Abstract
With a yearly production of about 39 million tons, brewer’s spent grain (BSG) is the most abundant brewing industry byproduct. Because it is rich in fiber and protein, it is commonly used as cattle feed but could also be used within the human diet. Additionally, it contains many bioactive substances such as hydroxycinnamic acids that are known to be antioxidants and potent inhibitors of enzymes of glucose metabolism. Therefore, our study aim was to prepare different extracts—A1-A7 (solid-liquid extraction with 60% acetone); HE1-HE6 (alkaline hydrolysis followed by ethyl acetate extraction) and HA1-HA3 (60% acetone extraction of alkaline residue)—from various BSGs which were characterized for their total phenolic (TPC) and total flavonoid (TFC) contents, before conducting in vitro studies on their effects on the glucose metabolism enzymes α-amylase, α-glucosidase, dipeptidyl peptidase IV (DPP IV), and glycogen phosphorylase α (GPα). Depending on the extraction procedures, TPCs ranged from 20–350 µg gallic acid equivalents/mg extract and TFCs were as high as 94 µg catechin equivalents/mg extract. Strong inhibition of glucose metabolism enzymes was also observed: the IC50 values for α-glucosidase inhibition ranged from 67.4 ± 8.1 µg/mL to 268.1 ± 29.4 µg/mL, for DPP IV inhibition they ranged from 290.6 ± 97.4 to 778.4 ± 95.5 µg/mL and for GPα enzyme inhibition from 12.6 ± 1.1 to 261 ± 6 µg/mL. However, the extracts did not strongly inhibit α-amylase. In general, the A extracts from solid-liquid extraction with 60% acetone showed stronger inhibitory potential towards a-glucosidase and GPα than other extracts whereby no correlation with TPC or TFC were observed. Additionally, DPP IV was mainly inhibited by HE extracts but the effect was not of biological relevance. Our results show that BSG is a potent source of α-glucosidase and GPα inhibitors, but further research is needed to identify these bioactive compounds within BSG extracts focusing on extracts from solid-liquid extraction with 60% acetone.
Highlights
In recent decades, there has been growing interest in the valorization of agri-food waste and agricultural by-products as a way of achieving sustainable food production
Our study aim was to prepare different extracts—A1-A7; HE1-HE6 and HA1-HA3 (60% acetone extraction of alkaline residue)—from various brewer’s spent grain (BSG) which were characterized for their total phenolic (TPC) and total flavonoid (TFC) contents, before conducting in vitro studies on their effects on the glucose metabolism enzymes α-amylase, α-glucosidase, dipeptidyl peptidase IV (DPP IV), and glycogen phosphorylase α (GPα)
Our results show that BSG is a potent source of α-glucosidase and GPα inhibitors, but further research is needed to identify these bioactive compounds within BSG extracts focusing on extracts from solid-liquid extraction with 60% acetone
Summary
There has been growing interest in the valorization of agri-food waste and agricultural by-products as a way of achieving sustainable food production. Some cinnamic acid derivatives were shown to be potent inhibitors of α-glucosidase from yeast and rat in vitro [15,16,19], and of pancreatic porcine α-amylase [17] These enzymes play important roles in digestion of nutritional polysaccharides; their inhibition reduces glucose liberation and lowers blood glucose levels. Strong antidiabetic effects were observed in another 30-day feeding study with 50 mg ferulic acid/kg body weight in type 2 diabetic rats, and it was shown that the treatment affected the activity of various glycogen metabolism enzymes including glycogen synthase (GS), glucokinase (GK), and glycogen phosphorylase (GP). The total phenolic (TPC) and total flavonoid (TFC) content of each extract was determined by photometric methods and their ability to inhibit the digestive enzymes α-amylase and α-glucosidase, DPP IV (an indirect modulator of insulin secretion), and the glycogen metabolism enzyme GPα was investigated in vitro, representing some parts of the human glucose metabolism
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