Toward a Spatiotemporal Model of Oxidation in Lipid Dispersions: A Hypothesis‐Driven Review

Response to "Liposome vesicle cannot be formed in non-aqueous phase"

In a recent letter to the editor Prof Khosravi-Darani responded to our paper ''Unravelling mechanisms of protein and lipid oxidation in mayonnaise at multiple length scales''. In our work, we observed liposomes in the continuous phase of mayonnaise. In the letter the objection was made that liposomes cannot be formed in a non-aqueous phase which, however, was not argued in our publication. As mayonnaise is an oil-in-water (O/W) emulsion and its continuous phase is aqueous, liposomes may be observed in this phase. Therefore, the objection from Prof Khosravi-Darani does not apply to our work.

Unravelling the effect of droplet size on lipid oxidation in O/W emulsions by using microfluidics

Lipid oxidation in emulsions is hypothesised to increase with decreasing droplet size, as this increases the specific oil-water interfacial area, where lipid oxidation is expected to be initiated. In literature, however, contradictory results have been reported, which can be caused by confounding factors such as the oil droplet polydispersity and the distribution of components between the available phases. In this work, monodisperse surfactant-stabilised emulsions with highly controlled droplet sizes of 4.7, 9.1, and 26 µm were produced by microfluidic emulsification. We show that lipid oxidation increases with decreasing droplet size, which we ascribe to the increased contact area between lipids and continuous phase prooxidants. Besides, a significant amount of oxygen was consumed by oxidation of the surfactant itself (Tween 20), an effect that also increased with decreasing droplet size. These insights substantiate the importance of controlling droplet size for improving the oxidative stability of emulsions.

Lipid oxidation in emulsions: New insights from the past two decades

Lipid oxidation constitutes the main source of degradation of lipid-rich foods, including food emulsions. The complexity of the reactions at play combined with the increased demand from consumers for less processed and more natural foods result in additional challenges in controlling this phenomenon. This review provides an overview of the insights acquired over the past two decades on the understanding of lipid oxidation in oil-in-water (O/W) emulsions. After introducing the general structure of O/W emulsions and the classical mechanisms of lipid oxidation, the contribution of less studied oxidation products and the spatiotemporal resolution of these reactions will be discussed. We then highlight the impact of emulsion formulation on the mechanisms, taking into consideration the new trends in terms of emulsifiers as well as their own sensitivity to oxidation. Finally, novel antioxidant strategies that have emerged to meet the recent consumer's demand will be detailed. In an era defined by the pursuit of healthier, more natural, and sustainable food choices, a comprehensive understanding of lipid oxidation in emulsions is not only an academic quest, but also a crucial step towards meeting the evolving expectations of consumers and ensuring the quality and stability of lipid-rich food products.

Antioxidant functions of caffeic acid and allylpyrocatechol in supramolecular oxidation of bulk oil: Role of acidic group in the mass transfer network

Antioxidant activities of caffeic acid (CFA) and allylpyrocatechol (APC) were measured in bulk oil and in emulsion (oil/water) to evaluate the effects of the carboxylic group on the antioxidant function. As an emulsifier, polyglycerol polyricinoleate (PGPR) was added to the bulk oil to assist in promoting the interfacial activity of antioxidants. The antioxidant activity of CFA in the bulk oil was 186.5% higher than that of APC. In the emulsion, however, APC showed 150.3% higher antioxidant activity. PGPR showed a synergistic effect on CFA (>31%) and APC (<18%) which arose from organizing and supporting the formation of oxidation-microreactors. Water molecules were generated as an oxidation byproduct throughout the oxidation course, and they facilitated the formation of supramolecular structures. Polar imbalance became more prominent in the molecular structure of CFA, due to the presence of the carboxyl group, thereby increasing the effective collisions between the antioxidant molecules and free radicals.

Shade of Innovative Food Processing Techniques: Potential Inducing Factors of Lipid Oxidation

With increasing environmental awareness and consumer demand for high-quality food products, industries are strongly required for technical innovations. The use of various emerging techniques in food processing indeed brings many economic and environmental benefits compared to conventional processes. However, lipid oxidation induced by some "innovative" processes is often "an inconvenient truth", which is scarcely mentioned in most studies but should not be ignored for the further improvement and optimization of existing processes. Lipid oxidation poses a risk to consumer health, as a result of the possible ingestion of secondary oxidation products. From this point of view, this review summarizes the advance of lipid oxidation mechanism studies and mainly discloses the shade of innovative food processing concerning lipid degradation. Sections involving a revisit of classic three-stage chain reaction, the advances of polar paradox and cut-off theories, and potential lipid oxidation factors from emerging techniques are described, which might help in developing more robust guidelines to ensure a good practice of these innovative food processing techniques in future.

