Article pp.379-387 du Vol.28 n°4-5 (2008)

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ADIV Association – 10, rue Jacqueline-Auriol – 63039 Clermont-Ferrand Cedex 02 – France.

Correspondence: Email: [email protected]

FOCUS : JSMTV

Producing stock-keeping units:

technological and sanitary aspects

L. Picgirard

RÉSUMÉ

Mise en œuvre des Unités de Vente Consommateur Industrielles : contrain- tes technologiques et sanitaires

La mutation de la profession de boucher associée à celle du comportement du consommateur a conduit à un accroissement important des rayons libre-service dans les grandes surfaces et des productions de viandes pré-emballées produi- tes par l’industrie. Ces viandes pré-emballées, appelées aussi UVCI (Unités de Vente Consommateur Industrielles), doivent avoir des durées de vie importantes pour permettre leur transport, leur distribution et apporter un délai d’utilisation suffisant pour le consommateur. Ces durées de vie longues peuvent être permi- ses par deux technologies de conditionnement : le conditionnement sous atmos- phère modifiée (MAP) ou le conditionnement sous vide (SV). Cependant, ces techniques ont apporté de nouvelles contraintes pour les industriels, en particu- lier techniques et sanitaires. Avant de commercialiser leurs viandes pré-embal- lées, les industriels doivent choisir un conditionnement approprié, adapté à leurs réseaux commerciaux et aux attentes de leurs consommateurs. Cet article pré- sente les avantages et les inconvénients de quelques technologies de conditionnement ainsi que leur impact sur la couleur de la viande, son exsudation et la croissance microbiologique de la flore présente. De nouveaux développements attendus pour les viandes conditionnées tels que les emballages actifs, l’emploi de nouveaux mélanges gazeux ou la prise en compte nouvelle des questions environnementales sont également abordés.

Mots clés

viande, conditionnement, uvci, couleur, microbiologie.

SUMMARY

Changes in the butchery profession and in customer behaviour have induced a big increase of self-service counters in supermarkets and of meat trays produced by industrial companies. These pre-packed meats must have long shelf lives to allow transport and retail to the stores and to provide enough storage for customers. Long shelf lives can be expected thanks to two technologies: modified atmosphere packaging (MAP) and vacuum packaging (VP). These technologies have induced new drawbacks for industrial companies and especially sanitary and technological restraints.

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Before selling meat trays, industrial companies have to choose an appropriate packaging, adapted to their commercial channels and to the customer expecta- tions. This article presents interests and limits of several packaging technologies, their influence on meat colour, drip losses, microbial growth. New developments expected for meat packaging such as active packaging, use of new gas mixture and respect of new environmental restraints are reported too.

Keywords

meat, packaging, stock-keeing units, colour, microbiology.

1 – INTRODUCTION

In France, livestock meat purchases have not evolved much during the past 10 years: 89.6 kg/years/person in 2007 and 90.4 kg/years/person in 1997. Veal meat and lamb meat consumption have clearly decreased (– 5.3% and – 4.2% during the period 2007/2006, respectively). Beef and pork meat have hardly risen (+ 2.6% and + 1.3% during the period 2007/2006) (Office de l’Elevage, i.e. French Livestock Agency, 2008). In this context, 73% and 75% of beef and pork meat household purchases are carried out in supermarkets. In the same time, there have been a lot of changes in the butchery profession: working difficulties, job depreciation, etc. Therefore, traditional counters, particularly in small supermarkets, have disappeared and self-service counters have progressed thanks to meat trays produced by industrial companies: these are the Stock-Keeping Units (SKU). With this evolution, the distribution of pre-packed products has changed. Store orders, prepared by companies, are collected on big platforms, before being transport to the different shops. Improving meat tray shelf lives has been necessary and tray packaging with high permeability films has been abandoned. Beef meat cuts produced by SNIV (a French meat company organization) members repre- sented 43,400 tons in 2005 and have increased by 96% since 2001. From year to year, the market share of pre-packed processed products purchases have grown so that SKU production concerns not only large firms but also small and medium companies.

Traditionally, these companies only produced vacuum packed muscles.

Currently, two technologies are available to propose SKU with a sufficient shelf life in self-service counters: modified atmosphere packaging (MAP) and vacuum packaging (VP). They have replaced traditional trays packed with high permeability films and produced by store workshops (film packaging).

