Introduction
Solid state fermentation (SSF) is the fermentation process where microorganisms are grow in an environment without free water, or with very low content of free water on solid substrate and complex material is converted into simpler forms, moreover this solid substratum itself act as carbon/energy source. Egyptians were reported to make bread using a notable technique SSF and has been used in Asian continent from the ancient time in 2000 BC. In natural condition with little moist or in near absence of water, the microorganisms obtain carbon, nitrogen and other nutrients for growth and show degradative activity. In recent years, SSF has shown much development in bio processing in food, pharmaceutical, textile, biochemical and bioenergy. Solid state fermentation is processed through fungi, bacteria or yeast.
With an increasing the expansion of agro-industrial activity, accumulates a bulk quantity of residues over every year. This agricultural based biomass is highly lignified as per the composition and having lignocellulosic in nature, that causing elimination issue with ultimate environmental pollution. Worldwide bid of animal based products are augmented in a blooming rate thus emphasizing the inevitability of applying strategies to improvise animal productivity. The major constraints in livestock’s sector have lack of availability of feed resources, poor quality of available feed sources, and high feed cost particularly in tropical countries. The worsening animal health and their sustainability have necessitates the use of certain substitute such as agricultural by products, crop residues and grasses as a feed source (a lignocellulosic biomass). These agro industrial residues used for animal feeds, having highly lignified fiber, poor in nitrogen and minerals, reduced digestibility and contains anti nutritional factors, owing to this these, are not to utilized judiciously and therefore they are receiving more consideration for quality control.  In view of high nutritional perspective these residues are not described as a waste but known to be as raw materials for further product formation and developments. With an advance, in a field of animal nutrition, animal nutritionist developed various physical, chemical and biological methods to overcome the problem associated with animal’s feed stuffs. As a biological treatment has fascinated an interest of researchers and it has become a widely discussed theme in a current period.
 The huge nutritious potential of agricultural based byproducts has generated an interest of nutritionist for utilizing these efficient ways and improves the production of animals. In concern to this, solid state fermentation (SSF) is a promising novel technique. The lignocellulosic structural characters of plant residues provides a solid support and act as a substrates for the microbial fermentation to produce a certain value added products through SSF. SSF has a wide scope in the field of animal nutrition in terms for the production of enzymes, bioactive components, organic acids, vitamins, and feed additives, bio transforming products, biological degradation and detoxification of agricultural residues/wastes. The research studies shown the inclusion of SSF biomass has a great impact on nutritive composition of feed, performance, hemo-biochemical status, gut morphology, gut microbiota, carcass attributes, rumen fermentation along with the reduction in enteric methane emission of ruminants, non ruminants animals and poultry birds.
Types of fermentation
There are two type of fermentation process namely solid state fermentation (SSF) and Liquid or submerged state fermentation (SmF). Solid state fermentation (SSF),  a  process that takes place in a solid matrix (inert support or support/substrate) without or with smaller quantity of free water. However,  moisture needed to support the growth and metabolic activity of microorganisms on solid substrate. On the other hand, in liquid-state fermentation (LSF) the substrate is solubilized or suspended as free particles in a large volume of water. The differentiating feature between SSF and SmF has been described in Table 1.

Table: 1 Differentiating points in SSF and SmF.
Features
SSF
SmF
Medium
Not free – flowing
Free flowing
Deepness
Shallow
Greater
Nutrients
Solid substrate
Employed
Water
Medium absorbs
Medium is dissolved
Temp., pH
Not uniform
Uniform
Contamination
Less
Higher
System
3 phase
2 phase
Intra particle resistances
Present
No such resistances
Culture distribution
Adhere to solid and grow
Uniformly distributed
Bioreactors
Small
Large
Measurements of biomass
Lots of difficulties
Online sensors are available
Product
Highly concentrated
Low concentration
Liquid waste
Not produced
High quantity
Solid state fermentation
Solid state fermentation (SSF) is recognized a biotechnological processes in which in the absence or near absence of free water organisms grow on non-soluble material or solid substrates. It involves microbial fermentation of byproducts with few processing steps. At the most general level, the major processing steps of SSF are not different from those of a submerged liquid fermentation (SLF) process. The processing steps of solid state fermentation involves are as follows.
General processing steps in SSF process       
  1. Inoculum preparation
  2. Substrate selection and preparation
  3. Bioreactor preparation
  4. Inoculation and Loading
  5. Bioreactor operation
  6. Unloading
  7. Downstream processing
  8. Waste disposal

Characteristics of the fibrous components of crop residues
The major portion of the agricultural residues are carbohydrates mainly lignocellulose. These Cellulose, hemicellulose and lignin bonding in the cell wall matrix need to be broken. The dietary fibers components of plant walls are influenced by both the content and physical characteristics of wall polysaccharides such as degree of crystallinity and polymerization thus not completely digested by enzymes of the animal’s digestive system. With increasing the plant maturity lignin content is also elevated and has directly impacted on digestibility of neutral detergent fiber (NDF) and it has a correlation with other nutrient utilization.
 Use of certain alternative options such as the agricultural crop residues and grasses (lignocellulosic biomass) as animal feed stuffs. If these are utilized judiciously this may provide enough energy and nutrients to the animals. However, high lignin content and lower digestibility, protein content and poor palatability of crop residues and grasses discourage their use as the sole animal feed. Lignin, being a cementing material in plant cell wall restricts the fullest accessibility of carbohydrates, the energy reserve, to the microorganisms inside the gut of ruminating animals. Among various microorganisms known for lignin degradation, white- rot fungi (majorly basidiomycetes) have been adjudged most promising lignin degraders and have been largely studied for bioconversion of plant residues into nutritionally digestible animal feed under solid-state fermentation (SSF) conditions.


