Keith Grainger - Wine Faults and Flaws

There are more species of bacteria than any other form of life. Bacteria can be single or multi‐celled microbes. Unlike yeasts, they do not have a nucleus and are classified as prokaryotes. A simple way of describing bacteria is by their morphology, i.e. their shapes. There are five groups of shapes; the two most important groups that may be found in wine are cocci that are round or oval shaped and bacilli that are rod shaped. These descriptors often appear as part of genus names. Some species of bacteria exist as individual cells; others group themselves together in pairs, rods, chains, or clusters.

Many species of bacteria grow only in aerobic conditions – these are termed obligate aerobes, and include the dreaded Acetobacter , which can turn wine into vinegar. There are also species, termed obligate anaerobes, which grow in anaerobic conditions. The presence of oxygen poisons some of the key enzymes of obligate anaerobes, so they will not grow in aerobic conditions. There are some obligate anaerobes which require a high level of carbon dioxide for growth, as will be the case during fermentation. Facultative anaerobes are organisms that manufacture adenosine triphosphate (ATP) by aerobic respiration in the presence of oxygen, but are able to change to fermentation or anaerobic respiration in the absence of oxygen. Bacteria that grow in an acid environment are termed acidophiles, but these are very rarely present in grape musts and wines as the pH is invariably higher than they can tolerate.

The pH of musts and wines varies considerably between extremes of pH 2.7 and 4.3 depending, inter alia, upon growing conditions including soil type, aspect, climate, and weather in the growing season. As grapes ripen, pH increases. A high pH will not only have a negative influence upon the taste profile, but increase the risk of bacterial growth. Although there are exceptions, the ideal pH of must for white wines lies in the range of pH 2.9–3.4, and for red wines pH 3.3–3.7, although in recent years this latter figure is often exceeded. Generally speaking the lower the pH the less the risk of the growth of unwanted microorganisms, but there are some bacteria, including acetic acid bacteria, that may grow in wines with a low pH.

Whilst the different species of bacteria vary considerably in the other conditions in which they thrive, they all require food, water, and suitable environmental conditions. During grape growing and winemaking bacteria have numerous sources of nutrients, including sulfur, carbon (particularly six carbon sugars), nitrogen, and phosphorus. Although, with some notable exceptions, sugars are largely consumed by yeasts during fermentation, wines that have completed all production processes will contain other nutrients, including typically between 80 and 230 mg/l of phosphorous. Whilst generally bacterial growth is slower at low temperatures, most species flourish in the wide range of 5 °C (41 °F) and 60 °C (140 °F). Pre‐fermentation heat treatments of red musts, including pre‐fermentation hot maceration, flash détente, and thermo détente which, depending upon the individual process, involve heating the must to between 75 and 85 °C (158–185 °F) will kill any bacteria present at that stage, as will pasteurisation, which may be also undertaken when the finished wine is bottled.

Although many species of bacteria are implicated in microbiological wine faults some, particularly certain lactic acid bacteria of the genus Oenococcus , perform a most useful role in the winemaking process, being responsible for the MLF, which leads to textural changes that are almost always desirable for red wines. The MLF is often undertaken for certain styles of white and sparkling wines too, but whether or not it is desired it is crucial that it never spontaneously takes place subsequent to bottling – see Chapter 9.

Faults of microbiological nature that may be found in wine include

Contamination with haloanisoles – see Chapter 3;

Brettanomyces related faults – see Chapter 4;

Excess volatile acidity – see Chapter 7;

Lactic bacteria and Pediococcus related faults – see Chapter 11.

The key tools in minimising the risk of the occurrence of microbiological faults are as follows.

In the vineyard:

Creating and maintaining open leaf canopies to help air‐flow;

Controlling pests such as European grapevine moth (Lobesia botrana), and vinegar fly (Drosophila melanogaster);

Preventing or controlling vine diseases such powdery mildew and downy mildew;

Preventing or controlling grape rots, such as Botrytis cinerea;

Picking only healthy and undamaged fruit;

Picking at an appropriate pH;

Harvesting as cool as possible and transporting to the winery without delay.

In the winery and cellar:

Sorting fruit to exclude rotten and damaged berries and materials other than grapes (MOGS);

Maintaining an appropriate pH in must and wine;

Scrupulous winery and cellar hygiene;

Using commercial preparations of S. cerevisiae for alcoholic fermentations and lactic acid bacteria strains for the MLF.

Careful oxygen management in wine – e.g. avoiding ullage in vats and barrels;

Controlled temperatures for fermentation, maturation, and storage;

Creating and maintaining a nutrient desert [22];

Controlled humidity in the winery (maximum 75%) and barrel store (maximum 80%);

Maintaining an appropriate level of molecular sulfur in wine.

Chemical faults result from unwanted chemical changes in wine and may be due to internal or external factors. Two of the most common faults of chemical origin are chemical oxidation and, conversely, reduced aromas or reduction. There are thousands of chemical reactions that take place during the winemaking processes; some of these can result in the synthesis of compounds noted for their off‐odours and flavours, whilst others give simple changes, the impact of which may include immediate or rapid product deterioration.

Faults that are generally regarded as being primarily of a chemical nature include

Excessive acetaldehyde – see Chapter 5;

Chemical oxidation – see Chapter 5;

Reduced aromas/reduction – see Chapter 6;

Iron haze and copper haze – see Chapter 10;

Eucalyptol – 1,8‐cineole – see Chapter 12;

Smoke taint related compounds, including guaiacol, 4‐methyl‐guaiacol, 4‐methyl‐syringol, m‐cresol, o‐cresol, and p‐cresol – see Chapter 12;

Brown marmorated stink bug related compounds, including trans‐2‐decenal – see Chapter 13.

The key tools in minimising the risk of the occurrence of chemical faults are as follows.

In the vineyard:

Maintaining sufficient, but not excessive, soil nitrogen levels;

Discontinuing the use of all chemical contact and systemic treatments well before harvesting;

Avoiding using agrochemicals that may break down into unwanted odours compounds such as 2,4‐dichlorophenol and store all agrochemicals away from possible sources of contamination;

Minimising sources of aerial pollution, including the burning of winery rubbish;

Minimising the vineyard presence of pests, particularly ladybug (Coccinellidae) and brown marmorated stink bug (Halyomorpha halys).

In the winery and cellar:

Careful oxygen management – e.g. avoiding ullage in vats and barrels and using gases to sparge equipment, but ensuring that adequate levels of dissolved oxygen are maintained;