(2) The main component of plant cell wall and intercellular layer is pectin, which is a macromolecular compound and insoluble in water. Pectinase and protease can be added to make the fruit wine clear and transparent without precipitation.

(3) According to the meaning of the question, the fermentation of aseptic wort by immobilized yeast cells failed, and the fermentation conditions met the operating requirements. The reason for the failure is the immobilization of yeast cells, which may be due to the low concentration of sodium alginate and the small number of immobilized yeast, resulting in too few fermentation strains and too little alcohol. It is also possible that when sodium alginate melts, the activated yeast solution is poured without cooling, killing the yeast strain.

(4)① By analyzing the experiments in the table, we can see that there are two groups of experiments, the experimental group and the control group, which follow the control principle of experimental design; The independent variables of this experiment are immobilized amylase and common amylase, and the reaction temperature conditions and reaction time are independent variables, and the independent variables are the same, so this experiment follows the principle of univariate in experimental design.

② The starch molecule is too big to contact with amylase through agar diffusion, so the reaction can't proceed and the starch can't be hydrolyzed, so 1 test tube will turn blue after adding iodine solution.

So the answer is:

(1) Acetobacter ferments to produce acetic acid.

(2) Pectinase and protease

(3)① The concentration of sodium alginate is too low, and the number of immobilized yeast is too small.

② When sodium alginate is melted, the activated yeast solution is poured directly without cooling.

(4)① Control principle and univariate principle (if one item is missing, no score)

② Because the starch molecule is too big to contact with amylase through agar diffusion, the reaction cannot be carried out.