Microwave assisted extraction advantages have been widely proved at bench scale. However, its implementation at an industrial level is yet a task to develop, as this technology still presents some limitations that have to be overcome. The main two problems for the scaling up of a microwave process are the penetration depth and the energy consumption. The penetration depth depends on the dielectric properties of the material and the frequency of the radiation. However, this latter parameter can be considered almost fixed, as only a reduce number of frequencies are worldwide allowed for heating purposes. So, the penetration depth is a factor difficult to improve, since it will require the change of the properties of the medium (for instance, adding an ionic salt to the medium). The energy consumption of an oven cannot be avoided, but its design can be optimised to increase its efficiency.
In order to surmount these disadvantages, a microwave pretreatment has been proposed as an alternative to microwave assisted extraction. During this pretreatment, the medium is irradiated with an intense pulse of energy to disrupt the cellular structure. The process then continues with a solid-liquid extraction (shorter than the conventional one) to draw out the extractable compounds.
The microwave pretreatment takes place in a tubular duct with its dimensions in accordance with the penetration depth, so that all the material is irradiated. The energy consumption effectiveness is accomplished by a thoughtful microwave cavity design taking into account the dielectric properties of the media. In this way, it is possible to get a more homogeneous temperature distribution, avoiding so the formation of overheated regions or hotspots. Figure 1 shows the heat distribution simulation for an optimised oven to treat a mixture of grape pomace, ethanol and water. This design has been executed and the result is the equipment depicted in Figure 2. This microwave oven is able to homogeneously irradiate 20 L/h of a grape pomace-solvent mixture up to a temperature of 120ºC (4 bar) in 3 minutes. This facility is highly convenient for the extraction of polyphenols from grape pomace, as it improves the product quality and quantity as well as allows to reduce the extraction time and the solvent consumption.
Ana Álvarez – Projects WineSense and Junta Castilla y León: EDU/592/2013