How to preserve food

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food preservation, any of a number of methods by which food is kept from spoilage after harvest or slaughter. Such practices date to prehistoric times. Among the oldest methods of preservation are drying, refrigeration, and fermentation. Modern methods include canning, pasteurization, freezing, irradiation, and the addition of chemicals. Advances in packaging materials have played an important role in modern food preservation.

Spoilage mechanisms

Food spoilage may be defined as any change that renders food unfit for human consumption. These changes may be caused by various factors, including contamination by microorganisms, infestation by insects, or degradation by endogenous enzymes (those present naturally in the food). In addition, physical and chemical changes, such as the tearing of plant or animal tissues or the oxidation of certain constituents of food, may promote food spoilage. Foods obtained from plant or animal sources begin to spoil soon after harvest or slaughter. The enzymes contained in the cells of plant and animal tissues may be released as a result of any mechanical damage inflicted during postharvest handling. These enzymes begin to break down the cellular material. The chemical reactions catalyzed by the enzymes result in the degradation of food quality, such as the development of off-flavours, the deterioration of texture, and the loss of nutrients. The typical microorganisms that cause food spoilage are bacteria (e.g., Lactobacillus), yeasts (e.g., Saccharomyces), and molds (e.g., Rhizopus).

Microbial contamination

Bacteria and fungi (yeasts and molds) are the principal types of microorganisms that cause food spoilage and food-borne illnesses. Foods may be contaminated by microorganisms at any time during harvest, storage, processing, distribution, handling, or preparation. The primary sources of microbial contamination are soil, air, animal feed, animal hides and intestines, plant surfaces, sewage, and food processing machinery or utensils.


Bacteria are unicellular organisms that have a simple internal structure compared with the cells of other organisms. The increase in the number of bacteria in a population is commonly referred to as bacterial growth by microbiologists. This growth is the result of the division of one bacterial cell into two identical bacterial cells, a process called binary fission. Under optimal growth conditions, a bacterial cell may divide approximately every 20 minutes. Thus, a single cell can produce almost 70 billion cells in 12 hours. The factors that influence the growth of bacteria include nutrient availability, moisture, pH, oxygen levels, and the presence or absence of inhibiting substances (e.g., antibiotics).

The nutritional requirements of most bacteria are chemical elements such as carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, magnesium, potassium, sodium, calcium, and iron. The bacteria obtain these elements by utilizing gases in the atmosphere and by metabolizing certain food constituents such as carbohydrates and proteins.

Temperature and pH play a significant role in controlling the growth rates of bacteria. Bacteria may be classified into three groups based on their temperature requirement for optimal growth: thermophiles (55–75 °C, or 130–170 °F), mesophiles (20–45 °C, or 70–115 °F), or psychrotrophs (10–20 °C, or 50–70 °F). In addition, most bacteria grow best in a neutral environment (pH equal to 7).

Bacteria also require a certain amount of available water for their growth. The availability of water is expressed as water activity and is defined by the ratio of the vapour pressure of water in the food to the vapour pressure of pure water at a specific temperature. Therefore, the water activity of any food product is always a value between 0 and 1, with 0 representing an absence of water and 1 representing pure water. Most bacteria do not grow in foods with a water activity below 0.91, although some halophilic bacteria (those able to tolerate high salt concentrations) can grow in foods with a water activity lower than 0.75. Growth may be controlled by lowering the water activity—either by adding solutes such as sugar, glycerol, and salt or by removing water through dehydration.

The oxygen requirements for optimal growth vary considerably for different bacteria. Some bacteria require the presence of free oxygen for growth and are called obligate aerobes, whereas other bacteria are poisoned by the presence of oxygen and are called obligate anaerobes. Facultative anaerobes are bacteria that can grow in both the presence or absence of oxygen. In addition to oxygen concentration, the oxygen reduction potential of the growth medium influences bacterial growth. The oxygen reduction potential is a relative measure of the oxidizing or reducing capacity of the growth medium.

When bacteria contaminate a food substrate, it takes some time before they start growing. This lag phase is the period when the bacteria are adjusting to the environment. Following the lag phase is the log phase, in which population grows in a logarithmic fashion. As the population grows, the bacteria consume available nutrients and produce waste products. When the nutrient supply is depleted, the growth rate enters a stationary phase in which the number of viable bacteria cells remains the same. During the stationary phase, the rate of bacterial cell growth is equal to the rate of bacterial cell death. When the rate of cell death becomes greater than the rate of cell growth, the population enters the decline phase.

