Copyright 1999
Chapter 23
LABORATORY WINE TESTING
About 30 tons of grapes are required to make 5000 gallons of wine. High quality fruit might cost $1500 per ton, so the grapes needed to fill a 5000-gallon tank can cost $45,000. Obviously, losing a tank of wine can be very expensive, so commercial winemakers depend on extensive laboratory wine measurements to reduce the number of gross wine failures. Laboratory measurements also help winemakers produce wines consistent in style and quality each season. Laboratory measurements are indispensable in commercial wineries, and they play an important role in most home winemaking programs.
Large wineries maintain extensive in-house laboratory facilities containing much expensive equipment. On the other hand, many small wineries cannot afford extensive measurement facilities because the equipment is too expensive. Most small wineries rely on basic wine tests that can be done with a minimum amount of apparatus, and they depend on the winemakers nose to provide much of the needed information.
Noses can detect and identify extremely small quantities of many different materials, so noses are wonderful measurement tools for quickly determining the condition of wine. Noses are always available (unless the winemaker has a cold), and they can be used quickly. An educated nose is the most valuable measurement tool any winemaker possesses.
The equipment and methods for doing six basic wine measurements are discussed below.
BRIX
Sugar in juice or wine is measured using the Brix scale. This scale was developed specifically for winemaking, and one degree Brix is equal to one gram of sugar per 100 grams of juice. Winemakers usually measure Brix with an optical refractometer or a hydrometer.
Sugar content is one of several criteria used to judge fruit maturity, and most grapes are picked when the sugar content is between 20 and 25 Brix. Winemakers start measuring grape sugars several weeks before harvest time, and they continue sugar testing until the fruit is picked.
Hydrometer readings are important during fermentation, and most winemakers measure Brix at least once each day to monitor the speed and consistence of their fermentations. Moderate drops in sugar each day shows normal fermentation, and if an unusual change in the sugar is observed, the winemaker can make corrections promptly.
Alcohol has a density less than water, so hydrometers read less than zero Brix when a completely dry wine is measured. Because of the accumulated alcohol, dry white table wines usually measure -1.8 to -2.4 Brix, and dry red table wines measure -1.4 to -2.2 Brix
Brix Measurement Materials
Sugar can be accurately measured using inexpensive ($20), short range, Brix hydrometers, and many winemakers use a set of three instruments. One instrument reads from 16 to 24 degrees Brix. The second instrument reads from 8 to 16 degrees, and the third hydrometer reads + 5 to - 5 Brix. All three hydrometers are calibrated in 0.1 Brix steps. Some home winemakers rely on a single, long range hydrometer with a zero to 30-degree scale. These long range instruments are inexpensive and readily available, but they are difficult to read accurately.
The following materials are needed to measure the sugar content of juice.
Brix hydrometers
A hydrometer cylinder
Brix Measurement Procedure
Hydrometers are made from very thin glass. They are fragile, and these instruments must be handled carefully. The following procedure can be used to measure Brix in juice.
1. Strain unclarified juice to remove the solids.
2. Fill the hydrometer cylinder within a few inches of the top
with juice.
3. Gently lower the hydrometer in the liquid and give it a slight
twist motion. The twist will release any bubbles sticking to the
glass.
4. When the hydrometer is floating freely, wait a few seconds and
then read the hydrometer scale. Read the scale in line with the
bottom of the meniscus curve.
5. Record the reading.
6. Discard the sample and rinse the cylinder and hydrometer with
clean water.
TITRATABLE ACID
Titratable acid (TA) is a measure of the sum of all the organic acids in juice or wine. In healthy wine, the major acids are tartaric and malic, but all wines contain small quantities of citric, succinic, acetic, butyric, lactic and other organic acids. In the United States, wine acidity is expressed as if all of the acids in the wine were tartaric acid. The titratable acid of juice ranges from 0.4 to 1.2 grams per 100 milliliters of liquid. One hundred milliliters of juice weighs approximately 100 grams, so "grams per 100 ml" is roughly equal to percent.
The taste of wine is strongly related to titratable acid. When wine contains too much acid, it will be very tart or even sour tasting. When too little acid is present, the wine often lacks freshness and tastes flat. Grapes grown in warm areas are usually low in acid, and additions of tartaric acid are often needed to produce balanced wine.
Titratable acid is used to judge maturity, and most winemakers monitor the titratable acid of the grapes for several weeks before harvest. TA is always measured when the grapes are crushed so any needed acid adjustments can be made before fermentation is started.
The titratable acid of normal wine slowly decreases as the wine ages, and a rise in wine titratable acid during the aging period is a danger signal to the winemaker. When the TA rises, acetic acid may be forming, and the wine may be turning into vinegar. Therefore, titratable acid is measured periodically from the time fermentation is complete until the wine is bottled. Small producers often measure titratable acid once each month, but many large producers measure the TA twice a month.
