By: Dr Mustapha ABA. Aquaculture Scientific Expert. Fish nutrition. Rabat. Morocco.
Feed processing techniques play a vital role in African aquaculture, since it is important to apply the appropriate processing technique to maximise production efficiency at the lowest possible cost. Fish feed is one of the most important factors in commercial fish farming in Africa, as diet can affect growth performance, flesh quality and feed wastage. Over the past decade, the use of extruded feeds for fish production has increased dramatically in Africa, as these diets have higher water stability, better flotation properties and higher energy content than other pelleted diets. Consequently, the main effects of feeding extruded diets to fish are followed by increased fish growth, better protein retention efficiency and digestibility, fatty acid-rich flesh quality, improved feed conversion ratio, economic gain with less pollutant discharges into aquatic systems.
Introduction
With the expansion of aquaculture worldwide, feed production for industrial aquaculture is also expected to increase, with 73.15 million tonnes of compound feed expected to be used by 2025 (Tacon, 2020). However, following the rapid increase in aquatic feed production on the African continent, certain problems have become more worrying, notably low feed efficiency and poor water stability, leading to feed waste and water pollution. Research in fish nutrition focuses mainly on dietary formulations and diets to improve feed conversion rates for economic gain, on fishmeal substitution methods, and less on feed processing (Aba , 2020).
The cost of feed is the biggest challenge facing aquaculture, and efforts are being directed towards reducing feed-related production costs (Abanikannda et al., 2021). In Africa, aquaculture practices and studies have shown that feed costs represent half or more of production costs, and this largely determines the viability and profitability of the fish farming enterprise (El-Sayed 2006; Aba et al., 2012; Khalil et al. 2019; Orimaye et al., 2019; Regupathi, 2019; Allam et al. 2020; Musa et al., 2021).
Sustainable aquaculture feed production is part of the fundamental objectives of the ‘Blue Transformation’ in aquaculture development suggested by the Food and Agriculture Organization of the United Nations (FAO) in 2022 (FAO, 2022). As an important basis for sustainable aquaculture, sustainable aquaculture feeds must be produced in terms of new raw materials or ingredients, energy efficiency, environmental and social compatibility (Cheng et al., 2023).
Feed development and management play a very important role in the growth and expansion of aquaculture. In fact, it is a major factor in determining the profitability of an aquaculture business (Aba, 2020). In formulating a fish feed diet, several combinations of ingredients can be used to achieve a particular target nutritional profile for amino acids, lipids, carbohydrates, minerals and vitamins (Aba et al., 2012).
However, feed technology is one of the least developed sectors of aquaculture, particularly in Africa (Aba, 2020; Malgwi et al., 2020). Certain feed parameters must be taken into account when choosing the best feed for a given species. These include protein level, energy concentration, digestibility, ingredient composition, feed particle size and type of processing (Aba, 2020).
Feed processing techniques play a vital role in aquaculture, as it is important to apply the appropriate processing technique to maximize production efficiency at the lowest possible cost, moreover, feed processing technology has received increasing attention in recent years (Wan et al., 2022), and it is one of the fundamental challenges facing the development and growth of aquaculture in Africa (Malgwi et al., 2020).
Pelleting and extrusion are the two main methods of pelleting animal feed (Kamarudin et al., 2018), these two processes are described carefully by Dr. El Abd and Dr. Abo Shady in their articles in this issue of the magazine.
Pelletizing
Pelletizing is the most common approach to preparing feed pellets, a relatively low-cost process that helps increase operational management efficiency, while reducing storage and transportation costs, as pelleting is done locally on the fish farm (Olusegun et al., 2017; Mircea et al., 2020). Pelleted feeds are dense and heavy and therefore sink, because the pelleting process is unable to reduce the bulk density of the incoming raw material, in other words, it is unable to gelatinize the starch to the extent that it affects the bulk density (Michael, 2021). The benefits of using pelleting equipment are not only to reduce acquisition costs but also to reduce dependency on fish feed traders (Nenciu et al., 2022). A pelletizer consists of a screw pump similar to a screw press or screw conveyor in which the feed is compressed and worked to form a semi-solid mass (Ikedudu et al., 2015; Malgwi et al., 2020). The feed is forced through a restricted opening (the die) at the discharge end of the screw. Pelletizing is the most common approach to preparing feed pellets, a relatively inexpensive process that increases nutrient utilization and helps increase operational management efficiency, while reducing storage and transportation costs, as pelleting is done locally on the fish farm (Olusegun et al., 2017; Mircea et al., 2020).
The pelletizing process involves creating pelleted feed by compressing the raw material, which is in powder form. This compaction occurs in the presence of moisture, temperature, and pressure. Pelleted feeds are dense and heavy and therefore sink because the pelleting process is unable to reduce the bulk density of the incoming raw material, in other words, it is unable to gelatinize the starch to the extent that it affects the bulk density (Michael, 2021).
The advantages of using pelleting equipment are not only reducing acquisition costs but also reducing dependence on fish feed traders (Nenciu et al., 2022).
Extrusion
Adopting the extrusion process to manufacture fish feed is the main way to ensure the production of high-quality and safe aquatic products for human health, which is also a trend for the progress of the animal feed industry in the future (Abou-Zied et al., 2015). Extrusion is a process in which feed is not only compressed, as in pelleting, but also “cooked”, which is why it requires higher levels of humidity, temperature and pressure than pelleting. During this process, gelatinized starch acts as a glue and helps increase the integrity of the feed during storage, handling, and stability in water (Michael, 2021). Protein and fiber also contribute to pellet binding, although to a much lesser extent than starch.
