Valorization of residual strawberry extrudate througt obtaining high added value compounds and methane production by anaerobic digestion

Abstract

Strawberry is used not only for direct consumption due to its excellent nutritional properties, but is also used for the manufacture of other products derived from the transformation of the fruit, such as jams, yogurts, and other dairy derivatives, soft drinks without alcohol, ice cream, cakes, flavoring substances, etc. Specifically, around 21 % of the total strawberry production is used to obtain these products. During the industrial processing to obtain the strawberry concentrate for these by-products, the strawberries are pressed and extruded using twin-screws up to several sieves with different mesh sizes. In this process, the pulp and the liquid part, which constitute the strawberry concentrate, are separated from the fibers part and the achenes, generating a residual fraction, named strawberry extrudate (SE). The strawberry extrudate fraction can reach around 7 % by weight of all manufactured strawberries, which must be managed and treated properly due to their high organic load and high polluting power. To take advantage of the main components of the strawberry extrudate (cellulose, hemicellulose, and lignin) and to recover high added value compounds (bioactive compounds), a previous treatment is required that allows breaking the physical and chemical barriers of its complex structural framework. With this previous treatment, the accessibility to the mentioned compounds should be increased, causing the solubilization of the compounds of interest, such as phenol compounds and sugars, thus allowing the separation of the phases (solid and liquid), and obtaining an easy recovery of high added value compounds. There are different extraction methods, such as hydrothermal pre-treatment, either at low and/or medium temperatures and pressures, as well as with steam at high pressure and temperature, and with rapid decompression, called “steam-explosion”. In the case of the agro-wastes resulting from the pressed and extruded of the strawberry, in this Doctoral Thesis a study has been carried out with different hydrothermal pre-treatments, under different conditions of pressure and temperature. After the subsequent hydrothermal pre-treatment, it is generated a liquid phase, rich in easily degradable carbohydrates, and phenol compounds with high added value that, due to their antioxidant nature, could be of interest to the pharmaceutical and food industry. After carrying out these hydrothermal pre-treatments, it is proposed a purification process of phenol compounds using an adsorbent resin already used by the food industry called Amberlite XAD 16. The high added value compounds obtained after hydrothermal pre-treatments, under different conditions of pressure and temperature, have been proposed for use as comonomers for a process called inverse vulcanization. Recently, several studies have been reported in the bibliography that demostrated the possibility of the use of some agro-waste, for its use as a comonomer in the inverse vulcanization process. Being used for the manufacture of the mentioned polymers elemental sulfur (S8) from petroleum refining processes. The use of copolymers of natural origin that come residues of the agro-food industry would lead to obtaining a polymeric material that would offer a high degree of biodegradability, antioxidant power, and antimicrobial properties. To close the recovery cycle of the strawberry extrudate, the remaining biomass after the hydrothermal pre-treatment and the purification of phenol compounds, must be treated adequately since it would still contain a large amount of organic matter. In these, the organic matter of the substrate is transformed into biogas, a mixture of carbon dioxide (CO2, 30-40%) and methane (CH4, 60-70%), whose high calorific value (5000-6000 kcal/m3) allows its use as biofuel or its use for the generation of electrical and thermal energy through cogeneration. All these reasons have promoted the study of the influence of hydrothermal hydrolysis pre-treatments in order to try to overcome the difficulties that could be encountered when anaerobically processing the residue from strawberry extrudate without treatments with a high content of phenol compounds.