Pedro Guerrero and Koro de la Caba
3.5 APPLICATIONS OF PROTEIN-BASED MATERIALS
3.5.2 b iodegradable F ilms
The use of proteins for film manufacturing fits well within environmental sustain- ability strategy� First, the large amount of waste produced by the food industry can be converted into great valuable materials to be reused into food packaging systems, eliminating waste management problems from the economic and environmental point of view (Mirabella et al�, 2014)� Second, the development of protein films able to extend the food shelf life can also reduce the environmental impact due to the reduction of food losses (Barlow and Morgan, 2013; Williams and Wikström, 2011)� Therefore, efforts should be carried out not only to reduce environmental impact from the packaging film itself, but also to enhance those functional properties of protein films that contribute to reduce food losses (Wikström et al�, 2014)� Finally, since governments have implemented legislation to reduce the amount of municipal waste sent to landfill, biodegradability/compostability is also one of the main focuses for choosing proteins as packaging materials (Shi et al�, 2010)�
Biodegradation provides the opportunity to degrade materials after useful life and enables to close the ideal life cycle, from cradle to cradle (Figure 3�9)� Under appropriate conditions of moisture, temperature, and oxygen availability, biodeg- radation leads to disintegration of the film with no toxic or harmful residue (Rhim et al�, 2013)� The biodegradability of the film during composting occurs in three phases: mesophilic, thermophilic, and maturation phases (Verbeek et al�, 2012)� The mesophilic phase is an exothermic process in which easily oxidizable compounds are broken down by hydrolase enzymes and temperature rises, leading to the growth of thermophilic bacteria� During the thermophilic phase, proteins are biodegraded into amino acids and ammonia; in this phase, thermophilic bacteria and fungi take over biodegradation process, characterized by oxidation� The change from meso- philic to thermophilic phase must be accompanied with a pH change from acidic to alkaline to maintain high degradation rates in large-scale composting facilities
Industrial processing By-product valorization Cradle to cradle Food packaging application Biodegradation
(Sundberg et al�, 2004)� After the active composting phases, compost requires a cur- ing period to develop a valuable soil for plant growth (Kale et al�, 2007)�
To measure the environmental impact of products from the extraction of raw materials to ultimate disposal, life cycle assessment (LCA) is a practical key tool (Bier et al�, 2011; Roy et al�, 2009; Siracusa et al�, 2014)� The principles and frame- work of LCA are described in ISO 14040 (ISO, 2006); however, some limitations are related to data availability since industrial production and commercialization of biodegradable films are currently very competitive (Alvárez-Chávez et al�, 2012; Iles and Martin, 2013)� Furthermore, when dealing with novel products, data can be derived from experiments at laboratory scale, which could change when scaling to industrial production (Hospido et al�, 2010)�
Nevertheless, LCA can be used to identify the most pollutant phases of the life cycle for bio-based films, as a first step prior to the analysis of the changes needed during the design of products and processes to minimize negative environmental impacts� Although the environmental assessment of protein-based biodegradable films is largely unreported, some works have recently been published related to plant proteins� Deng et al� (2013) carried out LCA on wheat gluten materials� The LCA results exhibited that wheat gluten films produced by extrusion and inciner- ated to recover energy were favorable from an environmental perspective� Benefits in climate change and fossil depletion over LDPE films were measured� Although wheat gluten films suffer from common problems for bio-based materials, such as land occupation, the overall environmental performance indicated that wheat gluten can provide a promising source for bio-based polymer production� With regard to soy protein films, Leceta et al� (2014) showed that the extraction of raw materials was the stage with the highest environmental burden� Additionally, soybeans cultiva- tion contributed to the environmental burden in land use category due to the use of glycerol, by-product from biodiesel production, as plasticizer� However, the end of life stage was the least pollutant phase for soy protein-based films due to their bio- degradable nature, which allows composting as the end of life scenario and provides environmental benefits (Rudnik, 2008)�
In spite of the fact that protein-based films seem to be more environmentally friendly materials when their origin and biodegradability are considered, the sus- tainability of biodegradable films must include all the stages of their life cycle, from cradle to cradle, as well as social and economic issues to assist in decision making (Philp et al�, 2013)� Therefore, recent developments were initiated to broaden LCA to life cycle sustainability analysis, an interdisciplinary framework for addressing knowledge from diverse scientific disciplines in such a way that the whole cause and effect can be assessed (Guinée et al�, 2011)�
3.6 CONCLUSIONS
Research and development of protein-based films and coatings in the food packaging field have been intensified, but their low presence in the market evidences that fur- ther studies must be carried out� The benefits of proteins can be numerous, but some drawbacks for potential applications need to be overcome� Protein-based films and coatings possess good barrier properties to oxygen, carbon dioxide, and lipids, but
not to water vapor� Therefore, the challenge for successful commercial implementa- tion of protein-based films and coatings is to improve and stabilize their functional properties during storage and use� In this context, extensive research is still needed on protein modification and processing methods�
Although protein-based materials are mostly processed in solution, this method is slow, requires large volumes of solvents, and thus, may be considered commercially less interesting than the dry process� Furthermore, protein-based materials devel- oped by thermomechanical molding have better properties and are economically more efficient since they are less time consuming� With regard to the modification of proteins, this is still a long way from being a common method in food packaging, but it is increasingly being recognized as essential for two main reasons� Firstly, proteins fulfill multipurpose functions in materials science and these functions can be served better by modified than by native proteins� Secondly, environmental concerns pro- mote the utilization of renewable and biodegradable materials� Furthermore, the use of by-products and waste streams from food processing industry for food packaging materials can positively impact the economics of food processes�
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