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.     

CHARACTERISTICS OF MUSCLE FROM TWO SPECIES OF BIGEYE SNAPPER, PRIACANTHUS TAYENUS AND PRIACANTHUS MACRACANTHUS

Composition and some properties of muscle from two species of bigeye snapper, P. tayenus and P. macracanthus, were investigated. Both species had a similar composition with the same myofibrillar protein content. However, muscle proteins from P. tayenus had higher thermal stability than those from P. macracanthus, as indicated by the higher enthalpy for transitions as well as the lower inactivation rate constant (K_D). Upon 15 days of iced storage, natural actomyosin Ca^2*‐ATP ase and Mg²⁺‐Ca²⁺‐ATPase activities decreased, whereas Mg²⁺‐EGTA‐ATPase activity increased, suggesting the denaturation of myosin, actomyosin and troponin/tropomyosin complexes, respectively. Increased surface hydrophobicity and decreased sulfhydryl groups indicated the denaturation possibly occurred via hydrophobic interaction and disulfide formation. Heading and eviscerating offish retarded the denaturation and physicochemical changes of proteins during iced storage. The results indicated that a rapid and proper post harvest handling was of importance to maintain the muscle quality of bigeye snapper.     

ANALYSIS OF SURIMI GEL PROPERTIES BY COMPRESSION AND PENETRATION TESTS

Gel properties of surimi of different quality were analyzed using compression and penetration tests. Parameters used to vary the surimi quality were species, freshness, freeze-abuse and added ingredients. Compressive force at failure was more discriminative than penetration force. The result of a failure response analysis suggests that the compression test is useful in assessing the cohesive property, while the penetration test is better for assessing density and compactness. Good correlations between compression and penetration force values were seen in surimi gels prepared from the same species with and without ingredients, while poor correlations were observed in the surimi which either underwent freeze-abuse or was prepared from different species.     

PURIFICATION AND CHARACTERIZATION OF TRYPSIN FROM PYLORIC CAECA OF BIGEYE SNAPPER (PRICANTHUS MACRACANTHUS)

Trypsin from the pyloric caeca of bigeye snapper was purified and characterized. Trypsin had an apparent molecular weight of 23.8 kDa when analyzed using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS‐PAGE) and substrate‐gel electrophoresis. The trypsin fraction consisted of three isoforms as evidenced by the appearance of three different bands on native‐PAGE. Optimal activity was observed at 55C and pH range of 8–11. The activity of trypsin fraction was completely inhibited by soybean trypsin inhibitor and was partially inhibited by E‐64 and ethylenediaminetetraacetic acid. CaCl₂ partially protected the trypsin fraction from activity loss at 40C, while NaCl (0–20%) decreased the activity in a concentration‐dependent manner. The apparent Michaelis–Menten constant (K_m) and catalytic constant (k_cat) were 0.312 mM and 1.06 s, respectively when Nα‐Benzoyl‐dl ‐arginine ρ‐nitroanilide was used as a substrate. Trypsin from the pyloric caeca of bigeye snapper generally showed similar characteristics to other fish trypsins.     

TEXTURE PROPERTIES OF GELLAN GELS AS AFFECTED BY TEMPERATURE

Gellan gels can be made very brittle, similar to agar gels, or very flexible, like gelatin gels. The entropy or enthalpy nature governing those gellan gel behaviors was studied by mechanical testing at temperatures varying from 2 to 62C. Both failure stress and strain for 1% low acyl and low acyl/high acyl mixed gellan gels decreased with increasing temperature, indicating that the hydrogen bonding contributed significantly to the stabilization of gellan gels in addition to the polyanion-calcium-polyanion bonding. Hydrophobic interactions were less important. The initial Young's modulus for two mixed high and low acyl gellan gels containing 2 mM Ca⁺⁺ increased with temperature from 2–42C, indicating entropy elasticity. Average molecular weight between adjacent crosslinks for these two mixed gels was larger than 10⁴. For other gels, the entropy elasticity was not a dominant mechanism for elastic force because of molecular weights between crosslinks and from the observation of negative temperature dependence of the modulus.     

THE THERMAL PROPERTIES OF POTATOES AND CARROTS AS AFFECTED BY THERMAL PROCESSING1

The effects of processing on the thermal properties of white potatoes and carrots were studied. The test samples were blanched, boiled, cooked and canned. Whole potato tubers were baked to study the effects of starch gelatinization. The thermal conductivity (k) and density (ρ) were measured and the specific heat (c_p) and thermal diffusivity (α) were calculated. Results showed that in general, α initially decreased and then increased during processing. Test samples were found to have a gain in α when their moisture content increased by more than 9%. The α of potatoes decreased after canning and increased after boiling. In carrots, a similar trend was also observed but to a lesser degree. The k of potatoes was unaffected after blanching or cooking. In all the processing baking study showed that gelatinization significantly decreased the k of potatoes.     

