INTRODUCTION

Gelatin is a popular collagen derivative primarily used in food, pharmaceutical, photographic and technical products. In foods, gelatin provides a melts-in-the-mouth function and to achieve a thermo-reversible gel property. Its clarity, bland flavor, emulsifying characteristics, stability, texture properties and the ability to be applied in a wide range of pH, makes it suitable to be used in confectionaries and dairy products (GMIA, 2001). In addition, it is recommended for used as a dietetic food, salt reducer, flocculating agent, protein enrichment and adhesives. In the pharmaceutical industry gelatin is generally used in capsules, tablets, haemostatic sponge, blood plasma substitutes, suppositories and vitamin encapsulation (GME, 2010).

Gelatin is obtained from the degradation of collagen, thus collagen-containing tissues are generally used as sources of gelatin. In mammals, birds and fishes, the most commonly used source of collagen for gelatin is obtained from body protein constituents of the skin, tendons, cartilage, bone and connective tissue, whereas in invertebrates, collagen is an essential constituent of the body wall (Balian and Bowes, 1977). Porcine and bovine gelatins are still the most widely used today; therefore the development of alternative sources of gelatin is one of the issues that have been given much priority. In addition to the health related issues that, bovine gelatin has a potential risk of spreading bovine spongiform encephalopathy (BSE), widely known as mad cow diseases and foot-and-mouth disease (FMD) (Jongjareonrak et al., 2005), it used is vitally limited by religious concerns. For instance, Hindus do not consume cow-related product (Karim and Bhat, 2009). Similarly, Islam considers all pork-related products to be non Halal and prohibited to be consumed. Thus researches into the used of some alternative source of gelatins are being pursued. Such researches include the exploitation of marine and poultry products. It has been established that fish and fish products in generally can be considered as Halal food as long as it does not contain toxins and poisons (Huda et al., 1999). Therefore, the objective of this review is to present the potentials of using fish bones and scales for gelatins and available technologies to improve upon the yield of fish gelatins.

RAW MATERIAL OF HALAL GELATIN

Gelatin is a product of rapidly growing market. In 2003, the world market for gelatin reached 278.300 tons; consisting of 42.4% from pig-skin origin, 29.3% bovine hides, 27.6% bones and 0.7% from other sources (GEA, 2010). In previous years, (Karim and Bhat, 2009) reported that the annual world output of gelatin increased to 326.000 tons with the highest source being pig-skin (46%), followed by bovine hides (29.4%), bones (23.1%) and other sources (1.5%). In such proportions, existing gelatins do not meet the demands of the Halal market. As such alternative sources of collagen for gelatin from other sources other than porcine and bovine have been studied. They include previous studies on fish skin, bone and fins collagen isolation by (Nagai and Suzuki, 2000), sea urchin by Robinson (1997), jellyfish by Nagai et al. (2000) and bird feet by Lin and Liu (2006).

The production of gelatin from fish waste is a topic that has gain much attention, especially from fish skin due to its properties and qualities. In addition to the nature of the fish, that is almost acceptable by all communities, it also provides a solution to the utilization of huge amounts of fish wastes produced by the fish industry. For instance Guerard et al. (2001) reported that, canned fish processing generates solid wastes composed of muscles after the loins have been taken, fish viscera, gills, flesh dark/dark muscle, head, bone, and skin, which can be as high as 70% of the original material. Whereas skin, scale and bone wastes consist of more than 30% of fish processing (Kittiphattanabawon et al., 2005). As the total world fisheries reaches about 141.6 million tons (FAO, 2006), with anticipated increases in subsequent years, it’s a worth taken effort to utilize the large quantities of fish waste into useful products such as fish gelatin.

PROCESSING OF GELATIN

Fish skin, the common source of Halal gelatin: Gelatin can be obtained in several ways. Johns and Courts (1977) demonstrated that, the breakage of cross-links and non-covalent bonds of collagen can be done by direct thermal treatment, use of acidic or alkaline and enzymatic pre-treatments. Acidic and alkaline pre-treatment is the most widely used method, and has advantage over the direct thermal pre-treatment that is carried out under high temperature (heating and autoclaving), which produces an gel inferior quality.

