Among preservatives, antimicrobials maintain food quality by mitigating microbial spoilage and enhancing the safety of foods. They further prevent the formation of toxic byproducts, such as aflatoxins, bacterial enterotoxins, and botulinum toxin by their bacteriostatic activities on microbes. Key microbes targeted include Aspergillus flavus and Aspergillus parasiticus molds; Escherichia coli, Staphylococcus aureus, and Clostridium botulinum bacteria.

Removing antimicrobials is not a commercially viable option for many food categories. In addition to shortening shelf life, this approach can lead to greater food loss across the supply chain, with a higher frequency of market withdrawals and food recalls. The potential damage to consumer confidence in the product or company would be irreparable, not to mention fraught with potential legal issues.

A more serious consequence of omitting antimicrobials and antifungal agents is the increased risk for Aspergillus and Penicillium fungi to produce mycotoxins in grain- and fruit-based foods. Mycotoxins can cause acute poisoning, cancer, liver disease, respiratory disease, and neural tube defects.

The battle against all of these bad bugs has gone botanical. Although in truth, only 5% of the 3,000 food additives permitted by the FDA are synthetic antimicrobials, naturally occurring antimicrobials from botanicals are gaining ground with food processors. The trend is being driven by consumers who want environmentally friendly processing, without allergens or potential carcinogens. 

While natural and botanical methods of preservation have been in use for centuries, the modern food processing system has relied heavily on synthetic food preservatives, such as nitrates, benzoates, sulfites, sorbates, and others. Once classified as safe for consumption when used at specified levels, these are now heavily scrutinized because of research on specific exposure conditions that raises concerns about their safety.

In addition to making foods and beverages safe, there also is a distinct financial advantage to food preservation. Microbial food spoilage is responsible for about 25% of food losses. Lately, food waste has become a significant concern among consumers as experts cite that as much as 40% of food grown and manufactured ends up being tossed. Food waste also is exacerbated by greater use and consumption of fresh, minimally processed, and ready-to-eat foods. In turn, this further increases the incidence of contamination by spoilage and pathogenic microorganisms.

Natural Advantage

To meet these myriad challenges, food and beverage makers have zeroed in on natural ingredients that can be readily used to delay color and flavor loss while extending shelf-life. For snack bars, cookies, chips, nuts, dressings, and sauces, common ones include citric acid (often from acerola, grapefruit, and lemons); ascorbic acid, a.k.a. vitamin C; and vinegar. Botanicals also are increasingly being recognized for their antimicrobial benefits. More food companies are using extracts derived from rosemary, celery, and tart cherries, thereby raising awareness of more effective alternatives from the plant kingdom.

Seeds, fruit, peels, leaves, and roots of plants are viable sources of powerful antioxidative and antimicrobial phenolic compounds. This is a class of innate phytochemicals that includes phenols, phenolic acids, anthocyanins, flavonoids, quinones, and tannins.

These compounds are distinguished by a benzoic ring in their chemical structure. Other phenolic compounds, such as chalcones, benzophenones, xanthones, stilbenes, and benzoquinones—all of which extractable from leaves, roots, fruits, bark and seeds—are effective against gram-positive bacteria like S. aureus and S. epidermidis.

Some of these botanically derived antimicrobials are particularly powerful. Phenolics extracted from Hibiscus acetosella (cranberry hibiscus) are effective against the growth of E. coli O157:H7, S. aureus, and even methicillin-resistant S. aureus (MRSA). It’s used commercially in milk and as a surface wash in hot dog applications against microbes such as L. monocytogenes.

Other types of botanical extracts that have potent antimicrobial activities are essential oils, terpenoids, glucosinolates, alkaloids, and thiols. All of these have a generally recognized as safe (GRAS) status in the US and have been granted the qualified presumption of safety (QPS) status in the EU.

Mix and Match

Botanical antimicrobials are not, however, “one size fits all.” Effective control of spoilage and foodborne pathogens requires matching the bioactives to the type of food product and the microbe targeted. The diversity of structures among botanical bioactives offers more than 12,000 known alkaloids, more than 10,000 phenolic compounds, and more than 25,000 different terpenoids. This makes selection and application both easy and difficult at the same time.

Botanical antimicrobials, depending on the presence or absence of nitrogen in their chemical composition, are water-soluble compounds. Examples are alkaloids and glucosinolates. There also are lipophilic compounds, such as terpenoids and phenolics, which are devoid of nitrogen.

The antibacterial action of phenolic acids is enhanced by using their ester derivatives, which act against gram-positive and gram-negative bacteria, including spore-forming bacteria, and can inhibit the production of microbial toxins and biofilm formation.

Roots and Berries

Alkaloids and thiols have potent antimicrobial functionality. Berberine, an isoquinoline alkaloid extract of barberry plants, is effective against E. coli and S. aureus, and Aspergillus species. It is applied to biodegradable polymers such as polylactic acid, chitosan, starch, and cellulose and qualifies them as so-called “green packaging.”

