In recent years, there has been an increased consumer interest in “clean labels,” both in the U.S. and globally. However, there is no legal or regulatory definition of a “clean label.” Customers and stakeholders may associate clean labels with foods that are minimally processed, devoid of artificial flavors, artificial colors and synthetic additives, or free of unexpected allergens.[1] Stephanie Lynch, vice president of sales and marketing for International Dehydrated Foods, notes: “What the consumer does not want to see on the label determines whether it’s considered cleanly labeled or not.… Consumers are more interested in what goes into their bodies than ever before, so anything that sounds like it wasn’t provided by Mother Nature is scrutinized.… Despite the fact that, in many cases, scientific studies find no real detrimental effects, public opinion drives preference and, eventually, regulation.”[2]

Consumer Responses
A 2013 Food and Health Survey conducted by the International Food Information Council (IFIC) foundation revealed that 36 percent of consumers indicated that they purchase foods and beverages because they are advertised as “natural” on the label.[3] Moreover, 27 percent of consumers made their purchase decisions on whether the labeled product is “organic.”[3] While clean labels may be associated with natural, organic and nongenetically modified organism claims, the term does not necessarily equate to natural or organic. For example, compounds such as potassium bicarbonate are allowed by the National Organic Program, but it is not considered a clean-label item. Consumers expect foods with a clean label to be transparent about ingredients (free of synthetic chemicals and made with simple ingredients that they can recognize). Based on the 2013 IFIC survey,[3] chemicals are the number one factor that American consumers consider in terms of food safety (84%), followed by foodborne illness from bacteria (79%).

“[A clean label] relies on consumer knowledge and awareness of a vast array of ingredients, and understanding what they are, where they come from, how they are used, why they are used and what impact, if any, they have on human health,” says Matthew Incles, market intelligence manager at UK-based Leatherhead Food Research.[2] So while there are few scientific data to support the rationale of seeking a clean label and little agreement about what a clean label really means, the consumer’s interest is a real and growing phenomenon that is driving a large response by the food industry.

To meet consumer demand for clean-label foods or synthetic, additive-free foods without jeopardizing shelf life or sensory and nutritional quality, food manufacturers are searching for alternative ingredients to replace the currently used synthetic food additives that have long been implemented and demonstrated to be effective. Ingredients such as sweeteners, colorants, artificial flavors and preservatives are most commonly targeted for removal or substitution. Transition from traditional preservatives to natural preservatives has been a major focus for many food manufacturers. For example, potassium sorbate, a traditional chemical antifungal preservative commonly used in products such as juice products, salad dressings, cheeses and baked goods, is being removed or substituted with commercially available, natural antimicrobials due to consumer undesirability. Another example is sodium benzoate, an antimicrobial considered most effective against yeast and bacteria. It is commonly used for low-pH products such as salad dressings and fruit juices. Like potassium sorbate, it is also gradually being removed or substituted with commercially available, natural antimicrobials. Since the removal of functional ingredients viewed by consumers as unnatural is ongoing, how are the functions that these ingredients provided being replaced?

Natural Antimicrobials
As previously discussed, the increasing consumer demand for healthy, minimally processed food has resulted in mounting interest in removing some specific ingredients. However, the function of these ingredients in the food must still be addressed. In many cases, the food industry is looking toward more natural ingredients that will provide the same or similar function. For increased food preservation and prolonged shelf life, the exploration and application of natural antimicrobial compounds is rising.

Antimicrobials are chemical compounds that are used as a preservation method to inhibit or inactivate pathogenic or spoilage microorganisms; they can be naturally present or purposefully added to food matrix, food packaging and food processing environments.[4] Natural antimicrobials serve the same function but are produced naturally or isolated from natural sources, most commonly from animal, plant or microbial origins.

