Antimicrobial Active Packaging Systems to Extend the Shelf Life of Lamb Meat

1. Introduction

Food and Agricultural organisation of the United nations (2017) reported that over 1.3 billion tonnes of food products gets wasted every year accounting for about one third of the total food production in the world. The meat food contributes over 20% loss of the total food produced and this loss occurred mainly at the later stages of supply chain i.e. retail losses. Gunders (2012) reported that retail losses of meat products are largely influenced by the short shelf life of the product and the reports from European Commission (2010) showed that the retail food waste contributes to 58% of the total food wastage in the EU countries, therefore signifying the importance to extend the shelf life of the meat products. Lamb meat is a very perishable food product with a shelf life of nine to ten days under MAP or VP (Williams, 1991) making it hard for commercialisation and exporting over distant countries. Therefore, the lamb meat industries are looking for technologies to extend the shelf life of raw and processed lamb meat products which would provide them huge economic benefits and help them in managing the food waste related problems.  Extending the shelf life of the lamb meat would benefit the lamb meat industries in establishing their product among the competitors and helps them in effective transport to long distant export markets and satisfy consumer requirements. This can solve major food wastage issues related to the short shelf life of the product in the supply chain.

The shelf life of packaged lamb meat is affected by several factors such as microbial, biochemical and sensory characteristics, of which, the microbial metabolic activity is the most predominant factor affecting the shelf life of these products as it is hard to produce a meat product without some degree of microbial contamination (Mills, 2012) that arises during slaughtering techniques. Both aerobic and anaerobic bacteria can grow on the surface of the meat, therefore, by Controlling the microbial growth using antimicrobial materials, the shelf life of lamb meat product can be extended. Recent Studies have reported the extension of shelf life of lamb by using the antimicrobial agents such as whey protein isolate, TiO­₂ nanoparticles, rosemary essential oil, food by-products, grape pomace and so on (Andres et al., 2017; Alizadeh Sani et al., 2017; Guerra -Rivas et al., 2016; Ortuno et al., 2015; Karabagias et al., 2011). The shelf life of the meat can also be affected by changes on the biochemical characteristics such as colour of the meat, lipid oxidation to a certain extent as these factors are responsible for the sensory changes in the meat thereby reducing its retail quality and shelf life. Currently techniques like chilling, addition of preservatives and packaging techniques are used to preserve meat products and extend their shelf life (Ortuno, Serrano and Banon, 2015).

The primary purpose of packaging is to contain, protect, preserve and provide entire information of the food product right from the manufacturing point to the consumer usage point (Cruz Romero and Kerry, 2017). Packaging systems used to package fresh and processed meat products, control microbial contamination thereby delaying the meat spoilage. In other words, they are simply responsible for the maintenance of the sensory qualities and other important parameters within the estimated product shelf life. Active packaging, according to the European Union Guidance to the commission Regulation No 450/2009, is a packaging system with an additional function apart from acting as a protective barrier from the external factors. Antimicrobial active packaging is the form of active packaging that are used to control the microbial growth using antimicrobial materials in the packaging system and are used in the form of coatings, edible films, sachets/ pads, polymer coated. The edible films or coatings are generally impregnated with the naturally occurring antimicrobial nano solubilisates and then surface coated on to the conventional polymers to control the growth of the microbes. Gelatin films containing antimicrobial materials Sodium octonoate and Auranta FV are used as carrier coatings on the commercial polymer surface to control the microbial growth on the meat surface and control its spoilage effects (Clarke et al., 2016). Rosemary essential oil known to have antioxidant and antimicrobial activity due to the presence of carnosic acid and used for the development of antimicrobial active packaging to extend the shelf life of lamb meat products (camo et al., 2008; Ortuno et al., 2015; Pineros- Hernandez et al., 2016).

However, to the best of our knowledge, rosemary nano solubilisates have not been used to develop antimicrobial active packaging and also no work have been reported on the use of gelatin film as a carrier of antimicrobial rosemary nano solubilisates, coated on the inner surface of conventional vacuum packaging pouches, to extend the shelf life of lamb meat. Therefore, the objectives of this study are

1. To develop conventional vacuum packaging pouches coated with gelatin as a carrier of antimicrobial substances and to determine its mechanical and barrier properties.
2. To assess the performances of the developed antimicrobial active packaging in extending the shelf life of the vacuum packaged lamb meat.

