‘Continuous in-flow sterilization’, better known as UHT (Ultra-High Temperature), was first sought patented during the 1880s, long before the technology became commercially viable with the arrival of the aseptic filling machines in the late 1950s. The UHT technology can be split into direct and indirect sterilization, depending on the specific method of heating and cooling of the product.
The clear advantage of the continuous in-flow sterilization when compared to the ‘in-container autoclave principle’ is that it offers higher capacity, better control of the process and less investment along with improved capacity vs investment.
The other clear rationale of the technology is that it offers a much shorter holding time, as in comparison to the widely used autoclaves. Where the processing temperature in autoclaves is approx. 120°C, the UHT operates at approx. 140°C, which allows for holding times down to 3-5 seconds. This achieves the same bacterial and spore destruction, however with only a third of the destruction of nutrients, meaning that the products appears fresher and tastes better.
This is due to the energy transfer principles where the direct UHT plant has a near-instant heating with the addition of the steam and also a near instant cooling as the condensate is removed by vacuum following the short and well defined holding time.
The indirect system has a far longer heating and cooling time as the energy is transferred from a medium, normally water, through a thin metal wall into the product.
It is important to mention that the direct UHT plant offers the best product quality whereas the indirect UHT plant consumes roughly only half of the steam and cooling resources.
In the early days of the industrial introduction when filling machines were introduced to cold aseptic filling, it was majorly the direct systems, both injection and infusion, that were used. Also, indirect systems developed over the years from being based on plate heat exchangers to tubular heat exchangers, each and all with their own set of advantages and disadvantages.
During the latter part of the 20th century the UHT technology has been further developed, and the direct and indirect technologies have matured with a better understanding of the pros and cons for each technology. This means that, often, they are also applied in a more discreet fashion based on the product and the end-product requirements.