Amylopectin - Interactions with Lipids and Proteins

Starch is a common ingredient in many products, influencing properties such as viscosity, stability, sensory properties and appearance. These properties and how they change with time in the process called retrogradation are dependent on the structure of starch and its interaction with other components such as lipids and proteins. Lipids, which are present in many starches and products, play an important role in the ageing stability of starch by inhibiting the retrogradation. The inhibitory effect of lipids is believed to be due to complex formation with starch. The formation of this complex was studied by determining the binding of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) to amylose and amylopectin using surface tension measurements and measuring the enthalpy of binding with isothermal titration calorimetry (ITC). The enthalpy of binding CTAB to amylopectin was in the same range as the binding of CTAB to amylose. The binding of CTAB to amylopectin was of Langmuir type while the binding to amylose was of the positive cooperative type. Amylopectin seems to be able to bind lipids in a similar way as amylose where amylopectin can be regarded as a collection of independent short amylose chains. The binding involves a configuration change of the polysaccharide from random coil to helix so that the polysaccharide is coiled around the hydrophobic part of the lipid.

The influence of molecular structure of amylopectin on retrogradation and interaction with lipids and proteins was studied by hydrolysing amylopectin with alpha- and beta-amylase. The amount of recrystallisation during storage was determined, with and without lipids and proteins present, using differential scanning calorimetry (DSC). The most important structural factor influencing retrogradation is the length of the exterior chains (ECL), which is more or less the same as the length of the A and short B chains that make up the individual clusters of amylopectin. The shorter the ECL, the less the sample retrograde, and no retrogradation could be seen for samples with number average exterior chain length (ECLn ) below 11 glucose units. There is also a critical chain length of the exterior chains with which the lipids can form a complex and thus inhibit the retrogradation; lipids can only inhibit retrogradation when the ECLn was above this length. The interaction of the whey proteins, beta-lactoglobulin and alpha-lactalbumin, with starch did not influence the retrogradation directly, but beta-lactoglobulin was able to bind lipids and thus remove the inhibitory effect of lipids on retrogradation.