| Precious marine products are nutritious, and the demand is increasing in resent years. The high pressure, low temperature and salty living environment result in their unique histological structure and physiological properties. It will be of great significance for enhancing the processing theory of precious seafoods to carry out a systematic research on the processing characteristics of precious seafoods, set up a texture control technique of precious seafoods, and develop bioactive peptides from precious seafoods.In view of the special processing characteristics of sea cucumber, scallop and oyster, we investigated the changes of chemical composition, histological structure as well as texture properties of sea cucumber and scallop during the heating process and of oyster during the vacuum freeze drying process in the present study. Meanwhile, antioxidant peptides were prepared from sea cucumber and oyster due to their abundant protein. The main research contents were summarized as below:(1) To determine the structural changes of collagen from sea cucumber body wall during heating process, pepsin-solubilized collagen (PSC) were purified and the changes were monitored by using ultraviolet (UV) absorption spectra and SDS-PAGE during the heating process ranging from 60℃to 100℃. Results indicated that the triple helical structure of collagen was destroyed by heat, resulting in an increased absorption at 235 nm. The time for complete degradation of a peptide chain of PSC decreased with increasing temperature. It was 8,6,1.5,0.83 and 0.33 h at 60,70,80,90 and 100℃, respectively.(2) Based on the self-assembly characteristics of collagen molecule on solid surface, the effects of different factors on aggregation state of PSC from sea cucumber body wall were investigated by using atomic force microscope (AFM). Results indicated that the aggregation degree of collagen molecule increased gradually with increasing concentration in either acetic acid buffer solution (pH 2.7), phosphate buffer solution (pH 7.2) and ultrapure water, and formed close-packed and sheet-like aggregates finally. After heating for 30 min at 40-80℃, the aggregation degree of PSC increased in acetic acid buffer solution. When the temperature was higher than 80℃, the aggregation degree of PSC decreased. The self-aggregation phenomenon was obvious after heating for 0.5-2.0 h at 80℃.(3) To determine the heat-processing characteristics of sea cucumber body wall, the histological structure was observed by using microhistological techniques, and the shear force, hardness, springness, chewiness and resilience were analyzed by using texture analyser (TA). Results indicated that the higher heating temperature used, the shorter consumption time needed for the mass of sea cucumber body wall reaching a plateau. It was 5.0,3.0,1.5,1.0 and 0.5 h at 60,70,80,90 and 100℃, respectively. When the temperature was higher than 60℃, the sea cucumber body wall began to absorb water. The higher heating temperature used, the shorter consumption time was needed for the appearance of water absorption phenomenon. It was 24,6,3 and 2 h at 70,80,90 and 100℃, respectively. When the sea cucumber was processed at temperature higher than 100℃, the water-absorbing capacity was obviously enhanced. The higher heating temperature used, the shorter time was needed to get the waterishlogged sea cucumber with two-fold mass. It was 35,25,20 and 10 min at 105,110,115 and 120℃, respectively. In the process of heating, the collagen fiber of sea cucumber became thin, swelled, aggregated and dissolved. The higher heating temperature used, the shorter time was needed for the appearance of aggregation phenomenon. It was 8.0,1.5 and 0.5 h at 60,80 and 100℃, respectively. After heating at a certain temperature for a certain period, such as 70℃for 34 h,80℃for 20 h,90℃for 8 h,100℃for 4 h,105℃for 2.5 h,110℃for 1.5 h and 120℃for 1 h, the shearing force and hardness of the sea cucumber body wall fell to less than 1000 g and 2000 g, respectively. When the heating temperature and time were beyond such points, the resilience decreased obviously, and the tissue of sea cucumber became too soft to process.(4) Based on the changes in structure of collagen protein, texture properties and sensory evaluation of sea cucumber body wall, the processing-controlling curves were obtained and the heat processing-controlling area was built up for sea cucumber body wall. The curves included a losing-water-balance curve (A), an absorbing-water-beginning curve (B), a high-temperature processing-controlling curve (B'), and a heat processing-threshold curve (C and C'). When waterishlogged sea cucumber was processed, the processing parameter located in the area between Curve B and Curve C or Curve B' and Curve C' should be employed.(5) To determine the heat-processing characteristics of scallop adductor, the denaturation of myofibrillar protein in heating process was investigated. Meanwhile, the changes in muscle fiber structure were studied by using light microscope, and the shear force, hardness, springness, chewiness and resilience were analyzed by using TA. Results indicated that the higher heating temperature used, the shorter consumption time was needed for complete denaturation of myofibrillar protein (denaturation percent≥90%, the adductor muscle become ripen). It was 240,30,20,7 and 3 min at 60,70,80, 90 and 100℃, respectively. The myofibrillar of scallop adductor showed a trend of curving-stretching-breaking change with prolonging heating time at different temperature. When heating temperature was 60,70,80,90 and 100℃, the time for appearance of myofibrillar breakdown was 360,150,70,50 and 20 min, respectively. Under these processing conditions, the adductor muscle became over cooked. When the heating temperature and time were beyond the points such as 60℃for 360 min,70℃for 150 min,80℃for 90 min,90℃for 50 min and 100℃for 20 min, the adductor muscle tissue became too loose to process.(6) Based on the denaturation of protein, texture properties and sensory evaluation of scallop adductor, the heat processing-controlling curves were obtained and the heat processing-controlling areas were built up. When heat-processing scallop adductor was produced, the processing parameters located in the optimum area such as 60℃for 4-6 h,70℃for 30-150 min,80℃for 20-90 min,90℃for 7-50 min and 100℃for 3-20min should be employed.(7) To illustrate the changes in texture properties of oyster in the process of drying and follow-up rehydration, the drying ratio, rehydration ratio, rehydration rate and histological structure of the direct freeze-drying oysters (Fresh+FD group), the pre-cooked freeze-drying oysters (Boiled+FD group) and the natural-drying oysters (Fresh+ND group) were investigated. Results indicated that freeze-drying treatment was favorable to maintain the intrinsic histological structure and better rehydration capacity than the natural-drying treatment.(8) Sea cucumber gelatin peptides were prepared by high-temperature pretreatment followed by enzymatic hydrolysis, and their antioxidant activities were also investigated. Results indicated that both of gelatin and gelatin peptides possessed strong DPPH radical scavenging capacity (SC50 was 0.97 mg/mL and 0.37 mg/mL, respectively), hydroxyl radical scavenging capacity (SC50 was 0.97 mg/mL and 0.37 mg/mL, respectively), and lipid peroxidation inhibitory capacity (IC50 was above 20 mg/mL and 8.56 mg/mL, respectively). Obviously, the antioxidant activity of gelatin peptides was significantly higher than that of gelatin.(9) Oyster meat was hydrolyzed with three proteases including papain, neutrase and alcalase, respectively. The hydrolysates were fractionated using a series of ultrafiltration membranes, and the resultant peptide fractions were evaluated for their antioxidant activity. Results indicated that the antioxidant capacity of oyster peptides was related to the molecular weight distribution and amino acid type, the fractions with molecular mass below 3 kDa showed stronger antioxidant activity. Besides, the alkaline peptides had greater capacity on scavenging DPPH radical.
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