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.