Abstract
In this study, heat and relative humidity (HRH) treatment was applied in highland barley for γ-aminobutyric acid (GABA) accumulation. Tibetan highland barley cultivars (25) were selected for comparison and analysis. HRH treatment could accumulate GABA in several hours with low moisture content and high temperature, and the grains were treated for 2.5 h at 65 °C in this study. The GABA content of processed grains under HRH optimal condition ranged from 26.91 to 76.28 mg·100 g−1, which was significantly higher than the initial content (12.78–43.00 mg·100 g−1). The highest GABA accumulation capacity was observed in two-row yellow cultivars (YT1), increasing from 36.52 to 76.28 mg·100 g−1. Correlation analysis showed that the accumulation of GABA after HRH treatment was positively and significantly (p < 0.05) correlated with the contents of protein (0.52), total free amino acids (0.68), threonine (0.53), serine (0.51), glutamate (0.69), glycine (0.49), alanine (0.46), cysteine (0.57), tyrosine (0.50), lysine (0.53), proline (0.40), and glutamate decarboxylase (GAD) activity (0.62), which were closely related to GABA-shunt pathway. The polyamines contents, diamine oxidase (DAO) and polyamine oxidase (PAO) activities, as the substrates and critical enzymes of polyamine degradation pathway, showed no significant correlation with GABA accumulation. The results suggested that the main pathway of GABA accumulation in highland barley under HRH treatment was GABA-shunt pathway.
Highlights
Highland barley (Hordeum vulgare L.) with naked caryopsis is a type of barley mainly distributed in Tibetan Plateau at an elevation of 1400–4700 m [1], and is a traditional staple food, occupying over 70% of the crop lands in Tibet [2]
The substrate and enzymes of polyamine degradation pathway showed no significant correlation with GABA accumulation
The results suggested that the main pathway of GABA accumulation under the heat and relative humidity (HRH) treatment might be GABA-shunt pathway
Summary
Highland barley (Hordeum vulgare L.) with naked caryopsis is a type of barley mainly distributed in Tibetan Plateau at an elevation of 1400–4700 m [1], and is a traditional staple food, occupying over 70% of the crop lands in Tibet [2]. Due to the richness of various types of nutrients and bioactive compounds, like dietary fiber, protein, β-glucan, and phenolic compounds, highland barley has attracted extensive attention on hyperlipidemia, diabetes, regulation of blood pressure, and anti-atherosclerosis effects [3–5]. The GABA content of Tibetan Plateau highland barley was 18–28 mg·100 g−1 [10], which was higher than rice germ (10.75–18.62 mg·100 g−1) [11], fava bean (0.30–20.60 mg·100 g−1) [12], soybean (2.99 mg·100 g−1) [13], and wheat (0.12 mg·100 g−1) [14]. A higher GABA accumulation of highland barley may be induced by extreme climatic conditions, such as intense UV radiation, drought, and hypoxia [15]. GABA is primarily synthesized via the decarboxylation of L-glutamate, catalyzed by glutamate decarboxylase (GAD) [16]. Another contributor of GABA synthesis is the degradation of putrescine (Put), spermine (Spm), and spermidine (Spd), catalyzed by diamine oxidase (DAO) and polyamine (PAO) [17]
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