Gelatin is divided into type A gelatin (acid pretreated) and type B gelatin (alkali pretreated) according to the processing. Although they are both extracted from collagen, there are some differences in their properties.
Type A gelatin is extracted from raw material that has undergone acid treatment. The light yellow color of gelatin is due to the Maillard reaction between protein and saccharide. The amino groups are protonated to inhibit the Maillard reaction in acidic solution, resulting in lighter color. The protonated amino groups make the molecules positively charged, so its isoelectric point is between pH 7 and 9. The amino acid of type A gelatin is almost the same as that of the raw material. It has a wide molecular weight distribution: the polypeptide chains are evenly distributed between a dozen KDa and several thousand KDa, with slightly more alpha chains at 100 KDa. It has a higher fat content because fat is not completely dissolved during acid treatment, while during thermal extraction it dissolves in the gelatin solution and is difficult to remove. Type A gelatin is more stable in an acidic environment.
Type B gelatin is extracted from raw material that is pretreated by alkali. The asparagine and glutamine are converted to aspartic acid and glutamic acid. The decrease of amino group and the increase of carboxyl group give its negative charges. Its isoelectric point is between pH 4.7 and 5.2. The amino groups are hydrolyzed partially during alkaline pretreatment, so it contains slightly less nitrogen element than the raw material. The molecular weight distribution is relatively narrow: alpha chains of 100 KDa are the main component with a few larger and smaller molecules. Nonetheless, there are still more molecules over 100 KD than in acid-treated gelatin, so alkali-treated gelatin is usually more viscous. Although the compositions of type A gelatin and type B gelatin are quite different, their Bloom values are similar. (The bloom value or gel strength of gelatin is mainly determined by the 100KD molecules, with some contribution to the gel strength from other components. The viscosity of gelatin is mainly determined by molecules larger than 100KD, and other components contribute less to viscosity.)The alkali solution can dissolve some non-collagen protein in raw material, especially albumin and globulin. Fat is removed after saponification by alkali pretreatment, so less fat is left in gelatin B. It is more stable in alkaline environment.
| Gelatin Type A vs Tybe B | ||
|---|---|---|
| Type A gelatin | Type B gelatin | |
| Isoelectric point | PH 7-9 | PH 4.7-5.2 |
| Bloom value or gel strength | Similar | Similar |
| Viscosity | Low viscosity | High viscosity |
| Amino acid content | Unchanged | asparagine→aspartic acid and glutamine→glutamic acid |
| Molecular weight distribution | Uniform distribution from a dozen KDa to several thousand KDa, with more abundant 100 KDa parts | Concentrated on 100 KDa, others less |
| Color | Yellowish | More yellowish than gelatin type A |
| Impurities | More fat | Less fat |
| Stability | More stable in acid | More stable in alkali |