Application of olive leaf extract and chitosan as natural preservatives for beef patties

Fresh beef is commonly consumed meat around the world. The beef consumption in Australia is high, with Australians consuming about 30% of total beef produced in Australia. At retail, modified atmosphere packaging (80% O2 + 20 %CO2) is used for packaging of fresh cut beef. This high headspace oxygen is used to maintain the myoglobin in oxymyoglobin (MbO2) form, which provides beef with desirable red colour. The high oxygen concentration inside the packages also promotes the oxidation of myoglobin into metmyoglobin, which leads to undesirable brown colour in beef. The high oxygen concentration inside packages also leads to lipid oxidation and aerobic microbial spoilage of beef, thus reducing its shelf life. To mitigate the unfavourable effect of oxygen on the quality attributes (pH, colour, protein quality, lipid oxidation and microbial load) of beef packed in high oxygen modified atmosphere, antioxidants and antimicrobial agents are used. Synthetic antioxidants and antimicrobial agents are still commonly used to extend the shelf life of beef packed in high oxygen modified atmosphere. With increasing awareness of healthy living, consumers are demanding application of natural preservatives fresh meat. Thus, systematic studies are required to assess the effectiveness of natural preservatives when high oxygen atmospheric packaging is used to extend the shelf life of fresh meat. Olive leaf extract is a rich source of polyphenols and possesses a higher antioxidant potential. Chitosan is a natural polymer with proven antimicrobial potential. This study aimed to evaluate the combined effect of chitosan and olive leaf extract on the shelf life of beef patties, when stored under high oxygen modified atmosphere packaging (80% O2 + 20 %CO2) for 15 days under refrigerated temperature (3.2oC). It proceeded by hypothesising that the combination of these two preservatives would be more effective than using alone as they have completing antioxidative and antimicrobial properties. Beef patties prepared from fresh cut beef were used as a model fresh meat product as they have greatly exposed surface area (thus sensitive to deteriorative effect of oxygen and microorganisms). Four treatments were prepared: control (water only), olive leaf extract (OLE) (0.2%, v/w), chitosan (CHI) (0.1%, w/w), chitosan (0.1% w/w) + olive leaf extract (0.2% v/w) (CHI + OLE). The experimental design was: 4 treatments × 6 sampling points × 2 patties per treatment per trial. The important quality attributes affecting the shelf life of patties including pH, colour (CIE L*, a*, b*, chroma, hue), oxymyoglobin (MbO2) and metmyoglobin (MetMb) contents, lipid oxidation (TBARS), total viable counts (TVC) and lactic acid bacteria (LAB) counts were evaluated by storing the patties under high oxygen atmospheric packaging (80% O2 + 20 %CO2) at refrigerated temperature (3.2oC). It was found that combination of chitosan and olive leaf extract resulted in a significant (p < 0.05) positive effect on the colour of beef patties (greater stability of MbO2 and less formation of MetMb), with high a* (redness) value as compared to that of the control. Lower oxidation of lipids (lower TBARS value) was observed in patties treated with OLE and CHI + OLE compared to those treated with CHI alone and control. It was observed that with the increase in lipid oxidation, the redness of the beef patties decreased, showing that oxidation also increased the oxidation of myoglobin and reduced colour stability. Significantly lower LAB counts were observed in beef patties containing CHI + OLE as compared to their individual use (OLE or CHI) and the control, indicating a complementary preservative effect of chitosan and olive leaf extract against LAB. There was a positive correlation between TVC and LAB counts, implying LAB were the prominent bacterial strain causing spoilage of beef patties stored under high oxygen atmospheric and refrigerated conditions. Among above-mentioned treatments, combination of CHI and OLE showed overall best effect in improving the shelf life of beef patties. This combined preservative can meet the requirement of natural label at the same time achieve the preservation (shelf life) requirement of beef patties when stored under high oxygen packaging atmosphere. This combination could be further applied to other fresh meats that are stored under high oxygen packaging conditions without needing synthetic preservatives. The outcome of this study can benefit the meat industry by providing an alternative to synthetic antioxidants, antimicrobial agents and achieving a reasonable extension of shelf life.