Microbial and sensory quality changes in refrigerated
minced goat meat stored under vacuum and in air
Y. Babji
a,*, T.R.K. Murthy
b, A.S.R. Anjaneyulu
b aCentral Institute for Research on Goats, Makhdoom 281 122, Mathura, IndiabIndian Veterinary Research Institute, Izatnagar 243 122, U.P., India
Accepted 11 August 1999
Abstract
Goat mince was packed and stored at 418C. Mince was analysed physico-chemically and microbiologically at intervals 0, 3, 7, 10, 21 and 28 days for packs stored aerobically and 7, 14, 21, 28 and 40 days for vacuum packs. Although the initial pH of mince was 6.5, meat pH and extract release volume declined during vacuum storage. The microbial pro®le of aerobically stored mince was signi®cantly higher than that in vacuum packs. The mean count of total aerobes of aerobic packages was higher than that of different groups of spoilage ¯ora in vacuum packed mince. The air packed mince comprised the climax population (log10cfu g
ÿ1
) of aerobic plate counts (9.0), psychrotrophs (8.8), coliforms (6.0), enterobacteriaceae (6.0), Pseudomonas(9.0), faecal streptococci (6.8), lactic acid bacteria (3.5), staphylococcal counts (5.5) and yeast and mould (4.0) counts, whereas bacterial (log10cfu gÿ1) pro®le of vacuum packed mince included total aerobic plate counts (7.0),
psychrotrophs (7.1),Pseudomonas(7.6) faecal streptococci (7.3) and lactic acid bacteria counts (3.1). The shelf-life was 28 days for vacuum packed mince whereas similar overall acceptability scores were observed at 3 days for aerobic packages. During storage putrid odours in aerobically packed mince and sulphide odours in vacuum packs were observed. High pH of mince and the initial heavy carcass contamination promoted the rapid multiplication of facultative anaerobes leading to spoilage of the mince.#2000 Published by Elsevier Science B.V. All rights reserved.
Keywords:Goat meat; Microbes; Vacuum packaging; Aerobic packaging; Spoilage; Shelf-life
1. Introduction
India possesses 1/5th of the global goat population producing more than 0.47 million tonnes of meat. Goat meat is the most widely consumed in this country and the extension of its shelf-life is important in marketing the product for internal consumption and
export purposes. Vacuum packaging to extend the shelf-life of raw meat is well established. The princi-pal objective of vacuum packaging is the partial or total inhibition of rapidly growing pseudomonads and allowance of dominance by ¯ora of low spoilage potential lactic acid bacteria. The composition of the climax population that consequently develops on the packaged meat will be of mixed (Gill, 1996) type and dictates the course of spoilage which is largely affected by relative numbers of various spoi-lage types in the initial ¯ora (Gill and Tan, 1979). *Corresponding author. Tel.:91-0565-763280; fax:
91-0565-763246.
E-mail address: [email protected] (Y. Babji).
However, dominance of lactic acid bacteria in the climax population will depend on initial ¯ora of lactics, pH of meat, level of initial contamination and prevalence or absence of facultative anaerobic biota (Grau, 1980, 1981). Initial microbial load, gas permeability of packaging ®lm and temperature of meat during storage play a signi®cant role in extend-ing the shelf-life of packaged meat (Gardner et al., 1967; Adams and Huffman, 1972; McMullen and Stiles, 1991). High pH (>5.8) meat tissue at 08C or below under O2depleted CO2atmospheres exhibited
no growth of facultative anaerobic bacteria and dom-inance by lactic acid bacteria (Gill and Harrison, 1989). Few studies have been conducted on vacuum packed lamb (Shaw et al., 1980; Moore and Gill, 1987; Sheridan et al., 1997). Compared with beef, shelf-life of vacuum packed lamb has been reported to be short. Gill and Jones (1994) reported a minimum shelf-life of 4 weeks for vacuum packed ground beef. The present study described physico-chemical, microbial and sen-sory changes in high pH (6.5) minced goat meat stored under vacuum in poly-ethylene pteraphthalate (PETPE) ®lm and the aerobic storage life in low density polyethylene (LDPE) ®lm at 418C.
2. Materials and methods
2.1. PETPE laminate
The PETPE ®lm was a two-layered laminate (Flex Industries, Noida, India) with a water vapour trans-mission rate (WVTR) of 1.6 g cmÿ2per 24 h at 80% relative humidity at 378C. One layer of this laminate was composed of PETPE with a thickness of 10mand the other layer was polyethylene. The gas transmission rate of the ®lm is not known. The PETPE bags were vacuum sealed using a Roschermatic packaging machine (Model VM 19S, Roscherwerke Gmbh, Ger-many) and refrigerated (418C).
