxes of plums are as effective as coatings having 20% lipid. Fruit

texture was not affected by coating after short-term storage at 20°C. However, for pro-

longed storage at 20°C, the coatings significantly reduced texture loss and internal

breakdown compared to uncoated and water-dipped plums. In order to improve the

moisture barrier of "Autumn Giant" plums, a coating containing more than 20% lipids

needed to be applied.

Sensory evaluation of edible films, coatings

and coated products

Sensory evaluation involves the use of human subjects, and has been conducted for as

long as there has been human life on earth. As it is practiced now in relation to food

products, sensory evaluation is of incomparable value to food companies producing

products for an ever-changing and expanding market. Sensory science is not an old

science; however, it has its landmarks, as discussed by Lawless and Heymann (1998).

Most significant was the change from individual "experts" to a panel of subjects to

discriminate among samples, describe what they perceive, or relate how they feel

about the products presented.

Through the years sensory methods have been modified, largely based on what worked

and what didn't work, by food scientists, psychologists, and statisticians. There has

been little testing of one method against another. Critical to the choice of any method

is a clear statement of objectives. When working with edible films and coatings, the

major questions are, "What are the sensory properties of the product before and after

coating?', "What changes occur during storage and or subsequent processing?', and

"Does the consumer notice and care about any difference that is perceivable?' It is

impossible in this brief chapter to discuss how all sensory methods might be applied

to coated products. Rather, we will direct you to excellent references such as Lawless

and Heymann's (1998) ambitious and thorough covering of sensory principles and

practices. For those new to sensory evaluation, Meilgaard

al. (1999) is a user-

friendly guide to sensory common sense, laden with excellent case studies. For assis-

tance in implementing sensory evaluation in quality control, Munoz

al. (1992) is