Abstract
Aged swim bladders from the yellow drum (Protonibea diacanthus) are considered collagen-based functional food with extremely high market value. The structural integrity of collagen may be crucial for its biological functions. In the current study, swim bladders with 25-year-old sequences were collected and found to be basically composed of collagen. Then, thermogravimetry (TG), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and attenuated total reflectance–Fourier transform infrared spectroscopy (ATR–FTIR) were conducted to evaluate the integrity of the peptide chain and triple helix in the collagen. The structures of microfibers and fiber bundles were revealed with atomic force microscopy (AFM), scanning electrical microscopy (SEM), and optical spectroscopy. The collagens in the aged swim bladders were found to have similar thermal properties to those of fresh ones, but the relative content of the triple helixes was found to be negatively correlated with aging. The secondary structure of the remaining triple helix showed highly retained characteristics as in fresh swim bladders, and the microfibrils also showed a similar D-period to that of the fresh one. However, the fiber bundles displayed more compact and thick characteristics after years of storage. These results indicate that despite 25 years of aging, the collagen in the swim bladders was still partially retained with structures.
Highlights
The swim bladder is a gas-filled regulating organ that can be found in most bony fishes
The total combined content of collagen and moisture in swim bladders was found to be higher than 90%
The swim bladders were basically composed of collagen, and, after years of aging, the integrities of the peptide chains were retained, but the collagen structures were affected
Summary
The swim bladder is a gas-filled regulating organ that can be found in most bony fishes. It is responsible for balance, floating, sinking, and protecting internal organs in fish [1]. In Asian countries, there is a long history of using the swim bladder as a luxury food with nutritional and medicinal value [2]. In the field of traditional Chinese medicine, it is believed that swim bladders can relieve gastrointestinal bleeding and postpartum hemorrhage. It has been reported that the declared annual import value of swim bladders has reached USD 264–394 million in Hongkong, similar to that of sea cucumbers, shark fins, and abalone [2]
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