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Content archived on 2024-05-18

Proactive biobased cheese packaging

Exploitable results

A biobased, low molar mass lactic acid-based compound may be used as a softening additive, thereby producing materials with increased flexibility. The addition of the flexibilizer may be performed in combination with the film extrusion process and an additional processing step is thereby avoided.
Polylactide/nanoclay films were produced primarily because of the reduced permeability to oxygen and water vapour that could be expected in fully exfoliated films. Nanoclay films were produced (a) by compounding with nanoclay and (b) by coating the material with multi-layers of nanoclay. Nanoclays compounded into PLA were obtained from Southern Clay Products (Texas, USA) and consisted of and consisted of Cloisites 10A, 15A, 20A, 25A and 30B. For nanoclay coating, alternating, multiple layers of organic acrylic polymers and inorganic clay was used. Nanoclays were combined in dry form with various polylactides, both "flexibilized" and "un-flexibilized" and extruded into a film using a pilot plant-scale twin-screw extruder. Analysis confirmed that nanoclay addition enhanced the thermal stability of PLA. Nanoclay was compounded with acceptable appearance. Compounding nanoclay with PLA resulted in a reduction of OTR to 70-99cm3/m2/24h/bar and of WVTR to 34-57g/m2/24h. These results were obtained on "un-flexibilized" PLA films extruded on a laboratory extruder and confirm that nanoclay compounding can reduce the permeability. However, the desired target values for cheese are not within easy reach. It also seems likely that use of nanoclays to reach the target permeability values would be even more difficult when combined with "flexibilized" PLA. These results are consistent with the observations of other researchers, in that extrusion of fully exfoliated PLA nanocomposites, and hence achievement of the full potential for permeability reduction in PLA films is critically dependent on a combination of factors including processing conditions, extruder characteristics, and selection of the most appropriate type of nanoclay. Initial testing of nanoclay coating of "flexibilized" PLA showed a significant reduction in OTR from 154cm3/m2/24h/bar for uncoated " flexibilized " PLA-1 to 14-19cm3/m2/24h/bar and 15-19cm3/m2/24h/bar for " flexibilized" PLA coated with 40 and 60 layers, respectively. The number of layers coated on PLA did not affect OTR. Nanoclay coating did not have any pronounced effect on WVTR, which was 74-75g/m2/24h and 83-99g/m2/24h for " flexibilized" PLA coated with 40 and 60 layers, respectively. In general, the adhesion of the nanoclay coating to the PLA surface was insufficient indicated by small spots of visual delaminating. However, adherence aspect may be solved during further optimisation. Microbiological tests on polylactide films produced on a pilot plant-scale extruder indicated that nanoclays incorporated into the polylactide films might positively influence properties by facilitating release of a cyclodextrin-encapsulated antimicrobial (e.g., AITC) within the films. Although this finding is complicated by the fact that the films used were likely not true nanocomposites, it deserves further attention in terms of testing with fully exfoliated nanocomposite films. If confirmed, it could have significance for food packaging and other applications.
The following materials have been evaluated in the course of the project: - Polylactide (PLA); - Flexibilized PLA; - PLA-polycaprolactone copolymer; - Alpha-cyclodextrin (CD); - Beta-CD; - Gamma-CD; - Acetylated alpha-CD; - Acetylated beta-CD; - Partially acetylated beta-CD; - Random methylated-beta-CD; - (2-Hydroxy)propyl beta-CD; - Imazalil/beta-CD; - Allyl isothiocyanate/alpha-CD; - Initial chitosan PR/d; - Chitosan lactate. The following methods have been applied in the course of the project: - Compostability testing program according to EN 13432 including material characteristics (heavy metals); - Biodegradation under controlled composting conditions; - Disintegration; - Effect on compost quality and ecotoxicity test (summer barley plant growth test and cress test); - Biodegradation in soil and water at 20 degrees Celsius (examined for several materials); - Anaerobic biodegradation of PLA at soil conditions (58 degrees Celsius). All methods resulted in useful information. As a general conclusion, it is noted that the developed packaging has the potential of fulfilling all requirements on compostability as required by EN 13432. The basic PLA material of the packaging has already proven to fulfil all these requirements. The influence of other parts, such as a plasma-treated layer, nanoclays or chitosan, on compostability should still be further examined. It is expected that a PLA packaging covered with a plasma-coated layer would probably also fulfil all requirements of EN 13432. However, further testing is required. Biodegradation is no issue for the plasma-treated layer, as it is an inorganic material, and the plasma coating will probably cause no negative ecotoxic effects. Furthermore, the coating is only applied in very thin layers (e.g. 20nm) and thus, will have only a limited effect on disintegration. Moreover, no insurmountable problems are expected when including nanoclays, chitosan lactate, and/or antimicrobial agents/CD complexes in the PLA packaging.
