Disclosed are substantially stable sterilized liquid protein supplements including a highly soluble protein source for use with human milk and other infant or adult feeding formulas. The sterilized liquid protein supplements have a more neutral pH, thereby inhibiting protein denaturation and reducing microbial growth. In one aspect, the sterilized liquid protein supplement comprises at least 250 mg of protein per 1 ml_, the protein has a degree of hydrolysis of 25% to 45%, a molecular weight median of less than 800 Daltons, and a dipeptide and tripeptide concentration of no more than 40% by weight total protein, wherein the sterilized liquid protein supplement has a pH of 3 to 8.