In recent times, fish skin is the most widely used fish raw material for making fish gelatin. In previous works, gelatin extraction from fish species have been carried out using Alaskan pollock skin (Zhou and Regenstein, 2004, 2005), yellow-fin tuna (Cho et al., 2005), Atlantic cod (Arnesen and Gildberg, 2006), bigeye and brownstripe red Snapper (Jongjareonrak et al., 2005), Channel catfish skin (Liu et al., 2008), shark cartilage (Cho et al., 2004), grass carp skin (Kasankala et al., 2007), Nile perch skin and bone (Muyonga et al., 2004), and many more.

Gelatin from acid-treated collagen, known as type A gelatin is the most widely reported type of gelatin derived from fish skin material. Karim and Bhat (2009) confirmed that acidic treatment is most suitable method to be applied for fish skin due to its less covalently cross-linked collagen. Apart from acidic pre-treatment for Nile perch skin and alkaline pre-treatment for big eye snapper as reported by Muyonga et al. (2004) and Benjakul et al. (2009), respectively. Pre-treatment can be done simultaneously using both acidic and alkaline treatment as showen by Zhou and Regenstein (2005). Zhou and Regenstein (2005) found that alkaline and acidic pre-treatments had positive effect on removing non-collagenous proteins and resulted in high gelatin yield and gel strength in Alaska Pollock gelatin. Furthermore, they also mentioned that alkaline treatment followed by acid neutralization provide a neutral or weak acid extraction medium that makes it possible to produce high gelatin yield.

The removal of non-collagenous materials has been a common preparatory step in collagen isolation and the extraction of gelatin. Nagai and Suzuki (2000) performed the removal of non-collagenous proteins with 0.1 N NaOH under 4°C. In fish skin gelatin production, this step is continued to swelling step using low concentration of either acid or alkali solution. Previous research carried out by Huda et al. (2004) indicated that, different concentrations of acetic acid (1, 2, 3 and 4%) during pre-treatment had no significant effect on sensory evaluation of the produced gelatin. Contrarily, Yang et al. (2007) mentioned that acid solution concentration had significant effect on yield of protein and viscosity of gelatin in their work involving channel catfish.

After pre-treatment process, the gelatin can be extracted with aqueous extraction and heating (by gentle and mild temperatures) treatment. The extraction can be performed at a temperature between 50-90°C for 1-6 h before it is separated, evaporated and usually freeze dried (Wangtueai and Noomhornm, 2009; Zhang et al., 2010). This step distinguishes between gelatin extractions processes and the isolation of collagen. In collagen isolation process, collagen is not denatured by heating, but is extracted using the acid repeatedly and then separated, most commonly by using salting process.

Several fish skin-based gelatin has been reported to have varied bloom value (gel strength) compared with food grade bovine origin. Benjakul et al. (2009) reported that gelatin derived from two species of bigeye snapper fish has bloom strength value of 227.73 and 254.10, which was lower when compared to gelatin from bovine bones (293.22). Furthermore Gomez-Guillen et al. (2002) found a bloom value of 350 and 340, for sole and megrim fish species, respectively. Although bloom values between fish skin gelatins and other gelatin sources vary, fewer works done on fish skin for producing gelatin reveals that fish skin is one of potential source of high quality gelatin. Fish bone and fish scale could also be a potential source of gelatin due to its similar collagen characteristic to fish skins as reported by Wang et al. (2008), who showed that collagen composition as isolated from the skin, scale and bone of deep sea redfish had similar amino acid profile.