Thiols such as allicin and its derivatives derived from garlic are effective against spoilage from yeasts, gram-positive and gram-negative foodborne pathogens, and Campylobacter jejuni. It has proven highly effective on chicken at a concentration of just 50mg/kg. Their sulfoxide content inhibits diverse microorganisms, as do horseradish, mustard seeds, and wasabi by virtue of their allyl glucosinolates.

Gallic acid, ferulic acid, rosmarinic acid, and p-coumaric have strong antimicrobial action at 1-2 ppm level applications. Gallic acid and ferulic acid are effective against E. coli, S. aureus, and L. monocytogenes; while ferulic acid and p-coumaric acid also work against Salmonella enteritidis. P-coumaric acid used at 0.5 ppm levels can control the growth of Alicyclobacillus acidoterrestris in apple juice.

In plant-based foods, citral, hexanal, and 2-(E)-hexenal from oregano (Origanum vulgare L.) and thyme (Thymus vulgaris) deter the growth of yeasts responsible for spoilage of fresh-cut fruits, soft drinks, and fruit-based salads.

Protecting Protein

In fish products, extracts from the seeds of the common grape (Vitis vinifera L.), polyphenols from tea (Camellia sinensis), essential oil from thyme, and extract of rosemary (Salvia rosmarinus) all can help delay the growth of lactic acid bacteria, Enterobacteriaceae, hydrogen sulfide-producing bacteria (HSPB), and low- and warm-temperature bacteria (psychrotrophics) that produce off-flavors.

In meat products like sausages and hot dogs, tannic acid, catechin, and ellagic acids inhibit spoilage bacteria during cold storage, while ethanol extracts of rosemary and clove (Syzygium aromaticum L.) reduce spoilage bacterial counts in raw chicken.

In plant-based analog foods, the choice is more complicated because it requires matching the botanical extract with the mode of failure. This is something the industry is still trying to sort out. The challenge is that it depends not only on the source of the ingredients but how they were processed, their storage and usage conditions, and the use of the finished products.

Replacing synthetic preservatives with botanical antimicrobials can allay consumer concerns of safety. However, these ingredients do possess limitations that hinder their widespread application within and across categories of foods. For example, sensory off-notes are the drawbacks of bioprotective cultures of lactic acid bacteria used to prevent fungal infections in cereal-based products and bacterial spoilage of cooked hams. Others can impart flavors that, while not necessarily unpleasant would be better suited to savory applications.

The cost of natural antimicrobials can be of concern to mainstream processors accustomed to the lower cost of synthetics. Ingredient technologists have come up with solutions for some of these challenges. A popular example is replacing the flavorful rosemary extract with rosmarinic acid, its key bioactive, which is odorless and flavorless. The consistency and low cost without the issue of color, flavor, or availability makes its use easier for makers of fruit beverages.

Inner Workings

The application of botanical antimicrobials requires a fundamental knowledge of the bioactive constituents in the source and its extract to understand potential interactions with other ingredients in the formulation. For example, crude essential oils (aromatic and volatile liquids extracted from botanicals) have greater antimicrobial activity compared to blends of their major components, suggesting trace components in crude oils have a role with a potentially synergistic effect.

Despite the potential for synergistic combinations of plant extracts, synergies are not always achieved. Identifying the right combination is essential. Of the commonly used essential oils of spices and herbs, only the combination of coriander oil (Coriandrum sativum) and cumin oil (Cuminum cyminum) has notable and significant synergistic interactions in terms of antibacterial activity. This further underscores the complexity and variability in the synergistic potential of plant extract combinations.

Antagonistic interactions are also possible: for example, pure carvacrol, the phenolic monoterpenoid extracted from oils of oregano, thyme, and bergamot, is a significantly more potent antifungal than crude essential oils. Confounding this complicated matter is the variance in the constituents due to differences in growing, harvesting, and extraction conditions.

Despite the potent action of botanical antimicrobials, their widespread acceptance is hampered by chemical instability, limited dispersibility in food matrices, limited availability of ready-to-use commercial preparations, and sometimes, the possibility of unacceptable flavor issues.

These challenges may be overcome by stabilization techniques such as nano-emulsions, encapsulation, inclusion in active packaging for enhanced activity of the bioactive compounds, and controlled release during storage. Synergistic mixtures of antimicrobials and the application of hurdle technologies offer protection from simultaneous or successive use of preservation techniques.

Fortunately, fruit and vegetable wastes, such as peels, seeds, pulp, and unprocessed residues from food processing, are rich sources of antioxidants and organic acids and offer a sustainability narrative to the user.

