While domestic microwave ovens can be found in most households nowadays, medium- and large-scale microwave systems are also widely used in the industrial processing of food. In the following, we are listing a few out of the many possible applications:
A widely used application of microwave systems in the food industry is the defrosting of food stuff, typically for food like meat, fish, poultry, vegetables etc, also known as "tempering" or "thawing". In order to conserve food and to extend the shelf life, a large number of different food types are kept in cold rooms at temperatures as low as - 40 ℃. However, before the food can be processed after the removal from the cold room it needs to be defrosted, but conventional methods like defrosting with hot air or water do not only take a long time, because energy transfer can only take place via thermal conduction, but also might negatively impact the food quality due to local overheating, water absorption etc. Microwave defrosting systems, on the other hand, offer much shorter thawing times because the microwave energy is able to penetrate the block of frozen food, heating up a much larger volume at the same time, thus cutting down the required time considerably.
Depending on the type of food and the size of the blocks to be tempered, microwave defrosting systems can be designed to either operate in batch mode, similar to a conventional oven, where one block of food is thawed at a time, or in continuous mode, whereby the frozen food is placed on a conveyor and then transported through a tunnel while being exposed to the microwave energy. If required, PLC systems can control the energy profile during the thawing process, maintaining an optimum balance between short thawing times and careful handling of the food.
Microwave systems are also very suitable for the pasteurisation and sterilisation of different types of food. In contrast to conventional systems, microwave systems offer shorter exposure times to high temperatures, even temperature distribution, conservation of vitamins and other important nutritions, while at the same time being highly effective in killing bacteria and other harmful micro-organisms. Depending on the type of food to be processed, two different systems are employed:
Liquid food like milk, fruit juices, edible oil etc is pumped through a specially designed applicator, where it is rapidly and evenly heated due to absorption of microwave energy. Depending on the actual process requirements, it might be held at the specific temperature for a certain period of time, before if enters a counter-flow heat exchanger where it transfers most of its thermal energy to the incoming liquid food, thus making the whole process highly energy efficient. The design of the plants does not only guarantee exact temperature and time profiles, but also short start-up times and a fast and safe wash-down at the end of the operation. Depending on the food to be pasteurised / sterilised, processing under increased pressure is available as an option.
Food which is too solid to be pump through an applicator is usually processed on a conveyor-type continuous microwave tunnel. While the food can be processed before packaging, the preferred way is to pack it first and then pasteurise / sterilise the already packed food, as this avoids any problems which might arise from contamination with micro-organisms during the final packaging stage. However, it should be noted that not all types of packaging are suitable for processing, packaging containing metal (e.g. metal tins, laminated packaging materials containing metal foils etc) would reflect the microwave energy and shield the food from it, making it unsuitable for this type of processing.
Please note that this process is also suitable for certain types of liquid food, e.g. packed drinks, as it can increase the shelf life, however the same limitations as above apply to the type of packaging.
The use of microwave systems for the baking and post-baking of food products offers several advantages, mainly very high energy efficiency, fast energy transfer, elimination of bacteria and mould spores to extend the shelf life, and others. Main applications include:
Due to the advantages listed above, industrial microwave baking is widely used, e.g. for the baking of bread, biscuits, cookies, cakes, pound cakes etc. The use of microwave energy increases the output and decreases the energy consumption, making it possible to offer the final product at a more competitive price. However, it should also be noted that baking via microwave energy alone differs considerable from the conventional baking process, where energy is only transferred to the surface of the product, mainly via radiation and convection, and for certain products also via conduction. For some products it is therefore necessary to combine different types of energy transfer, e.g. microwave and radiation via infrared heating, to maintain the look, feel and texture of the final product. Still, the usage of microwave energy will lead to the advantages mentioned above.
Post-baking mainly refers to the drying of the baked product to extend the shelf life. Because microwave energy is capable of penetrating the product, drying takes place much faster and with a higher efficiency than in conventional drying systems, making it the preferred choice. An added advantage is the capability of microwave energy to destroy bacteria, mould spores and other micro-organsims, further extending the shelf life of the final product, even under unfavourable conditions. Furthermore, drying via microwave energy can reduce or eliminate unwanted side effects, like cracks showing up on biscuits and cookies after the post-baking process.
An application closely related to post-baking is the drying of pasta, noodles and other food products which are traditionally difficult to process. As an example, when drying pasta with hot air alone, the moisture from the surface of the pasta is removed very quickly and efficient. However, because moisture migration from the inner part of the pasta to the surface is a very slow process, drying efficiency drops very quickly and reaching the final moisture content takes a long time. However, if a combined hot air / microwave drying system is used, the microwave energy penetrates the pasta and increases the vapour pressure inside, forcing the moisture content to migrate to the surface. Here it is quickly picked up by the hot air, leading to a much higher efficiency and a strongly reduced drying time.
Puff-drying is a special way to conserve food, mainly used for fruits. The fruits are first processed into slices or cubes of appropriate size, and are then loaded into a vacuum chamber. Once the Vacuum reaches the required level, microwave energy is introduced into the vacuum chamber, heating and evaporating the water contained inside the fruits. Because the drying takes place under vacuum, the boiling point of the water is strongly reduced, commonly used vacuums result in a boiling point of around 30 - 40 ℃, thus protecting the vitamins and other important nutritions of the fruits. Another effect of applying microwave energy under vacuum is that the fruit pieces become "puffy", i.e. because the reduced ambient pressure and the rapidly evaporating water they grow in size and get a spongy texture.
Puff-dried fruits don not only taste soft and crispy and at the same time, but also preserve most of the original vitamins and nutrients. Due to the low moisture content they also offer a very long shelf life as an added advantage.
Should you require any further information regarding the above applications please contact us.