Amino acids under various pH conditions impact the pasting properties of rice starch.

Scientists at Louisiana State University investigated how different pH treatments, in combination with amino acids, might affect the pasting properties of rice starch. It appears that different pH levels, along with amino acids, can alter the product's starch properties and stabilize the starch against retrogradation and cooking. Starch retrogradation, the cause of ordinary texture staling of the crumb, is usually slowed by adding certain compounds, such as starch-complexing agents, to the dough. Monoglycerides of fatty acids have been widely used as dough additives to retard staling in the finished loaf.

In experiments, starch samples were prepared by adding amino acids--aspartic acid, leucine, lysine and tyrosine--at 6% on a starch dry-weight basis to rice starch in buffers of pH 4, 7 and 10. The starch-to-buffer ratio was 1 to 2.5. The samples then were mixed for 1 minute.

The scientists dried the samples in an oven at 40 C for 4 hours. Then they measured pasting properties using a rapid visco analyzer. The pH and amino acid treatments all decreased the paste viscosities compared to the untreated control. For lysine-treated rice starch, peak, minimum and final viscosities decreased as the pH increased. At pH 10, the peak viscosity was 302[+ or -]7 cP (viscosity value). At pH 4, the peak viscosity was 1554[+ or -]50 cP for lysine-treated rice starch.

Without amino acid, the peak viscosity for the treatment at pH 4 was 766[+ or -]16 cP, but by adding amino acid, some of the viscosity could be recovered. For all other samples, all paste viscosities were greater at pH 7 than at pH 4 or pH 10.

The three different pH treatments led to an improvement in the cooking stability of rice starch even without amino acids, with the lowest breakdown at 78.5[+ or -]3.5 cP for pH 7. The lowest setback of 164[+ or -]1.4 cP, and hence the lowest chance for retrogradation, was found for the leucine-treated sample at pH 4. The setback for the untreated control was 1138[+ or -]2 cP.

Further information. Joan King, Department of Food Science, Louisiana State University, 111 Food Science Building, Baton Rouge, LA 70803; phone: [phone]; fax: [phone]; email: [e-mail].

There can be little doubt that the physical and chemical properties of individual starches can be keys to their commercial success. Research has shown that amino acids can change the functional properties of starches.