Tiny, yet impactful: Detection and oxidative stability of very small oil droplets in surfactant-stabilized emulsions

HYPOTHESIS

The shelf life of multiphase systems, e.g. oil-in-water (O/W) emulsions, is severely limited by physical and/or chemical instabilities, which degrade their texture, macroscopic appearance, sensory and (for edible systems) nutritional quality. One prominent chemical instability is lipid oxidation, which is notoriously complex. The complexity arises from the involvement of many physical structures present at several scales (1-10,000 nm), of which the smallest ones are often overlooked during characterization.

EXPERIMENTS

We used cryogenic transmission electron microscopy (cryo-TEM) to characterize the coexisting colloidal structures at the nanoscale (10-200 nm) in rapeseed oil-based model emulsions stabilized by different concentrations of a nonionic surfactant. We assessed whether the oxidative and physical instabilities of the smallest colloidal structures in such emulsions may be different from those of larger colloidal structures.

FINDINGS

By deploying cryo-TEM, we analyzed the size of very small oil droplets and of surfactant micelles, which are typically overlooked by dynamic light scattering when larger structures are concomitantly present. Their size and oil content were shown to be stable over incubation, but lipid oxidation products were overrepresented in these very small droplets. These insights highlight the importance of the fraction of "tiny droplets" for the oxidative stability of O/W emulsions.

Mechanisms of lipid oxidation in water-in-oil emulsions and oxidomics-guided discovery of targeted protective approaches

Lipid oxidation is an inevitable event during the processing, storage, and even consumption of lipid-containing food, which may cause adverse effects on both food quality and human health. Water-in-oil (W/O) food emulsions contain a high content of lipids and small water droplets, which renders them vulnerable to lipid oxidation. The present review provides comprehensive insights into the lipid oxidation of W/O food emulsions. The key influential factors of lipid oxidation in W/O food emulsions are presented systematically. To better interpret the specific mechanisms of lipid oxidation in W/O food emulsions, a comprehensive detection method, oxidative lipidomics (oxidomics), is proposed to identify novel markers, which not only tracks the chemical molecules but also considers the changes in supramolecular properties, sensory properties, and nutritional value. The microstructure of emulsions, components from both phases, emulsifiers, pH, temperature, and light should be taken into account to identify specific oxidation markers. A correlation of these novel oxidation markers with the shelf life, the organoleptic properties, and the nutritional value of W/O food emulsions should be applied to develop targeted protective approaches for limiting lipid oxidation. Accordingly, the processing parameters, the application of antioxidants and emulsifiers, as well as packing and storage conditions can be optimized to develop W/O emulsions with improved oxidative stability. This review may help in emphasizing the future research priorities of investigating the mechanisms of lipid oxidation in W/O emulsion by oxidomics, leading to practical solutions for the food industry to prevent oxidative rancidity in W/O food emulsions.

Metal Complexes of Omadine (N-Hydroxypyridine-2-thione): Differences of Antioxidant and Pro-Oxidant Behavior in Light and Dark Conditions with Possible Toxicity Implications

Omadine or N-hydroxypyridine-2-thione and its metal complexes are widely used in medicine and show bactericidal, fungicidal, anticancer, and photochemical activity. The redox activity of omadine complexes with iron, copper, and zinc on lipid peroxidation under light and dark conditions has been investigated. The monitoring of the oxidation of linoleic acid micelles, resembling a model of lipid membrane, was carried out using nuclear magnetic resonance (¹H-NMR). It has been shown that the omadine-zinc complex can induce the oxidation of linoleic acid under light irradiation, whereas the complexes with iron and copper are photochemically stable. All the chelating complexes of omadine appear to be redox-inactive in the presence of hydrogen peroxide under dark conditions. These findings suggest that omadine can demonstrate antioxidant behavior in processes involving reactive oxygen species generation induced by transition metals (Fenton and photo-Fenton reactions). However, the omadine complex with zinc, which is widely used in shampoos and ointments, is photochemically active and may cause oxidative cell membrane damage when exposed to light, with possible implications to health.

