Keywords:
Biotechnology, Fermentation, Solid-State Fermentation, Rumen,Ganoderma , Pleurotus, Ruminant Digestion, Acacia mellifera, Animal Feed, Pretreatment

Introduction

Growth and expansion in the agricultural and food sectors results in the generation and accumulation of a vast amount of agro-industrial waste every year. Agro-industrial waste is defined as “the waste generated during the industrial processing of agricultural or animal products or the waste obtained from agricultural activities” by Mirabella and co-workers [1]. Environmentally friendly or acceptable disposal methods of agricultural wastes and residues pose a significant scientific problem, especially when considering the chemical properties, recalcitrance, and abundance of lignocellulose. An increase of interest was also noted for alternative treatments other than expensive physical or chemical treatments, such as the use of white-rot fungi through solid-state fermentation to degrade lignin components in biomass, supported by an amassing number of published research on this subject [2]. The use of macrofungi, such as mushrooms, which naturally occur in nature as wood or organic litter decomposers, excrete nonspecific oxidative enzymes that degrade lignocellulosic materials, and thereby offer solutions that are being widely and readily applied for the biotransformation of these materials. Equally important, these organisms have shown efficient bioconversion and degradation of various types of agro-industrial/forestry by-products with low or no economic value to edible biomass [3].
Any biotechnological method in which living organisms are cultivated on non-soluble or solid material in the absence (or near absence of free water) is recognised as solid-state fermentation (SSF) [4]. Solid-state fermentation has been shown to be the most effective method to harness the power of these macrofungi for the purposes of delignification [4], conversion [3] and nutritional value addition [5]. White-rot fungi have been shown to digest or degrade lignin and sequentially break down the lignocellulose complexes, increasing nutrient availability for microbial fermentative utilisation during rumen digestion [6]. Basidiomycetous white-rot fungi, such asPleurotus ostreatus, digest lignocellulose in anticipation of producing fruiting structures (mushrooms) and display a unique strategy to colonise and modify the substrate in such a way that readily metabolizable cellulose is available when fruit bodies are produced. During mycelial growth they sequentially produce enzymes that degrade or modify lignin and change the enzyme target towards degradation of cellulose and hemicellulose during fruiting. Stopping this process before fruiting results in an organic substrate with fewer hemicellulose-lignin bonds and subsequently increases accessibility of cell-wall carbohydrates for rumen microbes to digest. The ultimate effectiveness of the fungal pretreatment hinges on factors such as biochemical characteristics of the material, the choice of fungal strain used and the amount of time of the fungal treatment [7].
The encroaching bush Acacia mellifera, locally known as Swarthaak (Figure 1), has formed vast impenetrable thickets in the central to northern parts of southern Africa, detrimentally impacting grazing forage [8,9,10,11]. Debushing efforts have somewhat alleviated the physical presence, yet mechanical removal of the encroachment only serves as temporary solutions. In order to increase sustainability, Lukomska and co-workers [11] refer to the necessity to find an alternative use for harvested biomass to offset the short-term income risk debushing holds for farmers. Converting harvested encroaching biomass to livestock, especially ruminant, fodder could provide sustainable solutions to multiple problems. The primary goal of the SSF design of this research is for maximal reduction or bioconversion of lignin with minimal loss in cellulose and hemicellulose as shown in Figure 2. This study presents a SSF method for the selective delignification of Acacia mellifera using basidiomycetous fungi, i.e., mushrooms, as the biological agents.