Properties of infrared heat-moisture treated amylose-lipid complex nanomaterials

Abstract

Nanotechnology involves the study and use of materials at nanoscale dimensions of ≤ 100 nm. This nanoscale length results in materials exhibiting physical and chemical properties that are significantly different from the properties of macroscale materials composed of the same substances. However, there are limited food compatible nanomaterials to be used in foods, or most of them are not considered as an edible or clean label. Current consumers’ trends are more towards ‘clean label’ food and food ingredients. This has prompted the development of ‘clean label’ starch with principles of green chemistry, for example using naturally occurring substances such as fatty acids. Amylose lipid complexes (ALC) has been produced at nanoscale when maize starch is wet heat processed with stearic acid at lab scale. The material is semi-crystalline and dispersed in aqueous system to form hydrocolloids, and this limits the use as nanofiller in biodegradable packaging systems. Heat moisture treatment using infrared energy can increase the crystallinity of starch. The aim of this project was to produce ALC nanomaterials at a pilot scale and to evaluate the effects of infrared energy heat moisture treatment on the properties of the isolated nanomaterials. It is anticipated that HMT will result in more crystalline material to be used as nanofillers. The nanomaterials were produced using maize starch with added stearic acid (1.5% w/w) pasted for a prolonged period of 130 min at 91℃ using Rheometer (lab scale) and Reactor (pilot scale). The nanomaterial was infrared heat moisture treated for 1, 2 and 3 hours at 110℃with 25% moisture content as a continuous method. Repeated infrared heat moisture treatment was carried out for three days at 1 hour each day with 25% moisture content at 110℃ using different cooling systems after the daily infrared treatments (room temperature, refrigeration temperature and liquid nitrogen) for 24 hours. The nanomaterial was analysed for thermal properties using differential scanning calorimetry (DSC), wide angle x-ray diffraction scattering (WAXS), thermogravimetric analysis (TGA) and dynamic mechanical thermal analysis (DMTA). The morphology (using scanning electron microscopy), flow properties and water absorption and solubility index were also determined. The lab scale synthesis had a significantly (p<0.05) higher yield compared to the pilot scale, with the yield percentage of 41% and 38% respectively. Both the lab scale and pilot scale isolated materials had an average size of ±45 nm and DSC endothermic peak at about 103℃, showing that the isolated nanomaterials contained amylose lipid complexes. Thus, ALC nanomaterials were scaled up successfully. The DSC for ALC nanomaterials treated with heat moisture followed by cooling showed higher endothermic peak temperature and hence the presence of type II ALC when compared to untreated ALC (control), that only had type I ALC. Wide angle X-ray scattering (WAXS) showed that the moisture treated ALC nanomaterials showed relatively higher crystallinity as compared to untreated ALC and the increase in crystallinity was correlated to the transformation of type I to type II ALC as evidenced by DSC and WAXS results. Infrared heat moisture treatment followed by cooling changed the structural, flow and morphological properties of isolated amylose lipid complex at nano level. SEM of the treated ALC nanomaterials showed more closely packed particles and protrusion on the surface which resulted in significantly (p<0.05) reduced water solubility and water absorption index compared to untreated isolated ALC nanomaterials. DMTA showed similar grass transition temperature, increased storage modulus and decreased tan delta peak broadness as opposed to that of untreated ALC nanomaterials. Treated isolated ALC nanomaterial resulted in significantly (p<0.05) reduced zero shear viscosity and hysteresis during flow property analyses at different shear rates. These changes were related to the formation of type II ALC observed in XRD and DSC; as well as the nanostructure. The crystallinity of isolated amylose lipid complex nanomaterials can be increased with infrared heat moisture treatment by structural transformation of type I of ALC to type II. This was because during infrared HMT, the helices acquired enough mobility to align around the remaining helices acting as nuclei. This leads into more ordered type II ALC crystals. They was also increased in crystallinity from type IIa into type IIb. The molecular re-ordering reduced the viscosity. Heat moisture treatment resulted in increased crystallinity of isolated amylose lipid complex nanomaterials. Therefore, ALC nanomaterials with increased crystallinity can be produced from infrared HMT that can be used in biodegradable packaging systems. Moreover, the reduced viscosity suggest that infrared HMT treated ALC can be used for encapsulation of flavours to be used in emulsion at higher solid content for higher encapsulation efficiency.