In order to pursue plasma coating on PLA films, contacts were established to leading research groups in Europe. Different coating methods were applied: Method 1: The trials revealed a problem when using specific siloxanes as precursors for deposition of SiOx. In particular, the requirement for a certain density of activated oxygen in the plasma in order to fully oxidize the siloxanes led to problems with film degradation. Since the plasma is in direct contact with the surface that is being coated, the activated oxygen species in the plasma can damage the film. Past experience using lower plasma input power had shown that films deposited from similar precursors under these conditions had not been effective barriers as they retained significant organic components. Therefore, it was concluded that this option was not likely to provide a solution. Plasma deposition of SiOx on polylactide films might still be possible, either by a series of experiments in which input power is optimised in a trade-off between barrier deposition and film damage, or alternatively through use of remote-source plasma technology in which the sample to be coated is not directly exposed to the plasma source. However, the latter option required equipment not available to the project. Method 2: Attempts were made to deposit SiOx coatings using plasma containing a mixture of hexamethyldisiloxane (HMDSO) and oxygen (O2) on flexible PLA films. Surface analysis subsequently showed that polylactide film surface modification had occurred. In particular, X-ray photoelectron spectroscopy (XPS) revealed that the carbon percentage on the film surface could be reduced from 60% in uncoated polylactide to 11% in the case of plasma-treated films. This finding indicates that a pure SiOx is not formed, however, some remaining organic component exists in the surface coating (i.e. chemically the film may be described as RSiOx). This is consistent with the suggestions from other researchers. Atomic force microscopy (AFM) also proved a surface structure similar to that expected on HMDSO plasma-treated films. Despite these promising results, permeability values for both oxygen and water vapor remained well above the target levels required. A possible explanation of this might be the film surface roughness, which could present problems in terms of placing a uniform SiOx coating of approx. 20nm thickness. Thus, it has become clear that a considerable amount of further process optimisation is needed, which should be pursued in a new project. Danish Polymer Center, Risø hopes to continue the collaboration with Aachen (method 2) under another project with a view to optimising the technology.
The Biopack partners have produced two public reports on: - Economic issues related to the developed materials (titled "Economic Evaluation of the Biopack Materials"). The results of the project and a description of future research and development in the area of biobased food packaging will appear in documentation (titled "The Biopack Project in a Nutshell - The Past, Present, and Future"). The reports are aimed at the food industry, packaging industry, and the universities. The reports also function as inspiration for decision takers within these areas. The reports will be available from http://www.biopack.org and from the coordinator.
