To add to Novalou's statement:
Malolactic Fermentation (MLF) put simply, is the conversion of L-malic acid into L-lactic acid, MLF is very important to the wine making process; especially to the final product, these conversions impart positive characteristics such as desirable aroma, flavor, and increased mouth feel, just to name a few.
When a wine has gone through a spontaneous MLF it does not mean that these bacteria will give us an MLF that we can predict, nor will it give us one that has the positive characteristics that we expect from MLF such as taste, mouth feel smoothness, aromas and sensory profiles that we want; It only means that lactic acid bacteria are present in the wine, and that the bacteria, not the winemakers, have the ultimate control over the quality of the finished product.
To make matters worse, if a spontaneous, but undesirable, strain of Malolactic bacteria becomes implanted in the winery, then all subsequent wine made in that winery, commercial or home winery, may be in danger of exhibiting the negative characteristics associated with that particular strain of bacteria.
It is crucial that we add known, reliable strains of the bacteria, and not rely on the strains that have become implanted in our wineries.
