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Managing potential microbial problems during crushing or pressing grapes

Introduction

The crushing or pressing of grapes may seem like one of the simplest parts of the winemaking process, however it is at this stage that problems may start which will affect the quality of the wine at a later stage. These problems are mainly related to the microbiology (and related chemistry) of what is happening in the must once the crushing or pressing has taken place.

Grapes skins

Problems can arise once the grape skin is broken. The precious skin of the grape berry has done well to protect its contents from rain, heat and microbial pests throughout the growing season. The skin of the berry has a waxy layer and can be host to damaging microbes such as Lactobacillus, Pediococcus, Acetobacter and others. Some of the resident microbes on grapes are beneficial; however many are not and can cause numerous problems if they are not managed correctly. Although resident yeasts may be wanted in the ferment, bacteria usually are not.

As soon as the skin is broken, the sweet contents of the grape are wide open for the sugar-munching microbes to get in, grow on the sugars and multiply. The problems that the unwanted bacteria can cause can be quite serious are deserve some attention.

Problem microbes found on grapes

Lactobacillus

The main problems that Lactobacillus species can cause in wine are:

  • sluggish or stuck ferments
  • excess acetic acid (volatile acidity)
  • they are also associated with mousy taint

Lactobacillus species have been shown to come into the winery from the vineyard, however only a few of the dozen or so species identified have been shown to be involved in sluggish or stuck ferments (1). Lactobacillus species are found more readily in wines with a pH greater than 3.5 and are sensitive to changes in pH and ethanol levels (2).

Pediococcus

These bacteria are can cause several problems in wine including:

  • off aromas (caused by the formation of diacetyl and acetoin)
  • bitterness (caused by the formation of acrolein)
  • increased viscosity by producing b-glucans (causing ropiness in extreme cases)

Pediococcus bacteria survive well above a pH of 3.5 and are commonly found on numerous plants and in the by-products of these plants. For example, just as in wine, they have also been shown to go from cabbage and then into sauerkraut (2).

Acetic acid bacteria

The Acetic Acid bacteria (AAB), Acetobacter and Gluconbacter, can cause several problems in wine. These include:

  • excess acetic acid
  • unacceptable levels of acetaldehyde
  • unacceptable levels of ethyl acetate

Gluconobacter are not very alcohol tolerant and do not live till the end of the fermentation, however Acetobacter species are far more tolerant and can survive to the end of the fermentation. All AAB are aerobic and so need some oxygen to survive. They are also very pH dependent and prefer pH above 3.5. Commercial vinegar production relies upon these bacteria due to their ability to generate high levels of acetic acid, an ability not favoured in wine production (2).

Managing bacteria at the crusher or press stage

Two of the main management tools available to a winemaker are the management of the pH of the must and the use of the antimicrobial agent sulfur dioxide.

pH adjustment

The control of pH at the crushing and pressing stage is of critical importance because the problem bacteria are more likely to grow and cause problems above pH 3.5. The relationship between pH and the effectiveness of sulfur dioxide is also a major factor as the higher the pH, the less effective sulfur dioxide is in controlling bacterial growth. This effect will be covered in more detail below.

It is important that pH of the fruit is checked and acid added to bring the juice to the correct pH level. This is best done as early in the crushing or pressing process as possible. The pH chosen will be dependent upon the grape variety, wine style to be made and natural acid in the fruit.

Although pH appears to be a simple measurement, it is not always easy to get an accurate result. Much care needs to be taken with the pH electrode and its calibration. The electrode plays a vital role and should be replaced without hesitation if there are any ongoing problems with its performance. Our experience over many years has been that it is rarely the pH meter that causes problems (although this can occur). The electrode is certainly the more delicate part of the apparatus and should be considered a consumable item. If it has not been well looked after during the off season it is best replaced just prior to vintage as it is not expensive and plays a critical role. Errors of just 0.1 of a pH unit can make quite a difference to the management of bacteria.

Addition of sulfur dioxide

To prevent the growth of bacteria once the grapes have been crushed or pressed it is important to have the correct amount of sulfur dioxide in the must as close to the time the skins are broken as is practical. This often occurs in the vineyard during the picking process by adding PMS (potassium metabisulfite) powder to the fruit in the picking bins.

The critical stage is as the skins are broken, either at the crusher or press. Sulfur dioxide is also usually added at this stage, normally as a solution into the press tank or fermentation vessel. This depends upon the winemaking process and if it is white or red wine being produced.

The level of sulfur dioxide present at this stage is critical. As reported earlier (3) it is the level of molecular sulfur dioxide that is one of the most important determinants of microbial activity, as it is the molecular form of sulfur dioxide that is the active molecule that acts upon bacteria. The level of molecular sulphur dioxide that is required to eliminate bacterial problems is 0.8 ppm of molecular sulphur dioxide.

The commonly used aspiration/oxidation method for determining the level of sulphur dioxide does not measure in terms of molecular sulphur dioxide. To achieve the recommended 0.8 ppm molecular SO2 the corresponding level of Free SO2 must be reached. The relationship between Free SO2 and molecular SO2 is shown in Table 1.

Table 1: The level of free SO2 needed to give 0.8 ppm molecular SO2 at various pH levels

pH free SO2
3.0 13
3.1 16
3.2 21
3.3 26
3.4 32
3.5 40
3.6 50
3.7 60

To give an example from Table 1:

To achieve 0.8 ppm Molecular SO2 in a wine of pH 3.2 a free SO2 of 21 ppm is required.

To achieve 0.8 ppm Molecular SO2 in a wine of pH 3.5 a free SO2 of 40 ppm is required.

As can be seen it is much easier to protect must of a lower pH from unwanted bacterial growth, and also more difficult to achieve the desired level of free SO2 the higher the pH goes.

Laboratory testing

It should be obvious that accurate laboratory testing prior to and during crushing or pressing is essential to avoid microbial problems from emerging at this stage of winemaking. The list of tests is short and these tests can and should be done at the winery so that fast results are available for making quick decisions about additions.

The main tests required are:

  • pH
  • free sulfur dioxide
  • titratable acidity
  • sugar level (hydrometer or refractometer for Baume or Brix)

Doing these tests is cost effective in even the smallest wineries as only rudimentary equipment is needed. However, it is critical that the operator is well trained and knows that their results are accurate.

Conclusion

As can be seen, to limit problems at the crushing or pressing stage of winemaking, careful attention to pH and sulfur dioxide levels is essential in limiting the damage that can be caused by common wine bacteria. Some of the potential problems that can arise include sluggish or stuck ferments, off flavours and aromas and excessive volatile acidity. Careful attention to the testing required in the winery lab is important so that good management of the crushing and pressing process and a good, clean fermentation follow.

References

1. Tinney, M; Lactobacillus, the cause of stuck fermentations, Wine Business Monthly, Sonoma USA, August 2002

2. Guerra, B; Examining the effects of wine bacteria, A review of the main bacteria that can spoil wine, Wine Business Monthly, Sonoma USA, July 2007

3. Howell G.N and Vallesi, M; Is your wine really protected? Using molecular sulfur dioxide levels as the yardstick, Australian and New Zealand Grapegrower and Winemaker, February 2004

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