Somatic Cell Count-Associated Immune Cell Shifts and Their Impact on Yoghurt Quality in Alpine × Beetal Crossbred Goats

Abstract

In goats, apocrine milk secretion sheds cytoplasmic particles and epithelial cells into milk as a normal physiological event, so uninfected udders sustain higher somatic cell counts (SCC). To characterise the immunophysiology underlying rising SCC and test whether it reaches a fermented product, forty-eight healthy early-lactation Alpine × Beetal crossbred does were stratified into six SCC classes (<4 to >12 × 10⁵ cells/mL; n = 8/class), the lowest serving as a physiologically normal reference rather than a low-mastitis group. Rising SCC brought a consistent immune cell shift: relative to the lowest class, neutrophils rose 39.4% and macrophages fell 45.0% in the highest class, raising the neutrophil-to-macrophage ratio 2.6-fold (P < 0.05), consistent with chemoattractant-driven neutrophil recruitment across the blood–milk barrier. Milk pH rose 9.8% and conductivity 24.7%, reflecting the same barrier disruption. For the yoghurt experiment, milk from the same group was pooled into low (<5 × 10⁵ cells/mL; n = 16), medium (5–10 × 10⁵; n = 16) and high (>10 × 10⁵; n = 16) SCC classes and monitored over 14 days of refrigerated storage. SCC level, storage day, and their interaction significantly affected every quality trait measured (P < 0.05). High-SCC yoghurt was already compromised at manufacture and worsened with storage: firmness was 48.8% lower and consistency 49.3% lower than low-SCC yoghurt, while cohesiveness fell 85.7%. Syneresis rose from 58.3% at Day 0 to 85.6% by Day 14, with high-SCC yoghurt shedding 15.0% more whey throughout. Despite lower acidity development (6.9% less), its pH was paradoxically higher (1.6%), pointing to a buffering effect of SCC-associated proteins rather than fermentation failure. Firmness and consistency were almost perfectly correlated (r = 0.988) and both strongly anti-correlated with cohesiveness (r ≈ −0.88), indicating one underlying loss of gel integrity. These findings implicate plasmin and somatic-cell-derived lysosomal proteases in degrading the casein network underlying yoghurt structure, showing that immune trafficking with rising SCC though physiologically distinct from bovine mastitis carries a measurable functional cost. Regular SCC monitoring is therefore as much a product-quality tool as an udder-health measure. Keywords: Somatic Cell Count; Yoghurt Quality; Goat Milk; Mastitis