[Award]: Microwave Working Party 3rd award to “Yield vs selectivity in grape pomace polyphenol microwave extraction”

Our PhD student Ana Álvarez was awarded third place in the scholarship competition at the Third Global Congress on Microwave Energy Application, hold in Cartagena (Spain) in July 2016. This prize was sponsored by the Microwave Working Group. A guild dedicated to formulate the path for microwave technology over the next decade. One of its mission statements is rewarding students at college level for innovative or new uses of microwave technology.

In particular, this awarded research discuss the extraction of polyphenols by a microwave pretreatment that not only boosts process efficiency, but also enables a balance with final product richness. An innovative approach whose main concern has been to overcome microwave technology limitations in order to scale-up the process at an industrial level.

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Abstract—The advantages of microwave extraction against conventional processes have been already widely discussed. However, nothing has been said about extraction selectivity. In this work, short microwave-pressure pretreatments to the conventional process have been assessed. The objective is to improve both yield and polyphenol selectivity. Microwave radiation greatly accelerates polyphenol extraction, while other compounds (mainly sugars and fibres) do not present such a considerable improvement. Thus, microwaves affect differently to each type of compounds. By controlling the kinetics of the extraction, it is possible to increase yield a 57%, enhance selectivity a 32% and reduce extraction time to a fifth. Shorter extraction times avoid the massive extraction of undesired compounds and so, the polyphenol richness decay is prevented. Microwave-pressure pretreatments allow to stop the extraction at an optimal time where both maximum yield and selectivity can be achieved; whereas this is not possible in conventional extraction. In the case of anthocyanins (group of special interest due to their antioxidant properties), the microwave boost is even more pronounced, as it is possible to obtain a final product 85% richer in these valuable compounds. This improvement is shown in Figure 1A.

Regarding biofunctionality, microwave extract compositions differ from conventional extracts, and so do their antioxidant activity. In vivo assays provide different antioxidant capacities than in vitro results. The latter are almost proportional to the polyphenol concentration, whereas cellular antioxidant analyses show that microwave extracts are able to preserve their antioxidant properties for longer than the conventional does. Therefore, an interaction between polyphenols and other substances that affect their bioavailability is likely.

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Fig 1. Anthocyanin selectivity and yield kinetics for (A) microwave preteatment of 300 W during 60 s and (B) conventional solid-liquid extraction at 60ᵒC.

These results suggest the convenience of implementing this kind of technology in order to improve product quality and assure the sustainability of winery industry by exploiting a value-less residue.

Ana Álvarez

WineSense (FP7-386 MC-IAPP)

Junta de Castilla y León EDU/592/2013 Ref: VA330U13

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