MLF is an interesting topic and the discussion of it is worthy of a separate thread but there a a couple of points that I would like to make here since the topic has come up.
Firstly, the MLF bacteria come in a multitude of varieties in the natural environment and all give off their own by-products. From bitter experience, I never, ever want to have a natural MLF occur in my cider again because the resulting aroma and flavour was, err, "contentious". I'm proud to make "real" cider but when customers start complaining about the last 100-gallon batch, I understand why the big boys *never* use a natural MLF. If they do use MLF then they'll buy in a particular strain of bacteria (I was offered a bag of it by Thatchers last September but declined it). I would recommend the route of using the correct strain of yeast to reduce malic acid levels if a reduction of acidity is required. It is far more reliable.
Secondly, you can't simply feed MLF bacteria by adding malic acid. Certainly it is "food" but there is a problem in that MLF bacteria don't like low pH levels (as found in eating apples - often in the 2.8-3.1 area). Malic acid is a strong acid and so, by adding more without knowing the starting pH level, there is the danger that you'll just kill the bacteria that your trying to feed, in the same way that oxygen is necessary for a person to live but pure oxygen will kill them (so I'm told; I have never tried that one - honest 'Guv).
Cheers,
Martin