Effect of packaging and storage on the sensory quality of cooked chicken menu items served from chilled vending machines

J Sci Food Agric 1989,48, 323-338

Effect of Packaging and Storage on the Sensory Quality of Cooked Chicken Menu Items Served from Chilled

Vending Machines

Helen Young," Halliday J H MacFieS and Nicholas Light5

Division of Food Studies, Department of Catering and Hotel Management, Dorset Institute of Higher Education, Poole, Dorset BN12 SBB, UK

(Received 29 June 1988; revised version received 22 July 1988; accepted 3 August 1988)

ABSTRACT

This study examined the effects of packaging on the sensory qualities of two cooked chilled foods (deep j i e d chicken drumsticks and chicken a la king) after extended chilled storage. Quantitative descriptive analysis was used with a trained taste panel to score the sensory attributes of the two products at various stages of chilled storage up to a maximum of 14 days at 0-3°C.

Storage had a significantly (P<O.O5) deleterious effect on all the sensory attributes of chicken drumsticks. Just over half the sensory characteristics of chicken a la king were significantly affected and these effects were mainly concerned with appearance. Vacuum and modijied atmosphere packaging extended the shelf life of both products in the sense that the changes in sensory attributes after 11-14 days were the same as in the cling-film-wrapped products after 4 days. The vacuum and modified atmosphere treatments were almost indistinguishable in their effects on the sensory attributes.

Key words: Sensory, packaging, shelf life, storage time, chilled foods, vacuum packaging, modified atmosphere packaging, chilled vending machines.

* Present address: OXFAM, 274 Banbury Road, Oxford OX2 7DZ, UK. $ To whom correspondence should be addressed at: AFRC Institute of Food Research- Bristol Laboratory, Langford, Bristol BS18 7DY, UK. 4 Present address: Devro Ltd, Moodiesburn, Chryston, Glasgow G69 OJE, UK.

323

J Sci Food Agric 0022-5142/89/$03.50 0 1989 Society of Chemical Industry, Printed in Great Britain

324 H Young, H J H MacFie, N Light

INTRODUCTION

Chilled food vending has been a method of food service in the United Kingdom since the mid 1960s, and significant growth has been seen over recent years (Light et a1 1987a). The major advantage of this food-service system is that it provides a 23.5 h service (30 min for cleaning and loading the machine) with a minimum of food-service staff. It differs from other catering systems, including traditional cook-chill catering, in that individually packaged food items are stored and displayed chilled in a food vending machine prior to their direct automated purchase by the consumer, who may also reheat the food in an adjacent microwave oven. Thus food vending represents a complex system with a number of processing parameters that will influence the end quality of the product.

The delayed effect of vacuum packaging and modified atmosphere storage (MAP) on the rate of microbiological spoilage of raw, cured and fermented meats is well documented. With the exception of McDaniel et at (1984) and the present authors (Young 1986; Young et a1 1987) no similar studies on cooked food have been carried out. McDaniel et a! (1984) found that storage of pre-cooked beef roasts in a 100% CO, atmosphere gave lower microorganism counts than vacuum packaging. Young et a1 (1987) reported similar findings using modified atmosphere packaging (MAP) and also vacuum packaging. Very little information on the sensory qualities of such stored cooked chilled food products, other than rancidity, has been reported.

The occurrence of rancidity in cooked meats, with the development of a characteristic flavour, within a few hours of cooking, is well documented as a result of both sensory and chemical analyses (Pearson et al 1983). Cooked chicken is especially vulnerable to pronounced flavour changes during refrigerated storage (Jacobson and Koehler 1970; Harris and Lindsay 1972; Pokorny et a1 1982; Dawson and Gartner 1983), but little work has been undertaken on a composite chicken dish, such as chicken a la king, where the addition of other ingredients may have an effect.

In cooked meats rancid flavours are frequently described as ‘warmed over flavour’ (WOF), and according to Enser (1985) are organoleptically different from rancidity in uncooked meats, probably because of the differences in the side reactions after hydroperoxide decomposition. The chemistry of the development of WOF in meats has been reviewed by Sat0 and Herring (1973). Uebersax et ul(1978) and Jantavat and Dawson (1980) reported that vacuum- and N,-packed chicken and turkey meats showed similar thiobarbituric acid values and less lipid oxidation than did the meats packaged with CO, and air.

