In this work, we use a tapered-optical-fiber-based evanescent field sensor for detecting the percentage of fat in milk. This measure is an important index of quality as it is correlated with the amount of protein present in this product. The tapered fiber was made by heating and stretching a relatively small length of optical fiber, so as to increase the sensitivity of the guided modes in the region thinned to changes in the refractive index of the external medium. The sensor principle is based on the modulation of the light intensity produced by the variation in the percentage of fat in milk.