Isolation of gelatin from fish bones and scales: There are slight differences in the process of isolating gelatin from fish skins, bones and scales due to differences in their characteristics. For bones and scales, demineralized (decalcified) treatment is a common process employed after removal of non-collagenous material prior to the acid solution treatment. This process can be carried out by immersion using compounds such as EDTA until the hard part of bones disappears. In carp samples, skipjack tuna, Japanese sea bass, ayu, yellow sea bream, chub mackerel, and bullhead shark, demineralization takes 5 days (Nagai and Suzuki, 2000a; Duan et al., 2009). Demineralization has also been achieved using 3% HCl at ambient temperature in Nile perch bones in approximately 9-12 days until a leached bone (ossein) is formed (Muyonga et al., 2004). This demineralization period is much longer when compared to acid treatments on skin samples of the same species which only took 16 h. Furthermore, Wangtueai and Noomhornm (2009) employed a low alkaline concentration(0.1-0.9%) at 30°C for 1-5 h to process lizardfish scales, whereas Arafah et al. (2008) used 4-6% HCl in 24-48 h demineralization period for snakehead fish bone. In addition to the demineralization process, raw materials from both porcine and bovine bones undergo a process of defatting (GEA, 2010). In fish bones, this process is done by using butyl alcohol, hexane or a detergent (Duan et al., 2009; Nagai and Suzuki, 2000a; Wang et al., 2008). Not only different in demineralization (Duan et al., 2009) used different condition to perform acid treatment at carp fish. For the skin and scale, 0.1 M NaOH in 1:8 (w/v) sample/alkali solution was used under stirred for 6 h, while for bone the ratio was set into 1:5 (w/v).

An alternative approach to substitute acidic or alkaline pre-treatment by using enzymes in the production of gelatin from grass carp has been demonstrated by Zhang et al. (2010). In their work they use protease enzymes (after the removal of non-collagenous part by NaCl and demineralization using HCl) at a neutral pH and 20-40°C for 1-12 h. This produced a good quality gelatin with gel strength of 172-219 g. Several methods for gelatin and collagen isolation from fish bones and scales are presented in Table 1.

CHEMICAL PROPERTIES OF BONE AND SCALES GELATIN

Table 2 summarizes the amino acid composition of bone and scale based gelatins. In general, the amino acid composition of both fish scale and bone is almost similar to fish skin-based gelatin, and showed slight differences with commercial gelatin. With the exception of gelatin from pigskin origin, all other gelatins do not contain aspargine and glutamine. In addition, amino acid composition of fish scales and bone varied, particularly in cysteine content. Amino acids from pigskin gelatin and bone gelatins (Nile perch bone, commercial bones) do not contain cysteine. Gelatin from fish’s bone and scale, in general have higher of imino acids (proline) content than the fish skin gelatin and almost the same with commercial gelatin from pigskin and bone. Muyonga et al. (2004) mentioned that the higher content of imino acid in Nile perch contributed to better gelling properties in their gelatin.

However, the content of hydroxyproline in fish skin gelatin is higher when compared with fish bone and scale gelatin as well as from commercial gelatin. For the content of glycine, which is the most common component in collagen, fish-based gelatin had lower quantities compared to those from mammalian sources (Wangtueai and Noomhornm, 2009; Zhang et al., 2010; Liu et al., 2008; Muyonga et al., 2004; Kasankala et al., 2007; Ledward, 2000), although Zhang et al. (2010) found a very high content of glycine in grass carp scale.

Arnesen and Gildberg (2002) mentioned that the lower concentration of hydroxyproline in fish compared to bovine and porcine accounts for the low gel strength in fish based gelatins. Nonetheless, Intarasirisawat et al. (2007) reported that heat-stable indigenous proteases were responsible for the degradation of gelatin molecules especially α and β-chains during extraction at elevated temperature; the results of this is low bloom value of gelatin.

Muyonga et al. (2004) compared gelatin extracted from young and adult bones of Nile perch and found that gelatin extracted from young bones had higher concentration of low molecular weight fraction compared to gelatins from old bones. Recent study carried out by Zhang et al. (2010) using grass carp scales and enzymatic treatment revealed that the lower the amino acids content of gelatin, the higher the α-chain and β-component.

PHYSICAL PROPERTIES OF FISH BONE AND SCALES GELATIN

The physical properties of fish bones and scales based gelatins are summarized in Table 3. The yield of gelatin extraction have been reported to range from 0.98-3.9% for bones and 9.1-10.9% for bovine gelatin was 322±4.56 (Wangtueai and Noomhornm, 2009).

