Brewers’ spent grains (BSGs) represent a significant by-product of beer production, yet their detailed compositional variation across brewery scales and beer technologies remains insufficiently explored. This study systematically characterized eleven BSG samples from Latvian breweries of diverse capacities and beer styles, assessing macronutrient profiles, amino acid content, phenolic compounds, and antioxidant activity. The results revealed considerable compositional heterogeneity: the moisture content ranged 71.9–83.9%, crude protein 17.8–30.83%, lipids 0.68–5.44%, and dietary fiber 59.61–71.64% of dry weight. All samples contained a complete set of essential amino acids, and the highest total phenolic content was observed in dark beer BSGs, which was up to 1.6 times higher than in light beer. Notably, a significant negative correlation was found between ABTS antioxidant activity and total phenolic content, indicating complex matrix interactions. These findings demonstrate a marked variation in BSG composition and underscore the impact of brewery parameters on nutritional and functional values, supporting efficient, tailored valorization strategies for functional food production.

Amylase activity in rye flour plays a crucial role in the production of rye bread. When preparing a scald in rye bread production, diastatic rye malt is utilized to augment the amylase activity of the rye flour. This study investigated the effects of the diastatic power (DP) and concentration of rye malt on the Falling Number (FN) and the rheological properties of rye flour. Additionally, it examined reducing sugars in the scalding process and fermentation. Mixolab results provided comprehensive data on dough properties at different temperature stages, highlighting significant changes in starch gelatinization and enzyme activity due to varying malt diastatic power and concentrations. The decline in the gelatinization index (C3-C2) indicated faster starch gelatinization with increased diastatic power. Adding rye malt significantly increased maltose content in the saccharified and fermented scald, promoting a favorable environment for lactic acid bacteria and yeasts. FN and Amylograph results showed that less active malt (DP 170, 179 °WK), at a 1.5% concentration, could achieve similar effects as the more active malt (DP 362, 408 °WK) at 0.5%. Adding rye malt to rye flour allows for the regulation of the flour’s rheological properties and FN, adjustable based on malt DP and concentration.