Properties and Applications of Gelatin

Gelatin is the heat-denatured, partially hydrolyzed form of native, insoluble collagen. It is a soluble amorphous mixture made up primarily of three types of free chains: α monomers (mol wt 100 kDa), β dimers (mol wt 200 kDa), and γ trimers (mol wt 300 kDa) (Kijowski, 2001) . The denaturation temperature of collagen varies by species and hydroxyproline content. During this process, the molecule’s hydrogen bonds are disrupted and its intramolecular (aldol condensation and Schiff base), intermolecular, and main-chain peptide bonds are hydrolyzed, causing the collagen triple helix to unravel (Eyre, 1987) . 

Commercial gelatin is obtained primarily from raw materials rich in type I collagen, most importantly pork skin and bones, beef hides and bones and calf skin, through a very controlled stepwise process that involves the chemical hydrolysis of collagen, followed by heating to denature the molecule to gelatin. Type A gelatins are obtained by the mild acid pretreatment of physiologically young forms of collagen (e.g., pig skins), which have high proportions of acidand heat-labile cross-links (Bailey & Light, 1989 ; Eyre, 1987 ; Stainsby, 1987) . Type B gelatins result from the more severe alkali pretreatment of the more highly cross-linked collagen from bone and cattle hides (Pearson & Gillett, 1999 ; Stainsby, 1987) . The isoelectric points of type A gelatins are typically in the pH 6–9 range and those of type B gelatins are at approximately pH 4.8–5.2. Because of this, type A gelatins carry a net positive charge in most food systems, whereas type B gelatins are positively charged in acidic systems and negatively charged in near-neutral systems (Stainsby, 1987) . After extraction, gelatin is clarified (by filtration), concentrated (by vacuum evaporation or membrane ultrafiltration), dried, ground, blended, and, sometimes, sterilized (Linden & Lorient, 1999 ; Stainsby, 1987) . 

Properties and Applications 

Gelatin is deficient in methionine and completely devoid of tryptophan (Bailey & Light, 1989) , both essential amino acids. However, it has excellent functional properties, such as gelling, melting (melts at<35°C), stabilization, film-forming, texturizing, and water-holding properties, which make it a very useful and desirable food ingredient for many applications. Gelatin gels, like those of collagen, are thermoreversible. Dried gelatins are characterized, graded, and commercialized on the basis of their gel strength expressed in Bloom units, defined as the force in grams required to press a 12.5 mm diameter flat-faced, sharp-edged, cylindrical probe 4 mm into 112.5 g of a 6 2 / 3 % (w/v) gelatin gel that has been aged 16–18 h at 10°C (Gelatin Manufacturers Institute of America, 2006) . Gelatin Bloom values typically range from around 100 Bloom for very weak gels to around 250 Bloom for firm gels (Rosenthal, 1999) .

Gelatin is currently used in a wide variety of meat products, such as aspics, canned hams, and canned sausages (Bailey & Light, 1989 ; Stainsby, 1987) . Gelatin and gelatin hydrolysates have also been proposed as external coatings to protect meat against color loss, aroma deterioration, and purge losses (Antoniewski, Barringer, Knipe, & Zerby, 2007 ; Krochta & De Mulder-Johnson, 1997 ; Villegas, O’Connor, Kerry, & Buckley, 1999) . This protective effect has been attributed to the action of gelatin as a moisture and oxygen barrier.