Are you trying to produce a shelf-stable, carbonated soft drink, alcoholic seltzer, or cannabis beverage? Here’s something to consider. Microbial spoilage of shelf-stable beverages is an important concern. Beverages have varying degrees of sensitivity to microbiological spoilage depending on intrinsic factors of the beverage such as pH, nutrient content (e.g., juice, vitamin, or micronutrient content), carbonation level, Brix, water quality (e.g., alkalinity and/or hardness), and preservatives. Spoilage events occur when microorganisms are able to overcome the beverage’s intrinsic factors and grow. The microorganisms’ ability to-overcome these hurdles can be influenced by, among other things, initial contamination level, temperature, and package integrity of the beverage against carbonation loss. This article will help you decided on the best technique for eliminating the risk of microbial spoilage and achieving shelf stability for many months.
Spoilage of Shelf-Stable Carbonated Beverages
Microbiological spoilage can result from one or more yeasts, bacteria, and/or mold microorganism. Yeasts and bacteria are capable of spoiling carbonated and non-carbonated beverages such as fruit drinks, teas, coffees, enhanced waters, etc. The ability of yeasts and certain bacteria to grow anaerobically enables their growth in carbonated beverages, while molds are restricted to aerobic metabolism, and therefore do not grow. Typically, spoilage by yeasts manifests itself as fermentation with gas and ethanol production, as well as sedimentation, off-flavors and odors, and loss of cloud or emulsion stability. Bacteria tend to produce off-flavors and odors with associated sedimentation. On the other hand, molds may survive but generally are not capable of growth in low oxygen environments and thus, do not spoil carbonated soft drinks except when carbonation is diminished. Yeasts such as Saccharomyces, Zygosaccharomyces, Candida, and Brettanomyces / Dekkera spp. are often responsible for spoilage incidents in common beverages, and acidophilic bacteria such as Lactobacillus, Leuconostoc, Gluconobacter, and Zymomonas spp. also occur.
Yeasts like Brettanomyces bruxellensis and Saccharomyces spp. can spoil cold-filled, carbonated beverages stored at ambient temperature or above, especially when beverage formulas contain nutrients from sweeteners and juice. In processing you have two choices; 1) either eliminate these spoilage microorganisms by heating (e.g., tunnel pasteurization), or 2) use preservatives to restrict their growth in the canned beverage. The most used, economical, and simplest way to prevent microbiological spoilage of such products is to formulate the products with the weak acid preservative, sodium benzoate and potassium sorbate. Natural preservatives such as cinnamic acid or glycolipids may be suitable substitutes, albeit with less efficacy than benzoate and sorbate. An expensive, but effective option that would mean no change to the ingredients text would be dosing each filled bottle with a compound called Dimethyl Dicarbonate (DMDC; Velcorin®) prior to sealing. DMDC inactivates microorganisms by destroying enzymes, and then the molecules dissipate into minute amounts of methanol and carbon dioxide. Don’t worry – the levels of methanol are trace amounts, and many flavor compounds used in beverages are dissolved in methanol at minute levels. Other than these antimicrobial options, cold-filled, carbonated juice beverage products can be pasteurized in-container in a tunnel pasteurizer. Time temperature requirements would depend upon container size, but there is a balance between achieving enough heat to kill spoilage microorganisms without destroying the flavor of the product or causing the can to rupture.
Processing or Preservation to Control Microorganisms
Protection against microbiological spoilage of beverages can be achieved using chemical preservatives and/or processing techniques such as hot filling, tunnel pasteurization, ultra-high temperature treatment (UHT) or pasteurization followed by aseptic packaging, and/or pasteurization followed by chilling the beverage. Generally, beverages with a pH<4.6 can be chemically preserved, heat processed, and filled into packages such that the product is not re-contaminated. For example, process techniques such as cold filling with chemical preservatives or pasteurization followed by cold-filling may be used to preserve this type of beverage. In a similar manner, this same beverage may be processed using non-preserved techniques such as hot filling, tunnel pasteurization, pasteurization followed by aseptic filling or even requiring the beverage to be chilled, i.e., under refrigeration following the pasteurization step. Beverages having a pH≧4.6 must be processed such that spores are destroyed using ultra-high temperatures followed by aseptic filling into packages or retorting sealed packages of product. Carbonated beverages cannot be UHT heated or hot filled, but certain beverage parts (i.e., syrups) can be pre-treated this way for later processing to beverage strength.