Goat meat (obtained from non-descript animals of 2 years age) was purchased 3±4 h after slaughter from a local meat shop. Leg portion was cut into chunks after removing the loose fat, fascia and connective tissue. Meat chunks from two to three animals were pooled for each trial. Mince was prepared in the laboratory by passing the chunks through an 8 mm plate of the mincer (Electrolux, Sweden), and two trials were conducted.
The minced meat was divided into 200 g portions and placed in individual bags of LDPE for aerobic storage or in PETPE bags for vacuum storage. The LDPE bags were refrigerated (418C) without seal-ing the ends of pouches. The aerobically stored packs were analysed at 0, 3, 7, 10, 21 and 28 days. The vacuum packed samples were analysed at 0, 7, 14, 21, 28 and 40 days of storage.
2.2. Analyses of packed mince
pH of mince was determined by grinding 10 g of mince from each storage pack with 50 ml of distilled water with frequent blending. pH was recorded by dipping the combined glass electrode of a digital pH meter (Century, Model LP 901) in the mince slurry (Trout et al., 1992). Three readings were made on each sample and the mean recorded.
A 10 g mince from each storage pack was ground with 200 ml of distilled water in a Warring blender and the volume of the slurry was made up to 250 ml in a volumetric ¯ask and ®ltered through Whatman No. 2 ®lter paper. 25 ml of ®ltrate was collected to which 75 ml of distilled water was added and titrated against 0.1 N NaOH with three drops of phenolphthalein. The amount of standardized 0.1 N NaOH required to neu-tralize the meat is expressed as total acidity (Konecko, 1979).
25 g mince was blended with 100 ml of distilled water in a Warring blender for 5 min. The resultant slurry was ®ltered through Whatman No. 2 ®lter paper and the amount of ®ltrate released in 15 min was expressed as extract release volume (Jay, 1964).
Minced meat was placed in sample holder of Lovi-bond tintometer (Model E, The Tintometer, Salisbury, UK) and secured against the viewing aperture. The colour of the mince was determined by adjusting the red and yellow units while blue units were kept constant (Froehlich et al., 1982).
2.3. Microbial analysis
food sample. Duplicate 0.1±0.5 ml or 1 ml inoculum of appropriate dilutions were spread with a sterile glass spreader on pre-poured plates or pour plated, respectively, on the following media: for enumeration of aerobic plate counts on spread plates of plate count agar (PCA, Hi-Media Laboratories, M 091, Bombay) which were incubated at 308C for 48 h; psychrotrophs on spread plates of plate count agar (PCA, Hi-Media, M 091) which were incubated at 418C for 10±14 days;Pseudomonascounts on spread plates of plate count agar (Pseudomonas¯uorescein agar with 1 ppm crystal violet, Hi-Media, M 120) which were incu-bated at 308C for 72 h; Staphylococcal counts on spread plates of Baird Parker agar (BPA, Hi-Media, M 043) which were incubated at 378C for 48 h; anaerobic plate counts on pour plates of plate count agar (PCA, Hi-Media, M 091) which were incubated at 358C for 24 h; Enterobacteriaceaecounts on pour plates of Violet Red Bile Glucose agar (VRBG, Hi-Media, M 581) which were incubated at 378C for 24 h; faecal streptococci on pour plates of Slanetz and Bartley agar (SBM, Hi-Media, M 641) which were incubated at 378C for 48 h; lactic acid bacteria counts on pour plates of MRS agar (MRS, Hi-Media, M 641) which were incubated at 308C for 120 h; yeast and mould counts on pour plates of potato dextrose agar (PDA, Hi-Media, M 096) with 2% antibiotic (Chlor-amphenicol and chlortetracycline, 1 : 1) and the plates were incubated at 258C for 120 h. Colonies were counted from each group of micro-¯ora and expressed as log per gram. The enumeration procedures as described by Speck (1975) were followed.
2.4. Sensory (objective) evaluation of packed mince
Goat meat mince in aerobic packages and under vacuum was assessed 2 h after opening the packs at each storage interval by a three member semi-trained panel for colour, odour, discolouration and over all acceptability traits (Gill and Jones, 1994; Gill and McGinnis, 1995). The colour acceptability of mince was scored on a 7-point scale with slight modi®cation in the procedure where 7 represents bright red and 1 indicates extremely dull red. The degree of discoloura-tion of the mince in the package was expressed in percent on a 5 point scale (1: no discolouration, 2: 1± 10%, 3: 11±25%, 4: 25±50%, 5: 50±100%). The odour acceptability of the mince was assessed on a 4 point
scale where 1 indicates no off-odour, 4 represents strong off-odour. The type of odours developed during storage was recorded. The overall acceptability of the mince was assessed on a 7 point scale where 7 represents extremely acceptable and 1 indicates extre-mely unacceptable.
