by Dr Ioannis Nengas, Research Director, Hellenic Centre for Marine Research, Greece

 

Ensuring food supply to a population that is expected to exceed nine billion before the middle of the century remains one of our biggest challenges, according to the Food and Agriculture Organisation (FAO, 2017). Aquatic organism farming is one of the world's fastest-growing food sectors, providing the planet with about half of all globally consumed fish. According to FAO, aquaculture will contribute to both food security and the economic well-being of large populations. Today, world production of aquaculture products has set a new record, representing 55 percent of the total seafood produced, amounting to a total of 200 million tonnes-per-year.

However, the expansion of aquaculture production requires a proportional increase in aquafeed production. So, the challenge that the aquaculture industry has faced over the last few decades is to identify sustainable and nutritious ingredients to support the sector's growth. The aquafeed industry has recognised for many years now that the utilisation of plant feedstuffs for the production of aquatic species is an essential requirement for the future development of aquaculture. Such plant feedstuffs must provide nutritious components that will efficiently grow aquatic species with minimal environmental impact and produce high-quality fish flesh to promote human health benefits in a cost-effective manner (Gatlin et al., 2017).

Although many plant-derived animal feed ingredients contain acceptable amounts of protein, essential amino acids, calories, certain minerals and vitamins, their use is still limited due to the presence of several endogenous antinutritional factors (ANFs) that adversely affect enzyme activity or the absorption of minerals and other nutrients (Rasha et al., 2011). Some of the antinutritional factors in plant ingredients can be partially or totally inactivated by heat processing, such as roasting, autoclaving, extruding or cooking, prior to inclusion in fish feeds (Francis et al., 2001). However, high heat compromises these ingredients' nutritional quality due to partial destruction of heat-sensitive nutrients and is not effective on some of these antinutritional factors.

One way to enable the extensive use of plant feed ingredients by increasing their nutritional value and minimising the antinutritional factors is by using biotechnological processes. These processes include solid-state fermentation (SSF) and the use of exogenous dietary enzymes.

Solid-state fermentation

Fermentation is a process involving microorganisms, substrates and specific environmental conditions that converts complex substrates into simpler compounds (Niba et al., 2009). Fermentation products will vary depending on the characteristic of the microorganisms, substrates and conditions used. Conditions include temperature, pH, dissolved O2 and CO2, operational systems, mixing and the length of the fermentation process (Renge et al., 2012). Fermentation improves the nutritional quality of feedstuffs by:

  • Lowering the fibres
  • Increasing protein and lipid content
  • Improving vitamin and mineral availability
  • Improving amino acid digestibility.

It has also been reported to increase feed palatability (Borresen et al., 2012). A major benefit of the process is that it decreases the antinutritional content in plant feed ingredients and mycotoxin levels (Niba et al., 2009; Canibe and Jensen, 2012).

SSF is the fermentation process that involves a solid substrate, such as plant ingredients, in the absence of liquid. SSF is generally exploited to produce fermented dry ingredients that can be added to basic feed mixes. Due to the low moisture content, the SSF method can only be carried out by a limited number of microorganisms, mainly fungi such as Aspergillus spp. and Rhizopus spp., although some bacteria, like Lactobacillus spp., can also be used (Supriyati et al., 2015). SSF-processed ingredients are more compatible with aquafeed production.

Fermented plant ingredients, especially those produced through SSF, as components in aquafeeds are potential raw materials and have gained the aquaculture industry's interest. Table 1 documents results from the incorporation of fermented plant feedstuff in diets of various aquaculture species.

Exogenous dietary enzymes

Aquaculture, like the rest of the animal production sectors, is trying to minimise feed costs and, for this reason, has recently turned its attention on the use of exogenous dietary enzymes in plant-ingredient-rich feeds. This is due to their capacity to improve nutrient digestibility, which is low because aquatic species lack the endogenous enzymes needed for breaking down the complex cell wall structure of plant ingredients and release nutrients. Another important benefit of their incorporation in feed formulations is the breakdown of antinutritional factors, such as fibres, phytate and non-starch polysaccharides (NSPs), which lower performance and compromise the health and well-being of the animals (Alsersy et al., 2015).

Exogenous enzymes as feed additives have been extensively studied for the poultry and pig feed industries, and their dietary incorporation is now common practice for the reasons above. However, for aquaculture species, research on exogenous enzyme supplementation has been focused mainly on phytase. Only recently has the interest and research into dietary carbohydrases, proteases and enzyme mix supplementation increased (Gatlin et al., 2017).

The primary function of exogenous carbohydrases is to hydrolyse complex NSPs present in plant feedstuffs. Furthermore, carbohydrase supplementation increases digestibility of energy-yielding nutrients, such as starch and fat. In addition, it is possible that carbohydrases also act to improve nitrogen and amino acid utilisation by increasing access to protein for digestive proteases (Tahir et al., 2008). As with other nutrients, carbohydrase enzymes are also involved in improving the availability of minerals in diets to the target organism. Moreover, carbohydrases may promote and support the growth of beneficial bacteria, thereby improving the gut and overall health of the animal (Adeola and Cowieson, 2011).

Apart from the breakdown of carbohydrases, it is essential to improve the availability of proteins within the plant ingredients by supplementing dietary proteases. Proteases comprise a class of enzymes that hydrolyse protein into smaller proteins, peptides and amino acids. Without proteases, these bonds cannot be easily broken, and proteins, therefore, could not be readily digested by fish and crustaceans.

There is an increasing number of publications on the use of exogenous enzymes dietary supplements or enzyme complexes, comprising mainly of carbohydrases and proteases with very promising results. Table 2 summarises the most recent findings.

With all these potential benefits from biotechnological novelties, today's formulator has more tools to nutritionally and economically optimise aquafeeds. Research is ongoing on the effects of different substrates, microorganisms, different methods of producing enzymes and feed ingredients, as well as the interaction of these innovations with the metabolism, growth, intestinal health and immune system of cultured aquatic organisms.

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