Varietal differences in nutrient, amino acid and mineral composition and in vitro rumen digestibility of grape (Vitis vinifera) pomace from the Cape Winelands vineyards in South Africa and impact of preservation techniques

Grape pomace, a by-product of the winemaking industry, is a potential source of animal feed but its nutrient and chemical composition is not sufficiently studied in South Africa. The current study investigated the changes in nutrient, amino acid and mineral composition, and in vitro digestibility of neutral detergent fiber of grape (Vitis vinifera var. Pinotage, Shiraz and Sauvignon Blanc) pomace after three drying treatments: sun (7d), freeze (72 h) and oven drying (72 h at 60 °C). Oven-dried Shiraz had the greatest dry matter, crude protein and the least ash content (P < 0.05), while sun-dried Shiraz had the highest neutral detergent fiber, acid detergent fiber and acid detergent lignin compared to other grape pomace treatments (P < 0.05). Freeze-dried Sauvignon Blanc had the highest starch content, whereas freeze-dried Shiraz had the highest ether extract content relative to other grape pomace treatments (P < 0.05). The in vitro digestibility of neutral detergent fiber at 24- and 48 h were highest in freeze-dried Pinotage followed by freeze-dried Shiraz and Sauvignon Blanc treatments (P < 0.05). Freeze-dried Pinotage had the majority of amino acids in highest concentration, followed by freeze-dried Shiraz (P < 0.05). Freeze- and oven-dried Pinotage exhibited higher contents of potassium, magnesium, sulfur, sodium, iron and aluminum than other treatments. Overall, sun- and oven-dried Shiraz had the higher dry matter, crude protein, neutral detergent fiber, acid detergent fiber and acid detergent lignin contents, freeze- and oven-dried Pinotage had the best mineral composition, while freeze-dried Pinotage had the best amino acid profile and in vitro digestibility of neutral detergent fiber at 24 and 48 h.