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Home , Food preservation, Water activity

Purdue extension FS-15-W

Food Entrepreneurship Series

Food Preservation Methods Department of are freshest and at optimum quality at the time of their harvest or slaughter. To maintain this ous ways to extend the period of time during which quality in foods that will be consumed later, the the food can be shipped, displayed in the store, foods can be preserved by cold, heat, chemical purchased by the consumer, and finally consumed. Katherine Clayton , or combinations of these methods. The physical and chemical composition of the food science Extension Cold usually means or freezing. food helps determine the type of process required outreach specialist Heating involves many processing methods, for preservation. Other factors that influence a such as , commercial sterilization, choice of preservation method are the desired end Deidre Bush product, type of packaging, cost, and distribution former Extension assistant and . Adding ingredients and processing by means of are also methods. Kevin Keener ways to preserve foods. food process engineer, The Role of and Extension specialist, and A food entrepreneur needs a basic understand- Acidity in Preservation professor of food science ing of the various preservation techniques before starting a business. The two most important chemical composition factors that affect how a food is preserved are converts harvested or raw water content and acidity. Water content includes foods into forms that are more easily stored and moisture level, but an even more important Department of Food Science consumed, and sometimes into a form that may measurement is water activity. Water activity (aw) 745 Mall Drive be more desirable. For example, wheat is pro- refers to the energy status of water in the food, West Lafayette, IN 47907 cessed into flour, which is used to make bread which affects whether or not chemical reactions www.foodsci.purdue.edu and pasta. can be processed into occur and/or will grow. The freeze-dried berries for use in cereals or cooked content of the food—such as , , , or to create preserves. starch—“binds” the water, making it less available. Foods can be generally classified as cereals, Foods with lower water activities are less prone , , dairy, and . Different to spoilage by microorganisms and have fewer food types are preserved and processed in vari- undesirable chemical changes occur during storage.

Water activity levels for growth of microorganisms

aw Microorganisms that grow at this water activity level 0.95 spp., Pseudomonas, Escherichia coli, , some

0.90 , Lactobacillus, monocytogenes, 0.87 Yeasts, 0.80 , Saccharomyces spp. 0.60 Some yeasts and molds FS-15-W Food Entrepreneurship Series • Methods Purdue extension

2 Water activity levels of Federal and state food regulations specify that a common foods shelf-stable product that does not require refrigera- tion or does not undergo sufficient heat treatment aw range Food must have a water activity equal to or below 0.85 or 0.95–0.99 Fresh , a natural pH equal to or below 4.6. Various changes 0.90–0.95 Bread can be made to product recipes in order to reach the 0.85–0.95 target water activity and/or pH level. For example, 0.80–0.91 Jams adding sugar or salt to a product can lower its water 0.75–0.90 , syrups activity. And adding —in the form of 0.60–0.90 Cakes, pastry or lemon can reduce pH. Reducing the water 0.60–0.75 Dried fruits activity below 0.85 or acidifying a food to reach a 0.20–0.35 Crackers pH level of 4.6 or lower will prevent the growth of harmful . Adding preservative compounds— The water activity of pure water is 1.0 (or 100% such as (to prevent bacteria and relative ). A dry cracker has a water activity growth) or (to prevent yeast of about 0.2, and jam has a water activity of about and growth)—can also aid in the preservation 0.85. A low level for water activity indicates less free process. water in the food. Neither Staphylococcus aureus nor any other can grow at a water activity level Preservation Techniques of 0.85 or below. Foods are often preserved with a combination of Acidity refers to pH, or level of hydrogen ions, formulation (added ingredients), processing (heating which is measured on a scale of 0–14. Products or cooling), and distribution methods (shelf stable, with a low pH (below 7.0) are acidic, and products refrigeration, freezing). When choosing the best with a high pH (above 7.0) are alkaline (basic). For method for preserving a product, food processors example, tomatoes have a pH range of 4.1–4.9, so must pay close attention to the pH and water activity they are acidic. Egg whites have a pH range of 7.6- while also considering how a preservation technique 9.6, so they are alkaline. A pH of 7.0 is considered will affect the quality of the finished product. neutral; water has a pH of 7.0. A pH level below 4.6 Preservation techniques commonly used for food inhibits the production of a deadly produced by processing are outlined below. More detailed de- Clostridium botulinum, which causes . scriptions of these commercial processing methods can be found in textbooks. pH levels for growth of microorganisms Refrigerating pH range of growth Potentially hazardous foods, or those foods with a pH greater than 4.6 and water activity greater than 5.5–8.0 Clostridium perfringens 0.85, must be kept below 40˚F. These foods include 4.9–9.3 Bacillus cereus cooked meat and poultry, and dairy products, 4.6–9.5 Escherichia coli eggs, products made with eggs, shellfish, and sea- 4.5–9.0 Salmonella spp. food. Foods that cannot be dried or canned or that 4.2–9.0 Clostridium botulinum need to maintain a fresh quality are also preserved 4.2–9.3 Staphylococcus aureus by refrigeration. Examples are perishable fruits and vegetables, meat and poultry, cheese, yogurt, fresh pH levels of common foods salsa, and soy milk. These products have a limited pH range Food because refrigeration only slows bacterial 7.1–7.9 Eggs growth; it does not prevent it. Mishandling during shipment and by the consumer can increase the rate 6.3–8.5 Milk of and should be accounted for in 5.3–5.8 Bread shelf-life determination. Additionally, these prod- 5.0–7.0 Meats ucts have a higher warehousing and shipping cost 4.8–7.3 Fish because of the need to maintain proper refrigera- 4.0–7.0 Vegetables tion. However, these products have high appeal to 3.3–7.1 Fruits consumers because they are fresh and convenient. 3.1–4.5 Berries FS-15-W Food Entrepreneurship Series • Food Preservation Methods Purdue extension