Targeting Interfacial Location of Phenolic Antioxidants in Emulsions: Strategies and Benefits

It is important to have larger proportions of health-beneficial polyunsaturated lipids in foods, but these nutrients are particularly sensitive to oxidation, and dedicated strategies must be developed to prevent this deleterious reaction. In food oil-in-water emulsions, the oil-water interface is a crucial area when it comes to the initiation of lipid oxidation. Unfortunately, most available natural antioxidants, such as phenolic antioxidants, do not spontaneously position at this specific locus. Achieving such a strategic positioning has therefore been an active research area, and various routes have been proposed: lipophilizing phenolic acids to confer them with an amphiphilic character; functionalizing biopolymer emulsifiers through covalent or noncovalent interactions with phenolics; or loading Pickering particles with natural phenolic compounds to yield interfacial antioxidant reservoirs. We herein review the principles and efficiency of these approaches to counteract lipid oxidation in emulsions as well as their advantages and limitations.

Physical and Oxidative Stabilization of Oil-In-Water Emulsions by Roasted Coffee Fractions: Interface- and Continuous Phase-Related Effects

Emulsions fortified with polyunsaturated fatty acids are highly relevant from a nutritional perspective; however, such products are prone to lipid oxidation. In the current work, this is mitigated by the use of natural antioxidants occurring in coffee. Coffee fractions with different molecular weights were extracted from roasted coffee beans. These components were positioned either at the interface or in the continuous phase of emulsions where they contributed to emulsion stability via different pathways. Coffee brew as a whole, and its high-molecular-weight fraction (HMWF), was able to form emulsions with good physical stability and excellent oxidative stability. When added post-homogenization to the continuous phase of dairy protein-stabilized emulsions, all coffee fractions were able to slow down lipid oxidation considerably without altering the physical stability of emulsions, though HMWF was more effective in retarding lipid oxidation than whole coffee brew or low-molecular-weight fraction. This is caused by various effects, such as the antioxidant properties of coffee extracts, the partitioning of components in the emulsions, and the nature of the phenolic compounds. Our research shows that coffee extracts can be used effectively as multifunctional stabilizers in dispersed systems leading to emulsion products with high chemical and physical stability.

Recent Trends in Improving the Oxidative Stability of Oil-Based Food Products by Inhibiting Oxidation at the Interfacial Region

In recent years, new approaches have been developed to limit the oxidation of oil-based food products by inhibiting peroxidation at the interfacial region. This review article describes and discusses these particular approaches. In bulk oils, modifying the polarity of antioxidants by chemical methods (e.g., esterifying antioxidants with fatty alcohol or fatty acids) and combining antioxidants with surfactants with low hydrophilic–lipophilic balance value (e.g., lecithin and polyglycerol polyricinoleate) can be effective strategies for inhibiting peroxidation. Compared to monolayer emulsions, a thick interfacial layer in multilayer emulsions and Pickering emulsions can act as a physical barrier. Meanwhile, high viscosity of the water phase in emulsion gels tends to hinder the diffusion of pro-oxidants into the interfacial region. Furthermore, applying surface-active substances with antioxidant properties (such as proteins, peptides, polysaccharides, and complexes of protein-polysaccharide, protein-polyphenol, protein-saponin, and protein-polysaccharide-polyphenol) that adsorb at the interfacial area is another novel method for enhancing oil-in-water emulsion oxidative stability. Furthermore, localizing antioxidants at the interfacial region through lipophilization of hydrophilic antioxidants, conjugating antioxidants with surfactants, or entrapping antioxidants into Pickering particles can be considered new strategies for reducing the emulsion peroxidation.

Model systems for studying lipid oxidation associated with muscle foods: Methods, challenges, and prospects

Lipid oxidation is a complex process in muscle-based foods (red meat, poultry and fish) causing severe quality deterioration, e.g., off-odors, discoloration, texture defects and nutritional loss. The complexity of muscle tissue -both composition and structure- poses as a formidable challenge in directly clarifying the mechanisms of lipid oxidation in muscle-based foods. Therefore, different in vitro model systems simulating different aspects of muscle have been used to study the pathways of lipid oxidation. In this review, we discuss the principle, preparation, implementation as well as advantages and disadvantages of seven commonly-studied model systems that mimic either compositional or structural aspects of actual meat: emulsions, fatty acid micelles, liposomes, microsomes, erythrocytes, washed muscle mince, and muscle homogenates. Furthermore, we evaluate the prospects of stem cells, tissue cultures and three-dimensional printing for future model system development. Based on this reviewing of oxidation models, tailoring correct model to different study aims could be facilitated, and readers are becoming acquainted with advantages and shortcomings. In addition, insight into recent technology developments, e.g., stem cell- and tissue-cultures as well as three-dimensional printing could provide new opportunities to overcome the current bottlenecks of lipid oxidation studies in muscle.

