is it possible that malolactic fermentation can start in wine which is allowed to age in barrels?

Answer 1

Yes, it can. In fact, under the right (or wrong) circumstances, it can even occur in the bottle. Usually wineries take precautions to control malolactic fermentation.

The bacterial fermentation of malic acid into lactic acid and CO2 is sometimes desired (for most red wines) and sometimes discouraged (such as to preserve acidity in white wines grown in warm climates).

Some wineries may actually encourage malolactic fermentation to occur in barrel. They can rely on naturally occuring bacteria or add their chosen culture. Other times, it may be more appropriate to prevent malolactic from occurring by adding sulfur dioxide. This can be done at the time of pressing, during the first fermentation, or as the wine is transferred to aging barrels.

Hope that helps. Cheers!

Answer 2

That seems to be the worry. You can rest assured that over 2000 years of wine making, barrels have been tried and found lacking.