Soyasaponins Resist Extrusion Cooking and Are Not Degraded during Gut Passage in Atlantic Salmon (Salmo salar L.)

Abstract

The stability of soyasaponins in fish feed formulations was investigated. The level of soyasaponin Ab, Bb, Bc, Ba-2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one (Ba-DDMP), Bb-DDMP, and Bc-DDMP was quantified in 15 samples of defatted soybean meal, two full fat soybean meals, and two soybean protein concentrates by reverse phase high-performance liquid chromatography. The total level of saponins in the 15 samples of commercial defatted soybean meal ranged from 4.8-6.8 micromol/g (5.1-7.0 g/kg). The two full fat meals contained 4.4 and 4.7 micromol/g whereas no saponins could be detected in the alcohol-extracted soybean protein concentrates. Fifteen batches of fish feed containing 20% defatted soybean meal were produced by twin-screw extrusion from the 15 different samples of defatted soybean meal. Extrusion did not reduce the total level of group B saponins, but the ratio between DDMP-conjugated group B saponins and non-DDMP-conjugated group B saponins was slightly reduced. A soybean-containing diet was fed to seawater adapted Atlantic salmon for 9 weeks. Yttrium oxide was included in the feed as an inert marker in order to estimate the disappearance of saponins during gut passage. High levels of intact non-DDMP-conjugated group B soyasaponins were found in feces whereas only low levels of DDMP-conjugated saponins could be detected. The overall disappearance of saponins was close to zero, and the concentration of intact saponins in dry feces reached levels several fold higher than dietary levels. The present work demonstrates that non-DDMP-conjugated group B soyasaponins resist extrusion cooking and remain intact during gut passage in Atlantic salmon. The latter is contrary to earlier findings in endothermic animals.

Effects of feed ingredients on nutrient digestibility, nitrogen/energy balance and morphology changes in the intestine of Nile tilapia ( Oreochromis niloticus )

“…Bone () suggested that SBM induced pathological changes in fish may be due to additive or synergistic impacts of several anti‐nutritional factors. Saponin levels in SBM range between 5 and 7 g kg ‐ 1 (Knudsen et al, ), but are very low or absent in other plant ingredients and thus insufficient to induce pathological changes in the intestine (Gatlin et al, ). This is in line with Madalla () that histopathological intestine was unaffected by morning leaf meal, cassava leaf meal and cassava root meal and Aanyu et al () that sunflower cake and cotton seed cake did not change intestinal fold length and numbers.…”

Criteria for Safe Use of Plant Ingredients in Diets for Aquacultured Fish

“…Saponins are also lost due to binding with cholesterol, forming an insoluble complex that cannot be absorbed (Malinow et al, 1977). In the only report of the fate of orally administered saponins in fish, Knudsen et al (2006) (approximately 2.6 times) of saponins in feces than in the diet, suggesting that bacterial degradation may be limited in fish. Francis et al (2005) summarise the results of several reports of the growth promoting effects of saponins in common carp and tilapia.…”

Feeds, water quality, gut morphology and digestion in Nile tilapia (Oreochromis niloticus)

“…Bone (2013) suggested that pathological changes observed in fish fed soybean meal may be due to additive or synergistic impacts of several antinutritional factors. Saponin levels in SBM range between 5-7 g kg -1 (Knudsen et al, 2006), but are absent or at much lower levels in other plant ingredients. They are usually insufficient to induce pathological changes in the gastrointestinal tract (Gatlin et al, 2007).…”