The effects of packaging on the growth of naturally occurring microflora in cooked, chilled foods used in the catering industry

Food Microbiology, 1987, 4, 317-327

The effects of packaging on the growth of naturally occurring microflora in cooked, chilled foods used in the catering industry Helen Young, Andrew Youngs and Nicholas Light*

D i v i s i o n o f F o o d S t u d i e s , D e p a r t m e n t o f C a t e r i n g a n d Ho te l M a n a g e m e n t , Dorse t I n s t i t u t e o f H i g h e r E d u c a t i o n , W a l l i s d o w n , Poole, Dorse t B H 1 2 5 B B , U K

Rece i ved 23 October 1 9 8 7

Two products, fried chicken drumsticks and chicken a la king, were selected as being representative of the type of cooked, chilled food used in the catering industry and in particular for chilled food vending. The growth of microflora in these products at 0 and 4°C was investigated with respect to the effectiveness of vacuum packaging and modified atmosphere packaging in restricting microbial growth. An investigation was also made of two representative species of the naturally occurring microflora of these food items by means of direct inoculation studies. The findings of the research clearly show that both vacuum packaging and modified atmosphere packing, when used at storage temperatures of O + 1 ° and 4 +__ 1°C, inhibit bacterial growth in cooked chicken menu items to levels below 5.00 loglo/g for up to 17 days. The direct inoculation studies indicated that the degree of this effect was dependent on the growth characteristics of the initial microflora. More important than the possible extension to shelf-life brought about by vacuum and modified atmosphere packaging is the value of these methods in ensuring high product quality in microbiological terms over short periods of chilled storage (up to 5 days). These results have importance in the catering industry with respect to the use of cooked, chilled foods.

Introduction

The use of cooked, chilled food products in both catering (Glew et al. 1979) and retail markets is becoming more popular. Studies of the microbiology of foods prepared using the so-called 'cook- chill' system in laboratory and operating conditions have been reviewed by Snyder and Matthews (1984). Little work has been reported.in this field, however, and some studies show considerable variabil- ity in cooked menu items produced simultaneously (Dahl et al. 1980) or

* To whom all correspondence should be sent. Present address: Devro Ltd, Moodiesburn, Chryston, Glasgow G69 0JE, Scotland.

0740-0020/87/040317 + 11 $03.00/0

between samples produced at different times by the same system (Nicholanco and Matthews 1978, Cremer and Chipley 1977, 1979, 1980) or even within a single sample of a menu item (Bunch et al. 1976). In the UK, the Department of Health and Social Security (DHSS 1980) has produced guidelines on the production of pre-cooked chilled foods based on available microbiological data which stipulate rigorous conditions for production and storage at 0-3°C for no more than 5 days including the day of production.

Packaging of foods, particularly vacuum packaging and modified atmosphere packaging (MAP), has been shown

© 1987 Academic Press Limited

318 H. Young et al.

to be beneficial in inhibiting microbial growth in some areas, especially in terms of raw, cured or fermented foods (see Mead 1983, Daniels et al. 1985) but little work has been carried out on the effects of such procedures on cooked, chilled foods. Vacuum packing has been used by caterers for packaging cooked foods, such as entree dishes, which are pasteurized after packing (Bjorkman and Delphin 1966, Minor 1982, Paulus et al. 1979). Under chilled storage conditions, pasteurized vacuum packed foods have been found to provide a longer shelf-life than untreated foods (Lott 1973, Pinaga et al. 1979, Paulus et al. 1979). An alternative to vacuum packing is MAP, which generally involves the flushing of packs with a combination of N2, CO2 and 02. The advantages of MAP are increased shelf-life of certain products, minimal damage to fragile foods otherwise crushed by vacuum packing and the opportunity of improved presen- tation. MAP has been used for retailing raw meats and fish, cured and fermented meat products, bakery goods and cheeses (Anon 1984). Differences in the effectiveness of vacuum and MAP systems have been shown, for example, McDaniel et al. (1984) found that storage of pre-cooked beef roasts in a 100% CO2 environment was preferable to vacuum packing from a microbiological viewpoint.

