Although ensiling is not a conventional process for the management of waste, it has found significant application in the reprocessing/ treatment and preservation of a variety of agricultural residues for use in animal feeding. Silage making is the (lactic) fermentation/storage of (forage) for use in animal (ruminant) feeding. Ensiling is a multistage process which ultimately results in low pH (< 4.0) products that have extended resistance to spoilage (and often having appealing flavour to ruminants). During ensiling, some bacteria are able to break down some cellulose and hemicellulose to their components sugars which are subsequently metabolized to low molecular weight acids, mostly lactic acid. This can also be encouraged by the use of appropriate mix of enzymes and microbial (lactic acid bacteria) silage inoculants (Okine et al., 2005; Aksu et al., 2004; Zahiroddini et al., 2004; Colombatto et al., 2004abc; Gardner et al., 2001). The lactic acid bacteria are also believed to produce bacteriocins that discourage the growth of and spoilage by unwanted populations. Efficient fermentation ensures a palatable and digestible feed. Production of good quality silage requires that anaerobiosis be achieved quickly to enable the lactic acid bacteria to develop and predominate, and in the process further bring down the pH of the mass. This discourages spoilage of the silage by putrefactive aerobic populations and ensures the retention of the most nutrients in the final product (Arvidsson et al., 2008).
Silage making starts with the impounding of the biomass and is initiated by aerobic populations. During this stage the aerobic organisms scavenge oxygen and bring about anaerobiosis. This phase is undesirable, because the aerobic bacteria consume soluble carbohydrates that should otherwise be available for the beneficial lactic acid bacteria. It also leads to the production of moisture, and heat generation, which if not properly managed are capable of destroying the process. Proteinaceaous materials may also be rapidly broken down during this phase and this can lead to loss of nutrients and the accumulation of ammonia (Slottner and Bertilsson, 2006). This phase may not be completely avoided but must be reduced to the minimum for successful silage making. To encourage rapid acidification during ensiling, fermentable sugars and lactic acid bacteria inoculants are often added to the silage (Okine et al., 2005; Yang et al., 2006). This is common during ensiling of protein rich feeds such as manure, slaughter house and fish wastes as well as many agricultural residues such as wheat straw, tomato or apple pomace and citrus waste (Santana-Delgado et al., 2008; Vazquez et al., 2008; Bampidis and Robinson, 2006; Denek and Can, 2006; Pirmohammadi et al., 2006; Yang et al., 2006; Volanis et al., 2006; Vidotti et al., 2003; Oda et al., 2002; Scerra et al., 2001; Chaudhry et al., 1998; Shaw et al., 1998).
In the anaerobic stage of ensiling, mixed populations of lactic acid bacteria predominate and metabolize fermentable sugars, producing lactic acid and reducing the pH of the mass to acidic levels. As the pH drops, minor acetic fermentation (if present) ends. This process continues until most of the available sugars have been consumed, and the pH has dropped to a level low enough to discourage bacterial activity. The duration of this stage varies with the nature of the biomass being ensiled, particularly, the initial concentration of fermentable sugars and the population of lactic acid bacteria. Ensiling by itself hardly leads to protein enrichment of the biomass except if mineral nitrogen such as urea is included. However, its capacity to achieve conservation of waste protein for use in animal feeding makes it important in schemes for the reuse of agricultural refuse.
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