The purpose ofthis study was to examine the effects of packaging and storage in a chilled vending machine on the sensory quality of deep-fried chicken drumsticks and chicken a la king. It is part of a larger study investigating other aspects of product quality resulting from a chilled food vending system (Young 1986; Light et a1 1987a,b; Young et a1 1987). Several previous studies on stored cooked foods have simultaneously examined sensory attributes alongside hedonic ratings for acceptability or preference. In this study trained expert panels were employed to qualify and quantify the sensory characteristics of the two menu items chosen for the

Sensory quality of chicken menu items j o m vending muchiries 325

research, and large consumer panels were used to determine acceptability. The latter will be reported elsewhere.

MATERIALS AND METHODS

Preparation of chicken a la king and chicken drumsticks

Both dishes were prepared under conditions of good catering practice, and all time and temperature limits were within those recommended by the guidelines for the preparation and storage of pre-cooked, chilled foods laid down by the Department of Health and Social Security (1980). Fried chicken drumsticks and chicken a la king were prepared as detailed by Young et a1 (1987). The conditions of frying for preparation of the chicken drumsticks were controlled by standardising the rate of adding fresh oil to the used oil in the fryer, ie after each frying session the oil was filtered and the fryer was cleaned out (800/, used oil and 20% fresh oil were used for each frying session). Operatives wore gloves and face masks for all stages of food preparation.

Packaging and storage

The three packaging treatments were cling wrap, vacuum packaging and modified atmosphere packaging (MAP). The cling-wrap film was supplied by Perfarap, High Wycombe (rates of transmission: water vapour at 38"C, 90% RH, 90 g m-' day- ' ; at 1 atm 25"C, 75% RH, 0, 5400mlm-2 day- ' ; CO, 35000mlm-Z day-'). Amilon bags were used for the MAP and vacuum packaging and were supplied by Otto Nielsen, St Albans (similar conditions as above, water vapour 6-log m-' day-'; 0 , 6 0 ml rn-' day-' and CO, 250 ml m-' day-'). The MAP and vacuum packaging were carried out with a Multivac A300. The gas composition used for MAP was 70% CO, and 30% N,. Portions (50 g) of chicken a la king or individual drumsticks were placed into polypropylene containers (12 x 9 x 4.5 cm) suitable for microwave reheating. The containers were either inserted into amilon bags if to be packed under modified atmosphere or were wrapped in cling film. The samples to be vacuum packed were placed directly into the amilon bags.

Storage in a chilled food vending machine

The samples were stored in a Rowe 448 chilled food vending machine (CFVM). Its operating temperature was monitored using a Grant miniature time/temperature recorder. The time of storage depended on the particular experiment.

Selection and training of panellists for sensory experiments

Sensory evaluation was by means of quantitative descriptive analysis (Stone et al 1974, 1980; BSI 1980). Prospective assessors were students or staff at the Dorset Institute of Higher Education, all of whom were unpaid volunteers.

Initial screening tests included recognition of the four basic tastes (Amerine et a1 1965); arrangement of four solutions of sucrose (0.3-1.2%) in order of strength; and recognition of five commonly encountered food odours (vinegar, vanilla, peppermint, almond and onion). Candidates who were not able to fulfil these

326 H Young, H J H MacFir, N Light

criteria were rejected. Successful volunteers went on to take part in the training programme, which consisted of eight 45-min sessions, during which the assessors identified the important sensory attributes of the different samples. Each panel member independently examined samples and recorded their findings, followed by a group session for reporting and discussion of individual findings. A sensory profile of the perceived characteristics of the product was established, consisting of the consensus definition of the important attributes. Evaluations were made by placing a vertical mark across an unstructured line 15 crn long and anchored at either end by terms indicating the extremes of each characteristic (Table 1).

Three separate panels were used for each product due to the large volume of work involved and the voluntary nature of the sensory panel.

A total of 15 people volunteered and successfully completed the screening tests for the chicken drumstick taste-panel training programme during which attendance dwindled leaving a core of eight assessors who participated in the actual experiment. The final panel for evaluating the chicken i la king consisted of six assessors from a total of 12 volunteers..

The descriptions of the sensory attributes of the two menu items agreed by the two sensory panels are shown in Table 1. It was noted that for chicken a la king the product should be inspected in its chilled state (as it would be perceived at the point of choice in the CFVM), as well as after microwave reheating.