Full-meal vending is a form of food service that is gaining importance in the UK. Many foods are prepared by the 'cook-chill' method in production kit- chens, are transported to remote vending sites and are stored in a chilled food vending machine (CFM). Consumers purchase the products at will from the CFM, normally re-heating them by means of microwave cooking. Although changes have been made in the design of some CFMs, a recent study of the temperatures in such machines showed considerable fluctuation, e.g. - I °C to + 16°C

(Young, 1986, Light et al. 1987). If food products are not sufficiently heat- processed before chilling and storage or are contaminated after cooking the risks of growth of food poisoning and/or food spoilage organisms under such conditions may be high.

The aim of the present study was to examine the effect of packaging, storage time and temperature on the type and number of micro-organisms present in food products typically found in the vending industry. Two chicken products were chosen for the study as chicken has been shown to be a good growth medium at chill temperatures (Barnes 1976) and has been used in previous microbiological studies (cooked chicken, Toule and Murphy 1978, Patterson and Gibbs 1973, chicken a la king, Angelotti et al. 1961). The packaging types of interest in the present study included cling wrap film, vacuum packaging and MAP.

Materials and Methods Preparation of menu items and packaging procedures Ingredients for the menu items chosen (chicken a la king and deep fried chicken drumsticks) were purchased in advance with the exception of the fresh vegetables in the chicken a la king. The meat was purchased frozen from Dorset Poultry Packers, Upton, Dorset, UK and held at below -20°C until required. Operatives wore face masks and gloves during preparation of the food.

Frozen chickens were defrosted overnight at 15-20°C and were washed under running cold water. After drying with absorbent paper, the chickens were roasted in a Ben- ham convection oven at 190°C until the internal temperature between the leg and thigh joint and in between the area joining the leg and thigh to the body was greater than 80°C. On removal from the oven chickens were jointed hot into two legs, two wings and one breast portions and chilled in a blast chiller (Foster Ltd, King's Lynn, UK) prior to storage.

Chicken a la king was prepared following a standard recipe under conditions of good

Microflora in packed, cooked, chilled food 319

hygienic catering practice. The sauce was made by a standard recipe and frozen peas, canned chopped pimento and chopped, cooked chicken was added. The mixture was boiled for 5 min and then was poured into 9 cm deep trays and chilled in the blast chiller prior to storage at chill temperatures.

Chicken drumsticks were obtained frozen. They were defrosted and washed as described above and trimmed. Thirty drumsticks were fried together in a deep frying unit for 15 min. Excess oil was removed by shaking and the drumsticks were placed into trays lined with absorbent towels and chilled in a blast chiller before being stored.

Samples were packaged in cling-film, under vacuum or by MAP. Cling film was supplied by Perfawrap, High Wycombe, UK and had the following rates of transmission; water vapour, 90 g/m2/24 h; 02, 5 400 ml/m2/ 24 h; CO2, 35 000 ml/m2/24 h. Amilon bags supplied by Otto Nielsen, St Albans, UK were used for vacuum and MAP. The transmission rates for this material were water vapour at 4°C, 0.13 g/m2/24 h; 02, 2.5 ml/m2/24 h.

50 g portions of chicken a la king or individual drumsticks were placed into poly- propylene containers (12 × 9 × 4-5 cm) suitable for microwave re-heating and were covered with cling film or packed under vacuum or modified atmosphere in a Multi- vac A300 packer. Gas composition for packaging under MAP conditions was 70% CO2 and 30% N2. This composition was used for the modified atmosphere storage in order to reduce the effect of pack collapse common when 100% CO2 is employed (Anon 1984).

Storage conditions Packaged samples were stored for up to 21 days at either 4 + I°C or 0 + I°C in chilled incubators. The operating temperatures of the chilled cabinets were monitored with thermocouples placed in the cabinets and attached to a Grant miniature temperature recorder (Grant Instruments, Cambridge, UK).

Microbiological analysis of chicken menu items Four samples of each product were removed for analysis on ten separate occasions (including the day of production and the last day of storage), over a 21 day period.

The chicken drumsticks were sampled as follows. Whole drumsticks were cling- wrapped, vacuum packed or packed under

modified: atmosphere and, aider the appropriate storage period, the surface of the drumstick was removed for sampling. The surface consisted of skin and flesh to a depth of 2-3 mm.

Samples of either food product were diluted tenfold in 0.1% peptone (pH 7.0) and blended in a Colworth Stomacher. 1 ml and 0.1 ml surface spread plates on PCA and also PCA plates prepared by the spiral plate technique (Gilchrist et al. 1973) using a spiral plater (Don Whitley Scientific, Shipley, W. York- shire, UK) were prepared using the first dilution. PCA plates were incubated aerobically at 20°C for 72 h.