2. Irish Lamb Meat Production

Lamb meat industries are the indigenous meat industry in Ireland contributing for about 3% of total food exports in the year 2016. Over 50,000 tonnes of sheep meat valuing approximately around € 240 million was exported in the year 2016 (Boardbia) with France (45%) and UK (25%) forming the major export market contributing 70% of the total Irish lamb exports. Irish lambs also account for about 7% of total meat product consumed in Ireland. Due to the small domestic market and huge demand in European Union market, lamb meats produced are mainly for the export purpose. This signifies the importance of shelf life extension of the lamb meat products for the effective export qualities.

 3. Shelf Life Characteristics of Lamb Meat

The shelf life of the lamb meat is characterised by the microbial, biochemical and sensory changes that occur during storage. These characteristic changes are dependent on the intrinsic and extrinsic factors of the lamb meat. The Intrinsic factors include pH, its oxidation- reduction potential, water activity (a_w), available sources of water, carbon sources, vitamins, minerals and other important constituent of the meat. Extrinsic factors include availability of oxygen, relative humidity, temperature and constituent of the storage atmosphere (Cutter, 2002). The proximate composition study of lean lamb meat indicated the level of moisture (72.9%), fat (4.7%), protein (21.9%) and other essential vitamins and minerals present which are suitable for the above characteristic changes in the meat (Williams et al., 2002).

3.1 Microbial Characteristics of Lamb Meat

Growth of microbes in lamb meat is influenced by its intrinsic, extrinsic and processing factors. Availability of enough nutrient content  and the suitable environmental factors makes the lamb meat as an excellent medium for the growth and survival of  many spoilage and pathogenic microbes on the meat (Cutter, 2002). These microbes utilises the sugar in the meat and breaks down the meat protein producing slime and off-odour, thereby reducing the meat quality and causes food borne disease on consumption. Salmonella, Pseudomonas, E. coli, Staphylococcus, Bacillus are some of the common bacteria that grow on the meat. The main microorganisms found in packed lamb meat are:  Entereobacteriaceae, Pseudomonas spp., Brochothrix thermosphacta, Lactic acid bacteria are the major bacteria that are found to grow on the lamb meat (Rubio et al., 2016).

Table 1 Main Spoilage Bacteria found in Chilled Packed Lamb meat (adapted from Mills et al., 2014).

In MAP lamb products, the composition of gaseous atmosphere, water activity, storage temperature and pressure inside the package determines the growth of the microbes (Cutter, 2002).

3.2 Biochemical Characteristics of Lamb Meat

During chilled storage of lamb meat biochemical changes such as pH, Colour and lipid oxidation can occur. These changes can effectively alter the eating quality of the lamb meat and contribute to the shelf life of these products

3.2.1 Changes in Lamb pH

pH is defined as the numerical measure of acidity or alkalinity of the solution on a logarithmic scale which in other words, is the measure of hydrogen ion concentration of the solution. The pH of lamb meat is an important parameter that can affect the eating quality of the product. The pH of the living sheep muscle is almost at neutral pH around the range 7.0-7.2. After slaughtering, the glycogen in the meat converts into lactic acid which brings down the pH around 5.4-6.0. The ultimate pH_u of the lamb determines the quality of the meat which is around 5.4-5.7. The variation in the ultimate pH of the meat has influence on the tenderness and water retaining capacity of the meat. Lower ultimate pH results in dry meat with reduced water holding capacity, less tender and pale coloured lamb product (Ronald Klont, 2005) Higher the ultimate pH, increased water holding capacity with the dark coloured and highly tender (softer texture) meat product. Also, the higher pH of meat above 5.8 increases the chances of microbial growth on the product surface declining the quality and shelf life of the product which is the major concern for the meat industries (Villarroel et al., 2003).

Table 2 Change in quality of Lamb with ultimate pH change. (Adapted from Lomiwes et al.)

3.2.2 Changes in Lamb Colour

Meat colour is one of the main sensory attributes that determines the freshness of the meat from the customer’s point of view (Brewer et al., 2002). In case of red meats like lamb, beef, the freshness is determined by the bright red colour of the meat. Studies report that consumers prefer to buy the bright red coloured meat than the meat that turns into brown colour upon storage (Calnan et al., 2013). The colour appearance of the meat product is due to the presence of the heme protein pigment named Myoglobin. Myoglobin on reaction with oxygen gives oxymyoglobin that is responsible for the bright red colour of the meat. Oxymyoglobin on oxidation turns out into metmyoglobin that results in the brown colour of the meat.

 oxygenation

         Purple Red

deoxygenation

Reduction Oxidation

Brown

Fig.1 Colour Formation in Meat adopted from O’ Sullivan and Kerry, 2012.