3 Freezing and color. This process is done to ensure Freezing can be used to preserve a number of food the treated food is safe for human consumption. products. Commercially is stored at -10˚F Pasteurization is most commonly used on liquids to 20˚F. Freezing halts bacterial growth, but does not such as milk and , with raw milk being the eliminate bacteria. If processed carefully, a frozen most common pasteurized food. High-temperature/ food product will maintain quality in color, texture, and short-time pasteurized milk is heated for 15 seconds flavor for a long shelf life. Frozen foods, such as large at 161˚F. Ultra-high-temperature pasteurized milk is cuts of meat that require thawing, are less convenient heated for 2 seconds at 280˚F. These different time/ than fresh foods. However, consumers perceive food temperature treatments for milk are equally effective items such as frozen meals and desserts as more at reducing harmful bacteria and many spoilage convenient than making them from scratch at home. microbes. In addition to making a product safer for In addition, frozen fruits and vegetables are perceived human consumption, pasteurization also increases as being fresher than canned. As with refrigeration, product shelf life. Most pasteurized products are commercially frozen foods have the disadvantage of stored under refrigeration conditions and are not higher storage and distribution costs in addition to the shelf stable. energy costs to initially freeze the food product. Thermal Processing (Low-Acid Drying (Traditional, Freeze-Dry, Spray-Dry) Canned Foods) Dehydrated foods have a long shelf life because Foods that are stored at room temperature and removal of moisture lowers the water activity to below sold in a sealed (metal, glass, or plastic 0.50 so that spoilage organisms cannot grow. Fruits laminate pouch) are heat processed to destroy and vegetables can be dried and sold as is, or they microorganisms that can either spoil the food or can be used in other dry products that have a long cause a health hazard. These thermally processed shelf life, such as cereal or granola bars. Traditional foods are called “commercially sterile”—or, more drying of foods utilizes heat, air, and time in various commonly, “shelf stable.” The time and temperature processes that allow removal of moisture to a desired needed to make a food shelf stable depends on level. Freeze-drying is a form of dehydration in which several factors, including the pH and physical nature the product is frozen and water is removed as vapor. Spray-drying is a method that rapidly dries a liquid slurry by spraying small droplets into a heated cham- ber. Milk that undergoes spray-drying is sold as pow- dered milk that can be reconstituted. The of moisture content by heat treatment in drying can be expensive, depending on the time required. Addition- ally, there is usually some loss of product quantity and quality associated with any drying method. Pasteurizing Pasteurization uses heat treatment for a short time to destroy harmful microorganisms that might be in a food without adversely affecting the food’s Pasteurization Methods Product Temperature Time 145˚F 30 min. Milk 161˚F 15 sec. 280˚F 2 sec. 155˚F 30 min. Juice When doing any type of thermal processing, it 180˚F 15 sec. is important to verify the final temperature of Egg in shell 130˚F 45 min. the product. Here a kitchen manager checks the temperature of a product being cooked in a steam- Note: Italics indicate most common pasteurization method for milk. jacketed kettle. FS-15-W Food Entrepreneurship Series • Food Preservation Methods Purdue extension

pH (all products have water 4 Product characterization FDA regulations Example activity greater than 0.85) natural pH equal to or less than Acidic lemons 4.6 formulated pH equal to or less 21 CFR 108.25 tomato salsa with green Acidified than 4.6 21 CFR 114 peppers & onions 21 CFR 108.35 Low-Acid pH greater than 4.6 canned green beans 21 CFR 113 Note: Some exceptions do exist in these regulations. Please refer to the Code of Federal Regulations for more information about these exceptions. of the food, type of container, and size of container. ally fermented food. Non-fermented products are For example, low-acid canned foods (those with a pH acidified by adding acid—for example, vinegar or

