Disclosed are concentrated, liquid, human milk fortifier compositions comprising from about 15% to about 45% by weight of protein, on a dry weight basis, and having a caloric density of from about 1.25 kcauml to about 6.0 kcau/ml, wherein the liquid human milk fortifier composition is added to human milk in a volume-volume ratio of from about 1:3 to about 1:9. These composition include embodiments comprising carbohydrate and fat, that are formulated with improved stability by selecting any one of the following variations: 1) certain whey-casein protein blends, 2) water insoluble calcium-containing materials, 3) protein hydrolysates, and 4) aseptically packaged concentrates. The liquid concentrates are especially useful for providing nutrition to preterm infants in neonatal intensive care units or similar other institutional setting, and to minimize the risk of introducing microbial contaminants such as Enterobacter sakazakii to infant feedings during preparation in such institutional settings.