From processing, manufacturing, transportation, storage to consumer purchase, food packaging plays a crucial role in ensuring food safety and maintaining quality. For a long time, petroleum-based plastic materials such as polystyrene, polyethylene terephthalate (PET), and polypropylene have been widely used as food packaging materials to protect food from external environmental damage and pollution. At the same time, it also brings convenience to food transportation and storage. However, plastic materials cannot completely protect food from environmental impact, so they cannot fully maintain product quality and safety. In addition, microplastics and nano-plastic materials produced by non-biodegradable plastics have caused environmental pollution, leading to serious ecological problems. Therefore, it is of great significance to develop biodegradable and low-toxic new biological materials as packaging materials.
In recent years, active food packaging films have attracted extensive attention from researchers and the food industry because they can avoid deterioration caused by microorganisms, dust, gas, light, moisture, etc. in the external environment. Some special packaging materials can be designed to release fungicides to inhibit microbial spoilage and extend shelf life.
As a biodegradable material, polyvinyl alcohol (PVA) has been widely studied in clinical applications, film production, food packaging, etc. It has the characteristics of non-toxicity, biodegradability, biocompatibility, hydrophilicity, good film formation, good transparency, good mechanical strength, and strong oxygen barrier ability. However, the high light transmittance, hydrophilicity, and water solubility of PVA films cannot meet the key criteria for food packaging, which limits its application. In order to expand the application range of polyvinyl alcohol, the preparation of functional polyvinyl alcohol films has become an important research topic. It is well known that the introduction of nanomaterials into PVA films is a good choice for the production of packaging materials. PVA structures contain hydrogen bond groups, which are suitable for including other active nanomaterials to form new hydrogen bonds, thereby improving its functional properties.
Carbon dots (CDs) are nanoparticles with a diameter of less than 10 nanometers. It has the characteristics of photoluminescence, good biocompatibility, stable water dispersion, outstanding reductivity, high fluorescence, excellent antibacterial ability, etc. Since CDs are non-toxic and environmentally friendly, it is mainly used for the detection/determination of antibiotics, heavy metal ions, pathogens, additives, pesticide residues, functional components, and nutrients. Some studies have confirmed that CDs can be used as an antimicrobial agent to extend the shelf life of food. It is worth noting that CDs, as a novel nanomaterial with a large number of oxygen-containing functional groups, have the potential to enhance the functional properties of polyvinyl alcohol hydrogels. In addition, the introduction of CDs makes PVA hydrogels have some characteristics such as fluorescence, antioxidant, and antibacterial properties.
Although there are some reports on CDs-enhanced PVA membranes, most of them focus on the enhancement of structural, optical and waterproof properties. The novelty of this study is to use CDs-enhanced PVA films as active packaging materials to prolong the storage period of fruits and fried foods. In this study, active CDs/PVA composite membranes containing different concentrations of CDs (0.00, 0.25, 0.50, 0.75 and 1.00%) were successfully prepared, and the morphology, mechanical properties, optical properties, water resistance, thermal properties, UV protection properties, antioxidant activities and antibacterial activities of the composite membranes were comprehensively determined. Based on the experimental results, the mechanism of adding CDs to enhance the film properties of PVA was proposed. Furthermore, the packaging performance of CDs-enhanced PVA films was tested by storing bananas, dates and fried meatballs.
In this study, multifunctional carbon dots (CDs) were prepared from banana pulp by an easy hydrothermal method. The diameter of CDs was 2.01~ 5.61 nm and the dispersion was good. The addition of CDs to PVA matrix produced an active membrane with improved thermal stability, mechanical properties and water resistance. The active membrane had effective UV blocking properties, antioxidant and antibacterial properties. The above properties of CDs/PVA active membrane were concentration-dependent. With the increase of CD concentration, the functional properties of the active membrane were improved. It was found that 0.50% CDs/PVA membrane had a scavenging rate of 97.08% for ABTS free radicals, while 1% CDs/PVA membrane had the largest scavenging rate for DPPH free radicals, which was 72.81%. The inhibition ring diameters of 0.50% CDs/PVA solution against Staphylococcus aureus (S. aureus), B. subtilis (B. subtilis) and E. coli (E. coli) were 9.52, 8.21 and 9.05 mm, respectively, and did not increase significantly with the increase of CDs concentration. Using 0.50% CDs/PVA membrane as active packaging, the shelf life of bananas, red dates and fried meatballs was significantly extended. These results suggest that CDs/PVA composite membrane is a promising active food packaging material.
Zhang Min, a second-level professor at Jiangnan University and a distinguished professor at Zhishan, is the director of the International Joint Laboratory of "Intelligent Processing and Preservation of Fresh Food" in universities in Jiangsu Province. He has been selected as a national talent and enjoys special allowances from the State Council. He has been engaged in high-tech research in fresh food processing for a long time, and has made pioneering research in the field of new technologies for efficient food processing in the past ten years, training nearly 200 graduate students/postdocs. Since 2007, he has served as the editorial board member/deputy editor of SCI publications such as Journal of Food Engineering; Drying Technology; International Agrophysics; Foods. Since August 2013, he has been awarded an honorary professor in the field of food science by the University of Queensland, Australia. In the past five years, he has presided over more than 20 national, provincial and ministerial-level and industry-university-research key projects; he has been authorized more than 230 invention patents at home and abroad, and won the Jiangsu Province "Ten Outstanding Patent Inventor Award" (2012) and Jiangsu Province Patent Inventor Award (2020); published more than 590 SCI papers as corresponding authors, with a total of more than 13,000 citations, an H-index of 70 (Web of Science), and was selected for the ELSIVER "China Highly Cited Scholars" list for 6 times. His research results have won the second prize of the National Science and Technology Progress Award, the first prize of the Jiangsu Province Science and Technology Progress Award, and the special prize of the China Federation of Commerce.
