Effect of oxygen- and carbon dioxide-enriched atmospheres on shelf-life extension of refrigerated ground pork

Meat Sci,'nce4 ~1980) 89-94

EFFECT OF OXYGEN- AND CARBON DIOXIDEENRICHED ATMOSPHERES ON SHELF-LIFE EXTENSION OF REFRIGERATED GROUND PORK

P. LOPEZ-LORENZO, P. HERNANDEZ, B. SANZ-PEREZ d~. J. A. ORDOI~IEZ

Departamento de Higiene y Tecnologda de los Alimentos, Facultad de Veterinaria, Universidad Complutense, Madrid 3, Spain (Received: 29 January, 1979)

SUMMARY

The oxidation oJthe lipids and myoglobm o f groundpork meat stored m oxygen- and carbon dioxide-en riched atmospheres at 1 ° C has been studied. Elevated oxygen levels (80-100 ~o) depressed myoglobin oxidation, increasmg the time to 50 ~ metmyoglobin jormation j r o m j o u r to about thirteen days. Twenty per cent carbon dioxide greatly reduced the rate o f lipid oxidation, extending the time to reach a TBA number 0,['5 j r o m five to about twelve days. Tocopherol and ascorbic acids were ejficient inhibitors o f lipid oxidation but citric acid was not.

INTRODUCTION

Three factors limiting chilled meat shelf-life are spoilage by psychrotrophic, slimeproducing bacteria (Clark & Lentz, 1972), lipid oxidation (Webb et al., 1972; Greene et al., 197 I) and metmyoglobin (met M b) formation (Solberg, 1970; Greene et al., 1971). Modification of the gaseous atmosphere surrounding the meat has proved to be a useful means of extending chilled meat shelf-life. Storage in COz-enriched atmospheres efficiently controls slime production in beef (Clark & Lentz, 1972), while enrichment with oxygen (80~o) inhibits metMb formation and maintains a desirable red surface colour in beef (Clark & Lentz, 1973; MacDougall & Taylor, 1975; Taylor & MacDougall, 1973) and in pork (Ordofiez & Ledward, 1977). Although it might be thought that oxygen enrichment must increase the rate of lipid oxidation, Ordofiez & Ledward (1977) found that oxidative rancidity progresses at 89 Meat Science 0309-1740/80/0004-0089/$02-25 ~ Applied Science Publishers Ltd, England, 1980 Printed in Great Britain

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P. LOPEZ-LORENZO, P. HERNANDEZ, B. SANZ-PEREZ, J. A. ORDOI~EZ

similar rates in fresh pork slices stored at 1 °C in either air or gas mixtures containing 80-100 ~ oxygen. They also suggested that storage in suitable O2/CO 2 mixtures could make lipid oxidation the factor limiting shelf-life. Lipid oxidation, being a surface phenomenon, is obviously of only limited importance in commercial cuts because of the favourable ratio of volume to surface area. However, it could become serious if mixtures of CO2/O 2 were to be used to preserve ground meat. This paper describes the influence of oxygen- and carbon dioxide-enriched atmospheres on microbial growth, lipid and myoglobin (Mb) oxidation in ground pork meat and the effect of tocopherol, citric and ascorbic acids on these spoilage parameters.