Titratable acid of wine or juice can be measured by several different analytical techniques. However, most wineries use a titration method. This measurement procedure is simple, and the laboratory apparatus is inexpensive.
Acid Measurement Materials
Titratable acid can be measured by a simple titration procedure using a calibrated (0.1 N) sodium hydroxide solution. Phenolphthalein solution is used as an indicator to show the titration end point.
The following materials are needed to measure the titratable acid of white wine, blush wine and juice.
5-ml serological (transfer) pipette
10-ml serological (transfer) pipette
250-ml Erlenmeyer flask
0.1 normal sodium hydroxide
1% phenolphthalein solution
distilled water
Titratable Acid Measurement Procedure
This procedure is satisfactory for measuring the titratable acid of juice or white and blush wines.
1. Draw 5 milliliters (ml) of juice or wine into the 5-ml
pipette.
2. Transfer the sample into the flask.
3. Add about 50 ml of distilled water and three or four drops of
phenolphthalein solution.
4. Fill the 10-ml pipette with 0.1 N sodium hydroxide solution.
5. Titrate with the sodium hydroxide while mixing the wine sample
by rocking the flask
6. Stop titration when the sample turns a faint pink.
7. Record the quantity of sodium hydroxide solution used.
8. Rinse the flask and pipettes with clean water.
Titratable Acid Calculations
Titratable acid can be calculated using the following formula.
A TA (%) = 0.15 X milliliters of sodium hydroxide used.
In the above formula, the titratable acid is given in grams of acid per 100 ml of wine (percent), and TA is expressed as if all of the acids in the wine were tartaric acid.
Alternative Procedure for Red Wines
The above procedure does not work well for dark red wines because the end point is very difficult to recognize in dark red wines. Diluting red wine samples with up to 200 milliliters of water makes the measurement easier, but most winemakers prefer to use the following procedure when testing dark red wines. However, a pH meter, ring-stand and a magnetic stirrer are required for this procedure (see pH measurement below).
1. Place the probe in a ring-stand clamp and arrange the stand
so the probe is centered above the stirrer plate.
2. Draw 5 milliliters (ml) of wine into the pipette and transfer
it into the beaker.
3. Add about 50 ml of distilled water and place the stir bar in
the beaker.
4. Place the beaker on the stirrer plate. Adjust the ring-stand
so the probe is emersed in the sample but do not allow the stir
bar to strike the end of the probe.
5. Turn the stirrer on.
6. Fill the pipette with 0.1 N sodium hydroxide solution.
7. Titrate the wine sample while watching the pH meter.
8. Stop the titration when the pH meter reads 8.2.
8. Record the quantity of sodium hydroxide used.
9. Discard the wine sample and rinse the probe, beaker, stir bar
and the pipettes several times with clean water.
Two examples of how titratable acid is calculated
Example # 1.
A white wine sample was titrated drop by drop. During titration, the sample was mixed by rocking the flask from side to side. A faint but persistent pink color was reached, and the pipette scale was read. In this example, the scale showed 5.3 ml of sodium hydroxide had been added to the sample. The titratable acid of the sample was calculated by multiplying 0.15 times 5.3. This gave a TA of 0.79 grams of acid per 100 milliliters of wine or 0.79 percent.
Example # 2.
A pipette was used to add 0.1 N sodium hydroxide solution to a red wine sample. The titration was done slowly, so the stir bar could keep the sample mixed, and the pH meter was carefully watched. Titration was stopped when the meter read 8.2. The pipette scale was read, and it showed 3.9 ml of sodium hydroxide had been added to the sample. The acidity of the red wine sample was calculated by multiplying 0.15 times 3.9 which give 0.58 grams per 100 milliliters (0.58 percent).
FREE SULFUR DIOXIDE
Winemakers try to maintain 20 to 40 milligrams per liter of free SO2 in their wine from the completion of fermentation until the wine is bottled, but the amount of free SO2 does not remain constant. Sulfur dioxide is consumed as it does its job, and the free sulfur dioxide slowly decreases with time. Consequently, winemakers measure the free sulfur dioxide in their wines every few weeks, and they make appropriate additions to maintain the sulfur dioxide near the desired level.
SO2 Measurement Materials
5-ml serological pipette
10-ml serological pipette
250-ml clear glass container
Dilute sulfuric acid (1 part acid in 3 parts water)
1% starch solution
0.01 N iodine solution
Distilled water
The starch indicator solution is made by mixing one half teaspoon of corn starch in about 200 ml of water and boiling the mixture for a few minutes. The starch solution can be used when it is cool, but a fresh solution should be made every week or so. Starch solutions are prepared commercially, and these solutions last for several months.
