The control of the microbial contamination during the sugar fermentation in the processes for obtaining alcohol is a very important action to increase the productivity of the alcoholic fermentation processes. The Saccharomyces cerevisiae yeast cells engage a very tough nutritional competition for the sugarcane juice with the bacteria (Lactobacillus sp and Acetobacter) and the wild yeasts. The proposed composition uses an antimicrobial agent of the guanidine family, such as for example, poly(hexamethyl biguanide), an antibiotic agent, and also a surfactant agent, to prevent the undesired microbial growth. The present invention further refers to the process for controlling the microbial contamination through the use of said agents.