Mineral availability is modified by tannin and phytate content in sorghum flaked breakfast cereals

Highlights

• Tannin and phytate content were affected by sorghum genotype.

• In vitro mineral availability was low in whole grain flour.

• Tannin and phytate content significantly reduced in breakfast cereals.

• In vitro mineral availability significantly improved in breakfast cereals.

• Flaking was effective in improving the nutritional properties of sorghum grain.

Abstract

Mineral availability from sorghum grain in some varieties may be low because of the presence of phytates and tannins. However, heat processing of sorghum may lower tannin and phytate levels and thus improve mineral availability. Sorghum has potential in the manufacture of gluten free breakfast cereals, therefore flaked breakfast cereals were manufactured from whole grains of three contrasting sorghum varieties and compared to that manufactured from whole grain wheat. The content of tannin, phytate, minerals (Ca, Fe and Zn), in vitro mineral availability and phytate: mineral molar ratios were determined in the raw whole grain flours and the cooked flaked breakfast cereal. For all grain varieties, the in vitro mineral availability of flaked breakfast cereal was higher than the raw flours, an effect most probably related to the concomitant reduction in tannins and phytate levels after processing. The in vitro mineral availability of the flaked wheat breakfast cereal was significantly higher than that of all sorghum breakfast cereals, with that manufactured from the brown sorghum IS8237C having the lowest value (p ≤ 0.05). Given that the sorghum varieties in this study gave lower mineral availability than wheat, other sorghum varieties now require evaluation to identify those with improved mineral availability.

Introduction

Breakfast is an important meal of the day for humans, since it provides energy and nutrients, such as dietary fiber, minerals and vitamins (Cauvain, 2011) and the risk of obesity and type 2 diabetes has been reported as inversely associated with breakfast meal intake and its quality (Pereira et al., 2011). In Australia for example, people eat breakfast on five or more days per week, and 50% of the time they select breakfast cereals to consume (Williams, 2002). Breakfast cereals are able to provide higher dietary fiber and calcium, and consumption of breakfast cereals are also associated with lower body weight (Louie, Dunford, Walker, & Gill, 2012). Breakfast cereals are commonly prepared from wheat, with very limited use of sorghum in developed countries (Akhter et al., 2012, Louie et al., 2012). However, compared to wheat, sorghum is a naturally gluten-free cereal which may suit to a wider range of consumers (Kaboodvandpour, 2015).

Sorghum (Sorghum bicolor (L.) Moench),belonging to the grass family, is the fifth leading cereal crop in the world after rice, wheat, barley and maize and widely cultivated in the world. In Australia, it is the third most important cereal crop after wheat and barley, with about 2.2 million tonnes production in 2013 (FAO, 2013). Sorghum is widely planted in sub-Saharan Africa, where the environment is too dry for other cereals, and in this region around 35% of it is utilized for human food (Dicko, Gruppen, Traoré, Voragen, & Van Berkel, 2006). However, the majority of sorghum grain is used for poultry feed and biofuel in Australia (Mahasukhonthachat, Sopade, & Gidley, 2010).

Sorghum grain contains various levels of tannins, which depend on sorghum genotype (Wu et al., 2016). Tannins have potential health beneficial effects, through antioxidant and anticarcinogenic activities, however they may also have anti-nutritional properties (Kumar, Sinha, Makkar, & Becker, 2010). Phytates are also found at varying levels in different sorghum grain varieties (Wu et al., 2016). Both tannins and phytate can strongly chelate minerals, such as calcium, zinc, and iron, to form insoluble complexes, which could reduce the bioavailability of these minerals (Kruger et al., 2013). In addition, phytate: mineral molar ratios have been considered as an indicator of the mineral availability (Lazarte, Carlsson, Almgren, Sandberg, & Granfeldt, 2015). The maximum desired molar ratios of phyate:calcium (Phy:Ca), phyate:iron (Phy:Fe) and phyate:zinc (Phy:Zn) are 0.17, 1 and 15, respectively, and the mineral availabilities are severely reduced when the molar ratios are higher than these values (Hurrell, 2004, Umeta et al., 2005, WHO, 1996). Low mineral availability in staple foods has potential to lead to malnutrition, and to physiological-pathological conditions, like osteoporosis, impairment of physical growth and iron deficiency anaemia (Akhter et al., 2012, Gibson, 2006). However, the negative influences of tannins and phytate on the absorption of minerals might be decreased by reducing their levels through processing (Kruger, Taylor, & Oelofse, 2012).

In addition, sorghum also contains resistant starch that can reduce postprandial glycaemia and insulinaemia which is of great importance for diabetes patients (Dicko et al., 2006). Recently, several foods have been developed with added sorghum flour, such as bread, pasta and extruded snack foods (Khan et al., 2013, Licata et al., 2015, Yousif et al., 2012). A postprandial study on whole grain flaked breakfast cereals has reported that participants had greater subjective satiety after consuming the product manufactured from red sorghum whole grain compared to the ones from whole grain wheat, white sorghum or brown sorghum (Stefoska-Needham, Beck, Johnson, Chu, & Tapsell, 2016). The satiety effect of the red sorghum flaked breakfast cereal may be mechanistically linked to significant differences in the participants' appetite hormone levels after consuming red sorghum cereal compared to the other products. Currently no research however has been reported on the mineral availability of sorghum breakfast cereals.

The purpose of the present study was to investigate the tannin, phytate, mineral levels (Ca, Fe and Zn), phyate:mineral molar ratios and in vitro mineral availability, of whole grain sorghum flour and processed flaked sorghum breakfast cereals. This research will provide valuable information to the food industry and consumers on the nutritional value of sorghum products, as affected by food processing.