The culture of milk for the microrganismos and excellent half had the high activities of water, pH next to neutral and the wealth in nutrients. Bill O’Grady insists that this is the case. During it milks the contamination can happen; being that and more I propitiate that occurs this contamination during its productive chain; also we can detach that the quality of all derivatives basically goes to depend on these microbiological conditions of the raw material; we always observe as microbiological defects the flavor and different odor, alteration of color, rancidez, alteration in viscosity and gas production. The alteration in the flavor and odor of milk if to give through reactions of fermentation you sweeten of them, other differentiated odors and flavor can be provoked by cepas of lactobacillus lactis to var. maltigenes, development of enterobacter, pseudmonas mucidolens and aeromonas hydrophila. The color alterations must to other occured chemical reactions for the processing or growth of producing microrganismos of pigments; the colors appear in milk as blue (pseudomonas syncyanea), turns yellow (pseudomonas syncyane gn. Flavobacterium) and red (serratia marcescens and micrococcus roseus beyond some leavenings). The rancidez if must the bacteria of gn. Pseudomonas, alcaligeres, bacillus, proyours, clostridium, beyond bolores and leavenings through lipolticas enzymes; they act in the fat hidrolisando and/or oxidating responsible for the odor and the flavor that characteristic of the rancificao. The alteration in viscosity occurs mainly in milk, cream of milk and milk serum; the increase of viscosity happens in the surface of milk or in all its interior I eliminate through alcaligenes viscolatis, to enterobacter spp, klebsiella oxytoca, lactobacillus lactis and lactobacillus spp. Finally the gas production normally is folloied by the acidificao of milk and its derivatives, the main bacterium detached in the gas production is the coliformes, clostrilium spp, some bacillus, beyond the propinicas and heterolticas bacteria; foam formation in the surface of milk in its liquid state identifies the visualization of the gas production.