Improving the nutritional value of rapeseed cake and wheat dried distillers grains with solubles by addition of enzymes during liquid fermentation

• The enzyme mixture GluXylPec gave a decrease in NSP and increased protein solubility in RSC.
• No clear effect was found of the enzymes tested on wheat dried distillers grains with solubles.
• The most pronounced effect of enzyme addition was observed in the initial phase of fermentation.

An increased production of biofuel during the last decade with a subsequent increased amount of co-products has led to an interest in utilizing these co-products as inexpensive alternatives to traditional ingredients for pig feed. Biofuel co-products are considered valuable sources of protein but the high amount of fibre resuts in low digestibility in pigs. An in vitro study was conducted to screen several enzyme mixtures, primarily carbohydrases, for their ability to solubilise non-starch polysaccharides (NSP) and protein in rapeseed cake (RSC) and wheat dried distillers grains with solubles (wDDGS) during fermentation. During the first 48 h of the experiment, denoted the initial phase, the mixture was fermented and samples were collected at 0, 24 and 48 h. Then, backslopping was performed every 24 h, where 50% of the mixture was replaced with fresh feed, water and enzyme, until a steady state cycle was reached. Samples were collected at 0, 6, 12 and 24 h after the last backslopping. Results on RSC showed a decrease during the initial phase of fermentation in the amount of total NSP (−31%, P < 0.001) and insoluble NSP (−42%, P < 0.001) in fermented RSC treated with the enzyme mixture composed by β-glucanase + xylanase + pectinase (GluXylPec). Carbohydrates released from the GluXylPec sample of RSC were fermented by the microorganisms present in the mixture, resulting in elevated amounts of lactic acid during the steady state phase (P ≤ 0.05) and acetic acid from 24 h of incubation to the end of the study (P < 0.01), and reduced pH levels from 24 h to the end of the study (P ≤ 0.01) compared to the Control were measured. Protein solubility in RSC increased in samples fermented with GluXylPec (+14% during the initial 48 h, P < 0.05). Samples fermented with phytase (Phy) showed a higher protein solubility, too (+18% during the initial 48 h, P < 0.01); and addition of phytase clearly reduced the level of phytate-P in RSC. Although all the enzymatic treatments showed a small reduction of total NSP content in wDDGS in the initial phase (P ≤ 0.01), and three enzymes [GluXylPec (6), CelXyl (9), and Xylmix (7)] reduced the level of soluble NSP in the initial phase (P ≤ 0.05), no increase in protein solubility or organic acid concentration was detected as a result of enzyme addition. For both fermented products, the biggest effect of enzyme addition was observed in the initial phase of fermentation. In conclusion, addition of β-glucanase, xylanase and pectinase during especially the first 48 h of liquid fermentation can be a strategy to disrupt NSP and increase protein solubility in RSC. The overall impact of enzymes on wDDGS was more ambiguous, though reduction of total NSP was observed for all enzymes. Addition of phytase resulted in an almost complete reduction of phytate-P in RSC during fermentation.