Impact of flour particle size on nutrient and phenolic acid composition of commercial wheat varieties

Highlights

• Particle size directly influence wheat flour proximate composition and phenolic acids content.

• Positive correlation between particle size and phenolic acids was observed.

• Negative correlation between particle size and carbohydrate and energy content.

• Uneven distribution of bran, germ and endosperm in different particle size fractions.

Abstract

In the present study, the effect of particle size (PS) on phenolic acid content and proximate composition (moisture, ash, protein, total fat, crude fiber and total carbohydrates) of whole-wheat flour (WWF) was determined. Five different particle size fractions <1.18 to>0.43, <0.43 to>0.30, <0.30 to>0.15, < 0.15 to>0.08 mm, and < 0.08 mm (ASTM recommended), of three commercial wheat varieties (Benazir, TJ-83, Imdad) were used for the present study. Total phenolic acid was determined using HPLC-DAD detections after base hydrolysis of the samples. Proximate composition of each particle size fraction of all three wheat flours were determined by recommended AOAC methods. In all wheat varieties, maximum quantities of total phenolic acids (162.12–554.16 mg/100 g) were obtained from <1.18 to>0.43 mm PS fractions. The maximum moisture content (8.12–8.98 g/100 g) was observed in the two lower PS (<0.15 mm) fractions while ash and protein were highest in the largest PS (<1.18 to>0.43) i.e. 2.10–2.77 mg/100 g and 10.9–11.8 mg/100 g respectively. The total fat (0.24-0.53 mg/100 g) and crude fiber (0.40-0.59 mg/100 g) were highest in <0.43 to>0.30 PS fraction of all wheat varieties. The average carbohydrates (g/100 g) and energy (kilojoules) were ranged from 77.4-79.9, 1502.0-–1518.8 for Imdad; 79.7-84.3, 1518.8-–1527.2 for Benazir; and 78.4-81.1, 1506.2-–1518.8 for TJ-83 wheat varieties. Pearson correlation showed negative correlation between particle size and carbohydrate and energy content and positive correlation with phenolic acids content. This study concludes that particle size has direct influence on the distribution of phenolic acids, carbohydrate, protein, crude fiber, ash, crude fat and moisture in the three wheat varieties.

Introduction

Particle size (PS) distribution is the oldest and most widely used technique for the classification of solid particles and is commonly used in industries and laboratories for the characterization of particulate material (Ndegwa et al., 2002). Particle size distribution plays an important role for the determination of physical and chemical properties for many processed food products, chemicals, colorants, paints, and pharmaceuticals (Noguera et al., 2011). Particle size distribution also influences taste, texture, appearance, and nutritional content as well as shelf life. The particle size has been analyzed by various methods such as sieving, digital imaging, light scattering, acoustics, sedimentation, and microscope (Patwa et al., 2014; Niu et al., 2014).

In cereals, whole wheat flour (WWF) particle size is an important factor affecting the qualities of whole wheat food products. Wang et al. in, 2016 investigated the effects of particle size on solvent retention capacity, expandability and cracker baking performance in two US soft wheat cultivars. Barros et al. (2010) compared the physical and rheological properties of refined and whole wheat tortillas. The presence of high quantity of bran may provide quality challenges in flour, i.e. the larger the flour particle size, the harder and less extensible the dough produced, and the more reduced the shelf stability. In addition, there could be reduction of nutrient content (i.e. protein, dietary fiber, vitamins etc.) as the quantity of bran is reduced in different PS fractions (Marquart et al., 2007).

Grinding/milling is critical for developing grain derived food products. While the milling procedures for refined wheat flour are well established, there is no standard method available for WWF (Doblado-Maldonado et al., 2012). Different milling techniques (e.g., stone, roller, hammer, and plate milling) result in production of WWFs with various particle sizes and functionalities (Prabhasankar and Rao, 2001; Patwa et al., 2014). While the typical particle size for endosperm is less than 150 μm, most bran particle sizes are generally larger than 500 μm before further fine grinding process (Rodriguez and Olivares, 2007). The reported range of particle size for wheat flour is in between 90−440 μm (Ting et al., 2016) which may play an important role in increasing or decreasing the phenolic and nutrient content of wheat flour. However, the influence of particle size on nutrients and phytochemicals content in finished food products have not yet been thoroughly reported.

There have been number of reports on the analysis of phenolic acids and other bioactive components in different cultivars of wheat grown and processed under different conditions. Chemically, phenolic acids in wheat can be subdivided into acids derived from either benzoic acid or cinnamic acid derivatives (Kim et al., 2006). Vanillic and salicylic acid, for example, are derivatives of benzoic acid while ferulic acid, the dominant phenolic acid in a wheat grain, and caffeic acid are derivatives of cinnamic acid (Abdel-Aal et al., 2001; Lu et al., 2017). Phenolic acids are predominantly found in the outer bran layer of a wheat grain (Beta et al., 2005; Gallardo et al., 2006; Moore et al., 2007; Lu et al., 2017). Though termed secondary metabolites, phenolic acids play an essential role in protecting plants from UV radiation (Stalikas, 2007), inhibiting pathogens (Abdel-Aal et al., 2001) and providing structural integrity to the cell wall (Klepacka and Fornal, 2006). Most phenolic acids in a wheat grain are insoluble and bound by ester and ether linkages with polysaccharides, such as arabinoxylan and lignin, in the cell wall (Liyana-Pathirana and Shahidi, 2007; Renger and Steinhart, 2000). A smaller portion is present in a soluble form (Stalikas, 2007). The bran layer is highly enriched in phenolic compounds, and phenolic acids are commonly extracted after alkaline hydrolysis (Krygier et al., 1982).

The nutritional composition of wheat flour is comprised of 8–13% protein, 1.0 % soluble protein, 72 % carbohydrates, 1.5 % fat, 2.5 % sugar, 0.5 % minerals and 12–13% moisture (Oberoi et al., 2007). The wheat kernel contains the embryo which is a concentrated source of minerals, protein and vitamins while endosperm is the starch storage portion of the kernel (Bozzini, 1988). Gluten and Gliadin are the main protein fractions, which are water insoluble and form a gluten network on hydration.

The impact of specific particle size on the phenolic acids and the nutritional quality of WWFs is still unknown. Therefore, the three objectives of the present study were: (1) to compare nutritional value as a function of among the different ASTM recommended particle sizes (i.e. > 1.18, <1.18 to >0.43, <0.43 to >0.30, <0.30 to < 0.15, and < 0.08 mm) for three wheat varieties, (2) compare phenolic acid concentration as a function of particle size of the flour, and (3) to determine the most suitable particle size for highest value of phenolic acid and nutrient content.