Thumbnail Image

Finding Value in Grape Pomace

Thunga, Neeharika
The production of grapes is increasing all around the world. A 7% annual increase in wine grape production, and the waste products associated with wine processing, occurred in 2013 in the US. Pomace is a predominant waste product of wine processing which can be processed into value-added products. The objective of this study was to evaluate potentially valuable components of grape pomace. Vacuum steam distillation was used to obtain essential oil from grape pomaces originating from red (`Merlot') and from white (`Muscat', Riesling', `Sauvignon Blanc' and `Traminette') wine grape cultivars. Free and glycosidically-bound aromatic compounds of fresh grape pomace (prior to and after distillation) and of the distillate was evaluated using Gas Chromatography (GC). Pomace remaining was forced air dried at 400C and utilized for further determinations. Oil from separated seeds of the above varieties was determined analytically and used to compare oil yield from bulk, mechanically separated seed of `Riesling' and `Red Zinfandel' using a mechanical oil press. Separated dry pomace components, seed oils and seed meals were analyzed for phytosterols and policosanols as trimethyl-silyl derivatives by GC. Free aromatics were higher in concentration than bound aromatics in all the pomaces. Phenethyl alcohol predominated in all the grape pomaces in the free aromatic fraction. The glycosidically-bound aromatic fraction was similar in distilled and non-distilled pomaces and the distillates obtained had a similar aromatic profile to the free aromatics of the grape pomace. Phytosterols and policosanols were notably enriched in grapeseed oils with oils containing 8 to 16 times more of these compounds than grape seeds. Mechanically pressed oils only contained about 4 or 5 times more phytosterols and policosinols than the original seeds; some thermal degradation of these compounds appeared to occur, probably due to frictional heat exposure during oil pressing. Phytosterols and policosanols were higher in seeds than skins/pulp and they were notably deplenished in seed residue after oil extraction. The most predominant phytosterol was β-sitosterol; campesterol and stigmasterol were also identified in varying concentrations. Eicosanol, tetracosanol and octacosanol were the major policosanols identified in most of the samples. The overall seed content in dried grape pomace was about 50% on a dry weight basis. The oil content for the grape seeds was in the range of 10-13% and mechanical oil pressing yielded about 70 % of the total oil within the seeds (about 10% of seed weight). Merlot seeds had the highest concentration of oil (about 13%) and Traminette had lowest (10%).