A bacterial population is expressed either per gram or per square centimetre of surface area. Rarely does the total bacterial population exceed 10 10 cells per gram. A population of less than 10 6 cells per gram does not cause any noticeable spoilage except in raw milk. Populations of between 10 6 and 10 7 cells per gram cause spoilage in some foods; for example, they can generate off-odours in vacuum-packaged meats. Populations of between 10 7 and 10 8 cells per gram produce off-odours in meats and some vegetables. At levels above 5 × 10 7 cells per gram, most foods exhibit some form of spoilage.

More fruit and veg than you know what to do with? Turn it into jam, preserved lemons, pickles and apple chips. Here’s how.

Need to know

  • Preserving, pickling and dehydrating seasonal fruit and veg means you can buy it when it’s cheap and in abundance and eat it throughout the year
  • These processes are straightforward, so all you need are basic kitchen equipment and utensils – just follow our top tips
  • We include recipes for delicious jam, preserves, pickles and dehydrated fruit

Has your vegie patch produced a bumper crop of tomatoes? Did your green-fingered neighbour leave a basket of carrots and cucumbers on your doorstep? Or perhaps you bulk-bought berries or your favourite apples at the greengrocer or supermarket? After all, not only is seasonal fruit and veg fresher and tastier, but it’s often cheaper too.

If you’ve eaten your fill of fresh produce and still have some left over, why not turn it into jams or pickles, which you can tuck away in the pantry and continue to enjoy all year round – in season or not?

Fiona Mair, CHOICE home economist, guides you through some simple ways to preserve the fruit and veg you have in abundance.

How to make jam

Making jam is a great way to use up leftover berries, citrus or other fruits (and even chilli!). And, according to Fiona, it’s a fairly simple process.

“Essentially all you’re doing is boiling fruit and sugar together until the mixture reaches a deliciously thick and sticky consistency,” she says.

To ensure you get the best results every time, follow her top tips.

Tips for making the perfect jam

  • Use fresh, seasonal fruit that’s washed and dried well. Underripe fruit is preferable as it’s higher in acidity and pectin, giving a better (more viscous) set. Ripened fruit is still fine, but will give a softer set.
  • Soften your fruit first to draw out the pectin, before adding the sugar. Make sure the fruit skin is heated and softened, as once you add the sugar the skins won’t soften further. Of all the fruit skins, citrus peel will take the longest to soften (between one and two hours).
  • Adding a tablespoon of butter to every kilo of fruit will help prevent any scum forming in the jam.
  • After you’ve added the sugar, stir over a low heat until the sugar has dissolved, then stop stirring and bring it to a rolling boil. This is when you should start your timer.
  • To test if your jam has reached its setting point, put a teaspoon of jam onto a saucer that has been in the freezer. Push your finger through the jam to create a ‘channel’. If the jam wrinkles and the channel stays in place it’s ready. If not, continue cooking.
  • Always pour jam into clean, sterilised jars. To seal them, turn the jars upside down for a few minutes.
  • Make sure you label and date your jars and store them in a cool, dark, dry place such as a pantry. Store in the fridge once opened.

Mould on jam

Question: If my jam has mould on the surface, can I just scrape it off and keep eating it?

Answer: No. Many people will tell you they’ve always done this and have come to no harm. But moulds, and the toxins they release, can penetrate more deeply than the eye can see – particularly in liquid or semi liquid foods such as jam – and can be harmful if eaten. It’s safer just to chuck it.

Updated, easy techniques for preserving fruits and vegetables

by Susan Moeller, AARP, August 4, 2020

Yuliia Chyzhevska / Alamy Stock Photo

En español | Oklahoma City gardener Bryan Wright typically hauls in 2,000 pounds of vegetables — a ton — every season from his urban garden. That’s not even counting his apples, berries, plums and other fruits.

How to make that bounty last? He cans, freezes, ferments and dehydrates it.

Wright runs the Black Urban Gardening Society, a nonprofit that teaches gardening and food preservation, and he uses his haul to feed his family of six as well as those in the community.

