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Ultra-high temperature (UHT) technology
Ultra-high temperature (UHT) process is still an essential step in dairy productions to increase the product’s shelf life by inactivating pathogenic or spoilage microorganisms, microbial developments and unwanted enzymes, and improving the sensory properties of dairy products. It also still presents the most efficient aseptic solutions ensuring UHT dairy products with longer-lasting quality and safety.
Nowadays, there has been an increase in global consumption by a growing population of consumers who have appreciated UHT-based dairy products’ benefits, such as their convenience and high quality and safety. Simultaneously, the increasing demands and fast progress on dairy product development propel the need for innovation in UHT processing technology.
The efficiency of the UHT treatment must be analysed through before or after sample characterisation and testing. In this way, an effective UHT process can be designed while retaining the original quality or even improved them. An inline measurement system is often desirable as it allows real-time and continuous monitoring of processes.
Advanced sensor technologies can be integrated into the flow-through processing line to help in continues monitoring.
State-of-the-art UHT process technology
UHT treatment subjects the dairy food samples to a high temperature (~135-155°C), heating in a short time regime (one to few seconds), enough to kill many bacterial endospores, and then followed by rapid cooling in a continuous-flow system. It is different from the conventional in-container sterilisation processes as the heating and cooling take place relatively quicker in a UHT process to avoid heat penetration problems. The continuous process also provides uniform product quality, which does not depend on the size of a vessel. These attributes are essential for dairy products containing heat-sensitive ingredients (particularly protein-based) and highly viscous food samples with poor heat transfer properties.
State-of-the-art UHT process technology
Why the need for UHT processing?
Storage of dairy products for a very long time can result in many chemical and physical changes. The major instabilities, such as age gelation, sedimentation, and creaming, end the shelf-life of the products immediately. There are ways to trigger such destabilisation processes during storage or length series of transport (in global exportation), for example, fluctuation of external conditions (pressure and temperature). In most general cases, these unwanted processes are often catalysed by the presence of microbes (e.g. Geobacillus stearothermophilus) and enzymes (e.g. Plasmin). In this case, the major purpose of UHT is to transform one product into a “commercially sterile” product where it also inhibits bacterial growth at ambient storage conditions. Sterilised product is important under aseptically packaged conditions that they should be resistant to spoilage for very long periods (around nine months or longer).
The current and future challenges.
Although it has been successfully applied for almost a century, certain aspects still present challenges for UHT processors. Existing challenges include the inevitable and undesirable alteration of sensory and nutritional properties of dairy products, limited applicability to hard cheese products due to deterioration of rennet coagulation and curd grain dehydration, and formation of fouling layers on heat exchangers. There are also adverse economic and environmental impacts in implementing UHT technologies in dairy plant productions. Mainly in the direct heating system, thermal generation and exergy destruction are costly. A large amount of water and detergent is also required in the process.
Direct versus indirect UHT system
Furthermore, due to intensive research in food research, there has been an exponential discovery of a new type of dairy food samples to sustain global demands and consumer preferences. Much of the new samples carry the risk of introducing unknown heat-stable microbial spores or heat transfer properties and thermal stabilities of ingredients. Such more considerable diversity presents new challenges for the manufacturers because of the requirements of new processing conditions or different UHT treatments while ensuring the benefits of high thermal treatment.
Recent advancements in UHT process technology focus on the engineering improvements and reconfigurations of the existing technology, its integration with packaging and filling technologies, and newly designed prototypes to extend applicability towards a new type of dairy food samples and reduce economic costs.
Features such as fine control of heating for less than a second is also an essential consideration for developments. Ideally, to have a highly efficient bactericidal effect, a holding time of 0.05 seconds is required. So far, a UHT technology capable of heating up to 160°C with less than a second holding time has been currently available. Such extreme fast heating enables a high level of vitamins to be retained in the product with minimum protein denaturation.
Integration of physical (temperature, pressure and flow-rate) measurement or indicators onto the processes is necessary for improving the UHT processing technology. The aim is to achieve to monitor and optimise the heat transfer process. These potentially enable better process safety and reduce maintenance costs due to surface fouling, overheating, and other lethal effects on the plant. Multiple temperature sensors and their management systems with very fast response times have been developed to precisely control constant temperature holding and temperature homogeneity. A pressure sensor is also installed to measure the pressure inside the closed system, which can be built up at a very high heating regime. Current integrated sensor systems allow simultaneous measurement of pressure and temperature. A magnetic flowmeter to measure the flow and velocity of the milk is necessary to guarantee the efficiency of heat transfer during the dynamic flow. These individual indicator components should have the flexibility to place anywhere on the plant and withstand the operational temperature.
A thermo-economic analysis is often helpful in designing these integrated systems and new prototypes to achieve the desired performance and characteristics of the UHT processor at an economically inexpensive cost and environmental impact.
UHT milk processing technologies are still the most suitable treatment for the commercial production of dairy products. It has improved dairy products giving consumers worldwide a convenient, tasty and nutritious liquid dairy milk option. Besides, the key to effective UHT treatment is to find the right balance between optimal temperature and holding time, the ‘kill rate’ of the microbes, and the preservation of desirable product characteristics. Then, the UHT heating process can be designed according to the defined characteristics of the product to be heated, which leads to achieving the desired results.
Anema, S. G., Age Gelation, Sedimentation, and Creaming in UHT Milk: A Review. Comprehensive Reviews in Food Science and Food Safety 2019, 18 (1), 140-166.
Chavan, R. S.; Chavan, S. R.; Khedkar, C. D.; Jana, A. H., UHT Milk Processing and Effect of Plasmin Activity on Shelf Life: A Review. Comprehensive Reviews in Food Science and Food Safety 2011, 10 (5), 251-268.
Deeth, H., 13 - Improving UHT processing and UHT milk products. In Improving the Safety and Quality of Milk, Griffiths, M. W., Ed. Woodhead Publishing: 2010; pp 302-329.
Eisner, M. D., Direct and indirect heating of milk – A technological perspective beyond time–temperature profiles. International Dairy Journal 2021, 122, 105145.
Lee, A. P.; Barbano, D. M.; Drake, M. A., The influence of ultra-pasteurisation by indirect heating versus direct steam injection on skim and 2% fat milks. Journal of Dairy Science 2017, 100 (3), 1688-1701.
Singh, G.; Singh, P. J.; Tyagi, V. V.; Pandey, A. K., Thermal and exergoeconomic analysis of a dairy food processing plant. Journal of Thermal Analysis and Calorimetry 2019, 136 (3), 1365-1382.
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