MATERIALS AND METHODS

Fresh pork meat (24 h post mortem) of rump or top round was purchased at a local •retail market. The meat (initial pH 5.5-5.7)was trimmed of external fat and ground in a stainless steel meat grinder. About 100 g of meat were placed in open Petri dishes in which several holes had previously been made. The Petri dishes were enclosed in hermetically sealed plastic containers (27 litres) and the atmospheres within the containers were maintained at the desired composition by continuous flushing with the appropriate gas mixtures. The relative humidity was kept at 99-3 ~ as previously described (Ordofiez & Ledward, 1977) and the whole system stored at 1 °C. If used in the modified atmosphere, the concentration of CO 2 was always 20 ~o, as it has been shown that this is the most appropriate level for inhibiting bacterial growth (Clark & Lentz, 1969). The percentage of Mb oxidised to metMb was estimated by the procedure of Broumand et al. (1958). The extreme values of the curve (100 ~ and 0 ~ metMb) were calculated for each meat sample after treatment with ferricyanide (1 ~) or dithionite (20 ~ ) (Ledward, 1970). Each determination was the mean of at least eight samples. Oxidative rancidity was measured by the reaction of malonaldehyde with thiobarbituric acid (TBA) as described by Tarladgis et al. (1960). Periodically, 5-7 samples of 10 g each were removed and individually submitted to malonaldehyde extraction. Two aliquots from each extract were submitted to the TBA reaction and the results averaged. The 5-7 values for each storage time so obtained were used to calculate the means and standard deviation shown in Fig. I. Total bacterial counts were carried out by the pour-plate method using plate count agar as the culture medium. Plates were incubated at 22 °C for three days. For their incorporation into ground meat, 22 ml of a 0.3 ~o (w/v) solution of ascorbic or citric acid in distilled water, or 0.075 ~ (w/v) of a-tocopherol in 95 ethanol, were added to 500 g of meat and the mixtures thoroughly stirred with a glass rod. This experiment was conducted under an atmosphere of CO2/O2 (20/80).

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91

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5

10

15

20

Storage Time (Days)

Fig. 1. Effect of storage on TBA number ( 4- standard deviation) in ground pork meat stored in air (A), air + CTC (I-1), 2 0 ~ CO 2 + 8 0 ~ air (11), 20% CO2 + 8 0 ~ 0 2 ( 0 ) and 02 + CTC (C)) at 1 °C.

pH determination proved that neither the ascorbic nor citric acid concentrations used affected the pH value of the samples. Addition of ethanol to control samples in the quantities used for tocopherol incorporation did not change the rate of metMb or malonaldehyde formation.

RESULTS AND DISCUSSION

Four different atmospheres were used: CO2/air (20/80), CO2/O 2 (20/80), oxygen and air. When CO 2 was not incorporated, chlortetracycline (CTC) (30 mg/100g) was added (3 ml of a 0-1 ~o aqueous solution per 100 g) to inhibit the microbial growth. As has already been shown for beef(Clark & Lentz, 1973; MacDougall & Taylor, 1975; Taylor & MacDougall, 1973) and pork slices (Ordofiez & Ledward, 1977), oxygen depresses Mb oxidation (Fig. 2). Greene et al. (1971) have claimed that consumers will reject beef at metMb concentrations greater than 40 ~. If this metMb value is also taken as the level at which the pork meat is refused, it is obvious that the shelf-life is, in this respect, increased from four to about thirteen days (Fig. 2) by oxygen enrichment (80-100~o). As already shown for slices, neither CO 2 (Ledward, 1970; Ordofiez & Ledward, 1977) nor CTC (Hutchins et al., 1967) affects the rate of metMb formation in ground pork meat. From Fig. 2 it is also seen that the rate of bacterial growth is much reduced by 20 CO 2 but is unaffected by oxygen. The shelf-life from a microbiological point of view would be extended by 20 ~ CO 2 from about eight to around sixteen to eighteen days

92•

P. LOPEZ-LORENZO, P. HERNANDEZ, B. SANZ-PEREZ, J. A. ORDONEZ

75

g .~

50

25.

5

I0

15

20

Storage Time (Days)

Fig. 2. Effect of storage on met Mb formation ( + standard deviation) in g r o u n d pork meat stored in air ( A ) , air + C T C (17), 20 ~ C O 2 + 80 ~ air (11), 20 ~ C O 2 + 80 ~ 0 2 ( 0 ) and 0 2 + C T C (C)) at i °C. The arrows indicate the time for the bacterial population to reach a level o f 106 (one arrow), l0 T (two arrows) and 10 8 (three arrows) organisms per gramme.

if a figure of 107-10 s bacteria per gramme is taken as the beginning of slime formation. This shelf-life extension is less than that reported by Ordofiez & Ledward (1977) for pork slices and probably reflects a heavier initial bacterial load in the ground meat. Contrary to previous findings on pork slices (Ordofiez & Ledward, 1977), storage in 20 ~ CO 2 greatly depresses the rate Of lipid oxidation (Fig. 1), extending the storage time required to reach a TBA value of 5 from five to about twelve days. No obvious explanation for the protection by CO 2 against lipid oxidation can be advanced but in our view the phenomenon deserves further investigation. Citric acid does not retard rancidity development during storage under C O 2 / O 2e n r i c h e d atmospheres. In fact, as already shown by Benedict et al. (1975), a marked 15

