The effect of processing conditions on the properties of gelatin from skate (Raja Kenojei) skins
ABSTRACT Effects of several conditions (liming concentrations, extraction solution pH, extraction temperature and extraction time) to extract gelatin from skate skin on the yield and quality properties were investigated. The optimum conditions for gelatin extraction are as follows; place skin in a lime solution of 1.5% (w/v) calcium hydroxide, extract with three volumes of water (pH 6.0) for 4 h at 50%, filter gelatin through activated carbon (250–350 mesh, 3%) and freeze-dry the colloidal suspension. The functional properties of skate skin gelatin produced by optimum extraction conditions were: gelling point 16.12 °C; melting point 19.30 °C; isoelectric point 6.45; and turbidity 6.98.
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ABSTRACT: Gelatin from skins of several marine species were compared on the basis of their rheological characteristics (viscoelasticity and gel strength) and chemical/structural properties (amino acid composition, molecular weight distribution and triple helix formation). Gelatins from flat-fish species (sole and megrim) presented the best gelling ability and the gels were more thermostable than those from cold-adapted fish (cod and hake). This different behaviour may be explained considering the amino acid composition, the α1/α2 collagen-chain ratio, and the molecular weight distribution. Thus, cod gelatin presented a lower alanine and imino acid content, and a decreased proline hydroxylation degree; cod and hake gelatins presented a low α1/α2 ratio (∼1); hake gelatin showed a highly significant decrease in β-components and other aggregates. The squid gelatin presented the most significant changes regarding amino acid composition and molecular weight distribution, most of these differences arising from the low solubility of the squid connective tissue. However, the squid gelatin showed viscoelastic properties intermediate between those from flat-fish and cold-adapted fish species. Circular dichroism analysis reveals that gelling involves a refolding of denatured collagen chains into the typical triple helix conformation and, conversely, unfolding upon reheating. Thermal folding and unfolding curves were similar to those of viscoelastic properties but showing a shift towards lower or higher temperatures upon cooling and heating, respectively. The folding process seem to be directly related in the stabilisation of the gels without disregarding its role in triggering the gelation process. Finally, gel strength evaluation revealed the importance of slow cold maturation.Food Hydrocolloids. 01/2002;
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ABSTRACT: Gelatin gels made from cod and megrim skins were formed using high pressure and subsequent cooling at 7°C during 16–18 h. Pressure treatments were carried out at two temperatures (20 and 7°C) and 200, 300 and 400 MPa. Turbidity, gel strength and viscoelastic properties of these gels were studied. Gelatin gels from megrim showed lower degree of turbidity, independent of the treatment, only small significant differences were observed amongst them. In cod gelatins turbidity decreased as pressure increased, being higher at higher temperature. Gel strength of gelatin gels from megrim was about three times higher than gels from cod. High-pressure treatments improved gel strength only in gelatin gels from cod. However, in gels from megrim, which has high gelling capacity in comparison to cod gelatins, gel strength values of pressurized gels were lower than the control. Melting and gelling points did not vary substantially with pressure although values tend to improve. Initial values of G′ and G″ at 5°C after setting during 16–18 h were higher for megrim gelatins than cod gelatins except for high-pressure treated cod (400 and 200 MPa, 20°C) that showed the highest value. Gelation mechanism appears to be different between pressure- and heat-induced gels and they affect different to bonds that stabilise or destabilise the gel network. The properties of the resulting gelatins are also influenced by the initial collagen (that is different depending on the fish species and amino and imino acid content) and by the concrete treatment process.Food Hydrocolloids. 01/2002;
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ABSTRACT: The proximate compositions of fresh and fermented skate skin were each 75.95% and 74.5% moisture, 22.7% and 21.8% protein, 0.5% and 0.7% lipid and 0.6% and 0.9% ash, respectively. The predominant minerals were potassium and phosphorus (i.e. 53.5 and 33.0 mg/100 g in fresh skin, and 10.46 and 10.51 mg/100 g in fermented skin, respectively). Amino acid concentrations were lower in the fermented skin compared with the fresh skin. Histidine, glycine, alanine and glutamic acid were the major free amino acids in both skins. Palmitic acid (C16:0) was the major fatty acid in both fresh (16.68%) and fermented (20.38%) skate skin. Omega-3 polyunsaturated fatty acids were higher in fresh skin (22.17%) and fermented skin (24.54%) compared with omega-6 polyunsaturated fatty acids. The predominant microflora present in the both fresh and fermented skin were Photobacterium sp. and Vibrio sp. Total plate counts for the fresh and fermented skin were 2.4x10(4) CFU/g and 7.7x10(7) CFU/g, respectively.International Journal of Food Sciences and Nutrition 03/2004; 55(1):45-51. · 1.26 Impact Factor