Investigation of the phenolic and antioxidant content in Australian grains using traditional and non-invasive analytical techniques

Recent years have seen the emergence of the concept of “functional foods”– where the value of food products is based on their health-benefiting properties in addition to their basic nutritional value. Globally, the functional food market is worth US $170 billion and is projected to grow at 7.5% p.a. over the next 10 years. In order to capitalise on this lucrative emerging market, producers and wholesalers need to demonstrate that their products contain high levels of these desirable compounds. This is typically assessed through time-consuming, expensive analytical techniques such as high-performance liquid chromatography (HPLC) or liquid chromatography-mass spectrometry (LC-MS). While these methods provide a high level of specificity and sensitivity, more rapid analytical techniques may be better suited to the routine, near-real-time analysis of large numbers of samples. Furthermore, there is currently a lack of basic context data on the typical levels of bioactive compounds that are found in many crops grown under Australian conditions, particularly for grain crops. This lack of context data makes it challenging to know whether a particular product would be considered high or low quality from a functional food perspective. Consequently, the first major aim of this project was to profile the typical levels of bioactive compounds present in economically significant grain crops grown in Australia – specifically faba bean, wheat, mungbean and chickpea. The major focus was on phenolic compounds, as these possess high levels of antioxidant activity and are found in relatively high levels in grain crops. Furthermore, this class of compounds is associated with a wide range of health benefits, particularly for the prevention of cardiovascular disease. Using spectrophotometric methods and HPLC analysis, moderate differences were found in the phenolic contents and antioxidant capacity of different varieties from each crop. This was particularly noted for the ten varieties of faba bean analysed, where there was a 121% difference in total phenolic content (TPC) between the varieties with the lowest and highest contents. This pulse also contained the highest total phenolic contents (258-571 mg GAE/100 g) and ferric reducing antioxidant potential (237-531 mg TE/100 g) of all crops investigated. The five mungbean varieties showed lower levels and more minor differences in phenolic content (79-105 mg GAE/100 g; 32% variation between varieties) and cupric reducing antioxidant capacity (498-584 mg TE/100 g; 17% variation), while the while the ferric reducing antioxidant potential did not differ significantly between varieties (14-20 mg TE/100 g). However, the content of numerous phenolic compounds (p-hydroxybenzoic acid, vanillic acid, caffeic acid, sinapic acid, trans-ferulic acid, cinnamic acid and vitexin) were significantly different between the mungbean varieties investigated. Similar observations were made for the chickpea samples, where there were moderate differences in total phenolic content (73-94 mg GAE/100 g; 29% variation) and ferric reducing antioxidant potential (25-40 mg TE/100 g; 62% variation) between varieties. Again, the content of most phenolic acids analysed by HPLC were significantly different between varieties. Although varietal differences were not examined for wheat, the TPC of the 65 samples was higher than mungbean and chickpea (130-180 mg GAE/100 g), while the ferric reducing antioxidant potential ranged from 14-64 mg TE/100 g. In addition to the varietal differences, the impact of growing location and season on phenolic content and antioxidant capacity were investigated in faba bean. Although these variables had no effect on the total phenolic content, the growing location did alter the levels of several individual phenolic compounds (protocatechuic, vanillic and chlorogenic acids, as well as the flavonoids vitexin and rutin). The second major aim of this project was to investigate the prospect of infrared spectroscopy as a rapid technique for the prediction of phenolic content and antioxidant capacity in Australian grain crops. Promising results were found for the estimation of total phenolic content and antioxidant capacity in faba bean and wheat flour, particularly using near-infrared spectroscopy. The NIR model for TPC showed an R2test of 0.66 and RMSEP of 76 mg GAE/100 g when applied to faba bean, and an R2test of 0.86 and RMSEP of 4 mg GAE/100 g in wheat. However, infrared spectroscopy was unable to predict the concentrations of these analytes in mungbean or chickpea flour. This may be due to additional matrix constituents obscuring the analyte signals in the infrared region, or a consequence of the lower phenolic/antioxidant contents in these crops. Nevertheless, the overall results suggest that infrared spectroscopy could be used for the estimation of total phenolic content or antioxidant capacity (i.e., prediction of high or low contents) in certain grain crops. This technique could potentially be applied for the routine screening of bioactive constituents, helping Australian producers to capitalise on the growing domestic and international functional food markets. Monitoring bioactive compound levels in Australian grain – either through traditional or non-invasive analytical techniques – could provide an additional level of quality assurance for producers of functional food crops and help maintain Australia’s global recognition as a producer of high-quality food.