Lipid degradation during grain storage: markers, mechanisms and shelf-life extension treatments

Abstract

Introduction: One of the challenges with whole grain ingredients, foods and meals is that they typically have a shorter shelf-life compared to equivalent products that contain refined grains, without a bran layer. This is primarily due to lipid degradation that leads to rancidity and is a major impediment to the further development of whole grain products. The consequent reduction in shelf life is due to the production of molecules with unpleasant scent and flavour caused by the lipid degradation which result from chemical or enzymatic modification of (primarily) bran-based lipids. The rate and extent of lipid degradation depends upon many factors including: the composition and structure of the seed/grain, the types of lipid present, the moisture content and any external catalysts such as the presence of some metals or simply exposure to light. Materials and methods: This work includes a comprehensive study of the different biological seed groups used in the food industry when creating different whole grain products included under the “whole grain” and “whole meal” label. The 11 samples selected were barley, red sorghum, white sorghum, teff, red quinoa, white quinoa, buckwheat, sunflower, flax, white chia and black chia. The samples (n=1188) were stored ground and intact under three different storage conditions at three different temperatures (4, 23, and 38°C) for up to 4 months to examine the reactions occurring in the samples: from the fresh seed (fatty acid composition), followed by lipid degradation, from hydroperoxides (hydroperoxide value), up to the volatile compounds production (head space analysis) at days 0, 2, 5, 10, 15, 20, 30, 60 and 110. Multivariate analysis was performed to identify the differences between samples, storage conditions and lipid composition. Based on these results: furan 2-ethyl-, furan 2-pentyl-, pentanal, heptanal, octanal, nonanal and 2-heptanone are the analytes that showed a statistical relation (p<0.05) and were strongly positively correlated with storage time were studied in detail. The goal was to find molecules that show a stable increase with time and could potentially be good quantitative markers of lipid degradation, so with future work and relating with sensory studies they could become markers of rancidity. A selection of samples were then used in an experiment to evaluate options to extend the shelf-life of ground whole grains, using natural treatments. The samples selected were: barley, quinoa white and flax, and they were treated with natural antioxidants (lemon myrtle and rosemary-tocopherol) and thermal treatments (dry heat and wet heat), compared with a chemical antioxidant (BHA) during storage for 14weeks at room temperature in closed vials. Headspace analysis was performed after 2, 4, 8 and 14weeks. Results: The results show that the seed fatty acid composition has a strong impact on lipid stability, except in the case of flax and chia black which based on the fatty acid composition should have been very unstable but were not. Endogenous antioxidant capacity can result in high stable samples, since they play an important role against oxidation and ROS (Reactive Oxygen Species) which may explain this effect. The results also show a difference between storage conditions, where most of the samples stored at 4oC are more stable than the ones at 23 and 38oC. Once ground, lipid oxidation was enhanced for all samples, for example chia white once ground showed quick lipid degradation even when stored at 4oC. There is a clear separation between botanical origins, so even though cereals and pseudo cereals are treated the same in the food industry, they have different responses to storage treatments. Therefore, in general they should not be treated as if they were equal since that could reduce the shelflife of the product. It has been seen that the state of the raw material is a key factor on the stability since there is a big difference between storing the samples ground or intact. The markers that showed a positive statistical relation to storage time as well as being common in cereals and pseudo cereals are furan 2-ethyl-, furan-2 pentyl-, pentanal, heptanal, octanal, nonanal, and 2-heptanone. Further detailed study of these 7 suggests that the best general quantitative markers of lipid degradation are furan 2-pentyl and 2-heptanone. These two analytes were quantified and used to determine the stability of samples after being treated with natural antioxidants and thermal treatments. The results from the natural treatments to extend the shelf-life of the ground grains show that rosemary-tocopherol extract is the best results followed by dry heat. BHA and wet heat are the least effective treatments, in some cases acting as pro-oxidants. From the results of this project, several recommendations are made in order to extend the shelf-life of whole grain products and maintain high-quality grain ingredients.