2.5. Statistical analysis
Data were analysed statistically for all the para-meters according to procedures described by Snedecor and Cochran (1967). Analysis of variance were used to assess pH, total acidity, ERV and microbial counts based on a linear model that included aerobic and vacuum storage conditions and storage time within storage condition. The difference between the means were tested by critical difference. The sensory evalua-tion scores (colour, discolouraevalua-tion, odour and overall acceptability) and Lovibond colour units (red, yellow and green) of minced goat meat were tested using chi-square analysis. The hypothesis that the initial values are not affected due to aerobic or vacuum storage was tested for each parameter.
3. Results
The mean pH, total acidity and extract release volume of minced goat meat under aerobic and/or vacuum storage at 418C is shown in Table 1. The initial pH of minced goat meat used in this study was high (6.5). The extract release volume decreased and total acidity increased during storage in both aerobic and vacuum packages. There was no signi®cant effect of storage period on pH of meat under both the storage conditions (Table 1).
The microbial growth in the mince stored aerobi-cally or under vacuum is shown in Figs. 1±9. Analysis of variance of the data indicated a signi®cant effect of the storage condition on the counts of all the microbial groups except for lactic acid bacteria and yeasts and moulds. The lactic acid bacteria and Pseudomonas
from day 7 to 28 in the vacuum stored mince. There was no signi®cant difference between the initial and the day 28 counts of the vacuum stored samples for all the microbial groups except for total aerobes, psy-chrotrophs andPseudomonas.
Yeast and mould counts did not form a major part of the prevalent micro-¯ora during storage both in aero-bic (log 4.0) and vacuum (log 2.0) at day 28. Overall,
aerobically stored samples had higher microbial counts than that of vacuum packed samples.
The vacuum packed goat meat was bright red at 7 days and moderately bright red at 14, 21 and 28 days of storage whereas aerobically stored samples were slightly bright red at 0, 3, 7 days and slightly dull red at 10, 21 days and dull red at 28 days (Table 2). Vacuum packed goat meat had no discolouration up to 14 days
Table 1
pH, total acidity and extract release volume (ERV) of minced goat meat under aerobic or vacuum packed (PETPE laminate) storage at refrigeration temperaturea
Storage condition Storage time (days) pH Total acidity ERV (ml) Initial 6.530.18a 0.700.10a 41.000.35a Aerobic 3 6.380.01a 1.000.00b 38.500.35a,b
7 6.110.005a 1.000.00b 36.001.00b
10 6.130.17a 1.300.00c 24.500.50c
21 6.250.01a 1.300.00c 30.250.25d
28b NDd NDd NDd
Vacuum 7 6.000.01a 1.300.00b 24.000.50b
14 6.100.00a 1.200.00b 25.500.50c
21 6.070.01a 1.400.00b 32.500.50d
28 6.100.01a 1.200.00b 24.500.50e
40c 6.030.00a NDd NDd
aMeans followed by the same alphabets do not differ signi®cantly (P< 0.05). bSpoiled due to putrefactive odours.
cSpoiled due to sulphide odours. dND: Not done.
storage and at 21 and 28 days there was a slight discolouration 1±10% in the pack. The aerobically stored samples showed 25±50% discolouration at 7 days and 50±100% discolouration at 21 days. The vacuum packed samples were spoiled by sulphide odours at 40 days while the aerobically stored packs spoiled by putrefactive odours at 28 days. Thus, on 28th day, the vacuum packed samples were extremely
acceptable while the aerobically stored samples were unacceptable.
Chi-square analysis of the data also indicated sig-ni®cantly higher discolouration scores in aerobically stored mince (Table 2), signi®cantly higher yellow colour units for vacuum packs (Table 3) and lower overall acceptability scores (Table 2) for aerobically stored samples when compared with the initial values.
Fig. 2. Counts onPseudomonasF-agar of minced goat meat under aerobic or vacuum storage at refrigeration temperature.
4. Discussion
Data on microbial and sensory evaluation of minced goat meat stored at 418C indicated a shelf-life of 28 days under vacuum compared with similar overall acceptability scores at 3 days observed in aerobically stored samples. The low storage life of aerobically stored samples was due to loss of colour. Shaw et al. (1980) also reported that the colour scores of refri-gerated lamb loins in aerobic storage were lower than
that under vacuum for 6 weeks. Vacuum packed goat meat mince maintained bright red colour up to 40 days even when the meat was spoiled. Gill and Jones (1994) observed that the master packed ground beef in anoxic state continued to maintain its bloom up to 32 days although its shelf-life was only 21 days. The spoilage of vacuum packed mince by sulphide odours has also been reported by earlier workers (Hanna et al., 1979, 1983). Greenish discolouration of the drip might be due to oxidation of heme to greenish myoglobin or by
Fig. 4. Total aerobic counts of minced goat meat under aerobic or vacuum storage at refrigeration temperature.
formation of sulphmyoglobin between myoglobin and hydrogen sulphide (Nicol et al., 1970; Taylor and Shaw, 1977).
