Manat Chaijan

Physicochemical changes of tilapia (Oreochromis niloticus) muscle during salting

The effect of wet and dry saltings on the physicochemical changes of tilapia (Oreochromis niloticus) muscle was investigated. Dry salting resulted in the higher rate of salt uptake into tilapia muscle facilitating the faster decrease in Aw (p<0.05). The pH of both dry and wet salted fish muscles tended to decrease throughout the salting time and the lower pH was found in dry salted fish (p<0.05). The increase in the protein content in the salting medium was found during wet salted tilapia production (p<0.05). The TCA-soluble peptide content tended to decrease with increasing the salting time in both salting methods (p<0.05), suggesting a leaching effect of the salting medium or the exudative loss occurred in salted tilapia. Wet salting caused the greater formation of metmyoglobin in tilapia muscle when compared to dry salting at all time points (p<0.05) and the content of metmyoglobin increased as salting time increased in both salting methods (p<0.05). A lowered metmyoglobin with a lowered redness index of dry salted tilapia muscle was found, indicating the continuous oxidation of metmyoglobin to other hypervalent derivatives and hence the discolouration of salted tilapia. Lipid hydrolysis and oxidation of tilapia meat occurred with varying degrees in both salting methods and these changes depended on salting time. Dry salting resulted in a higher oxidation of tilapia muscle lipid as indicated by the higher PV and TBARS throughout the salting period when compared with that of wet salting (p<0.05). In conclusion, the physicochemical changes of tilapia muscle during salting are governed by the salting method and the salting time applied.

24kDa Trypsin: A predominant protease purified from the viscera of hybrid catfish (Clarias macrocephalus×Clarias gariepinus).

Trypsin was purified to homogeneity from the viscera of hybrid catfish (Clarias macrocephalus×Clarias gariepinus) through ammonium sulphate fractionation and a series of chromatographies including Sephacryl S-200, Sephadex G-50 and DEAE-cellulose. It was purified to 47.6-fold with a yield of 12.7%. Based on native-PAGE, the purified trypsin showed a single band. The molecular weight of purified trypsin was estimated as 24kDa by size exclusion chromatography and SDS-PAGE. The optimum pH and temperature for N(α)-p-tosyl-l-arginine methyl ester hydrochloride (TAME) hydrolysis were 8.0 and 60°C, respectively. Trypsin was stable to heat treatment up to 50°C, and over a pH range of 6.0-11.0. Trypsin was stabilized by calcium ion. The trypsin activity was strongly inhibited by soybean trypsin inhibitor and N-p-tosyl-l-lysine chloromethyl ketone and partially inhibited by ethylenediaminetetraacetic acid. Activity decreased continuously as NaCl concentration (0-30%) increased. Apparent Km value of trypsin was 0.3mM and Kcat value was 92.1S(-1) for TAME. The N-terminal amino acid sequence of 20 residues of trypsin was IVGGYECQAHSQPPTVSLNA, which is highly homologous with trypsins from other species of fish.

Interrelationship between myoglobin and lipid oxidations in oxeye scad (Selar boops) muscle during iced storage

The interrelationship between myoglobin oxidation, lipid oxidation and discolouration in oxeye scad fish during iced storage were investigated. The myoglobin autoxidation rate increased with increasing storage time up to 12 days (p < 0.05) and remained constant thereafter (p > 0.05). Increase in metmyoglobin correlated well with a blue shift from 410 to 408 nm for myoglobin. The soret band of myoglobin decreased with a concomitant decrease in the redness index (p < 0.05). During storage, the extractable haem iron decreased (p < 0.05), while the non-haem iron increased (p < 0.05). Hydrogen peroxide and ferrylmyoglobin concentrations had increased at the end of storage (p < 0.05). The conjugated diene (CD) and peroxide value (PV) of oxeye scad lipids tended to stabilise during the initial phase of storage, increased in the differentiation phase and had declined at the end of storage. However, thiobarbituric acid reactive substances (TBARS) increased markedly (p < 0.05). Overall, lipid and myoglobin oxidations in oxeye scad occurred in a concurrent manner and each process appeared to enhance the other.