Selective growth media were used to determine the composition of the microflora as follows.

Pseudomonas on pseudomonas selective media (PSM) with CFC supplement (PSM; Oxoid). Plates prepared on the spiral plater and 1 ml and 0-1 ml spread plates were incubated at 20°C for 72 h.

Clostridia on sulphite polymyxin sulpha- diazine (SPS; Difco). 1 ml pour plates were prepared and incubated anaerobically at 37°C for seven days.

Lactobacilli on Mann Rogosa and Sharpe agar (MRS, Oxoid). 1 ml layer plates were prepared and incubated anaerobically at 30°C for three days.

Yeasts and moulds on oxytetracycline glucose yeast extract agar (OYGE; Oxoid). 1 ml pour plates were incubated aerobically at 20°C for five days.

Brochathrix thermosphacta on STAA agar (Gardner 1966). The pH of the tenfold dilution of each sample was recorded.

Preparation of inocula, inoculation of samples and microbiological analyses Two cultures isolated from the chicken products were selected for examination, a presumptive Pseudomonas from PSM and a presumptive Lactobacillus from MRS agar. Both colonies were examined for cellular morpho!ogy, gram stain reaction, oxidase reaction, catalase reaction and ability to use glucose. The lactobacilli was further examined for production of CO2 from glucose, production of ammonia from arginine and growth at 37 ° and 45°C. The pseudomonads were examined for hydrolysis of arginine, fluorescent pigment and by inoculating API20E galleries (API Laboratory Products Ltd, Basingstoke, UK) for the identification

320 H. Young et al.

of Enterobacteriaceae and other Gram negative reds.

Both organisms were grown at 20°C in I0 ml nutrient broth supplemented with 0.25% glucose. The cultures were centrifuged at 3000 g for 10 rain and the liquid decanted. The supernatant was washed in 0-1% peptone, centrifuged as before and resuspended to a total volume of 10 ml. Appropriate dilutions were prepared in 0-1% peptone from which 0-1 ml PCA spread plates were prepared and the optical density read at 650 nm. A standard curve was prepared showing bacterial concentration against optical density.

Chicken drumsticks were prepared by the method described above. The inocula were prepared as before and diluted to give 10s-10 s organisms per ml using optical density measurements at 650 nm and the standard curves described above. The surface of the chilled drumsticks was inoculated with 1 ml of either inoculum. The drumsticks were then either cling wrapped, MAP or vacuum packed. Uninoculated drumsticks were also packaged for use as controls. All samples were stored at 4 -+ 1°C in an incubator for 21 days. Four drumsticks from each treatment (cling wrapped, MAP or vacuum packed and also uninoculated or inoculated with either lactobacilli or pseudomonas) were examined on storage days 0, 4, 7, 14 and 21.

Any off odours present on opening the packs were noted and the surface pH recorded by means of a surface electrode. The surface of the same area that had been inoculated was cut away for sampling, diluted tenfold in 0.1% peptone and homogenized for 30 s in a Colworth Stomacher. PCA plates were prepared and incubated as described above.

The selective medium PSM with CFC supplement was used to determine the number of pseudomonads present on the control samples on the first and last day of storage and on the drumsticks inoculated with Pseu- domonas throughout storage. Similarly, MRS agar was used to determine the number of lactobacilli present on the control samples and samples inoculated with lactobacilli.

R e s u l t s

Figures 1-4 show the mean results obtained for TPC of the two chicken products packaged in cling film, vacuum packs and MAP when stored for various periods up to 21 days at 0 + I°C and 4 +

5 m __----- m-~m

~4 .1o

d

3

21 , l i I i o io 20 30

Doys of storoge

Fig. 1. The growth of naturally occurring microflora (TPC) on chicken drumsticks stored at 0 _.+ I°C for 21 days. Each data point represents the mean of four determinations. D, cling film wrapped samples; O, vacuum packed samples; m, samples packed under an atmosphere of 70% COs, 30% N2 (MAP).

