1. Introduction
Gelatin, a partially hydrolyzed protein product derived from animal collagen, is soluble in hot water and forms a reversible gel widely used in the food, medicine, beauty, and photography industries [
1,
2,
3].
Commercial gelatin is a raw material sourced from collagen and bone (or related byproducts) of pigs, cattle, and other animals. However, in recent years, animal diseases among cattle and pigs, such as mad cow disease (bovine spongiform encephalopathy, BSE) and foot and mouth disease (FMD), have brought into question the safety of the source materials used for gelatin [
4,
5]. During the Malaysia and Halal Food Marketing Strategy Forum in 2015, it was mentioned that there are 1.6 billion Muslim people across the world, accounting for nearly a quarter of the world’s population, and this number is expected to increase. The market demand for halal food is expected to increase as well. Muslim people follow strict religious rules and therefore, cannot eat pig-derived gelatin [
6]. In addition, other countries, such as India and countries in East Asia that follow Buddhism, have also restricted the use of pig and cattle gelatin materials in food processing because of specific religious beliefs [
2].
According to the 2015 data of the Fisheries Agency of the Executive Yuan Agricultural Committee [
7], the tilapia production capacity in Taiwan is approximately 70,472 metric tons. Tilapia is primarily used in processed products, such as frozen fish fillets; most of the byproducts of processed foods were deemed feed waste in the past, thus leading to environmental pollution and the waste of resources. If more byproducts can be further extracted from this resource, for example, the use of fish skin extract to make the raw materials necessary to prepare fish gelatin—which would serve as an alternative to gelatin derived from terrestrial mammals—the value of processing aquatic products would increase and environmental pollution would decrease [
8,
9]. Therefore, fish gelatin has become a useful aquatic raw material. However, the rheological properties [
10,
11], gelation and melting temperatures, and gel strength [
12] of fish gelatin are lower than that of mammalian gelatin because of the reduced hydroxyproline, proline, and imino acid content [
2,
13]. These properties limit fish gelatin’s application scope.
To improve the functional properties of fish gelatin, studies have examined whether adding various agents, such as sugars, salts, and glycerol, can modify the functional properties of fish gelatin [
14,
15]. However, adding such agents may affect subsequent processing recipes. Ultraviolet (UV) radiation technology is widely used in the sterilization of raw food materials and food contact surfaces. UV radiation technology uses non-free radiation and is a non-thermal processing technology, with such advantages as being cost effective and environmentally friendly, rendering it suitable for use in the food industry [
16]. Exposing fish gelatin to UV radiation can significantly increase its gel strength, and this increase is proportional to the radiation dose [
17,
18]. Current research has focused on short-time UV-light-irradiated gelatin, with exposure times of 0 to 1 h [
18,
19], 0 to 1.5 h [
17], and 0 to 3 h [
20]; no long-term radiation treatment has yet been reported.
This study used commercially available tilapia fish skin gelatin powder as a raw material to investigate the effects of UV irradiation treatment on fish skin gelatin powder (UVFGP) for durations of 0 to 6 h and to determine the optimum processing conditions, with the objective of using UVFGP to replace mammalian gelatin. The functional properties of gelatin and the optimum irradiation treatment conditions were analyzed through gel strength analysis, colorimetry, differential scanning calorimetry (DSC), sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy.
3. Materials and Methods
3.1. Materials
Tilapia skin gelatin and porcine skin gelatin were purchased from the Jellice Pioneer Provate Limited Taiwan Branch (Pingtung, Taiwan). Each 100 g gelatin sample was sealed in a 1 kg polyethylene (PE) bag. All chemicals, namely acrylamide/bis-acryamide 30% solution, ammonia persulfate (APS), 0.5 M Tris-buffer (pH 6.8), 1.5 M tris-buffer (pH 8.8), sodium dodecyl sulfate (SDS), and N, N, N0, N0-tetramethylethylenediamine (TEMED), for the electrophoresis were purchased from Bio Basic Inc. (Toronto, ON, Canada).
3.2. Tilapia Gelatin Powder Pretreatment
This study adopted the method used by Sung and Chen [
27], with few modifications. Fish gelatin powder (50 g) was placed on a stainless sheet plate (21 × 29 cm) with a powder thickness of nearly 1.5 mm. The UV light C tube (Model Allkill-01, 253.7 nm; 30W, PJLink, Taipei, Taiwan) was used as the UV light source, and the lamp tube clamp was fixed on a steel frame; the irradiation distance was adjusted to 30 cm for the UV irradiation treatment. The UV light irradiation pretreatment equipment of the fish skin gelatin was placed on a table in an aseptic operation room, and the UV irradiation durations were 1, 2, 3, 4, 5, and 6 h. All experimental treatments were conducted in triplicate. The irradiated samples were sealed into PE bags at room temperature (25 °C) until subsequent analyses.
3.3. UV Irradiance Measurement
UV radiation dose measurements were performed per manufacturer instructions. Using a UV light meter (ST512, Sentry Industries Inc., Hillburn, NY, USA) with a spectral range of 220–275 nm, illuminance (mw/cm
2) measurements were performed using UV light at nine points across the stainless sheet plate and averaged.