Application of SSF in Animal Nutrition                                                                                   
Solid state fermentation has an extensive scope and a novel technology in the field animal nutrition for utilizing these highly lignified by products. SSF having wide no of applications (Table 3) includes enzyme production, bioactive metabolites, organic acids production, vitamins, biological degradation of anti-nutritional factors from the various byproducts and animal feed stuffs. Enzymes are important products obtained from microorganisms and useful for human as well as animals and birds. Enzyme production is higher in solid state fermentation (Pandey et al. 1999) [44]. Plant cell wall has two phases including micro-fibrilar phase, it contains micro fibrils of cellulose and second is matrix phase (non-crystalline phase) which contains polysaccharides (Pectin and hemicelluloses), proteins and phenolic compounds. Recently renewed interests have been seen in enzyme production, mainly celluloses, xylanases, Xylanases, Laccases etc. Besides bacteria, fungi are considered the best source of enzyme production through the SSF. The various substrates and microbes used for the production of various products used for feed stuffs are shown in Table 4 and 5.
Table: 3 Applications of SSF in animal nutrition
Economic Sector
Application
Examples
Industrial   Fermentation
Enzymes production
Amylases, amyloglucosidase, cellulases,
proteases, pectinases, xylanases, glucoamylases
Bioactive products
Mycotoxins, gibberellins, alkaloids, antibiotics, hormones
Organic acid production
Citric acid, fumaric acid, itaconic acid, lacticacid
Biofuel
Ethanol production
Miscellaneous compounds
Pigments, biosurfactants, vitamins, xantham
Agro-Food
Industry

Biotransformation of crop residues
Traditional food fermented (Koji, sake, ragi, tempeh), protein enrichment and single cell protein production, mushrooms production.
Food additives
Aroma compounds, dye stuffs, essential fat and organic acids
Environmental control



Bioremediation & biodegradation of  hazardous  compounds
Caffeinated residues, pesticides, polychlorinated biphenyls (PCBs)
Biological detoxification of agroindustrial wastes
Coffee pulp, cassava peels, canola meal, coffee
husk

                                                              (Source: Guerra et al. 2003[21]; Mienda et al. 2011[36]).
Table: 4 Microorganisms used for SSF
Microorganisms
Substrates/ Solid supports
Source
Bacteria
Amycolatopsis mediterranean
MTCC 14
GOC and COC
Vastrad and Neelagund (2011a,b)[71; 72]
Pseudomonas spp. BUP6
GOC, COC, SOC, and CSC
Faisal et al. (2014)[17]
Bacillus licheniformis
MTCC 1483
Wheat straw, sugarcane bagasse, maize straw, and paddy straw
Kaur et al. (2015)[24]






(Continued table 5)
Fungi
Aspergillus niger
                 
Rice bran, wheat bran, black gram bran, GOC, and COC
Suganthi et al. (2011)[63]
Aspergillus oryzae
Soybean meal (waste)
Thakur et al. (2015)[68]
Rhizopus arrhizus and Mucors ubtillissimus
Caorn cob cassava peel, soybeans, wheat bran, and citrus pulp
Nascimento et al. (2015)[40]
Aspergillus niger
Rice bran, wheat bran, black gram bran, GOC, and COC
Mahalakshmi and Jayalakshmi, (2016)[28]
Aspergillus terreus
Palm oil cake
Rahman et al. (2016)[49]
                                                                                                              
Table: 5 Substrates used for SSF                                                                                  
Enzymes
Microorganisms
Substrates/ Solid support
Source
Lipase
Candida rugosa
Groundnut oil cake (GOC)
Rekha et al. (2012)[52]
Pectin methyl
Esterase
Pseudomonas notatum
Wheat bran and orange peel
Gayen and Ghosh (2011)[19]
Lipase
Pseudomonas aeruginosa
Linseed oil cake (LOC)
Dharmendra, (2012)[14]
α-Amylase
Aspergillus niger
Orange peel
Sindiri et al. (2013)[58]
α-Amylase
Aspergillus oryzae
Coconut oil cake (COC)
Ramachandran et al. (2004)[50]
α-Amylase
Bacillus sp.
Rice bran
Sodhi et al.(2005)[61]
α-Amylase
Bacillus sp.
Corn bran
Sodhi et al. (2005)[61]
α-Amylase
Aspergillus niger
Rice bran, wheat bran, black gram bran, and soybean
Akpan et al. (1999)[1]
Invertase
Aspergillus niger
Fruits peel waste
Mehta and Duhan (2014)[35]
                                                                                                            (Source: Sadh et al. 2018)
SSF is most imperative method used to improve the availability digestibility of fibrous crop residues by relaxing the lignocellulose network along with increasing other nutrients digestibility. Further, it resultant in to improved rumen fermentation (TVFA) range of 10 to 15 % and feed efficiency of animals. Reported studies prove incorporation of SSF ingredients at the rate of 5-20% in the ration of both ruminants and non ruminants could be improves growth, production, health status with reduced methane production and economics of feeding. However, research is needed to developed methodology for making it more economical, huge biomass production at farmer's door steps. In addition, need to produce genetically modified strains of microbes, develop proper controlling parameters and experimentation for optimize the level of SSF to increase the productive performance in various species of ruminants and non ruminants animals and poultry birds.
Dr. A.B. Parmar and Dr. V. R. Patel,
College of Veterinary Science and A.H.
Navsari Agricultural University, 
Navsari, Gujarat, India



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