Experimental design and statistical analysis

The experiment incorporated a split plot design with subunit treatments in Latin squares (Cochran and Cox 1957). Factor A, the whole units, consisted of packaging (cling, vacuum and MAP), factor B, the subunits, consisted of length of storage (0,4, 7, 11 and 14 days), and factor C consisted of the assessors (eight evaluated the chicken drumsticks and six evaluated the chicken a la king). The position of the three packaging types was randomly allocated in each replication. To account for the fact that there were only three levels of storage time (0, 4, 7 days) for the cling- wrapped products and five levels (0,4, 7, 11, 14 days) for the MAP and vacuum- packed products, it was decided to use two 3 x 3 Latin squares for the cling products and one 5 x 5 Latin square for both the M A P and vacuum products. The Latin squares were randomised and the first column of each square was assigned to the first replication of the experiment. The design was repeated for each assessor. Thus, each column of the Latin square determined the order of presentation to the subjects. One block of three or five samples was presented once, and at any one session assessors would examine no more than two blocks.

The effects and interactions of the factors packaging, storage and replications were estimated separately for each assessor to examine their discriminative ability. The analysis was then repeated on the combined data to provide a panel summary for each attribute.

RESULTS Evaluation of performance of individual assessors Inconsistency of scoring denotes that the replicate means of a particular attribute differed significantly. Discrimination of assessors is measured in terms of the

Sensory quality of chicken menu items from vending machines 321

TABLE 1 The attribute descriptive terms selected for each product

Sensory attribute End-point labels

Chicken drumsticks

Appearance 1. Degree of shrivelling 2. Brightness 3. Dry appearance of flesh 4. Compact appearance of flesh

5. Natural fried chicken odour 6. Rancid odour 7. Blaqd odour

8. Juiciness of flesh 9. Degree of chewiness

Odour

Texture

10. First bite

Flavour 11. Fresh fried chicken flavour 12. Rancid flavour 13. Bland flavour

Chicken a lu king

Appearance 1. Granular/pasty 2. Gelatinous texture 3. Depth of colour 4. Sauce coating properties

(peas/pimentos) 5. Appearance of meat 6. Shade of pea colour 7. Pea colour range 8. Surface oil on reheated sample

9. Sauce odour Odour

10. Depth of chicken odour

Texture 11. Sauce texture 12. Meat texture 13. Variability of pea texture

Flavour 14. Staleness 15. Acidic flavour 16. Degree of sweetness 17. Degree of saltiness 18. Chicken flavour of meat

Low score High score

No shrivelling Not bright Not dry Moderately compact

Weak Absent Weak

Not juicy Low Less resistant

Weak Weak Weak

Absent Not set Weak

No coating Broken up Fresh pea colour Not variable Absent

Fresh Weak

Coarse Breaks down readily Not variable

Absent Absent Weak Weak Bland

Very shrivelled Very bright Very dry Very compact

Strong Strong Strong

Very juicy High More resistant

Strong Strong Strong

Present Well set Full

Coats well Whole Olive-khaki Variable Present

Stale Strong

Smooth Chewy Variable

Present Present Strong Strong Full

H Young, H J H MacFir, N Light 328

TABLE 2 Panel performance: number of attributes for which assessors show significant differences (inconsistency) across replicates and among storage times within a packing type

(discrimination)

A s s e s s o r Inconsistency Totul Discrimincition Totul

Vac M A P Cling Vuc M A P Cling

Chicken drumsticks 1 2 2 3 7 1 7 3 11 2 0 0 0 0 11 9 13 33 3 1 6 2 9 8 3 8 19 4 2 0 0 2 10 9 9 28 5 6 4 2 12 3 4 8 15 6 0 0 0 0 9 6 4 19 7 0 0 0 0 8 9 9 26 8 0 I 0 1 5 6 10 21

1 2 3 4 9 1 1 4 6 2 1 4 3 8 4 2 4 10 3 0 1 0 1 3 4 5 12 4 5 2 3 10 1 6 1 8 5 6 2 0 8 6 3 2 11

Chicken u la king

number of significant F ratios comparing different storage times within a packaging type. The results are summarised in Table 2.

