Agricultural and Food Processing Residues
Agricultural residues primarily comprise of stalks and leaves that are generally not harvested from fields for commercial use. Sugar cane bagasse, corn stover (stalks, leaves, husks, and cobs), wheat straw, rice straw, rice hulls, nut hulls, barely straw, sweet straw' bagasse, olive stones, etc. are some of the examples of agricultural residues. The use of agricultural residues for biorefinery is beneficial as it eliminates the need of sacrificing arable lands. The wastes such as animal manure (from cattle.
chicken, and pigs) are also included within the agricultural residues. The refuge- derived waste generated from either domestic or industrial sources is another source of biomass.
Large amounts of lignocellulosic wastes are generated through agro-industrial activities each year and these materials are underused and disposed off in the environment without any treatment, leading to serious environmental pollution problems. These agricultural wastes can potentially be bioconverted into value-added products such as pulp, animal feed, and biofertilizer through the action lignin-degrading enzyme secreting fungi, such as mushrooms. Currently, biofuels such as bioethanol, biodiesel, biohydrogen, and methane from lignocellulosic biomass are produced from agro-wastes rather than from energy crops, as they compete with food crops.
Oil prices volatility and limited fossil resources are pushing the chemical industry’s giants - but not exclusively - towards the use of alternative raw materials: agricultural and food processing residues. Agricultural residues constitute a major part of the total annual production of biomass residues and are an important source of energy for domestic as well as industrial purposes. Sometimes residues are used as fuel, but a large amount is burnt in the field.
1.4.1.2.1 Rice
Rice straw: In many countries rice straw is burnt in the field with the ash used as inorganic fertilizer. Relatively small quantities are used as animal fodder, animal bedding, raw material for pulp, paper and board making, or building material.
Rice husk: Husks are often burnt at the rice mill to get rid of the husk but in some countries it used extensively for power generation in large rice mills and also for the brick industry as a source of energy.
1.4.1.2.2 Cotton
Cotton stalks are often burnt in the field as leaving them in the field may result in damage to future crops due to diseases, infestation, etc. Part of them is possibly to be used as domestic fuel and raw materials for energy (10, 11).
1.4.1.2.3 Sugar Cane
Bagasse and sugar cane tops and leaves are the main residues of which the former is normally used as an energy source for steam generation, while the latter is normally used as cattle feed or is burnt in the field. Most sugar factories burn all the bagasse they generate even at very low efficiency. This is done to ensure that all bagasse is burnt, as dry bagasse is known to be a fire hazard. In some countries bagasse is also used as raw material for the furfural or pulp, paper, and board industries. Increasing the combustion efficiency in the sugar industry could result in the saving of considerable quantities of bagasse which either could be sold to pulp and paper factories or used to generate power products as well (12).
Food Processing
Until the end of the 20th century, disposal of food wastes was not considered as a matter of concern. Particularly, increase of food production without improving the efficiency of the food systems was the prevalent policy. This consideration increased generation of wasted food along supply chains. In the 21th century, escalating demands for processed foods have required identification of concrete directions to minimize energy demands and economic costs as well as reduce food losses and waste. Today, food wastes account as a source of valuable compounds and deal with the prospects of feeding fast growing population. Perspectives originate from the enormous amounts of food-related materials (food ‘losses’, ‘wastes’, ‘by-products’, or ‘wasted by-products’), which are discharged worldwide and the existing technologies that promise not only the recovery but also the recycling and sustainability of valuable ingredients inside food chain. The prospect of recovering valuable compounds from food by-products is a story that started few decades ago. Citrus peel was one of the first by-products to be utilized for the recovery of essential oils and flavo- noids, and their re-utilization as additives and flavourings in foods and fruit juices. Even earlier, solvent extraction had been applied to recover oil from olive kernels, which are one of the by-products derived from olive oil production. Nowadays, olive kernels are considered an established commodity similar to olive fruit, whereas researchers focus on the recovery of polyphenols from olive mill wastewater. Over the last decade, several companies have started commercializing the latest process and are ambitious to turn this waste into valuable compounds. In the field of animal- derived side streams, cheese whey constitutes the most intensely investigated food by-product and represents a successful reference of valorization. Protein concentrates and various sugar derivatives are the prominent compounds derived from this source, as reflected by the numerous processes and products existing in the market.
These commercially available applications inspired the scientific community to intensify its efforts for the valorization of all kind of food by-products for recovery purposes. Besides, the perpetual disposal of high nutrition proteins, antioxidants, or dietary fibres in the environment is a practice that could not be continuing for a long time within the sustainability and bioeconomy frame of the food industry. Indeed, the depletion of food sources, the fast growing population, and the increasing need for nutritionally proper diets do not allow considering other alternatives. As a result, a large number of projects have been initiated all over the globe and across scientific disciplines, whereas the existence of numerous scientific articles, patents, congresses, and industrialization efforts has emerged as a wealth of literature in the field.
Despite this plethora of information and the developed technologies, the respective shelf products remain rather limited. For instance, only few companies across the globe are activated in this field, whereas many of them are in an early stage and have not developed their process in a commercial manner yet. This is happening because the industrial implementation of food waste exploitation for the recovery of valuable components is a complex approach that needs careful consideration of numerous aspects. Waste emerges seasonally and often in large quantities and is prone to microbial spoilage. Therefore, drying or immediate processing is required. In addition, the concentration of target compounds varies significantly, which may be a challenge for subsequent standardization of extracts or products. Scale-up of processes developed on laboratory or pilot-plant scale needs to be appropriately designed to retain the functional properties of the target compounds. Finally, the product to be developed needs to meet the high expectations of the consumers in an increasingly competitive market.
Following these considerations, a commercially feasible product can be generated only if a certain degree of flexibility and alternative choices can be adapted in the developing methodology. Experience has shown that a project focused on the recovery technologies without investigating and establishing particular applications is doomed to fail because the final product might not be as beneficial as initially expected. In addition to these challenges, regulatory issues still exist and constitute a severe problem for the industry. Without doubt there is a market need for nutraceuti- cals and functional foods. However, marketing of such products using health claims requires comprehensive research activities to unambiguously demonstrate their health benefits. These studies are costly and constitute an impediment especially to small- and medium-sized as well as spin-off and spin-out companies. So far, only a small number of compounds and products have been approved, whereas the overwhelming majority of health claims has been declined by regulatory bodies (13).
