Preserving emulsions with plant-based antioxidants
By Rebecca Guenard
- Food manufacturers are eager to develop functional foods by adding healthy polyunsaturated fatty acids to new products, but they must first develop a natural antioxidant capable of protecting the lipids in an emulsion.
- Researchers are using AI to help them more quickly identify protein peptides that can act as antioxidants.
- Synergistic phospholipids and phenolic compounds from plant extracts are other natural antioxidants researchers are exploring.
Saturated fats were once the preferred lipids for baked goods and other packaged foods. They are not easily oxidized, and their chemical stability means foods stay fresh longer. However, saturated fats have been linked to cardiovascular disease, diabetes, and certain cancers, and physicians recommend that we minimize our intake of saturated fats. Current dietary guidelines recommend substituting polyunsaturated fatty acids (PUFAs), particularly omega-3s.
Consequently, formulators are eager to incorporate omega-3s into more food products, especially since studies show that western consumers do not get enough seafood in their diets. But unlike saturated fats, PUFAs are easily oxidized. The accumulation of harmful oxidation by-products, such as aldehydes and ketones, deteriorate the foods’ taste, color, smell, or texture, leading to a shorter shelf life and increased waste. After decades of studying lipid oxidation, food chemists still face the challenge of protecting unsaturated lipids. With changing consumer preferences, the need for a solution grows particularly important.
“For many years, EDTA has been used as the preservative for emulsions,” says Eric Decker, food science professor at the University of Massachusetts in Amherst, Massachusetts. “EDTA works great, but everyone is trying to get away from it.”
Consumer demand for clean labels and a growing interest in functional foods are motivating experts to identify natural preservatives for emulsified systems to incorporate healthy lipids into everyday food products without increasing the risk of spoilage. A lipid emulsion represents a unique antioxidation challenge since the increased surface area of the droplets provides more sites to initiate decomposition. Researchers have made progress toward developing naturally sourced compounds that act as preservatives for emulsified lipids, but these compounds have physical and financial limitations. This article describes the latest approaches for preserving lipids in emulsified systems.
Lipid oxidation in emulsified systems
The fatty acid composition of an oil determines how susceptible the lipids are to oxidation. Lipids that are rich in polyunsaturated fatty acids, like omega-3s, are beneficial for health, but they are more prone to oxidation because of their unsaturated bonds. By preventing light, oxygen, or metals from initiating the reaction cascade that leads to the eventual breakdown of the fatty acid molecules, scientists can protect the vulnerable lipids from decomposition.
Lipid oxidation actually occurs faster in an emulsion than in a bulk oil. A lipid droplet surrounded by water-based solution, as in salad dressing and mayonnaise, is routinely under attack from dissolved oxygen and metals in the continuous phase. At the same time, hydroperoxides from inside the droplet also pose a threat. The trace amount of hydroperoxides that exists in the oils after refining migrate to the droplet interface and react to produce lipid radicals which initiate and propagate oxidation. This surface reactivity quickly leads to a breakdown of the lipids in the emulsion.
Scientists are eager to develop a means of protecting vulnerable PUFAs while still incorporating them into food. “Most people will agree the chemistry of oxidation of emulsions happens mostly at the water-oil droplet interface,” says Decker. “If you can change that interface in certain ways, you can slow down the oxidation.” A range of techniques have been attempted to engineer oil droplet surfaces and limit oxidation.
Casein and whey protein are popular oil-in-water emulsifiers due to their antioxidant properties, but they cannot be used in animal-free foods and researchers continue to search for plant-based options. (https://doi.org/10.1021/acs.jafc.8b02871). Lecithins are a complex mixture of polar lipids—such as phospholipids, glycolipids, and sphingolipids—and residual triacylglycerols produced as a by-product of oil refining. There is growing interest in using these polar lipids as emulsifiers given their natural source and antioxidant activity. Amphiphilic biopolymers extracted from botanicals are another natural ingredient used to stabilize oil-in-water emulsions. The size of these compounds allows room for both hydrophobic and hydrophilic regions that arrange at the interface and stabilize the mixture. Researchers are still searching for the right kind of molecules that will serve double duty to preserve the lipid by shielding it from oxidation.
Corn gluten meal, sorghum, potato rapeseed, and canola have all been reported as good sources of enzymatically hydrolyzed proteins that possess antioxidant activity. The proteins are reduced to smaller amino acid chains when cut at peptide bonds. The bond cleavage unfolds the protein, improving its solubility and emulsifying capacity. Since the resulting peptides are less prone to conformational folding, reactive groups are available for scavenging free radicals and chelating iron.
Statistical modelling has determined that the effectiveness of the peptides resulting from the enzymatic hydrolysis of proteins depends on multiple factors: protein source, hydrolysis conditions, enzyme type, and emulsion pH. The trial and error associated with finding the optimal condition for an appropriate protein can be costly and time consuming. A research group from the Technical University of Denmark in Kongens Lyngby, Denmark, is developing machine learning algorithms to understand and then predict patterns of antioxidant activity in a peptide, by evaluating its amino acid sequence (https://doi.org/10.1038/s41598-020-78319-w).
Using similar technology, the research group first identified antioxidant active peptide sequences in proteins found in potato juice. By combining proteomics and bioinformatics, the team could quickly reveal the peptides with the molecular functionality needed to serve as a preservative and an emulsifier (https://doi.org/10.1038/s41598-019-57229-6). The computer algorithm highlighted 40 peptides that were likely candidates. Of those, the group tested the most active examples in a fish oil-in-water emulsion to see how well they protected its fatty acids. After successfully identifying functional peptides (Fig. 1), particularly in a food-processing waste stream like potato juice, the research team developed the machine-learning software that will hunt down useful peptides in a myriad of untapped sources.
