Grape Seed Nitrogenous Components and Possible Contribution to Wines

Abstract

Seeds of 25 grape varieties were isolated and the extractability of total nitrogen, amino acids, proteins and phenols from the seeds was investigated. Whole or ground seeds of five selected varieties — Chardonnay, Riesling, Koshu, Cabernet Sauvignon, and Muscat Bailey A — were extracted at 15°C, 20°C, and 25°C for several hours to 10 days using water, aqueous ethanol, and wine-like model solutions. Fresh whole seeds and an extracting solution were mixed at a ratio similar to the ratio of seed weight to juice weight in each grape variety and free amino acids, proteins and peptides were determined. The protein extracted was a small part of that present and this is attributed at least partly to the high tannin (protein precipitant) present in the seed extracts. The total nitrogen and free amino acids, as well as phenols, extracted from seeds increased with increasing ethanol concentration in the range of 0 to 40%, while proteins decreased. As the temperature increased, total nitrogen was extracted more, but free amino acids and proteins varied with the variety. On the fifth day the average contents of total nitrogen, free amino acids, and proteins extracted for the five varieties were 0.29, 0.35, and 0.41 mg/g of fresh seeds into a model solution containing 5% ethanol, and 0.80, 1.43, and 0.84 mg/g into 40% ethanol (both at pH 3.0), respectively. The proteins extracted were rich in acidic amino acids, and their amino acid compositions varied appreciably with the ethanol concentration. The possible significance of these results regarding wine composition, stability, and flavor is discussed.