Effect Of Steam Treatment On Chemical Changes Over Storage Of Intermediate Wheatgrass (Thinopyrum Intermedium) Refined, Partially Refined, And Whole Flour

Abstract

Intermediate wheatgrass (IWG, Thinopyrum intermedium) is a perennial grain that has great environmental benefits such as reduced soil and water erosion due to its extensive root system. IWG has shown promise in its development as a food ingredient with high nutritional value. An understanding of its grain storage stability is essential to its commercial viability and success in food use. Grains are typically processed into flour and stored until use. While whole grains can be stored for eight to twelve years, flour has a significantly shorter shelf life. The storage stability of IWG flour has not yet been addressed. IWG has a relatively high fat content compared to wheat, which poses a potential issue regarding shelf life, as over time rancidity and decreases in antioxidant activity may occur. Thermal treatment, such as steaming, may be beneficial to increase grain shelf life by inactivating enzymes that are involved in lipid rancidity, namely lipase and lipoxygenase. Additionally, reducing the bran content via refinement may also prolong shelf life by decreasing fat content, but this would also reduce dietary fiber and antioxidant content, negatively affecting the praised nutritional value of IWG. An understanding of IWG flour storage stability and ascertaining methods to improve its stability is necessary for IWG’s commercialization and success. The objective of this work was to determine the effect of steam treatment, bran content, prior grain storage, and relative humidity on enzymatic activity, antioxidant content, antioxidant activity, and measures of hydrolytic and oxidative rancidity over IWG flour storage. Compositional analysis of IWG and hard red winter wheat (HRW) samples, harvested in 2016 and 2017, was conducted prior to storage using standard AOAC and AACCI methods. IWG grains were subjected to steam treatment in a sieve over a boiling water bath (100°C for 120 seconds), and then were left to equilibrate at room temperature for 24 hours. IWG grains (those that were steamed and those left as-is) were milled into whole, partially refined (75% bran), and refined flour. HRW grains were milled into whole and refined flour. Flour samples were stored at ambient temperature at 43% and 65% relative humidity (RH) for up to nine months. Samples were analyzed periodically for changes in enzymatic activity, antioxidant content and activity, and measures of hydrolytic and oxidative rancidity. Over storage, lipase (determined spectrophotometrically), and lipoxygenase (ferrous oxidation-xylenol orange assay) activities were evaluated. Carotenoid and hydroxycinnamic acid contents were determined by high performance liquid chromatography (HPLC). Antioxidant activity was evaluated using 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging and leucomethylene blue (LMB) assays. Hydrolytic rancidity was evaluated by measuring free fatty acids (FFA, titration), meanwhile oxidative rancidity was evaluated by measuring peroxide value (acetic acid-chloroform method), and hexanal content (static headspace gas chromatography). Compared to HRW, IWG had significantly higher protein, insoluble fiber, total dietary fiber, antioxidant content, and fat content. IWG had higher lipase yet lower lipoxygenase activity compared to HRW. The direct steam treatment utilized resulted in a significant reduction in lipase and lipoxygenase activity, without a significant reduction in antioxidant content. Overall, direct steam treatment and partial refinement had a positive effect on storage stability of flours from freshly harvested grains. Positive effects included reduced enzymatic activities and lower contents of rancidity products over storage, as well as preserved antioxidant content and activity over storage. The free fatty acid content increased over storage, but to a lesser extent in flour from steamed grains, confirming that steam treatment was effective in partial inactivation of lipase. Similarly, hydroperoxides increased over storage, but flour from steamed grains had lower hydroperoxide contents over time compared to flour from not steamed grains, confirming that steam treatment was effective in delaying oxidation product formation due to partial lipoxygenase inactivation and reduced level of free fatty acids. Partial refinement resulted in significantly lower measures of rancidity over storage, with lower enzymatic activities, compared to whole flours. Antioxidant content and activity were not greatly impacted by steaming or partial refinement, and were mostly preserved during storage. Results demonstrated that in comparison to HRW, IWG’s superior antioxidant content and activity contributed to the delayed appearance of oxidative rancidity measures. Results also demonstrated that flours stored at 43% RH had a slower progression of rancidity compared to samples stored at 65% RH. In IWG flours from one-year-old grains, steam treatment had a positive impact with regard to storage stability, preserving antioxidant contents and delaying the development of rancidity. However, with regard to lipase and lipoxygenase activity and antioxidant activity as measured by DPPH and LMB, steam treatment and refinement did not have a consistent, significant impact. The impact of grain storage prior to milling was positive in general, with lower enzymatic activities and similar antioxidant contents at the beginning and end of storage, and overall lower contents of free fatty acids, hydroperoxides, and hexanal at the end of storage in the resultant flour compared to flour from freshly harvested grains. Overall, the reduction of enzymatic activity by steam treatment resulted in lower contents of hydrolytic and oxidative rancidity products over storage. Partial refinement also resulted in lower contents of rancidity products and enzymatic activities over storage, while also retaining benefits linked with bran including dietary fiber and antioxidant content. Additionally, flours stored at 43% RH had a slower progression of rancidity compared to those stored at 65% RH. Thus, both steam treatment and partial refinement, combined with low RH storage conditions, may be useful to prolong the shelf life of IWG flour.