Discoloration and Lipid Deterioration of Farmed Giant Catfish (Pangasianodon gigas) Muscle during Refrigerated Storage

Discoloration and lipid deterioration of farmed giant catfish (Pangasianodon gigas) muscle during 14 d refrigerated storage were investigated. Lipid deterioration, lipolysis, and lipid oxidation in both dorsal and ventral muscles increased as storage time increased. A progressive formation of primary lipid oxidation products monitored by the increase in conjugated dienes (CD) was observed (P < 0.05) and the increase in thiobarbituric reactive substances (TBARS), an index of secondary lipid oxidation products, was noticeable throughout the storage (P < 0.05). The pH of both dorsal and ventral muscles tended to increase as storage time continued (P < 0.05). A gradual increase in free fatty acid (FFA) formation was found within the first 10 d of refrigerated storage (P < 0.05), suggesting hydrolysis induced by lipases and phospholipases. However, a sharp decrease in FFA content was observed at the end of storage. Refrigerated storage also resulted in changes in redness index of both dorsal and ventral muscles. These changes were coincidental with the changes in metmyoglobin content. Therefore, the discoloration and lipid changes in giant catfish muscle during refrigerated storage depended on the muscle type and might be related to the difference in composition between dorsal and ventral muscles.

Characterisation of muscles from Frigate mackerel (Auxis thazard) and catfish (Clarias macrocephalus)

Frigate mackerel (Auxis thazard) and catfish (Clarias macrocephalus) can be used as alternative sources for surimi production. However, the functionality of surimi is species-dependent. This study aimed to characterise certain chemical and physical compositions of dark and ordinary muscles from these species. Catfish, particularly ordinary muscle, was composed of higher contents of lipid and carotenoid than Frigate mackerel muscle (p<0.05) but ordinary muscle from Frigate mackerel had the highest phospholipid content (p<0.05). Both dark and ordinary muscles of Frigate mackerel had greater contents of myofibrillar proteins than had catfish muscle (p<0.05). Myosin heavy chain and actin were predominant proteins found in both muscle types of both species. Dark muscle from Frigate mackerel had the highest sarcoplasmic protein content, especially extractable myoglobin (p<0.05). Muscles from Frigate mackerel had greater content of sodium chloride than had catfish (p<0.05). The highest contents of iron, copper and selenium were found in Frigate mackerel dark muscle (p<0.05). The pH of ordinary muscle from both species was higher than that of dark muscle (p<0.05). Frigate mackerel, especilly dark muscle, exhibited the most dark-red colour, as shown by the lowest L(*) and b(*) values with the highest a(*) value and redness index (a(*)/b(*)) (p<0.05).

Chemical deterioration and discoloration of semi-dried tilapia processed by sun drying and microwave drying

ABSTRACT Drying methods can affect the chemical deterioration and discoloration of fish muscle. This study aimed to monitor the effect of microwave drying at different effective powers in comparison with sun drying on the physicochemical changes of semidried tilapia. Lipid deterioration, protein oxidation and discoloration of semidried tilapia (Oreochromis niloticus) muscle processed by sun drying and microwave drying were investigated. Results of the study revealed that the pH of microwaved tilapia was higher than that of the sun-dried tilapia (p < 0.05) and the pH increased with increasing microwave power (p < 0.05). The sun drying possessed higher degree of lipid oxidation as indicated by thiobarbituric acid-reactive substances (TBARS) with a greater level of lipolysis when compared to microwave drying (p < 0.05). Higher protein carbonyl content was in microwaved tilapia compared to sun-dried one (p < 0.05) and this content increased with increasing microwave power (p < 0.05). Sun drying resulted in a brown discoloration correlated with the highest metmyoglobin formation (p < 0.05), whereas microwave drying, particularly with high power, caused a green discoloration of semidried tilapia as revealed by a negative a* value with a significantly decreased redness index (p < 0.05). Therefore, lipid deterioration, protein oxidation and discoloration of semidried tilapia muscle were strongly influenced by drying methods.

