Drying food is one of the oldest methods of preserving food However this method of preservation is still very relevant in today's world With more and more dried food products coming onto the market the need for new innovative drying technologies is increasing The principle of dehydrating foods to inhibit the growth of microorganisms is the platform on which new methods such as drying using superheated steam are evolving Superheated steam drying SSD is an innovative method of processing food Its various applications mean that this drying technique has a wide range of possible opportunities and disadvantages In this report I will be looking at how this technique is used in the food industry to process food In particular I will be discussing the principle behind this technology its kinetic modelling and how superheated steam drying is now being applied to process food beyond drying it One area that has garnered a lot of interest in relation to SSD is its capacity to decontaminate certain foods such as wheat grains This review will also strive to investigate the effect of SS drying on the nutritional quality of foodstuffs SSD as an alternative to hot air drying overcomes some of the problems faced by HAD while also favouring the environment
The principle of superheated steam drying The basis of superheated steam SS is that the steam must first be saturated Saturated steam is formed when the temperature of the water is raised to its boiling point In order for the saturated steam to become superheated steam it must be heated beyond its boiling point at a correlative pressure When the steam reaches a temperature above its boiling point it is then referred to as unsaturated or superheated steam Condensation occurs in saturated steam when the temperature falls Unlike saturated steam condensation does not occur in SS so long as the temperature is kept above saturation temperature This allows for the temperature of SS to fall a little at times without condensation occurring One of the main attractions of superheated steam processing is that the water evaporated from the product is incorporated into and becomes part of the drying medium The SS is used to raise the temperature of the product being processed so that the water in it changes state from liquid to gas steam Drying is a process which characteristically requires vast amounts of energy to sustain it SSD differentiates itself from other drying methods in this regard The steam generated is useful to processing plants in other capacities
Therefore the energy consumption of SSD seems to be reduced if the processing plant can use the waste or excess steam for other purposes Typically superheated steam under atmospheric conditions of pressure has a temperature bracket of between 100oC and 150oC SS processing is most commonly used for foods which are stable at varying temperatures This includes foods with a high starch content such as banana slices soybeans rice and potato chips Nimmol et al 2007 Prachayawarakorn et al 2006 Wu et al 2016 Pimpaorn et al 2007 However SSD can also be conducted at atmospheric pressure low pressure or high pressure When using SSD for processing fruit and vegetables for example low pressure is required to prevent nutritional losses Nimmol 2007 The process of how SSD is achieved is shown in Fig 1 SSD takes place in a closed circuit system which means that the circuit is constructed as a closed loop system The closed loop system allows for the excess steam to be collected and reused in a different capacity in the plant The process is started by steam being generated in a steam generator This steam is then directed to a heating chamber where it becomes superheated steam and fed into a drying unit In the drying unit heat from the SS is absorbed by the product being dried and the resulting evaporated moisture from the product becomes part of the drying medium As shown in the schematic in Fig 1 it is then that the purged steam can be repurposed A heat exchanger can be employed to recover energy from the steam for other applications Steam which is not being used elsewhere but is only slightly superheated can also be recycled through a fan and superheated once again R Sehrawat et al Innovative
Food Science and Emerging Technologies 34 2016 285 301 287 Fig 1 Diagram of superheated steam dryer Advantages of superheated steam drying When compared with drying methods like hot air drying HAD SSD has a number of clear advantages SSD allows for energy savings because evaporated air becomes part of the drying medium in HAD moisture laden air has to be eliminated and new air must be heated to replace it The rate of evaporation of water in SSD is higher than in HAD as stated by Ezhil 2004 Later Sehrawat et al 2016 agreed with this stating that because air wasn t present in SSD there is no resistance to the evaporated material diffusing in its own vapour The benefits of SSD as drying process are very interesting Even without comparing it to other methods of drying this drying medium has a lot of potential both for consumers and in industry The oxygen free nature of SS processing reduces the risk of explosions and fires Thus making manufacturing environments safer Without oxygen oxidation cannot occur this is important for foodstuffs like fruit which can lose valuable vitamins due to oxidation Using SSD processing technology allows for subsequent processing of the product such as blanching sterilisation deodorising and pasteurising Karimi 2007
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