The Chemistry, Physics and Science of Food Preservation

Whether you call it pickling, canning or jarring there’s more to food preservation than a recipe 

jars

Many people engage in canning and pickling foods.  Everything from jams to jellies, vegetables and fruits and even meats and fish.  The process seems simple but there are some critical things to understand related to the chemistry and physics of home canning.

pH 

pH is a measure of the acidity or the alkalinity of foods.  Acidic foods are foods with a natural source of acid. A good example is the citric acid in oranges, lemons, limes and even tomatoes.  Citric acid is the primary source of Vitamin-C in these foods and this form or acid also has very good food-preservation values.  Germs don’t like acid and the presence of citric acid inhibits microbial growth.

Acidic Foods:

Acidic Foods

  • Citrus fruits
  • Tomatoes
  • Corn
  • Blueberries
  • Cranberries
  • Strawberries
  • Mulberries
  • Olives
  • Currants
  • Lentils
  • Winter squash

There are also dairy products that fall into the acidic category including:

  • Yogurt
  • Cream cheese
  • Sour cream
  • Butter
  • Milk
  • Cottage cheese
  • Ice cream
  • And hard cheese

While these foods are naturally acidic that property alone will not make them food-safe from a canning standpoint, but they will preserve better than their alkaline counterparts.

Another natural form of acid is acetic acid.  We commonly refer to it as “vinegar.”  Any vinegar recommended for canning has an acetic acid level of 5%.  The other 95% is water.  White vinegar and apple-cider vinegar are the most popular vinegars for canning.  The addition of vinegar helps to preserve a highly alkaline food like cucumbers.

Vinegar effect

The pH scale is measured from 0 to 14.  0 is the most acidic and 14 the most alkaline.  No foods register on the extreme ends of the pH scale.  Something with a pH of 0 is battery acid, and a pH of 14 is the alkaline measure of concentrated lye.  Most foods range from a pH of 4 to 10.

Alkaline Food Examples

Alkaline food examples

  • Root vegetables like carrots, turnips, potatoes and most others.
  • Cruciferous vegetables like broccoli, cabbage, cauliflower, and Brussels sprouts.
  • Legumes like peas and beans
  • Leafy greens from spinach to mustard greens and wild greens like dandelion and plantain.
  • Garlic
  • Hot peppers
  • Meats, fish and seafood

The thing to keep in mind is the natural pH of your food source.  If you’re canning tomatoes both the processing time and the need for added acids like vinegar is diminished.  On the other hand, high alkaline foods like beans and meat need longer processing times and often have the addition of natural preservatives like salt, mustard seed and vinegars.

Mustard seed

Mustard seed

How can salt and sugar help with food preservation?

Pickling salt

Pickling salt

Not many people like the idea of chemicals in our food but we use them all the time without a thinking.  The two most common chemicals used in food preservation are sodium chloride and sucrose or fructose.  To put the chemistry into a simpler context, sodium chloride is salt, and sucrose is sugar.  Fructose is a form of natural sugar found in plants and most notably in honey.

Salts and sugars whether in solid form or dissolved in a liquid cause something called an “osmotic effect.”  Osmosis is the transfer of molecules between one thing and another in an effort to create a balance when they come in contact with each other.  This transfer of chemicals to a food item creates a hostile  environment that kills and prevents the growth of bacteria and molds.

Both salt and sugar will kill and inhibit the growth of bacteria and mold but in order to saturate a piece of meat or a fruit or vegetable the food has to be surrounded by the salt and sugar to permeate the item.  As it does so, the food environment becomes toxic to the bacteria or mold.  The amount of salt or sugar to inhibit the growth of bacteria is less than for mold.  That’s why we’ll sometimes see mold growing in a canned or fermented food that we assumed had been properly processed.