The brown colour formation of the lamb meat on storage i.e. the formation of the metmyoglobin is favoured by various factors such as rate of auto- oxidation of myoglobin, rate of oxygen utilisation by the meat, lipid oxidation, partial pressure of the oxygen present, muscle oxidative capacity pH, temperature and microbial population in some cases (Mancini and Hunt, 2005). After slaughtering, the iron molecules present in the lamb meat becomes free ion which forms metal chelates. Thus, upon storage, these free ferrous ions promote the oxidation of oxymyoglobin resulting in the discolouration of the lamb meat.

Population of aerobic microbes like Pseudomonas spp. which grows on the surface of the lamb meat creates oxygen demand in the logarithmic phase of their growth, causing the oxidation of metmyoglobin (Walker, 1980). When lamb meat is vacuum packaged, the unavailability of oxygen may result in the rapid discolouration of the lamb meat, however, unpacking the meat resulted in the blooming of the colour of the meat because of its exposure to the atmospheric oxygen.

3.2.3 Lipid Oxidation in Lamb meat

The flavour of packaged lamb meat is can be affected by the oxidation of lipids or fatty acids present. Compared to beef or pork, the high content of ω-3 polyunsaturated fatty acids in the lamb leads to the easy oxidation of fats, resulting in the reduced shelf life of the product (Banon et al., 2011). Modified Atmospheric packaging (MAP) containing high concentrations of O₂ increased lipid oxidation and developed off-flavours in the lamb meat packs (Kerry et al., 2000). The oxidation of lipid in fresh meat produces off- odours which is significantly due to the rapid increase in the development of rancidity of meat. This have a minor role in the discolouration and reduction of other quality aspects of the meat. The oxidation occurs when the meat is exposed to the excess concentration of oxygen which is catalysed in the presence of enough light and catalysts like free iron molecules (Campo et al, 2006). Meat products packaged with high O₂ and the conditions favouring the above situations will result in the development of rancidity of meat product producing off-odours on unpackaging.

3.3 Sensory Characteristics of Lamb meat

Sensory of the meat generally described as tenderness and juiciness of the meat which is partly associated with the water holding capacity and muscle structure of the meat. Studies showed the consumer’s interest on paying high price for tender meat products in the market thus making tenderness as one of the most important qualitative characteristic of the meat product (Miller et al., 2001) and was also reported as the main parameter determining the consumer’s satisfaction on eating (Pipek et al., 2008). It is associated with the connective tissues present in the meat and the strength of these connective tissues are dependent on the age, species of the animal slaughtered, muscle type and its location. Pre-slaughter handling, transportation stress, animal diet also plays the key role in meat tender quality. Other factors like lipid oxidation, pH, protein oxidation also have an influence over the meat tenderness and juiciness. Protein oxidation plays a major role in meat quality as they cause both physical and chemical alterations like loss of enzyme activity, breakdown of amino acids, reduction of protein solubility range, etc., all of which contributes to the reduction of meat tenderness and juiciness. Studies on beef packed inside MAP packaging system reveals that the high O₂ content in the packaging induced the protein oxidation in the meat thereby decreasing the meat tenderness and shelf life (Zakrys et al., 2008).

4.  Packaging for Lamb Meat

The commonly used meat packaging systems to preserve the lamb meat products are Modified Atmospheric Packaging(MAP), Vacuum packaging(VP) systems and overwrapping systems. Vacuum packaging and MAP are used for long term cold storage while overwrap packs used for retail displays and for short term cold holding (Kerry et al., 2006). Recent studies in the advancement of packaging system had resulted in the development of new packaging technologies namely active packaging, intelligent packaging, nanomaterial packaging and edible films packaging systems.

4.1 Modified Atmospheric Packaging

Modified Atmospheric packaging is an advanced system of packaging meat products with Nitrogen (N₂), Oxygen (O₂) and Carbon dioxide (CO₂) used as primary gases. . These Primary gases are either used alone or as mixtures; however, oxygen and Carbon dioxide are the predominantly used MAP gases for packaging fresh lamb meat. The gas combination commonly used is 70% O₂ and 30% CO₂ respectively (Cruz Romero and Kerry, 2017). High O₂ concentration in the packaging helps in prolonging the metmyoglobin formation which in turn ensures the maintenance of the bright red colour of the lamb meat, however, high concentration of O₂ produced lipid oxidation in meat. The high level of CO₂ is responsible for the inhibition of microbial growth on the meat surface which ensures the extension of shelf life of the product (McMillin, 2008).

Table 3. Properties of Primary gases used in MAP (adapted from Cruz-Romero and Kerry, 2017).