Natural biopolymer-based nanocomposite films for packaging applications

Concerns on environmental waste problems caused by non-biodegradable petrochemical-based plastic packaging materials as well as the consumer's demand for high quality food products has caused an increasing interest in developing biodegradable packaging materials using annually renewable natural biopolymers such as polysaccharides and proteins. Inherent shortcomings of natural polymer-based packaging materials such as low mechanical properties and low water resistance can be recovered by applying a nanocomposite technology. Polymer nanocomposites, especially natural biopolymer-layered silicate nanocomposites, exhibit markedly improved packaging properties due to their nanometer size dispersion. These improvements include increased modulus and strength, decreased gas permeability, and increased water resistance. Additionally, biologically active ingredients can be added to impart the desired functional properties to the resulting packaging materials. Consequently, natural biopolymer-based nanocomposite packaging materials with bio-functional properties have a huge potential for application in the active food packaging industry. In this review, recent advances in the preparation of natural biopolymer-based films and their nanocomposites, and their potential use in packaging applications are addressed.     

Food applications of active packaging EVOH films containing cyclodextrins for the preferential scavenging of undesirable compounds

Novel ethylene-vinyl alcohol copolymer (EVOH) films containing beta-cyclodextrins (βCD) with potential application in active food packaging have been tested as materials for the preferential retention of undesired food components. The films were immersed on pasteurized milk and UHT milk and stored at 4 and 23 °C, respectively. The films containing βCD presented a significant reduction in cholesterol concentration, achieving a 23% reduction in UHT milk exposed to EVOH films containing 30% βCD. Despite the immobilization of the βCD and the large molecular size of cholesterol, 15% of the βCD molecules added to the films were involved in the formation of βCD/cholesterol inclusion complexes. In another set of experiments, the films were used to reduce the presence of aldehydes (substances which develop as a result of oxidative processes) in packaged fried peanuts. The films containing βCD brought a significant reduction in hexanal, reaching a 50% decrease over short periods (1-5 weeks). At longer storage times (10 weeks) the retention capacity of the developed films was exhausted and no differences were observed between the samples.     

磁性聚乙烯醇微球固定化α-淀粉酶的研究

磁性聚乙烯醇微球为载体,采用戊二醛交联法固定化α-淀粉酶,并对固定化酶的理化性质等进行了研究。结果表明,磁性固定化α-淀粉酶的总活力、蛋白载量、比活、活性回收率分别为1107.89U/g微球、125.36mg/g微球、8.84U/mg蛋白质和37.96%;固定化α-淀粉酶的反应最适温度和最适pH分别为110℃和7.0;固定化α-淀粉酶对金属离子Mg2+、Fe2+、Zn2+和Cu2+的抑制作用的忍耐性比自由酶的明显提高;α-淀粉酶被固定化后其热稳定性、操作稳定性、pH稳定性均比自由酶的明显提高。固定化α-淀粉酶在4℃,pH7.0的缓冲液中保存30d,其活力仍保持最初活力的91.6%,这比其自由酶的高12.3%。     

溶菌酶在羊毛织物上的固定化

以羊毛织物为载体,研究了戊二醛交联固定溶菌酶的工艺条件。结果表明,戊二醛交联工艺的最佳条件为：戊二醛0.2%,交联时间2 h。溶菌酶固定化的条件为：溶菌酶5 g/L,固定化时间6 h,固定化pH值7.0,固定化温度4℃。在此工艺条件下制得的固定化溶菌酶的操作稳定性较好,使用6次后相对酶活仍保持在30%左右。     

Immobilization of humic acid in nanostructured layer-by-layer films for sensing applications

Humic acids (HAs), naturally occurring biomacromolecules, were incorporated into nanostructured polymeric films using the layer-by-layer (LbL) technique, in which HA layers were alternated with layers of poly(allylamine hydrochloride) (PAH). Atomic force microscopy (AFM) revealed very smooth films, with mean roughness varying from 0.89 to 1.19 nm for films containing 5 and 15 PAH/HA bilayers, respectively. The films displayed electroactivity, with the presence of only one reduction peak at ca. 0.675 V (vs Ag/AgCl). Such a well-defined electroactivity allowed the films to be used as highly sensitive pesticide sensors, with detection of pentachlorophenol (PCP) in solutions at concentrations as low as 10(-9) mol L(-1).     