If the tool is successful, finding proteins to use as antioxidants in emulsified systems will get easier. However, thus far protein hydrolysates work best at a neutral to high pH, while emulsified foods are typically acidic. So, scientists continue to search for other options.
A perennial succulent plant native to tropical areas of the world and used in the traditional medicine of several different cultures has recently shown promise as a possible antioxidant for fish oil-in-water emulsions. The polyphenolic compounds contained in the plants are known to exhibit pharmacological activity, such as inhibiting tumor development, bacteria growth, and inflammation. Phenolic compounds are known to scavenge free radicals and chelate metal ions, in addition to participating in other types of antioxidant reactions.
Last year, Pascual García-Pérez, a recent doctoral recipient at the University of Vigo, in Vigo, Spain, along with a team of collaborators, developed a plant extract from the genus Bryophyllum to test its antioxidant capabilities in a fish oil-in-water emulsion (https://doi.org/10.3390/plants9081012). “The methodology is working quite well,” says García-Pérez.
Polyphenols are synthesized by plants as a result of secondary metabolism. Such compounds are not typically involved in the plant’s growth and often function as a way for the plant to defend itself against stress. As such, they do not exist in large concentrations in the plant. García-Pérez used a biotechnology technique, known as plant tissue culture, to grow Bryophyllum extracts obtained from the plant (Fig. 2).
After acquiring a sufficient amount of plant extract, the group analyzed its effectiveness for minimizing oxidation within the emulsion. Using a simple binary system of fish oil and water, they first determined that the phenolic compounds would partition as necessary between the two phases. García-Pérez says they were surprised by how well the extract dissolved in both phases, giving them confidence it would reside at the interface of an emulsion. After adding a surfactant and producing an emulsion, they found that the oxidative stability of the emulsion increased with greater Bryophyllum extract concentration. Finally, they evaluated a range of emulsion compositions, pHs, and temperatures to determine conditions where the extract worked most effectively.
“We saw that the application of the antioxidants caused a delay in the accumulation of conjugate dienes which are a by-product formed after omega-3 oxidation.” García-Pérez says.
García-Pérez acknowledges that although they are encouraging, the experiments his team performed are preliminary. The researchers plan to identify the particular polyphenols within the plant extract that act as antioxidants in the emulsion, as well as their distribution between lipid and aqueous regions. The team will also determine if there are any compounds in the extracts that create synergistic activity—in other words, compounds that restore an antioxidant’s reactivity after it has been depleted.
“It is essential to develop several strategies to prevent oxidation while also taking into account the current consumer demand on food naturalness,” says García-Pérez.
Another strategy under development is amplifying the lipid’s own natural protection system. The antioxidant compounds, tocopherols, occur naturally in vegetable oils. Their protective ability is limited, however, and the Decker group is investigating ways to extend it by combining the tocopherols with other antioxidants. The researchers have proved that phosphatidylethanolamine (PE) and phosphatidylserine (PS), phospholipids found in lecithin, regenerate tocopherol’s oxidizing potential (https://doi.org/10.1021/acs.jafc.8b00677).
Tocopherols are amphipathic molecules with hydrophobic side chains extending from aromatic rings. They exist in different forms depending on the number and position of methyl groups in the ring. A molecule’s form determines its surface activity and thus its effectiveness at acting as an antioxidant in an emulsion. Earlier research showed that the delta-tocopherol is more surface active than alpha-tocopherol. The team combined these two forms with the two phospholipids in different combinations and evaluated whether synergistic antioxidant activity resulted.
“We found that these phospholipids are effective when combined with tocopherols and can sometimes triple the shelf life compared to tocopherol alone,” says Decker. The formation of lipid oxidation products, hydroperoxide and hexanal, was delayed by nearly a week when the tocopherols were mixed with phosphatidylserine. Since it is more polar, the delta-tocopherol form combined with PS was especially effective at preserving lipids in an emulsion (Fig. 3), but alpha- and mixed tocopherols also worked well.
Though these results are encouraging, PE and PS are currently too expensive to use as food additives, and commercial lecithin contains a low quantity of the two phospholipids. The research team is working on enzymatically modifying lecithin to increase its PE and PS content. Preliminary experiments show that lecithin modified to have higher PE concentration can synergistically interact with naturally occurring tocopherols in refined oils and increase oxidative stability.
Cost continues to be the greatest challenge for developing natural antioxidants for a variety of food systems. Decker says he regularly preaches to companies that they should view natural preservatives in an emulsion as a set of hurdles. “If you add one barrier you get a certain amount of oxidative protection and when you add a second, distinct barrier the lipids get even more stable. And then you add a third mechanism and so on,” he says. Putting all those hurdles in place can increase the guarantee on freshness, but it comes at a cost. In addition, more additives are counter to consumer demand and complicate manufacturing the food at the plant.
Until researchers can find one natural preservative that can do the job of many, food manufactures will have to continue relying on EDTA. Decker says it is still the prominent preservative for most commercial emulsions. “It has no flavor. It is super cheap and super effective,” he says. “It is really hard to replace.”
About the Author
Rebecca Guenard is the associate editor of INFORM magazine at AOCS. She can be contacted at firstname.lastname@example.org.
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