Agro-based waste-/co-products as feedstocks for polyhydroxyalkanoate biosynthesis

Authors: Ashby, Richard - Rick; Liu, Cheng Kung

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 5/25/2021
Publication Date: 10/28/2021
Citation: Ashby, R.D., Liu, C. 2021. Agro-based waste-/co-products as feedstocks for polyhydroxyalkanoate biosynthesis. In: Sarker, M., Liu, L.S., Yadav, M., Yosief, H., Hussain, S., editors. Conversion of Renewable Biomass and Bioproducts. Washington DC: ACS Press, ACS Symposium Series 1392. p.261-286.

Interpretive Summary: Polyhydroxyalkanoates (PHA) are renewable, degradable bioplastics that mimic the properties of many petroleum-based plastics. Unfortunately, their use has been hindered because PHA yields are currently not sufficiently large to overcome their high production costs. Many research efforts have been undertaken to improve this situation. These efforts have primarily focused on process engineering, genetic/metabolic engineering and on the utilization of low-value, high-volume renewable coproducts as fermentation feedstocks. In this chapter we have discussed some of the more well-known agro-based coproducts that have been utilized. While these coproducts are generated in large quantities and are comparatively cheaper than the purified/refined carbon sources typically used in PHA fermentations, the use of each of these materials is fraught with drawbacks that impede the desired goals. Whether it be the need for pretreatment processes, overcoming multi-substrate issues or the lack of the proper enzymes, overcoming these hurdles has necessitated the implementation of additional measures that impart increased cost to the overall production process. While utilizing these inexpensive waste-/co-products is a step in the right direction, it is highly probable that success in creating a successful economic process for PHA production through fermentation will require a combination of judicious feedstock selection, development and utilization of engineered bacterial strains and enhanced production processes.

Technical Abstract: The term ‘sustainability’ in the context of the environment implies a homeostatic relationship between abundant natural resources and limits to irreversible ecological damage from those resources. To that end, many agricultural waste- and co-products have been examined as feedstocks for the development of eco-friendly products. Unfortunately, bio-based production technologies are generally considered to be more cost prohibitive than many existing petro-based systems resulting in limited industrial-scale efforts. In fact, raw material costs for bio-based syntheses are reportedly as high as 50% or more of the total production costs. To improve the economics of bio-based synthetic processes, low-value, high-volume agro-based feedstocks have been exploited to produce sustainable high-value products (e.g., biofuels, biopolymers etc.). One class of biopolymers that have received much attention are the polyhydroxyalkanoates (PHA; bacterial polyesters) because of their property similarity to many common petro-based polymers (e.g., polypropylene and polyethylene) and their propensity to be completely biodegradable in microbially-active environments. In an effort to decrease their production costs, many agro-based feedstocks (or their waste-/co-products) have been tested as cheaper alternatives for the fermentative synthesis of PHAs. These feedstocks include waste fats and oils, crude glycerol, whey, molasses and lignocellulosics among others. This chapter focuses on the utilization of these feedstocks for PHA biosynthesis, as well as identifying certain bacterial strains capable of PHA synthesis and the types of PHA produced from these abundant agro-based materials.