Polyphenols are considered as high value products by cosmetic and food industry due to, mainly, their great antioxidant power. Therefore, new extraction techniques have been investigated in recent years in order to find how to preserve the extract quality during the process. Microwave assisted extraction has been found to provide good results. It allows to reduce extraction time and the amount of solvent required. However, during this work, microwave radiation has been used as a pretreatment for a conventional process: instead of irradiating the sample up to a mild temperature (difficult industry implementation), an intensive pretreatment has been employed. In it, temperature is raised up to close to the solvent’s boiling point by a short microwave pulse; then, the sample is rapidly cooled down and undergoes a brief solid-liquid conventional extracion. An innovative aspect of this research is that the combination of microwaves and pressure has also been studied. As pressure increases, solvent boiling point rises too. This enables extraction to be carried out at a higher temperature, and in consequence, extraction kinetices are accelerated.
It has been found that a microwave pretreatment greatly enhances initial extraction velocity and yield. In the case of the total phenolic content of the extract, initial velocity is between 9 and 29 times higher with microwave (MW) and microwave-pressure (MW+P) pretreatment than conventional extraction. Also, a 18-50% richer extract is obtained by the use of these pretreatments. Similar behaviours have been observed for the main polyphenolic groups (flavonoids, non-flavonoids and anthocyanins).
To sum up, a kinetic analysis has been done over the extraction of polyphenols from grape pomace. Microwave and microwave-pressure pretreatments favour their extraction: kinetics are accelerated and the extract is able to maintain its properties due to the short processing time. Microwaves enable such a rapid heating that it prevents the sample from being exposed to high temperature for a long time, and thus, avoids the degradation of these thermolabile compounds. Therefore, a microwave pretreatment has been proven to be a better alternative rather than conventional solid-liquid process for the extraction of active compounds.
Ana Álvarez – Project WineSense