# Cocoa Bean Shell as Promising Feedstock for the Production of Poly(3-hydroxybutyrate) (PHB)
## 1 Introduction  
The growing environmental issues derived from organic waste generated during food processing make the development of new alternatives essential to transform these residues into value-added products [1]. In this sense, the EU Waste Framework Directive (European Parliament and Council, 2008, Directive 2008/98/EC) prioritizes residue prevention followed by recycling and valorisation and considers waste disposal as the most unfavourable option [2]. Additionally, the Circular Economy Action Plan adopted in 2020 by the European Commission constitutes one of the main building blocks of the European Green Deal, Europe's agenda for sustainable growth. Therefore, in the current context, industries are in need of the development of green technologies that allow the targets included in European policies to be achieved to move a sustainable society forward [2; 3; 4].  
Almost 20 tons of waste are generated for each ton of dry cocoa beans produced; due to the rising demand of cocoa products in the last 10 years, nowadays, the cocoa industry has to address increasing environmental concerns [5]. Cocoa bean shell (CBS), the outer part that cover the cocoa bean, is a by-product, which is usually discarded as waste, generated during the cocoa roasting process. This residue means between 10-20% of the total cocoa bean weight, although these percentages can vary depending on the type of cocoa beans  
mboxciteB6-applsci-2132561,B7-applsci-2132561,B8-applsci-2132561. Although the composition of cocoa bean shell is also quite variable and depends on the origin and the processing of cocoa beans, among other factors, CBS is constituted by carbohydrates (including fructose, glucose, melibiose, sucrose, and raffinos), dietary fibre, theobromine, phenolic compounds, fats, and other components [1; 9].  
Due to environmental concerns related to plastic pollution and fossil resources depletion, the interest in biodegradable polymers as a novel and sustainable alternative is rapidly increasing [10; 11]. Polyhydroxyalkanoates (PHAs), a class of biodegradable and biobasedpolymers, are linear polyesters produced in nature by bacterial fermentation, which can be accumulated in microorganisms in form of intracellular granules in response to unstable environmental conditions [12]. These polymers are reducing-power storage materials in various microorganisms and act as a reserve for energy/carbon, so bacteria utilize them when the external supply is insufficient [13; 14; 15; 16]. Poly(3-hydroxybutyrate) (PHB), the most common PHA, can be used as an alternative to synthetic polypropylene due to its similar structural and mechanical properties [13; 17; 18]. PHB is commonly produced intracellularly by microorganisms included in the genera _Bacillus_, _Azotobacter_, _Pseudomonas_, and _Rhizobium_; these bacteria can accumulate between 60-80% of their total weight as PHB [19]. _Bacillus_ is one of the most employed genera to produce PHB, due to its high replication stability, predominance in nature, and lack of lipopolysaccharidesaccharides, which makes PHB extraction much easier [20].  
One of the major limiting factors that have to be faced for the industrial production of PHB is associated with carbon source expenditure, reaching in some cases 50% of the total production cost [18]. Therefore, it is necessary to explore potential alternatives to find new, easily available, as well as cheap, carbon sources. Lignocellulosic biomass, such as cocoa bean shell, after adequate pre-treatment of hydrolysis could be a promising alternative renewable source to be employed as a substrate of fermentation.  
The aim of the present work has been to employ CBS as a carbon source to obtain PHB by means of _Bacillus firms_ CECT 14. CBS was hydrothermally treated, so that the resulting broths could be used as a substrate in the fermentation processes. To the best of our knowledge, this is the first research on PHB production from CBS, expanding the possible alternatives of valorisation of this lignocellulosic by-product.