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Isolation of Highland Barley Starch
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Highland barley (HB) is a nutritious crop with excellent health benefits, and shows promise as an economically important crop with diverse applications. Starch is the main component of HB and has great application potential owing to its unique structural and functional properties. Physical techniques, enzymatic hydrolysis, and other procedures can be used to decompose the cell wall of HBS, which consists of cellulose, hemicellulose, and pectin. Following the release of the cellular contents, proteins can be separated using appropriate procedures to yield pure HBS.

highland barley starch extraction structure

1.Introduction

Highland barley (HB; Hordeum vulgare L. var. nudum Hook. f.) is a variety of barley that belongs to the grass family and is an annual herb, which can be classified into two-, four-, and six-row HB based on the number of ridges, as well as white, black, and purple HB based on color. The characteristics of HB include a high degree of cold tolerance, a short growing period, wide adaptability, early maturity, and high yield, and it is suitable for cultivation in the cool climate of the plateau [1]. In 2019, Tibet and Qinghai, the major HB planting areas, produced 792,900 and 144,100 tons of HB, respectively, accounting for over 80% of total HB production.
In comparison with other cereal crops, HB had better nutritional value as it contains higher protein, vitamin, and fiber contents, especially β-glucan (Table 1). β-glucan serves a variety of physiological roles, such as reducing blood glucose and fat levels, lowering cholesterol, preventing colon cancer, and boosting immunity [2]. Owing to its rich nutritional value and unique taste and flavor, HB has been processed and formulated into many different types of food products, including noodles [3], bread [4], biscuits [5], cakes [6], vinegar [7], and wine [8].
Table 1. Composition of highland barley grain.
Table
Nutrients Summary of Research Results References
Starch HB had lower levels of starch (58.1–72.2%) than wheat (70–75%), corn (65–74%), and rice (~80%) [9][10][11][12]
Protein The protein content of HB was 8.20–20.80%, similar to wheat (8–20%), and higher than rice (6–7%) and corn (6–12%) [13][14][15][16]
Lipid The crude lipid content in HB was about 2.01–3.09%, which was higher than rice, but lower than corn, sorghum and oat [16]
Fiber HB contained 12.8–17.2% fibers, higher than most cereals, especially β-glucan [17][18]
Mineral The mineral content of HB was 1.46–2.20%, similar to normal staple foods, such as rice, wheat and corn [19]
Vitamins HB had about 39.0–379.7 mg/kg vitamin E, and 30.4–1327.4 mg/kg vitamin B, which was higher than the average of maize (3.9–36.3 mg/kg) and wheat (0.16–13.55 mg/kg) [20][21][22]
The primary component of HB is starch, which accounts for 58.1–72.2% of the dry weight and generally comprises 74–78% amylopectin, and up to 100% in a few varieties [9]. The structures and properties of starch have a significant impact on the quality of HB products. For example, a higher content of amylopectin enables HB flour to have better freeze-thaw stability and can be added to other flours to enhance the quality of noodle products. These quality characteristics are key factors affecting the processing of HB noodle products. Therefore, a better understanding of the structure and functionality of HBS may help expand the application of this starch in food and other industries.

2. Isolation

Physical techniques, enzymatic hydrolysis, and other procedures can be used to decompose the cell wall of HBS, which consists of cellulose, hemicellulose, and pectin. Following the release of the cellular contents, proteins can be separated using appropriate procedures to yield pure HBS. The current methods for extracting HBS are the dry, wet, and wet-dry combination approaches.

2.1. Dry Extraction

Dry extraction is a useful process for enriching certain nutrients (starch, protein, lipids, and β-glucan) in barley. Pearling [23][24], roller milling [25][26], milling followed by air classification [27][28], and milling followed by sieving [29][30] are some of the dry fractionation procedures reported by researchers. Liu et al. [30] stated that pearling has a considerable impact on the efficiency of subsequent milling procedures. The milling process and the barley genotype had a substantial influence on the effectiveness of sieving for nutrient enrichment and recovery rates. Pearling alone was the optimum strategy for enriching protein, whereas, for β-glucan and starch enrichment, a combination of pearling and milling followed by sieving was the optimal choice. Compared to wet extraction, dry fractionation uses less energy and water and preserves the natural structure and function of the components. However, these methods are currently not capable of producing high-purity isolates (>90%) [31]. It is noteworthy that during the milling process, the starch granules are inflicted to various forces, which cause them to break into smaller particles, called milling damaged starch (MDS) [32]. Due to its unique structure, MDS has a considerable impact on the quality of the final starchy products. For example, MDS has higher water absorption capacity and enzymatic hydrolysis rate, and is easily fermented by yeasts. During bread making, the suitable amount of starch can improve the quality of the dough, while excessive starch can lead to sticky dough [33][34][35].

2.2. Wet Extraction

Wet extraction of HBS frequently requires the use of alkaline and enzymatic techniques. Soaking with lye can degrade or loosen the protein around the HBS, weaken the HBS-protein combination, and dissolve the protein to obtain high-purity starch. Yang et al. [36] extracted HBS by soaking the grains in NaOH solution using a 1:8 ratio at 30 °C for 8 h. Although the alkaline extraction is straightforward, multiple washes, centrifugation, acid neutralization following alkaline washing, wastewater treatment, and desalination are components of the alkaline extraction technique and are time-consuming. HBS extracted by the alkali method is characterized by its low gelatinization temperature, poor thermal stability, low degree of retrogradation, and a relatively smooth surface [37].
The enzyme cellulase is commonly used for starch extraction, and it acts on the cell wall of HB to rupture and disintegrate it, enabling the complete release of cellular contents, which is beneficial for the separation of starch and protein [38]. Extraction of highland barley starch using alkaline protease and neutral protease has also been reported. Zhao et al. [39] used response surface methodology to optimize the neutral protease extraction process of highland barley starch, and the optimal extraction conditions included an enzyme concentration of 140.79 U/g, an extraction temperature of 45.01 °C, and a hydrolysis time of 2.57 h. The enzymatic method can extract starch directly from seeds and retain most of the grain’s natural properties with a gentle separation process and a relatively smooth surface of the isolated starch granules. HBS extracted by enzymatic method has relatively rough surface with high gel hardness and gelatinization temperature [37].
The presence of β-glucan is a concern, as it absorbs a considerable quantity of water during washing and increases the slurry viscosity, making it difficult to separate in subsequent steps. Technology has been developed for separating high-purity starch from HB grains with different amylose contents. It is possible to separate starch and fiber fractions from whole barley flour in a semi-aqueous medium (50% ethanol) without affecting the viscosity of β-glucan. The majority of the starch isolates thus obtained had high purity large particles, with yields ranging from 22–39%. More significantly, the extraction efficiency of the β-glucan component was 77–90%, indicating that separation from the starch component during processing was efficient [40].

2.3. Combined Wet and Dry Extraction

Barley was subjected to milling and air classification operations to identify the graded fractions with high starch content based on particle size and composition, and the fractions were further separated based on the wet technique. The combination of wet and dry approaches can minimize water consumption and centrifugal load while increasing starch yield. During the milling and air classification procedures, most of the β-glucan is transferred to other components, which is advantageous for further separation and purification of starch.

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Subjects: Food Science & Technology
Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Jingjing Xie
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Yan Hong
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Update Date: 08 Feb 2023
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