While vacuum or modified atmosphere packaging materials (made from polyethylene or other plastic-based materials) can improve the stability and safety of raw or processed meat, consumers are demanding more environmentally friendly packaging and more natural products. In this article, recent research is presented that demonstrates the feasibility and commercial application of a variety of bio-based polymers or ‘bio-polymers’. These packaging solutions are made from a variety of materials, including renewable/sustainable agricultural commodities.
By Professor C. Cutter
The benefits of ‘bio’ packaging
When used in packaging, bio-based materials have been shown to prevent moisture loss, drip, reduce lipid oxidation and improve the flavour attributes of a product. In addition, the handling properties, colour retention and microbial stability of foods can be enhanced. Films made from such materials may also serve as gas and solute barriers and complement other types of packaging by improving the quality and shelf life of food. For example, bio-based packaging materials may be formulated to reduce drip loss of meat products, while also retarding microbial growth by lowering the amount of water within a package. Further to this, bio-based packaging materials appear to be excellent vehicles for incorporating a wide variety of additives, including antioxidants, antimicrobials, colours and nutrients to meat.
Research into anti-microbial delivery
Over the years, our laboratory has investigated the use of bio-based packaging materials as a delivery system for antimicrobials. The research has focused on controlling microbial populations associated with meat. In an early study, lean and adipose beef carcass tissues were innoculated with the meat spoilage organism Brochothrix thermosphacta (BT) (approx. 4.50 log10 CFU/cm2) and left untreated (U) or treated with 100 μg/mL of the bacteriocin, nisin (N), calcium alginate (A) or 100 μg/mL nisin immobilised in a calcium alginate gel (AN). Tissue samples were refrigerated after treatments and bacterial populations and nisin activity were determined at 0, 1, 2 and 7 days. U, A and N treatments of lean and adipose tissues did not suppress bacterial growth (>6 log10 CFU/cm2 by day 7), while treatments of lean and adipose tissues with AN suppressed bacteria (>2.42 log10 CFU/cm2 by day 7). Bacteriocin titres from both tissues were higher in AN versus N samples after the 7 day incubation. This study demonstrated that immobilisation of nisin in a gel may be a more effective delivery system of a bacteriocin to the carcass surface than direct application alone.
In another complimentary study, sterilised, lean and adipose beef carcass tissues were inoculated with BT, left untreated (U), or treated with 100 mg/mL nisin (N), calcium alginate (A), or 100 mg/mL nisin immobilised in a calcium alginate gel (AN). Treated tissues were aseptically processed into ground beef and populations of BT as well as nisin activity were determined during refrigerated storage (4°C) at 0, 7, and 14 days. At day 0, bacterial populations of U- and A-treated ground beef were 3.24 and 3.17 log10 CFU/g respectively. Ground beef treated with N exhibited populations of 2.80 log10 CFU/g while AN significantly suppressed the organism to undetectable levels (<1.30 log10 CFU/g) at day 0. In contrast to high nisin titres from AN-treated ground beef at day 0, nisin titres were undetectable in N-treated ground beef. By day 7, BT had grown to 7.18, 7.04, and 6.92 log8 CFU/g in U-, A-, or N-treated ground beef, respectively, while AN-treated ground beef exhibited significantly different populations of 6.56 log10 CFU/g. By day 7, nisin titres from AN-treated ground beef were considerably diminished. At day 14 of the study, all treatments exhibited bacterial populations <7 log10 CFU/g and nisin titres were virtually undetectable in any of the ground beef samples. While the growth of BT could not be effectively suppressed for the full 14 days, the research demonstrated that the application of nisin in alginate gels to meat surfaces does afford some immediate protection against undesirable bacteria when these surfaces are processed into ground beef.
Using a different matrix, collagen films were soaked in a nisin solution and dried to produce biologically active nisin-incorporated collagen films (NICF). Frankfurters were wrapped with NICF or collagen films without nisin (Control), vacuumed packaged, heated (30 min, 100°C), cooled and inoculated with approximately 3 log10 CFU/g of either the foodborne pathogen, Listeria monocytogenes (LM) or BT. Innoculated, NICF and control frankfurters were subjected to refrigerated storage (4°C) for up to 14 days or temperature abused (24 h, 25°C) and refrigerated (4°C) for up to 14 days. Immediately after treatments and following refrigerated storage at days 4, 7, and 14, BT was reduced by more than 1.4 log10 CFU/g, whereas LM was not reduced by more than 0.60 log10 CFU/g. Following temperature abuse and 14 days of refrigerated storage, BT and LM were reduced by approximately 1 log10 CFU/g. This research was the first to demonstrate that the incorporation of nisin into a collagen film has an effect on bacteria associated with ready-to-eat meat products.
Discovering optimal packaging materials
In a recent study, the optimal combination of different bio-based packaging ingredients was researched. The effects of different concentrations of pullulan (an edible, mostly tasteless polysaccharide polymer), the organic product glycerin, the polysaccharide xanthan gum and the vegetable gum locust bean were studied. The qualities of the packaging assessed in the study were thickness, transparency, ultimate tensile strength, elastic modulus, elongation and puncture force. Statistical analysis demonstrated that pullulan and glycerin significantly influenced the desirable properties of films for use in packaging. Using predictive models, contour plots and the characteristics of commercial low-density polyethylene oxide (LDPE) as constraints, an optimal pullulan composition was found. These films were infused with sakacin A and evaluated for their ability to inhibit LM on a ready-to-eat food. Sakacin A-containing (1 mg/cm2) pullulan films demonstrated improved antimicrobial activity in vitro, as compared to the direct application of sakacin A alone. When experimentally innoculated surfaces of cooked turkey breast were treated with sakacin A-containing pullulan films, LM populations were reduced 3 log10 CFU/g after three weeks under refrigerated storage. These results demonstrate the possibility of using sakacin A-containing pullulan films to inhibit or reduce LM on the surface of a ready-to-eat food.
These research studies clearly demonstrate the potential for utilising bio-based packaging materials to improve the microbial quality and safety of many products, including fresh or processed meat.
The author
Catherine N. Cutter, Ph. D.
Associate Professor
Department of Food Science
Pennsylvania State University
University Park, PA 16802, USA
Tel: +1 814-865-8862
email: cnc3@psu.edu
