Although there has been an increasing demand for fish gelatine due to its religious and safety advantages over pig and bovine sources of gelatine, the main limiting factors of its widespread use lies in its technofunctional properties—i.e., the lower gel strength and melting temperatures compared to those for mammalian gelatines. This poses a challenge for commercial exploitation, and various approaches have been proposed to date to overcome these issues. Ultraviolet (UV) irradiation represents a physical, cost-effective, non-thermal, and environmentally friendly technology that has received increased attention in the food sector during recent years. Bhat and Karim [67
] have investigated the effect of UV irradiation (at 30 and 60 min interval lengths) on the gel strength of fish gelatine granules. They concluded that the irradiated samples exhibited significant improvements in the gel-strength, a reduction in viscosity, as well as changes in the melting enthalpy. These results indicate the possibility of using simple UV radiation as a method to improve cold fish gelatine properties. In their more recent work, Bhat and Karim [68
] have also investigated combination of UV irradiation and addition of sugars (ribose and lactose) on the properties of fish gelatine based films. Their results indicated that films with added ribose showed decreased solubility after UV treatment and exhibited higher swelling percentages than films with added lactose. Otoni et al. [69
] have also noted an improvement in functional properties of of fish gelatines from cold- (cod, haddock, pollock) and warm-water (tilapia) fish as a consequence of UVB radiation exposure.
Gelling properties of fish based gelatines may further be modified by use of various chemical agents which induce molecular crosslinking, such as glutaraldehyde [70
], as well as by creating mixtures with various non-gelatine systems such as pectin [71
]. Besides from natural polymers, several synthetic polymers have been used to create gelatine hybrid hydrogels. Zohuriaan-Mehr et al. [72
] have reported a number of organic (PEG-dialdehyde, acrylamines, EDTAD, poly(acrylic acid)), and inorganic (kaolin, silica gel) compounds which can affect gel strength, solubility, and hydrophobicity of such composite hydrogels. Another means of improving gelling properties of fish gelatine is to introduce enzymatic crosslinking using transglutaminase. This enzyme catalyses the formation of crosslinking bonds between γ-amide groups of glutamine and ϵ-amino groups of lysine. Baltic cod gelatine treated with transglutaminase was shown to be able to withstand heating in boiling water for 30 min without melting [48
]. As a collagen denaturation product, gelatine contains many divalent metal ions such as calcium, copper, iron, and zinc. These ions can form ionic bonds with the gelatine carboxylic acid groups, thus influencing the organization of the gelatine network. Removal of those metal ions by means of ion-exchange may improve further crosslinking between gelatine molecules, as demonstrated by Xing et al. [73
] who purified gelatine solutions using Chelex resin to replace divalent metal ions with sodium ions prior to crosslinking by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC). On the other hand, the effect of different salts on the rigidity or melting temperature of animal gelatines has also been researched previously [74
]. Koli et al. [75
] have optimized a method for improving fish gelatine extracted from Tiger-toothed croaker (Otolithes ruber
), using combination of three co-enhancers (MgSO4
, sucrose, and transglutaminase). By addition of co-enhancers at optimal concentrations in their experiments, the gel strength and melting point were improved from 170 to 240.89 g and 20.3 to 22.7 °C, respectively. Due to their better acceptability by consumers, natural compounds and extracts can also be used to improve gelatine properties. Araghi et al. [76
] examined the effects of natural phenolic cross-linkers (ferulic and caffeic acid) on fish gelatines. In their study, caffeic acid had notable effects in decreasing solubility, water vapour permeability, and oxygen permeability of fish gelatine films. Natural phenolic compounds may therefore be used as a natural ingredient for increasing safety of gelatine-based biodegradable packaging, by improving their barrier and physicochemical properties. Another natural material, chitosan nanoparticles (CSNPs), with excellent physicochemical properties, is known to be environmentally friendly, and bioactive, has been researched for improving properties of fish gelatine based films. Hosseini et al. [77
] have created novel bio-nanocomposite films by addition of CSNP particles (created by ionic gelation between chitosan and sodium tripolyphosphate) into fish gelatine film matrix. Newly created films had significantly increased tensile strength and elastic modulus, and decreased water vapour permeability compared to fish gelatine films.
With the exception of its inferior physical properties when compared to mammalian counterparts, fish derived gelatine intended for food use often possesses undesirable sensory properties characterized by an unpleasant “fishy” flavour [78
]. Sae-leaw, Benjakul, and O’Brien [79
] have investigated the effects of defatting and tannic acid incorporation during extraction on the properties and fishy odour of gelatine obtained from seabass skin. They concluded that defatting by pre-treatment with citric acid and isopropanol and subsequent incorporation of tannic acid during the extraction prevented lipid oxidation and the subsequent development of volatile compounds and fishy odours in the resulting gelatine. The intensity of fishy odour may also increase if the storage of frozen raw materials is prolonged before processing, due to formation of volatile aldehydes and alcohols [78
]. Therefore, delays in processing should be avoided in order to minimize formation of undesirable odour and further loss of technofunctioal properties of gelatine.