CHANGES IN PHYSICAL-PROPERTIES OF ACID MI LK GELS BY ADDITION OF LACTOPEROXIDASE

Addition of lactoperoxidase (LPO) to yogurt softens the hardness of th e product. The objective of this study is to clarify the cause of this phenomenon. A mixed culture (Lactobacillus delbrueckii subsp. bulgari cus and Streptococcus thermophilus, 1:1 ratio) was inoculated into rec onstituted skim milk and incubated. After coagulation, the hardness of the gel was measured using a rheometer (Rheoner, Yamaden Inc., Japan) . When hypothiocyanite ion (OSCN-), an oxidation product formed during peroxidase-catalyzed oxidation of thiocyanate (SCN-), was added to th e culture before incubation, the hardness of the gel decreased, To exa mine the effects of OSCN- on the hardness of milk gel without any bact erial or proteolytic activities, reconstituted skim milk was acidified by glucono-delta-lactone (GDL). The hardness of GDL-gel was also decr eased by addition of OSCN-. The reactivity of sulfhydryls in beta-lact oglobulin was decreased by addition of OSCN-. From these results, it w as suggested that OSCN- modified the milk protein, especially beta-lac toglobulin, inhibiting the development of gel network in the LPO-treat ed yogurt.