HEARD IT THROUGH THE GRAPE WINE

PROBLEM:

One early fall morning you go out to check your grapevine as you do most mornings. You discover an unusually large number of ripened grapes. You pick the grapes and prepare them for storage. However, you have so many grapes that you cannot make any more jelly. Your cupboard is too full. You have a wine cellar that needs to be cleaned out and put to good use for a change, so you decide to use some of the extra grapes to try your hand at making wine.

BACKGROUND INFORMATION:

Fermentation is an energy-releasing form of metabolism where both the substrate (initial electron donor) and by-products (final electron acceptors) are organic compounds. The fermentation of glucose to ethanol involves a series of enzymatic biochemical reactions where ethanol acts as the final electron acceptor.

For more information about fermentation and winemaking, please refer to Appendix B.

OBJECTIVE:

The student will be able to observe the processes of fermentation. The student will also be able to calculate the percent sugar content, pH and titratable acidity.

MATERIALS:

  • Concentrate from a local brew store or cans of concentrated juice from the grocery store. Different fruit and fruit blends can be used. Do not purchase any concentrate with sulfur dioxide present. Thaw the juice overnight.
  • Hydrometer (purchased from a local brew store)
  • pH meter
  • Large gallon container
  • Fermentation lock (purchased from a local brew store)
  • Wine yeast (purchased from a local brew store)
  • Potassium metabisulfite (KMBS) 1g/gallon aqueous solution
  • Yeast nutrient (purchased from a local brew store)

    TIME:

    Preparation of wine approximately 1 hour.
    Fermentation process several days to weeks.

    PROCEDURE:

  • Mix one can of concentrate with one can of water in a glass jar. Leave a large headspace at the top of the container to allow room for the fermentation and adjustments (if necessary). Allow juice to equilibrate to room temperature.
  • Check the percentage of soluble solids (sugar content) by suspending a hydrometer in the concentrate. The goal for sugar content is 20-22%. If the reading is low add more concentrate. If the reading is high add more water.
  • Check pH of the concentrate. (Goal pH is 3.3-3.5)
  • Determine titratable acidity
  • Place 5-ml juice or wine in a 250-ml beaker and add 125 ml deionized water.
  • The pH meter should be warmed up and standardized as for pH determination.
  • Place a stirring bar in the diluted sample, place on a stirring plate, and carefully place the electrode in the diluted sample (do not let stirring bar strike electrode).
  • Titrate the sample to pH 8.2 with 0.1 N NaOH and record the ml NaOH required.
  • Acidity can be expressed as a particular acid or as milliequivalents of acid. Acidity in this experiment is expressed as % tartaric acid since this is the predominate acid in most grape cultivars (except muscadine).

    Acidity (as % tartaric) = (ml NaOH) (N NaOH) (meq wt.) X 100
    Acidity (as % tartaric) = (ml NaOH) ( Sample size (ml)

    Acidity (as % tartaric) = (ml NaOH) (0.1) (0.075) X 100
    Acidity (as % tartaric) = (ml NaOH) ( 5 ml

    NOTE: Adjustment of Acidity

    In Arkansas (and many other warm areas) acidity is usually low, and as a consequence, many wineries will raise the acid level of their juice or wine. This is accomplished by adding legal amounts of food grade acid such as citric or tartaric. If acidity adjustments are made, the final acidity cannot exceed 0.80% by law.

    EXAMPLE:

    A winemaker wishes to raise the acidity (% tartaric acid) of a wine from 0.60% to 0.75% using citric acid. Acidity must be increased by 0.15% (0.75-0.60).

    1% = 1g                   or            37.85g
    1% = 100 ml                              gal.

    0.15% x 37.85 = 5.68g citric acid per gallon of wine.

    Thus the winemaker would add 5.68-g citric acid per gallon of wine.

    In some areas of the U.S. especially the cooler areas (Ohio, New York), acidity is excessive. Many wineries in these areas wish to lower the acidity. This is usually accomplished by one of four methods: addition of sugar and water (amelioration); precipitation of acid with calcium carbonate, Acidex or similar compounds; ion exchange, and malolactic fermentation (conversion of malic to lactic acid by bacteria).

  • 1 gram of dry yeast per gallon of juice is added in the following manner. (Wine yeast can be purchased at any brewery shop or by mail order, do not use bread yeast.) Use 10 ml of 105 o F water per gram of yeast. Temperature is critical. Pour the hot water in a warm flask and slowly pour the yeast onto the water. Do not stir the water. The yeast will slowly fall to the bottom of the container. After 10 minutes, swirl the flask in a circular motion. Let the flask sit for an additional 10 minutes and swirl again before adding to the juice. If you have to wait more than 20 minutes, add a spoonful of sugar to the yeast.
  • Juice should be at room temperature. Add the yeast to the juice. Stir gently or swirl the container. Add 1 gram of the yeast nutrient per gallon of juice. Nutrients are extremely important for the complete fermentation. Diammonia phosphate can also be added at the same rate if commercial nutrients are not available.
  • Stopper a fermentation lock on the container. Fill the lock with a potassium metabisulfite solution. Allow juice to ferment at 70 o F for the first few days and then move to 60 o F for storage. Swirl or agitate the jar daily (if possible once in the morning and once in the evening).
  • After vigorous fermentation ceases but prior to stopping completely, siphon the wine into smaller flasks and leave very little headspace in the flask. This prevents oxidation and the development of off-flavors from extended contact with the lees (sediment). Allow the wine to sit without stirring to allow sediments to settle. Do not siphon the sediments into the new flasks. Stopper a fermentation lock onto each container.
  • Fermentation is complete when all of the sugar has been utilized and converted to alcohol and carbon dioxide. You will see no more bubbles rise out of the fermentation lock.
  • Measure the pH of the wine. Potassium metabisulfite (KMBS) addition is needed to achieve a free sulfur dioxide level of 0.8 ppm. Higher pH wines require the addition of more KMBS to achieve 0.8 ppm free sulfur dioxide. Use the charts on the last page to determine the amount of KMBS need for your wine.
  • Filter the wine through a pre-filter if necessary and place in wine bottles. Seal the bottles tightly to avoid oxygen leaks.

    QUESTIONS:

    Why must you use brewer's yeast for winemaking rather than bread yeast?


    An anaerobic environment is necessary for fermentation. Why is a fermentation lock used rather than just sealing the jar?


    What is the chemical reaction for fermentation?


    FOR ADDITIONAL INFORMATION CONTACT:

    Dr. Justin Morris
    Dr. Renee Threlfall- Enology, Food Science, U of A- Fayetteville,
    rthrelf@comp.uark.edu

    CHART 1: FREE SULFUR DIOXIDE (SO 2 ) LEVELS FROM pH of 3.0 to 4.0
    pH wine ppm of SO 2
    3.0 13
    3.1 16
    3.2 21
    3.3 26
    3.4 32
    3.5 40
    3.6 50
    3.7 63
    3.8 72
    3.9 79
    4.0 99

    CHART 2: FORMULA FOR ADDING POTASSIUM METABISULFITE (KMBS)

    (ppm)(0.00675)=grams/gallon
    ppm of SO 2 Grams KMBS/gal
    100 0.6570
    90 0.6075
    80 0.5400
    70 0.4725
    60 0.4050
    50 0.3285
    40 0.2700
    30 0.2025
    20 0.1350
    10 0.0675

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