ISSUE FOCUS FEED & ADDITIVE MAGAZINE November 2025 35 have been studied primarily for their potential to improve gut health, their impact on ABC-4 and acid diffusion rate is a point of consideration when integrating them into diets. Protein sources are the macro-ingredients that potentially have the largest impact on the ABC-4 value of a feed. Their higher ABC-4 values (0 to 1000, most commonly 400 to 600 meq/kg) compared to starch and fiber sources, are explained by the higher buffering capacity of peptides due to the presence of free amine groups (lysine, arginine and histidine) and free carboxylic groups (glutamic and aspartic acid). Raw materials containing the same protein type (e.g., soy protein) have higher ABC-4 values as the protein content increases (Figure 2). Different protein types (soy, potato, wheat gluten, …) with the same crude protein content have different ABC-4 values due to their different amino acid and structural composition. Research (Stas et al. 2024) indicates that some protein sources exhibit a wide range of ABC-4 values, with different batches of the ‘same’ raw material having significantly different buffering capacities (e.g., fish meal). For these, it is advisable to make an ABC-4 determination on the individual batches and not rely on literature values, especially if their inclusion levels in the feed are high. Other sources (e.g., blood plasma powder) have relatively similar batchto-batch protein levels and uniform ABC-4 values. Milk-derived ingredients vary in ABC-4 (0 to 700 meq/kg), due to differences in protein and mineral content and in the type of processing. Pure lactose has no acid buffering capacity. Milk powders and sweet whey powders have quite uniform ABC-4 values, due to their relatively constant protein level. In whey permeates, crude ash and especially phosphorus content, correlate positively with ABC-4 values. Fats have almost no acid buffering capacity when present in their natural (triglyceride) form and are of little relevance with respect to the ABC-4 calculation of a feed. Free fatty acids, either added on purpose or generated by fat source deterioration, are minor contributors of acidity (small negative ABC-4 value). THE DEVIL IS IN THE MICROS Due to their high absolute buffering capacities, some of the micro feed ingredients may have a large impact on the ABC-4 of a diet (Figure 3). Oxides, (bi)carbonates (5000 to 20000 meq/kg) and phosphates are the most relevant minerals for ABC-4. Zinc oxide (ZnO) deserves particular attention because of its very high capacity to neutralize acids and its potential to form poorly soluble zinc salts upon reaction with organic and phosphoric acids. As can be seen from Figure 1, the ABC-4 value of ZnO can be highly variable, depending on manufacturing conditions, levels and types of impurities (Image 1) and particle size. Because ZnO has a significant impact on the ABC-4 of a feed (even at low inclusion levels), it is definitely one of the raw 600 550 500 450 400 350 300 30 35 40 45 50 55 60 65 ABC-4 (meq H+/kg) Crude protein (%) Figure 2. Correlation between protein content (%) and ABC-4 value (meq H+/kg) of (unfermented) soy protein sources, showing the high contribution of protein to ABC-4 Image 1. Impurities in zinc oxide (ZnO) lead to significant differences in color and reactivity. Whereas pure ZnO is white (left), the presence of iron (middle, right) causes strong brown discoloration and hampers the reaction with acids, leading to lower ABC-4 values.
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