Objective  Potato juice of freeze-thaw process is the liquid by-product derived from the production of purple potato flour by solid-liquid separation method, and it is rich in carbohydrates and anthocyanins. In order to increase the utilization value of the liquid by-product, the enzymolysis juice was added as supplementary ingredient into wort to prepare dark beer by fermentation.

Method  Single factor test combining orthogonal experiment was performed to optimize the fermentation conditions. The contents and activities of antioxidant components in the dark beer were evaluated.

Result  The optimum fermentation conditions for dark beer preparation with addition of purple potato enzymolysis liquid were as follows: Content of potato enzymolysis liquid was 25% (φ), initial pH was 6.5, inoculation of 2% (φ) M21 beer starter, and fermentation for 10 d under the temperature of 15 ℃. The prepared beer was amber color and crystal clear, had harmonious flavor and mellow taste. Its sensory score was 86.9, alcohol content was 5.84%, total acid content was 15.6 mL/L beer, total reducing sugar content was 18.69 mg/mL, chroma was 37.71 EBC, total phenol content was 360.2 mg/L, chlorogenic acid content was 685.7 mg/L, and anthocyanin content was 208.4 mg/L. The total reduction capacity of 10-fold diluent beer was equivalent to that of 0.150 mg/mL vitamin C, the scavenging rate of DPPH radical was 49.9%, which was equivalent to that of 0.043 mg/mL vitamin C, and the scavenging rate of ABTS⁺ was 83.9%.

Conclusion  Addition of purple potato juice of freeze-thaw process can partially replace wort for brewing of dark beer with high antioxidant activities.