Abstract The physicochemical characteristics of dietary fiber help determine its functional role in the gastrointestinal tract. Specifically, hydration behavior modifies the digesta milieu and viscosity, potentially impacting nutrient utilization. However, the equilibration time required to accurately assess fiber hydration properties in vitro is not well established. The objective of this study was to characterize time-dependent changes in water binding capacity (WBC), water swelling capacity (WSC), and water holding capacity (WHC) among fiber sources differing in processing origin. Fiber sources were categorized as either intrinsic (unground soyhulls, wheat middlings, distillers dried grains with solubles, soybean meal, and oat fiber) or isolated (cellulose-fine, cellulose-coarse, extract-free cellulose, lignocellulose, enzymatically processed soyhulls, and isolated citrus pectin). Each source was evaluated for WBC, WSC, and WHC in triplicate at 2, 4, 6, 8, and 24 h using the most widely cited in vitro procedures. Data were analyzed using mixed-effects models with time, fiber type, and their interaction as fixed effects, and fiber source as a random effect. Slope and segmented-regression analyses were used to describe hydration rate and identify potential breakpoints in time-dependent behavior. Mean WBC values across all sources ranged from 2.4 to 6.7 g water per g of dry matter and increased with time from 3.2 ± 0.4 g/g at 2 h to 5.9 ± 0.6 g/g at 24 h (P = 0.04). Isolated fibers had greater WBC than intrinsic fibers (5.8 vs. 3.6 g/g; P = 0.03) and exhibited steeper hydration slopes, indicating continued water uptake beyond 8 h. Lignocellulose and enzymatically processed soyhulls displayed the greatest WBC responses, while intrinsic sources such as wheat middlings and distillers dried grains reached equilibrium earlier with minimal changes after 6 h. The WSC of sources ranged from 4.5 to 11.0 mL/g and generally increased with time (P = 0.04), with isolated citrus pectin (10.4 ± 0.8 mL/g) and lignocellulose (9.8 ± 0.7 mL/g) exhibiting the greatest swelling. Intrinsic fibers averaged 6.3 ± 0.5 mL/g and showed limited time-related increases, suggesting that intact cell-wall structures limited water expansion. The WHC varied among sources (2.8–7.1 g/g) and rose modestly over time (P = 0.07). Isolated fibers maintained higher WHC through 24 h compared with intrinsic fibers, reflecting their more open, porous structure and greater capacity for water retention. Collectively, hydration of intrinsic fibers reached equilibrium rapidly, supporting shorter incubation times; in contrast, isolated fibers continued to hydrate beyond 8 h, indicating that shorter durations may not capture their full capacity.
Ashton et al. (2026) studied this question.