Age-Related Properties of Aquaponics-Derived Tilapia Skin (Oreochromis niloticus): A Structural and Compositional Study

Abstract

In the last two decades, fisheries and fish industries by-products have started to be recovered for the extraction of type I collagen because of issues related to the extraction of traditional mammalian tissues. In this work, special attention has been paid to by-products from fish bred in aquaponic plants. The valorization of aquaponic fish wastes as sources of biopolymers would make the derived materials eco-friendlier and attractive in terms of profitability and cost effectiveness. Among fish species, Nile Tilapia is the second-most farmed species in the world and its skin is commonly chosen as a collagen extraction source. However, to the best of our knowledge, no studies have been carried out to investigate, in depth, the age-related differences in fish skin with the final aim of selecting the most advantageous fish size for collagen extraction. In this work, the impact of age on the structural and compositional properties of Tilapia skin was evaluated with the aim of selecting the condition that best lends itself to the extraction of type I collagen for biomedical applications, based on the known fact that the properties of the original tissue have a significant impact on those of the final product. Performed analysis showed statistically significant age-related differences. In particular, an increase in skin thickness (+110 µm) and of wavy-like collagen fiber bundle diameter (+3 µm) besides their organization variation was observed with age. Additionally, a preferred collagen molecule orientation along two specific directions was revealed, with a higher fiber orientation degree according to age. Thermal analysis registered a shift of the endothermic peak (+1.7 °C) and an increase in the enthalpy (+3.3 J/g), while mechanical properties were found to be anisotropic, with an age-dependent brittle behavior. Water (+13%) and ash (+0.6%) contents were found to be directly proportional with age, as opposed to protein (-8%) and lipid (-10%) contents. The amino acid composition revealed a decrease in the valine, leucine, isoleucine, and threonine content and an increase in proline and hydroxyproline. Lastly, fatty acids C14:0, C15:0, C16:1, C18:2n6c, C18:3n6, C18:0, C20:3n3, and C23:0 were revealed to be upregulated, while C18:1n9c was downregulated with age.

Keywords: Tilapia; aquaponic; collagen extraction; skin.

Figure 1

Representative DSC thermograms of ST and BT.

Figure 2

Coronal (Z-Y), sagittal (X-Y), and transversal (X-Z) view (a) of ST (b) and BT (c), where Z was the axis parallel to the fish backbone. Tilapia skin dermis can be observed in the transversal reconstruction. Sample size: 8 mm.

Figure 3

Three-dimensional rendering reconstruction of mCT scans of ST (a) and BT (d), their coronal section (b,e, respectively) in colored scale, and their relative intensity profile (c,f, respectively).

Figure 4

SEM imaging of ST (a) and BT (b) in dehydrated state, representative of the different skin thickness according to age (Magnification: 300×) and of ST (c,e) and BT (d,f) in hydrated state, showing the wavy-like structure of Tilapia skin collagen fibers (Magnification: 500× and 2000×).

Figure 5

(a) A typical 2D WAXS diffraction pattern on type I collagen from animal tissue, in particular equine tendon, in which the equatorial (black arrow) and meridional (red arrow) directions are clearly distinguished. In the 2D diffraction patterns of BT (b) and ST (c), the diffraction signal is characterized by four lobes along two directions (black arrows). (d) The azimuthal integration along the equatorial diffraction signal of both BT (black profile) and ST (green profile) are shown.

Figure 6

In the 2D diffraction patterns of BT (a) and ST (c) the equatorial and meridional signals are shown by black and red arrows, respectively. The radial integration along the equatorial (b) and meridional (d) direction for both ST (green profile) and BT (black profile) are shown. Magenta vertical dotted bars refer to Ultralene^® sachet diffraction.

Figure 7

The 2D SAXS patterns of BT (a) and ST (b) are shown. In the corresponding profiles (c,d), the black dotted lines indicate the fibrillary periodicity.

Figure 8

Representative stress–strain curves of ST (a,c) and BT (b,d) skin. Plots (a,b) were obtained in longitudinal direction, (c,d) in the transverse direction.