The change from deboning to processed products was not an easy one. Simi- larly, meat tray producers had to modify their processes in order to obtain the long shelf lives expected by customers. Technological, hygienic and economical drawbacks appeared after setting up SKU production facilities in industrial companies.

This article gives an overview of these problems after a short description of current packaging technologies.

2 – THE DIFFERENT PACKAGING TECHNOLOGIES

Currently, two kinds of technologies are available for producing SKU: modified atmosphere packaging (MAP) and vacuum packaging (VP). Modified atmosphere

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packaging consists of injecting a gas mixture in the place of the surrounding atmosphere. It preserves meat colour and stabilizes the microbial development of packed products. Gas mixture composition packaging depends on commercial channels and required shelf lives. Thus, two kinds of gas mixture are often used: blending with 70% oxygen and 30% CO₂ (10% of nitrogen can be added to avoid trays collapsing) and an oxygen-free mixture composed of 50% to 70% nitrogen and 50% to 30% CO₂. Gas mixture with oxygen is used for sales at self-service counters. Shelf lives are approximately 10 to 12 days for meat cuts and 6 to 7 days for minced meat. Oxygen makes it possible to retain myoglobin, the pigment responsible for meat colour, in its oxygenated state, i.e. bright red. CO₂ avoids bacterial growth.

Therefore, the meat keeps an attractive aspect for customers. Gas mixtures without oxygen are more often used for catering commercial channels (foodservice): restau- rants, school or hospital canteens, etc. Final customers do not see raw meat in trays before cooking, so therefore a bright red meat colour is not necessary. Its colour becomes purple red with shelf lives of up to 14 days for meat cuts and approximately 8 to 9 days for minced meat.

With these two kinds of gas mixture, two technologies can be used:

– Vacuum/injection technology: gas is injected into the trays after removing the surrounding atmosphere via a vacuum process;

– The “Flow pack” system which consists of flushing surrounding atmosphere with the required gas mixture. BDF^® from Sealed Air Cryovac is the only significant application of this technology for fresh meat.

Vacuum packaging consists of wrapping a product in a barrier film without atmosphere. The vacuum level fluctuates between – 3 and – 5 mbar. Two technologies can be used: vacuum in a bag or in a thermoformed pocket or “skin” technology. This consists of heat wrapping the upper film of the packaging and applying it to the meat with moderate force. Also, the film takes on the exact shape of the meat like a “skin” without deforming the meat. An example of this technology is the Dar- fresh^® system from Sealed Air Cryovac. This technology, which is expensive, is more suited to fragile or up-market products. Shrinkable films can now be used for vacuum thermoforming. This is the case of the Formshrink^® technology proposed by Multivac. When vacuum-packed, oxygen-free meat has a purple red colour and myoglobin is in its reduced state, desoxymyoglobin. Thus, VP in a vacuum bag or in a thermoformed pocket is used for the foodservice market. Skin technology is usu- ally be used for selling produce to final customers in super or hypermarkets. In this case, the meat in the skin film is wrapped in a cardboard case or sealed with another promotional film: it hides the dark aspect of meat. In both cases, usual shelf lives are approximately 14 to 21 days for meat cuts.

3 – MEAT COLOUR

The main characteristics of meat expected by customers buying packed meat cuts are colour (Renerre and Labadie, 1993) and tenderness. Colour has a direct impact on purchasing patterns. Tenderness provides regular customers. Taste, drip losses, lipid stability have to be taken into account but they are of minor importance.

Thus, two kinds of shelf life can be defined: commercial shelf life, the period during which the product keeps suitable organoleptic characteristics (aspect, odour, fla-

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vour) and regular microbial shelf life. But, we often note that meat cuts in trays (SKU) have an unstable colour before the end of the shelf life has been reached.

The colour of meat is due to myoglobin, a natural pigment of the muscle.

Myoglobin can be present in 3 forms. The bright red oxygenated form is “oxymy- oglobin”. It is the main form when partial oxygen pressure exceeds 13%. The purple (reduced red) form is “desoxymyoglobin”, which dominates when partial oxygen pressure is below 0.2%. The oxidized form, “metmyoglobin”, has a green or grey colour. It appears for low partial oxygen pressure between 0.2% and 13% and for muscles with a high oxygen consumption rate and with poor reducing capacities (McMillin, 2008). In this case, fresh beef meat containing 20% metmyoglobin is detected by customers (MacDougal, 1981) or is rejected when the metmyoglobin rate reaches 40% (Green, Hsia and Zipser, 1971). When meat is damaged by spoilage bacteria, the green colour of meat is due to sulfomyoglobin (spoilage with bacterial H₂S) or to cholemyoglobin (spoilage with bacterial H₂O₂).

