Antioxidant Activity of Rye Bran-Derived Alkylresorcinols: Effect of Alkyl Chain Length on Activity in Foods
Open Access
- Author:
- Elder, Andrew Stuart
- Graduate Program:
- Food Science
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 19, 2021
- Committee Members:
- Ryan John Elias, Dissertation Advisor/Co-Advisor
Ryan John Elias, Committee Chair/Co-Chair
John Neil Coupland, Committee Chair/Co-Chair
Joshua D Lambert, Committee Member
Andrew David Patterson, Outside Member
John Neil Coupland, Dissertation Advisor/Co-Advisor
Robert F Roberts, Program Head/Chair - Keywords:
- Alkylresorcinols
Antioxidant
Bulk Oil
Cut-off Theory
Emulsion
Iron Chelation
Lipid Oxidation
Low-Moisture Cracker
Partitioning
Radical Scavenging Capacity - Abstract:
- Lipid oxidation is the major cause of chemical deterioration in foods leading to the loss of saleable product and increased food waste. The oxidation of lipids also results in the loss of their nutritional benefits, the formation of toxic compounds, and the development of rancid aromas and off-flavors. Antioxidants are added to foods to retard the rate of lipid oxidation and extend their shelf life. Currently, the food industry is under tremendous pressure to replace synthetic antioxidants with natural alternatives to align with consumer demands. While many natural antioxidants have been identified, very few have been successfully adopted by the food industry due in part to their lower efficacy than synthetic antioxidants. In addition to this, most research on natural antioxidants fails to screen their activity in foods requiring the food industry to extensively, and expensively, test their application. The localization of antioxidants within foods greatly affects their ability to participate in lipid oxidation reactions. While foods have a variety of different physical structures, many contain an aqueous phase, a lipid phase, and an interfacial region between the two. At the interfacial region, water-soluble prooxidants can readily react with the lipid substrate. Antioxidants that localize to the interfacial region display stronger antioxidant activity because they are concentrated at the site of lipid oxidation. One novel strategy to improve the efficacy of natural antioxidants is to graft them with alkyl moieties to increase their hydrophobicity, giving rise to surface activity and promoting their localization to the interfacial region. While fine-tuning antioxidant hydrophobicity to optimize antioxidant activity for a given food has been shown to be an effective strategy to control lipid oxidation, the products are no longer natural. One potential solution to this is through the use of alkylresorcinols (ARs), a homologous series of amphiphilic phenolipids. ARs are novel natural antioxidants that contain a meta-substituted dihydroxyl phenolic ring and an alkyl chain that ranges in length from 13 to 27 carbons. Found in the bran layer of cereal grains, ARs represent a low-cost class of natural antioxidants that can be derived from waste streams. Currently, there are conflicting reports on the efficacy of ARs which is due to previous studies exclusively testing their antioxidant activity using simple in vitro assays. These assays do not correlate with antioxidant activity in real foods because they fail to account for antioxidant localization. To address this shortcoming, the antioxidant activity of ARs was investigated in bulk oils, model oil-in-water food emulsions, and model crackers. To determine what hydrophobicity promotes localization to the interfacial region of each model food system, the effect of alkyl chain length on the antioxidant activity of individual AR homologues (i.e. C17:0, C19:0, C21:0, C23:0, C25:0) was also investigated. My first study set out to determine the viability of using ARs as natural antioxidants to control lipid oxidation in oil-in-water emulsions. A rye bran extract containing ARs was found to inhibit lipid oxidation reactions. The majority of the ARs were associated with the dispersed lipid phase and those in the aqueous continuous phase were associated with surfactant micelles, perhaps inhibiting their interaction with water-soluble prooxidants. The rye bran extract was able to scavenge oxygen radicals but was unable to chelate iron. These results indicate that a rye bran extract containing ARs can function as a radical scavenging antioxidant treatment in emulsions. My next study set out to determine if ARs were the active compounds in the rye bran extract and to determine the effect of alkyl chain length on the antioxidant activity of ARs in bulk oils and oil-in-water emulsions. ARs were found to inhibit lipid oxidation reactions indicating that they were responsible for the observed antioxidant activity of the previously studied rye bran extract. In bulk oils, the antioxidant activity of ARs decreased as alkyl chain length increased. In emulsions, optimum antioxidant activity was observed at intermediate alkyl chain length (C21:0). It was found that there was no effect of alkyl chain length on the rate of loss of ARs in bulk oils but longer alkyl chain homologues were lost more rapidly in emulsions. The radical scavenging capacity of ARs decreased as alkyl chain length increased but ARs were unable to chelate iron. These results indicate that intrinsic properties (e.g. radical scavenging capacity) are responsible for the antioxidant activity of ARs in bulk oils while physicochemical phenomena (e.g. antioxidant partitioning) drive the antioxidant activity of ARs in emulsions. My final study set out to determine the effect of alkyl chain length on the antioxidant activity of ARs in low-moisture crackers. The antioxidant activity of ARs increased as alkyl chain length increased, with optimum activity at an alkyl chain length of C23:0. It was found that there was no effect of alkyl chain length on the rate of loss of ARs, however, a majority of ARs remained even at advanced stages of lipid oxidation. These results indicate that a portion of the ARs were localized at the lipid phase and longer alkyl chain AR homologues (C23:0, C25:0) were likely more concentrated here due to their higher lipid solubility, resulting in stronger antioxidant activity. This suggests that ARs are effective antioxidants in low-moisture foods likely due to their hydrophobic nature promoting them to localize at the lipid phase, the purported site of lipid oxidation in the model cracker system. This work has demonstrated the viability of using ARs as natural antioxidants in a variety of model food systems. It has been shown that different AR homologues display optimal antioxidant activity depending on the food system. This is a result of the physical structure of these food systems dictating what hydrophobicity (i.e. alkyl chain length) promotes localization to the relevant interfacial region of the food system where lipid oxidation reactions occur. This unique property of ARs provides versatility for the food industry to selectively use AR homologues that exert optimal antioxidant activity for their given food application which cannot be achieved through the use of other conventional natural antioxidants.