Types of salts and their uses and why

  • Kosher salt.  Unlike iodized salt, Kosher salt does not have iodine. It has a coarse texture with large, clear crystals and dissolves fast.  It is often used for dry curing of meats and fish because of its quick dissolving characteristics.
  • Sea salt.   Sea salt varies and are made from the evaporation of sea-water.  Because sea water carries other chemicals the color can be brown to red to pink to white.  It is often used for fermenting vegetables because it contains sufficient nitrites to both aid the fermentation process and further inhibit the growth of bacteria by compromising their cell membranes.
  • Pickling salt.  Pickling salt is more concentrated than other types of salts.  It is 100% sodium chloride making it the purest salt and does not have added anticaking agents which would affect the color of pickled foods.  It is also used for fermenting and as a classic brine for meats, fish and vegetables. It is fine grained and clear white.
  • Traditional table salt.  Table salt if often iodized which means a trace of the chemical iodine has been added to the salt.  It is rarely used for food preservation because the iodine can add an off-flavor, but its chemical composition is mostly sodium chloride with a trace of iodine.

Types of sugars for food preservation

  • White granulated sugar.  White sugar is pure sucrose.  It is used as a dry rub on meats usually with added salt or dissolved in water or fruit juice to make a solution.  It is not as potent against bacteria as pickling salt, but when combined with salt or when used with acidic components like vinegar or fruits it has excellent preservative characteristics if used in the proper concentration.
  • Honey is amazing as a food preservative.  Archaeologists in Egypt discovered a jar of honey in a tomb that was 3,000 years old.  Talk about shelf-life!

A Little Canning History

A little canning history

The concept of canning food to preserve it was a bit of an accident.  A French confectioner named Nicolas Appert discovered that when heat was applied to food in sealed jars preserved the food.  He had no idea why it worked, but it did.

In 1806 the French Navy tried this technique on a wide range of foods including meats, vegetables, fruits and milk.  Once again, it worked but the mechanism was still a mystery.

In 1810 and Englishman by the name of Peter Durand used tin cans for the first time applying the same principles.  Each experiment succeeded but the reason it worked remained a mystery until Louis Pasteur.

In 1864, Pasteur discovered the relationship between bacteria and food spoilage.  His approach to food canning was known as pasteurization and it is still the primary approach to contemporary food preservation.

Just before Pasteur’s discovery a man by the name of Raymond Chevalier-Appert began to process food in a pressure canner which raised the temperature above 212° F. The ability to raise the temperature of boiling water to 250° F. effectively killed the Clostridium Botulinum bacteria which is one of the most virulent form of food borne bacteria: botulism.  It wasn’t until the 1920’s that the significant effects of botulism was understood.

The 2 Approaches to Food Processing

Hot water bath pot, mason jars, and jar tongs

Hot water bath pot, mason jars, and jar tongs

Food processing for home canning takes place in one of two ways.  Either a hot water bath of boiling water in a pot, or a hot water batch of boiling water in a pressure cooker.  The pressure cooker raises the heat and does the most effective job of killing any bacteria present.

The temperature of boiling water is 212° F.  It never gets hotter because the atmospheric pressure allows the oxygen in water to be released as bubbles and it cannot increase in temperature.  A pressure cooker contains the pressure and water will reach a boiling point of 250° F.  This was Raymond Chevalier-Appert discovered.

Pressure canning also calls for more time across various recipes.  A standard amount of time in a hot water bath in a pot is 10 minutes.  Foods processed in a pressure canner will be in the pressure canner for 15 minutes or more.

The Chemistry of pH and the Physics of Boiling Water are a Critical Combination.

It’s all about killing any bacteria present in the food and inhibiting its growth.  The boiling process kills the bacteria and the natural acidity of the food or the addition of an acid like vinegar and spices like salt and mustard seeds inhibit any bacterial growth.

Once a jar is opened it should be kept refrigerated or in a root cellar.  The low temperatures will continue to inhibit the growth of microbes.

Recipes for Canning that Demonstrate these Concepts

Chili

chili

Chili

Canned chili has the potential to be a hot-bed for the growth of bacteria.  The alkaline nature of both the meat and beans creates a prime habitat for microbes.  That’s why chili needs to be pressure canned at a 250° F. temperature.

