Grape phenolic compounds and bioactivity: Modification of maceration techniques for wine and agro-waste extraction for active polymers

Abstract

Phenolic compounds are contained in many plants and are therefore in the foods and beverages that are consumed. These compounds are found in all parts of the grape with some tissues having greater concentrations than others. Interest in phenolic compounds comes from epidemiological, in-vitro and in-vivo studies on the effects of these compounds. The effects observed can be attributed to the antioxidant, antimicrobial, and antiviral activity of the compounds. The potential health benefits associated with phenolic compounds have driven research into maximizing their extraction. Wine, a fermented beverage from grapes, is known to have high levels of these compounds, especially in red varietals. It would be advantageous to increase phenolic levels in white wines so that a greater quantity would be available in the diet of white wine drinkers. The challenge in increasing phenolic levels is maintaining wine varietal characteristics. In an attempt to do this, two varietals were selected; Sauvignon blanc and Chardonnay. These two varietals were subjected to various maceration techniques: cryogenic, carbonic, extended skin contact, and combinations of these techniques, in order to increase the phenolic levels within the wines. One maceration technique, cryogenic maceration, provided a significant increase in phenolics and antioxidant activity, while maintaining the varietals characteristics, which was established by an experienced tasting panel. In addition to significantly increasing the antioxidant profile, it was found that the cryogenic maceration also increased the concentration of certain varietal thiols in the Sauvignon blanc wines. Within both the wine varieties the enhancement of the aromas needed to be balanced with the tactile aspects presented by the phenolic loading. Perceived aroma increases can occur by two possible mechanisms, an increase in the concentration of the chemical providing the attribute or a lowering of the perceived aroma threshold. The process of the cryogenic maceration is to produce cellular membrane disruptions, this in turn results in the release of more compounds, aromatic and other, to the juice. The mere increase in aroma compounds can account for the sensory attribute increases, however the phenolic concentration plays an important role in altering the perception thresholds of aroma compounds. The wine studies demonstrated this aspect utilizing the sensory panel to determine enhanced attributes. These increases were then correlated to phenolic increases via previous studies looking at this aspect. Additionally, it was established that the ethanol concentration did not have an affect on the aroma attributes in the reported studies. At the same time, winemaking processes generate large quantities of waste, which remain under-utilized as there are still many phenolic compounds contained in the final grape marc left over after pressing off the juice. On the other hand, the presence of phenolics presents a problem for the composting of the waste marc, as they can cause phytotoxicity. A method was designed to extract these compounds to allow for more immediate use of the marc for horticulture uses. The water extract contained phenolics, sugars and other residual compounds. To convert this extract into a viable product, the extract solution was purified, selecting for the phenolic compounds, which were then dried to form a powder. This product, consisting largely of phenolic tannin molecules, was then characterized to determine its composition and compare it to commercially available extract products. The water based extract performed equally well compared to commercially available grape tannin in antioxidant activity and antimicrobial activity against gram-positive bacteria, and outperformed the commercial product in antimicrobial activity against gram-negative bacteria. Additionally, the water-based extract contained a three-fold lower amount of residual sugar. The results of the antioxidant and antimicrobial studies gave encouraging results for the application of the extract in packaging materials to protect foods. Initial studies were performed using solution blending with ethyl cellulose to establish antioxidant activity and binding mechanisms. It was demonstrated that the antioxidant activity of the grape tannin/ ethyl cellulose blend remained after processing and was significantly increased at a concentration of 0.5% added extract. It was proposed that the phenolics were integrated within the ethyl cellulose structure through hydrogen bonding. The excellent results from the ethyl cellulose work prompted studies on extract incorporation into polyolefins as they are more commonly used polymers in the packaging industry. The polyolefins presented the challenge of the high temperatures required to melt blend the materials, as phenolics are known to degrade at high temperatures. However, only the blending of PET required a high temperature that adversely affected the phenolics, resulting in their thermal degradation, which was observed by the low antioxidant activity and the resulting deep purple color of the blend. The other polyolefins, high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and polypropylene (PP), all showed promising results with low leachable solids, superb antioxidant activity significant even at 1% grape tannin incorporation, and minimal to no changes in physical properties.