The colour of packed meat cuts depends directly on the gas blending used, especially partial oxygen pressure, gas/product (v/v) ratio, storage temperature, lighting intensity and the nature of the light source. There is also a strong influence concerning the muscle which is used, especially its pH, its metabolic activity (oxida- tive or glycolytic), its NADH reserve, on meat colour stability (Mancini and Hunt, 2005). NADH is a natural anti-oxidant in muscles; it decreases after animal death.

Thus, the animal’s diet is directly responsible for muscle colour, and modifying the anti-oxidant potential. According to Legrand and Rennere (1998), the cattle diet sup- plementation with vitamin E could improve the colour of meat cuts packed with modified atmosphere containing oxygen. According to Lynch et al. (2002), heifers fed with a finishing diet containing silage or concentrates have loins and strip loins with better colour stability than heifers fed only with grass. Higher linolenic acid content and lower tocopherol content in fat tissue can explain the differences between the two categories of animals. It is assumed that lipid and protein oxidation have a good correlation. Despite these results, it is not easy for the food industry to control animal feeding.

Finally, microbial growth also influences, directly or indirectly, the colour instability of packed meat cuts.

4 – MAP WITH OXYGEN

MAP with 70% to 80% oxygen is the most difficult technology to control and colour instability phenomena are frequent. From a microbial point of view, there is enough oxygen to enable the growth of aero-aerobic flora, such as pseudomonas, or the growth of anaerobic germs such as lactobacillus sakei (Ecolini et al., 2006).

Internal studies steered by ADIV (a French meat technical centre), have demon- strated that the microbial evolution of modified atmosphere packed meat cuts was different from that of vacuum packed meat cuts, stored in the same conditions. The growth of the total viable count, lactic flora and enterobacteriaceae was lower but the development of pseudomonas or brochothrix thermosphacta, germs often responsible for colour spoilage, was higher. So, an optimal rate of CO₂ has to be determined. CO₂ rates below 15% do not accurately curb microbial growth. Rates higher than 40% can induce trays collapsing because meat lipids and water are able to absorb this gas. After all, CO₂ levels above 50% or 60% provide no or little micro-

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bial gain (Mac Millin et al., 1999; Gill and Tam, 1980). If spoilage bacteria evolution determines directly the aspect of the finished product, the process obviously affects it. Technological factors concern as much raw material as process parameters.

These factors are summarized in table 1.

Table 1

Factors affecting meat colour stability.

Also, myoglobin stability is different between muscles. It depends on their metabolic activity, i.e. their capacity to use oxygen before animal death, and on their reducing potential due the natural NADH system. Thus, longissimus dorsi, longissi- mus lumborum, obliquus externus abdominis, tensor fasciae latae and semi tendino- sus are colour-stable muscles. Semi membranosus and rectus femoris have intermediate stability. Psoas mjor, gluteus medius, supraspinatus, triceps brachii and diaphragma medialis are rather unstable muscles (Renerre, 1984; Mc Kenna et al., 2005).

Moreover, a study steered by ADIV and financially supported by INTERBEV and the Office de l’Elevage, is proving that the meat ageing method (carcass or vacuum packed) and ageing time have a noticeable effect on the colour stability of meat cuts in trays. A higher sensitivity of sirloins compared to strip loins has also been demon- strated. Colour stability is not improved by ageing temperature. Ageing on carcasses improves the visual acceptability of meat cuts compared to muscles aged in a vacuum bag, especially for strip loin (figure 1). Optimal ageing time for muscles before slicing seems to be 6 days maximum.

Raw material Process

Type of muscle Slicing and storage temperature

Ageing methods Gas/product ratio

Ageing temperature and ageing time Slicing delay Gas mixture

Figure 1

Visual acceptability (in days) of meat cuts in trays (70%O₂/30%CO₂) versus ageing method, ageing period and muscle (ADIV, 2008).