Effect of polyglycerol polyricinoleate on the inhibitory mechanism of sesamol during bulk oil oxidation

In this study, effects of sesamol on improving the oxidative stability of sunflower oil and its oil-in-water emulsion was investigated. To investigate the kinetic parameters related to the initiation and propagation stages of oxidation, a sigmoidal-model was used. Sesamol exhibited higher antioxidant activity in sunflower oil-in-water emulsion than that of sunflower oil. In both sunflower oil and sunflower oil-in-water emulsion, the inhibitory effect of sesamol against lipid oxidation continued even after the induction period. To improve the efficiency of sesamol in sunflower oil, polyglycerol polyricinoleate (PGPR) was incorporated into the functional environment of the sesamol. Sesamol exhibited a synergistic effect with PGPR during both initiation (synergistic effect of 68.87%) and propagation (synergistic effect of 36.84%) stages. Comparison of the size of reverse micelles in samples containing PGPR with those without PGPR revealed that PGPR can enhance the efficiency of sesamol by increasing the acceptance capacity of lipid hydroperoxides in reveres micelles structures. This can result in enhancing the effective collisions between sesamol and lipid hydroperoxides in the presence of PGPR. The water produced as a major byproduct of oxidation played a key role on the antioxidant activity of sesamol alone or in combination with PGPR during oxidation process.

Alkyl chain length modulates antioxidant activity of gallic acid esters in spray-dried emulsions

Lipid oxidation is a well-recognized issue in dried food emulsions, such as infant milk formula. Antioxidants can be used to mitigate this issue; however, their efficiency in such complex systems is far from understood. In this study, antioxidant polarity is varied through the alkyl chain length of gallic acid esters (0-16 carbon atoms) incorporated to O/W emulsions that are subsequently spray-dried. During processing and subsequent storage of the samples, antioxidants with more than eight carbon atoms are effective. Both for encapsulated fat and surface free fat, we observe a slight cut-off effect, meaning that beyond eight alkyl groups, a more nonpolar antioxidant is slightly less effective. Depending on the antioxidant polarity, lipid oxidation is faster either in the encapsulated or in the surface free fat. The insights obtained contribute to understanding lipid oxidation in low moisture food emulsions, and thus lead to effective antioxidant strategies.

Effect of triacylglycerol structure on the antioxidant activity of γ-oryzanol

In this study, the triacylglycerol structure of vegetable oils was removed and the effects of this removal were observed on the antioxidant efficiency of γ-oryzanol. A sigmoidal-model was used for calculating kinetic parameters relevant to the initiation and propagation phases during the peroxidation of soybean, corn, sesame, and olive oils as well as their fatty acid methyl esters. Removing the triacylglycerol structure caused an increase in the antioxidant activity of γ-oryzanol (26.49%) by affecting both inhibitory mechanisms, i.e. hydrogen-donating (7.80%) and electron-transfer (14.72%). Unexpectedly, the antioxidant performance of γ-oryzanol continued even when the induction period had ended. During the propagation phase, the highest antioxidant activity was observed in the fatty acid methyl esters of soybean oil (3.86) based on hydroperoxides decomposition. An evaluation of how the endergonic activated complexes formed could indicate that the removal of the triacylglycerol structure increased the effective collisions between the γ-oryzanol molecules and free radicals.

