Write a one page summary of one of the following newer antimicrobials: natamysin, nisin, microgard (Links to an external site.)Links to an external site. (TM), or sodium lactate
Write a one page summary of one of the following newer antimicrobials: natamysin, nisin, microgard (Links to an external site.)Links to an external site. (TM), or sodium lactate
Foods deteriorate in quality due to a wide range of reactions including some that are physical, some that are chemical, some enzymatic and some microbiological. The various forms of spoilage and food poisoning caused by microorganisms are preventable to a large degree by a number of preservation techniques, most of which act by preventing or slowing microbial growth. These include freezing, chilling, drying, curing, conserving, vacuum packing, modified atmosphere packing, acidifying, fermenting, and adding chemical preservatives. This is section 13, Chemical food protection. We’ll look at the other food preservation mechanisms in future modules.
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From the moment a food source is harvested in begins to deteriorate. It is estimated that 25% of the worlds food is lost to microbial decay annually. This equals more than a billion dollars per year.
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Salting as a means of preserving foods predates written history. The Mesopotamians were known 3000 B.C.E. generally used salt to preserve meat and fish. Early Roman writers such as Cato (234‐149 B.C.E.) clearly explained the need to salt perishable meats and vegetables to preserve them. We have already seen in past chapters that salt binds water reducing water activity and also is toxic to enzymatic and DNA processes in cells.
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Today, nearly all manufactured foods have different chemical preservatives because it makes financial sense. Food additives maintain or improve freshness, safety, nutritional value, taste, texture, or appearance. Consumers demand and enjoy a food supply that is flavorful, nutritious, safe, convenient, colorful and affordable.
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In the United States, neither the Food and Drug Administration (FDA) nor the U.S. Department of Agriculture (USDA) has labeling claim rules for “natural.” The FDA explicitly discourages the food industry from using the term. The Food, Drug, and Cosmetic Act prohibits labeling that is false or misleading, but does not give any specifics. The USDA’s Agricultural Marketing Service has a standard for organic food, but there is no legal definition for natural foods. Despite no legal U.S. definition for natural foods, there are numerous unofficial or informal definitions, none of which are applied uniformly to foods labeled “natural”.
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So, are food chemical additives safe?
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In 1958, the US created food additive legislation stating that the FDA must approve all food additives and ingredients that are not generally recognized as safe. The GRAS list contains food additives that have been used historically and have shown no hazards during that time. Easily recognized examples are sodium chloride, sucrose and acetic acid. Less easily recognized are many of the common food preservatives such as sodium benzoate, potassium sorbate, and calcium propionate.
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The U.S. FDA maintains a list of over 3000 approved, generally recognized as safe, food ingredients including food additives. The European Union uses a more international code called “E” numbers. E‐numbers are the code numbers that are used to identify food additives that have been shown to be safe, and which have been authorized for use in the EU. The E numbers 200 through 299 refer to those additives that are antimicrobial preservatives. Many countries that operate under the international standard called CODEX also use E numbers.
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Before we get started, let’s look a moment at the antimicrobial effects of chemicals. First there can be no effect at all. Growth of microorganisms proceeds normally. Second, a chemical may slow growth some. Third, a chemical may stop growth. In this case it is called bacteriostatic or fungistatic for bacteria and fungi respectively. Chemicals that are sporistatic prevent spores from germinating. The forth level is death. It is referred to as microbiocidal, bactericidal, fungicidal, or sporicidal.
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It is important to note that individual antimicrobial chemicals or other factors can make growth difficult for a microbe. In this case it is termed a hurdle. Basically, their growth or progress is slowed. If a hurdle is large enough, growth cannot occur and this is termed a barrier. Sometimes multiple hurdles can be created that prevent growth and then are considered a barrier.
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Let’s first look at the antifungal chemicals benzoate, sorbate, propionate, and parabens. They are primarily used in foods to prevent spoilage from yeast and molds. Their use generally increases quality (no spoilage) and shelf life. It is interesting to note that all of these chemicals are naturally produced in foods and are easily broken down in human metabolism. All are on the US FDA GRAS list, yet the “all natural – no preservative” folks ascribe many ailments to these compounds. However, if yeast were allowed to spoil a food they have the ability to produce some harsh chemicals naturally including: propanol, isopropanol, butanol, and amyl alcohol.
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The weak acid antimicrobial theory is based on the weak acid (benzoate, sorbate or propionate) in its undissociated or acid form is lipophilic and can easily pass through the cells membrane. Once inside it can disassociate back into an anion and a proton hydronium ion. Disassociated acids and excess protons accumulate inside the cell reducing intracellular pH. As the intracellular pH lowers, cell metabolism slows and then stops. Since only undissociated acids pass through the lipid membrane, the pH of the food will determine the effect. The white box is a chart of the percent undissociated acid of benzoate at different pH levels. Resistance to weak organic acids is based on either an increase in cytoplasmic buffering or the cells ability to shed excess protons.
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Sodium benzoate was first preservative allowed by the FDA under GRAS. It inhibits yeasts and molds at usage level ~0.1%. It is most effective against yeast, less so for mold. It has little affect on bacteria. The antimicrobial activity of benzoates are related to its undissociated form. It is most undissociated at very low pH. It is 60% active at pH 4, but only 1.5% active pH 6. Therefore its highest usage is in acid
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foods. Its main antimicrobial effect is believed to be disrupting lipids in cell membranes. It is used at 0.1% and levels above that may result in a peppery flavor.
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Sorbate also works best undissociated and will disrupt cell membranes. However, its pKa is higher than benzoate. It will be effective below pH 6. Sorbate is most effective against molds and is permitted to 0.2%. Therefore it is common to combine both sorbate and benzoate in many acid foods. Sorbate has some incidental antibacterial effects, although these are minor. Sorbate
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may be used as a second hurdle to enhance other antimicrobials such as nitrite for Clostridia.
