The following descriptions of microorganisms involved in traditional fermentation processes are correct: (a) In the process of brewing, the longer the sealing time, the more fermentation.

A, with the deepening of fermentation, nutrients are gradually consumed, metabolic wastes are gradually accumulated, the living conditions of yeast are gradually deteriorated, and the number of yeast deaths increases, entering a recession period. A is wrong;

B. When producing fruit vinegar, yeast turns sugar into alcohol, and then acetic acid bacteria turn alcohol into acetic acid. There is synergy between the two, and B is correct;

C, yeast is eukaryote, and the nuclear membrane covers 9 nuclei, which is C error;

D, lactic acid bacteria breathe anaerobically under anaerobic conditions, and the product is lactic acid, not alcohol and carbon dioxide. D is wrong.

Therefore, choose: B.