6

o_

E ..~ m .......~- m

~;4 / o,

21 , , , i , 0 I0 2 0 3 0

Doys of s toroge

Fig. 2. The growth of naturally occurring microflora (TPC) on chicken drumsticks stored at 4 + 1°C for 21 days. Each data point represents the mean of four determinations. D, cling film wrapped samples; O, vacuum packed samples; B, samples packed under an atmosphere of 70% CO2, 30% N2 (MAP).

I°C. As can be seen, growth at 0 _ 1°C is slower than at 4 + 1°C, as might be expected, and, in general, lag phases are lengthened by conditions of storage util- ising vacuum packaging or MAP. The details of these results are outlined below in the following sections.


5

o

~4 d z

3

2 I I I I I 0 I0 2 0 3 0

Doys o f s to rage

Fig. 3. The growth of naturally occurring microflora (TPC) on chicken a la king stored at 0 + I°C for 21 days. Each data point represents the mean of four determinations. [7, cling film wrapped samples; . , vacuum packed samples; I , samples packed under an atmosphere of 70% COs, 30% N2 (MAP).

8

-J •

O ~ O - - I I I I ' t P - - O ~ f "

21 i l u I J 0 I0 2 0 SO

Days of s to rage

Fig. 4. The growth of naturally occurring microflora (TPC) on chicken a la king stored at 4 + I°C for 21 days. Each data point represents the mean of four determinations. 1:3, cling film wrapped samples; O, vacuum packed samples; I , samples packed under an atmosphere of 70% CO2, 30% N2 (MAP).

Effects of packaging on microbial growth - cling wrap film

Figures 1 and 3 shows that~ at 0 + 1°C, cooked chicken drumsticks and chicken a la king wrapped in cling film exhibited little or no microbial growth in the first five days (2.64 loglo/g and <3.47 logzo/g

for chicken drumsticks and chicken a la king respectively). At this temperature the TPC of the chicken a la king remained less than 5.00 loglo/g for 16 days (Figure 3) and less than 5-00 loglo/g for 18-19 days in chicken drumsticks (Figure 1).

At 4 _+ 1°C, however, both products showed evidence of more extensive growth (Figs 2 and 4). In terms of the recommended maximum acceptable TPC (5-00 loglo/g) laid down in the DHSS guidelines on pre-cooked chilled foods (1980), the drumsticks showed counts in excess of 5-N0 loglo/g by day 15 (Fig. 2) and the chicken a la king by day 7 (Fig. 4). In previous work we noted that minced cooked chicken showed TPCs over 5-00 log~o/gby day 10 at 0 _+ 1°C and by day 6 at 4 _+ 1°C (Young 1986).

Effects of packaging on microbial growth - vacuum packaging In the vacuum packed samples the effect of temperature of storage was reduced. In the chicken a la king, the difference between the TPC lag phase at 0 _+ 1 and 4 _+ I°C had become negligible and the TPC at 0 -+ 1 and 4 + I°C were approximately within one log cycle of each other throughout storage (Figs 3 and 4). The vacuum packed chicken drumsticks stored at 4 + I°C had a lag phase of 13 days (Fig. 2), which was increased to 17 days at 0 + I°C (Fig. 1). After 21 days storage the mean TPC at 4 + I°C was loglo 5.40/g (Fig. 2). At 0 + I°C after 20 days storage the mean TPC was only 3.80 loglo/g (Fig. 1). As growth in the lag phase at 0 _+ l°C was only recorded for a few days at the end of storage it was not possible to compare logarithmic growth rates.

The figures illustrate the extended lag phases observed and restricted growth in the later stages of storage, especially in the chicken a la king.

322 H. Young et al.

Effects of packaging on microbial growth - MAP In the MAP chicken a la king microbial growth was observed aider 11 days at 4 + I°C (Fig. 4), but the TPC did not exceed 4-5 loglo/g at any point in the 21 day storage time and in fact the TPC dwin- dled towards the end of storage. Little or nb growth occurred at 0 + I°C until day 21 when the TPC was 3.86 loglo/g (Fig. 3). In the MAP chicken drumsticks the lag phase at both storage temperatures was less distinct than in previous experi- ments (Figs 1 and 2). At 4 + I°C the TPCs rose to above 4.0 loglo/g after four days and remained at this level until 20 days, when they rose slightly to 4.92 loglo/g (Fig. 2). The TPC at 0 _+ I°C was always within one log cycle of the TPC at 4 + I°C and growth did not exceed 5.00 loglo/g at any point during storage at either temperature. Figures 2 and 4 summarise the data obtained for both products at 4 _+ I°C and illustrate the delayed lag phase and, most particularly in the chicken a la king, the reduced rate of growth after extended storage.

