Years ago, as the clean label movement began in earnest, a high-profile, heart-healthy potato chip with a claim of “natural” was launched. Impressed by media accolades, I sampled some chips. Although the product was still within its “best by” date, I noticed a rancid taste.
The product’s development, driven by enthusiasm and consumer demand, was one of likely many that avoided added antioxidants without taking shelf life challenges into account.
Since then, clean labels have been enabled, in part, by advances in food technology. Food processors are working toward improved label-friendly solutions to oxidation, and ingredient vendors strive to provide sophisticated antioxidant ingredients with consumer-friendly names.
Components taking part in antioxidant systems can be categorized as primary antioxidants, secondary antioxidants, chelators, quenchers, oxygen scavengers and antioxidant regenerators¹. These components interfere with various stages of oxidation reactions.
Some ingredients have abilities in more than one category, and ingredient combinations can act synergistically. For example, chelators that bind pro-oxidation metals such as iron and copper, whether synthetic or natural (e.g., EDTA, phytic, phosphoric and citric acids) are often combined with primary antioxidants.
The physical properties of a food matrix as well as where antioxidant molecules are located within that matrix have a great impact on oxidation. Foods and beverages differ in the amount and type of fat (e.g., degree of saturation); in their processing, such as exposure to high temperatures and air; in their pH; water activity (aw); and exposure to pro-oxidants such as metals and light, to name just a few factors.
Natural chemicals and overlooked antioxidants
Primary antioxidants function as free radical scavengers and reducing agents. These reducing agents "donate" or lose an electron to another chemical species, such as an unsaturated lipid in redox chemical reactions.
Synthetic sources of primary antioxidants include TBHQ, BHT, PG and BHA. Other primary antioxidants include tocopherols and ascorbic acid, which can be obtained from both natural and synthetic sources. Mixed tocopherols, rosemary, sage and green tea are examples of antioxidants from natural sources.
At a Clean Label Conference presentation, Fereidoon Shahidi, university research professor at Memorial University of Newfoundland, listed categories of antioxidants found in nature. They included phenolic acids, phenylpropanoids, tocols (tocopherols and tocotrienols), flavonoids, bisoflavones, coumarins, tannins (condensed and hydrolysable) and others such as carotenoids, phospholipids, amino acids and protein hydrolysates, vitamin C, and so on. Under some circumstances, certain antioxidants can also be pro-oxidants.
Well over 5,000 polyphenolic compounds have been identified in plants. They perform functions that include attracting pollinators, participating in a plant’s wound-healing and protecting against predators as well as their own oxidative stress, Shahidi noted.
While consumers may shy away from “chemical-sounding” names, commercially popular and consumer-friendly rosemary extracts contain key antioxidant chemicals including carnosic acid, carnosol and rosmarinic acid. These and other antioxidants have varying polarity, which enables them to be beneficial in a range of applications due in part to their affinity for lipid or aqueous phases.
Depending on a final product's required sensory profile and composition, formulated products can take advantage of oxidation-fighting components. Examples include those contributed by spices and herbs (e.g., flavonoids, phenolic acids, coumarins); oils and oilseeds (e.g., tocopherols, lignans, phospholipids, phenolic acids); cereals and grains (e.g., flavonoids, phenolic acid esters, lignans, sterols); and fruits and vegetables (e.g., ascorbic acids, carotenoids, hydrolylated carboxylic acids) among others.
At the 2017 Clean Label Conference, Eric Decker, a professor at the University of Massachusetts-Amherst, pointed to several overlooked antioxidants such as protein and reducing sugars.
Beta-lactoglobulin, a protein, is effective in very small amounts. If beta-lactoglobulin is hydrolyzed into peptides, the antioxidant activity increases dramatically. Some amino acids found in proteins and peptides can both chelate metals and scavenge free radicals. Hydrolyzing proteins exposes amino acids in the interior of the protein molecule to the greater food matrix and increases their efficiency.
Unfortunately, hydrolyzing proteins can produce bitter peptides. One way to decrease bitterness without decreasing antioxidant activity is to react peptides with glucose while heating, which creates Maillard products. Maillard products in themselves are not great antioxidants; the benefit of the process is that less bitter peptides that have not lost their antioxidant activity are created. One downside is that chelating activity is lost at less than a pH 5.0, Decker explained.
Aw has a great impact on oxidative stability. Oxidation rates may increase when a product has an aw either below or above an optimal value. Sugars can lower the aw of a product, such as in a health bar. However, monosaccharides are also reducing agents and, like ascorbic acid, can donate a proton to a free radical, thus stopping an oxidative process.
The monosaccharide glucose functions as a very effective antioxidant. Maltose, a reducing disaccharide, also works. It has the advantage of being less sweet than glucose, but roughly twice as much has to be added for the same antioxidant effect.
Customizing natural antioxidants
Plant extracts from teas, oregano, sage, melissa, rosemary and many others are processed by ingredient vendors for greater functionality in food products. Suppliers take steps like standardizing, deodorizing and decolorizing the original raw materials.
Oxidation of processed meats, bakery, beverages and other products produce undesirable results. However, the process occurs for somewhat different reasons. Since blending antioxidants can increase their efficacy, and since the properties of individual antioxidant molecules (e.g., lipid vs. aqueous solubility) vary, vendors take advantage of this by developing blends for specific applications.
For example, XtraBlend RN is a blend of spinach and rosemary extracts designed by Naturex. It is a natural alternative to the chelator EDTA and its primary antioxidants also scavenge free radicals. The ingredient is intended for use in emulsified sauces.
To help verify Xtrablend RN’s ability, a model mayonnaise system containing 70 percent rapeseed oil was stored at both room temperature and 40 degrees C. In this shelf life study, a marker for off-taste due to secondary oxidation, 2-4 heptadienal, occurred at a significantly lower level than the control and at a level comparable to an EDTA-containing mayonnaise. The Xtrablend RN-containing mayonnaise’s peroxide value, a measure of by-products of primary oxidation, was lower than either the control mayonnaise or the one with EDTA, the company says.
Naturex is familiar with a range of raw material sources and uses its expertise to create blends by selecting and varying the concentrations of various extract-based antioxidant components, says Catherine Bayard, Naturex’s global category director for food preservation. For just one example, the antioxidant carnosic acid found in rosemary extracts is a powerful active compounds that shows great protection against oxidation for a variety of meat products.
Designing optimal blends for an application is complex. Large oil-water interfaces that occur in emulsified oil-in-water sauces, dressings and soups and in the reverse water-in-oil emulsions in margarine and other lipid-based spreads generally increase risk of lipid oxidation. However, the optimal antioxidant blend for each may be different.
As formulators strive to “go natural,” food processors have numerous choices beyond the traditional synthetic options. Suppliers can offer more varied and creative antioxidants than in years past.
¹Oxidation in Foods and Beverages and Antioxidant Applications, Woodhead Publishing, 2010, Editors: Eric Decker Ryan Elias D. Julian McClements