I recently received a question regarding the performance of the Accuvin Malic Acid test kit.  A customer had prepared a 75 m/L malic acid standard from a quantity of repackaged malic acid he had purchased.   He assumed the malic acid was the DL-malic acid form, which is about 50% active in enzymatic tests like the Accuvin test kit. 

He tested the sample using the Accuvin kit, and assumed the result would be about 30 mg/L (actually 37.5 mg/L).  The actual reading was 75 mg/L. 

What could have happened?  The malic acid he used had been repackaged.  It most likely was the L-malic acid form, which is 100% active in the enzymatic test, and not the DL-malic acid form he expected.

Chemically, the two forms are identical.  They differ in some physical properties. For example, the melting point of the L-malic acid form is 101C – 103C, while the melting point for the DL-malic acid form is 130C – 133C.

The key difference is biological.  DL-malic acid, a mixture of D-malic acid and L-malic acid, is the form most readily available commercially, and is the most economical. In DL-malic acid, the D-malic acid form will affect the acidity just like the L-malic acid form, but it will not later undergo malolactic fermentation. Thus, only about half can later be converted to the softer-tasting L-Lactic Acid; the other half is microbially stable. It should be added that the D-malic acid form has caused a slowing of primary fermentation by some strains of yeast. Also, especially when MLF is not planned, with malic acid additions there is a risk of developing green acid taste.

The bottom line:  plan your chemical additions.  If you want to make an acid adjust only without malolactic fermentation, DL-malic acid is a viable option.  If you want to adjust acid levels prior to primary fermentation, and/or if you are planning MLF, it might be better to stick to tartaric acid or use only the L-malic acid form.