Except for assessor 5 the panel was generally consistent in scoring chicken drumsticks. The three attributes that caused most difficulty were bland odour, rancid odour and degree of chewing. In the chicken a la king, acidic flavour and chicken flavour were the most difficult attributes for the assessors to score consistently. Bearing in mind the larger number ofattributes in thechicken a la king profile (18), the level of consistency for this product is about the same as for the drumsticks.

In terms of discrimination, assessors 2, 4, 7 and 8 were the best for chicken drumsticks and assessor 1 only showed discrimination of the MAP samples. Assessors showed much less discrimination in the chicken a la king attributes but no one assessor was particularly different. It was concluded that all assessors in both panels were showing adequate consistency and discrimination, and so they were included in the panel analyses described in the next section.

Sensory analysis of chicken drumsticks

The mean scores for each sample and attribute are presented in Table 3. All sensory attributes were significantly (P < 0.05) affected by storage time, with the greatest effect occurring between 0 and 4 days of storage. The fried chicken flavour and odour scores were the attributes most influenced by storage time, showing large losses with increased storage.


TABLE 3 Panel means (along a 15-cm scale) for sensory attributes of chicken drumsticks classified by

packaging type and length of storage

Pack Storage Degree of Brightness' Dry Compact Fried Rancid (days) shrivelling' appearance' appearance' chicken odour'

odour ' Vacuum 0 2.97 4.94' 1.35" 2.11" 6.23' 0.54"

4 2.34 2.76" 4.34' 4.17' 2.39" 1.99' 7 2.53 3.26" 4.17' 4.94' 2.29" 2.49'

I 1 2.25 2.47" 4.58 ' 4.98' 1.56" 3.38' 14 3.18 2.20" 4.64' 4.92 ' 1.52" 3.96'

MAP 0 2.76" 4.98 2.03" 2.3 5.41 ' 0.78' 4 3.84' 3.73 ' 3.77' 3.70 2.46" 2.14" 7 3.66".' 3.58' 4.04 ' 4.12 1.94" 2.39"

11 3.75 ' 2.74" 4.66 ' 4.27 1.32" 4.75 ' 14 4.12' 3.20b 5.00' 4.35 1.14" 4.20h

Cling 0 2.19" 5.03 I .74" 2.33" 5.95' 0.80' 4 4.33b 3.05" 3.80' 3.85'.' 1.81" 3.58' 7 4.48' 2.96" 4.42' 4.54' 1.76" 4.50'

SED 0.92 1.05 0.95 0.99 0.95 0.66

TABLE 3-contd.

Pack

Vacuum

MAP

Cling

SED

Storage Bland Juiciness Degree of First Fried Rancid Bland (days) odour'.' of jlesh chewiness'.' bite' chicken flavour'.' f l a ~ o u r ' , ~

,flavour1.'

0 1.01 a 5.93' 4 4.20' 2.42" 7 3.92' 1.68'

I 1 3.79' 1.48" 14 3.71' 1.92"

0 1.56" 5.73b 4 3.34' 3.67a

I I 3.12' 2.28" 14 3.64' 2.23"

0 0.80" 6.01' 4 2.89' 2.40" 7 2.68' 2.03"

1 .oo 1.00

7 3.38' 2.760

1.91 * 3.83".' 4.95 ' 5.29' 5.01'

2.01 3.18b,C 4.38' 4.48' 4.78'

2.28" 4.45'' 4.86'

0.93

1.23" 5.93' 3.68' 1.90" 4.82' 1.60" 5.26* 1.12" 4.85' 1.10"

1.82" 5.49' 2.91" 1.79" 4.27' 1.81" 4.29' 1.20" 4.33' 1.18"

1.77" 5.86' 4.31' 1.74" 4.71' 1.23"

0.86 0.58

0.57" 1.65",b 1.57".' 2-78'.< 3.34'

0.90" 2.47" 2,24" 4.42' 3.67 ' 0.55" 2.94 'J 4.27 ' 0.86

0.96" 4.28' 4.18' 4.31 4.30'

1.49" 3.84' 4.51 ' 3.46 ' 3.84'

0.92" 3.18'' 3.18'

0.82

Means in same column within packaging type bearing unlike superscripts differ significantly ( P < 0.05).

' Significant storage effects present. Significant storage times packaging interaction present.

Significant packaging effects present.