Growth on selective media In the cling film wrapped chicken a la king there was a steady increase in counts on PSM which corresponded with an increase in TPC. By the end of the storage period the microflora was dominated by the Pseudomonas species.

Throughout the work little or no growth was seen on OYGE agar and counts of yeasts and moulds were never above 2-5 loglo/g. Similarly, there was no appreciable growth on SPS. The growth on MRS agar of samples from vacuum packed chicken a la king was limited to samples stored at 4 __+ I°C for 21 days and represented a small proportion of the total population (the TPC). Little or no growth occurred on the other selective media and so the dominating bacterial species remains unknown. By the 60th

day of storage the number of lactobacilli on the vacuum packed chicken drumsticks had risen to 6.0 loglo/g, but were still tenfold less than the TPC.

Counts were made on STAA agar for the determination of numbers of Bro- chothrix thermosphacta in vacuum packed chicken drumsticks stored at 4 ___ I°C for 60 days and 6-0 loglo/g were detected. Brochothrix thermosphacta was not present on MAP drumsticks stored for the same time but numbers of lactobacilli were similar to the TPC and, therefore, the latter dominated the microbial population in these samples.

Direct inoculation of chicken drumsticks The Pseudomonas species used in this study was gram negative, motile, oxida- tive, catalase positive, oxidase positive, hydrolysed arginine and did not flu- oresce under ultra violet light. It was identified as 'Pseudomonas fluorescens, excellent identification' according to the API20E system.

The strain of Lactobacillus occurred as stubby cocci, was non motile, Gram positive, catalase negative and oxidase negative. As it produced gas from glucose, ammonia from arginine and grew at 37°C but not at 45°C, it was presumed to be a betabacterium (heterofermenta- tive).

The pseudomonas strain grew rapidly without a lag phase on the cling wrapped chicken drumsticks, rising from 3.0 loglo/g to more than 8.0 loglo/g after four days (Fig. 6). The counts on PSM mir- rored the TPC (Figs 5 and 6), which indicates that Pseudomonas dominated the population in the cling wrap samples. Even when numbers of the pseudomonas strain exceeded 10 loglo/g no off-odours were detected. Therefore, this species of pseudomonas must have been .non-proteolytic, which was confirmed by a negative reaction to the gelatin lique- factin test on API20E strip.


12

8

6 O ~ ~ ~ ~ J U ~ n n . u ~ n p

4

21 n l n I O 10 20 30

Days of storage

Fig. 5. Total plate counts for samples of chicken drumsticks inoculated with a Pseu- domonas species and stored at 4 + I°C for 21 days. Each data point represents the mean of four determinations. [:], cling film wrapped samples ; . , vacuum packed samples; n, samples packed under an atmosphere of 70% CO2, 30% N2 (MAP).

12

.9

u

0

_ J

I0

8

6

4

0

\ 1 - - 1 / I ~ 1 [ = I ~0 20 Oays of storage

3 0

Fig. 6. Bacterial growth on PSM of samples from chicken drumsticks inoculated with a Pseudomonas species and stored at 4 + l°C for 21 days. Each data point represents the mean of four determinations. [], cling film wrapped samples; O, vacuum packed samples; n, samples packed under an atmosphere of 70% CO2, 30% N2 (MAP).

In the absence of 02 (vacuum packs) growth of the pseudomonas strain was reduced ten thousand fold (Fig. 6). After 21 days of storage the PSM counts ranged between 6.03-6.93 l o g j g , which was one hundred fold less than the TPC, therefore suggesting that another organ-

ism was present (Fig. 6). As this pattern developed prior to the end of storage, the vacuum packed samples were analyzed on day 21 on MRS agar, which gave a count of 7.38 loglo/g, which suggested that lactobacilli were the dominant species.

Where CO2 was present (MAP) the growth of the pseudomonas strain was inhibited (Fig. 6); after four and seven days of chilled storage the numbers of pseudomonads were reduced to below 3-00 loglo/g. Counts were nearly a 1000 fold lower than the TPC at day 21, once

8

9 7

6

o~5 . J

4

3 a I I I n 0 I0 20 30

Days of storage

Fig. 7. Total plate count for samples of chicken drumsticks inoculated with a Lacto- bacillus species and stored at 4 + I°C for 21 days. Each data point represents the mean of four determinations. [3, cling film wrapped samples; O, vacuum packed samples; m, samples packed under an atmosphere of 70% CO2, 30% N2 (MAP).

more indicating that another species dominated growth.

