INHIBITION OF GEL WEAKENING OF THREADFIN BREAM SURIMI USING THAI LEGUME SEED PROTEINASE INHIBITORS

Partially purified proteinase inhibitors from cowpea (Vigna unguiculata (L) Wasp), pigeon pea (Cajanus cajan (L.) Millsp.) and bdmbara groundnuts (Voandzeia subterranea (L.) Thou) effectively inhibited sarcoplasmic modori‐inducing proteinase extracted from threadfin bream muscle in a concentration dependent manner. Incorporation of these proteinase inhibitors into threadfin bream surimi partially inhibited autolytic degradation and increased the gel force and deformation. Combination of setting and incorporating proteinase inhibitors from cowpea and bambara groundnut var. HY at the level of 30 Kcunits/g resulted in an increase in gel force and deformation by 60% and 26%, respectively. However, the lightness and whiteness of surimi gels decreased slightly when the proteinase inhibitor was added at a level of 30 kunits/g.     

PHYSICOCHEMICAL AND BIOCHEMICAL CHANGES IN WHOLE LIZARDFISH (SAURIDA MICROPECTORALIS) MUSCLES AND FILLETS DURING FROZEN STORAGE

The physicochemical and biochemical changes in whole lizardfish (Saurida micropectoralis) muscles and its fillets kept in air and under vacuum during frozen storage at ?20C for 24 weeks were investigated. The formaldehyde (FA) and dimethylamine contents increased with a concomitant decrease in trimethylamine‐ N‐oxide (TMAO) content as the storage time increased (P < 0.05). The Ca²⁺–adenosine 5′‐triphosphatase activity continuously decreased with a coincidental decrease in salt‐soluble fraction. The disulfide bonds were increasingly formed throughout the storage (P < 0.05). The surface hydrophobicity increased and reached the maximum at week 12 with a subsequent decrease up to the end of storage. In general, the higher changes were observed in samples kept under vacuum than those kept in air. With the same atmosphere used, the whole fish showed slightly higher changes than the fillets. A marked increase in TMAO demethylase (TMAOase) activities was observed up to 12 weeks, followed by the continuous decrease up to 24 weeks of storage. The produced FA might play an important role in inducing protein denaturation and/or aggregation in lizardfish. The TMAOase activity as well as the FA formation could be reduced to some extent with the removal of internal organs and storage in the presence of oxygen. However, a detrimental effect of oxygen, especially on the promotion of lipid oxidation, would be an obstacle.     

EFFECT OF CHICKEN PLASMA PROTEIN AND SOME PROTEIN ADDITIVES ON PROTEOLYSIS AND GEL-FORMING ABILITY OF SARDINE (SARDINELLA GIBBOSA) SURIMI

The effect of chicken plasma protein (CPP) and various protein additives on autolysis and gel‐forming ability of sardine (Sardinella gibbosa) surimi was investigated. CPP and other protein additives showed inhibitory activity toward autolysis of sardine surimi incubated at 70C in a concentration‐dependent manner. Porcine plasma protein (PPP) and egg white (EW) were more effective in proteolysis prevention than CPP and other protein additives. Breaking force and deformation of both modori and kamaboko gels increased when CPP and other protein additives were added at levels up to 2% (P < 0.05). Nevertheless, PPP and EW showed a greater gel‐strengthening effect than CPP and other protein additives (P < 0.05). Addition of CPP and other plasma proteins resulted in decreased whiteness, especially with increasing amount (P < 0.05). However, no change in whiteness was observed with gels containing EW and soy protein isolate (SPI) (P > 0.05). Proteolysis of sardine surimi can be retarded by the addition of CPP and protein additives, leading to increased gel‐forming ability.     