The Interplay of Ascorbic Acid with Quinones-Chelators—Influence on Lipid Peroxidation: Insight into Anticancer Activity

Ascorbic acid is a multifaceted compound that can perform both antioxidant and pro-oxidant activities in the redox reactions induced by transition metal ions, so its role in nature and especially in the human body is still the subject of debate. In the present study, we have examined the influence of ascorbic acid on lipid peroxidation in a model system that mimics the cell membrane, namely micelles of linoleic acid (LA), induced by chelate complexes of iron and copper ions with quinone-chelator 2-phenyl-4-(butylamino)-naphtholquinoline-7,12-dione (Q1). This quinone effectively generates reactive oxygen species and semiquinone radicals inside cancer cells via a cycling redox reaction. Here it was demonstrated that in the absence of quinone-chelator ascorbic acid significantly accelerates the lipid peroxidation induced by both Fe(II) and Cu(II) ions. It has been shown also that Q1 chelate complexes with Fe(II) and Cu(II) ions are redox active in the LA micelles oxidation. No effect of ascorbate was detected on the reactivity of chelate complex with Fe(II) ions. On the other hand, ascorbate performs pro-oxidant activity in Q1-Cu(II) complex induced reaction. We can conclude that ascorbate-driven redox cycling of Q1 may promote its anti-tumor activity.

Advances in the control of lipid peroxidation in oil-in-water emulsions: kinetic approaches†

Large efforts have been, and still are, devoted to minimize the harmful effects of lipid peroxidation. Much of the early work focused in understanding both the lipid oxidation mechanisms and the action of antioxidants in bulk solution. However, food-grade oils are mostly present in the form of oil-in-water emulsions, bringing up an increasing complexity because of the three-dimensional interfacial region. This review presents an overview of the kinetic approaches employed in controlling the oxidative stability of edible oil-in-water emulsions and of the main outcomes, with particular emphasis on the role of antioxidants and on the kinetics of the inhibition reaction. Application of physical-organic chemistry methods, such as the pseudophase models to investigate antioxidant partitioning, constitute a remarkable example on how kinetic methodologies contribute to model chemical reactivity in multiphasic systems and to rationalize the role of interfaces, opening new opportunities for designing novel antioxidants with tailored properties and new prospects for modulating environmental conditions in attempting to optimize their efficiency. Here we will summarize the main kinetic features of the inhibition reaction and will discuss on the main factors affecting its rate, including the determination of antioxidant efficiencies from kinetic profiles, structure-reactivity relationships, partitioning of antioxidants and concentration effects.

Ascorbate-and iron-driven redox activity of Dp44mT and emodin facilitates peroxidation of micelles and bicelles

BACKGROUND

Iron (Fe)-induced oxidative stress leads to reactive oxygen species that damage biomembranes, with this mechanism being involved in the activity of some anti-cancer chemotherapeutics.

METHODS

Herein, we compared the effect of Fe complexes of the ligand, di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT), or the potential ligand, Emodin, on lipid peroxidation in cell membrane models (micelles and bicelles). These studies were performed in the presence of hydrogen peroxide (H₂O₂) and the absence or presence of ascorbate.

RESULTS

In the absence of ascorbate, Fe(II)/Emodin mixtures incubated with H₂O₂ demonstrated slight pro-oxidant properties on micelles versus Fe(II) alone, while the Fe(III)-Dp44mT complex exhibited marked antioxidant properties. Examining more physiologically relevant phospholipid-containing bicelles, the Fe(II)- and Fe(III)-Dp44mT complexes demonstrated antioxidant activity without ascorbate. Upon adding ascorbate, there was a significant increase in the peroxidation of micelles and bicelles in the presence of unchelated Fe(II) and H₂O₂. The addition of ascorbate to Fe(III)-Dp44mT substantially increased the peroxidation of micelles and bicelles, with the Fe(III)-Dp44mT complex being reduced by ascorbate to the Fe(II) state, explaining the increased reactivity. Electron paramagnetic resonance spectroscopy demonstrated ascorbyl radical anion generation after mixing ascorbate and Emodin, with signal intensity being enhanced by H₂O₂. This finding suggested Emodin semiquinone radical formation that could play a role in its reactivity via ascorbate-driven redox cycling. Examining cultured melanoma cells in vitro, ascorbate at pharmacological levels enhanced the anti-proliferative activity of Dp44mT and Emodin.

CONCLUSIONS AND GENERAL SIGNIFICANCE

Ascorbate-driven redox cycling of Dp44mT and Emodin promotes their anti-proliferative activity.