“We grow this food … and we can’t eat it all right now, so we have to preserve it,” he says.

Easy, safe techniques

Most gardeners don’t go as big as Wright, but still find themselves inundated with more cucumbers, tomatoes, zucchinis and berries than they might be able to eat during the growing season.

Luckily there are easy food preservation techniques that are safe and tasty. Renewed interest in preservation is being driven by the local food movement and, after unpredictable pandemic shortages, people want more control over food sources, experts say.

Preservation also helps people without regular access to reasonably priced fresh food get the most out of what they have. In California, for example, the San Luis Obispo County branch of the University of California Cooperative Extension teaches low-income families how to use the oven to dehydrate fruit and other foods.

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"That was the original basis of food preservation, right? To extend the resources that families knew they weren’t going to have access to later in the year,” says Katherine Soule, assistant vice provost for UC extension and the head of the San Luis Obispo office.

Whether you want to put aside enough to last the winter or just process a few jars of jam, you need to understand the basics. For example, be wary of heirloom recipes or outdated advice, experts say.

"People think they know how to preserve but a lot has changed in the last 10, 20 years,” says Noreen Goff, a master food preserver with UC extension in Amador and Calaveras counties. Master food preservers are extension volunteers trained as educators.

Two dependable preservation sources are county extension services, which are backed by university research centers, and the National Center for Home Food Preservation (NCHFP), supported by the Cooperative State Research, Education and Extension Service and the U.S. Department of Agriculture. Nonprofit organizations like the Black Urban Gardening Society or Tilth Alliance in Seattle, Washington, also offer food preservation classes.

Most importantly, only preserve what you like and will actually eat. Otherwise, it’s just a waste of time and resources, experts say.

Wright uses a fermentation process with a natural starter to make natural fruit sodas for his kids, among other foods. In St. Clair County, Alabama, Philip Meade, 51, who shares a 1/8-acre garden with his housemate, makes pickled corn, a sauerkraut-style dish that he learned from his mother. He puts sweet corn in a 5-gallon bucket in a vinegar and salt brine, covers it loosely and waits. It takes about a month, he says, to get “a real good pickle.”

If you want to begin preserving food, here are some basic techniques. Consider what’s most appropriate for the specific food and your own time, storage, tastes and dietary restrictions.

1. Freezing

This is the most nutritious method because you can capture food at the peak of its vitamin value and you don’t add sugar or salt, says Roxana Ehsani, a registered dietician and consultant in Las Vegas. Some foods, like beans, require a quick bath in boiling water to stop enzymes from causing decay; others, like berries, can be frozen without blanching. Most foods will last at least three to six months. The only downsides are space and the cost of running the freezer.

2. Pickling

This easy process preserves fruits or vegetables in a brine of salt, vinegar, seasoning and sometimes sugar. It’s possible to can pickles for longer storage but many basic recipes keep well for weeks in the refrigerator. Some ingredients, such as beets, need to be precooked; others, like green beans and zucchini can be used raw, according to the University of Minnesota Extension Service. Make sure you follow exact, up-to-date directions for particular ingredients and use a vinegar that’s at least 5 percent acetic acid.

3. Pressure canning

All canning requires special glass jars, lids and lid rings and processing, the stage where filled jars are heated to kill microorganisms such as the bacteria that cause botulism, a serious nerve disease. Low-acid foods like beans or carrots that have a pH level of 4.6 or higher must be processed in a pressure canner that heats above the boiling point. Pressure canners cost $100 to $200 and hold about seven quart jars. Canned foods last up to 18 months, says Ann Supa, 58, of Johnson City, New York, who works in the nutrition department at Cornell Cooperative Extension and is a master food preserver. “Pressure canners are super easy to use,” she says. “I don’t know why people are so afraid of them.”

4. Hot water-bath canning

Acidic foods with a pH under 4.6, such as tomatoes, pickles and fruit jams can be canned and processed in a large pot of boiling water or an atmospheric steam canner. These resemble a tight-fitting cake cover on top of a skillet. Steam canners cost under $100.

