Effect of laboratory milling on properties of starches isolated from different flour millstreams of hard and soft wheat
Introduction
Wheat (Triticum aestivum L.) is one of the most important daily staples consumed in China in different forms such as noodles, steamed bread and various bakery products. It is classified and traded as hard or soft based on the endosperm hardness and texture (Morris, 2002, Tsilo et al., 2011). In addition to high amount of nutrients in grains, wheat products have desirable palatability and digestibility. They also have their characteristic flavour, aroma and colour due to the occurrence of Maillard reaction during high-temperature processing (Mulla, Bharadwaj, Annapure, & Singhal, 2010). Wheat grains consist of endosperm, bran and germ. The inner endosperm is primarily composed of starch granules and proteins, which account for about 65–75% and 8–20% of the grain dry weight, respectively (Dupont & Altenbach, 2003). The peripheral part of wheat grain is generally referred to as bran and germ, which are rich in vitamins, minerals, antioxidants and dietary fiber (Dewettinck et al., 2008). Most wheat grains consumed by human generally undergo an important processing procedure, namely roller milling to produce flour for various uses. Commercial wheat milling generates various flour fractions and mill streams that can be recombined to provide flour with distinct processing characteristics (Liu, Ohm, Hareland, Wiersma, & Kaiser, 2011).
Wheat flour production includes two main steps, namely break of wheat grains to separate endosperm from bran and germ, followed by the reduction of endosperm into flour. The milling is a gradual mechanical process to produce flour through a series of break and reduction rolls. This results in the production of different types of flour streams containing endosperm, bran and germ in varying proportions. The starch in flour from each break and reduction system is subjected to differential compressive and shear forces, which result in the different extent of damage of starch granules (EI-Porai et al., 2013, Li, Hasjim, et al., 2011, Liu, Ma, et al., 2011, Liu, Ohm, et al., 2011, Pauly et al., 2013, Sakhare et al., 2013, Sakhare et al., 2012). The changes in starch morphology, crystalline and molecular structure that are caused by milling damage can greatly affect the functional properties such as swelling, solubility, pasting, gelatinization, retrogradation and in vitro digestibility. As the major component in wheat grains, the functional properties of starch granules in milled wheat flour can greatly affect the quality of final wheat-based products (Ghodke et al., 2009, Goesaert et al., 2005, Mulla et al., 2010).
There have been many studies on the effects of milling on starch structure and functionality as reviewed by Li, Dhital, and Hasjim (2014), however, they were conducted mainly on isolated starch granules. Milling of isolated starch granules can cause starch damage at different structural levels, including the fragmentation or rupture of intact granules, disruption of crystalline structure, and degradation of starch molecules (Li et al., 2014). In contrast, the effects of milling of cereal grains on structure and functionality of starch granules remain largely unknown. In particular, no information is available regarding the properties of starch from different flour millstreams.
In the present study, two wheat varieties of Beijing 0045 (hard grain) and Zhongmai 175 (soft grain) were chosen to understand the effects of laboratory milling on the structure and functionality of starch granules isolated from different flour millstreams. The aim of the present study was to evaluate whether the starches isolated from different flour millstreams of hard and soft wheat have different properties, and whether the damaged starch is a major factor that influences the functional properties of isolated starches. Such information would be very useful in the preparation of flour blends either by selecting or omitting particular stream/streams for the production of specific mill fractions used in different wheat-based food products.
Section snippets
Materials
Zhongmai 175 and Beijing 0045 are the leading soft and hard wheat varieties in the Northern China Plain Winter Wheat Region, respectively. Sound grain samples from 2011–12 season in Beijing from the same field were provided by the Institute of Crop Science of Chinese Academy of Agricultural Science. The grain hardness of Zhongmai 175 and Beijing 0045 was 13.6 and 57.6, respectively, based on the test of Single Kernel Characterization System (SKCS 4100, Perten Instruments, Sweden). The protein
Damaged starch content in wheat flours and isolated starches
The damaged starch content in Beijing 0045 and Zhongmai 175 flours from different mill streams by Buhler laboratory mill was in the range of 3.4–15.7% and 1.8–6.0%, respectively. Flours from Beijing 0045 presented significantly higher damaged starch content than flours from Zhongmai 175 at the same break and reduction millstream (Table 1). This was consistent with the previous findings that starch granules of hard wheat are generally subjected to more severe damage than those of soft wheat
Conclusions
Starches from different flour millstreams of hard and soft wheat were investigated for comparison of their damaged starch content, granular morphology, relative crystallinity and functional properties such as swelling power, starch solubility, thermal transition, pasting properties and in vitro digestibility. Meanwhile, the effect of two different millings on damaged starch content and properties of starches isolated from different flour millstreams was also investigated. This is the first
Acknowledgements
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (31371720, 3140151) and the Natural Science Foundation of Tianjin (13JCYBJC38100). Shujun Wang also greatly appreciates the financial support of “Haihe Scholar Program” (000050401) and Tianjin “1000 Youth Talents Plan” from Tianjin University of Science & Technology.
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These authors contributed equally.