Appearance Shrivelling of the chicken skin was most pronounced in the stored cling-wrapped samples and least apparent in the vacuum-packed samples, where the drumsticks stored for 14 days were not significantly different from the fresh sample (Table 3). Interaction between storage time and packaging was significant for shrivelling, indicating that the effect of storage time was dependent on the packaging.


Storage for 7 days produced an average significant increase (P<O-O5) in the shrivelling score of 2.29 k 0.92 for cling-wrapped samples, compared with an increase of 0.9 f 0.92 for MAP products and a decrease of 0.44 k 074 for vacuum- packed samples. For all packaging types the meat appeared drier and the fibres more compact with increased storage time, thechange being most noticeable from 0 to 4 days’ storage.

Odour Fried chicken odour did not differ significantly between packaging types, but significantly ( P < 0.001) decreased between 0 and 4 days for all packaging types. The rancid odour score of the cling-wrapped drumsticks significantly increased on average by 3.7 k 0.66 after 7 days’ storage, compared with 1.95 066 for vacuum samples and 1.61 k0.66 for MAP samples. This is shown in Fig 1.

The terms ‘bland odour’ and ‘bland flavour’ were included on the score sheet during one of the last training sessions, when the assessors felt it would be helpful to be able to score the absence of the fried chicken or rancid odour or score the presence of a mellow underlying chicken odour not characterised by the other attributes. The vacuum and MAP samples tended to be more bland in odour than the cling samples. After storage for 4 days the bland odour score significantly increased (P<O.O5) on average by 3-19kO.75 in vacuum samples, 1.78+0-75 in MAP drumsticks and 2.09 + 075 in cling-wrapped drumsticks, after which time the score did not significantly alter in any samples.

Texture The degree of chewing required to reduce the samples to a form that was easily swallowed increased with storage time in all samples. The greatest change was seen

‘1

0 2 4 6 8 10 1 2 14 16

Storage time (days)

Fig 1. Chicken drumsticks: mean panel score for intensity of rancid odour plotted against storage time for the three packaging types. (-n- Vacuum packed; -+-, MAP; -=-, cling wrapped).


in the vacuum-packed samples, whose score significantly increased on average by 3.04+0.93 after 7 days as compared with 2.37+093 for MAP drumsticks and 2.58 + 093 for cling-wrapped drumsticks. First bite refers to the degree of resistance presented by the meat fibres during the first molar incision. This became more resistant and drumsticks became less juicy with increased storage time, with the main effect occurring between 0 and 4 days and levelling off after 7 days.

Flavour Fried chicken flavour was more affected by storage time than any other attribute studied, giving the largest F value. The greatest mean decrease in fried chicken flavour occurred between 0 and 4 days: 4 0 + 0.58 for vacuum-packed samples, 3.7+0,58 for MAP samples. As fried chicken flavour became weaker the rancid flavour became stronger; increasing on average by 1.1 +0-86 in the vacuum samples, 157 + 0.86 in MAP drumsticks and 2.39 + 0.86 in cling-wrapped drumsticks after 4 days’ storage. Rancid flavour developed more rapidly during storage in cling-wrapped drumsticks than in MAP or vacuum-packed drumsticks (Fig 2). Unlike cling-wrapped drumsticks, where the largest increase in rancid flavour was between 0 and 4 days, the largest increase in rancid flavour in the MAP and vacuum drumsticks occurred between 7 and 11 days, thus demonstrating an inhibitory effect of MAP and vacuum packaging on lipid oxidation.

Sensory analysis of chicken a la king

The sensory task of scoring a more complex product was clearly more difficult. The standard errors in Table 4 are larger than those in Table 3 and the storage effects are only revealed by averaging across the three packaging treatments. In contrast with the chicken drumsticks not all the sensory attributes of chicken a la king selected by

0 2 4 6 8 10 1 2 1 4 1 6

Storage time (days)

Fig 2 Chicken drumsticks: mean panel score for intensity of rancid flavour plotted against storage time for the three packaging types. (-B-, Vacuum packed; -+-, MAP; -m-, cling wrapped).


the assessors were affected by storage time. Most were related to the appearance of the cold chicken a la king and in particular the appearance of the sauce.

