Introduction
Utilizing agricultural and industrial side streams offers a pathway to more sustainable salmon feed. While fisheries and aquaculture by-products are nutritionally valuable, agricultural side streams can serve as viable alternatives to traditional ingredients like soy protein concentrate (SPC) and gluten. Brewer's spent grain (BSG), the primary by-product of the brewing process, offers a potential sustainable solution for aquaculture feeds. Approximately 3.4 million tons of BSG are generated annually in the EU, with Germany accounting for about 2 million tons . Although the brewing process extracts starch, leaving residual lipids, fibers, and proteins, dried BSG contains only 20–30% protein , too low for significant inclusion in salmon diets. Further processing can yield in a high-protein BSG concentrate (HP-BSG, ~50% protein) possibly suitability in salmon feed. However, HP-BSG retains a high non-starch polysaccharide (NSP) content of approximately 26%. Dietary fiber fractions can affect intestinal peristalsis, as previously shown in ballan wrasse (Le et al., 2021). Consequently, this project aimed to analyze how HP-BSG and specific fibers (β-glucans and arabinoxylans) modulate peristalsis in the Atlantic salmon intestine.
Material and Method
Boluses of fish meal (FM), HP-BSG and SPC as well as the NSPs arabinoxylan and β-glucan were subsequently introduced into an ex vivo intestine preparation (n=6 per treatment) mounted in a Ringer's solution bath. The intestinal movements were recorded, and changes in intestinal shape analyzed over time using in-house software. Peristaltic patterns are visualized in a heatmap and were identified and counted visually. Further, the time of bolus passage was recorded. Differences in contraction frequency between treatments were analyzed using anova or non-parametric alternatives. A Kaplan-Maier equation was applied to bolus passage times with log-rank test to compare between treatments.
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Figure: Heatmap showing the different peristaltic patterns
Results and Discussion
When comparing the passage rates of HP-BSG, SPC, and FM boli no significant differences between treatments were found (log-rank test, p = 0.23). Passage time is dictated by gut contractions, which can travel in a retrograde (slowing passage) or anterograde (accelerating passage) direction . In both midgut and hindgut, anterograde and reterograde contraction frequency remained unaffected by bolus type (1-w aov, p = 0.95).
However, comparing the effects of NSPs to FM yielded unexpected results with β-glucan exhibiting a longer passage time than FM, with arabinoxylan in-between (KLM, p<0.01). This contrasts findings in ballan wrasse, where nutrient-rich boli (protein and lipid) were retained by retrograde peristalsis, resulting in longer passage times than cellulose boli . In the current study, NSP boli did not affect retrograde contractions in the hindgut (p = 0.407). Anterograde contractions increased in the midgut with FM compared to β-glucans or arabinoxylans (Kruskal-Wallis test, p = 0.001; FM vs AX, p = 0.003; FM vs BG, p = 0.004). No bolus effects were observed in the hindgut. Bolus composition did not affect contraction length (p > 0.05).
Overall, the peristaltic braking mechanism of slowing passage rates in response to nutrients was not observed in the A. salmon intestine. Conversely, FM passed faster than β-glucan. These findings suggest that in Atlantic salmon the physical attributes of the bolus may exert a greater impact on peristaltic mechanisms than its chemical composition.
Acknowledgment
This research is part of the project: "Improving salmon feed sustainability with high protein brewers spent grain", funded by the Norwegian Seafood Research Fund (FHF# 901988).
References
Karlsen, F., Skov, P.V., 2022. Review – Potentials and limitations of utilising brewer's spent grain as a protein source in aquaculture feeds. J. Clean. Prod. 357, 131986. https://doi.org/10.1016/j.jclepro.2022.131986
Le, H.T.M.D., Lie, K.K., Etayo, A., Rønnestad, I., Sæle, Ø., 2021. Physical and nutrient stimuli differentially modulate gut motility patterns, gut transit rate, and transcriptome in an agastric fish, the ballan wrasse. Plos One 16, e0247076. https://doi.org/10.1371/journal.pone.0247076
Le, H.T.M.D., Lie, K.K., Giroud-Argoud, J., Rønnestad, I., Sæle, Ø., 2019. Effects of Cholecystokinin (CCK) on Gut Motility in the Stomachless Fish Ballan Wrasse (Labrus bergylta). Front Neurosci-switz 13, 553. https://doi.org/10.3389/fnins.2019.00553
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