Heat stability of indigenous milk plasmin and proteases from Pseudomonas: A challenge in the production of ultra-high temperature milk products

The worldwide demand for ultra-high temperature (UHT) milk products is increasing, as these products are interesting for export. Their shelf-life at ambient temperature is usually around 4–6 months; however, for products to be exported, a longer shelf-life is necessary. Spoilage of sterile UHT milk products is often caused by proteases, in particular plasmin and proteases from Pseudomonas. These proteases are highly heat-stable and are able to resist UHT heating processes. This review summarises the data on the inactivation behaviour of the plasmin system and of Pseudomonas proteases. Based on the data presented, heating processes and alternative methods are proposed that sufficiently reduce the protease activity in the final product to achieve a shelf-life of the UHT products of up to one year.