Brewer's Spent Grain Flour: Chemical Composition, Functional Properties, and Influence on Gut Microbiota
Brewer's spent grain (BSG) flour, in an in vitro fermentation model with human fecal microbiota, modulated microbial composition and increased short-chain fatty acid production, favoring genera associated with gut health.
| Outcome | Grade | Direction | Effect | Studies |
|---|---|---|---|---|
| Gut microbiota composition (16S rRNA) | D | ▲ Favorable | relative abundance increase in Bifidobacterium and Lactobacillus vs negative control; no SMD or CI reported | 1 |
| Total short-chain fatty acid (SCFA) production | D | ▲ Favorable | increased vs negative control; absolute values and CI not reported | 1 |
| Post-fermentation pH | D | ▲ Favorable | pH decrease vs negative control; absolute values and CI not reported | 1 |
| Prebiotic index | D | ▲ Favorable | positive index, comparable to inulin for some parameters; no CI reported | 1 |
| Total dietary fiber content (composition) | C | — Neutral | 46.9% dry weight basis; descriptive, no comparator CI | 1 |
| Antioxidant capacity and total (poly)phenolic compounds | D | — Insufficient | 3.8 mg GAE/g total polyphenols; in vitro antioxidant assay only, no CI | 1 |
| Glucose diffusion retardation index (GDRI) | D | ▲ Favorable | GDRI > 0 vs control; absolute values and CI not reported | 1 |
Context
BSG represents 85% of brewing by-products and is largely discarded as animal feed. Its high dietary fiber and (poly)phenolic content warrants investigation as a food ingredient with potential prebiotic activity. Compositional and fermentability data are prerequisites for any clinical or industrial application.
What the study showed
BSG flour contains 46.9% total dietary fiber (dry weight basis), 24.1% protein, and 3.8 mg GAE/g total (poly)phenolic compounds. In vitro fermentation increased relative abundance of Bifidobacterium and Lactobacillus and reduced fecal pH, indicating organic acid production. Total SCFA production was elevated versus negative control, with acetate, propionate, and butyrate predominating; specific absolute values and confidence intervals were not reported. The calculated prebiotic index was positive, comparable to inulin for some parameters.
How it was done
Mixed observational/experimental study: physicochemical characterization of BSG flour (9 industrial batches, single brewery, Spain) combined with a static batch in vitro fermentation model using human fecal microbiota from healthy donors (exact n not precisely specified in available text). Fermentation was conducted for 24 hours at 37°C under anaerobic conditions; microbiota analyzed by 16S rRNA sequencing.
Effect magnitude
The study did not report standardized effect sizes (SMD, RR, OR) or confidence intervals for microbiological outcomes; differences were described relatively without robust inferential testing for all outcomes.
Limitations
Exclusively in vitro model: does not reflect gastric/intestinal digestion, absorption, or in vivo interindividual variability. BSG from a single brewery and a single malt type (Pilsen lager), limiting generalizability. Number of fecal donors and interindividual variability not detailed in the available text. No formal bias assessment tool applied (RoB 2 not applicable; ROBINS-I not used). No human or animal groups preclude direct clinical inference.
In clinical practice
Insufficient evidence to recommend BSG as a prebiotic in clinical practice. The nutritional profile (high fiber, moderate protein, phenolics) is compatible with use as a fiber-enriching food ingredient. Any application requires controlled clinical trials.
What is still missing
Randomized controlled trials in humans to confirm prebiotic effects, effective dose, gastrointestinal tolerability, and impact on relevant metabolic outcomes such as glycemia, lipids, and inflammatory markers.