3.1. Gelatin Composition and bloom strength of gelatin gels
The composition of (waste skin gelatin) WS and (waste grilled skin gelatin) WG are tabulated in Table 1. The protein content, moisture and ash content were 81.37 %, 9.96 and 0.41 % respectively for the lyophilized chicken skin gelatin. The evaluation of the difference in the composition of the extracted gelatin from chicken skin fresh (WS) and grilled (WG) is necessary to understand the properties of the prepared films.
Table 1
Composition of gelatin and bloom strength of WS and WG
|
Yield, %
|
Moisture, %
|
Protein, %
|
Ash, %
|
Bloom, g/m2
|
WS
|
17.3±1.01
|
9.96±0.06
|
81.37±0.32
|
0.41±0.03
|
247 ±1.31
|
WG
|
14.9±0.94
|
3.4±0.02
|
77.13±0.28
|
1.3±0.06
|
179 ±0.85
|
The gel strength of WS is higher than WG 28.2 % probably due to the thermal treatment of WG through the grilling process. The gel strength of extracted gelatin from fish or chicken gives 181 and 263, respectively[24]. In addition, horse mackerel gelatin shows bloom strength 280 [25] and 177 g [11]. The low value of fish gelatin may be due to hydroxyproline content being relatively low in fish skin [26]. The hydroxyproline and proline can form hydrogen bonding with free water and triple-helix of gelatin structure [15]. Also, the bloom strength of gelatin is affected by many factors like chemical handling of gelatin, type, form, concentration, and source of gelatin, besides the thermal history of the extracted gelatin [11]. In addition, the value of bloom strength is directly proportional to the gelation point, melting, and gelation time of the extracted gelatin.
3.2. Differential Scanning Calorimetry
One of the most popular tools to investigate thermal behaviors in food ingredients such as gelatin is Differential scanning calorimetry (DSC). Recently, endothermic Tm of bovine gelatin was characterized during the thermal investigation of extracted gelatin [27].
Figure 1 shows the thermographic trend of WS and WG. Detectable peaks are calculated to determine onset temperature (To), offset temperature (Tf), maximum temperature (Tmax) and heat of energy (ΔH). The chicken waste gelatin exhibits two endothermic peaks one for glass transition temperature Tg ≈ 74°C and the other for melting. The melting peak is detected at 130.7°C (To), 143.6°C (Tf), 136.4 (Tmax), and 25.61 J/g (ΔH).
WG presented two endothermic peaks with a little shift than fresh chicken skin gelatin. The melting peak is calculated with thermal indices 126.6°C (To), 148.6°C (Tf), 137.8 (Tmax), and 26.12 J/g (ΔH). There is a detectable broadness of WG melting peak with temperature difference (Tf -To) 22°C where the difference of WS melting peak is 12.9°C. Additionally, the intensity of Heat flow response for WS is higher than WG. This is can be attributed to the thermal treatment of chicken skin through the grilling process.
3.3. Particle size and zeta potential
The particle size of WS and WG was measured according to Intensity-weight with uniform bell shape Gaussian distribution. The double desolvation process can be produced unique gelatin particles in nano-scale size.
The particle size distributions with unimodal form are illustrated in Fig. 2. Moreover, the zeta potential at low voltage was measured in an aqueous solution (pH 7). Over many measurements, the average zeta potential value was calculated as shown in Fig. 2. the stability of gelatin particles through eleven measurements is perfect and nearly the same.
Table 2
Particle size, zeta potential, and nitrogen content of WS and WG
|
Mean particle size, nm
|
Polydispersity
PDI
|
Zeta potential, mV
|
Nitrogen content, %
|
WS
|
63.5
|
0.365
|
16.98
|
11.71
|
WG
|
75.5
|
0.401
|
21.86
|
8.27
|
Table 2 summarized the mean particle size and zeta potential of WS and WG. The surface area of lyophilized gelatins was tabulated. The results give excellent clues on the nanoparticle size for both types of extracted gelatins [28]. The mean diameter of WS and WG nanoparticles were 63 and 75 nm respectively. That reflects the high precision preparation method of nanoparticle gelatin. The zeta potential measurements presented the positive charge on gelatin particles with 16.8 and 21.8 mV for WS and WG respectively. The nitrogen content was decreased for WG than WS which can be indicated to thermal treatment of WG.
