No abstract

The quality changes of shelled Pacific oysters (Crassostrea gigas) were examined in relation to the effects of superchilling storage at -1 °C for 28 days by measuring changes in biochemical properties (microbial analysis, ATP-related compounds, pH, free amino acids) and sensory evaluations in this study. The results indicated that microorganism growth was significantly inhibited during superchilling storage. Adenosine diphosphate (ADP) and adenosine monophosphate (AMP) accumulated while ATP rapidly decreased in the adductor muscle. Adenosine triphosphate (ATP) and ADP were the primary components in the other 3 tissues including mantle, gill, and body trunk of oysters, and they remained relatively stable over time. The pH and adenylate energy charge (AEC) in the adductor muscle could be utilized as freshness indicators for shelled oysters. However, there were no significant differences (P >0.05) among the free amino acids during whole storage. According to the sensory evaluations, oysters could be alive and tolerated up to 21 days at -1 °C storage. The study demonstrated that superchilling storage at -1°C could better maintain the eating quality of shelled oysters and the shelf life was extended to 21 days.

Spotted mackerel were stored at 5 °C for 0-5 days. To reveal changes in freshness and provide the fundamental knowledge of sarcoplasmic reticulum (SR), structural and biochemical changes were investigated by optical microscope, transmission electron microscopy and biochemical detection, including pH, myosin Ca 2+ -ATPase activity, adenosine 5 0 -triphosphate (ATP) content, SR recovery field and SR Ca 2+ -ATPase (SERCA) activity. The result exhibited that SR swelled on day 1 and ruptured after 2 days. Muscle structure showed significant changes from day 0 to day 5. SERCA activity and SR recovery field were 0.46 lm Pi/min/mg and 1.05 mg/g at day 0 and 0.106 lm Pi/min/mg and 0.109 mg/g at day 2, respectively, highly corresponding to the decline of pH, myosin Ca 2+ -ATPase activity and ATP content with a significant difference from day 0 to days 1 and 2. The biochemical properties of SERCA exhibited the maximum activity at pH 6.8-7.0 (0.408 lmol Pi/min/mg). Treated at pH 5.5 for 80 min or incubated at 35 °C for 40 min inactivated SERCA of 80%. Therefore, keeping at a freezing temperature of 5 °C and maintaining SR functionality was essential to delay freshness decrease.

The changes of sensory and biochemical characteristics on the fresh and frozen‐thawed scallop adductor muscle during storage at 4°C were discussed in this study. The Quality Index Method (QIM) scheme for evaluating scallop adductor muscle as raw materials for sashimi was proposed for the first time. The results of sensory evaluation showed that frozen‐thawed scallop adductor muscle within zero to one day of refrigeration can be happily accepted by consumers, indicating the superiority of freezing for long‐distance transportation, although the triangle test confirmed that there are still sensorial differences between fresh and frozen‐thawed scallop adductor muscle. The microscopic observation of myofibrils extracted from scallop adductor muscle suggested that the myofibrillar protein which constitutes myofibrils has suffered some extent change due to freezing and thawing, even though the head region of myosin remained stable judging by the fact that there was no significant difference in Ca2+‐ATPase activity (p > 0.05). The changes of adenosine triphosphate (ATP) and its related compounds, and pH value during storage can be regarded as indicators to differentiate fresh and frozen‐thawed scallop adductor muscle. The changes of Mg2+‐ATPase activity indicated that the interaction between myosin and actin was weakened by the freezing and thawing process. Practical Application: The QIM scheme can be used to evaluate the scallop adductor muscle as raw materials for sashimi. The mechanism of quality changes in the frozen‐thawed scallop adductor muscle was discussed in combination with the destruction of myofibrils, ATP degradation and the decrease of pH value. This study has positive significance for improving the quality of frozen‐thawed scallop adductor muscle by combining the changes of sensory and biochemical characteristics.

Thermal stability of silver carp actin in summer and winter as well as the effect of myosin binding was investigated. Actin denaturation was detected by chymotryptic digestion with 1/200 (w/w) of myofibrillar protein (Mf). The temperature heated for 30 min and reached 50% denaturation (T1/2) of actin was 47°C for summer and 42°C for winter at 0.1 M salt concentration. Denaturation rate of winter actin was eight, six, four, and three times more quickly than summer one under 42, 43, 45, and 48°C, respectively. T1/2 changed to 43°C for summer and 37°C for winter at 0.5 M salt concentration. With the rise of salt concentration from 0.1 to 2.0 M, the actin denaturation rate increased 10 ~ 60 times, but the difference between the two seasons disappeared above 1.5 M NaCl. T1/2 for both summer and winter actin fell to around 40°C when actin was separated from myosin by adding magnesium pyrophosphates (Mg‐PPi). Denaturation rate was promoted by 3 ~ 8 times with Mg‐PPi higher than that without it, besides actin in summer and winter showed similar denaturation rate. It could conclude that the difference in thermal stability of actin depends on the stability of myosin when binding together. Summer and winter actin in silver carp Mf showed similar thermal stability after being separated from myosin. Novelty impact statement The differences in thermal stability of silver carp actin in winter and summer were studied for the first time. The interaction between myosin and actin under different conditions was studied by changing the salt concentrations, adding Mg‐PPi, etc. It simulated the actual manufacturing of surimi products to some extent. Understanding seasonal changes in actin stability will help food processors in thermal processing design to produce textured fish meat products based on the raw materials harvested.