Toward online dairy milk fouling detection: A review highlighting critical gaps and future sensing insights

Dairy milk fouling remains a persistent constraint on thermal processing efficiency, product quality, and food safety. However, industrial fouling control still relies predominantly on conservative cleaning schedules and indirect process indicators that respond only after substantial performance loss. This review synthesizes current understanding of dairy fouling mechanisms, heat-exchanger configurations, and emerging online sensing strategies with explicit reference to Technology Readiness Level (TRL) progression. It first outlines how protein-, mineral-, fat-, and biofilm-derived deposits interact with indirect and direct heating geometries to create system-specific fouling loci and sensor constraints. It then critically evaluates thermal, ultrasonic/acoustic, optical, electrochemical, electrical resistance and impedance, tomography, ohmic, QCM-based, and synchrotron imaging approaches for in situ fouling detection, benchmarking each against hygienic integration, early-stage sensitivity, and compatibility with plate heat exchangers, tubular systems, and direct steam injection plants. The analysis highlights that most modalities remain confined to laboratory or pilot scale (TRL 2-4), with no single sensor alone currently capable of resolving both fouling layer growth and thermal properties under full-scale ultra-high-temperature conditions. In response, the review proposes a multimodal, tiered sensing philosophy that couples complementary modalities to deliver condition-based monitoring architectures tailored to each heat-exchanger type. By mapping critical knowledge gaps and integration barriers, this work provides a structured roadmap for advancing from proof-of-concept sensors toward robust, real-time fouling monitoring in industrial dairy processing.