Physicochemical, functional and structural properties of flour and starch from dry heat-treated wheat

Thesis (PhDFoodSc)--Stellenbosch University, 2024.

ENGLISH ABSTRACT: Dry thermal treatment techniques as a pre-processing method for whole wheat or flour are gaining popularity for enhancing wheat properties. The effect of heat treatment on flour functionality is of critical importance for achieving the desired outcome. This study aimed to investigate the effect of forced convection roasting on the physicochemical, functional and structural properties of whole wheat kernels, flour and starch. Firstly, chemical-structural changes in whole wheat and flour, due to roasting, were investigated by determining the effect of roasting conditions and wheat types on shortwave infrared (SWIR) spectral data using ANOVA simultaneous component analysis (ASCA). Secondly, optimum roasting conditions were determined to (1) enhance flour viscosifying properties and (2) maintain protein integrity of the produced flour. Finally, ¹H time domain (TD) nuclear magnetic resonance (NMR) was used to investigate the molecular dynamics of the starch-water systems during heating and to differentiate between starch isolated from roasted and unroasted wheat as well as maize starch. Changes in whole wheat and flour due to roasting were characterised by determining the effect of roasting temperature, roasting frequency and wheat types on SWIR spectral data by applying ASCA. Roasting at different conditions significantly affected the chemical-structural properties of moisture, protein, starch, lipids and enzymes in wheat and/or flour as observed by the main variations in the loading line plots. Wheat roasting at 115 ºC and 140 ºC significantly affected protein stability in the white flour as well as in the whole wheat flour and kernel, respectively. Milling as well as roasting at 115 ºC and 140 ºC were responsible for starch damage observed in the white and whole wheat flour, respectively. Significant differences between hard and soft wheats were also indicated. Wheat roasting at 115 ºC and 65 Hz was predicted, by response surface methodology (RSM), to be the optimum conditions that would improve flour viscosifying properties. Optimally roasted wheat showed grain puffing, slight damage to the starch granule surface and destruction of the protein matrix. Compared to an unroasted sample, lowered relative crystallinity along with formation of amylose-lipid complexes were observed. Increased viscosities, compared to the control, were attributed to the combined effect of starch and protein properties affected by heat treatment. RSM predicted 108 ºC and 80 Hz as the optimum wheat roasting conditions that would maintain protein integrity. Increased viscosities, compared to the control, were ascribed to conformational changes in protein structures. Proteins were observed to be more heat sensitive than starch. At these optimum conditions, the starch granule structure and relative crystallinity remained relatively unaffected. TD-NMR experiments accurately demonstrated the interaction between starch and water, and the distribution of hydrogen atoms in the starch-water systems. The change in relaxation times enabled differentiation between unroasted, optimally roasted wheat starches and maize starch. Results obtained by TD-NMR, thermal and rheological analysis methods were comparable. The pulse sequences and signal processing methods used identified key characteristics associated with the structural (crystallinity), thermal (onset of gelatinisation) and rheological (maximum stiffness) properties of different starches. TD-NMR was shown in this study to be an effective and simple method to perform crystallinity measurements and study starch gelatinisation.

AFRIKAANSE OPSOMMING: Droë termiese behandelingstegnieke as 'n voorverwerkingsmetode vir volkoring of meel neem toe in gewildheid vir die verbetering van koring eienskappe. Die effek van hittebehandeling op meelfunksionaliteit is van kritieke belang om die gewenste uitkoms te bereik. Die studie het ten doel gehad om die effek van geforseerde konveksie rooster metode op die fisies- chemiese, funksionele en strukturele eienskappe van volkoringpitte, meel en stysel te identifiseer. Eerstens was die chemiese-strukturele veranderinge, as gevolg van rooster, in volkoring en meel ondersoek deur die effek van rooster toestande en koringtipes op kortgolf infrarooi (KGIR) spektrale data te bepaal deur gebruik te maak van ANOVA gelyktydig komponentanalise (ASCA). Tweedens, die optimale roostertoestande wat (1) die meel viskosifiseerende eienskappe verbeter en (2) die proteïenintegriteit van die geproduseerde meel behou is bepaal. Laastens was ¹H tyddomein (TD) kernmagnetiese resonansie (KMR) gebruik om die molekulêre dinamika van die stysel-water mengsels tydens verhitting te ondersoek, asook om te onderskei tussen stysel geïsoleer vanaf geroosterde en ongeroosterde koring, sowel as mieliestysel. Veranderinge in volkoring en meel, as gevolg van rooster, is gekenmerk deur die effek van rooster temperatuur, roosterfrekwensie en koringtipes op KGIR-spektrale data te bepaal deur ASCA toe te pas. Rooster by verskillende toestande het die chemiese-strukturele eienskappe van vog, proteïene, stysel, lipiede en ensieme in koring en/of meel beduidend geaffekteer, soos waargeneem deur die belangrikste variasies in die ladingslyn plotte. Koringrooster by 115 ºC en 140 ºC het die proteïenstabiliteit in die wit meel sowel as in die volkoring meel en pit onderskeidelik, beduidend beïnvloed. Die maal van koring sowel as rooster by 115 ºC en 140 ºC was verantwoordelik vir die styselbeskadiging waargeneem in die onderskeidelike die wit en volkoringmeel. Beduidende verskille tussen hard en sagte koring is ook waargeneem. Koringrooster by 115 ºC en 65 Hz is, volgens reaksie oppervlakmetodologie (RSM), voorspel as die optimale toestande wat meel viskosifiserende eienskappe sal verbeter. Optimaal geroosterde koring het gewys dat korrelpof, geringe skade aan die styselgranuleoppervlak en vernietiging van die proteïenmatriks na vore kom. In vergelyking met ‘n ongeroosterde monster was laer relatiewe kristalliniteit tesame met die vorming van amiloos-lipied komsplekse waargeneem. Verhoogde viskositeit, in vergelyking met die kontrole, is toegskryf aan die gekomineerde effek van stysel- en proteïeneienskappe wat deur hittebehandeling beïnvloed is. RSM het 108 ºC en 80 Hz voorspel as die optimale koringroostertoestande wat proteïen integriteit sal handhaaf. Verhoogde viskositeite, in vergelyking met die kontrole, is toegeskryf aan struktureleveranderinge in die proteïen strukture. Daar is waargeneem dat proteïene meer hitte-sensitief is as stysel. Die styselgranulestruktuur en relatiewe kristalliniteit het by hierdie optimale toestande redelik onveranderd gebly. Die interaksie tussen stysel en water, en die verspreiding van waterstof atome in die stysel-water sisteme is akkuraat met TD-KMR eksperiemente gedemonstreer. Die verandering in die ontspanningstye het dit moontlik gemaak om te onderskei tussen ongeroosterde, optimaal geroosterde koring stysels asook mieliestysel. Die resultate verkry deur TD-KMR, termiese en reologiese metodes is vergelykbaar. Die polsreeks en seinverwerkingsmethodes wat gebruik is, het kenmerke geïdentifiseer wat verband hou met die struksturele (kristalliniteit), termiese (aanvang van gelatienisering) en reologiese (maksimum styfheid) eienskappe van die verskillende stysels. In hierdie studie het TD-KMR getoon om ‘n effektiewe en eenvoudige methode te wees om krialliniteitsmetings te neem en om styselgelatiniseering te bestudeer.

Doctorate