It is not always easy to determine if the appearance of bubbles following primary fermentation is the onset of actual MLF or merely a wine “outgassing” as it warms. Knowing whether malolactic fermentation has started allows the detection of problems when they are small and require less extensive and expensive intervention. Grapes naturally contain no L-Lactic Acid. A small amount can be produced by some yeasts.  An L-Lactic Acid level above 40 mg/L is considered a confirmation that malolactic fermentation has begun.