Whey protein hydrolysates were generated at different total solids (TS) levels (50–300 g/l) using the commercially available proteolytic preparation Debitrase™ HYW20, while enzyme to substrate ratio, pH and temperature were maintained constant. Hydrolysis proceeded at a faster rate at lower TS reaching a degree of hydrolysis (DH) of 16·6% at 300 g TS/l, compared with a DH of 22·7% at 50 g TS/l after 6 h hydrolysis. The slower breakdown of intact whey proteins at high TS was quantified by gel-permeation HPLC. Reversed-phase (RP) HPLC of hydrolysate samples of equivalent DH (~15%) generated at different TS levels indicated that certain hydrophobic peptide peaks were present at higher levels in hydrolysates generated at low TS. Sensory evaluation showed that hydrolysates with equivalent DH values were significantly (P<0·0005) less bitter when generated at 300 g TS/l (mean bitterness score=25·4%) than hydrolysates generated at 50 g TS/l (mean bitterness score=39·9%). A specific hydrophobic peptide peak present at higher concentrations in hydrolysates generated at low TS was isolated and identified as β-lactoglobulin f(43–57), a fragment having the physical and chemical characteristics of a bitter peptide.