Table 6 presents the UV irradiation exposure durations and corresponding UV radiation doses. The UV irradiation dose per unit area can be calculated by multiplying the average irradiance with the continuous irradiation duration, per the formula used by Craik et al. [
28]:
3.4. Gel Strength of Gelatin Solution
This study followed the method developed by the British Standard Institute in 1975 [
29]. The gelatin powder was dissolved in distilled water at 60 °C to obtain a 6.67% (
w/
v) colloidal solution; it was then poured into a test bottle (3.8 cm diameter × 2.7 cm high). The prepared solution was placed in a 10 °C refrigerator for cooling and kept for 16–18 h. After completing the sample preparation, it was immediately removed for testing of its gel strength using a texture analyzer (TA.XT2, Stable Micro Systems, Godalming, Surrey, UK) and a P/0.5 cylindrical probe of diameter 1.27 cm. The test speed was 0.5 mm/s and the penetration distance was 4 mm. These gel strength measurements were performed by analyzing the force–time curve of the gelatin solution in triplicate.
3.5. Color Measurement of Gelatin Powder
The CIELAB color (L*, a*, b*) and total color difference (ΔE) of tilapia skin gelatin powder was measured using a colorimeter (Chroma meter CR-410, Konica Minolta, Japan). After using a whiteboard for calibration, the fish gelatin powder was placed in a scanning spectrophotometer sample tray, followed by L*, a*, and b* determination. L* values range from 0 to 100 and represent the lightness of the color; L* = 0 represents black and L* = 100 represents white. Values for a* and b* range from −60 to 60, with negative and positive a* signifying green and red, respectively, and negative and positive b* signifying blue and yellow, respectively [
30]. All tests were performed in triplicate. The measured L*, a*, and b* values were substituted into the following formula to calculate the total color difference (ΔE) [
31].
3.6. DSC Analysis of Gelatin Powder
The DSC (Mettler, Toledo, Switzerland) analysis of gelatin powder was conducted using a method adopted from Rahman et al. [
32], but with slight modifications. The sample (5 mg) was placed in a dedicated analytical 40 μL standard aluminum crucible, sealed with the lid of a standard aluminum crucible and weighted. The samples were scanned from −60 °C to 250 °C at a heating rate of 20 K/min; the reaction gas was N
2, which was released at 50 mL/min. The thermal stability analysis of the gelatin powder was performed in triplicate to record the change of heat enthalpy (ΔH) during the test; the onset temperature (To), peak temperature (Tp), and enthalpy data values were also recorded.
3.7. SDS-PAGE Analysis of Gelatin
This study adopted a method reported by Lin et al. [
33]. The gel solution was composed of acrylamide (running gel with 8% and 15% (
v/
v)) and stacking gel (4% (
v/
v) of acrylamide). After the gel solidified, 20 μg of protein was pipetted into each well, and a voltage of 60 volts was applied. Subsequently, the gel was stained using Coomassie brilliant blue R250, and the molecular weight was determined using a molecular-weight size marker. The molecular size was relatively small because it was a fast protein.
3.8. FTIR Spectral Analysis of Gelatin Powder
This study adopted the method reported by Benjakul et al. [
34]. Fish gelatin powder and potassium bromide were uniformly mixed at a ratio of 1:100 and dried in an oven at 50 °C for 24 h. The mixed powder was then removed, transformed into pellets using a hydraulic press machine, and subjected to FTIR spectrometry (FT-IR, Perkin Elmer, Spectrum One, San Diego, CA, USA) at 25 °C ± 2 °C and a scan resolution of 4 cm
−1; scanning was performed 32 times. The spectral wave number was in the IR range of 650–4000 cm
−1.
3.9. Raman Spectroscopy Analysis of Gelatin Powder
This study adopted a version of the method reported by Sarbon et al. [
24]. This analysis did not require pretreatment of the fish gelatin powder samples, which were directly characterized using a Raman spectrometer (Acuscan 1500, Acutech scientific Inc., San Diego, CA, USA). The wave number range was 200–2000 cm
−1, and a 785 nm laser was used as the excitation source. The samples were analyzed at a laser power of 100 mW, and all assays were performed in triplicate.
3.10. Statistical Analysis
All experiments were repeated at least thrice. The results were analyzed using one-way analysis of variance (ANOVA), and Duncan’s new multirange tests by using SPSS (Version 12.0, SPSS, 2000, Chicago, IL, USA) with the significance level set at p < 0.05.
4. Conclusions
According to the gel strength and color analysis, the color of the fish gelatin powder gradually changed from bright white to slightly yellowish after irradiation with UV light. The gel strength and the melting peak temperature of gelatin increased significantly and was the highest when irradiated for 2 h; the color underwent noticeable chromatic aberration. When the samples were irradiated for more than 2 h, the increase in gel strength and melting peak temperature was insignificant and was accompanied by a substantial yellowing phenomenon. Thus, irradiation for 2 h is optimal.
The peak temperature of melting and cracking, as determined through DSC analysis, indicated that fish skin gelatin has poor thermal stability compared with commercially available porcine gelatin powder. Nevertheless, fish gelatin powder modified through optimal UV irradiation exhibited improved melting peak temperature and thermal stability similar to that of porcine gelatin powder, but the cracking peak temperature did not change significantly.
Gel electrophoresis and optical spectral analyses revealed that with increased dose of UV radiation, the macromolecules of the gelatin powder degraded and accumulated at the bottom. The FTIR and Raman spectra also confirmed this finding. Thus, FTIR and Raman analyses can be used to differentiate between gelatin powders from different sources.