As well known, the unique relationship between volume/surface area of creating nanomaterials will be generated a novel character [29]. the prepared nano-gelatin with particle size less than 80 nm was shown a promising enhancement in physical properties of prepared nono-film. The zeta potential can give a clue for homogeneity and well dispersion of gelatin solution. This is directly related to the film performance and stability of collided particles during the casting process with a proper zeta value in the moderated stable range from 10 to 40 mV [30]. Particle size and zeta potential are vitally characterized by nanomaterials especially gelatin dispersed in an aqueous medium.
3.4. Mechanical properties
The mechanical properties of gelatin films were presented in Table 3. The tensile strength (TS) of WG and WS-based films were not significantly different from each other. This was also observed regardless of the gelatin concentrations used. However, TS values for films, manufactured from both sources of gelatin used in this study, significantly increased when the concentration increased from 4 % to 6 %. No further significant increases occurred when gelatin concentration increased from 6 % to 8 %.
Table 3
Mechanical properties of WG and WS gelatin films
|
|
Thickness, µm
|
TS, MPa
|
E, %
|
4%
|
WS-gelatin
|
51
|
3.1
|
57
|
WG-gelatin
|
43
|
2.3
|
48
|
6%
|
WS-gelatin
|
52
|
4.9
|
37
|
WG-gelatin
|
49
|
4.3
|
34
|
8%
|
WS-gelatin
|
57
|
5.1
|
46
|
WG-gelatin
|
55
|
4.7
|
38
|
Means in the same column for 5 replicated samples with significant difference (P < 0.05) |
TS was unaffected by the gelatin source. TS for films containing 8 % gelatin concentration manufactured using WS presented greater film strength than films manufactured from WG. This also happened to be the film with the greatest TS (20.42 N) of all films tested. The concentration of gelatin used also significantly affected TS, whereby the higher the concentration of gelatin used, the greater was TS.
No significant differences were observed for Elongation (E) values from films manufactured from both sources of gelatin and at gelatin concentrations between 4 % and 6 %. However, at an 8 % gelatin concentration, E values for films derived from WG had greater E than analogous films manufactured from WS. However, when gelatin concentrations ranged from 4 % to 6 %, E properties decreased, irrespective of gelatin species origin.
3.5. Water vapor permeability (WVP)
WVP values are indicated in Figure 3. WG and WS gelatin behaved differently from each other concerning WVP. This is maybe revealed to heat treatment of chicken skin within the grilling process. WG-based films containing a gelatin concentration of 4 % had lower WVP values when compared to WS-derived gelatin film equivalents. WVP values for both WG and WS-derived gelatin films had similar values by using 6 % and 8 % gelatin concentrations were used to manufacture films. The films manufactured from WG showed no significant differences from each other for WVP as the concentration of gelatin used to produce the films increased. However, films manufactured from WS showed increased resistance to WVP as the gelatin concentration increased from 4 % to 6 %, but not at any other concentration used.
WS skin with 4 % concentration possessed the lowest WVP value, below 60 g·mm/kPa·d·m2. it was noted that as the gelatin concentration increased in films, so too did the requirement for greater plasticizer addition. This consequently led to an increase in WVP, but this was especially apparent for films manufactured from WS. It is well recognized that the addition of plasticizer to a protein-based film mixed solution causes the protein network to become less dense and more permeable [31], however, the plasticizing effect noted in this study is interesting in that it appears to affect the same protein source in different ways, depending on the differences associated with the protein-based on its species origin.
3.6. Oil uptake (OU)
OU of films manufactured from WG- and WS and used at a 4 % concentration showed no significant differences (Figure 4). The films manufactured using both 6 % and 8 % gelatin showed significant differences in oil uptake by both films which resulted in differences in weight gain. The films manufactured from WS had more resistance to oil uptake. In terms of gelatin concentration, no significant differences were observed within each species-defined film type for OU.