Tocopherols as antioxidants in lipid-based systems: The combination of chemical and physicochemical interactions determines their efficiency

Lipid oxidation is a major concern in the food, cosmetic, and pharmaceutical sectors. The degradation of unsaturated lipids affects the nutritional, physicochemical, and organoleptic properties of products and can lead to off-flavors and to the formation of potentially harmful oxidation compounds. To prevent or slow down lipid oxidation, different antioxidant additives are used alone or in combination to achieve the best possible efficiency with the minimum possible quantities. In manufactured products, that is, heterogeneous systems containing lipids as emulsions or bulk phase, the efficiency of an antioxidant is determined not only by its chemical reactivity, but also by its physical properties and its interaction with other compounds present in the products. The antioxidants most widely used on the industrial scale are probably tocopherols, either as natural extracts or pure synthetic molecules. Considerable research has been conducted on their antioxidant activity, but results regarding their efficiency are contradictory. Here, we review the known mechanisms behind the antioxidant activity of tocopherols and discuss the chemical and physical features that determine their efficacy. We first describe their chemical reactivity linked with the main factors that modulate it between efficient antioxidant capacity and potential prooxidant effects. We then describe their chemical interactions with other molecules (phenolic compounds, metals, vitamin C, carotenes, proteins, and phospholipids) that have potential additive, synergistic, or antagonist effects. Finally, we discuss other physical parameters that influence their activity in complex systems including their specific interactions with surfactants in emulsions and their behavior in the presence of association colloids in bulk oils.

Lipid oxidation in emulsions and bulk oils: a review of the importance of micelles

Lipid oxidation is a major cause of quality deterioration in food products. In these foods, lipids are often present in a bulk or in emulsified forms. In both systems, the rate, extent and pathway of oxidation are highly dependent on the presence of colloidal structures and interfaces because these are the locations where oxidation normally occurs. In bulk oils, reverse micelles (association colloids) are present and are believed to play a crucial role on lipid oxidation. Conversely, in emulsions, surfactant micelles are present that also play a major role in lipid oxidation pathways. After a brief description of lipid oxidation and antioxidants mechanisms, this review discusses the current understanding of the influence of micellar structures on lipid oxidation. In particular, is discussed the major impact of the presence of micelles in emulsions, or reverse micelles (association colloids) in bulk oil on the oxidative stability of both systems. Indeed, both micelles in emulsions and associate colloids in bulk oils are discussed in this review as nanoscale structures that can serve as reservoirs of antioxidants and pro-oxidants and are involved in their transport within the concerned system. Their role as nanoreactors where lipid oxidation reactions occur is also commented.

Evaluation of oxygen partial pressure, temperature and stripping of antioxidants for accelerated shelf-life testing of oil blends using 1H NMR

Lipid oxidation compromises the shelf-life of lipid-containing foods, leading to the generation of unpleasant off-flavours. Monitoring lipid oxidation under normal shelf-life conditions can be time-consuming (i.e. weeks or months) and therefore accelerated shelf-life conditions are often applied. However, little is known on their impact on the lipid oxidation mechanisms. In this study, different oxygen partial pressures (P_O2; 10 and 21%), temperatures (20, 30 and 40 °C), and the removal of antioxidants through stripping of the oil were tested to accelerate lipid oxidation. Increasing the incubation temperature of stripped oil blends from 30 to 40 °C reduced the onset of lipid oxidation from 4 to 2 weeks, whereas the P_O2 had no impact. Surprisingly, at room temperature, an increase in P_O2 resulted in a longer onset time (10 weeks under 10% oxygen, 15 weeks under 21% oxygen). We hypothesize that this is due to a shift in (initiation) mechanism. In non-stripped oil, an increase in P_O2 from 10 to 21% decreased the onset time from 16 to 10 weeks (40 °C). Temperature elevations and stripping led to a shift towards more trans-trans diene hydroperoxides, as compared to the cis-trans conformation. Additionally, oil stripping led to an increase in oxidized PUFAs with three or more double bonds in which the hydroperoxide group is located between the double bond pattern, instead of on the edge of it. Lastly, it was shown that small additions of LC-PUFAs (0, 0.3, 0.6, 1.2 and 2.3%, w/w) accelerate lipid oxidation, even in relatively stable stripped oils. In conclusion, increased P_O2 and slightly elevated temperatures hold fair potential for accelerated shelf-life testing of non-stripped oils with a limited impact on the lipid oxidation mechanisms, whereas stripping significantly changes propagation mechanisms.

