Grain quality of Australian wild rice (compared to domesticated rice)

Abstract

Wild rice may be an important resource for rice food security. Studies of the grain quality of wild rices may facilitate their use in rice breeding. Australian wild rice populations have been shown to be genetically distinct from those found elsewhere indicating that they may be a potential source of valuable alleles for rice improvement. To date, two taxa belonging to the A genome clade have been described in Australia: wild rice taxa A (Oryza rufipogon like) and wild rice taxa B (Oryza meridionalis like). To explore the grain quality of these wild rices from their natural environment, a collection from eleven sites within 300 kms north of Cairns, Queensland, Australia at the beginning of the dry season in May 2014 and 2015 was evaluated. Analysis of the physical traits of three Australian wild rice taxa revealed that the wild A genome taxa were of a size that could be classified as extra-long paddy rice with grains that were long or medium while Oryza australiensis was categorized as a long paddy rice with short grain. Australian wild rice grains were coloured and varied from light red brown to dark brown while domesticated rice is brighter. Due to the difficulty of obtaining sufficient mature wild rice seeds as well as the colour of the wild rice, further analysis of grain quality was conducted on unpolished wild rice. Starch molecular structures of Australian wild rice were very different from those of domesticated rice. They had a high amylose content with a larger proportion of shorter amylose chains and a lower proportion of amylopectin chains DP ≤ 10 to DP ≤ 24 lead to a higher gelatinization temperatures as well as requiring a longer cooking time. However, the starch molecular structures of Australian wild rice meet the criteria for slowly digestible starch indicating a potentially higher nutritional value. Cooking behaviours of the unpolished rice were related to the different bran layer characteristics and starch molecular structures. Cooked O. australiensis has the lowest hardness even though it had a higher amylose content and required a longer cooking time. Cooked wild rice taxa B also had a lower hardness compared to unpolished commercial rices, suggesting that the histological structures of the wild rice grain caryopsis may differ from that of domesticated rice. However, the amylose content still played an essential role contributing to stickiness of the cooked unpolished rice. Pasting properties of these unpolished rice flours suggested less retrograded starch in pastes of Australian wild rice especially in O. australiensis. Among the wild rice A genome taxa, wild rice taxa A had a lower pasting viscosity than wild rice taxa B early in gelatinization. However, the final viscosity of the former was slightly higher. Furthermore, the pasting behaviour of wild rice taxa B was more similar to that of Oryza sativa L.cv. Nipponbare (japonica rice). The eating characteristics of wild rice taxa B compared to commercial wild rice (Zizania aquatic L), Oryza sativa L.cv. Nipponbare, and seven unpolished commercial rices demonstrated that wild rice taxa B, with a red bran colouring, had good cooking behaviours and did not exhibit any ‘undesirable’ attributes such as earthy/root vegetable aroma, grassy aroma, bitterness or metallic. The aroma, flavour, and texture attributes of this wild taxa were similar to that of brown bran rice. The grain quality of coloured Australian wild rice has a high potential for commercialization and would provide a new source of genes for rice breeding and diversification of rice production and quality because of their very different starch molecular structures, cooking behaviours and eating characteristics compared to domesticated rice.