Evaluation of the Chemical and Functional Stability of Intermediate Wheatgrass (Thinopyrum intermedium) over Storage and in Response to Steam Treatment

Abstract

Annual crop farming is degrading the environment at a faster rate than it can be restored, causing soil erosion and water run-off with subsequent loss of nutrients and biodiversity. Yet, the world population currently relies on annual cereal grains to supply 40% of their calorie needs and 60% of their protein needs. The high dietary demand for cereal grains, together with the high rate of soil degradation from annual farming and growing consumer interest in sustainably-sourced food creates space in the market for a more sustainable grain. Perennial intermediate wheatgrass (IWG), Thinopryum intermedium, is a promising grain to fulfill this role, owing to its good flavor, breeding potential, and superior environmental benefits due to its extensive root system and long growing season. Understanding the storage stability of IWG and identifying ways to improve its stability will not only help incentivize farmers to plant IWG, but will also help make IWG competitive against existing grains on the market. The objectives of this study were: (1) evaluate the effect of steam treatment on antioxidant content and activity, enzyme activity, and progression of hydrolytic and oxidative rancidity in IWG compared to hard red wheat (HRW) over storage at different temperatures; (2) evaluate the effect of steam treatment of IWG grains on the functionality of its whole flour over storage at different temperatures; (3) determine the overall safety of the grains by assessing presence of chemical residues, such as pesticides and mycotoxins, heavy metals, allergens, and anti-nutrient factors. Pre-storage, compositional analysis of IWG and HRW was carried out following official AOAC and AACCI methods. Steam treatment was carried out by subjecting 30 g aliquots of IWG and HRW groats to 100°C and 95% relative humidity conditions in a proofing oven for 60 minutes. Steamed and non-steamed samples were stored at 45°C, ambient (22 ± 2°C), and 4°C at 0.43 water activity for 6 weeks, 6 months, and 12 months respectively. Samples were analyzed periodically for lipoxygenase and lipase activity, hydroxycinnamic acid content, carotenoid content, antioxidant activity, and indicators of hydrolytic and oxidative rancidity, including free fatty acids and hydroperoxides, respectively. Lipoxygenase activity pre- and post-steam treatment was analyzed using the ferrous oxidation-xylenol orange (FOX) assay, and lipase activity was determined spectrophotometrically using a copper soap assay. Hydroxycinnamic acids and carotenoids were quantified using high performance liquid chromatography. Antioxidant activity pre- and post-steam treatment was analyzed using 1,1-Diphenyl-2-picryl-hydrazyl (DPPH) radical scavenging and leucomethylene blue (LMB) assays. Free fatty acids and hydroperoxides were quantified according to AOAC titration methods 940.28 and 965.33, respectively. Functionality parameters were also measured at the beginning, middle, and end of storage at each storage temperature. Rheological and mixing properties were assessed using a Farinograph® and a texture analyzer equipped with a Kieffer rig. Starch pasting properties were monitored using a MicroVisco-Amylograph®. Bread baking tests were performed according to AACCI 10-10.03 method. IWG had significantly higher protein, insoluble fiber, and fat content than HRW, along with a higher lipase activity. HRW had a relatively higher lipoxygenase activity than IWG. IWG also had significantly higher hydroxycinnamic acid and carotenoids concentrations than HRW, along with higher antioxidant activity. The steam treatment employed in the present study did not inactivate lipoxygenase and lipase. Throughout storage, lipase activity significantly decreased in non-steamed grains over ambient (22 ± 2°C) storage but otherwise was retained over accelerated (45°C) and refrigerated (4°C) storage. However, an after-ripening effect was evident in lipoxygenase activity at ambient and 45°C temperatures, with significant increases in activity over storage. Hydroxycinnamic acid content was retained throughout storage, with prolonged storage inducing increases to its content, as well as increases to antioxidant activity at higher storage temperatures. Carotenoid content decreased throughout storage in IWG and HRW, to a greater degree at higher storage temperatures, as was expected due to its high susceptibility to oxidation. Development of oxidative and hydrolytic rancidity in IWG was minimal throughout storage of groats. Although the steam treatment was not able to effective in inactivating enzymes, it demonstrated a minimal, but positive effect on antioxidant activity and content, as well as a slight inhibitory effect on hydroperoxide formation over storage. IWG demonstrated a significantly higher antioxidant content and antioxidant activity and lower hydroperoxide content than HRW throughout storage. In terms of hydrolytic rancidity, IWG showed higher lipase activity and free fatty acid concentration than HRW throughout storage. IWG showed increases to dough development time, resistance to extension, and loaf volumes over storage, denoting an increase in dough strength, although no significant increases to dough stability time were evident. IWG also had improvements to starch pasting viscosities over storage, including peak viscosity and hold viscosity, at all temperatures. The effects of steaming on functionality were temperature-dependent, with positive effects on dough development time and resistance to extension during storage at 45°C and improvements to starch pasting viscosities during storage at ambient temperature. Steaming appeared to have a negative impact on functionality at refrigerated storage temperatures. Steaming did not have an effect on bread loaf volumes of IWG over storage. Although overall functionality of IWG remained inferior to HRW throughout storage, HRW had significant decreases in several functionality parameters over storage. While steaming had a positive effect on some of these parameters (e.g. extensibility, loaf volume), it had a negative impact on others (e.g. starch pasting viscosities). Bread firmness was not significantly affected over storage in either grain. The present study was the first to evaluate the storage stability of IWG and investigate steam treatment as a mode of improving storage stability of IWG. The results of the study provide additional incentives to farmers and food manufacturers alike by highlighting IWG’s competitive storage stability. The present study demonstrated not only its high tolerance to storage but in some regards, its positive response to storage. Furthermore, this research formed the foundation for establishing a more effective method of steam treatment in a currently ongoing storage study on IWG flour.