Starch-menthol inclusion complex: Structure and release kinetics

Flavoring ingredients are often the most expensive ingredients in food formulations, and their stability and release behavior are significant factors for quality and acceptability of food products. Among flavoring compounds, aroma compounds draw a lot of attention in research, as their volatile nature makes their release difficult to control. In the present study, we employed pre-formed V-type starches to molecularly encapsulate an aroma compound, i.e., menthol. We examined high amylose maize starch of six different V-type crystalline structures (containing 6, 7 and 8 glucose units per helical turn), namely V-6, V-7, and V-8-types, to encapsulate menthol and thus, to control the release behavior. Rehydration of V-6a at a relative humidity of 0.75 lead to its conversion to V-6h (V-h), while rehydration had only minor influence on the XRD patterns of anhydrous forms of V-7 and V-8. Successful inclusion complexation was characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and gas chromatography-mass spectrometry (GC-MS). Upon complexation with menthol, the XRD pattern of V-6h shifted to V-7, whereas all other V-subtypes retained their original crystalline structures. An endotherm with peak temperature at ca. 118 degrees C appeared after inclusion complexation, corresponding to the dissociation of starch-menthol inclusion complexes. Endothermic enthalpy, with the exclusion of V-6a, and GC-MS quantification agreed that the total menthol content in inclusion complexes followed the order of V-6h > V-6a > V-7a> V-7h > V-8a > V-8h, with the helices initially of the V-6h-type being the most effective in encapsulating menthol molecules. The controlled release characteristics of the encapsulated aroma compound were investigated in the presence and absence of pancreatic alpha-amylase. The presence of amylase increased both the release rate and the amount of menthol released within 2 h. The release rates were different among samples made from different subtypes, due to their different association mechanisms with menthol, including physical adsorption on the surface of starch or in the amorphous phase, inter-helical entrapment, and intra-helical inclusion complexation. The Higuchi model was fitted to understand the release kinetics and showed that the constant Y for menthol release from different V-type ICs followed the order of V-8h > V-8a> V-7h > V-6a > V-7a > V-6h.