Antimicrobials derived from animal origins are primarily recovered from vertebrates. Most of these antimicrobials are polypeptides. Animal-derived antimicrobials, such as conalbumin, lactoferrin, lactoperoxidase and lysozyme, have been extensively studied and shown to be effective against various foodborne pathogens and spoilage microorganisms. Davidson et al.[4] reviewed scientific research data regarding these compounds with regard to their efficacy against spoilage and pathogenic organisms, their methods of evaluation and their application in various foods as well as the development of novel delivery systems and incorporation with other hurdles.

Certain microorganism-derived metabolites have also shown antimicrobial properties.[5] Bacteriocins are generally low-molecular-weight polypeptides produced by Gram-positive bacteria that can inhibit the growth of other similar or closely related microorganisms. Nisin, for example, is produced by certain strains of Lactococcus lactis and is more effective against Gram-positive bacteria than Gram-negative bacteria. Nisin was also approved in the U.S. in 1988 for inhibiting the growth of Clostridium botulinum spores and toxin formation in pasteurized cheese spreads (21 C.F.R. 184.1538). Natamycin, another bacteriocin produced by Streptomyces natalensis, was approved in the U.S. in 1982 to inhibit mold growth when applied to finished cheese products at levels not to exceed 20 mg/L (21 C.F.R. 172.155).

Plant components such as herbs and spices are commonly used in foods as flavoring and fragrance agents. The essential oil components of leaves, seeds, flowers and bulbs are secondary metabolites formed in plants. They are also known to have antimicrobial properties and play a role as a natural defense system in plants against phytopathogens and insects.[4,6] Cinnamic aldehyde, for instance, is the major antimicrobial component of cinnamon. Phenolic compounds eugenol, carvacrol and thymol are the major antimicrobial compounds found in cloves, oregano and thyme, respectively. These four essential oil components have been studied extensively and have shown great antimicrobial activity against a broad range of foodborne pathogens, such as Listeria monocytogenes, Salmonella and Staphylococcus aureus, as well as spoilage microorganisms.[7] Garlic and onion also contain compounds that express strong antimicrobial properties against many pathogens including Bacillus cereus, C. botulinum type A and Shigella.[4] Additionally, many essential oils from herbs and spices have acquired Generally Recognized As Safe (GRAS) status by the U.S. Food and Drug Administration.

Practice Recommendations Before Commercial Application of Natural Antimicrobials
When making the decision to develop or reformulate to achieve a clean label in lieu of using artificial antimicrobials, a number of challenges must be overcome. For existing products, the implications of such a change on consumer perception are critical, yet the manufacturer must still ensure efficacy of the new additive. Moreover, manufacturers must also address regulatory concerns (e.g., labeling requirements, usage limits, GRAS status) and balance them with consumer acceptance of the product and efficacy. One such regulatory consideration is microbial control. For example, the U.S. Department of Agriculture Food Safety Inspection Service (USDA-FSIS) allows specific product cooling rates based on an ingoing sodium nitrite concentration of 100 ppm.[8] However, FSIS has taken the position that “neither celery powder (whether in a pre-reduced form or with a bacterial nitrate-reducing culture) nor other natural sources of nitrite alone are approved for use in 9 CFR 424.21(c) as curing agents” and elaborates that “it would not be appropriate for establishments producing products containing natural sources of nitrite alone” to use stabilization procedures allowed for cured products under Appendix B guidelines.[9] Substituting an alternate, clean-label ingredient such as celery powder for sodium nitrite, therefore, would require inoculated pack studies to demonstrate compliance with stabilization performance standards for preventing the growth of spore-forming bacteria. With so many factors contributing to the ability to market a product with a clean label, one can see why it is useful to devise a process for approving or rejecting natural alternatives in an efficient and cost-effective manner. Working with a reputable research services lab can help streamline this process and expedite your go-to-market process when working with natural antimicrobials.

A movement toward clean-label processes or ingredients may be perceived as positive by consumers, yet formulation and process changes can affect safety or quality and alter product characteristics, which consumers may perceive as inconsistency in the product. To expound on this, the sensory impact of many natural antimicrobial ingredients is often a significant barrier to entry into the clean-label arena. For example, some aromatic compounds such as rosemary extract or essential oil of oregano often have a negative sensory impact on neutral or sweet products but may pair well with savory sauces or meats. Cinnamic aldehyde, on the other hand, may have useful application only in sweeter products that are complemented by the classic cinnamon flavor and aroma.

