Source thermoreversible gel from starch.

Gelatin is one of the hydrocolloids or water-soluble polymers that can be used as a gelling, thickening or stabilizing agent in jelly desserts, dressings, confectionery, dairy and meat products. An important functional property of gels formed from gelatin is their thermoreversible nature. During the production process, gelatin is fluid. But the end-product is firm, and it gives a special mouthfeel.

The animal origin of gelatin is a major driving force behind the search for suitable substitutes derived from microorganisms or plants, such as starch. However, the substitution of gelatin by starch is hampered because starch does not normally form thermoreversible gels. Scientists at Holland's TNO Research Institute wanted to obtain gels from starch products that have thermoreversible properties using enzymatic treatment.

It's possible to obtain new or enhanced functionalities of starch by enzymatically changing its structural properties. The enzymes that can do this include the glucosyltransferases, such as phosphorylases that elongate amylopectin side chains, and branching enzymes that increase the number of branching points.

The investigators combined their expertise in screening microorganisms for thermostable starch-modifying enzymes and their knowledge of advanced methods for analytically and functionally characterizing starch products. They found that another type of enzyme, a-1,4-a-1,4 glucosyltransferases (amylomaltase: Amase), is able to change the side-chain distribution of amylopectin. It could well achieve the desired type of functional modification.

Starch-modifying enzymes should preferably be used at higher temperatures to prevent retrogradation. In order to obtain thermostable enzymes, a number of thermophilic organisms were screened for the presence of glucosyltransferases. Researchers detected Amase activity in cell-free extracts of Thermus thermophilus HB8. The enzyme was purified and characterized. The molecular weight of the purified enzyme turned out to be 54 kDa. The enzyme was active as a monomer with an optimum activity at 75-|C and pH 6.5.

Scientists cloned the enzyme and brought it to overexpression to produce enough enzyme to modify starch on a 100-kg scale. Upon incubation with Amase the viscosity of starch pastes decreased dramatically. However, the reducing power of the starch remained unchanged, indicating that hydrolysis did not occur. The maximum absorption wavelength of the complex of some of the Amase-modified potato starches was lower than that of the native starch and corresponded to that of amylopectin.

Repeated heating and cooling of Amase-modified starch solution caused reversible changes in the gel structure markedly different from that found in unmodified starch. The modified starch behaves as a thermoreversible gel. So we have a food grade material prepared from native starch using an enzymatic process.

Further information. Doede Binnema, TNO Nutrition and Food Research, Rouaanstraat 27, 9723 CC Groningen, The Netherlands; phone: [phone]; fax: [phone]; email: [e-mail].