Herein disclosed is a process for solubilizing protein from a biological source selected from the group consisting of animal tissue and keratinous material and combinations thereof, the process comprising: forming an alkaline slurry comprising the biological source and a first alkali comprising hydrated lime or quicklime; heating the alkaline slurry to a first temperature of at least 50° C. for a first treatment time to solubilize protein while minimizing degradation of amino acids and obtain a first treated slurry; separating from the first treated slurry a first liquid product and a solid product, wherein the first liquid product comprises the solubilized protein and calcium; and removing at least a portion of the calcium from the liquid product by adjusting pH to higher than 8 to precipitate calcium and produce a low-calcium liquid product.