Utilization of curdlan (CD), polyvinyl alcohol (PVA), and thyme essential oil for meat preservation
Utilization of curdlan (CD), polyvinyl alcohol (PVA), and thyme essential oil for meat preservation
Introduction
The growing consumer preference for preservative-free or naturally preserved food products has prompted the food industry to shift towards incorporating preservatives derived from herbs and microorganisms, as opposed to artificial additives, in their manufacturing processes. This research explores the prospective use of essential oils as natural antimicrobial agents to diminish the presence of prevalent spoilage and pathogenic bacteria in meat and poultry products. The antimicrobial characteristics of plant essential oils stem from key bioactive constituents, including phenolic acids, terpenes, aldehydes, and flavonoids, which are inherent components of these oils. They can be acquired from different plant components, such as buds, flowers, seeds, leaves, roots, peels, fruits, barks, and woods, solely through physical extraction and isolation methods like pressing and distillation. The antimicrobial activity of essential oils is efficacious through diverse mechanisms, including alterations to the fatty acid profile and structure of cell membranes, heightened cell permeability, impact on membrane proteins, and inhibition of functional properties associated with the cell wall. Various microorganisms, including bacteria, mold, and yeast, play a role in the deterioration of meat and meat products. Listeria monocytogenes is identified as the primary causative agent accountable for severe illnesses in both humans and animals. With the usage of thyme oil blending film, the antioxidant activity and antibacterial activity were improved and shelf life was extended up to 10 days. Curdlan (CD), an exopolysaccharide, is formed by glucose linked through β-(1,3)-glycosidic bonds and is synthesized by bacteria like Alcaligenes spp., Agrobacterium spp., Paenibacillus spp., Rhizobium spp., and the yeast Saccharomycescerevisiae etc. Curdlans and PVA is used to make the film in conjugation with thyme oil.
Curdlan (CD)/polyvinyl alcohol (PVA)/Thyme essential oil Film preparation:
The Curdlan (CD)dissolved in water was weighed and stirred at 90 °C for 30 min to prepare CD solution (5 wt%); similarly, curdlan was dissolved in water, which contributes to the preparation of curdlan water suspension (5 wt%). Further, curdlan water suspension with the CD solution was gently mixed according to the mass ratio of the two is 5:0, 4:1, 3:2, 2:3, 1:4, 0:5. Glycerin (20 % of dry matter) and Tween 80 (20 % of dry matter) were added, with the mixed solutions adjusted to pH 4.0 using lactic acid. Then the final mixture was stirred evenly and homogenized at 8000 rpm for 30 s. Fifty milliliters of film-forming liquid was immediately poured into a polytetrafluoroethylene plate (18 cm × 18 cm) by solution extension method and allowed to heat at 100 ℃ for 2 min, and cool at room temperature. After drying at 25 ℃ and RH50 % for 24 h, the film was peeled off.
Utilizing thyme oil for the preservation of chilled meat.
Equal portions (65 g each) of chilled meat, specifically black pork tenderloin, were segmented and packaged with diverse blending films. These five sample groups, each employing distinct blending films, were then refrigerated at 4 °C for a duration of 20 days. Treatments used are, Control: Pure curdlan film; L.0: CD/PVA = 4/1; L.1: 1.0 % thyme essential oil composite film; L.2: 1.5 % thyme essential oil composite film. L.3: 2.0 % thyme essential oil composite film.
Analysis of antibacterial activity
Total plate count (TPC) and Escherichia coli enumeration were conducted for each treatment group following the standard procedures outlined in Chinese Standards GB4789.2-2016 and GB 4789.3-2016. In this process, 10 g of pork samples was blended with 90 mL of distilled water and subsequently subjected to serial dilution. One milliliter of the diluted sample was then spread on a colony counting plate to assess total plate counts (TPC), followed by incubation at 37 ± 1 ℃ for 24 h. For Escherichia coli enumeration, the samples were incubated on plates containing eosin methylene blue medium for 48 h, and bacterial colonies were counted under the same conditions, with results expressed as log10 colony forming units (cfu/g).
Results
At 4 ℃, the total bacterial count in the samples from each treatment group increased as the storage time extended. However, in comparison to the control group, the microbial growth in the four treatment groups (L.0, L.1, L.2, and L.3) was significantly suppressed. Initially, during the storage onset, the control group exhibited a rapid increase in both total colony count and Escherichia coli, while the composite film-treated group experienced inhibited microbial growth. After six days of storage, the total bacterial count in the control group, indicative of spoiled meat, exceeded 6.0, and Escherichia coli surpassed 4.0. As the concentration of thyme essential oil increased, the L.0 group showed signs of spoilage by the 8th day, with the total bacterial count exceeding the limit of 10^6 CFU/g on the 10th day, while L.1 deteriorated by the 10th day. The shelf life of L.2 and L.3 extended up to fourteen days. The blending film incorporating thyme essential oil exhibited stronger antibacterial effects, primarily attributed to the active components in thyme essential oil.
In this study, a novel blending film was created using curdlan and polyvinyl alcohol. The findings indicate that the optimal mechanical properties for the composite film are achieved when the curdlan to polyvinyl alcohol ratio is 4:1, resulting in a tensile strength of 11.81 MPa and an elongation at break of 189.31%. Additionally, the water vapor permeability (WVP) of the blending film was enhanced. Furthermore, the functional attributes of the curdlan/polyvinyl alcohol blending films were fine-tuned by incorporating thyme essential oil. It was confirmed that the presence of thyme essential oil significantly improved the elongation at break of the blending films, although the WVP remained unaffected. Notably, the blending film exhibited an enhanced antioxidant property when infused with thyme essential oil. Ultimately, the use of this new blending film extended the shelf life of chilled meat by 10 days. This research lays a theoretical foundation for the application of blending films in food preservation.
Zhang, Y., Zhou, L., Zhang, C., Show, P.L., Du, A., Fu, J. and Ashokkumar, V., 2020. Preparation and characterization of curdlan/polyvinyl alcohol/thyme essential oil blending film and its application to chilled meat preservation. Carbohydrate polymers, 247, p.116670.