THERMAL PROPERTIES OF PASSION FRUIT JUICE AS AFFECTED BY TEMPERATURE AND WATER CONTENT

The specific heat, thermal conductivity and density of passion fruit juice were experimentally determined from 0.506 to 0.902 (wet basis) water content and temperatures from 0.4 to 68.8C. The experimental results were compared with existing and widely used models for the thermal properties. In addition, based on empiric equations from literature, new simple models were parameterized with a subset of the total experimental data. The specific heat and thermal conductivity showed linear dependency on water content and temperature, while the density was nonlinearly related to water content. The generalized predictive models were considerably good for this product but the empiric, product‐specific models developed in the present work yield better predictions. Even though the existing models showed a moderate accuracy, the new simple ones would be preferred, because they constitute an easier and direct way of evaluating the thermal properties of passion fruit juice, requiring no information about the chemical composition of the product, and a reduced time of the estimation procedure, as the new empiric models are described in terms of only two physical parameters, the water content and the temperature.     

MICROSTRUCTURE AND PHYSICAL PROPERTIES OF QUARG CHEESE AS AFFECTED BY DIFFERENT HEAT TREATMENTS

The effects of different heat treatments on the rheological properties, microstructure and yield of production of quarg cheese were investigated. Three levels of time–temperature combination were used: 72C–16 s, 82C–5 min, 90C–5 min. Rheological properties were studied by means of frequency sweep experiment with the measurement of elastic modulus ( G' ). Microstructures of samples were observed by scanning electron microscopy (SEM). Maximum and minimum G' and cheese yield were obtained at 90 and 72C, respectively ( P <  0.05). Yield at 82 and 90C was higher than 72C but difference between heat treatment at 82 and 90C was not significant ( P <  0.05). G' values at 82 and 90C were higher than that at 72C. The difference between 82 and 90C was initially low; subsequently increase in frequency made G' at 90C higher than 82C. SEM micrographs of samples at 82 and 90C were closely similar and no differences were observed between them. Micrographs showed the regular microstructure with branched clusters and numerous small pores at 82 and 90C, whereas micrographs of heat treatment at 72C had larger protein aggregates and fewer but larger pores. This study showed that heat treatments at 82 and 90C were more suitable for quarg cheese making than that at 72C.

PRACTICAL APPLICATIONS

Quarg cheese is one of the most popular acid-coagulated cheeses. This study shows that heat treatment at 82C–5 min is preferred for quarg production compared with other heat treatments. Contribution of denatured whey proteins in gel network led to higher cheese-making yield as well as high nutrition value. Heat treatment at 82C is also suggested in economic and energy consumption aspect.     

THERMO-RHEOLOGICAL PROPERTIES AND COOKING YIELD OF SAUSAGE-TYPE PRODUCTS AS AFFECTED BY LEVELS OF FAT AND ADDED-WATER1

Consumer interest in low fat foods has led meat processors to introduce low fat sausages with high added-water. The purpose of this study was to use dynamic thermo-rheological techniques to study the influence of fat and added water on sausage batters and cooking yields during heating. Twenty different sausage batters were formulated to 15–30% fat and 18–35% added water. Batters were heated in a thermal scanning rheometer from 20–80C in increments of IC/min. Rheograms of all batters showed a gradual decrease in storage modulus (G') as temperature increased from 20–59C. Significant changes in G' occurred in the range of 60–75C leading to the formation of rigid gel networks. Batters with levels of fat ≤ 24% and added water > 24% or levels of fat and added water ≥ 27% showed increased cooking yields. Rheological measurements suggested that levels of added water influenced the G' of batters during heating and that high levels of added water could be used in sausage formulations without adversely affecting product yield.     

PHYSICAL AND SENSORY PROPERTIES OF MEATSTICK PRODUCTS AS AFFECTED BY FORMULATION AND DRYING: SENSORY-INSTRUMENTAL RELATIONSHIPS

Shelf‐stable meats are of interest to industry and are important in military ration development. Stability is commonly achieved through control of pH and water activity. However, reduction in moisture as a means to lower a_w can significantly affect texture. Other ingredients, such as fat, can also potentially affect texture and possibly mitigate hardening due to lowered moisture. Meatsticks dried to three water activities, 0.88, 0.90, and 0.92, each from batters at three fat levels, 14, 17, and 21%, were produced, yielding products varying in both moisture and fat content. Products were subjected to mechanical and sensory testing and image and color analysis. Meatstick modulus, shear force, L* value, and percent of area of cut surfaces covered by separated fat were correlated with sensory assessments of interior hardness, hardness with skin, color, and visual fat, respectively. These parameters were significantly influenced by composition, particularly fat and protein contents and fat‐protein interaction.     

TENSILE PROPERTIES OF BEEF SEMITENDINOSUS MUSCLE AS AFFECTED BY HEATING RATE AND END POINT TEMPERATURE

The effects of heating rate and end point temperature on tensile properties of beef semi-tendinosus muscle were studied. Samples of meat heated in glass tubes in a water bath to simulate oven roasting at 93 °C had lower (P < 0.01) shear values than those heated to simulate oven roasting at 149 °C. Samples heated to 60 and 70°C had lower (P < 0.001) shear values than samples heated to 50°C. Instron breaking strength values varied (P < 0.001) with end point temperature, but were not significantly affected by the rate of heating. Significant (P < 0.001) interactions between the end point temperature and heating rate, as well as between the end point temperature and fiber direction suggested that Instron breaking strength measurements may be more sensitive to changes in tenderness than Warner Bratzler shear measurements.