INGREDIENTS:

  • 2 pounds of ground beef
  • 2 tablespoons of olive oil
  • 1 can of diced tomatoes
  • 1 large onion diced
  • 2 green peppers diced
  • 1 jalapeno diced (optional)
  • 4 cloves of garlic minced
  • 2 cans of kidney beans
  • 2 cans of chili beans
  • 1 tablespoon of chili powder
  • 2 teaspoons of cumin
  • 3 teaspoons of pickling salt or Kosher salt
  • 2 cups of water
  • 1 tablespoon of white sugar

DIRECTIONS:

  1. Brown the ground beef in a skillet.
  2. Drain the browned ground beef and reserve.
  3. Brown the onion, peppers, jalapeno until the onions are translucent.
  4. Sauté the garlic for one minute.
  5. In a large pot combine the ground beef, sautéed vegetables and all of the other ingredients.
  6. Bring to a gentle boil and then simmer for 15 minutes. Taste to adjust seasoning.

CANNING DIRECTIONS FOR CHILI:

  1. Add enough water to a pressure cooker to immerse any jars you are planning to can.
  2. Bring the water to a boil and add the jars and seal the lid tightly. Process for 20 minutes.
  3. Release the pressure stop and let vent for the time recommended by the pressure cooker manufacturer.
  4. Carefully open the pressure cooker and lift the jars with jar tongs.
  5. Let rest and cool for 3 hours at room temperature.
  6. Serve or store.

STEWED TOMATOES RECIPE

Stewed Tomatoes

Stewed Tomatoes

Stewed tomatoes are a classic canning recipe that usually takes place in the Fall when our gardens are over-flowing with tomatoes.  Then again, you can always buy them at the store at any time of year.  This is a classic breakfast dish and also serves as a base for many recipes.

Because of the natural acidity of tomatoes, they can be canned in a water-bath in a pot at 212° F.  This recipe also has onions and bits of celery which are alkaline, but the addition of some salt and vinegar plus the acidity of the tomatoes helps the preservation properties.

INGREDIENTS:

  • 5 pounds of fresh tomatoes
  • 1 large onion thinly sliced
  • 6 stalks of celery diced
  • 2 tablespoons of pickling salt or Kosher salt
  • 1 tablespoon of oregano
  • 1 teaspoon of cracked black pepper
  • 2 cloves of garlic finely diced (optional)
  • 2 tablespoons of apple-cider vinegar

DIRECTIONS:

  1. Bring a pot of 2 gallons of boiling water to a full boil.

    Setup for removing tomato skins and deseeding

    Setup for removing tomato skins and deseeding

  2. Drop the tomatoes into the boiling water for 2 minutes.

    Tomatoes in boiling water for one minute

    Tomatoes in boiling water for one minute

  3. Combine cold water and ice in a large bowl and drop the tomatoes into the ice-water. The skins will split.

    Tomato skins split after immersion in ice water

    Tomato skins split after immersion in ice water

  4. Under cold water in a sink, carefully remove the skins.
  5. Cut the top core from the skinned tomatoes and slice into chunks.

    Tomatoes skinned and chopped

    Tomatoes skinned and chopped

  6. Sauté the onion and celery until the onions are translucent. Add the garlic for the final minute.
  7. Combine the tomatoes, onions, celery and garlic in a large bowl and toss with the other ingredients.
  8. Refrigerate overnight to let the seasonings and flavors meld.

CANNING DIRECTIONS:

  1. Bring a 3-gallon pot of water to a boil filled with 2-gallons of water

    Hot water bath pot

    Hot water bath pot

  2. Add the jars to the boiling water using jarring tongs.
  3. Boil for 10-minutes.
  4. Remove the stewed tomatoes from the water with the tongs and let rest for 3 hours.
  5. Serve or store in a pantry or the refrigerator.

Understand the Science and Any Recipe is Easy

It’s all about temperature and the chemistry of ingredients like acids, seasonings like salt and proper processes.  Anyone can preserve foods if you understand the concept and apply the science.

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