With these concerns in mind, it is important for a manufacturer to consider the effects of a clean label on the sensory quality of a product and to conduct product testing tailored to the manufacturer’s specific objectives. For example, when launching a new product, a manufacturer may be interested in testing product quality based on consumer acceptability (also known as a “liking/hedonic test”). Alternatively, a reformulation of an existing product or an attempt to simulate a competitor product may warrant a “difference from control” analysis (also known as a “triangle test”) or comparative (A/B) testing. Consumers may be asked to simply rate how similar products are or to state a preference, depending on the manufacturer’s objectives.

The importance of addressing consumer acceptability is immeasurable, but an evaluation of ingredient efficacy must also be investigated. Depending on the intended purpose of an antimicrobial ingredient, efficacy against spoilage or pathogenic organisms may be desired. During product development, full-scale evaluation of one or multiple ineffective alternatives can quickly add a significant cost to the process of manufacturing a clean-label product, and for this reason, it is considered a best practice to screen candidate ingredients for efficacy. Ideally, in vitro assays would serve as an inexpensive means of establishing eligibility of ingredients based on efficacy against spoilage organisms or pathogens the manufacturer wishes to control. Ingredients may be tested to establish the minimum inhibitory concentration or minimum lethal concentration of the compound against target microorganisms,[4,10] or efficacy testing may be conducted to measure logarithmic reduction over time. Where standards exist for quantification of an antimicrobial ingredient, as with the common GRAS antimicrobial nisin,[11] laboratory testing to verify the potency of an antimicrobial may also be desired. These screening processes will allow the manufacturer to eliminate poor candidates early, before investing valuable resources into testing multiple variables with finished product.

While it serves as an important and cost-effective screening tool, in vitro testing cannot be assumed to replace testing of an antimicrobial substance in the finished product. The complexity of the product, and factors such as pH, water activity, concentration of lipids and proteins, or product homogeneity, can all vastly affect the performance of an antimicrobial ingredient.[4,10] It is therefore of paramount importance to validate the efficacy of an antimicrobial or a preservative under expected application conditions. Efficacy of an antimicrobial may be determined in a laboratory setting with challenge studies, wherein the laboratory inoculates the product of interest with the target microorganism(s) at predetermined levels, stores the product under desired storage conditions and monitors microbial response in the product over shelf life. If several candidates are still under consideration for use after screening via in vitro assays, a small-scale exploratory challenge study may assist in limiting the number of candidate materials or establishing specific combinations or concentrations of antimicrobial ingredients that are effective and maintain desirable product quality. The purpose of such a study is not to fully validate the performance and acceptability of each ingredient but rather to create an opportunity to eliminate poor candidates and select the best candidates for further testing in the finished product. Inclusion of this intermediate step often results in an overall cost savings, as it precludes large-scale testing of ineffective antimicrobials. At this stage, the cost and scalability of candidate materials might also be assessed. Those products that are ineffective, too costly to justify usage or do not scale well to full production should be eliminated from consideration.

Once selection of one or more viable antimicrobials or preservatives has been made based on acceptance criteria including cost, sensory impact and preliminary efficacy testing, a confirmatory test evaluating the true performance of the selected ingredient(s) in the finished product will be necessary. This is often conducted in the form of microbiological challenge studies or process validations, and shelf-life testing. The National Advisory Committee on Microbiological Criteria for Foods (NACMCF) has established recommended criteria for conducting microbiological challenge studies.[12] Studies can be designed based on these guidelines and adapted as necessary to suit the testing objectives. Regulatory considerations at this stage are also critical, as testing the product with antimicrobial concentrations above regulatory limits is not beneficial. More extensive quality shelf-life testing is also an advised aspect of confirmatory testing and may incorporate monitoring of microbial quality, sensory quality, physical changes and/or chemical changes during and beyond the anticipated shelf life of the product. Appropriate quality indicators are product- and packaging-specific, and the study design should reflect this. Testing under expected or advised storage conditions is important, but since consumer handling cannot be controlled, it is also prudent to test the product under abuse conditions (e.g., elevated temperature).