Natamycin release from alginate/pectin films for food packaging applications

Single and composite films based on alginate and pectin containing natamycin as active agent were prepared and the release behavior in water and the diffusion coefficients were evaluated. The influence of natamycin on physical attributes of the films was also investigated. Addition of natamycin promoted an increase in soluble matter in water, in the water vapor permeability and in the opacity and a decreased in tensile strength when compared to films without the added anti-microbial agent. The natamycin mass released by immersion of the film in water fitted well to Fick’s second law diffusional model, with effective diffusivity values ranging from 3.2 × 10⁻⁹ (single pectin films) to 9.2 × 10⁻¹² cm²/s (single alginate films). The values of the diffusional exponents ranged between 0.5 and 1.0, suggesting that the transport process had non-Fickian (anomalous) characteristics. The single alginate films exhibit more suitable attributes for application in packaging than the single pectin and the composite films.     

Novel natural phenolic compound-based oxygen scavenging system for active packaging applications

An oxygen scavenging system containing a natural phenolic compound, pyrogallol with sodium carbonate, was developed and analyzed as a possible oxygen scavenger. The effect of several parameters, including the amount of pyrogallol and sodium carbonate, relative humidity and storage temperature, on the oxygen scavenging capability were investigated. The initial, glass vial headspace oxygen content (%) of 21.1 % (v/v) decreased to 0.26 % after 8 days of storage at room temperature when the oxygen scavenging system used a 1:1 (w/w) ratio of pyrogallol (250 mg) and sodium carbonate (250 mg). Both pyrogallol and sodium carbonate were required for optimum oxygen scavenging, otherwise the oxygen scavenging ability decreased. The oxygen content (%) decreased further to 6.55 % (v/v) when the amount of sodium carbonate decreased from 250 to 166 mg, which yielded a 2:1 ratio. In the present study, pyrogallol (250 mg) and sodium carbonate (250 mg) had highest oxygen scavenging capacity of 51.81 mL O₂/g and an oxygen scavenging rate of 6.48 mL O₂/g day. The oxygen absorption kinetics rate of pyrogallol and sodium carbonate confirmed that the material has good efficiency for use as an oxygen scavenger. Results indicated that the pyrogallol based oxygen scavenging system with moisture activation can be used as an effective oxygen scavenger for low water activity food packaging applications.     

Incorporation of partially purified hen egg white lysozyme into zein films for antimicrobial food packaging

Lysozyme, partially purified from hen egg white by precipitation of non-enzyme protein with ethanol and lyophilized after dialysis, was incorporated into zein films. The recovery and specific activity of the enzyme after partial purification varied between 45% and 72% and 2173 and 3448 U/mg, whereas the activity of the lyophilized enzyme varied between 2900 and 3351 U/mg. The partially purified enzyme was very stable and lost almost no activity in lyophilized form or in zein films stored at −18 and 4 °C for up to 8 and 4 months, respectively. During partial purification and in zein film preparation, ethanol treatment caused 123–137% and 132–315% activation of the enzyme, respectively. In zein films incorporated with 187–1318 U/cm² (63–455 μg/cm²) lysozyme, the release rates at 4 °C, changed between 7 and 29 U/cm²/min, increased at high lysozyme concentrations. Zein films incorporated with partially purified lysozyme showed antimicrobial effect on Bacillus subtilis and Lactobacillus plantarum. By the addition of disodium EDTA, the films also became effective on Escherichia coli. The results of this study showed that the partially purified lysozyme may be used in antimicrobial packaging to increase food safety.     

Barrier properties of corn zein nanocomposite coated polypropylene films for food packaging applications

The feasibility of corn zein nanocomposite (CZNC) coatings as an alternative to synthetic polymer barrier layer on polypropylene (PP) films was examined. The effect of layered silicate content in the CZNC layers on the barrier and surface hydrophobicity of the CZNC-PP films were investigated. Incorporation of organomodified montmorillonite (OMMT) by solution intercalation into zein matrix significantly improved oxygen and water vapor barrier of coated PP films. The barrier properties were also investigated theoretically by using various phenomenological permeability models. Tortuous permeation path formed by the fine delamination of nanoclays was found to be responsible for the barrier improvements in zein layers. In conclusion, durable CZNC-PP laminates were developed. CZNC coating of PP films has reduced the oxygen permeability nearly four times, while water vapor permeability reduced by 30% with 5 wt.% OMMT content in 5.9 μm corn zein coating.     