3.7. Topographic investigation of gelatin films
The surface morphologies of the prepared films were characterized utilizing scanning electron microscopy (SEM). This helps to understand the effect of chicken waste sources (i.e. fresh or grilled skin).
It can be observed from the SEM images shown in Fig. 5 that the films exhibited uniform surface morphology. Gelatin-based fresh chicken skin wastes film was presence more homogenized and smoother than that based on grilled chicken skin wastes. This is maybe due to some impurities that remain in gelatin films based on WG or thermal degradation for the polymer chains during the grilling process.
3.8. Study the overall migration of chemical substances from the prepared films
The migration of any chemical substances from the prepared films has been studied according to the EU Regulation Nr. 10/2011. The stimulants were selected carefully to represent different food natures. As shown in Table 4, the prepared films showed highly acceptable migration limits. The overall migration (OM) from the WG and WS gelatin films were ranged from 0 up to 0.3 mg/dm2. The regulation limits the accepted level up to 10 mg/dm2.
Table 4
The overall migration from WG and WS gelatin films
Method
Replicates
|
Migration into
10% v/v ethanol (simulant A)
mg/dm2
|
Migration into
20% v/v ethanol (simulant A)
mg/dm2
|
Migration into
3% w/v
acetic acid
(simulant B)
mg/dm2
|
Migration into
Olive oil
(simulant D2)
mg/dm2
|
|
WS8%
|
WG8%
|
WS8%
|
WG8%
|
WS8%
|
WG8%
|
WS8%
|
WG8%
|
1
|
0.1
|
0.1
|
0.4
|
0.2
|
0.3
|
0.2
|
0.0
|
0.1
|
2
|
0.2
|
0.3
|
0.2
|
0.3
|
0.1
|
0.3
|
0.0
|
0.0
|
3
|
0.0
|
0.2
|
0.3
|
0.3
|
0.3
|
0.1
|
0.0
|
0.2
|
Mean
|
0.15
|
0.2
|
0.3
|
0.26
|
0.23
|
0.2
|
0.0
|
0.15
|
Acceptable
|
10
|
10
|
10
|
10
|
10
|
10
|
10
|
10
|
3.9. Total heavy metals determination in various simulants
The migration of heavy metals in various stimulants (i.e. Simulant A: 10 % v/v ethanol, Simulant B: 20 % v/v ethanol, Simulant D2: rectified olive oil) had been investigated. The traced metals were cadmium, lead, mercury, chrome, aluminum, arsenic, and antimony. The levels of migrated heavy metals in different simulants were in the accepted levels as shown in Table 5 [32]. The highly acceptable levels of overall migration and the traces of the heavy metal ensure the prepared gelatin films are appropriate as a food-contact layer.
Table 5
The determination of migrated heavy metals traces in various simulants
|
Conc. in
3% acetic acid, µg/L
|
Conc. in
20% ethanol
µg/L
|
Conc. in
10% ethanol
µg/L
|
Conc. in
Olive oil
µg/L
|
Acceptable
Value
µg/L
|
|
WS8%
|
WG8%
|
WS8%
|
WG8%
|
WS8%
|
WG8%
|
WS8%
|
WG8%
|
|
Cd
|
0.03
|
0.06
|
0.01
|
0.02
|
0.01
|
0.01
|
0.008
|
0.01
|
5
|
Pb
|
0.10
|
0.13
|
0.06
|
0.07
|
0.02
|
0.04
|
0.01
|
0.02
|
10
|
Hg
|
0.07
|
0.09
|
0.03
|
0.05
|
0.02
|
0.03
|
0.01
|
0.01
|
12
|
Cr
|
0.11
|
0.16
|
0.09
|
0.11
|
0.03
|
0.05
|
0.01
|
0.02
|
50
|
Al
|
0.11
|
0.13
|
0.05
|
0.09
|
0.06
|
0.08
|
0.01
|
0.01
|
40
|
As
|
0.10
|
0.14
|
0.08
|
0.13
|
0.08
|
0.12
|
0.01
|
0.01
|
10
|
Sn
|
0.63
|
0.82
|
0.39
|
0.56
|
0.28
|
0.34
|
0.01
|
0.01
|
1200
|