The most common preservation systems for acidic, shelf-stable carbonated and non-carbonated soft drinks rely on weak acid preservatives (e.g., benzoic and/or sorbic acid). Benzoic and sorbic acids (and salts thereof) effectively inhibit yeast, bacteria, and molds with only few exceptions. Weak acids in beverages exist in equilibrium between their dissociated and undissociated forms which is dependent upon the dissociation constant of the acid (pKa) and the beverage pH. The pKa for benzoic acid is 4.19 and the pKa of sorbic acid is 4.76. A beverage pH below the pKa of the particular acid pushes the equilibrium towards the undissociated form. The undissociated form is more efficacious against microorganisms, and therefore, weak acid preservatives are most effective in the low pH range. The preservation properties of weak acids may be enhanced by the addition of chelating compounds to the beverage. For example, common chelating compounds added to beverages include calcium disodium ethylenediaminetetraacetic acid (EDTA) or one or more of the polyphosphates, such as sodium hexametaphosphate (SHMP). In high nutrient non-carbonated products, such as those beverages containing juice, vitamins, and/or minerals, the weak acids are more likely to exert inhibition if used in conjunction with preservative enhancers.
Weak acid preservation systems, however, have limitations. Genetic adaptation and subsequent resistance by microorganisms may be one of the biggest concerns. Certain yeasts, such as Z. bailii, Z. bisporus, C. krusei, and S. cerevisiae, have specific genes that enable them to resist the weak acid preservatives and grow, despite their presence and regardless of the co-presence of EDTA or SHMP. Some bacteria, such as Gluconobacter spp., are also thought to be preservative resistant. The levels of weak acids necessary to overcome this resistance have been shown to be far beyond regulatory limits on use levels. Spoilage of preserved teas, juice-containing beverages, and carbonated beverages is commonly due to preservative-resistant yeasts.
Weak acids are also known to impart a throat or mouth burn when used at high levels. Although there are certain shelf-stable beverages where this may be acceptable, often this sensory perception is considered negative. In addition, non-government organizations and also some international government agencies have raised concerns regarding the use of weak acid preservatives in beverages and foods. Finally, consumers sometimes take note of ingredients and certain niche markets prefer preservative-free products.
Natural preservatives may be alternatives to traditional ways of preserving beverages. Preservatives that could be labeled as natural could also eliminate hot-fill requirements for unpreserved shelf-stable preservative-free beverages. Thus, it would be desirable to validate a natural preservative and/or a preservative system that inhibits growth of microorganisms.
Summary
Each of the options mentioned can result in a shelf-stable carbonated sodas either by killing yeast and bacteria at the point of production or by formulating with preservatives to inhibit yeast growth and bacteria growth in the product at ambient temperature. There are pros and cons for each option, and constraints of the beverage or manufacturing capabilities will often determine which options are most suitable. Some of the methods mentioned may be protected by patents, and so a thorough legal review (i.e., patent search) is recommended before implementation to avoid chances of patent infringement. Although options given below are likely to prevent yeast and bacterial growth, testing is recommended to verify shelf-stability. Challenge studies may be necessary to validate that a preservative system or processing strategy is indeed adequate to produce a shelf stable beverage.
References
FDA. 2001. Food additives permitted for direct addition to food for human consumption. Dimethyl dicarbonat. Fed. Rg. 66(45):13653.
Lawlor, Kathleen A., James D. Schuman, Peter G. Simpson, and Peter J. Taormina. “Microbiological spoilage of beverages.” In Compendium of the microbiological spoilage of foods and beverages, pp. 245-284. Springer New York, 2010.
Golden, D.A., R.W. Worobo, and C.S. Ough. 2005. Dimethyl Dicarbonate and Diethyl Dicarbonate.” In Antimicrobials in Food, pp. 305-326. CRC Press, Boca Raton, FL.
Stratford, M. et al., Fruit Juices, Fruit Drinks, and Soft Drinks, In The Microbiological Safety & Quality of Food (eds. B. M. Lund, T. C. Baird-Parker, and G. W. Gould, Aspen Publishers 2000).
Pitt, J.I. and A.D. Hocking. 2009. Yeasts. In Fungi and Food Spoilage, pp 357-382. Springer, New York.
Piper, P. et al., Weak Acid Adaptation: The Stress Response that Confers Yeasts with Resistance to Organic Acid Food Preservatives, 147 Microbiol. 2635-2642 (2001).
Dai, Yumei, Mark D. Normand, Jochen Weiss, and Micha Peleg. “Modeling the efficacy of triplet antimicrobial combinations: yeast suppression by lauric arginate, cinnamic acid, and sodium benzoate or potassium sorbate as a case study.” Journal of Food Protection® 73, no. 3 (2010): 515-523.
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