- The objectives of this project were to investigate the chemical composition of five varieties of wine grape pomace (WGP) skins obtained in the Pacific Northwest of the United States with the emphasis on dietary fiber (DF) and polyphenolic compounds, and further to evaluate the feasibility of developing WGP extract based edible films in terms of their physicochemical, nutritional and antimicrobial properties. Research studied two varieties of white WGP (WWGP) skins (vinifera L. cv. Morio Muscat and cv. Muller Thurgau) and three varieties of red WGP (RWGP) skins (Vitis vinifera L. cv. Cabernet Sauvignon, cv. Pinot Noir and cv. Merlot). DF was measured by gravimetric-enzymatic method with sugar profiling by HPLC-ELSD. Soluble polyphenols were extracted by 70% acetone/0.1% HCl/29.9% deionized water and measured spectrophotometrically.
The major composition of insoluble DF (IDF) were Klason lignin (7.9-36.1% DM), neutral sugars (4.9-14.6% DM), and uronic acid (3.6-8.5% DM), which weighed more than 95.1% of total DF in all five WGP varieties. RWGP was significantly higher in DF (51.1-56.3% DM) than those of WWGP (17.3-28.0% DM), but lower in soluble sugar (1.3-1.7% DM in RWGP vs. 55.8-77.5% DM in WWGP) (p<0.05). RWGP contained larger amount of soluble pectin (water soluble, chelator soluble and hydroxide soluble) (5.1-5.6% DM) compared with that of WWGP (3.2-4.1% DM). Compared with WWGP, RWGP had higher values in total phenolics content (21.4-26.7 mg GAE/g DM in RWGP vs. 11.6-15.8 mg GAE/g DM in WWGP) and DPPH radical scavenging activity (32.2-40.2 mg AAE/g DM in RWGP vs. 20.5-25.6 mg AAE/g DM in WWGP) (p<0.05). The total flavanol and proanthocyanidin contents were ranged from 31.0 to 61.2 mg CE/g DM and 8.0 to 24.1 mg/g DM, respectively for the five WGP varieties. This study not only demonstrated that the skins of WGP can be ideal sources of DF rich in bioactive compounds, but also built up the baseline data for developing innovative utilizations of both red and white wine grape pomace skins.
Because of its highest amounts of water soluble pectin (4.5 mg GUAE/g DM) and total phenolic content (25.0 mg GAE/g DM) among five WGP varieties, RWGP (cv. Merlot) was selected to develop WGP extract based films accompanying the study on their physicochemical, nutritional and antibacterial properties. Pomace extract (PE) was obtained by hot water extraction and had a total soluble solid of 3.6% and pH 3.65. Plant based polysaccharides, low methxyl pectin (LMP, 0.75% w/w), sodium alginate (SA, 0.3% w/w), or Ticafilm® (TF, 2% w/w), was added into PE for film formation, respectively. Elongation at break and tensile strength of the films were 23% and 4.04 MPa for TF-PE film, 25% and 1.12 MPa for SA-PE film, and 9.89% and 1.56 MPa for LMP-PE film. Water vapor permeability of LMP-PE and SA-PE films was 63 and 60 g mm m⁻² d⁻¹ kPa, respectively, lower than that of TF-PE film (70 g mm m⁻² d⁻¹ kPa) (p<0.05). LMP-PE film had higher water solubility, indicated by the haze percentage of water after 24 h of film immersion (52.8%) than that of TF-PE (25.7%) and SA-PE (15.9%) films, and also released the highest amount of phenolics (96.6%) than that of TF-PE (93.8%) and SA-PE (80.5%) films. PE films showed antibacterial activity against both Escherichia coli and Listeria innocua, in which approximate 5 log reductions in E. coli and 1.7-3.0 log reductions in L. innocua were observed at the end of 24 h incubation test compared with the control. In conclusion, wine grape pomace extract based edible films with the addition of a small amount of commercial polysaccharides showed attractive color and comparable mechanical and water barrier properties to other edible films. The films also demonstrated their antioxidant and antimicrobial functions.
The results from this study provided guidance on the utilizations of WGP skins based on their chemical compositions, and also demonstrate the possibility of developing innovative packaging materials using WGP skins extracts that may be used as colorful wraps or coatings for food, pharmaceutical or other similar applications.