Asymmetrical flow field-flow fractionation with online multidetection is aviable tool to investigate colored red wine colloids

Despite its relevance for wine quality and stability, red wine colloids have not still been suffi ciently investigated, an occurrence due to the lack of suitable analytical techniques to study them as they are present in wine.Recently, asymmetrical Flow Field-fl ow Fractionation (AF4) with online multidetection has been tested as a new analytical tool to thisaim, revealing its suitability for quantifi cation, fractionation, and characterization of wine colloids in native state [1]. With the aim tocharacterize red wines in relation to their colloidal composition, AF4 technique was applied to 24 monovarietal Italian wines kept inbottles for 2 years and produced without any fi ltration, oak contact, fi ning treatments, malolactic fermentation or ageing on yeast lees.AF4 analysis allowed to quantify wine colloids, and to characterize them in terms of dimensions (by MALS) and absorbance (A280 & A520 nm). MALS revealed that each wine contained several colloids’ populations of diff erent sizes (from 10 to 130 nm), butmost of them showed sizes in the range 20 – 40 nm. The comparison by AF4 analysis of the A280-absorbing species present in wholewines with that of wines containing only species larger than 5 kDa (which were considered as colloids) allowed to calculate for eachwine the percentage of molecules involved in the assembly of colloidal particles. This calculation showed that in the diff erent samplesthe percentage of colloids varied from 1 to 44% of the total A280 absorbing compounds, indicating the diversity of the wines. Giventhat the A280 signal is mostly due to phenolics and proteins, these data indicate that very diff erent percentages of these compounds participate in the formation of particles in the 20 – 40 nm sizerange. This means that phenolics necessarily need to be associated with other wine components to form particles of those dimensions.This association should involve proteins and polysaccharides [1]. The A520 data indicated the presence of pigments in the colloidalfraction. These pigments are likely to be constituted of tannin-anthocyanins complexes (polymeric pigments). Therefore, given theabsence of species with sizes <20 nm, an association of these colored complexes with other colloidal-forming compounds seemsnecessary, the obvious candidate being proteins as they are known to strongly interact with tannins. Our results suggest that the colorof red wines is due, in addition to free oligomeric pigments, also to colloidal particles formed by these latter bound to proteins, andthat the quantity of these particles is highly variable in wines from diff erent origin. How the presence of proteins aff ects the stabilityand evolution of red wines’ color remains to be investigated, keeping into consideration also the contribution of wine polysaccharides, which have been previously found to be part of the red wine colloidal particles.