In winemaking, grape juice is turned to wine during the fermentation process by the action of a number of essential beneficial microorganisms -namely bacteria. Sometimes, though, harmful bacteria also populate the fermentation vat, spoiling the wine in the process.
As part of her doctoral research, researcher Lucia Blasco of MTT developed new methods based on DNA identification for rapidly and accurately identifying detrimental lactic acid bacteria and acetic acid bacteria during the earliest stages of the wine fermentation process.
In her study, Blasco worked with different DNA fragments, i.e. probes, which bind themselves to the DNA of detrimental bacteria allowing the detection of whole cells, and so-called specific primers, which are used to replicate bacterial DNA. She applied FISH, PCR and 16S-ARDRA techniques in the DNA identification and compared how effectively bacteria marked with probes, or their DNA, can be identified using these methods.
"The FISH technique, which utilises fluorescence and can be used to directly identify individual bacterial cells in grape juice or wine, proved the most effective," says Blasco.
Lactic acid bacteria -- a key culprit of bitterness and mustiness
Detrimental lactic acid bacteria and acetic acid bacteria can exist on the surface of grapes prior to crushing; the subsequent grape crushing process then increases the propagation potential of these micro-organisms. In addition, harmful bacteria can also contaminate the grape juice via the winemaking equipment and piping.
As Blasco points out, some lactic acid bacteria are extremely beneficial to the fermentation process. Others, on the other hand, acidify the wine and give it an undesirably bitter palate and musty odour.
"Harmful lactic acid bacteria can also form biogenic amines in the wine which can cause headaches, allergic reactions and blood pressure fluctuations. At worst, they can be carcinogenic," explains Blasco.
In turn, acetic acid bacteria spoil the wine by turning it to vinegar.
Keeping the baddies in check
If harmful bacterial cells are identified early enough in the wine, their numbers can be kept to a safe level. Effective control methods include, for example, treatment with sulphur dioxide gas, filtering or covering with an inert gas of the type used in the food industry for food preservation. The detrimental bacterial DNA identification method developed by the PhD study for use in the wine fermentation process is also suitable for industrial-scale application.
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