Appearance Eight of the 18 sensory attributes pertained to appearance; five referred to the sauce, one to the meat and two to the peas. All the appearance characteristics were significantly ( P < 0.05) affected by storage time and the greatest changes occurred

TABLE 4 Panel means (along a 15-cm scale) for sensory attributes of chicken a la king classified by

packaging type and storage time

Pack Storage Granular' Gelatinous' Depth of Sauce Meut Pea (days) colour'12 coating' appearance',2 colour','

~~~

Vacuum 0 4 7

11 14

MAP 0 4 7

11 14

Cling 0 4 7

SED

3.40" 4.13 5.69".' 5.47 5.39",' 5.28 6.60' 635 5.46"~' 6.79

4.06 4.19 5.18 6.01 5.20 5.62 5.78 5.92 4.86 7.02

3.94 4.69 6.15 5.92 5.41 6.48

1.56 1.86

7.99' 7.10 5.16".' 4.42 4.60" 4.45 3.32" 3.92 4.00" 3.84

7.01' 7.36' 4.34",' 4.65' 3.86" 4.90b 3.49" 5.09' 3.68" 3.62"

6.41 6.9 1 3.99 4.5 1 4.04 4.85

1.63 1.72

7.49 5.68 5.13 5.25 455

6.77 471 4.89 4.15 4.14

6.94 499 564

1.84

2.06" 3.61"s' 4.61 a,'

4.68' 4.08",'

1.74" 4.52 ' 5.52' 6.07 6.39 ' 2.82 4.75 4.51

1.21

TABLE 4-contd.

Puck Storage Range of' Surface Sauce Chicken Sauce Meat (days) pea colour' oil' odour' odour texture' texture

~~~~ ~

Vacuum 0 2.98 6.40 4 5.10 4.68 7 486 4.12

11 5.13 5.42 14 3.93 4.09

MAP 0 2.49 5.08 4 5.19 6.16 7 4.61 5.31

11 5.00 5.80 14 4.86 5.76

Cling 0 3.75 5.33 4 5.65 4.57 7 5.20 3.58

SED 1.47 1.65

4.21 3.97 5.93 3.60 5.37 3.63 5.59 3.23 6.16 3.69

3.64 4.64 4.93 4.06 5.39 4.14 5.44 4.10 5.49 3.62

4.24 4.78 5.79 3.41 5.43 3.00

1.59 1.29

7.95 6.40 6.89 5.87 6.18

7.32 6.23 6.36 6.14 6.54

7.46 6.94 6.29 2.11

5.09 5.28 5.49 5.02 4.89

4.64 5.50 4.17 5.36 5.50

5.17 5.19 6.10

1.71

Sensory quality of’ chicken memi itrim from vending muchinrs 333

TABLE 4-contd.

Pea texture

Vacuum 0 4 7

11 14

MAP 0 4 7

11 14

Cling 0 4 7

SED

4.77 4.92 5.34 5.15 5.79

5.15 6.49 4.94 5.87 7.02

5.20 5.1 6 5.3 1

1.74

Stale flavour‘

4.3 I 5.83 5.37 5.94 5.54

4.22 4.44 4.70 4.5 I 6.52

3.95 5.49 5.36

1 3 5

-

Acidic Sweetness ,pavour

4.30 3.83 3.93 4.55 3.90 4.63 4.02 4.32 5.27 4.61

4.24 4.21 4.56 4.73 4.44 4.02 4.33 4.41 4.85 5.42

3.80 3.69 4.31 4.02 4.29 3.86

1.37 1.32

Saltiness

4.70 4.00 4.38 4.29 4.32

4.90 4.25 4.54 4.25 4.33

4.54 3.55 4.13

1.38

Chicken jlauour

4-79 4.22 4.11 7.77 4.15

5.60 4.64 4.97 3.8 1 4.19

5.34 4.19 3.84

1.46

Means in same column within packaging type bearing unlike superscripts differ significantly (P<0.05). ’ Significant storage effects present.

Significant packaging treatment effects present.

between 0 and 4 days, after which time results varied with different attributes. The granular or pasty appearance of the sauce was characterised by the appearance of visible grains and was accompanied by liquid separating out from the sauce. The sauce gel became significantly ( P < 005) firmer (more ‘set’) with increased storage, and the depth of colour was reduced. The ability of the sauce to coat the peas and pimentos was reflected in theattribute ‘sauce coating’, which decreased significantly between 0 and 4 days, after which time it levelled off and was not significantly affected by packaging type.

