Total Phenolic Content of Red and White Grape Juice

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Determining Total Phenolic Content of Grape Juice using Visible Spectrophotometry

Formal Food Analysis Laboratory Report
February 7, 2013

By Carla Reed
Lab partner: Nina Jung

Total Phenolic Content of Grape Juice using Visible Spectrophotometry
The health benefits of red wine are well known and consumption has been encouraged more over the years because of the populations increasing obesity rates and the inherent complications thereof; including coronary heart disease and high blood pressure. Since the phenolic compounds in wine combats these negative effects and originate from the grape itself the focus of this study was the total phenolic content in red and white grape juice; measured in gallic acid equivalents
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The concentrations of total phenolic compounds are written in Gallic Acid Equivalents (GAE) in units of milligrams of gallic acid per 100 milliliters of juice. Educational institutes frown upon buying students alcohol, even for scientific purposes, so white and red grape juices were used in this experiment rather than wine to determine the total phenolic content by visible spectrophotometery.
The overall objective of this experiment is to measure the total phenolics’ content in grape juice using a standard curve to find the sample’s phenolic concentration in milligrams of gallic acid equivalent for every 100 milliliters of juice.

Materials and Methods
This experiment used the Gilson pipetman Neo, Genesys 20 spectrophotometer, Folin-Ciocalteu’s phenol reagent, saturated Na2CO3 solution as well as the requisite test tubes, volumetric flasks, analytical scale and graduated cylinder necessary to obtain accurate results. The first step required the preparation of the gallic acid stock solution. This was made by quantitatively transferring 80 milligrams of gallic acid into a 250 mL volumetric flask, dissolving it into solution by using 50 mL methanol, bringing it up to volume with deionized water and mixing the solution vigorously. The final concentration of the stock solution should be approximately 320 µg/mL, depending on the mass of gallic acid measured. The working solutions that

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