REVISED UNDERSTANDING OF LIPID OXIDATION MECHANISMS

This project seeks to provide a new base of information about how lipids oxidize and cause rancidity and degradation in oils and foods. Most attention for food safety and stability currently focuses on microbial contamination and inactivation. However, once bacteria, yeasts, and molds have been eliminated, chemical oxidation of lipids becomes the major driver of degradation in foods, leading to substantial loss of sensory quality and nutritional value during storage as well as to generation of potentially toxic products and outright food destruction as oxidation progresses. Oxidation problems were largely ignored during the no/low fat food era, but new recognition of important roles of lipids in health is forcing reformulation of foods with higher contents of polyunsaturated fatty acids. These physiologically essential fatty acids are highly oxidizable in themselves and they cause extensive co-oxidation of other food molecules, particularly proteins. Lipid oxidation causes significant loss of processed, stored foods as well as emergency food aid products, particularly those targeted to regions with high ambient temperatures. Lipid oxidation is also intimately involved in many pathologies in vivo, including aging, atherosclerosis, Alzheimer's disease, and cancer. Thus, understanding lipid oxidation reactions and being able to control these processes is critically important for health, for stabilizing foods, for reducing food costs and food losses, and for maintaining food safety. Unfortunately, basic information about how lipids oxidize is outdated and incomplete so stabilization efforts in the food industry have encountered many hurdles. This program previously identified several alternate pathways that make lipid oxidation much more complex than previously thought and explain some of the problems encountered in stabilizing foods. The current project extends these studies to additional lipids in more complex systems, identifying more classes of products, some of which have toxic potential. Results will provide the first new understanding of lipid oxidation in nearly 50 years, from which more effective measures to prevent this degradation can be developed. Learning how oxidation reactions shift under different conditions will make it possible to deliberately tailor food formulations, processing methods, packaging, and distribution practices to limit degradation of food flavors, textures, and nutritional quality, as well as reduce food loss. Learning which factors have strongest directing effects or most negative impact on product quality provides invaluable guidance for redesigning industrial processing, formulations, and packaging to stabilize highly oxidizable lipids. Learning what oxidation products to look for under various conditions guides development of improved analytical methods to accurately determine extent oxidation in both foods and tissues during manufacturing, storage, and distribution of foods. These outcomes should greatly reduce food loss during storage and distribution of regular and emergency food supplies world-wide. Optimized methods for quantitating oxidation in foods integrated in a battery of tests will generate the most accurate picture of degradation yet available and provide critically-needed tools for quality control in labs and on-line. While focused on food degradation and stabilization, the chemistry elucidated is applicable also to toxicology, medicine, plant physiology and pathology, and personal products and cosmetics industries.Detailed information about how fast lipids oxidize and what products are formed will be obtained in model systems of pure lipids to simplify analyses and make it possible to identify key products. Model lipids to be studied are methyl esters of linoleic and oleic acids, the most prevalent polyunsaturated and monounsaturated fatty acids in foods, and their oil counterparts trilinolein and triolein, respectively. Lipids will be oxidized first in simple model systems of oils alone, then progress to oils dispersed on glass fiber filter paper to mimic lipids in solid food matrices, and finally oils in emulsions to determine effects of water phases and provide increasing structure that more closely represents actual food systems. Model lipids will be oxidized in two modes (open, as would occur with permeable or no packaging, and closed, as would occur in sealed impermeable packaging), at three temperatures - 25, 40, and 60 deg C (room, elevated, and accelerated storage temperatures), under different levels of oxygen and other conditions. Samples will be withdrawn periodically and analyzed for products from multiple oxidation pathways, including conjugated dienes and hydroperoxides, epoxides, hydroxylipids, volatile and non-volatile aldehydes and ketones, dimers and polymers, and volatile products. Particular attention will be given to epoxides and hydroxylipids which have toxic potential. Class analyses will focus on quantitating how much of a specific product is generated under different conditions, while chromatographic analyses with mass spectrometry detection will identify structures of products generated. A very new method that uses liquid chromatography couple with high resolution mass spectrometry will be applied to oils for the first time to identify total lipid products, including structures not previously recognized. Product patterns and kinetics will then be integrated to derive active reaction pathways and assess potential approaches for inhibition as well as consequences of modification (will blocking one pathway accelerate another pathway and cause more but different damage?). Chemometric statistical analyses will be applied to sort out relationships between reaction conditions and dominant reaction pathways and products. The same analyses are being applied to all other foods being studied in our laboratory in order to determine how accurately the model systems represent oxidation in real foods.