Extraction of nanostructured starch from purified granules of waxy and non-waxy barley cultivars

Highlights

• Waxy and non-waxy barley flours were selected for the extraction of starch nanoparticles.

• Organic and alkali treatments were adopted to purify the barley waxy cultivar.

• Nanoparticles with reduced mean diameter and thermal stability originated from waxy starch.

• Amorphous nanoparticles derived from waxy and non-waxy barley starches were extracted.

Abstract

Waxy and non-waxy barley flours were selected as native material for the extraction, by acidic hydrolysis, of starch nanoparticles (NPs), with the main aim of evaluating how their different amylose/amylopectin ratio affected NPs structural characteristics. To this scope, two approaches based, respectively, on the use of NaCl/toluene (called organic treatment) and of sodium hydroxide solutions (called alkali treatment) were adopted in order to purify the waxy flours to give starch granules. Morphological investigations of waxy flour after alkali and organic treatments, before and after acetone washing, showed that the isolation methods of the starch did not affect the general morphologic characteristics of starch granules. The method found to provide the best outcomes in terms of thermal and morphological properties was also applied to purify the non-waxy barley. The results confirmed that waxy starch characterized by granules of smaller dimensions (WS) gave amorphous nanoparticles (WNP) with reduced mean diameter and thermal stability with respect to NPs obtained from non-waxy barley flour.

Introduction

Barley ranks at the fourth position of cereal crops produced and cultivated worldwide, after wheat, maize and rice, with an harvested area of 47 Mha for a total production of 141 Mt (FAO, 2018). The grains, hulled in most cultivars, are mainly devoted to animal feed, food and malting, but non-food uses are increasing, especially in the industry of composite materials. De-hulled grains are mainly constituted by carbohydrates, proteins, β-glucans, and fatty acids, and their contents are about 60–70%, 9–20%, 3–8%, and 1–2%, respectively (Kong et al., 2016).

The most abundant carbohydrate present in the de-hulled barley grain is starch, which consists of over 50% of total dry weight (Asare et al., 2011), composed by amylopectin and amylose. The molecular structure of amylose is simpler than amylopectin, showing a linear structure with few α-1,6-branches (Bertoft, 2017). Differently, amylopectin, which, in general, is the major component of starch, has considerable shorter chains and a high number of α-1,6-branches, which can constitute about the 5% of the molecule, resulting in a very complex structure (Bertoft, 2017). Structural analyses showed as barley starch is constituted by granules of different shape and form, which can be large (15–19 μm) and disc-shaped (A-granules) or small (3.1–3.7 μm) and spheroidal (B-granules) (Mäkelä et al., 1982). A- and B-granules are distinguished on the basis of the organization of the external chains of the amylopectin molecules.

The ratio between amylopectin and amylose, the shape, size of starch granules, fine structure, arrangement and proportion of crystalline and amorphous fractions, are dependent on the genotype and, furtherly, they can be also conditioned by the environmental conditions during grain filling (Beckles and Thitisaksakul, 2014; Zhu, 2017). Obviously, all these factors may play a significant function for the determination of final starch structure.

Starch in normal genotypes consists of around 25–30% amylose and 70–75% amylopectin (Källman et al., 2015). Differently, waxy genotypes, so called because of the waxy appearance of the endosperm, contain no or very little amylose (Gérard et al., 2001; Shi et al., 1998; Yoshimoto et al., 2000). Amylose molecules with large branches are associated with the amorphous region of granules, while amylopectin molecules with short branches involve the crystalline region; for that reason, an elevated content of amylopectin in starch granules induces an increase of crystallinity (Naguleswaran et al., 2013; Cheetham and Tao, 1998).

In order to obtain substrates of interest to be used in nanoparticles-based technologies, starch is to be purified to give granules, which, in turn undergo to appropriate chemical treatments which allow to obtain nanoparticles (NP). In general, many different approaches can be applied to starch in order to obtain granules, while their conversion in NPs is generally carried out by acidic hydrolysis. However, it is to be made clear that differences in starch granule size and structure may affect the outcome of the process in terms of starch purification, characteristics and nanocrystal yield (Sanchez de la Concha et al., 2018).

The aim of the present research was to obtain nanoparticles (NP) from waxy barley flours and then to compare them which those extracted from a non-waxy barley flours. In fact, no information is available in the literature on the characteristics of NPs of waxy barley flours, also in relation to those shown by non-waxy barley flours. To this scope, two approaches were adopted in order to purify the waxy flours to give starch granules (an alkaline- and an organic-based method). Then, the method found to give the best results was also applied to purify the non-waxy barley. Acidic hydrolysis was carried on the purified granules in order to remove their amorphous regions and to give nanoparticles derived from waxy- and non-waxy- flour, that were deeply characterized.

We aimed to demonstrate that nanometric size materials can be obtained from different types of barley starches, purified applying different and relatively simple and green methods, opening new promising horizons in the field of food–related applications, where the starch nanoparticles have a prominent role in nanoencapsulation and controllable release of active compounds, in addition to the use as a filler in coating applications and biodegradable polymers. The use of amorphous starch nanoparticles is indeed convenient and their chemical modification can be considered to fulfil essential demands, when a controlled release is required, such as carrier and slow-release agent in pharmacy and cosmetics, other than delivery of pesticides, fertilizers and chemicals in agriculture.