greater than 4.6 and water activity greater than 0.85) citric acid—to the product. In the past, the majority of need to be heated at a high temperature (240oF) to acidified foods were fermented in the home, but as ensure destruction of Clostridium botulinum. In most demand grew for a large, consistent food supply, the cases a pressurized cooker is required to reach such direct addition of acid has become more common. high temperatures. This allows food processors to increase quality, con- Before low-acid canned foods, the sistency, and manufacturing speed. Some common food processor must meet U.S. Food and Drug non-fermented products widely available today are Administration (FDA) regulations. See “Registering pickled beets and dill pickles (cucumbers that have Food Processes and Facilities with the U.S. Food been pickled. and Drug Administration” (below) for more informa- Depending on the microorganisms used in fermen- tion. tation (i.e., bacteria and/or yeast), carbohydrates Acidifying (Acidified Foods) are converted into , gas, or alcohol. Thus, fermentation is not solely used on low-acid fruits and Adding acid to a product with a initial pH greater than vegetables. Fermentation is also commonly used to 4.6 in order to reduce it below 4.6 is called acidifica- produce dairy products – yogurt and cheese – as tion. This results in an acidified food. Low-acid fruits well as and , but be aware that these prod- and vegetables (those with a pH greater than 4.6) are ucts are subject to different regulations than other often preserved by acidification. acidified foods. Before manufacturing acidified foods that are not refrigerated, the food processor must meet U.S. Food and Drug Administration (FDA) regulations. Cucumbers can be preserved See “Registering Food Processes and Facilities with by a fermentation or non- the U.S. Food and Drug Administration” (below) for fermentation process, both of more information. which acidify the cucumbers and prevent the growth of Registering Food Processes and harmful bacteria. Facilities with the U.S. Food and Drug Administration In order to begin processing low-acid canned foods or acidified foods, a food processor must Acidified fruits and vegetables (sometimes referred first register the processing location with the U.S. to as “pickled foods”) can be either fermented or Food and Drug Administration (FDA) and submit non-fermented. Fermented products are those with for review a Form 2541 showing the scheduled an initial pH greater than 4.6 that are placed in a process, or food-processing conditions. Prior to this (saltwater) solution in which bacteria (native or filing, it is recommended that the scheduled process added culture) convert carbohydrates to acid. This be reviewed by a process authority, which is an acidifies the product, making it shelf stable without individual or group of professionals recognized by refrigeration. is an example of a tradition- the FDA to have specific expertise in the methods used for food preservation. In addition, the processor FS-15-W Food Entrepreneurship Series • Food Preservation Methods Purdue extension

5 must attend an FDA-approved Better Process Control Additional Resources School. Garbutt, John. Essentials of Food . Scheduled processes may now be filed electronically, London: Hodder Headline Group, 1997. so the processor can track the status of their sub- Hurst, William C., A. Estes Reynolds, George mission. Filing of a scheduled process with FDA for Schuler, and P.T. Tybor. “Getting Started in the an acidified or low-acid product does not constitute Specialty Food Business” (Extension Bulletin approval by FDA. It is the responsibility of the proces- 1051). University of Georgia College of Agriculture sor to determine and assure that the process used and Environmental Sciences, 1997. adequately meets food-safety requirements before Smith, Durward, and Jayne E. Stratton. product use. For more information, visit this website: “Understanding GMPs for Sauces and Dressings” http://www.fda.gov/Food/FoodSafety/Product- (Extension Publication G1599). University of SpecificInformation/AcidifiedLow-AcidCannedFoods/ Nebraska-Lincoln Extension, 2006. EstablishmentRegistrationThermalProcessFiling/ Instructions/ucm2007436.htm References U.S. Food and Drug Administration Foodborne Pathogenic Microorganisms and Natural Handbook, www.fda. gov/food/foodsafety/foodborneillness/ foodborneillnessfoodbornepathogensnaturaltoxins/ badbugbook/default.htm Acidified and Low-Acid Canned Foods Guidance for , http://www.fda.gov/Food/ GuidanceComplianceRegulatoryInformation/ GuidanceDocuments/AcidifiedandLow- AcidCannedFoods/default.htm U.S. Department of Agriculture For more information, please refer to other Refrigeration and , www.fsis.usda.gov/ publications in the Food Entrepreneurship Fact_Sheets/Refrigeration_&_Food_Safety/index.asp Series: FS-14-W, Organic Foods FS-16-W, Regulations for Indiana Food Processing FS-17-W, Using an Approved Kitchen to Prepare Food for Sale FS-18-W, Using a Home Kitchen to Prepare Food for Sale All of these publications are available at the Purdue Extension Store, www.the-education-store.com.

Purdue Agriculture REV 3/12

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