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Storage Time (Days]

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Effect o f citric ( A ) and ascorbic ( Q ) acids and ct-tocopherol t O ) on T B A n u m b e r ( + standard deviation) in g r o u n d pork m e a t stored in 2 0 ~ 0 2 + 8 0 ~ 0 2 at 1 °C. Control (A).

SHELF-LIFE EXTENSION OF REFRIGERATED PORK

93

acceleration occurs. However, both tocopherol and ascorbic acid are efficient inhibitors of lipid oxidation in ground pork meat stored at chill temperatures under CO2/O 2 (20/80) atmospheres (Fig. 3). A set of experiments was performed to follow the rate of metMb formation in air of samples which had been initially stored under CO2/O 2 (20/80) atmospheres. Figure 4 shows that the longer the storage in CO2/O 2, the higher is the rate of Mb oxidation when the samples are subsequently exposed to air.

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i

,

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Storage Time (Days)

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Fig. 4. MetMb formation ( _+ standard deviation) after 0 (I-q), 2 (A), 4 (ll), 6 ( 0 ) and 8 (©) hours of exposure to air of samples of ground pork meat stored in 20 ~ CO 2 + 80 ~ O~ at 1 °C for the time shown.

Ordofiez & Ledward (1977), on the basis of their work on whole slices of pork

bicepsfemoris and longissimus dorsi, suggested that the use of an O2/CO 2 mixture to improve the colour stability of pork and reduce microbial proliferation may not be particularly useful because of the tendency of pork meat to become rancid. Our results on ground pork do not support this view. In ground meat stored under O:/CO2-enriched atmospheres the 45-50 ~o metMb limit is reached before a TBA value of 5 is achieved. Furthermore, the addition of ascorbic acid or tocopherol can further inhibit lipid oxidation. The differences between the results reported here and those of Ordofiez & Ledward (1977) may well be due to the different rates of lipid oxidation in the various pork muscles already observed by these authors, and the probable different rates of pigment oxidation in intact and ground muscles (Ledward & MacFarlane, 1971). In our view, what may well limit the usefulness of the CO2/O2 mixture is the tendency of Mb in the samples so preserved to oxidise when subsequently exposed to air.

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P. LOPEZ-LORENZO, P. HERNANDEZ, B. SANZ-PEREZ, J. A. ORDOi~EZ

CLARK, D. S. & LENTZ, C. P. (1969). Can. Inst. Fd. TechnoL J., 2, 72. CLARK, D. S. & LENTZ, C. P. (1972). Can. Inst. Fd. Technol. J., 5, 175. CLARK, D. S. & LENTZ, C. P. (1973). Can. Inst. Fd. Technol. J., 6, 194. GREENE, B. E., HSlN, I. & ZIPSER, M. W. (1971). J. Fd. Sci., 36, 940. HUTCmNS, B. K., LIu, T. H. P. & WATTS, B. M. (1967). J. Fd. Sci., 32, 214. LEDWARD, D. A. (1970). J. Fd. Sci., 35, 33. LEDWARD, D. A. • MACFARLANE, J. J. (1971). J. Fd. Sci., 36, 987. MACDOUGALL, D. B. & TAYLOR, A. A. (1975). J. Fd. Technol., 10, 339. ORDOI~IEZ, J. A. 8/.~LEDWARD, D. A. (1977). Meat Sci.~ 1, 41. SOLBERG, M. (1970). Can. Inst. Fd. Technol. J., 3, 55. TARLADGIS,B. G., WATTS,B. M., YOUNATHAN,M. Y. & DUGAN, L. (1960). J. Am. Oil Chem. Soc., 37, TAYLOR, A. A. & MACDOUGALL, D. B. (1973). J. Fd. Technol., 8, 453. WEBB, R. W., MARION, W. W. & HAYSE, P. L. (1972). J. Fd. Sci., 37, 853.