The TPC of the drumsticks inoculated with lactobacilli were very similar to the counts on MRS agar on all packaging types indicating that the lactobacilli were the dominant micro-organism throughout storage (Figs 7 and 8).

The growth rate of the lactobacilli was slower than that of the pseudomonas strain on the cling film wrapped samples

324 H. Young et al.

3~ I i [ i I l 0 IO 20 30

Ooys of storage

Fig. 8. Bacterial growth on MRS agar of samples from chicken drumsticks inoculated with a Lactobacillus species and stored at 4 _+ 1°C for 21 days. Each data point represents the mean of four determinations, n, cling film wrapped samples; @, vacuum packed samples;., samples packed under an atmosphere of 70% CO2, 30% N2 (MAP).

and the TPC at the end of storage were one hundred fold less (Fig. 7). Thus, if the two organisms were present in the same numbers initially, pseudomonas would be expected to dominate the population in the cling wrapped sample and lactobacilli in the MAP and vacuum packed samples.

Growth was negligible in the uninoculated control samples, with the exception of the cling wrapped drumsticks on days 14 and 21. No off-odours were detected on these drumsticks. The strains respon- sible for this growth remain unidentified as growth on the selective media was negligible.

Discussion

The microflora of cooked foods consists of (a) heat-resistant organisms which sur- vive the cooking process and may be either undamaged or heat-damaged and unable to grow normally at chill temperatures and (b) organisms present as a result of post-cooking contamination. In practice, the composition of the microflora will be determined by the standards

of hygiene and control exerted in the production unit. The microbiological data from previous studies of catering products vary enormously, even when the differing nature of the food products is taken into account (Bunch et al. 1976, Nicholanco and Matthews 1978, Dahl et al. 1980, Cremer and Chipley 1977, 1979, 1980). This is not surprising as standards of hygiene are not uniform throughout the industry. Despite these difficulties, it is vital to establish the effects of various factors, particularly temperature and storage atmosphere, on the shelf-life of cooked, chilled foods to enable practitioners to optimize systems from the point of view safety.

The results in the present work show that reduction in storage temperature and use of controlled environments through packaging led, in general, to lengthening of the lag phase for microbial growth and a limitation of growth even after 21 days storage. The effect of storage at very low chill temperature 0 + I°C) in increasing the length of lag phase and reducing microbial growth in chicken a la king was greatest in the cling wrapped products and less in the MAP and vacuum packed products. In the cling wrapped product the lag phase was extended from 5 to approximately 10 days and the growth rate reduced at 0 ___ I°C, whereas in the MAP and vacuum packed products the difference in lag phase and growth rate was relatively less. One reason for this could be that the aerobic strains of bacteria growing in the cling wrapped samples were more sus- ceptible to changes in temperature than those strains in the MAP and vacuum samples.

These results agree with the work of Spahl et al. (1981), who found a temperature increase from 2 to 5°C resulted in a greater increase in growth of psychro- trophic organisms on pork chops in the control environment (air) than in CO2


containing environments. In contrast, Nielsen (1983) found that aerobic plate counts and counts of Brochothrix thermosphacta of vacuum packed bologna sausage at 2 and 5°C were similar.

Although Patterson and Gibbs (1973) detected small numbers of clostridial vegetative cells on freshly cooked chicken we were unable to detect any in our samples. The absence of Clostridia in the present study suggests that, if clostridial contamination was present on raw materials, the heat t reatment used was sufficient to destroy those vegetative cells present in raw food, that there was very little post-processing contamination and the temperature of storage was sufficiently low to prevent growth from residual spores.