Molecularly encapsulated preservatives were developed for further incorporation into PLA-films. The most promising antimicrobial, allyl isothiocyanate (AITC), was encapsulated into CDs. Complexation technology was tailored for volatile and non-volatile preservatives and elaborated on the gram scale. The technology was successfully scaled-up to kg/batch size. The CDs were thoroughly characterized. The complexation technology has been successfully modified for micronized complexes, and specifications for the components have been established. Incorporation of approx. 10% AITC/alpha-CD was sufficient to prevent mould growth on cheese during prolonged storage. The CD-assisted preserving PLA films can be regarded as water-activated, controlled-release biodegradable food packaging materials. The release of the molecularly encapsulated (CD-entrapped) AITC preservative into the vapour microenvironment is governed primarily by the actual humidity of the environment and temperature. This kind of "smart" packaging film for cheese products has not before been made or used. The technology will cause an insignificant increase of the price of PLA films, as the applied cyclodextrin is not expensive. From an environmental standpoint it is a special advantage of the packaging material, that both the preservative (AITC) and the cyclodextrins are of plant origin - they both entirely decompose in nature. The end users of such kind of controlled-release preserving packaging material will be food-packaging companies. The actual price availability and approval status of the selected preservative and cyclodextrin make this Biopack films a feasible, inexpensive environmentally safe, novel food packaging material, especially suitable for such products, that are to be transported long distances and under high humidity/high temperature conditions.
The following methods were implemented for PLA-based materials: A. Mechanical properties: - Tensile strength; - Elongation at break; - Young's modulus at standard and realistic food packaging conditions. B. Thermal properties: - Thermogravimetric analysis (TGA); - Differential scanning calorimetry (DSC). C. Stability of polymers: - Moisture sorption at different RH and temperatures; - Molecular weight changes at different RH and temperatures. D. Barrier properties: - Water vapour transmission rate; - Oxygen transmission rate; - Carbon dioxide transmission rate. E. Optical properties: - Light transmission. F. Bioactivity tests: - Release rate of antimicrobial compound from cyclodextrin; - Assays for evaluation of antimicrobial activity with respect to direct and indirect contact with the food product; - Microbial growth on the PLA-based materials. G. Migrational properties: - Total migration; - Specific migration of allyl isothiocyanate and lactic acid. H. Compostability: - Biodegradation; - Disintegration; - Compost quality – Ecotoxicity; - Evaluation of heavy metals. I. Compatibility of PLA and foods: - Evaluation of oxidative quality changes by determination of lipid oxidation, loss of nutrients, colour changes, etc.
Two very interesting features of chitosan in relation to food packaging are its very high gas barrier and antimicrobial properties. The following chitosan forms were evaluated: Initial chitosan, microcystalline chitosan (MCCh), chitosan salts (lactate, acetate, glutamate, maleate), and modified chitosan salts (lactate, acetate, glutamate). Chitosan coating blends made of modified chitosan lactate and modified MCCh. These forms were prepared according to procedures elaborated at IWCh. Selected chitosan forms were used for: - Coating of PLA film surface; - Compounding with PLA granulate. In the process of coating PLA film surface, the chitosan was applied as a one-sided coating, double-sided coating, and by inserting chitosan between two layers of PLA film and pressing the layers together. Chitosan was successfully coated on PLA-based materials. All chitosan forms displayed a distinct effect on the oxygen permeability of coated PLA with a resultant OTR below 4cm3/m2/24h at 23 degrees Celsius, 0% RH (100mm thick materials). The OTR was lowered considerably in comparison to uncoated, flexible PLA materials. Earlier studies on mechanical properties of modified PLA/chitosan films showed poor adhesion of chitosan to the PLA film surface and heterogeneity of the coating, but the moisture sensitivity of the latest modified MCCh coating on PLA showed a significant improvement compared to these earlier chitosan types tested, as no disintegration was seen even after the chitosan coating was conditioned at 38 degrees Celsius and 100% RH. Chitosan seemed not to influence the WVTR, which was 101g/m2/24h for the chitosan-coated PLA films. The selected chitosan types exhibited antimicrobial activity. It was found that both chitosan salts and modified chitosan salts demonstrated high bacteriostatic and bactericidal activity against E. coli. In the case of MCCh only bacteriostatic action was observed. All chitosan forms exhibited biological activity within the investigated incubation times and temperatures. Among the tested microorganisms, the highest relative inhibition (80-100%) was found for Penicillium roqueforti and Penicillium nalgiovense, and for Kluyveromyces marxianus and Debaromyces hansenii. These fungi and yeast are associated with the cheese microflora. Evaluation of chitosan-coated PLA films proved that the antimicrobial effect against the tested yeast and mold strains, i.e. P. nalgiovense, P. roqueforti, P. commune, P. caseifulvum, and K. marxianus was regained during processing of the films. Chitosan lactate is biodegradable. Further evaluation of the other modified chitosan forms is required. Finally, compounding of PLA granulate with chitosan powder was evaluated. It was noted that only chitosan preparates characterized by good thermo-stability could be applied for preparation of PLA/chitosan composites with suitable properties. The investigations on thermal properties proved very good thermo-stability for initial chitosan but some chitosan forms displayed a little worse thermo-stability. In most cases the degradation occur in temp. 200-299 degrees Celsius so these values are acceptable for manufactures of PLA/chitosan composites (temp. not higher than 180 degrees Celsius are used). The mechanical and barrier properties of chitosan compounded PLA films were not sufficient to meet the requirements with respect to cheese packaging, and the compounding process should be further optimised. The PLA/chitosan materials are recommended for products requiring medium and high oxygen barriers, and where the water vapour transmission rate is not critical.