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Propionate, like sorbate is most effective against molds. Its activity is less against yeasts and there is little activity against bacteria. Propionates has the higher pKa than sorbate and is effective at pH levels below 6. It is most often used in breads, cakes, and cookies.
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Parabens are related to the benzoates. Like benzoates, they are believed to disrupt lipids and cell membranes. They have the highest pKa and will work at some of the highest pH levels in foods. However, they also have an off‐flavor making their use minimal.
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Let’s take a look at the antimicrobial fumigants sulfur, ethylene oxide and propylene oxide.
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Fumes from burning sulfur have been used since the ancient Egyptian and Roman times to sanitize fruits. Its been in continued use for more than 2000 years. When the SO2 gas combines with water it forms sulfurous acid (H2SO3) as the antimicrobial. When dry forms of sulfur are used, such as sulfite or bisulfite, they end up forming the same sulfurous acid. Sulfurous acid acts at the cell membrane and interacts with protein enzymes causing inactivation. Bacteria are most susceptible, while some yeasts and molds are slightly resistant. This fact explains its use in winemaking. Sulfur would more selectively kill off bacteria and leave the natural yeast to perform the wine fermentation. Sulfur works best applied on acid foods and therefore, it most often still applied to fruits. In the USA sulfur is not used as much anymore due to better general sanitation and the fact that sulfur residues can cause chemical sensitivity reactions in approximately 10% of humans.
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Ethylene and propylene oxides are very effective fumigant antimicrobials. They are microbiocidal, sporocidal, and fungicidal. The fungi are killed in one or more hours of contact time, while it takes longer to kill bacteria and spores. Ethylene oxide is commonly used in many foods and non‐food sterilizations for items that cannot withstand the heat of autoclaving.
The U.S. spice industry uses Ethylene oxide to eliminate pathogenic microbial contaminants such as Salmonella and E.coli in spices. When applied in a validated process, Ethylene oxide can be extremely effective in eliminating Salmonella and E. coli as well as reducing overall bacterial load, yeast and mold, coliforms, and other pathogens. Although exact numbers are difficult to determine, ASTA estimates that between 40% and 85% of spices in the U.S. are treated with Ethylene oxide each year. The main advantage of Ethylene oxide is that its use on spice generally has no significant impact on the appearance or flavor of the spice. The main concern over Ethylene oxide use is the production of ethylene glycol residues. Ethylene glycol is quite toxic. Due to that fact, propylene oxide has gained usage as an alternative. It is permitted for fruits, nuts, spices, and grains.
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Several hundred years ago it was discovered that sea salt would provide a pleasing red color to smoked and dried meats. Because of the color reaction it became more common to use. Later it was realized that meats smoked and dried using sea salt did not result in “sausage poisoning”. Eventually scientists figured out that sausage poisoning was caused by Clostridium botulinum and that the active chemical in sea salt was nitrate. Nitrate was microbially broken down into nitrite. Nitrite inhibits important iron containing enzymes in the Clostridia group that prevents cell germination from a spore. When nitrite is transformed into nitric oxide it than affects meat color. The maximum usage levels in meats are 500 ppm for nitrate and 200 ppm for nitrite.
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In the 1970’s it was discovered that nitrosamine compounds may lead to cancer. Scientists quickly looked at many foods for their presence. Bacon was determined to result in nitrosamines when cooked on high heat. In response the amount of nitrite in bacon was dropped to a maximum of 120 ppm. Some companies lower that further by using iron‐ binding or chelating compounds such as sodium ascorbate, EDTA, or polyphosphate to enhance the anti‐Clostridial activity of nitrite.
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The effects of organic acids on microbial growth inhibition have been discussed. Two specific organic acids have effective antimicrobial abilities against Listeria. These are lactates and diacetates. The primary mode of action is that they are transported into the cell where they interfere with the acid/pH stasis of the cell. First the cell must expend energy to try and remove them and second the accumulation begins to affect metabolism. At lower concentrations these organic acids are listeriostatic. As the concentration inside the cell rises they become listeriacidal. Lactates and diacetates are usually used in tandem in cured ready to eat meat products such as hot dogs. The usage levels are usually between zero and three percent for lactate and zero and 0.25 percent for diacetate.
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Naturally occurring substances such as salt, rosemary extract, sugar, vinegar, alcohol, hops, and diatomaceous earth are also used as traditional preservatives.
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As discussed previously many intrinsic factors can affect growth. These include pH, water activity, nutrient content. Sugar, salt, and other humectants can reduce the water activity inhibiting growth. Acids can reduce the pH reducing growth. Alcohol is another compound that inhibits bacteria and some yeasts. With all of the concerns over the health status of antimicrobial usage, manipulation of common food ingredients would definitely be an all “natural” means of preservation.
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Here are some of the more novel antimicrobials that are claimed as “natural”. These include eugenols a class of plant oils from clove, cinnamon, nutmeg, basil and bay leaf. Allicins are organic sulfur compounds that are antimicrobial in garlic. Natamycin is an antifungal agent produced during fermentation by Streptomyces natalensis. Bacteriocins are mostly small proteins produced by bacteria that inhibit other bacteria. Nisin is a 34 amino acid long protein produced by Lactococcus lactis. It has an unusual broad spectrum of activity. Nisin has been approved for use in some foods. The trick to get around this restriction is to use natural fermentates. These are glucose or dairy grown cultures of L. lactis spray dried. Fermentates are considered “fermented glucose” or “fermented milk” and not a preservative. Bacteriophage use as antimicrobials is a relative new approach. Nearly all species have specific bacteriophage that infect and kill them. This provides a safe and targeted approach to microbial control.
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