Davidson and Webb (1973) found the microflora of vacuum packed cooked chicken to be dominated by lactic acid bacteria after storage at 7, 24 and 37°C. In the present work lactobacilli were shown to be a dominant species in samples stored under all conditions for both products investigated. However, after 60 days storage at 4 + I°C Brochothrix thermosphacta was present in the same numbers as lactobacilli in vacuum packed samples although this species was not present in MAP samples. In the latter case lactobacilli dominated the microbial population. The differences in effect of packaging on microbial growth in the two products are partly due to the different nature of the initial microflora. Had pseudomonas species been present on the cling wrapped whole drumsticks initially, the TPC at the end of storage may have been greater, thus showing a greater difference between the cling wrapped and the MAP and vacuum packed samples. It was for this reason that it was decided to conduct a further experiment to demonstrate the effect of packaging on the growth of specific micro-organisms.

By inoculating samples with known numbers and types of micro-organisms prior to storage, the composition of the initial microflora could be standardized. Two strains of bacteria were chosen for study; first, a strain of pseudomonas, which was the predominant micro- organism on the cling wrapped products at the end of storage and was isolated from a chicken drumstick stored at 0 -+- I°C for 19 days. This strain was identified as a non-proteolytic Pseudomonas fluorescens. Shaw and Latty (1981) found that 82 isolates of non-fluorescent pseudomonads from meats contained a low incidence of strains producing extracel- lular enzymes. Secondly, a strain of lactobacilli which was isolated from an MRS agar plate prepared from a vacuum packed drumstick stored for 60 days at 4 + I°C. As both organisms had previously been isolated from cooked chicken, both in our pilot studies and by other workers (Davidson and Webb 1973, Toule and Murphy 1978), they were thought to be representative of potential post cooking contaminants.

The results of this experiment clearly show that the effect of packaging in restricting growth is dependent on the type of bacteria present on the packaged food. In the presence of a mixed microflora the inhibitory or stimulatory effects of modifying storage atmospheres may be masked. If, however, the two organisms studied are taken as representative of common contaminants of cooked foods then microbial growth may be predicted to be least in MAP products, followed by vacuum packed products and greatest in cling wrapped or unwrapped products.

In a study of vacuum packed sliced luncheon meats inoculated with lactobacilli and Brochothrix thermosphacta, Egan et al. (1980) concluded that a count of 8.00 loglo/g did not mean the product was spoiled. Spoilage depended on the exact nature of the product under test,

326 H. Young et al.

the strains of bacteria chosen, the conditions of storage and, in taste panel tests, the judges' expectations. Similarly Silla and Simonsen (1985), who examined the shelf-life of cured cooked meat products, found that the end of shelf-life was not closely related to a certain bacterial count, whether measured on PCA, MRS

"or STAA. They concluded that bacterial limits were not valid unless combined with organoleptic criteria.

The development of the microflora in stored cooked foods is dependent on a variety of factors, some of which are of more concern to food producers than others, as they exert a greater influence on microbial growth and also because they inhibit microbial growth without altering the product itself. Temperature and storage atmosphere are two such factors.

Our results in the present study show

that control of the temperature of storage to below +3°C and the applica- tion of methods such as vacuum packing and MAP can safely extend the shelf-life of cooked, chilled foods used in the catering industry. More importantly, the use of controlled temperature storage, below +3°C, combined with packaging techniques can result in a high degree of confidence in the microbiological safety of cooked, chilled foods over the storage life recommended by the UK DHSS guidelines on pre-cooked chilled foods (5 days).

Acknowledgement The authors wish to thank MAFF for financial support of this project, Dr R. Tomlins for advice and support and Mrs Philippa Hudson for excellent technical support.

References Anon (1984) Update on the gas flush pack. Food Manufacture 59 (3), 22, 23, 25, 65. Bjorkman, A. and Delphin, K. A. (1966) Sweden's Nacka Hospital food system centralizes

preparation and distribution. Cornell Hotel and Restaurant Administration Quarterly 7, 84-91.

Bunch, W. L., Matthews, M. E. and Marth, E. H. (1976) Hospital chilled food service: acceptability and microbiological characteristics of beef soy loaves when processed according to systems procedures. J. Food Sci. 41, 1273-1276.

Cremer, M. L. and Chipley, J. R. (1977) Satellite food service system assessment in terms of time and temperature conditions and microbiological and sensory quality of spaghetti and chilli. J. Food Sci. 42, 225-229.

Cremer, M. L. and Chipley, J. R. (1979) Time and temperature, microbiological and sensory quality of meat loaf in a commissary food service system transporting heated food. J. Food Sci. 44, 317-321.