Table 1:	Procedures employed to isolate fish bones and scale gelatin/collagen

Table 2:	Amino acids composition of several gelatins from fish bones and scales (/100 residues)

This study also mentioned that, the optimum conditions for gelatin extraction by alkaline pre-treatment was achieved using NaOH solution at a concentration of 0.51%, 78°C for 3.10 h treatment time and 3.02 h extraction time. Cheow et al. (2007) reported that gelatin from sin croaker and shortfin scad had low gel strength (of 124.94 and 176.92 g, respectively) compared to bovine gelatin 239.98 g (9.76±0.12 mg 100 g).

Table 3:	Physical properties of several gelatins from fish bones and scales (/100 residues)

Lower bloom value might be the biggest problem for gelatin from fish origin, although some works have indicated that fish skin had higher gel strength than bovine and porcine gelatin (Arnesen and Gildberg, 2002; Cho et al., 2005).

Gelatins of fish bone and scales origin also have lower setting and melting point which has been reported to range from 13.3-19 and 20.7-26.9, respectively; as well as the viscosity (28.2 and 30.0) compared with gelatins of bovines and commercial fish origin, which ranges from 22.5-25.3 and 26.3-31.6, respectively as well as the viscosity (40.0 and 46.0 mSt), yet the isoionic point of fish bone ad scale gelatin are stable at 7.0-7.2 (Muyonga et al., 2004; Liu et al., 2008).

IMPROVEMENT OF FISH ORIGIN GELATIN

The low yields of gelatin obtained from fish by-products compared to gelatin from other sources are issues of concern. A number of studies have been carried out to address this challenge. For instance, Gudmunsson and Hafsteinsson (1997) mentioned that the quality of gelatin can be controlled to the desired standard by manipulating pre-treatment and processing conditions. The same researchers also reported that, a treatment combination of citric acid, low concentration of sulfuric acid and sodium hydroxide resulted in a higher yield (14%) compared to 11% when a high concentration of citric acid (>1%) and sulfuric acid, and NaOH (>2%) were used. Arafah et al. (2008) also showed that, a higher concentration of acid, together with increased extraction temperature did lower the gelatin yield of mackerel fish skin. Zhang et al. (2010) used enzymatic treatment for grass carp scales and concluded that, gelatin from grass carp scales can be made into good quality gelatin which will have high gel viscoelastic property at lower temperature and good quality gel strength (276±12 g) compared to commercial porcine gelatin. Aewsiri et al. (2009) reported that fish products can be subjected to bleaching to enhance the quality of the gelatin. Thus in their study, they employed H₂O₂ as a bleaching agent in gelatin production from cuttlefish, and found higher yield, brighter color and effective increase in gel strength. Fernandez-Diaz et al. (2003) mentioned that gelatin extracted from lower temperature storage fish skin had higher gel strength compared to samples that stored at higher temperature. Bhat and Karim (2009) also observed that UV irradiation increased the gel strength of fish gelatin.

CONCLUSION

Production of gelatin from fish bones and scales are important alternative source for fish skin gelatin. Although the resulting yield from fish bone and scales gelatin is lower than that obtained from fish skin, the quality of gelatin produced is not inferior when compared. Nonetheless several studies have indicated that gelatins produced from fish bones and scales have acceptable gel strength (bloom value). Weak gel strength and low melting point, makes gelatin derived from fish unable to be used completely to replace the role bovine and porcine gelatin plays. With the development of research, various solutions such as enzyme-aided processes, combination of acid-alkali solutions and gelatin bleaching processes have been found to improve the quality of gelatin from fish bone and scales.

Preparation of gelatin from fish by-products is a way of utilizing the huge waste created by the fish industry into useful products. It also has the advantage of being accepted with ease as Halal and Kosher food.

ACKNOWLEDGMENTS

The first author is grateful to the Institute of Postgraduate Studies, Universti Sains Malaysia for the opportunity given him to pursue a Ph.D programme through USM Fellowship Scheme. Both authors are also grateful for the support given by the Universti Sains Malaysia for running research in the area of fish processing technology.