The new supreme discipline: direct heating

Classic UHT (ultra-high temperature) systems, such as the VarioAsept from Krones, are equipped with a plate or tube heat exchanger for preserving products. The latter operates according to the principle of indirect heating: in a tube heat exchanger, cold product flows in an inner tube, while hot water flows in the opposite direction through a sheathed jacket tube in order to heat the product to the desired temperature. Such indirect heat exchangers constitute a readily used technology in beverage processing, particularly in terms of their investment costs and higher energy recovery.

As Stefan Höller, Head of Product Management & Development Processing Units at Krones, knows, there are also weaknesses with regard to thermal stress. While the product is only heated to the ultimate maximum temperature for a few seconds in the UHT process, "The individual pre-heating steps and heat-holding for 120 seconds at 90 degrees Celsius for protein denaturation and subsequent cooling to the filling temperature last for a total of several minutes", he explains. However, chemical processes that affect the taste begin as of around 60 degrees Celsius. Höller: "It is therefore imperative to keep this period of time as short as possible." So the alternative for demanding products such as plant-based drinks or milk beverages is direct heating. In this process, heating and cooling take place within just a few seconds.

Krones has developed a separate module for this: the VarioAsept D's heat-holding phase only lasts for a few seconds, too, but time-intensive pre-heating in the critical temperature range can be extensively forgone in this case. The Krones process technology team already showcased the direct heating module at Anuga FoodTec back in 2022. It is now a firmly established part of the Neutraubling-based system manufacturer's portfolio. The method is not only suitable for milk and plant-based drinks, but also for products such as cream or ice-cream mix, which are more difficult to heat due to their higher viscosity or their high dry matter content.

The VarioAsept D operates using either the injection or the infusion method as the customer so chooses. Both methods use steam as the heating medium after the product has first been pre-heated to approximately 80 degrees Celsius. In the injection method, an adjustable nozzle is used to inject the steam directly into the product, abruptly heating it up to 145 degrees Celsius. This temperature is maintained for around five seconds, making the method particularly suitable for low-viscosity products such as milk and mixed milk beverages. Its advantages include lower investment costs and a smaller footprint for the corresponding module. Conversely, infusion enables the product to be heated virtually in free fall. In this process, it falls in a controlled manner through a steam-filled infusion chamber, during which it is heated up to the required target temperature of 145 degrees Celsius. The ingenious part is that the product does not come into contact with the hot stainless steel surfaces, as a result of which its taste is barely affected. The product is also degassed at the same time. Infusion is therefore regarded as the supreme discipline for demanding products with high quality requirements and is optimally suited for plant-based drinks, which are difficult to heat.

"Because we are able to offer VarioAsept D as a separate module, our customers are supplied with an end-to-end solution from a single source", emphasises Höller. Krones' inventiveness is also reflected in the solution, because "our engineers have improved and rethought numerous details in order to enable the energy-efficient production of high-quality products." The development of VarioAsept D was focused first and foremost on energy efficiency, because the method requires a comparatively high amount of energy to raise the product temperature from 80 degrees Celsius to 145 degrees Celsius in a matter of seconds.

Of course, the topic of energy recovery in direct heating is not a fundamentally new one in the beverage sector, but many approaches have so far failed because of the low temperature level of the available waste heat. The Krones team therefore revolutionised the core element of the VarioAsept D – the direct heating process itself – for the energy saving concept. What is called 'high gravity heating' was implemented based on the 'high gravity brewing' process that has proved its worth in brewing technology. In this, the product, i.e. the oat drink or the mixed milk beverage, is fed in concentrated form into the direct heating system. This means that the input product's water content is lower than that of the end product. The secret of this concentration process is revealed during subsequent cooling. In order to obtain the desired end product, the steam introduced during direct heating is removed again from the product in equal parts in the conventional process. Unlike in high gravity heating, in which water has to be subsequently added to the previously concentrated product. Krones solves this by simply leaving a defined part of the steam in the product during cooling. This therefore enables a reduction in the amount of energy required during both direct heating and in upstream processes such as mashing, for instance.

The benefits of this concept to food producers can be found at multiple points throughout the process. For instance, the end product itself can be used as a heat source, as a result of which the separate addition of steam in the heat exchanger can be forgone. Due to the higher temperature at which the product emerges from the flash cooler, the oat drink or the mixed milk beverage itself can serve as an energy source by using it in the counter-flow principle of the heat exchanger to pre-heat the product that has not yet been heated. Additional steam can therefore be forgone for this production step. The product output temperature of 80 degrees Celsius no longer has to be achieved during cooling. Instead, it is sufficient to lower it to just 95 degrees Celsius, as a result of which the cooling effort in the condenser is reduced. The temperature of the cooling water from the flash cooler can therefore be increased by up to 15 degrees Celsius. This considerably higher temperature level enables the upstream processes to be supplied in part with the released energy. The waste heat merely has to be stored temporarily in a buffer tank.

"We have succeeded in optimising the proven processes so that the available energy streams are intelligently recycled", explains Dr Thomas Oehmichen of the Krones Units and Components Product Development Team. "We're looking at the bigger picture, beyond the boundaries of the VarioAsept D direct heating system, because we are using hotter cooling water for mashing, for instance." This holistic approach enables the entire oat or mixed milk beverage production process to be optimised in terms of its energy use. High gravity heating is one example of how concepts that have been established for years in the brewing industry can be used in the liquid food sector. And beverage and liquid food companies are also benefiting from this innovativeness. This energy saving concept is enabling them to reduce the amount of energy needed to manufacture their products and therefore contribute to more sustainable production.