Abstract: The objective of this study was to investigate the relationship between the hydrolysis degree of lactose combined with heat treatments and Maillard reaction in milk. Fresh milk was hydrolyzed with neutral lactase to obtain low-lactose milk with different hydrolysis degrees. Then, the products were treated by various heating methods and the degree of Maillard reaction was assessed. The glucose concentration and the hydrolysis degree of lactose in milk at different hydrolysis times were measured by the glucose oxidase method. Furosine and 5-hydroxymethylfurfural (5-HMF) contents and browning degree (OD values) were determined by using high-performance liquid phase chromatography (HPLC) and ultraviolet spectrophotometry, respectively. The results showed that the glucose content in milk was increased with prolonged time of lactose hydrolysis, and glucose concentration slowly increased from 0.00 to 1 721.33 mg /100 mL. Similarly, lactose hydrolysis percentage increased from 0% to 70.33%, reaching a level higher than 50% after 2.0 h heat treatment. Furosine content in milk showed a rising trend with extended heat treatment (P < 0.05). Milk subjected to hydrolysis for more than 3.0 h and then heat treatment at 75 ℃ for 30 min had a furosine content exceeding 190 mg/100 g protein, while hydrolysis for 0.5 h and longer followed by heat treatment at 75 ℃ for 15 s resulted in a furosine content more than 12 mg/100 g protein. 5-HMF was not detected in fresh milk or in milk hydrolyzed and heat treated at 75 ℃ for 30 min. The content of 5-HMF in hydrolyzed milk increased with the extension of hydrolysis time followed by heat treatment at 75 ℃ for 15 s (P < 0.05). Likewise, the browning degree significantly increased with the extension of hydrolysis time (P < 0.05). Browning reaction in milk treated at 75 ℃ for 30 min was significantly higher than that at 75 ℃ for 15 s. Therefore, enzymatic hydrolysis of lactose coupled with heat treatment can promote Maillard reaction and thus affect protein quality in milk.

Key words: milk, lactose hydrolysis, Maillard reaction, furosine, 5-hydroxymethylfurfural