There were decreases in pH and ERV during storage under vacuum. In normal pH meats, decrease in pH and increase in ERV (Sutherland et al., 1975) or no signi®cant changes in pH or ERV (Turner, 1960; Pearson, 1968) have been reported. Total acidity increased during storage under vacuum. Sutherland
et al. (1975) observed decrease in total alkalinity in vacuum stored beef.
The highly variable shelf-life of vacuum packed fresh meat has been attributed to differences in pH (Rousset and Renerre, 1991), ®lm permeability (Hodges et al., 1974) and initial contamination and storage temperature (Bell and Garout, 1994). In the present study, the initial pH of meat was very high (pH 6.5) with high enterobacteriaceae counts
Fig. 6. Staphylococcal counts of minced goat meat under aerobic or vacuum storage at refrigeration temperature.
(log 4.3 gÿ1). Meat with very high pH (>6.0) had a very short shelf-life even in vacuum packaging (Bem et al., 1976) as enterobacteriaceae and Brochothrix
compete better with rapid bacterial breakdown of amino-acids producing unpleasant smelling products such as H2S or ammonia (Newton and Gill, 1981). In
our study, the growth of lactic acid bacteria on the vacuum packed meat in the beginning of spoilage was very slow (log 3.1 gÿ1 at 28 days) unlike previous studies (Egan, 1983; Dainty and Mackey, 1992). The
slow growth of lactic acid bacteria may be due to high initial pH of meat and growth dominance by faculta-tive anaerobic biota (enterobacteriaceae). These results are in accordance with Grau (1980, 1981) on high pH vacuum packed meat.
The increases in Pseudomonas counts in the ®rst week of vacuum storage were in agreement with Sutherland et al. (1975). There were increases in total aerobic and psychrotrophic populations as the condi-tions were not completely anoxic in vacuum
packa-Fig. 8. Coliform counts of minced goat meat under aerobic or vacuum storage at refrigeration temperature.
ging due to oxygen ingress through the plastic ®lm (Nottingham, 1982). The low coliform and faecal streptococcal counts in vacuum compared with the aerobic storage were in accordance with Sutherland et al. (1975) on vacuum packed beef. The staphylococcal counts at 28 days of vacuum storage were lower than in aerobic storage. However, Venugopal et al. (1993) reported unusually high staphylococcal counts in
vacuum packed beef possibly due to high initial contamination.
5. Conclusion
Vacuum packaging improves the colour and micro-biological shelf-life of high pH minced goat meat
Table 2
Colour, discolouration, odour and overall acceptability scores of minced goat meat under aerobic or vacuum packed (PETPE laminate) storage at refrigeration temperaturea
Initial 5.00a 1.00a 1.00a 7.00a
Aerobic 3 5.00a 1.00b 1.00a 7.00b
7 5.00a 4.00b 1.00a 5.00b
10 4.00a 4.00b 1.00a 4.00b
21 4.00a 5.00b 1.00a 3.00b
28 2.00a 5.00b 4.00a 1.00b
Vacuum 7 7.00a 1.00a 1.00a 7.00a
14 6.00a 1.00a 1.00a 7.00a
21 6.00a 2.00a 1.00a 7.00a
28 6.00a 2.00a 1.00a 7.00a
40 ± 2.00a 4.00a 1.00a
aNumbers followed by the same alphabets, placed on the basis of chi-square values, do not differ signi®cantly (P< 0.05). b7: Bright red; 4: Slightly dull; 1: Extremely dull.
c1: No discolouration; 5: 50±100% discolouration. d1: No off odour; 4: Strong off odour.
e7: Extremely acceptable; 4: Neither acceptable nor unacceptable; 1: Extremely unacceptable.
Table 3
Lovibond colour units of minced goat meat under aerobic or vacuum packed (PETPE laminate) storage at refrigeration temperaturea
Storage condition Storage time (days) Lovibond colour units
Red Yellow Green
Initial 5.4a 3.1a 3.0a
Aerobic 3 5.4a 4.2a 3.3a
7 5.6a 4.2a 3.2a
10 6.5a 4.5a 3.5a
21 5.4a 5.4a 4.4a
28 2.2a 3.3a 1.3a
Vacuum 7 7.4a 6.5b 3.2a
14 6.6a 6.3b 4.5a
21 6.4a 5.4b 4.3a
28 6.5a 5.5b 4.3a
40 NDb NDb NDb
when compared with aerobic storage by maintaining colour and restraining microbial growth with increased acceptability scores.
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