Objectives

Introduction

The procedures of making grape wine at home are quite straight forward. Grape juice is simply inoculated with a package of yeast starter culture purchased from a supermarket. Primary fermentation lasts for approximately one week; during that time most of the sugar originally present in the juice is converted to ethanol and yeast cells, with the evolution of carbon dioxide. The excess yeast cells are then removed from the juice along with other sediment, and a slower secondary fermentation is allowed to proceed to develop the final flavor. Sugar may be added to the original must to achieve the desired alcohol content or to modify the flavor. The type of wine can be classified according to the color of the wine. Another classification is based on the starting sugar content, as listed in Table 1.

Table 1. Classification of wines according to the sugar content.

     -------------------------------------------------------
     Type               Specific Gravity  Sugar Content(wt%)
     -------------------------------------------------------
     Dry Wine            1.085 - 1.100         21 - 25
     Medium Sweet Wine   1.120 - 1.140         29 - 33
     Sweet Wine          1.140 - 1.160         33 - 37
     -------------------------------------------------------

Use of a Hydrometer

The proper way of using a hydrometer is to spin it gently in the grape juice, must, or wine for which the specific gravity is to be measured. Twisting of the hydrometer removes most of the air bubbles from its surface which can invalidate the measurement. Do not drop the hydrometer into a container from a height, for the hydrometer is likely to be broken. A graduated cylinder is usually used for this purpose. The sugar content of the sample in weight percent is read from the meter which in turn indicates the potentially achievable alcohol content in the final fermented product. Two readings are required to estimate the alcohol content in a fermented wine. The first reading is taken at the beginning of fermentation. The second reading will tell one the amount of the remaining sugar at the time of the reading. Thus, the current alcohol content is simply the present PA value minus the starting PA value.

Because the density of a liquid is a function of the temperature, Table 2 gives the correction that one should add to the specific gravity readings to obtain the corresponding values at 60 º F, the temperature at which the meter is calibrated.

Table 2. Temperature correction to the specific gravity reading.

      ----------------------------------------
      Temperature(&$186;F)   Correction(sp.g.)
      ----------------------------------------
             50
             60                    0.000
             70                   +0.001
             77                   +0.002
             84                   +0.003
             95                   +0.005
            105                   +0.007
      ----------------------------------------

List of Reagents and Instruments

A. Equipment

• Graduated cylinder
• Test tubes
• Erlenmeyer flasks or bottles
• Rubber stoppers
• Tygon tubing
• Hydrometer
• Balance

B. Reagents

• Grape juice
• Sucrose
• Active dry wine yeast, strains of Saccharomyces ellipsoideus
  Montrochet, Redstar brand
  Universal Foods Corporation
  Milwaukee, Wisconsin, 53201

Procedures

1. Prepare Starter Yeast Culture
   • Mix 1 g of dry wine yeast culture in 100 ml of grape juice. See Note 1.
   • Let the yeast grow in a loosely capped container at room temperature for 24 hours.
2. Primary Fermentation
   • Add enough sugar to grape juice to prepare the following 4 substrates, about 1 liter each:

          -------------------------------------------
          Run              Conc. of Extra Added Sugar
          -------------------------------------------
          A                          0.0g/l
          B                        100.0g/l
          C                        200.0g/l
          D                        300.0g/l
          -------------------------------------------
     

   • Measure the specific gravity and PA value for each of the starting substrates with a hydrometer. This is the initial PA value which will be used later to estimate the alcohol content.
   • Inoculate each bottle with 20 ml of the starter yeast culture prepared in the previous step. See Note 2.
   • Plug the juice bottle with a rubber stopper. A piece of Tygon tubing is extended from the stopper to provide a vent for the evolved carbon dioxide. The other end of the tubing is dipped in water in a small test tube taped to the bottle. The water prevents the entry of oxygen, which alters the metabolism of the yeast and spoils the wine. At the same time, carbon dioxide can escape from the bottle.
   • Ferment at room temperature for one week.
3. Secondary Fermentation
   • At the end of one week, decant the juice from the bottle to clean individual temporary containers.
   • Measure the PA values for each of the substrate with a hydrometer. Estimate the alcohol content by subtracting the present PA value from the initial PA value.
   • Discard the sediment and wash each bottle with water.
   • Pour the juice back into the cleaned bottle. Put back the cleaned assembly of rubber stopper and Tygon tubing.
   • Ferment slowly for another 4-6 weeks.
   • Measure the PA values as before when it is ready for consumption.
4. Taste the wine and celebrate the end of the course in biochemical engineering laboratory!
5. Try out other fruit juices.

Notes

1. Fresh grapes may be crushed to obtain the juice. Most wineries add sulfur dioxide either as a gas or as a solid salt to prevent the growth of other yeasts and bacteria which cause spoilage. Sulfur dioxide is also produced by Saccharomyces during fermentation. The simplest source of juice is the supermarket shelf. Use the "All Natural" variety with no added preservatives or sugar.
2. Dry wine yeast may be directly added to grape juice from a package at the level of 1 g per 4 liter. For best results, first suspend 1 g of dry wine yeast in 10 ml of warm water at about 35 ºC. Then add the suspended culture to grape juice.

Questions

1. Compare the estimated values of alcohol content with those measured independently either by a wet chemistry method or a gas chromatograph. Does the hydrometer indeed give a good estimation for the ethanol content in wine?
2. Comment on ways to improve the experiment.

References

1. Cornelius S. Ough, Chemicals used in making wine, C&EN, January 5, 1987.