Evaluation of antioxidant capacity and vitamin E content in barley grains (Hordeum vulgare L.) and the impact of processing and storage

Abstract

Antioxidants, including vitamin E, may have a positive effect on human health but may be affected during storage, malting and/or other processing such as making pita bread. Those components are reported to be higher in barley compared to wheat and other grains. Therefore, as well as being a source of fibre, barley has potential for use in foods. However, there is limited knowledge about how vitamin E and antioxidant capacity might change in the barley food chain; and; whether there is genetic variability for these and what the genetic basis of differences between barley genotypes might be. Therefore, this research aimed to address the following questions: Do different barley genotypes have different antioxidant capacity, vitamin E content and isomers? What is the genetic basis of any differences in vitamin E content and antioxidant capacity observed? How are vitamin E and antioxidant capacity affected by common industrial practices for storage and processing (pearling, malting and baking) for barley genotypes high in those components? Vitamin E content and antioxidant capacity were measured in 25 barley genotypes using high performance liquid chromatography (HPLC) and ability to scavenge DPPH radicals, respectively. Vitamin E comprises eight isomers: α-, β-, γ-, δ-tocopherol (T) and α-, β-, γ-, δ-tocotrienol (T3). As expected, α-tocotrienol (α-T3) and α-tocopherol (α-T) were the predominant tocol isomers. Vitamin E content and antioxidant capacity varied significantly among those genotypes. Vitamin E ranged from 8.5 to 30.8 μg/g dry weight (DW) while ascorbic acid equivalent antioxidant capacity (AEAC) varied from 57.2 to 158.1 mg AEAC/100 g fresh weight (FW). Generally, lower vitamin E content or antioxidant capacity was observed in all v coloured (Jet, Sumire mochi, ICARDA 16, ICARDA 19, ICARDA 26, ICARDA 35 and ICARDA 39) or hulless genotypes (Jet, Sumire mochi and Macumba) except for the hulless variety Finniss. Results suggest some genotypes are potential candidates for breeding of barley cultivars with high vitamin E content or antioxidant capacity. To determine the genetic basis of differences in vitamin E content and antioxidant capacity, the measurement of these compounds was conducted across two years for the Amaji nijo x WI2585 and Tadmor x ERApm mapping populations respectively. Quantitative trait loci (QTLs) were detected for vitamin E, two major isomers and two minor isomers. QTLs were identified on chromosome 7H for vitamin E (LOD=3.4 and 4.2), 7H for α-T (LOD=3.32), 5H and 7H for α-T3 (LOD=3.74 and 3.90 respectively), 2H for β-T (LOD=3.27), 4H and 5H for β-T3 (LOD=3.76 and 3.50 respectively) and 2H for γ-T (LOD=3.10). Some QTLs overlapped each other and associated markers have also been linked to other traits such as frost, salt and black point tolerance, all of which have all been correlated to antioxidant capacity. However, no QTL was found for antioxidant capacity in the Tadmor x ERApm mapping population. Four months storage at 10°C decreased antioxidant capacity in all genotypes except the coloured genotypes (ICARDA 19, ICARDA 26, ICARDA 35, ICARDA 39, Sumire mochi and Tadmor) but vitamin E content was increased in all genotypes. Vitamin E content was significantly lower in the steeping, germination and kilning stages of malting for all genotypes compared to the unprocessed samples. However, the antioxidant capacity in the malt was higher than in the unprocessed samples for the majority of the genotypes. A strong correlation (r=0.9, n=14, p<0.05) between antioxidant capacity before and after malting indicated that barley varieties which have higher antioxidant capacity at harvest retain their antioxidants after malting. The same trend was observed in pita bread made with a substitution of wheat flour with barley flour made from whole grain, pearled grain (at 10%, 15% or 20% pearling) or malt. Higher antioxidant capacity and vitamin E content were observed in barley flour, even when made from pearled grain, and consequently they remained higher in barley pita after baking. Pitas made with barley flour from malt, Finniss whole grain or 15% pearled WI2585 were acceptable to consumers and retained similar physical and sensory properties to those in the control pita indicating the potential use as functional food. In summary, this study has shown that (i) certain barley genotypes have potential for use in pita bread due to their high antioxidant capacity and/or vitamin E content at harvest and the maintenance or increase of that during storage and processing; and (ii) the identification of QTL and genetic variability in vitamin E will allow barley breeding programs to develop varieties with greater vitamin E content. Further research could identify candidate genes responsible for vitamin E and antioxidant capacity in barley grains and should also ensure the bioavailability of vitamin E and antioxidant capacity to confirm its functionality in the human diet.