Where all three packaging types were compared, significant ( P < 005) differences in depth of colour were found. However, even the day 0 samples were scored differently, suggesting that differences were partly due to the periods of time elapsing between assessors’ scoring of groups of samples. The replications were scored significantly differently, P < 0.05, and so were included in the calculation of the error term, which reduced the significance of the effect of packaging from 0.025 to 0.05.

The MAP chicken a la king appeared significantly more broken up than the vacuum-packed or cling-wrapped samples.

The pea colour changed significantly with storage time in all packaged samples, becoming less pea green and more olive with time. The original pea-green colour was lost at a significantly (P<O.O5) higher rate in the MAP samples than in the vacuum-packed samples (Fig 3).

After reheating the product in the microwave, assessors evaluated the amount of

334 H Young, H J H MurFie, N Light

c

E"

0 2 4 6 8 10 1 2 1 4 1 6

Storage time (days)

Fig 3. Chicken a la king: mean panel score for intensity of pea colour plotted against storage time for the three packaging types. f-n-, Vacuum packed; -+-, MAP; -W- , cling wrapped).

oil on the surface of the chicken a la king samples. The effect of scoring between replications was greater ( P < 0.025) than the effect of packaging ( P < 0-05). This inconsistency in the assessors' mean scores was also reflected in the different mean scores assigned to the standards. During reheating in the microwave uneven heating patterns may have caused some of the variation between replications.

Odour Between 0 and 14 days of storage the odour of the sauce in all packed samples became significantly ( P < 005) more stale.

Texture Neither the rate of breakdown of the meat on chewing nor the pea texture were significantly influenced by any of the effects examined in the study (storage time, packaging, replication or assessor).

Flavour Only stale flavour was significantly affected by length of storage time.

DISCUSSION

In previous studies by the authors the management of foods in the vending industry and control of temperature in chilled food vending machines were shown to vary considerably, leading to potential loss in quality of stored chilled foods (Young 1986; Light et a1 1987a,b). The use of vacuum and modified atmosphere packaging techniques with good hygienic practice was shown to allow a potentially longer microbiological shelf life in cooked, chilled foods (Young et a1 1987).


The present study was concerned with the effects of these packaging methods on the sensory quality of cooked, chilled foods. Both vacuum and modified atmosphere packaging significantly deferred the sensory changes observed in cling-film- wrapped cooked, chilled chicken drumsticks and chicken a la king. Changes in odour and flavour predominated in the deleterious effects of storage observed in chicken drumsticks. Pokorny et a1 (1982) also found that changes in odour and flavour of cooked poultry stored at 4°C were more pronounced than changes in other sensory properties. They showed that storage of fried chicken and duck increased the perceived intensity of gluey, fishy, rancid and stale flavours.

The present finding that the majority of the change occurs in the first 4 days confirms the results of Harris and Lindsay (1972) on precooked poultry stored at 6°C for up to 5 days.

The skin of fried chicken drumsticks became wrinkled during storage when wrapped in cling film; this effect was ameliorated by vacuum packing. The higher permeability of the cling film might be expected to allow greater surface evaporation, possibly resulting in the skin contracting and producing a more shrivelled appearance. The pattern of ‘drying out’ was also true of the dry appearance of meat fibres even in vacuum packs. This is in contrast to the results of McDaniel et a1 (1984) who found that vacuum-packed beef roasts retained the ‘healthy bloom appearance’ that they had acquired immediately after cooking, whereas those packed under CO, or a mixture of gases lost this bloom after 7 or 14 days. Shrinkage of meat fibres under the skin might also be responsible for the shrivelled appearance of skin in samples. The extra permeability of the cling film may also account for the rancid odour.

The chicken a la king was clearly a more complex product than the chicken drumsticks, consisting of a number of component parts (meat, sauce and vegetables), which were studied in both the cold and reheated state, and this may have accounted for the lower levels of discrimination exhibited by the assessors. Changes in texture and appearance were more pronounced in this product than in the drumsticks.

The pea colour varied from a fresh pea green to olive through to khaki, and the colour of the peas was not uniform throughout the sample-hence the term ‘pea colour range’. The colour degradation was attributed to the conversion of chlorophyll to pheophytin (due to the replacement of the central magnesium atom in the tetrapyrrole ring by hydrogen and the consequent formation of dull olive brown pheophytins). Other colour reactions may also have occurred but would have been limited compared with pheophytinisation, for example the rupture of the tetrapyrrole ring to form colourless end products.

