Effect of stone and roller milling on physicochemical, functional and structural properties of sifted wheat flour

Date
2021-03
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Stone milling, which is one of the oldest forms of wheat flour milling, makes use of two stones to grind wheat grain to a flour. Stone milling is often associated with whole wheat flour, however a range of stone milled flour, including white flour, is found in supermarkets and other retail points across South Africa. Very few studies have focussed on white stone milled flour, and this study set out to provide fundamental knowledge regarding this. The first research chapter studied the effect of stone and roller milling on the physicochemical, functional and structural properties of white flour. Two hard wheat grain samples, i.e. a South African commercial wheat cultivar and an imported Canadian commercial blend, were milled using stone and roller milling methods. The results obtained from the samples indicated that the milling methods used often had the largest effect on the flour samples, larger than the wheat types or the interaction of wheat type and milling method. Sieving the stone ground wheat meal through a 212 μm sieve (as per South African wheat flour regulations) proved ineffective in removing the bran particles, as indicated by the significantly darker colour and significantly higher ash content compared to the roller milled flour. The sifted stone milled flour had an extremely low flour yield compared to the roller milled flour. This may be because the stone mill crushes the entire wheat kernel to produce a whole wheat flour and does not separate the bran and endosperm as is the case with the break system of a roller mill. The ash content of the sifted stone milled flour was significantly higher than that of the refined roller milled flour. The stone milled flour also had a significantly higher water absorption capacity than the roller milled flour, which was due to the significantly smaller median flour particle size, higher starch damage and ash content. The higher ash content suggests a higher aleurone and bran content, thus a higher arabinoxylan content and water absorption capacity. Stone milled samples had significantly higher falling numbers than roller milled samples. Significant differences between stone and roller milled flour for both wheat samples were seen for the alveograph P (tenacity), L (extensibility), P/L (curve configuration ratio) and W (deformation energy). The tenacity and curve configuration ratio were significantly higher, and the extensibility and deformation significantly lower for stone milled flour compared to the roller milled flour. The mixograph midline peak time and peak height were mostly affected by the wheat types, however the stone milled flour samples had significantly lower values for both these aspects compared to the roller milled samples. The pasting properties obtained with the Rapid Visco Analyser (RVA) also significantly differed between milling methods: the stone milled flour had lower pasting viscosities and peak time, but a higher pasting temperature. The scanning electron micrographs (SEM) provided insight into the qualitative aspects of the flour samples and illustrated that stone milled flour samples were less uniform with visible mechanical damage to the starch granules. The second research chapter aimed to determine the adherence of three commercial stone milled flour samples and one roller milled sample to South African wheat flour and fortification regulations. The samples were sourced from points of retail in the Western Cape, South Africa, and the moisture content, ash content, bran content, crude protein content, CIELab colour and presence of iron were determined. The protein content suggests that the flour samples could be suitable for bread production, yet the bran content of the stone milled flour samples were too high to be classified as white bread wheat flour. The packaging of the roller milled flour samples indicated that the products were fortified with micronutrients, yet one batch did not indicate a presence of iron. None of the stone milled samples indicated a presence of iron, nor did the packaging contain fortification claims or information as stipulated in the regulations.
AFRIKAANSE OPSOMMING: Die metode om koring met klippe te maal is een van die oudste vorms van koring maal. Twee klippe word teen mekaar geskuur om die koring tot ‘n meel, oftewel steengemaalde meel, te maal. Steengemaalde meel word dikwels geassosieer met volgraanmeel, alhoewel ‘n wye reeks steengemaalde meel, insluitend witmeel, in supermarkte en ander verkoopspunte in Suid-Afrika verkoop word. Die doel van hierdie studie is om fundamentele kennis rakende wit steengemaalde meel in te samel omdat min vorige studies daarop fokus. Die eerste navorsingshoofstuk bestudeer die effek van steen- en rollermaalmetodes op die fisikochemiese, funksionele en strukturele eienskappe van witmeel. Twee koringmonsters, naamlik ‘n Suid-Afrikaanse kommersiële koringkultivar en ‘n ingevoerde Kanadese kommersiële mengsel, is gemaal met steen- en rollermetodes. Die resultate dui aan dat die maalmetodes die grootste effek op die monsters gehad het in vergelyking met die effek van die koringtipes of die kombinasie van koringtipe x maalmetode. Die gesifde steengemaalde meel het ‘n laer opbrengs gelewer teenoor die rollermeel. Die wit steengemaalde meel is geproduseer deur die volgraanmeel deur ‘n 212 μm-sif (soos per Suid-Afrikaanse meel regulasies) te sif, maar dit was oneffektief in die verwydering van die semel. Die hoë semelinhoud is aangedui deur die betekenisvolle verskil in kleur- en asinhoud. Die rede vir die hoë semelinhoud is as gevolg van die steenmeul wat die hele koringkorrel maal om volgraanmeel te produseer. Dit verskil van ‘n rollermeul omdat dié ‘n stap het wat die semel en endosperm skei voor die endosperm verklein word tot meel. Die asinhoud van die steengemaalde meel (van beide koringmonsters) was te hoog om as witbroodkoringmeel, volgens Suid-Afrikaanse regulasies, geklassifiseer te word. Die steengemaalde meel het ook ‘n hoër betekenisvolle waterabsorpsiekapasiteit gehad as die rollermeelmeul as gevolg van die betekenisvolle hoër styselbeskadiging en asinhoud, asook die betekenisvolle kleiner mediaan meelpartikelgrootte. Betekenisvolle verskille tussen die steen- en rollermele se alveograaf P (weerstand van die deeg tot verlenging), L (rekbaarheid), P/L (kurwekonfigurasieverhouding) en W (deformasie-energie) is waargeneem. Die steengemaalde meel se P en P/L was betekenisvol hoër, en die L en W betekenisvol laer as dié van die rollermeel. Die verdikkingseienskappe van die Rapid Visco Analyser (RVA) het ook betekenisvol verskil tussen maalmetodes: die steengemaalde meel het laer verdikkingsviskositeite en piektye gehad, asook ‘n hoër verdikkingstemperatuur. Die skanderende elektronmikrograwe (SEM) het die kwalitatiewe aspekte van die meelmonsters aangedui, asook die oneenvormigheid van die steengemaalde meelmonsters en die meganiese skade wat die stysel ondergaan het. Die tweede navorsingshoofstuk se doel was om die nakoming van drie kommersiële steengemaalde meel- en een rollermeelmonster tot Suid-Afrikaanse koringmeel- en fortifiseringsregulasies te bepaal. Die monsters is afkomstig van supermarkte en ander verkoopspunte in die Wes-Kaap, Suid-Afrika. Die vog-, as-, semel- en proteïeninhoud, asook die CieLab-kleur en ysterteenwoordigheid is bepaal. Die proteïeninhoud dui aan dat die meelmonsters geskik is vir broodproduksie, alhoewel die asinhoud te hoog was om die steengemaalde meel te klassifiseer as ‘n witbroodmeel. Die verpakking van die rollermeelmonsters dui aan dat die produkte gefortifiseer is met mikronutriënte, alhoewel een monster nie ‘n ysterteenwoordigheid aangedui het nie. Geen van die steengemaalde monsters het ‘n teenwoordigheid van yster aangedui nie.
Description
Thesis (MScFoodSc)--Stellenbosch University, 2021.
Keywords
Wheat products, Milling industry, Flour, Commercial stone millers -- Analysis, Flour mills -- South Africa, Flour -- Quality, UCTD
Citation