Characteristics of ascorbate peroxidase in malt

Malt and Grains Session
Makoto Kanauchi, Miyagi University Taihaku-ku, Sendai-city, Miyagi, Japan
Co-author(s): Charles Bamforth, University of California, Davis, CA, USA

ABSTRACT: Oxidation promotes off-flavors and haze in beer, as well as impacting parameters such as wort separation. Increasingly attention has been drawn to the extent to which oxidative reactions occur in the brewhouse. Several oxidase and peroxidase enzymes may play a role in promoting such reactions. Thiol oxidase and oxalate oxidase levels increase during germination. Thiol oxidase produces disulfide (-s-s-) bonds in proteins that result in reduced lautering rates. Both oxidases produce hydrogen peroxide in the reactions they catalyze, and this reactive species can be the precursor of the even more reactive hydroxyl. Barley develops a number of peroxidases capable of consuming peroxide by reaction with polyphenols in the mash; however, some argue that it would be desirable to preserve such molecules in the beer where they can function as antioxidants. Our attention has therefore turned to an enzyme not hitherto studied in barley, namely ascorbate peroxidase. Levels of this enzyme were highest at 5 days of germination, coinciding with the highest level of ascorbic acid. Furthermore, enzyme activity during germination was higher when the grain was sprouted under an atmosphere of 15% CO2. The optimum pH of the enzyme was 5.5, and 40% of the activity survived 30 min at 50°C, although the enzyme would not survive at mash conversion temperatures. The enzyme, a monomer with a molecular weight of 25,000, is inhibited by iron, mercury, and copper.

Makoto Kanauchi graduated from the Tokyo University of Agriculture in Tokyo, Japan, in 1996 and received a Ph.D. degree in bio-regulation control from that university in 1999. He worked in Charles Bamforth’s laboratory in the Department of Food Science and Technology, University of California at Davis (1999–2003). Subsequently, he was employed at the Institute of Food Science in Fuji Oil Co. Ltd. in Moriya, Ibaraki, Japan, as a researcher (2003–2005). Since 2005, he has been at the Department of Food Management, Miyagi University. He has also been a lecturer in enzymology and alcoholic beverages (mainly spirits and wine) at the Tokyo University of Agriculture since 2005.