• bacteria occupy every ecological niche on earth: fresh water, salt water, soil, acidic hot springs, deep in the Earth’s crust, on and in organic matter, including plant and animal bodies
• bacteria are abundant: there are typically 40,000,000 bacterial cells in a gram of soil, 1,000,000 in a millilitre of fresh water; they form much of the world’s biomass (there is an estimated five nonillion [5×1030] bacteria on Earth)
• it is estimated that approximately 1% of all bacteria have been described
• bacteria are small, so small that you cannot see them without the assistance of a microscope at 1000x magnification. Most are about 0.5 - 5.0 micrometers (mm) in length (a micrometre is one thousandth of a millimetre). One exception is Thiomargarita namibiensis, a bacterium found in ocean sediments off the continental shelf of Namibia, which is 0.1-0.5 mm in length, and can be seen with the naked eye
• there are approximately 10 times as many bacterial cells as human cells in the human body (a large proportion are found on the skin and in the digestive tract)
• without bacteria we would not be able to ingest some of the nutrients in our food
• bacteria are economically important and are used in numerous industrial processes: waste processing, bioremediation, oil spill clean-ups, biological pest control, and production of therapeutic proteins
• bacteria are used in fermentation processes to produce a large array of foods, including yoghurt, cheese, pickles, sauerkraut, soy sauce, cocoa, coffee, vinegar and wine.

This article will focus on the bacterial component of the microflora that is associated with winemaking, considering first the ‘good guys’ then the winemakers’ foes.

Bacteria – the winemaker’s friend
The role of bacteria in winemaking is well established. Nearly 150 years ago, the father of microbiology, Louis Pasteur, noted the presence of bacteria in fermenting grape juice, and their capacity to produce lactic acid. Some 30 years later, in the 1890s, Müller- Thurgau showed that bacteria, not yeast, were required to reduce acidity in wine, by a process we now know as malolactic fermentation (MLF). The importance of MLF became evident in the production of Burgundy and wines from California, where it was realised that MLF improved wine quality; practices were subsequently employed to ensure of its occurrence.

It was not until the late 1960s that the principal bacterium responsible for MLF was characterised. At that time it was identified as Leuconostoc oenos, later to be reclassified as Oenococcus oeni. Over the next 40 years, O. oeni’s role in winemaking was studied intensely. What we have learnt from this research is that deacidification of wine is not the only benefit of MLF; this process contributes in several other ways to wine quality. For example, the metabolism of malic acid is the removal of a potential carbon source for spoilage bacteria, thereby imparting microbial stability to wine. In addition, the production of secondary metabolites by O. oeni augments the sensory qualities of wine; in other words, MLF imparts aroma and flavour attributes to wine.

Fermentation-derived compounds, originating from yeast and bacterial metabolism during fermentation of grape must, include many different types of volatile chemicals. Some of these compounds are found in wines at or above their sensory threshold. These compounds contribute flavours and aromas described as buttery, fruity, berry, floral to soap, fusel, solvent to vinegar, rancid and cheesy. Data on the effects of MLF on volatile fermentationderived compounds in wines (from numerous different red and white wine trials) show that there is a consistent increase in most of the ethyl esters and a decrease in acetate-esters. The concentration of fermentation-derived compounds is also affected by ...

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