The aim of the life cycle analysis (LCA) was to quantify and evaluate the total environmental load of the life cycle (production-use-disposal) of the Biopack cheese packaging and comparing it with a traditional cheese packaging. The chosen functional unit is one packaging for sliced, semi-hard cheeses. The life cycle inventory consisted of the following steps: - Conversion of whey to lactic acid; - Conversion of lactic acid to PLA; - Waste options for PLA. The following recommendations can result in a lower environmental load (of the life cycle of the packaging): - Using gas as energy source for the production phase; - Building the production unit of lactic acid close to the whey production site, i.e. the cheese plant; - Promoting (thermophilic) anaerobic digestion as the preferable waste management option for the PLA packaging. The LCA may provide input to databases used for life cycle inventories.
Material Selection and Developments: Several different polylactides (PLA) have been produced in the course of the project: - Linear poly (L-lactide); - Slightly branched polymer with long-chain branching obtained by applying melt-modification during a secondary processing step; - Copolymer of L-lactide and epsilon-caprolactone; - Four-arm branched homopolymer based on L-lactide. The materials have been thoroughly characterised (by GPC, NMR, DSC, TGA) and tested with regards to processing, mechanical and barrier properties. One problem with the manufactured polymers was the high content of residual monomers, which was found to result in a higher water uptake and accordingly in a poor stability of the processed samples. This problem was solved later on in the project by optimisation of the manufacturing process. The lower content of the residual lactide caused a more brittle material, which had to be solved by introducing a softening agent, which was based on a lactic acid copolyester resin. Materials with good mechanical properties could be produced when applying the softening agent. Transparent materials could be produced from the PLA resin. The material produced and tested showed acceptable properties except for the barrier properties, which was found to be too poor for cheese packaging materials. Especially the water vapour transmission was too high. A solution to the poor barrier properties was sought by means of the following approaches: - Plasma treatment of the PLA film; - Coating/compounding with chitosan; - Coating/compounding with nanoclays (preparation of nanocomposites). The PLA materials were mainly based on annually renewable, non-gmo resources. Migration levels of the PLA materials complied with current EU legislation and the PLA materials were in compliance with the EN13432 on compostability. B. Processing and Processability: The polylactides are generally considered to be processable by using standard processing equipment and film extrusion, injection moulding and thermoforming was possible to perform on all the materials produced in the project. However, the inclusion of cyclodextrin proved to be difficult. The results indicate that there is a potential for polylactide based materials for use as cheese packaging materials, but the barrier properties need to be further improved or storage time reduced. The PLA materials have good sealant qualities and may be printed on using standard equipment. C. Application sectors: The polylactide materials proved suitable for short-term storage of chilled food and storage of foods with low moisture at room temperature. The use temperature is -30 degrees Celsius to 70 degrees Celsius depending on moisture content.

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