Nutritional, structural and functional properties of infrared heat-moisture treated maize starch and maize meal with added stearic acid

Abstract

The current production trends are towards using emerging technologies in the food industry to make processes more efficient, less energy-intensive and more environmentally friendly. Infrared energy has the potential to replace conventional heating during the modification of starch and starch-containing foods using the heat-moisture treatment. The modification of starch and starchy foods to increase resistant starch could reduce rapidly digestible starch. This is beneficial as resistant starch can contribute towards alleviating diet-related conditions such as obesity and non-communicable diseases such as type-2 diabetes. This study, therefore, investigates the effect of adding stearic acid followed by heat-moisture treatment using infrared energy on the structural, nutritional and functional properties of both maize starch and maize meal. Stearic acid (SA) was added at 1.5% (w/w) level to maize starch and maize meal. Conventional heat-moisture treated samples were heat-moisture treated at 110°C for 16 h in a conventional oven. Infrared heat-moisture treatment was conducted using infrared hot air tunnel oven at 1000 W for three different periods (1 h, 2 h and 3 h). The raw (uncooked) maize starch and maize meal were analysed for proximate composition, nitrogen solubility index, water absorption and solubility index, pasting & textural properties and light microscopy. After pasting, both the pasted maize starch and maize meal samples were freeze-dried and analysed for in-vitro starch digestibility (IVSD), X-ray diffraction scatterings (XRD), thermal properties (DSC), scanning electron microscopy (SEM). Structural (XRD & FTIR), morphological (SEM & TEM) and thermal (TGA and DMTA) characteristics of unpasted infrared heat-moisture treated (IRHMT) maize starch (heat-moisture treated with and without stearic acid) were also determined. Infrared combined with SA induced changes in starch at a micro, nano and molecular level. At a micro level, light microscopy showed increased starch granule aggregation for IRHMT samples with SA. SEM also showed protrusion on the starch granular surface for maize starch with IRHMT plus SA. After pasting, scanning electron micrographs showed partial granular disintegration of granules from IRHMT starch with added SA compared to significant granular disintegration of conventionally heat-moisture treated starch (CHMT). At nano level, TEM showed more closely packed blocklets with the starch granules of IRHMT starch with SA compared to untreated maize starch. At a molecular level, FTIR showed that IRHMT of raw starch increased 1047/1022 cm-1 band ratio, suggesting an increase in the crystallinity of IRHMT starch with and without SA. Also, IRHMT raw starch with SA produced a new distinct XRD peak at 2θ=20°, which was assigned to amylose-lipid complexes. These structural changes in terms of relative crystallinity explained the increased thermal and thermo-mechanical stability of IRHMT starches with SA observed by DSC & DMTA. IRHMT starches with SA exhibited higher gelatinisation temperature and increased enthalpy of the first and second melting endotherms in raw samples corresponding to starch gelatinisation and melting of type I amylose-lipid complexes, respectively. DMTA showed increased glass transition temperature. Also the storage modulus of IRHMT modified starch (E′) was two-fold that of native starch. IRHMT starch with SA showed broader tan delta peaks compared to untreated starch. Infrared HMT was similar to conventional HMT since both treatments resulted in significantly (P < 0.05) reduced final viscosity and reduced in vitro starch digestibility in maize meal and maize starch with stearic acid. The resistant starch (RS) content increased with SA-IRHMT, while long pasting resulted in the highest slowly digestible starch (SDS) content in IRHMT starch and maize meal without stearic acid. IRHMT 2h on maize meal with SA resulted in a medium EGI maize meal. The increased RS can be related to the presence of V-type polymorphs (Type II amylose-lipid complexes) observed in XRD and DSC and the increase in relative crystallinity. The increased 1047/1022 cm-1 band ratio from FTIR also supported the increased crystallinity of IRHMT starches with and without SA. The above results suggest that infrared HMT changes the functional and nutritional properties of maize starch and maize meal. Infrared HMT also has the potential to replace conventional HMT in the development of lower GI, higher value-added functional starch foodstuffs for the management of diet-related non-communicable diseases.