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On an industrial scale, to avoid untimely colour spoilage of meat cuts, carcasses have to be deboned and muscles sliced 2 to 3 days post mortem. Unfortunately, meat cuts tenderness is not optimized. On the one hand, the muscle ageing time is too short. On the other hand, the ageing process is stopped by oxygen. MAP with oxygen is bad for beef meat tenderness (Seyfert et al., 2005) but also on pork meat (Lund et al., 2007). The problem is due to the formation of protein cross-links, issued from oxidation mechanisms.

Concerning process parameters, flow pack technology or vacuum/injection technology give similar results. In both cases, gas volume has to be 1.5 to 2 times higher than the meat volume to avoid tray collapsing. This phenomenon completely disap- pears when gas volume is 2 to 3 times higher. Tray collapsing depends on product shape too (surface/volume ratio), product temperature, and the meat pH and CO₂ rate used. CO₂solubility is very high in meat (approximately 1.8L CO₂/kg of meat). It increases with the CO₂ percentage in the gas mixture, with meat pH or at cold temperatures. Sealed Air Cryovac has developed a new packaging technology, Mirabella® that can preserve meat cuts during 10 to 12 days with a gas volume 1 only to 1.5 times higher than the product volume. This system consists of sealing two films on the tray: the first one, permeable to oxygen, can touch the meat, the second one, impermeable, is tightened on the tray surface (RIA, 2007). This technology tightens meat cuts and thus enables it to be displayed vertically on counters.

Trays volumes are reduced and transport costs lowered.

5 – MAP WITHOUT OXYGEN

Shelf lives and microbial development with MAP without oxygen or with VC are equivalent. The main gain of this technology in comparison with VC is to limit drip losses during trays storage. Thus, meat cuts packed with 50% N₂/50% CO₂ modified atmosphere have drip losses of about 1% after 14 days of storage whereas vacuum packed meat cuts lose around 3.6% (Picgirard, 2007).

By using MAP without oxigen, the meat colour can be quickly altered, especially with minced meat. For this product, problems can occur only a few hours after packaging: the product becomes irreversible grey. To avoid a grey colour apparition, the residual oxygen rate has to be reduced to a sufficiently low level because oxygen rates between 0.15% and 2% favour discolouration (Mancini and Hunt, 2008).

Using O₂ scavengers can be a solution for limiting discoloration phenomena. Thus, to optimize meat colour, the oxygen rate has to be reduced to 500 ppm within 0.7 hours with the use of scavengers. Other authors recommend 1% to 1.5% O₂ absorption per hour (McMillin, 2008). Nevertheless, using O₂ scavengers can have negative effects: for maximal efficiency, free gas flow has to surround the plastic bag placed in the tray and the cost of scavengers is expensive. Furthermore, using metal detectors after trays sealing can be disturbed by scavengers which contain ferrous components. Creating or improving O₂ scavenger-films could be a good solution for replacing current bags (Coma, 2008).

Like for MAP with oxygen, muscle stability has a deep impact on the colour of meat cuts. Therefore, the colour stability of psoas major is lower than that of longis- simus lumborum. Storage temperature and storage duration of vacuum-packed muscles have a significant influence. Rib steaks packed with free-oxygen modified atmosphere, retain an attractive red colour after 14 days storage at + 4°C, even if

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they come from muscles stocked during 75 days at – 1.5°C. If rib steaks are vacuum packed, visual acceptability remains correct only 8 days at + 4°C (Picgirard, 2007).

In these conditions, it seems better to slice muscles quickly after removing the vacuum bag and to avoid storing carcasses longer than 3 days. The microbial quality of raw materials is very important for limiting meat cut spoilage, especially when vacuum packed muscles are stocked at low temperatures during long periods before slicing.

Concerning packaging parameters, the gas/product ratio is the same as that used for MAP with oxygen. Nevertheless, flow pack technology cannot be used because of the too high residual oxygen content in the tray. The microbial evolution of modified atmosphere packed meat cuts without oxygen can be compared with that of vacuum packed ones. Low oxygen contents are not sufficient to enable spe- cific growth of spoilage bacteria on pork meat cuts (Jeremiah et al., 1992).

