Vol. 65, No. 1, 2000—JOURNAL OF FOOD SCIENCE 129 © 2000 Institute of Food Technologists Food Engineering and Physical Properties Proteolytic Degradation of Tropical Tilapia Surimi J. YONGSAWATDIGUL, J.W. PARK, P. VIRULHAKUL, AND S. VIRATCHAKUL ABSTRACT: Proteolytic degradation of tropical tilapia surimi was biochemically and rheologically characterized to identify a group of proteinase(s) responsible for its textural degradation. Proteolysis of tilapia surimi occurred as the temperature increased and attained the highest activity at 65 °C. Smaller proteins with molecular weight of 116-97 kDa were noted as a result of myosin heavy chain (MHC) degradation. MHC completely disappeared when incu- bated at 65 °C for 4 h. Proteolysis was significantly inhibited by soybean trypsin inhibitor (SB) and leupeptin (LE). Storage modulus (G') of surimi gels mixed with either SB or LE was higher than other inhibitors indicating that serine type proteinase(s) were involved in proteolysis of tropical tilapia surimi. Key Words: tilapia, surimi, enzymatic degradation, serine proteinase Proteolytic degradation of fish muscle proteins can be over- come using various strategies including food grade enzyme in- hibitors (Morrissey and others 1993), a rapid heating method (Yongsawatdigul and Park 1996), or high hydrostatic pressure techniques (Chung and others 1993). The optimal processing parameters can be determined based on the knowledge of pro- teolytic degradation patterns of fish species. Our objectives were: (1) to investigate degradation patterns of tilapia surimi at various incubation times, temperatures and salt concentration, and (2) to identify the group of proteinase(s) involved in proteol- ysis of tilapia surimi. Results and Discussion Optimum time and temperature of proteolysis Proteolytic activity linearly increased with incubation time at 65 °C for up to 240 min (Fig. 1). Proteolysis of tilapia surimi did not take place until the temperature > 40 °C, then it gradually in- creased to 50 °C and rapidly reached a maximum at 65 °C (Fig. 2). Proteolytic activity was not detected at 70 °C (Fig. 2), which was likely due to thermal denaturation of proteinase(s). Protein patterns of tilapia surimi for varied incubation times at 65 °C are shown in Fig. 3. Degradation of MHC was noted at 30 Introduction T ILAPIA IS A FISH WITH MILD FLAVOR AND WHITE FLESH. IT IS ONE of the most frequently aquacultured freshwater fish in the world. Park and others (1990) reported that blue tilapia (Areo- chromis aureus) surimi produced excellent gels. According to our recent comparative study with Alaska pollock and Pacific whiting (Klesk and others 2000), the gel quality of tropical tilapia (Tilapia niloticus) surimi was comparable to gel strength of Alaska pollock when heated at 90 °C for 15 min and was better than Pacific whit- ing surimi without enzyme inhibitors. Setting effects, accompa- nied by a decreased content of myosin heavy chain (MHC) and an increased shear stress value, were also noticed when pre-in- cubated for 1 h at 40 °C. These results indicate that tilapia can produce high quality surimi. When incubated at 60 °C for 30 min, a gel-weakening phenomenon was observed. This could limit its use, particularly in surimi-based products manufactured at slow heating. It has been shown that endogenous proteinases are responsi- ble for the gel weakening phenomenon; these proteases vary among fish species. Serine proteinases were found to be respon- sible for textural breakdown of threadfin bream (Nemipterus bathybius) (Toyohara and Shimizu 1988), oval-filefish (Navodon modestus) (Toyohara and others 1992), and lizardfish (Saurda sp.) (Suwansakornkul and others 1993). The enzyme was heat stable and effectively hydrolyzed MHC at pH 7.0 and 60 °C. Cathepsin L, a cysteine proteinase, hydrolyzed muscle proteins of Pacific whiting and caused severe textural degradation in whiting surimi (An and others 1994b; Morrissey and others 1993). The enzyme exhibited optimum activity at 55 °C and pH 5.5 and its molecular weight was found to be 28,800 daltons (Seymour and others 1994). Arrowtooth flounder is another species exhibit- ing high cystein proteinase activity, which was presumably relat- ed to an infection of the parasite, Kudoa thyrsitis (Greene and Babbitt 1990). In chum salmon surimi, gel quality was improved when L-trans-epoxysuccinylleucylamino (4-guanidino) butane (E-64) was added, indicating involvement of heat stable cysteine proteinase(s) in textural degradation (Saeki and others 1995). Heat stable alkaline proteinase also involves textural degrada- tion of surimi gels. A distinct characteristic of alkaline proteinase is its high activity in alkaline condition (pH 8.5 to 9.1) at 60 to 65 °C (Iwata and others 1974; Boye and Lanier 1988; Makinodan and others 1985). The enzyme shows maximum activity toward MHC, actin, and tropomyosin at 60 °C. JFS: Food Engineering and Physical Properties Fig. 1—Autolytic activity of tilapia surimi incubated at 65 °C