Mono- and multilayer active films containing lysozyme as antimicrobial agent

Active packaging materials able to release antimicrobial compounds into foodstuff can be used in order to avoid or slow down the bacterial growth during storage. In this work the use of two techniques to control the release of the chosen active compound (lysozyme) from a polymeric material into the foodstuff is proposed: a monolayer cross-linked PVOH film and a multilayer structure made of cross-linked PVOH layers are developed and studied. Lysozyme release tests into water were performed in order to compare the release kinetics from the investigated films. Results suggest that by means of both structures it is possible to control the rate at which lysozyme is released from the PVOH film. The antimicrobial activity of lysozyme released from the investigated films was tested against a suspension of Micrococcus lysodeikticus. Results show that the incorporation of lysozyme into PVOH does not lead to a loss of activity of the enzyme.

Industrial relevance

The increased use of gently processed foods requires packaging to be an integral part of the preservation concept. Consequently additional antimicrobial activity from the packaging material can aid in shelf life extension.This paper concentrates on the release rate of lysozyme, a naturally occuring antimicrobial agent (eg. salvia, mothers milk, raw milk), from multilayer films. A comperision of mono- and multilayer films containing lysozyme regarding their effectiveness on M. lysodeikticus as target organism was also performed. Both aims were met leading to a controlled release of lysozyme with no loss of activity.     

Nanocomposites for food packaging applications

Most materials currently used for food packaging are non-degradable, generating environmental problems. Several biopolymers have been exploited to develop materials for eco-friendly food packaging. However, the use of biopolymers has been limited because of their usually poor mechanical and barrier properties, which may be improved by adding reinforcing compounds (fillers), forming composites. Most reinforced materials present poor matrix–filler interactions, which tend to improve with decreasing filler dimensions. The use of fillers with at least one nanoscale dimension (nanoparticles) produces nanocomposites. Nanoparticles have proportionally larger surface area than their microscale counterparts, which favors the filler–matrix interactions and the performance of the resulting material. Besides nanoreinforcements, nanoparticles can have other functions when added to a polymer, such as antimicrobial activity, enzyme immobilization, biosensing, etc. The main kinds of nanoparticles which have been studied for use in food packaging systems are overviewed, as well as their effects and applications.     

Characterization and antimicrobial activity studies of polypropylene films with carvacrol and thymol for active packaging

Antimicrobial active films based on polypropylene (PP) were prepared by incorporating thymol and carvacrol at three different concentrations: 4, 6 and 8 wt.% of both additives as well as an equimolar mixture of them. A complete thermal, structural, mechanical and functional characterization of all formulations was carried out. SEM micrographs showed certain porosity for films with high additives concentrations. A decrease in elastic modulus was obtained for the active formulations compared with neat PP. The presence of additives did not affect the thermal stability of PP samples, but decreased PP crystallinity and oxygen barrier properties. The presence of thymol and carvacrol also increased stabilization against thermo-oxidative degradation, with higher oxidation induction parameters. Finally, thymol showed higher inhibition against bacterial strain present in food compared with carvacrol, leading to higher antimicrobial activity. The obtained results proved the permanence of certain amounts of the studied additives in the polymer matrix after processing making them able to be used as active additives in PP formulations.     

The selection of plastics films for food packaging

The packaging of perishable foodstuffs is a form of transfer engineering provided to collate contents and prolong their shelf-life. For many commercial purposes a life of a few weeks or months is adequate, and flexible wrappings meet this need. They impede oxygen and water vapour from the contained micro-climate and so retard the activity of microbes and of reactions which spoil flavour. The highest barrier properties are not always needed—they can sometimes be harmful by promoting anaerobic conditions; so informed selection is necessary to secure optimum value. Barrier properties can be produced over a range of about four decades. Maximum impedance is provided when steric hindrance in the polymer prevents the chain-folding and the spherulitic crystallisation characteristic of simple hydrocarbons, but this makes fabrication more difficult. Minimal barriers use atactic lyophilic polymers with high proportions of plasticiser, or hydrophilic polymers. Further extension of the range can be produced, if necessary, by perforation, or by combination of dissimilar polymers (by coating, co-extrusion or lamination).     

溶菌酶及其应用研究

本文介绍了溶菌酶的来源、结构、性质、作用机制、提取方法、酶活力的测定及其在食品工业、医学和酶工程中的应用;并对利用溶菌酶时存在的问题进行了分析总结,对溶菌酶的应用前景进行了展望。     

无菌纸盒包装应用新领域

以无菌纸盒包装食品和饮料并不是新的技术,随着人们膳食结构的变化,方便食用的产品渐趋普遍,而为了吸引更广泛的消费者,生产商便不断开发新式的产品与包装,可用于盛载微波炉食品如汤类的无菌纸盒包装,便是其中的一项成果,从而把纸盒包装的发展,推向更高更广的领域。