The effect of ultrasounds in syrah wine quality is not dependent on the ripening or sanitary status of the grapes

Abstract

Different studies have demonstrated that the application of ultrasounds (US) to crushed grapes improves chromatic characteristics of the wines (1,2), increases their polysaccharide content (3) and some aroma compounds are also favored (4,5). The US facilitate the extractability of phenolic compounds from grapes to must-wine, due to the erosion of the skin cell walls caused by the ultrasound-generated cavitation bubbles. It is also known that phenolic extractability increases during grape ripening due to the natural degradation of the cell walls during a maturation progresses, the cell walls being thicker and more rigid in the less ripen grapes. Our objective is to determine if the application of US to grapes of different maturity level leads to the same positive outcome as regards chromatic, phenolic and aroma compounds and sensory properties. Moreover, sometimes, during the late ripening status and due to adverse climatic conditions, some level of botrytis contamination may appear in the grapes and the effect of US in this partially botrytised harvest have not been discussed. The trials were carried out with Syrah grapes at two different ripening levels (12 and 14ºBaume) and with the most ripen grapes, grapes with two different sanitary status were treated, healthy grapes and 25% botritized grapes, treating 200 kg of each type of grapes with a semi-industrial scale high power ultrasound equipment working a sonication frequency of 30kHz. Physicochemical and chromatic parameters were analyzed by spectrophotometry and high-performance liquid chromatography and volatile compounds by gas chromatography-mass spectrometry. Also a sensory analysis of the wines at the moment of bottling was conducted. The results showed that, independently of the grapes ripening or sanitary status, the application of US to crushed grapes led to wines with improved chromatic characteristics, the volatile compounds concentration being less affected, although the sensory analysis showed clear differences, the wines made from sonicated grapes being preferred and reaching higher scores in most of the sensory parameters evaluated. In conclusion, this study on a semi-industrial scale demonstrated that the application of US to Syrah grapes improves the organoleptic characteristics in wine. The positive effect also observed in the less ripen grapes lead us to consider the sonication of the grapes as a tool for reducing wine alcohol content by harvesting less ripen grapes since the lack of natural phenolic extractability may be compensated by the sonication of the grapes.