Theme Feed processing

More and more countries throughout the world are investing in aquaculture facilities to cover their need for future food/animal protein supply. The fish  used in aquafeed in turn leads to increased pressure on diminishing marine resources. It is therefore important to increase the knowledge on the use of additional plant ingredients in fish feed. Kraugerud carried out five different studies to further investigate the physical and nutritional properties of polysaccharides in extruded fish feed.

 

Effect on digestibility
The first study looked at the degree of gelatinisation of the pellets. A random sampling of commercial grower diets of diameter ~12 mm for Atlantic salmon, showed differences in gelatinisation indicating differences in origin of starch source and/or processing conditions. The degree of gelatinisation is an essential factor when it comes to utilising the binding properties of the starch and to increase its digestibility. The second study investigated the effect of the nonstarch polysaccharides (NSP) fraction. Kraugerud showed that diets with added NSP showed no reduced digestibility of macro nutrients compared to diets without NSP. However, digestibility was reduced in fish fed the soybean meal (SBM) diets. The SBM diet also induced morphological changes in the distal intestine, while this was not the case for the other diets.

 

The third study was designed to investigate how diets, and their ingredients, responded to identical operating variables in the extruder. Furthermore the same feed formulations were processed at two different levels of specific mechanical energy (SME), to see how the diets responded to differences in treatment severity. In addition, feed for a digestibility trial (fourth study) was produced to achieve a fixed product density. Different plant ingredients were included in fish meal based diets at 140 – 240 g kg-1 inclusion.

 

The experimental ingredients were corn gluten, defatted soybean, defatted sunflower, dehulled lupin, defatted double-low rapeseed, whole field pea, whole and dehulled faba bean, whole wheat, and naked oat. To reach the same level of macro nutrients in the diets, wheat starch and cellulose were used as fillers. In addition to the ten plant meal containing diets, a reference diet with FM, FO, starch and cellulose was made. It was shown that sunflower meal for example, increased breaking force of the pellets. The diets with the protein-rich ingredients performed better in physical quality than the diets with the starch-rich.

 

Pre-treatment
The fourth study, feeding ~0.7 kg Atlantic salmon in sea water with the diets produced in the third study, showed that numerous plant ingredients are suitable for use in diets to Atlantic salmon. Additional treatment may be required for selected ingredients, such as sunflower and rapeseed to
increase digestibility. However, it was observed that the inclusion of dietary cellulose was related to a decrease in lipid digestibility. Based on the observation that certain ingredients would benefit from additional pre-treatment to improve nutritional performance, a fifth study was planned to assess the effect of an integrated pre-treatment step on physical properties of fish feed. Four plant ingredients, soybean, rapeseed, field pea, and faba bean formed part of fish meal based diets with levels of macronutrients similar to that of commercial salmonid diets.

 

For the two diets with soybean and rapeseed inclusion, wheat was included as a starch source. The feed mash was either subjected to conventional conditioning (75°C, 62 s, 270 g kg-1 total moisture) before being processed in a co-rotating twin screw extruder, or the plant fraction of the mash was incubated (45°C, 45 min, 400 g kg-1 total moisture) before adding the FM fraction and subsequently conditioning and extruding the total feed mash as described above. Two levels of screw speed were tested (325 or 225 rpm). The results showed that incubation of the plant meal fraction can modify processing responses and physical properties of the extruded feed.

 

The hardness of the diets with starch-rich ingredients increased as a result of incubation, while this was not the case for diets with protein-rich ingredients.

 

Conclusion
Overall, the studies reported in this thesis showed that processing responses were unique for different plant ingredients. No single ingredient or ingredient group outperformed the other in physical properties, although the sunflower diet led to harder pellets.

 

Source: Feed Mix Volume 16. No. 5