PORCINE PLASMA PROTEINS AS GEL ENHANCER IN BIGEYE SNAPPER (PRIACANTHUS TAYENUS) SURIMI

Cohn's fraction I‐S from porcine plasma showed the highest transglutaminase activity, compared to fractions I. 11+III, IV, IV‐l. The optimum temperature for incorporating monodancylcadaverine into dimethylated casein was 45C. Plasma transglutaminase in fraction I‐S was activated by calcium chloride but was inhibited by N‐ethylmaleimide, ethylenediaminetetraacetic acid, and ammonium chloride. The addition of fraction I‐S into bigeye snapper surimi resulted in a substantial increase in gel breaking force and deformation, particularly in the presence of calcium chloride and thrombin. No changes in whiteness and water holding capacity were observed in surimi gel with the addition of 0–0.5% of fraction I‐S. Fraction I‐S was found to catalyze nondisulfide covalent cross‐linking of myosin heavy chain. The combination of endogenous and plasma transglutaminase enhanced surimi gelation.     

PROPERTIES OF CYSTEINE PROTEINASE INHIBITORS FROM BLACK GRAM AND RICE BEAN

Cysteine proteinase inhibitors (CPI) were purified to 59 and 54 fold from black gram (Vignaraungo (L.) Hepper) and rice bean (Vignaumbellata Thunb.), respectively, by using heal treatment, followed by chromatography on a carboxymethyl (CM)‐papain‐Sepharose affinity column. The purified inhibitors were highly inhibitory to papain and Pacific whiting cathepsin L in a concentration dependent manner. They were detected as a dark band on tricine‐SDS‐PAGE gel stained for inhibitory activity. The apparent molecular weights of purified CPI from black gram and rice bean seeds were estimated to be 12, 000 daltons. The purified inhibitors were thermostable up to 90C and active in the neutral and alkaline pH ranges.     

CHITOSAN AFFECTS TRANSGLUTAMINASE-INDUCED SURIMI GELATION

Effect of chitosan on barred garfish (Hemiramphus far) surimi gel was studied in the presence of EDTA and microbial transglutaminase (MTGase). An increase in breaking force of surimi gels added with 1.0% prawn shell chitosan indicated the gel enhancing effect of chitosan on the heat‐induced gelation of fish myofibrillar proteins. However, gel‐forming ability of surimi containing chitosan was inhibited in the presence of EDTA, especially at higher concentration. Therefore, the enhancing effect of chitosan was possibly mediated through the action of endogenous transglutaminase (TGase) during setting, resulting in the formation of protein‐protein and protein‐chitosan conjugates. In general, addition of MTGase remarkably increased both breaking force and deformation of surimi gel (P<0.05). However, enhancing effect of MTGase was retarded in the presence of chitosan, resulting in lower magnitude of breaking force and deformation (P<0.05). Scanning electron microscopy showed that chitosan particles were uniformly dispersed in the gel matrix. A tightly associated gel network was formed in surimi containing MTGase, whereas a large number of voids were noted in gels with EDTA. These results suggest that chitosan acted as a surimi gel enhancer in combination with endogenous TGase in fish muscle, but hindered gel formation in the presence of MTGase.     

ACCELERATED PROTEOLYSIS OF SOY PROTEINS DURING FERMENTATION OF THUA-NAO INOCULATED WITH BACILLUS SUBTILIS

Thua‐nao is a traditional Thai‐fermented soy product. It was prepared by the conventional method and also with starter inoculation. Inoculated soybean exhibited a higher rate of fermentation than did the natural fermentation as indicated by greater rate of pH increase, higher extent of proteolysis, ammonia‐nitrogen content and nitrogen solubility. Sodium dodecyl sulfate‐polyacrylamide gel electrophoresis patterns indicated the extensive degradation of soy proteins during fermentation. Activity staining of Thua‐nao extract suggested that proteolysis of soybean was mediated by various proteinases. Active proteinases with estimated molecular weight (MW) of 29,000, 27,000 and 19,000 Da were present during natural fermentation of soybeans. However, proteinases with MW of 40,000 and 29,000 Da were predominant in inoculated soybeans and like the activity bands observed with spent culture broth of Bacillus subtilis BIOTEC 7123. The results suggested that proteinases released by the dominant species in the inoculum, especially B. subtilis, play an important role in proteolysis of soy proteins during fermentation. Inoculation with B. subtilis resulted in an increased proteolysis, which is useful for subsequent reactions leading to development of Thua‐nao characteristics.