Opportunities and Limitations of Natural Antimicrobials

Originally Published: July 9, 2014
Last Updated: July 9, 2014
Oregano extract is an example of a plant-sourced natural antimicrobial.

July 9, 2014, Global Food Forums — The following is an excerpt from the Ingredion sponsored “2013 Clean Label Conference Report.”

2013 Clean Label Conference-Kathleen Glass-Antimicrobial Alternatives

Click to view PDF of chart.

The primary function of food antimicrobials is food safety; the secondary function is shelflife extension. “In order to be effective as an antimicrobial,” explained Kathleen Glass, Ph.D., Associate Director of the Food Research Institute at University of Wisconsin Madison, “several factors need consideration.”

“Concentration of active compounds, antimicrobial solubility, dissociation constant, food composition (e.g., fat, moisture, hydrophobic proteins, free iron, pH, salt, water activity),synergistic effects between antimicrobials, processing, cooling, and storage temperature and times all affect antimicrobial effectiveness,” Glass continued.

A key characteristic of antimicrobials is amphiphilcity. An amphiphilic antimicrobial is partially lipophilic, with ability to pass through cell membranes; and it is also partially hydrophilic and, thus, is soluble in the aqueous phase. Sodium chloride is a conventional antimicrobial that reduces available water. Others include organic acids and their salts, such as lactate, acetate, diacetate and antimycotics (both acid and salt forms), like sorbate, benzoate and propionate. Nitrite, phosphates and some antioxidants are also included.

To be considered a “natural antimicrobial,” it is generally understood that the compound must be naturally occurring or directly extracted using simple methods, chemical reactions or naturally occurring biological process. No petrochemicals or genetic engineering can be used, explained Glass. No processing could be used that would not be done in a home kitchen. Antimicrobials from natural sources include microbial, plant or animal sourced compounds. Microbial sources include fermentation byproducts,
like organic acids and other primary metabolites, such as bacteriocins like nisin; competitive cultures, bacteriophages and natamycin (pimaricin); and minerals
and gases, like sodium chloride and 100% CO2 or CO. Plant sources include spices, extracts, essential oils, oleoresins, natural wood-smoke components, natural nitrate or nitrite and fatty acids. Animal sources include lysozyme, chitosan, lactoferrin and milk lactoperoxidase.

Fermentates are commercially available, proprietary ingredients that are derived from culturing sugar or milk and spray-dried. Often, they are blends of organic acids like lactic, propionic and acetic. These may or may not contain bacteriocin activity, and their byproducts depend on what starter cultures are used (for example, Propionibacterium,
Lactococcus, Pediococcus, etc.). The substrate and controls, such as temperature, oxygen and nutrient availability, also help determine the fermentation byproducts.

Organic acids in their undissociated form enter the cell, lowering its internal pH, denature proteins, disrupt proton motive force, inhibit membrane transport and starve cells.

Chelating metal ions can cause sub-lethal injury to pathogens and enhance efficacy of other antimicrobials. Organic acids and salts have optimized efficacy with
lower pH values (<5.5, near pKa) and lower temperatures (4 vs. 7 or 10°C)—except when the pH is <4.6; then, combined stress with higher temperatures increases inactivation rate. Combining with other antimicrobials also optimizes efficacy.

Bacteriocins are polypeptides that inhibit other closely related species. They are the byproducts of lactic acid bacteria fermentation, such as nisin, pediocin and reuterin.
Active against Gram-positive bacteria, they bind to receptors, which affects pore formation, causing leakage of molecules and cell death of pathogens. Bacteriocins
are bacteriocidal but have some disadvantages. Bacteriocins may be inactivated by proteolytic enzymes in raw foods, and some microbes have developed resistance.

Additionally, they are less effective in high-fat foods, and they also may inhibit beneficial competitive microflora. Bacteriocins work best in low-fat foods, with pH <6, and in combination with other antimicrobials.

Plant extracts, spices and glycerides used as antimicrobials are native compounds that protect the plant. They can be extracted with water or ethanol and concentrated. Common plant extracts used in foods that provide flavor and antimicrobial activity include
cinnamon, thyme, mustard, cloves and oregano. Antioxidants commonly used in foods that also provide antimicrobial activity include dried plum, rosemary, tocopherol, (vitamin E) and ascorbate (vitamin C).

Disadvantages of plant extracts include variability due to variety, extraction methods and agricultural practices. They can also partition into the fat phase, which tends to make them less effective and also may impart strong odor, flavor or color. There may be unknown toxicological effects at higher concentrations. Activity may also decrease after heating
some extracts.

Clean label antimicrobials can be applied to a wide variety of foods. Typically, they are ingredients familiar to consumers, yet they can enhance the safety of foods. Optimization
of ingredients can reduce usage levels, improve sensory attributes and be cost-effective.

CLC-Glass chart-online-pdf

Kathleen Glass, Ph.D., Associate Director, Food Research
Institute, University of Wisconsin-Madison.
http://fri.wisc.edu/, kglass@wisc.edu