Cremer, M. L. and Chipley, J. R. (1980) Time and temperature, microbiological and sensory assessment of roast beef in a hospital food service system. J. Food Sci. 45, 1472-1477.

Dahl, C. A., Matthews, M. E. and Marth, E. H. (1980) Survival of Staphylococcus aureus in beef loaves, potatoes and frozen and canned green beans after microwave heating in a simulated cook/chill food service system. J. Food Prot. 43,916-921.

Daniels, J. A., Rajagopalan, K. and Syed, S. H. R. (1985) A review of the effects of CO2 on microbial growth and food quality. J. Food Prot. 48, 532-537.

Davidson, C. M. and Webb, G. (1973) The behaviour of Salmonella in vacuum packaged cooked cured meat products. Can. Inst. Food Sci. Technol. J. 6, 41-44.

DHSS (1980) Guidelines on precooked chilled foods. HMSO, London. Egan, A. F., Ford, A. L. and Shay, B. J. (1980) A comparison of Microbacterium

thermosphactum and lactobacilli as spoilage organisms of vacuum packed sliced luncheon meats. J. Food Sci. 45, 1745-1748.

Gardener, G. A. (1966) A selective medium for the enumeration of Microbacterium thermosphactum in meat and meat products. J. App. Bact. 29, 455-460.


Glew, G., Berg, R. and Yin Tam (1979) Some effects of the presence or absence of O2 on stored pre-cooked chilled chicken and porkmeat. In Advances in Catering Technology (Ed. Glew, G.) London, Applied Science Publishers.

Light, N. D., Young, H. and Youngs, A. (1987) Operating temperatures in chilled food vending machines and risk of growth of food poisoning micro-organisms. Food Sci. Tech Today. 1 (4), 252-256.

Lott, G. (1973) Mikrobiologische untersuchungen von fleisch fertiggerichten, die nach dem Nacka - Verfahren hergestellt wurden. Praventivmedizin 18, 371-374.

McDaniel, M. C., Marchello, J. O. and Tinsley, A. M. (1984) Effect of different packaging treatments on microbiological and sensory evaluation of pre-cooked beef roasts. J. Food Prot. 47, 23-26.

Mead, G. C. (1983) In Food Microbiology A dvances and Prospects, Soc. Appl. Bact. Symp. series 11. (Eds Roberts, T. A. and Skinner, F. A.) New York, Academic Press.

Minor, L. J. (1972) Today's food production systems. Cornell Hotel and Restaurant Administration Quarterly 13, 43-56.

Nicholanco, S. and Matthews, M. E. (1978) Quality of beef stew in a hospital chilled food service system. J. Am. Diet. A. 72, 31-37.

Nielsen, H. J. S. (1983) Influence of temperature and gas permeability of packaging film on development and composition of microbial flora in vacuum packed bologna type sausage. J. Food Prot. 46, 693-698.

Patterson, J. T. and Gibbs, P. A. (1973) Observations on the microbiology of cooked chicken carcasses. J. App. Bact. 36, 689-697.

Paulus, K., Piekarski, J. and Leistner, L. (1979) Chilled meals: possibilities and limits. In Advances in Catering Technology, 2nd Int. Symp. on Catering Systems Design (Ed. Glew, G.) London, Applied Science Pub.

Pinaga, F., Carbonell, J. V., Valles, S. and Ampara Feria, M. (1979). Pasteurization of ready to serve meals: quality changes during refrigeration of beef goulash in air atmosphere. Revista de Agroquimica y Technologiade Alimenta 19, 520-532.

Silla, H. and Simonsen, B. (1985) Shelf-life of chilled, cooked and cured meat products. Fleischwirtschaft 65, 66.

Snyder, P. O. and Matthews, M. E. (1984) Microbiological quality of food service menu items produced and stored by cook/chill, cook/freeze, cook/hot hold and heat/serve. J. FoodProt. 47, 876-885.

Spahl, A., Reineccius, G. and Tatini, S. (1981) Storage life of pork chops and CO2-containing atmospheres. Appl. Environ. Microb. 41,816-818.

Toule, G. and Murphy, O. (1978) J. Hyg. 81,161-169. Young, H. (1986) Factors affecting the shelf-life and acceptability of cooked, chilled foods. PhD

Thesis, Dorset Institute of Higher Education.

Documents

The effects of packaging on the growth of naturally occurring microflora in cooked, chilled foods used in the catering industry