Functional properties of pH-shifted protein isolates from bigeye snapper (Priacanthus tayenus) head by-product

ABSTRACT The functional properties of pH-shifted protein isolates from bigeye snapper head were evaluated. Alkaline isolate showed a superior salt-solubility and gel forming ability to acid counterpart as indicated by a regular gel structure (i.e., imaged by scanning electron microscope) with higher gel strength and lower expressible drip (p < 0.05). Acid isolate exhibited higher surface hydrophobicity (p < 0.05) and thereby improved interfacial properties. Emulsifying activity index of acid isolate was lower than commercial whey protein and egg-white (p < 0.05) but its emulsion stability was better (p < 0.05). Both protein isolates had lower foamability than commercial proteins but their foam stability was not different (p > 0.05).

Aqueous two-phase partitioning of liver proteinase from albacore tuna (Thunnus alalunga): Application to starry triggerfish (Abalistes stellaris) muscle hydrolysis

ABSTRACT The potential of aqueous two-phase system (ATPS) for the purification and recovery of proteinase from albacore tuna (Thunnus alalunga) liver was explored. Influence of phase compositions such as type of phase forming salts, PEG molecular weight, concentration of salt and PEG, pH of the system, and NaCl addition on partitioning of proteinase was investigated. ATPS comprising PEG1000 (25%, w/w) and NaH2PO4 (20%, w/w) at pH 7.0 provided the best condition for the maximum partitioning of proteinase into the top phase and gave the highest purification factor (5.58-fold) and specific activity (20.65 unit/mg protein). The yield of 89.99% was obtained. The addition of NaCl up to a final concentration of 6% (w/w) decreased the degree of purification and enzyme recovery of proteinase. Based on electrophoresis and activity staining, the fractionated proteinases had the MW 21, 24, 30, and 34 kDa. The effect of fractionated proteinases on starry triggerfish (Abalistes stellaris) muscle hydrolysis was also studied. Fractionated proteinases were able to hydrolyze triggerfish muscle in a dose-dependent manner. Overall, results demonstrated the feasibility of ATPS for the recovery and purification of proteinase without the need for multiple steps, and the obtained proteinase can be further in preparation of protein hydrolysate.

Removal of Lipids, Cholesterol, Nucleic Acids and Haem Pigments During Production of Protein Isolates from Broiler Meat Using pH-shift Processes

Abstract The effect of acid and alkaline pH shift processes on removal of total lipids, cholesterol, nucleic acids and haem pigments during production of protein isolates from broiler meat was investigated. The gel-forming ability of resulting protein isolates were evaluated in comparison with raw broiler meat and water washed broiler meat. Significant reduction of total lipids, cholesterol, nucleic acids and haem proteins was obtained from both pH shift processes (p < 0.05). Acid process recovered more protein with less total haem pigments resulting in a greater breaking force and whiteness of the isolate gel compared to alkaline counterpart (p < 0.05). However, protein isolate gels showed inferior deformation and water holding capacity to washed mince gel (p < 0.05). Therefore, the pH shift processing could be used to produce a functional protein isolate with low nucleic acids, haem pigments and lipids and, thereby, reduced cholesterol level. The protein isolates, particularly acid version, still had good gelling properties.

Autolysis and Characterization of Sarcoplasmic and Myofibril Associated Proteinases of Oxeye Scad (Selar boops) Muscle

ABSTRACT The autolytic profile of oxeye scad mince was characterized. Mince showed higher proteolytic activity than washed mince. The highest autolysis was observed at 65 and 60°C for mince and washed mince, respectively. Both mince and washed mince showed the optimum pH for autolysis at pH 9.0, and their activities decreased with increasing NaCl concentration (0–3.5%). Autolysis of washed mince was markedly inhibited by soybean trypsin inhibitor (SBTI), suggesting that myofibril-associated proteinase was serine proteinase. Sarcoplasmic proteinase was characterized to be heat-activated alkaline proteinase having the optimal pH and temperature of 9.0 and 60°C, respectively. The activities were stable at pH range of 8.0–11.0 at 20–40°C. The crude proteinase was inhibited by N-p-tosyl-L-lysine chloromethyl ketone, SBTI, and phenylmethanesulfonyl fluoride, suggesting the predominance of serine proteinases, especially trypsin. NaCl suppressed the activity while β-mercaptoethanol, dithiothreitol, and CaCl2 activated the activity. Therefore, trypsin-like proteinase is a major endogenous proteinase responsible for autolysis in oxeye scad muscle. The present results can be used as scientific guidelines to predict the gel strength of surimi made from oxeye scad muscle.