The extent of pheophytinisation is dependent on the processing temperature, length of storage, storage temperature and product pH (Fennema 1975). The first three of these variables were standardised among samples but the pH of the surface of the MAP chicken a la king was lower (5.75k0.1) than the vacuum-packed chicken a la king (6.2+0.1), probably due to the solubilisation of CO, in the product surface. The pH difference may have accounted for the greater degree of pheophytinisation in the MAP samples.

Other changes in appearance which had an important effect were those observed


in the sauce of the chicken la king. The increase in granular appearance of the sauce was attributed to retrogradation of starch, where the individual molecules were not sufficiently entangled to form a gel and so the growing micelles eventually caused precipitation and resulted in a curdled or granular appearance. After the initial significant increase in granular appearance during storage, the scores levelled off and even decreased slightly after 14 days. The gelatinous appearance of the sauce referred to the transformation of the sauce from a sol into a gel due to the intermolecular bonding between starch molecules resulting in a three-dimensional network entrapping substantial amounts of water. Cremer et ul (1985) attributed clumping of cooked noodles after 2 weeks of refrigerated storage to gelatinisation of starch. They also found that the colour of freshly prepared sauce appeared close to white but was significantly ( P < 0.01) more yellow after 2 and 4 weeks of refrigerated storage and reheating. Even though the other flavour attributes selected by assessors were not significant, they may still be useful descriptive terms for chicken a la king, particularly if the raw materials or the recipe formulation are the variables under study. These results agree with those ofCremer et a1 (1985) who found that no component of flavour in chicken in sauce and noodles prepared in a hospital food service system was affected by storage for up to 4 weeks at 0.2 k 0.6"C. The absence of rancid or warmed-over flavours (WOF) which were characteristic of the chicken drumsticks may have been due to the masking effect of the sauce and accompanying vegetables. Enser (1985) pointed out that the development of WOF is inhibited in many cooked products by the addition of spices, herbs, vegetables or certain cereal extenders.

The effect of packaging on sensory quality was less in chicken a la king when compared with chicken drumsticks. This was almost certainly because the chemical reactions causing flavour changes in chicken drumsticks (mainly fat oxidation) were influenced by the storage atmosphere whereas the latter would have had little effect on starch retrogradation and pheophytinisation which were the major sensory changes in chicken a la king.

Vending operators or caterers using cooked, chilled foods may be under the impression that if the cook-chill system is operated in accordance with the DHSS (1980) guidelines, then they will obtain a 5-day shelf life on their products. Feasibility studies incorporating product development and testing are well beyond the scope of the majority of caterers prior to purchase of equipment. Thus, caterers must learn by default that product quality deteriorates rapidly between 1 and 4 days and is very product dependent.

The present study has allowed the major sensory changes occurring in two cooked, chilled chicken products to be quantified in a reliable manner. The sensory changes in cling-wrapped chicken drumsticks occurred at a faster rate than in the MAP or vacuum-packed samples. Thus the use of the latter packing methods may improve the organoleptic quality of cooked menu items. However, MAP techniques may increase the rate of certain degradative changes such as pheophytinisation, and so the application of this technique will be limited to certain product types.

The significance of a number of attributes relating to minor components of chicken a la king (pea colour, sauce odour, etc) emphasizes the difficulties inherent in assessing quality changes in multi-component foods without consumer trials. For

Sensory quality of chicken menu items )om vending machines 331

example, if peas were removed from the product or the observed colour reaction was controlled by elevating the pH, would this substantially improve the perceived storage time? Similarly, since many consumers are accustomed to eating cooked meats that have been refrigerated for at least 24 h, during which warmed-over flavours would have developed, is it possible that these flavours are considered as desirable attributes of cold cooked meats? To answer these and other questions a consumer trial was undertaken and will be described in a later publication.

ACKNOWLEDGEMENTS

The authors would like to thank the Ministry of Agriculture, Fisheries and Food for financial support for this project and Drs A Youngs and R Tomlins for advice and help throughout the course of this work.

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Effect of packaging and storage on the sensory quality of cooked chicken menu items served from chilled vending machines