5. Dehydration

Drying makes food easy to store but can lower some nutrients, like vitamin C, although vitamins A and B remain, Ehsani says. Wright dehydrates sliced okra because his kids love it. Goff dries tomatoes and then grinds them into powder to add flavor to soups. You can use the oven or even the sun and countertop dehydrators start at under $100. Goff recommends the kind with a temperature gauge and fan.

6. Fermentation

This ancient technique uses a fruit or vegetable’s own bacteria to break down the natural sugars, making food easier to digest and forming lactic acid that controls harmful bacteria. It also encourages probiotics that are good for gut health, says Ehsani. Wright calls fermentation easy and foolproof. When fermenting, be sure to use a loose cover on your containers so natural gases escape or you’ll risk an explosion.

There’s never better a better time to learn how to safely preserve food at home. Home food preservation helps you stock up and save money, whether you’re growing your own food or buying in bulk.

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Your shelves are brimming, your freezer is overloaded, and your tomatoes, berries, and greens are nearing their expiration date. What to do? This is the perfect time to try canning and other simple food preservation techniques—and they’re easier than you might think. Experiment with some of the following:


Water-bath canning is relatively simple, and unlike pressure canning, doesn’t require special equipment. It should only be used for high-acid foods, like tomatoes—high-acid mixtures are more resistant to bacteria. If you’re using low-acid vegetables like cucumber or green beans, use a brine to increase acidity. For shelf-stable methods, the pH level should be below 4.6; test the foods you’re canning with a strip of pH paper to be sure. Use Mason or Ball jars and lids, not recycled jars, to ensure the best seal and avoid cracking during heating. Pint jars are better for beginners since the smaller quantities heat more evenly and thoroughly throughout.

Best for: tomatoes, brined vegetables (cauliflower, cucumber, carrots, turnips, radishes, green beans, beets).

How to:
sterilize jars and lids by immersing them in boiling water for a few minutes, or run them through the dishwasher on the hottest setting. Line the bottom of a large, heavy pot with a clean dishtowel. Bring a second large pot of water to a boil. While water is boiling, prepare foods:

  • For tomatoes: blanch first to remove skins, coarsely chop, add 1 tablespoon lemon juice, and cook for five minutes until softened. Pour tomatoes and juice into sterilized jars—use a funnel to prevent food from spilling on the threads and interfering with the seal—and leave about 1/2 inch of room at the top. Run a sterilized rubber spatula around the inside of jars to remove air bubbles and screw on lids.
  • For pickled vegetables: combine equal parts vinegar and water (white wine vinegar, rice vinegar or apple cider vinegar are best) in a small pot to make a brine. Add salt and bring to a boil. Slice cucumbers and cut vegetables into 1/2-inch pieces and pack firmly into sterilized jars. Cover with hot brine, leaving 1/2 inch of room at the top, and screw on lids.

Stand filled and sealed jars in the towel-lined pot and fill the pot with boiling water, making sure water completely covers jars. Boil for 10 to 15 minutes, adding more boiling water if needed to keep jars submerged. Turn off heat and let stand for 10 minutes, then remove jars and let cool for 12 hours. After cooling, check the seals by pressing down on the center of each lid. If the lid pops up, it’s not sealed; just store in the refrigerator.

Quick Pickling

If water-bath canning is too involved, and you have a little extra refrigerator space, try quick pickling. This method (also called refrigerator pickling) is fast and easy, and because it uses a brine to reduce pH, it’s appropriate for a variety of vegetables. While quick-pickled foods aren’t shelf-stable, this method can extend the life of many vegetables by days or weeks.

Best for:
cucumbers, onions, cabbage, green beans, carrots, zucchini, radishes, beets.

How to: trim and cut vegetables and pack firmly into clean pint-sized jars, adding herbs, spices, garlic or peppercorns for flavoring, and leaving 1/2 inch of space from the top of the jar. Pour boiling brine over vegetables to cover completely and screw on lids. Let cool to room temperature, then store in the refrigerator for 24 to 48 hours before using. Keep refrigerated for up to a month.


Jams, fruit spreads, preserves, and chutneys are the best way to extend the shelf life of fruits. Berries, peaches, oranges and other fruits can be canned using the water-bath method if limited refrigerator space is an issue. Or you can make simple refrigerator preserves. Adding sugar increases acidity, extending shelf life.

Best for:
berries, apples, pears, peaches, cherries, apricots, oranges, plums.