A 0.01 normal iodine solution is a weak solution, and weak iodine solutions are not stable. Weak iodine solutions should be stored in dark brown glass bottles, and the bottle should be tightly stoppered and stored in a dark place. Even then, the calibration (normality) of the iodine solution will change slowly.
Free SO2 Measurement Procedure
The following procedure can be used to measure the amount of free sulfur dioxide in juice or wine.
1. Draw exactly 10 milliliters (ml) of wine into the pipette
and transfer the wine into a small, clear glass container.
2. Add about 5 ml of diluted sulfuric acid (H2SO4) to the wine
sample.
3. Add 2 or 3 ml of the starch solution to the sample.
4. Immediately fill the 5-ml pipette with the iodine solution and
titrate the wine sample until a faint purple color is reached.
This is the end point. The purple color will fade in a few
seconds, but do not add more iodine.
5. Record the quantity of iodine solution used to reach the end
point.
6. Rinse the pipettes and the glass container several times in
clean water and place them in drying racks.
Most commercial wineries routinely use the iodine method to measure sulfur dioxide in white and blush wines. However, this method is not very accurate when used with red wines because phenolic materials in red wines also react with the iodine. These side reactions can give erroneously high results, and measured values of free sulfur dioxide can be more than 20 percent too high. Besides the side reactions, the end point is difficult to recognize in dark red wines. Generally, a distinct purple color does not occur when red wines are titrated. Instead, a slight darkening of the wine color indicates the end point. Diluting red wine with distilled water often makes the end point easier to recognize, and as much as 150 ml of water is sometimes used. Viewing the diluted red wine with a strong yellow sidelight is often helpful. Some winemakers have a 75-watt, yellow "bug light" in a desk lamp specifically for measuring sulfur dioxide in red wines. Measuring sulfur dioxide in dark red wines with the iodine method requires some practice.
Sulfur Dioxide Calculations
The amount of free sulfur dioxide in the wine sample is calculated from the measured volume of the iodine solution used in the titration and from the normality of the iodine. The formula for free SO2 can be written as:
SO2 = 3200 X volume of iodine X normality of iodine.
In the above formula, the free sulfur dioxide content of the wine sample is given in milligrams of sulfur dioxide per liter of wine (mg/l).
Two examples of how free sulfur dioxide is calculated
Example # 1.
A faint purple color was produced when 2.2 milliliters of 0.01 N iodine solution was added to 10 milliliters of white wine. The free SO2 level of this wine was obtained by multiplying 3200 times 2.2 (the quantity of iodine solution) times 0.01 (the normality of the iodine solution). The multiplication (3200 X 2.2 X .01) gives 70.4 milligrams per liter of free sulfur dioxide.
Example # 2.
A dark red wine sample was diluted with about 100 milliliters of distilled water. When 1.2 ml of 0.01 N iodine solution was added to the wine sample, the color darkened slightly. The free SO2 level of this red wine was calculated by multiplying 3200 times 1.2 times 0.01. This measurement gave 38.4 milligrams per liter (mg/l) of free sulfur dioxide.
pH
pH is one of several parameters used to judge fruit maturity, and most winemakers start monitoring the pH of the grapes several weeks before harvest time. pH also gives the winemaker important information about how much sulfur dioxide is needed to control microbes. Wine stability characteristics are dependent on pH.
Red wine color is influenced by pH. Red wines with low pH values have more and better red color. Wines with high pH values have a dull, less attractive color. Winemakers monitor wine pH every few weeks until the wine is bottled.
pH Measurement Materials
A pH meter with a three-digit resolution is the only practical way of measuring pH, and an instrument with a separate probe on a 36-inch cable is a great convenience for wine measurements. All pH meters should be calibrated just before use, and most instruments have long warmup times.
pH meter
pH probe
7.00 standard pH solution
4.00 standard pH solution
100-ml beaker
ring stand
magnetic stirrer
stir bar
pH Measurement Procedure
The following procedure can be used to measure the pH of any wine, must or juice.
Meter Calibration
1. Place the probe in the ring-stand clamp and arrange the
stand so the probe is centered above the stirrer plate.
2. Place 30 or 40 milliliters of the 7.00 standard solution in
the beaker.
3. Place the beaker on the stirrer plate. Adjust the stand so the
probe is emersed in the sample but do not allow the stir bar to
strike the end of the probe. Turn the stirrer on.
4. Adjust the "calibration" knob until the meter reads
7.00.
5. Discard the standard solution and rinse the beaker.
6. Place 30 or 40 milliliters of the 4.00 standard solution in
the beaker.
