Unravelling the mechanism of annealing in pulse and high amylose maize starches

Native starches are not widely used in the food industry due to their poor functional properties. Consequently, these starches are modified to improve their thermal stability, resistance to shear thinning and repeated freeze-thaw stability. Presently there is considerable interest in pulse starches that are physically modified due to their improved functionality and can be used in any kind of food applications. Annealing (ANN) is a physical modification technique that refers to the treatment of starch in excess water (> 65%) or at intermediate moisture contents (40-50%) for a specific period of time at a temperature between glass transition temperature and the onset (To) temperature of gelatinization. Annealing facilitates improved alignment and registration of double helices within the crystalline lattice and growth and perfection of existing crystallites. However, the mechanism by which these changes are achieved in starches of different amylose content is poorly understood. Against, this background my research aims to understand the structural changes that occur within the crystalline and amorphous regions of lablab bean (LB), navy bean (NB), rice bean (RB), tepary bean (TB), velvet bean (VB), wrinkled pea (WP) and Hylon VII (HVII). Structural changes on ANN were monitored by microscopy, ATR-FTIR, WAXS, 13C CP/MAS NMR, DSC, RVA and susceptibility towards acid and enzyme hydrolysis. The results showed that ANN increased onset (To) and conclusion (Tp) and decreased gelatinization temperature range (Tc-To). The increase in Tp and decrease in Tc-To suggest that ANN improved double helix register by decreasing heterogeneity within the crystalline lamellae. However, gelatinization enthalpy (∆H) remained unchanged in some starches (LB, NB, RB, WP) but increased in others (HVII>VB>TB) on ANN. Molecular order at the granular surface increased in LB, NB and HVII starches on ANN. Crystallinity of WP and HVII remained unchanged on ANN, but increased in the other starches (TB>LB>RB~VB~NB). On ANN, amylolysis remained unchanged in LB, increased in TB, but decreased in the other starches (WP>HVII>RB~VB>NB). Acid hydrolysis remained unchanged in WP and HVII, but decreased in the other starches (LB~VB>RB>NB~TB). The results showed that organization of starch chains within the amorphous and crystalline domains of native starches influenced reorganization of starch chains on ANN. The data obtained in this study will provide a new way of modifying pulse starch structure without recourse to chemical reagents that could improve the health-related quality of pulse starch based foods.