6 – VACUUM PACKAGING

For supermarkets, vacuum packaging in bags is not very widespread. Skin packaging especially Darfresh^® technology is more frequent. It brings a real added value thanks to a more attractive presentation of the meat cuts with less exudate. The microbial evolution of skin packed products is the same as that of in-bag vacuum packaging or modified atmosphere packaging without oxygen. Thus, the microbial growth of pseudomonas or brochothrix is curbed compared to modified atmosphere packaging with oxygen and this inhibition will be efficient all the earlier as the raw material user delay will be long. Vacuum packaging has two main advantages in comparison with MAP with oxygen: lesser discoloration provided that packaging films with low oxygen permeability are used, and meat tenderness optimisation. Nevertheless, discolouration can occur on the periphery of meat cuts if the muscle before slicing has been stored for too long and if the expected shelf life of meat cuts is superior to 14 days. With skin or vacuum packaging, meat tenderization continues thanks to ageing. Tenderisation will more efficient for meat cuts with a longer shelf life. Nevertheless, vacuum packaging induces significant exudates: roughly 3% after a 14-day storage period. Shrinkable films or skin technology can be used to reduce exudation of 30% to 50% and 66% to 80% respectively. Finally, vacuum or skin packaging concerns only boneless meat cuts. Strengthened films or textiles (“bone guards” - for example) make it possible to pack meat with bones without damaging the visual aspect.

7 – THE FUTURE FOR MEAT CUT PACKING

In the future, tenderness and the colour of meat cuts will have to be optimized thanks to packaging, whilst taking into account environmental aspects. There are several potential solutions:

1) using anti-oxidants;

2) using combined technologies;

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3) using carbon monoxide (CO);

4) using active packaging.

Because of regulations, the use of anti-oxidants only concerns meat prepara- tions such as burgers, marinated meats, meat-balls, kebabs or meat products.

Indeed, if an ingredient or an additive is added to meat raw material, the word

“meat” cannot be used for the end product. To be able to use the term “meat”, only changes in cattle feeding can be allowed for improving colour stability of meat cuts.

For meat products, only a few natural ingredients exist for curbing myoglobin oxidation. On the contrary, additives such as lactate, acetate or ascorbate have interesting effects even if they have to be added with ideal rates.

The use of combined technologies already occurs with a new technological development from Sealed Air Cryovac: Bloom^® (Viande Magazine, 2006). It unites skin presentation with the red colour of MAP. Two films have to be used. The skin film is oxygen permeable. It takes the exact shape of the meat cut and enables meat to retain its red colour. The outer sealing film ensures tray impermeability and can be used to put commercial advertisements on it.

Carbon monoxide (CO) seems to be a very interesting technology, as it ensures tenderness and the colour of meat cuts. It combines with myoglobin to form a red stable pigment: carboxymyoglobin. Generally, carbon monoxide levels in trays are 0.4% and are used with 20 to 30% CO₂ and 70% to 80% oxygen. The technology was approved by the FDA in 2004 but the European Parliament iforbade its use in 2008. A 0.5% CO rate is safe for customers but it induces two major problems. The meat can retain its bright red colour even if it contains a large amount of pathogenic bacteria or spoilage germs which are dangerous for customers. Gas can be dangerous for operators who use it in processed meat workshops.

Active packaging results in the best improvements. In the place of O₂-scavenger bags, scavenger films are easier to use. Such films exist but they have to be acti- vated by heat or UV light. Moreover, their absorption capacity is lower than that of scavenger bags (Coma, 2008). In addition to O2-scavengers, bags emitting stabiliz- ing or preservative compounds are available: CO₂ generators, H₂O₂ generators, chlorine dioxide generators. Anti-microbial compounds can also be added to packaging films; metal (iron), organic acids (lactic acid, propionic acid), enzymes (glu- cose-oxydase), bacteriocins (nisin, pediocin, lacticin) or essential oils (Coma, 2008).

To use these molecules in Europe, they must not have an impact on the quality of packed products.

Finally, environmental impact must be taken into account when creating new packaging technologies. During the past few years, a lot of efforts has been made to reduce films thickness and consequently, film volumes. Except for oxo-biodegradable films which are petrol-based, real biodegradable materials with correct water- proof properties are not available. Poly-lactic acid (PLA), the most well-known biodegradable film, can only be used to make trays for film packaging or “flow pack”packaging because its oxygen permeability is too high. We can imagine significant fields of action for improving current packaging technologies or for creating new ones unless the rocketing of petrol prices induces shorter commercial channels and shorter shelf lives, thus leading to the use of more basic materials such as paper or cardboard.

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