7. Place the beaker on the stirrer plate and adjust the stand so
probe is emersed. Do not allow the stir bar to strike the end of
the probe. Turn the stirrer on.
8. Adjust the "slope" knob until the meter reads 4.00.
9. Discard the standard solution and rinse the beaker.
pH Measurement
10. Place 30 or 40 milliliters of wine or juice in the beaker.
11. Place the beaker on the stirrer plate. Adjust the stand so
the probe is emersed in the sample but do not allow the stir bar
to strike the end of the probe. Turn on the stirrer.
12. Wait a few seconds, then read the pH meter and record the
value.
13. Rinse the probe, beaker and stir bar in clean water.
RESIDUAL SUGAR
Fermentation can restart in wine containing sugar anytime, so winemakers always measure the residual sugar contents of their wines shortly after fermentation to be sure no sugar remains. Large wineries use a wet chemistry method, but small wineries and home winemakers use Clinitest tablets to measure residual sugars in wine. This measurement method is simple, and it provides reasonable accuracy when the residual sugar content is less than about 2 percent. Clinitest tablets are sold in most large drugstores. Tablets and a special color chart can also be ordered from winemaking supply companies.
Residual Sugar Measurement Materials
The following equipment can be used to measure the residual sugar content of any table wine.
eye dropper
small clear container
Clinitest tablets
Clinitest color chart
Residual Sugar Measurement Procedure
This procedure can be used to measure the residual sugar content in any wine containing less than 2 percent sugar. Wines with higher values of sugar can be measured by diluting the wine appropriately. Measurements are made by comparing the color of a treated wine sample to the color on a calibrated chart. However, the color pigments in red wines do not seem to cause significant errors.
Directions and a color chart are provided with the Clinitest tablets. The directions should be followed carefully.
1. Draw a small amount of wine into the eye dropper.
2. Place 5 drops of wine in the clear container.
3. Rinse the eye dropper carefully (several times) with clean
water.
4. Draw a small amount of distilled water into the eye dropper.
5. Place 10 drops of distilled water in the clear container.
6. Place one Clinitest tablet in the container.
7. After the boiling stops, wait 15 seconds then read the sugar
content by comparing the color of the liquid to the colors on the
chart.
8. Rinse the eye dropper and the small container several times
with clean water.
Clinitest tablets are sensitive to moisture, so bottles should be kept tightly sealed.
CHROMATOGRAPHY
Most winemakers measure the malic acid content of their red wines when fermentation is complete. They want to know ML fermentation is complete and if all the malic acid is gone. An additional treatment may be needed to stabilize the wine when malic acid remains, and paper chromatograph is a simple and reliable means of determining what kinds of organic acids are present in wine. Yeasts produce small quantities of lactic acid during primary fermentation, so all wine contains some lactic acid. Therefore, the progress of ML fermentations should be judged by the disappearance of malic acid, not by the appearance of lactic acid. ML fermentation is considered finished when the malic acid spot completely disappears.
ML Chromatography Materials
The following materials are needed to measure ML fermentation status in any wine.
A 1-gallon wide mouthed jar
ML solvent
ML paper
Micro pipettes (capillary tubes)
standard malic acid solution
ML Measurement Procedure
The following procedure can be used to identify and estimate the quantities of tartaric, citric, malic and lactic acids in wine or juice.
1. Place a sheet of chromatography paper on a clean surface.
Use clean hands and try to hold the paper by its edges.
2. Make a light pencil line along a long side about 3/4 inches up
from the bottom, and then make marks about 1 1/4 inches apart
along the pencil line. Label each mark either as the malic acid
standard or for each of the wine samples being measured. The
first mark on the left is commonly used for the malic acid
standard.
3. Use capillary tubes to draw up a small quantity of the acid
standard and wine samples. Do not allow any contact between the
tubes.
4. Touch the end of each tube to the mark on the line very
quickly. The wet spots should not be larger than about 1/4 inch
in diameter.
5. Allow the spots to dry and then repeat the above step twice (3
applications). Let the dots dry between each application.
6. Pour enough ML solution into the wide mouthed jar to cover the
bottom. The solvent should cover about 1/8 inch of the bottom
edge of the paper. The solvent should not come up to the pencil
line.
7. When all of the dots are completely dry, staple the short
sides of the paper together forming a cylinder.
8. Stand the paper cylinder upright in the jar with the pencil
line near the bottom and place the cover on the jar.
9. Let the jar stand until the solvent front has climbed nearly
to the top of the paper (six or more hours).
10. Remove the paper from the jar. Pour the ML solvent back into
the original container, wash and dry the jar and discard the
capillary tubes.
Remove the staples from the sheet of paper and hang the paper in a well-ventilated place. The chromatogram can be read when the paper is completely dry.