INACTIVATION OF YEAST AND FILAMENTOUS FUNGI BY THE LACTOPEROXIDASE HYDROGEN-PEROXIDE THIOCYANATE SYSTEM

The antifungal activity of the lactoperoxidase (LPO) system with gluco se oxidase (GOD) as source of hydrogen peroxide was determined in salt solution and in apple juice. The test organisms Rhodutorula rubra and Saccharomyces cerevisiae were cultivated aerobically in apple juice, Mucor rouxii was grown on wort agar adjusted to pH 4.5. Aspergillus ni ger and Byssochlamys fulva were kept on malt extract agar. Spores of t he filamentous fungi were harvested by suspension in salt solution sup plemented with Tween 80(R) and checked microscopically. The antifungal activity of the combined enzyme system was tested with initial counts of approx. 10(5) cfu.ml(-1) (yeast cells or spores) suspended in salt solution supplemented with 25 mg.1(-1) thiocyanate and 20 g.1(-1) glu cose or in apple juice supplemented with the same amount of thiocyanat e. The tests were performed with 25 ml of the medium in 100 ml Erlenme yer flasks shaken at 28 degrees C under aerobic conditions. Inactivati on was achieved for all test organisms in both media. The yeast strain s were found to be least stable while B.fulva was most resistant. A co mbination of 5 U.ml(-1) LPO with 0.5 to 1 U.ml(-1) GOD was sufficient for complete inactivation of this mold in salt solution within 2 h. Th e enzyme system also showed antifungal activity in apple juice at acid pH (3.2), although its effectiveness was reduced. In this medium, B. fulva was inactivated by 20 U.ml(-1) LPD and 1 U.ml(-1) GOD within 4 h . R. rubra and S. cerevisiae were unable to survive in apple juice at 5 U.ml(-1) LPO combined with 1 U.ml(-1) GOD. For inhibition by GOD alo ne, higher amounts of this enzyme were needed and even then only M. ro uxii and R. rubra have been affected within the concentration range te sted (maximum 3 U.ml(-1)).