Compared to pelleted diets, extruded feeds increase water stability and durability (Adedeji et al., 2017), decrease antinutrient content, improve nutrient utilization, and reduce water eutrophication due to lower nitrogen and phosphorus removal (Ma et al., 2017), all of which are due to the higher levels of heat, moisture, and pressure used during the extrusion process that also contribute to improved microbiological safety of the feed (Okelo, 2006; Adedeji et al., 2017).
The use of extruded feeds in aquaculture is known to be more environmentally friendly and efficient than conventional pressed sinking pellets (Aba et al., 2012; Abanikannda et al., 2021). The quality of both extruded and pelleted feeds in addition to ponds, can also affect the environmental impact of cage aquaculture (Wu et al. 1999; Musinguzi et al. 2019). Today, we have knowledge and ingredients that allow us to manufacture fish and shrimp feed without marine flour, with results equal to those of traditional diets, in addition, compared to pelleting, extrusion allows the use of a greater variety of ingredients with a greater inclusion of some of them, knowing that each type of extruded feed is effective for specific fish species depending on their feeding habits (Xie et al., 2018), which contributes to sustainability in aquaculture.
Extruded feed contains high energy due to the high lipid content in the feed, which affects the growth performance of fish (Aba et al., 2012), with a better use of carbohydrates (gelatinized starch) especially in carnivorous fish (Krogdahl, 2005).
Extruded feeds remain on the water surface for a longer period of time, providing a greater nutritional advantage than pelleted feeds (Welker et al., 2018). In order to stabilize feeds in water and extend the floatation time of feeds, manufactured fish feeds must contain high-quality binding agents (Muo et al., 2016; Nenciu et al., 2022). If the water stability of pellets is achieved, there will be less waste than in conventional techniques, which is very beneficial for aquaculture systems.
Comparison of Pelleted and Extruded Feeds in Aquaculture
The following table summarizes the comparison of pellated and extruded feeds based on various aspects relevant to aquaculture.
| Pelleted Feed | Extruded Feed | |
| Production process | Grinding, mixing, compressing, cooling and bagging | Grinding, mixing, regrinding, conditioning, high pressure cooking, expanding, drying, cooling and bagging |
| Physical characteristics | Dense pellets, sink quickly | Usually floating |
| Size | Limits on smaller diameters with uneven cuts | Allows you to have all the grain sizes |
| Formulation Mode | Limited ingredient inclusion | Inclusion of a wide variety of ingredients |
| Buoyancy | Usually sikns | Float |
| Stability | Disintegrates quickly | Integrity lasts a long time |
| Digestibility | Medium to low due to cooking time | Excellent due to the gelatinization of starch |
| Fish Performance | Average to Good growth | Better growth |
| Feed Conversion Ratio | Average to higher FCR | The lowest FCR. |
| Addition of lipids | Limit, excess can cause the pelleted feed to disintegrate | High, allows the incorporation of large quantities of lipids |
| Energy content | Low energy content due to low fat content | High energy content through the incorporation of high lipid content |
| Nutrient Leaching | Can be raised | Minor |
| Transport and Storage | May be affected by humidity | Excellent, moisture resistant |
| Cost of Production | Lower | Raised due to equipment |
| Wear of the equipment | Faster | Slow |
Results of research on extruded feeds
The results obtained from several studies show that the growth performance of fish fed with extruded pellets was significantly higher than that of fish fed with pelleted diets.
Extruded feed gave better performance for several fish species; Clarias gariepinus (Abanikannda, 2021). In tilapia in Kenya, during a study on cage culture of Nile tilapia (Musa, et al., 2021), extruded feed gave better economic gains with lower environmental impact than pelleted feed.
The use of extruded feeds in rainbow trout, in addition to having better growth performance, improves waste disposal and reduces the contribution to pollution in effluents, (Aba et al., 2012, Aba et al., 2014, Welker, et al., 2018). The high gelatinization of starch resulting from extrusion, contributes to increasing starch utilization and improves the durability in water and the integrity of the nutrients of the feed (Aba et al., 2014, Wan et al., 2022).
The utilization of nutrients by aquatic animals is largely determined by the level of activity of digestive enzymes in the intestine, which can be affected by exogenous and endogenous factors, and diet is one of the main exogenous factors (Lu et al., 2020). Thus, the release of enzymes is related to the gastric evacuation time, which is prolonged for extruded feed (Gao et al., 2019; Wan et al., 2022).
Several studies indicate that the gut microbiota is closely related to the growth, nutrient absorption, and immunity of organisms (Xiong et al., 2017, Hoseinifaret al., 2018, Wan et al., 2022). In addition, extruded feed improved the richness of gut microbiota and reduced the proportion of harmful microbial taxa, hence the beneficial application value of extruded feed in fish aquaculture (Wan et al., 2022). In addition, extrusion leads to higher digestion and improved feed absorption, which affects fish performance (Barreto-Curiel et al., 2018, Hasyimi et al., 2020).
One of the great advantages of extrusion is the ability to manufacture feeds of different particle sizes according to the size of the fish mouth which is proportional to the size of the fish. In addition, pellet size can affect the amount of feed a fish can ingest over a given period. Pellet size has also been shown to be a limiting factor in allowing fish to ingest enough pellets to maintain maximum growth (Aba, 2024).
Conclusion
The selection of the types of feeds that will be used during the fish rearing cycle will guide the feed management strategy adopted and, therefore, improve the efficiency of diet utilization by fish. However, the trend is that in the future, all feeds will be extruded, as machines are becoming more efficient and costs are closer. In this context, small-scale African aquafarmers can come together in the form of an association or cooperative in order to equip themselves with extruder machines to make their aquaculture production profitable. These machines will also allow these farmers to use a variety of local ingredients in the manufacture of feed and thus contribute to the sustainability of aquaculture in Africa.
Source: Aquaculture Feed Magazine Africa. Volume 1, Issue 3, 2024.
