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Sulfite Management

We recently sent several samples of customer wine to Mosti Mondiale's laboratory for analysis.  When we got the results, we were surprised by the very low levels of free sulfur dioxide (SO2) in the samples.  The levels reported by Mosti's lab will appear later in this article.  The low level is a concern because of the importance of free SO2 to the winemaker.  Too little SO2 and the wine is unprotected.  Too much SO2 and the taste is affected and the wine has the aroma of a burnt match.
 
SO2 has many uses in winemaking.  It is used to retard the growth of bacteria and any wild yeast that may be present.  SO2 is also used as an anti-oxidant that combines with free oxygen in the juice and wine to prevent the effects of oxidation on the wine.  It is also used as a sanitizing rinse of any equipment that comes in contact with the juice or wine.  During bulk aging and once the wine is in the bottle, SO2 affords protection from oxidation and spoilage microbes. 
 
The amount of SO2 in the wine is measured in parts per million (ppm) or milligrams per liter (mg/l).  These measures are the same and are used interchangeably.  When SO2 is added to the wine, a portion will bind with other compounds in the wine and it is referred to as bound SO2.  The portion of the addition that does not become bound is referred to as free SO2.  The sum of the bound and free is the total SO2.  Over time the free SO2 will become bound SO2 and the protection provided will diminish.  It is essential to monitor levels during bulk aging and to ensure adequate levels of free SO2 at bottling for long term bottle aging.
 
A very small portion of the free SO2 is known as molecular SO2.  The molecular SO2 is the most important component as it is responsible for the anti-microbial and the anti-oxidant activity.  There is a relationship between the pH of the wine, the free SO2 and the molecular SO2.  Therefore, if the pH and free SO2 are known, the molecular SO2 can be determined.  A molecular value of 0.5ppm will allow malolactic fermentation to proceed.  A molecular value of 0.8ppm will inhibit the malolactic bacteria and provides effective protection.  At a molecular value of 2.0ppm the wine is provided maximum protection, however, it is close to the level at which sulfur dioxide can be detected by the senses.  In general the target for red wines is 0.5ppm molecular and for white wines 0.8ppm molecular.
 
The following table shows the amount of free SO2 needed in parts per million (ppm) to obtain the target molecular value at varying pH levels. 
 

pH

0.5ppm

0.8ppm

2.0ppm

2.90

7

11

27

2.95

7

12

31

3.00

8

13

34

3.05

9

15

38

3.10

10

16

42

3.15

12

19

47

3.20

13

21

52

3.25

15

23

58

3.30

16

26

65

3.35

18

29

73

3.40

20

32

82

3.45

23

37

91

3.50

25

40

102

3.55

29

46

114

3.60

31

50

128

3.65

36

57

143

3.70

39

63

161

3.75

45

72

180

3.80

49

79

202

3.85

57

91

226

3.90

62

99

254

3.95

71

114

385

4.00

78

125

319



The following table shows the results of the wines sent to Mosti Mondiale for laboratory analysis:

Wine Type

pH

Free SO2

Red Wine

3.27

9ppm

Pinot Noir - All Juice

3.36

14ppm

Grand Barolo - Masters Edition

3.69

14ppm

Barolo - Vinifera Nobel

3.64

5ppm

Zinfandel

3.76

8ppm

Sauvignon Blanc - All Juice

3.27

20ppm

 
All of the wines fell short of the target molecular value.  Since the level of free SO2 will slowly decrease with time as it becomes bound, these wines do not have the level of molecular SO2 necessary for long-term bottle aging.
 
The Wine Maker's Toy Store is planning to purchase the Hanna Instruments model HI 84100 Mini Titration System for Total Sulfur.  This will allow for the determination of the free and total SO2 in a must or wine with an accuracy of 5%.  As wines made in the store are tested, the results will be added to the Winemaking Log and Database.  We are also planning on comparing test results using this equipment with the results of other readily available methods for testing free SO2.  Other testing will be to monitor free SO2 levels on wines being bulk aged to determine the timing and amounts of SO2 additions.
 
We understand that every batch of wine is different and that what is true for one is not necessarily true for all.  We also know that without testing each batch, the best we can hope for is to be close using the information we have available.  Our goal at the Wine Maker's Toy Store is to provide the winemaker with information they can use to make the best wine possible. 

 

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