Fraction Extraction

Novel extraction methods such as ultrasound-assisted extraction, microwave-assisted extraction, pressurized liquid extraction, and critical carbon dioxide extraction are efficient in extracting valuable biomolecules from food waste without the concern of hazardous materials in the supply chain. The use of tropical fruit and vegetable waste as a source of natural antimicrobials against foodborne pathogenic contamination appeals to consumers, as do the antimicrobials derived from grape juice and banana processing.

Extracts from peels of sweet orange (Citrus sinensis) and pomegranate (Punica granatum) have antimicrobial effects against Pseudomonas aeruginosa, Serratia marcescens, E. coli, S. aureus, Bacillus subtilis, Bacillus cereus, and Klebsiella pneumoniae. Limonene and punicalagin concentrations in citrus and pomegranates peels can prolong the shelf life of baked goods and add a mild fruity aroma to the product. Addition at 5% in ground beef and chicken applications protects the product without changing its taste. Black chokeberry (Aronia melanocarpa) extracts effectively inhibit S. aureus, E. coli, and Streptococcus pyogenes that cause spoilage during raw pork meatball production.

Dairy products have short shelf lives because of their susceptibility to contamination. Lemon peel extract (at a 4% level) effectively extends the shelf life of yogurts like labneh (a thick cheese-like yogurt) for up to 3 weeks without detection of B. cereus, Listeria monocytogenes, S. aureus, B. subtilis, E. coli, S. typhimurium, P. aeruginosa, C. albicans, and A. fumigatus and enhances the flavor of the product.

Pomegranate peel extract at 0.5-1% in the fruit added to yogurt and smoothies helps inhibit pathogenic bacteria and fungi such as L. monocytogenes, P. aeruginosa, K. pneumoniae, Aspergillus niger, and Candida glabrata. Avocado (Persea americana) peel extract works like synthetic antioxidants in beverage applications such as ready-to-drink teas, where the dark color of the extract is not of concern.

Paradoxically, fiber extracted from recycled mushroom stalks have an antimicrobial impact on a broad spectrum of spoilage microorganisms, including bacteria, yeasts, and mold. This positively charged fiber interacts with the negatively charged membranes of spoilage microorganisms to physically shear them and cause intracellular leakage, which kills the microorganisms. The soluble fiber extract is effective at application-dependent dosages in beverages, dairy, plant-based alternatives, sauces, spreads, and dips and over pH levels ranging from 2.0-6.5. It is a heat-stable product, surviving processes like pasteurization.

Educating the trade and consumers on the mode of action, safety and regulatory implications, and the environmental effects of production methods can pave the way for more effective, safe, and sustainable food preservation strategies in the future.

Kantha Shelke, PhD, CFS, is a senior lecturer at Johns Hopkins University and principal of Corvus Blue LLC, a Chicago-based food science and research firm specializing in industry competitive intelligence, expert witness services, and new product/technology development and commercialization of foods and food ingredients for health and wellness. Contact her at kantha@corvusblue.net.

Call in the Cavalry

One of the oldest methods of controlling for illness-causing microbes is to overwhelm those “bad guys” with an army of the “good guys.” “In food microbiology, we use a combination of refrigeration, generation of low pH through fermentation, and the inoculation of the beneficial bacteria to prevent growth of unwanted microbes,” explains dairy fermentation scientist Dana McCurdy, PhD. “We call this approach the ‘hurdle effect’. We might also add sweeteners such as honey—an antimicrobial in its own right—to balance sugar content and lower the water activity. Data from a recent study on contamination in kefir grains show that kefir will eventually counteract undesired microbes. The study is still pending publication, but preliminary results with non-pathogenic Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and others appear to support this effect.”

Key Statistics

The global food preservatives market surpassed USD3 billion in 2023 and is projected to be approximately USD4.64 billion by 2033 (CAGR of 4.50% from 2024 to 2033). The U. food preservatives market, valued at USD650 million in 2023, is projected to reach around USD1.03 billion by 2033 (CAGR of 4.70% from 2024 to 2033). The food preservatives market in the U.S. is driven by a growing demand for clean-label and natural preservatives. North America dominated the global market with 31% of market share in 2023. Conventional foods held the largest market share of 68% in 2023, but the fastest growth is in the clean-label segment. While meat and poultry products dominate the food preservatives market (32% of market share in 2023), the beverages segment is predicted to be the fastest growing segment during the forecast period.

Effective and Safe

Effective antimicrobial measures already have sharply improved prevention of one particularly dangerous microbe in foods, Listeria monocytogenes. Applications in ready-to-eat luncheon meats have, for example, been a major contributor to reducing incidents of the potentially fatal foodborne disease listeriosis by as much as 96%, according to a 2023 report by the USDA.

Fermentation Solutions

While meaningful advances in innovation have expanded the library of botanical antimicrobials for food and beverage manufacturers, switching to an expensive natural alternative might not be as easily effected by makers of value brands. Fermentation and enzyme technologies are helping to recreate nature-identical bioactives at a fraction of the cost.