Towards Oxidatively Stable Emulsions Containing Iron-Loaded Liposomes: The Key Role of Phospholipid-to-Iron Ratio

To encapsulate soluble iron, liposomes were prepared using unsaturated phospholipids (phosphatidylcholine from egg yolk), leading to high encapsulation efficiencies (82–99%). The iron concentration affected their oxidative stability: at 0.2 and 1 mM ferrous sulfate, the liposomes were stable, whereas at higher concentrations (10 and 48 mM), phospholipid oxidation was considerably higher. When applied in oil-in-water (O/W) emulsions, emulsions with liposomes containing low iron concentrations were much more stable to lipid oxidation than those added with liposomes containing higher iron concentrations, even though the overall iron concentration was similar (0.1 M). Iron-loaded liposomes thus have an antioxidant effect at high phospholipid-to-iron ratio, but act as pro-oxidants when this ratio is too low, most likely as a result of oxidation of the phospholipids themselves. This non-monotonic effect can be of crucial importance in the design of iron-fortified foods.

Oxidation in Low Moisture Foods as a Function of Surface Lipids and Fat Content

Lipid oxidation is a major limitation to the shelf-life of low moisture foods and can lead to food waste. Little is known of whether the surface lipids in low moisture foods are more susceptible to oxidation since they are exposed to the environment. Therefore, the purpose of this research is to compare the rate of oxidation in surface and total lipids. Lipids in crackers were found to be in a heterogeneous matrix with proteins and starch, as determined by confocal microscopy. However, unlike spray-dried powders, both surface and interior lipids oxidized at similar rates, suggesting that the cracker matrix was not able to protect lipids from oxidation. Increasing the fat content of the crackers increased oxidation rates, which could be due to differences in the lipid structure or higher water activities in the high-fat crackers.

Quantitative and Predictive Modelling of Lipid Oxidation in Mayonnaise

Food emulsions with high amounts of unsaturated fats, such as mayonnaise, are prone to lipid oxidation. In the food industry, typically accelerated shelf life tests are applied to assess the oxidative stability of different formulations. Here, the appearance of aldehydes at the so-called onset time, typically weeks, is considered a measure for oxidative stability of food emulsions, such as mayonnaise. To enable earlier assessment of compromised shelf-life, a predictive model for volatile off-flavor generation is developed. The model is based on the formation kinetics of hydroperoxides, which are early oxidation products and precursors of volatile aldehydes, responsible for off-flavor. Under accelerated shelf-life conditions (50 °C), hydroperoxide (LOOH) concentration over time shows a sigmoidal curvature followed by an acceleration phase that occurs at a LOOH-concentration between 38–50 mmol/kg, here interpreted as a critical LOOH concentration (CC_LOOH). We hypothesize that the time at which CC_LOOH was reached is related to the onset of aldehyde generation and that the characterization of the LOOH-generation curvature could be based on reaction kinetics in the first days. These hypotheses are tested using semi-empirical models to describe the autocatalytic character of hydroperoxide formation in combination with the CC_LOOH. The Foubert function is selected as best describing the LOOH-curvature and is hence used to accurately predict onset of aldehyde generation, in most cases within several days of shelf-life. Furthermore, we find that the defining parameters of this model could be used to recognize antioxidant mechanisms at play.

Natural particles can armor emulsions against lipid oxidation and coalescence

Traditional functional ingredients, such as conventional emulsifiers (surfactants, animal-derived proteins), and synthetic antioxidants may become obsolete in the development of clean-label, plant-based, sustainable food emulsions. Previously, we showed that tailor-made antioxidant-loaded particles can yield both physically and oxidatively stable emulsions, and we expected that natural particles with related properties could also show these beneficial effects. Here, we investigated Pickering emulsions prepared with natural plant particulate materials. Particles that showed weak aggregation in acidic aqueous media, indicating a relatively hydrophobic surface, were able to physically stabilize oil-in-water emulsions, through either Pickering stabilization (powders of matcha tea, spinach leaves, and spirulina cake), or an increase in viscosity (pineapple fibers). Matcha tea and spinach leaf particle-stabilized emulsions were highly stable to lipid oxidation, as compared to emulsions stabilized by conventional emulsifiers. Taking this dual particle functionality as a starting point for emulsion design is, in our view, essential to achieve clean-label food emulsions.

