World Aquaculture - December 2014

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62 DECEMBER 2014 • WORLD AQUACULTURE •
Single Cell Proteins: a Novel Approach
in Aquaculture Systems
Vivekanand Bharti, P. K. Pandey1 and Satish Kumar Koushlesh
The rapid growth in aquaculture and
scarcity of fishmeal contribute to the high prices
of aquafeeds. Currently fishmeal and soybean
meal are the most common feed ingredients in
aquafeeds, representing 40-60 percent of total
variable costs. A serious challenge remains in
reducing input costs through reducing the level
of fishmeal in feeds and palliating pressure on
natural fish populations.
Aquaculture producers are seeking
alternatives to reducing input costs to enhance
profitability. Furthermore, consumers have
become more health conscious. The practice
of organic farming in aquaculture is one
current response to these drivers. Among
various alternative strategies available today, the exploitation of
microorganisms in aquaculture minimizes the use of antibiotics and
represents a source of high-quality protein at low cost.
Microorganisms in aquaculture production are used as live
organisms, killed organisms or extracted nutrients in aquafeed.
Microorganisms such as algae, bacteria, yeast, molds and higher fungi
—grown in large-scale culture systems or in biofilm/biofloc systems
—can be used in aquaculture. The application of microorganisms or
its products as single-cell protein (SCP) in aquafeeds is one of the best
approaches to enhance aquaculture sustainability.
Single Cell Protein
Single cell protein broadly refers to microbial biomass or protein
extract used as a food or feed additive. Yeast has been used as a
source of protein in human food from ancient times. Microorganisms
contain high levels of protein, fats, carbohydrates, nucleic acids,
vitamins and minerals (Table 1). The importance of yeast and other
microorganisms has been realized and further research has been
directed toward its utilization in aquafeeds. Efficient exploitation of
SCP of microbial origin can replace up to 50 percent of the fishmeal
(Dhevendaran et al. 2013). Substrates such as whey starch, cellulose
hydrocarbon, alcohols and molasses have been used to produce SCP.
There are several technologies for production
of SCP at the industrial level and the basic
steps are illustrated in Figure 1. Technically the
production of microbial biomass is achieved
either by a submerged or solid state fermentation
process.
Production of Single-Cell Protein
Microorganisms can use inexpensive
inorganic compounds, such as ammonium salts
and carbon, to generate energy for metabolism
and growth. Waste inorganic resources can be
converted into protein biomass through rapid
growth of microorganism on the substrate.
Cheap and abundantly available agricultural
and industrial wastes can be used for SCP production. Solid state
fermentation utilizes solid substrates such as bran, bagasse and paper
pulp while submerged fermentation uses free-flowing liquid substrates,
including molasses and broths. Sterilization is not required because
pathogenic microorganisms are not used for production of SCP for
aquafeeds.
Sources of SCP in Aquaculture
Numerous microorganisms with a high nutritional value can be
used in aquafeeds and these are discussed below.
Algae. Algae such as Chlorella sp., Chondrus sp., Scenedesmus
sp., Spirulina sp. and Porphyrium sp. can be applied as feed
supplements or substitutes for conventional protein sources (fishmeal
and soybean meal) in aquaculture. The application of algal biomass in
animal feeds, including aquafeeds, is about 30 percent of the current
world algal production. Euglena gracilis is one of the most preferred
algae for SCP production because it has a high protein content and
high digestibility by fish (Baker and Gunther 2004). Diets containing 5
percent Spirulina platensiscan replace Artemia nauplii in Litopenaeus
schmitti larvae culture (Jaime-Ceballos et al. 2005). Algae contain 40-
60 percent protein, 7 percent mineral salts, chlorophyll, bile pigments,
fiber and nucleic acid content (4-6 percent). Production of algae is
TABLE 1. Nutrient composition (as percent dry weight) of the main group of microorganisms used for single-
cell protein production (Miller and Litsky 1976).
Component Fungi Algae Yeast Bacteria
Protein 30-45 40-60 45-55 50-65
Fat 2-8 7-20 2-6 1-3
Ash 9-14 8-10 5-10 3-7
Nucleic acids 7-10 3-8 6-12 8-12
Substrate
Fermentation Nutrient
Filtration
Drying
SCP
Submerged fermentation
Semisolid fermentation
FIGURE 1.Production of single-cell protein
(SCP).