Markers for Potential Grape Juice Concentrate Browning

The purpose of this project was to generate a preliminary data base that will predict browning in white grape juice concentrate produced in the San Joaquin Valley. We focused on a number of reported marker compounds [a-amino nitrogen (AAN), hydroxymethylfurfural (HMF)] as well as pH, brix, percent insoluble solids, total phenolics, and color. Since certain inorganic ions are thought to be involved with nonenzymatic browning, the following levels were also measured: potassium, magnesium, calcium, phosphate [grape juice minerals], copper, and iron. Browning of the product may be indirectly affected by the amount of concentration [water removal]. Samples that were stored at lower brix levels may have some decreased browning when reconstituted to 16.0° brix. Percent insoluble solids in the juice does seem to have some correlation with the color of the juice. Our results show a slight correlation with AAN and browning in the Sept. 94 juice data and Jan. 95 concentrate data. The preliminary data points to a strong correlation between decreased color and higher total phenolics in white grape juice concentrate. These data indicated that as time progresses the correlation between decreased color and higher phenolic levels becomes stronger. The color of white grape juice concentrate may decrease with higher phosphate levels. It appears that higher calcium levels may be associated with darker colored white grape juice concentrates. These trends are similar to the magnesium trends in white grape juice concentrates. The correlation between the level of copper in white grape juice concentrates and darkening of the concentrate in this preliminary study also increased with time. A larger data pool is needed to statistically predict browning precursors in Californian white grape juice concentrate.