Summary
Consumer awareness of food ingredients and the desire for simple, natural foods have pushed food manufacturers to develop products with clean labels. While the term “clean label” is very broad in scope, one key element that consumers may focus on is the usage of alternatives to synthetic or “chemical-sounding” antimicrobial ingredients. This may include antimicrobial ingredients derived from animal-, plant- or microbial-based sources. Natural alternatives to negatively perceived additives, however, may not be as effective as traditional methods for preserving the safety and quality of foods, and extensive testing is often necessary to demonstrate the applicability and efficacy of such alternatives. It is therefore critical to consider the implications of developing a clean-label product, taking into account the effects such a change may have on sensory quality and microbiological control, while also maintaining regulatory compliance.

Initially, several natural antimicrobials may be considered plausible candidates for reformulation, but extensive testing of each separate ingredient can quickly become prohibitively expensive. It is therefore advised that manufacturers screen materials in several phases. Early indications of undesirable sensory attributes such as aroma or flavor can quickly eliminate some candidates, and in vitro efficacy testing against target microorganisms can also result in early elimination of poor contenders or help establish the most appropriate concentration(s) for use. The best candidates from these early phases may be further screened with small-scale exploratory testing in the laboratory. Those antimicrobials that are selected as viable options based on cost, scalability, sensory impact and efficacy through these screening processes would finally be subjected to more extensive testing, including challenge studies, process validations and quality shelf-life testing, which may be used to monitor chemical or physical changes in the product as well as sensory and microbial quality over the shelf life of the product. The final testing stage can be quite extensive, so failure at this phase is far more impactful in time and money than it is at earlier stages in this process. Early elimination of poor candidates is therefore beneficial to the manufacturer, and working with a reputable research services laboratory can make your product innovation project run more smoothly, allowing you to meet consumer demands for a cleanly labeled product.   

Wei Chen is a research project leader at Mérieux NutriSciences’ Food Science Center in Crete, IL.

Heather Hart is the research microbiology manager at Mérieux NutriSciences’ Gainesville, FL, laboratory, where she designs and oversees process validation, antimicrobial efficacy and microbial challenge studies. Both can be reached at silliker@mxns.com.

References
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3. www.foodinsight.org/articles/2013-food-and-health-survey.
4. Davidson, PM, FJ Critzer and TM Taylor. 2013. “Naturally Occurring Antimicrobials for Minimally Processed Foods.” Annu Rev Food Sci Technol 4:163–190.
5. Juneja, VK, HP Dwivedi and X Yan. 2012. “Novel Natural Food Antimicrobials.” Annu Rev Food Sci Technol 3:381–403.
6. Bakkali, F et al. 2008. “Biological Effects of Essential Oils – A Review.” Food Chem Toxicol 46:446–475.
7. Burt, S. 2004. “Essential Oils: Their Antibacterial Properties and Potential Applications in Foods – A Review.” Int J Food Microbiol 94(3):223–253.
8. www.fsis.usda.gov/OPPDE/rdad/FRPubs/95-033F/95-033F_Appendix%20B.htm.
9. askfsis.custhelp.com/app/answers/detail/a_id/1775/~/use-of-celery-powder-and-other-natural-sources-of-nitrite-as-curing-agents.  
10. David, JRD, LR Steenson and PM Davidson. 2013. “Expectations and Applications of Natural Antimicrobials to Foods: A Guidance Document for Users, Suppliers, Research and Development, and Regulatory Agencies.” Food Prot Trends 33(4):238–247.
11. www.fao.org/3/a-i0971e.pdf.
12. NACMCF. 2010. “Supplement: Parameters for Determining Inoculated Pack/Challenge Study Protocols.” J Food Prot 73(1):140–202.