How to:
combine chopped fruits or mashed berries with unrefined cane sugar in a medium pot and cook till thickened; add spices like cinnamon, cardamom or vanilla for extra flavor. For savory apple or pear chutneys, use vinegar, ginger, garlic and spices. Use the water-bath canning method for shelf-stable preserves. For refrigerator preserves, pour hot preserves into clean pint jars, screw on lids and let cool before storing in the refrigerator. Most refrigerator preserves will last for three to six weeks after opening, depending on the acidity of the fruit or how much sugar you use in the mixture.


This is one of the best ways to preserve greens, herbs or hard fruits like apples and pears. It’s best and easiest with a dehydrator; most are relatively inexpensive, and you can find small, simple versions for as little as $40. Or you can use an oven on the lowest setting, ideally below 180 degrees.

Best for:
apples, pears, zucchini, broccoli, onions, mushrooms, bell peppers, eggplant, kale, collard greens, herbs.

How to:
thinly slice foods with a sharp knife or a mandolin, arrange them in a single layer on dehydrator trays, and dry for 8 to 16 hours depending on the food. Or arrange on parchment-lined baking sheets and dehydrate in an oven on the lowest setting for 6 to 10 hours. Herbs can be air-dried; just tie them in a bundle with twine and hang them in a cool, dark location, away from direct sun to retain color and flavor.

When you preserve food, you are taking action to stop it from breaking down, as it does naturally. You are killing or preventing the growth of microorganisms.

Today, we will explore how you can preserve some of the fresh produce you buy or grow. By following a few easy steps, you can still eat those tasty fruits and vegetables when they are out of season.

You can preserve late-summer fruits and vegetables in four basic ways. These include freezing, canning, pickling and drying or dehydrating.


Freezing is the simplest way to save produce. It is important to freeze the produce as quickly as possible, and to do so in freeze-grade bags or other containers.

Eugenia Bone has written books about food and nature. She notes that you should cool down food before you freeze it.

One problem to avoid is freezer burn, which can affect the taste and feel of food. To prevent this, use plastic bags, wraps or containers designed for the freezer.

Good late-summer produce to freeze includes green beans, cabbage, apples and plums.


There are two main ways to can produce: boiling hot water baths and pressure canning.

A boiling water bath involves putting food in glass canning jars and then heating the jars in a pot of boiling water. The heat forces air from the glass jars and frees the food from bacteria and microorganisms.

You can then seal the jars.

This method works best with naturally acidic foods like fruits.

Pressure canning food requires a pressure canner. The pressure canner you use should be based on the kind of cooking equipment you have and the amount of food you plan to can.

Whichever method you use, always test the seal that keeps out fresh air.

Good late-summer fruits for water bath canning include apples, berries, cherries, figs and peaches.

Good foods for pressure canning include carrots, corn, okra and bell peppers.


There are many methods of preserving food in vinegar, salt brine or a similar mixture.

You can pickle whole vegetables, like green beans or okra. Another possibility is to make chutney or relishes. This is done by adding salt, pepper or other seasonings to small, cut pieces of vegetables.

Good late-summer foods to pickle include cucumbers, hot chili peppers, watermelon rinds and eggplants.


A final way to preserve food is by drying it. Whatever food you are drying should be just ready to eat and not have bruises.

When drying, try to find specific directions for each food to ensure quality and safety.

Drying methods include air drying, oven drying and using a dehydrator.

A dehydrator is probably the best choice. If you think you will be drying foods regularly, think about investing in an electric dehydrator.

Good late-summer produce to dehydrate includes apples, mushrooms, grapes and hot and sweet peppers.

When you get a chance, let us know how you like to preserve foods. What are the popular methods in your country? Write to us in the Comments Section of our website

I’m John Russell.

And I’m Anne Ball.

Katie Workman reported on this story for the Associated Press. John Russell adapted it story for VOA Learning English. George Grow was the editor.

Words in This Story

preserve – v. to prevent (food) from decaying

picklev. to preserve or protect food in a liquid solution

acidicadj. sharp-tasting; containing acid

chutney – n. a thick liquid that is made from fruits, vinegar, sugar, and spices

relish – n. a seasoned mixture that is used to add taste to other foods

oven – n. an enclosed space for cooking and heating food