Quantitative Spatiotemporal Mapping of Lipid and Protein Oxidation in Mayonnaise

Lipid oxidation in food emulsions is mediated by emulsifiers in the water phase and at the oil–water interface. To unravel the physico-chemical mechanisms and to obtain local lipid and protein oxidation rates, we used confocal laser scanning microscopy (CLSM), thereby monitoring changes in both the fluorescence emission of a lipophilic dye BODIPY 665/676 and protein auto-fluorescence. Our data show that the removal of lipid-soluble antioxidants from mayonnaises promotes lipid oxidation within oil droplets as well as protein oxidation at the oil–water interface. Furthermore, we demonstrate that ascorbic acid acts as either a lipid antioxidant or pro-oxidant depending on the presence of lipid-soluble antioxidants. The effects of antioxidant formulation on local lipid and protein oxidation rates were all statistically significant (p < 0.0001). The observed protein oxidation at the oil–water interface was spatially heterogeneous, which is in line with the heterogeneous distribution of lipoprotein granules from the egg yolk used for emulsification. The impact of the droplet size on local lipid and protein oxidation rates was significant (p < 0.0001) but minor compared to the effects of ascorbic acid addition and lipid-soluble antioxidant depletion. The presented results demonstrate that CLSM can be applied for unraveling the roles of colloidal structure and transport in mediating lipid oxidation in complex food emulsions.

Interrelated Routes between the Maillard Reaction and Lipid Oxidation in Emulsion Systems

In foods, the Maillard reaction (MR) and lipid oxidation lead to the formation of several molecules through interrelated chemical pathways. MR and lipid oxidation products were investigated in model oil-in-water emulsions consisting of canola oil, water, and Tween 20, a nonionic surfactant, with glucose and phenylalanine. The presence of 1% Tween 20, either in emulsion or as a control surfactant solution, sped up the formation of N-(1-deoxy-d-fructos-1-yl)-phenylalanine and of phenylacetaldehyde. Overall, the formation of MR products was up to sixteen times higher in emulsions than in an aqueous system without a surfactant. The formation of conjugated dienes, total aldehydes, hexanal, and (Z)-2-octenal was reduced down to six times when MR products were present in the emulsion. These results confirm that the formation of MR intermediates is influenced by the reactants' location, and the presence of a discrete nonpolar environment (oil droplets or surfactant micelles) promotes MR volatile formation through Strecker degradation.

Radical Scavenging Activity of Olive Oil Phenolic Antioxidants in Oil or Water Phase during the Oxidation of O/W Emulsions: An Oxidomics Approach

Omics approaches are recently being applied also in food lipid oxidation, to increase knowledge of oxidation and antioxidation mechanisms. The so-called oxidomics throws a wider spot of light on the complex patterns of reactions taking place in food lipids, especially in dispersed systems. This research aimed to investigate the radical scavenging activity of olive oil phenolic antioxidants (OPAs) in O/W emulsions, as affected by the phase in which they were added. This allowed one to assess whether different behaviors could be expected from antioxidants originally present in phenolic-rich olive oils compared to natural antioxidants added in the water phase during emulsion production. Hydroperoxide decomposition kinetics and the analysis of volatile pattern provided an outline of antioxidation mechanisms. Though being effective in slowing down oxidation when added both in the oil and water phase, OPAs interfered in different ways with oxidation pathways, based on the phase in which they were added. OPAs added to the water phase were more effective in slowing down hydroperoxide decomposition due to the hydrophilic radical initiator. On the other hand, OPAs present in the oil were more effective in preventing radical propagation, with relevant consequences on the volatile pattern.

Evaluation of PBN spin-trapped radicals as early markers of lipid oxidation in mayonnaise

Quality deterioration of mayonnaise is caused by lipid oxidation, mediated by radical reactions. Assessment of radicals would enable early lipid oxidation assessment and generate mechanistic insights. To monitor short-lived lipid-radicals, N-tert-butyl-α-phenylnitrone (PBN), a spin-trap, is commonly used. In this study, the fate of PBN-adducts and their impact on lipid oxidation mechanisms in mayonnaise were investigated. The main signals detected by Electron Spin Resonance (ESR) were attributed to L-radicals attached to 2-methyl-2-nitrosopropane (MNP), one of three degradation products of the PBN-peroxy-adduct. The second degradation product, benzaldehyde, was detected with Nuclear Magnetic Resonance (¹H NMR), in line with MNP-L adduct generation. For the third class of degradation products, LO-radicals, their scission products were detected with ¹H NMR and indicated that LO-radicals have a major impact on downstream oxidation pathways. This precludes mechanistical studies in presence of PBN. Degradation products of PBN-adducts can, however, be used for early assessment of antioxidants efficacy in oil-in-water emulsions.