і і 2

Vasiliki Kachrimanidou, Nikolaos Kopsahelis, Colin Webb ,

Apostolis A. Koutinas1’*

^Department of Food Science and Human Nutrition, Agricultural University of Athens, Athens, Greece, 2School of Chemical Engineering and Analytical Science, University of Manchester, Manchester, England,

United Kingdom

*Corresponding author email: akoutinas@aua. gr

OUTLINE

Introduction 419

PHA Structure and Properties 420

PHA Production Integrated in Biorefinery Concepts 421

Valorization of Biodiesel Industry By-Products 424

Valorization of Second-Generation Bioethanol Industry By-Products 427

Valorization of By-Product Streams from Food Industries 427

PHA Production from Winery By-Products 428

PHB Production from Confectionery and Bakery Industry Waste Streams 429

PHB Production from Whey 430

Conclusions and Future Perspectives 430

References 430

INTRODUCTION

The imminent depletion of fossilized raw materials and increasing environmental concerns has paved the way toward the development of a sustainable bio-based economy. Biorefinery concepts constitute a significant aspect of the future bioeconomy era where renewable raw materials, such as widely available ligno — cellulosic biomass in conjunction with industrial by-products and waste streams, will be utilized for the production of value-added commercial products, including biofuels, chemicals, biodegradable polymers and antioxidants among others. However, the establish­ment of a new industrial sector is a difficult task not only
because of the viability of new technological advances but also because of the transition from the nonrenewable to the sustainable era should occur smoothly in order to avoid job losses and economic turmoil. A smooth transi­tion can be achieved through the integration of sustain­able processing schemes in those conventional industrial plants that generate waste and by-product streams suitable for bioconversion or green chemical conversion into value-added products.

Petroleum-derived plastics constitute an everyday commodity used mainly for packaging and disposable materials. According to the U. S. Environmental Protec­tion Agency, 31 million t of waste was generated in 2010, while only 8% of the total plastic waste was

Bioenergy Research: Advances and Applications http://dx. doi. org/10.1016/B978-0-444-59561-4.00024-3

(Bhubalana et al., 2010; Cavalheiro et al., 2012). PHAs can be categorized according to the number of carbon atoms into short-chain-length PHAs (monomers containing three to five carbon atoms) and medium-chain-length PHAs (monomers containing more than six carbon atoms) (Asbhy et al., 2011; Du et al., 2012). A historical overview of PHA research and industrial applications is presented in previ­ous publications (Solaiman et al., 2006; Verlinden et al., 2007; Du et al., 2012).

As previously mentioned, one of the most important advantages of PHAs is their biodegradability and biocompatibility. Under aerobic conditions, PHAs are degraded to carbon dioxide and water, while under anaerobic conditions, methane and water are the final products. Hence, these compounds can be utilized from various microorganisms living in soil and water as carbon source for their growth, without toxic effects to the environment.

PHB was the first member of the PHA family that was identified after isolation from Bacillus megaterium (Lemoigne, 1926). It can be produced by many bacterial strains (especially various strains of C. necator) in high concentrations (more than 150 g/l) and intracellular content (more than 80% on a dry weight basis) from commercial carbon sources (mainly glucose as well as fructose and sucrose) and starch hydrolysates (Ryu et al., 1997; Yu et al., 2003). In addition, the physical properties of PHB are similar to polypropylene. How­ever, the brittle and thermally unstable nature of PHB limits its commercial applications and constitutes one of the major reasons that have prevented its production in large-scale operations. The high crystallinity of PHB (55—80%), associated with the formation of large spher — ulites, is the main reason that causes the brittle nature of PHB. It should be stressed though that the applica­tion of appropriate processing methodologies could reduce the undesirable mechanical properties of PHB, which could be used for the production of ductile films (Barham et al., 1992). Furthermore, the molecular weight of the PHB homopolymer produced by many bacterial strains, under varying fermentation condition and utilization of different feedstocks may also result in a biopolymer with improved characteristics (Kusaka et al., 1999).

P(3HB-co-3HV) was the first copolymer of the PHA family that was identified and subsequently produced on industrial scale by Imperial Chemical Industries using a R. eutropha strain. The incorporation of 3HV units in different proportions in the copolymer by this R. eutropha strain was only possible after the addition of propionic acid as a carbon source precur­sor that induced the metabolic synthesis of 3HV units. The production of P(3HB-co-3HV) also demonstrated that it is feasible to alter the properties of PHAs by controlling fermentation conditions. For instance, the addition of increasing propionic acid concentra­tions during PHA accumulation results in increasing proportions of 3HV units (expressed as mol%) in the P(3HB-co-3HV) copolymer. In this way, it was demonstrated that the incorporation of 3HV units in the P(3HB-co-3HV) copolymer results in improved mechanical properties (Byrom, 1987; Choi and Lee,

1997) .

Since the identification and commercial production of P(3HB-co-3HV) copolymers, research has focused on the identification or modification of microbial strains capable of producing PHA copolymers without addition of carbon source precursors or the production of different PHA copolymers, with addition of carbon source precursors, containing two, three or four mono­mers that demonstrate desired properties (Madden et al., 2000; Loo et al., 2005; Koller et al., 2007a). For instance, the archeon Haloferax mediterranei accumu­lates 72.8% (w/w) of P-(3HB-co-3HV) that contains 6 mol% 3HV units directly from whey sugars, while it produces the terpolymer P(3HB-co-3HV-co-4HB) when it is supplemented with 3HV and 4HB precursors (Koller et al., 2007a). Table 24.1 presents the specific properties of various PHAs compared with major petroleum-derived plastics. Nowadays, it is widely accepted that PHA physical properties can vary from brittle PHB homopolymers with high crystallinity to flexible PHA copolymers with lower crystallinity, such as P(3HB-co-3HV) and P(3HB-co-3HHx), to elastic PHA copolymers, such as P(3HB-co-4HB) and P(3- hydroxyoctanoate-co-3-hydroxydecanoate) (Wolf et al., 2005; Whitehouse et al., 2006). In the last 30 years, PHAs have been identified as potential biopolymers for a wide spectrum of end-uses including food pack­aging, flushable hygiene products, tissue engineering applications, adhesives, agriculture and biocomposites (Wolf et al., 2005).