Ultra-high-temperature (UHT) milk is widely consumed because of its extended shelf life, the absence of refrigeration before package opening, and its suitability for long-distance distribution. However, physicochemical and biochemical changes during storage may reduce product stability and limit shelf life. Among these changes, age gelation, characterized by the irreversible formation of a three-dimensional protein network, is one of the major quality defects affecting UHT milk.



This review summarizes studies published up to April 2026 on the roles of endogenous and exogenous proteolytic enzymes, processing factors influencing UHT milk gelation, and current approaches for preventing or delaying this phenomenon. Relevant studies were identified through systematic searches of the Web of Science, Scopus, and PubMed databases.



Current evidence indicates that proteolysis mediated by the plasmin system and heat-resistant bacterial proteases, particularly AprX, is the principal mechanism underlying age gelation. Plasmin destabilizes casein micelles through the hydrolysis of β-casein and αS-caseins, whereas AprX accelerates gelation by degrading κ-casein. Raw milk quality, thermal processing, and storage conditions are key factors influencing proteolysis and gelation. Effective preventive strategies include optimizing preheating and UHT processing, maintaining high raw milk quality, preserving the cold chain, minimizing raw milk storage time, and using approved stabilizing agents when appropriate.

Overall, age gelation is a multifactorial phenomenon driven by interactions between proteolytic activity, processing conditions, and storage environment. Integrated control of these factors is essential to improve product stability, extend shelf life, and maintain the quality of UHT milk.