Preserving the Harvest Safely: Canning, Storage and Maple Syrup

How to preserve what you grow without taking risks, with the safety rules first and the altitude, yield and storage numbers that go with them.

The safety rules come first

Home canning is safe when you follow tested methods and unsafe when you improvise. The main danger is botulism, a rare but serious illness caused by a toxin that grows in low-acid, oxygen-free conditions, such as a sealed jar of improperly processed vegetables. You cannot see, smell or taste the toxin, so safety has to come from the process. Use recipes tested by your state extension service, the USDA or the National Center for Home Food Preservation, and follow them exactly. Do not change the ingredients, the amounts, the jar size or the processing time of a tested recipe.

Three rules cover most situations. First, match the method to the food. Second, adjust for your altitude. Third, check every jar before you eat it.

Choose the method by acidity

Acidity decides how a food must be processed. High-acid foods, which have a pH of 4.6 or lower, such as most fruits, jams, pickles and tomatoes with added acid, can be processed in a boiling water bath. Low-acid foods, such as vegetables, meat, poultry, fish and most soups, must be processed in a pressure canner, which reaches temperatures above boiling that are needed to destroy the spores that cause botulism. A boiling water bath is never safe for low-acid foods.

Tomatoes sit on the boundary. Modern varieties can be borderline in acidity, so tested recipes call for adding lemon juice or citric acid to each jar. Do not skip it. Some other methods are not recommended: canning in an oven, using a microwave, open-kettle canning, or processing jars in a dishwasher do not give safe results.

Adjust for altitude

Water boils at a lower temperature as altitude increases, so food must be processed longer in a water bath or at higher pressure in a pressure canner to reach the same safety margin. The canning altitude calculator does the adjustment for your elevation. The usual adjustments from the National Center for Home Food Preservation are:

Altitude (ft)	Boiling water bath: add to the processing time
0 to 1,000	No change
1,001 to 3,000	5 minutes
3,001 to 6,000	10 minutes
6,001 to 8,000	15 minutes
8,001 to 10,000	20 minutes

Altitude (ft)	Dial-gauge canner (psi)	Weighted-gauge canner (psi)
0 to 1,000	11	10
1,001 to 2,000	11	15
2,001 to 4,000	12	15
4,001 to 6,000	13	15
6,001 to 8,000	14	15

These are the standard general values, and a tested recipe may specify something different, in which case the recipe wins. Have your dial gauge tested for accuracy each year, usually free at an extension office, and see the pressure converter if you need to read your canner in kilopascals.

Plan quantities

Knowing how much produce you need saves a trip. As a rough guide, a quart of tomatoes uses about 2.5 to 3.5 pounds of fresh fruit, a quart of green beans 1.5 to 2.5 pounds and a quart of peaches 2 to 3 pounds. The canning yield calculator turns pounds of produce into an expected number of quart or pint jars.

Good canning habits

Start with clean, unchipped jars, new lids and a canner in good order.
Use fresh, firm produce, and can it within a day or two of picking.
Leave the headspace the recipe calls for, remove air bubbles and wipe the rims.
Tighten the bands fingertip-tight, and do not overtighten.
Let a pressure canner vent steam before you pressurize, and let it cool naturally.
Cool jars undisturbed for 12 to 24 hours, then check the seals. A good lid is concave and does not flex.
Remove the bands, wash the jars, label with the date and store them in a cool, dark, dry place.

When to throw a jar away

Discard, without tasting, any jar with a bulging or leaking lid, spurting liquid on opening, cloudiness, mould, an off smell or any sign of spoilage. Dispose of suspect low-acid food carefully so that people and animals cannot eat it. If you are unsure about a jar, do not eat it.

Root cellars and cool storage

Some crops store without processing if conditions are right. Roots and cabbage like it cold and humid, onions and garlic like it cold and dry, and winter squash likes it cool and dry. Typical targets are:

Crop	Temperature	Humidity
Carrots, beets, parsnips, turnips	32 to 40 F	90 to 95%
Potatoes	38 to 40 F	90%
Cabbage	32 F	90 to 95%
Apples	30 to 35 F	85 to 90%
Onions and garlic	32 to 40 F	65 to 70% (dry)
Winter squash and pumpkins	50 to 55 F	50 to 70%

The root cellar planner sizes storage for the crops you plan to keep. Keep apples away from potatoes and other vegetables, since the ethylene they release speeds ripening and can cause sprouting. Check stored crops regularly and remove any that begin to rot.

Making maple syrup

Maple sap runs in late winter and early spring when nights freeze and days warm up. Sugar maples have the sweetest sap, around 2 percent sugar, but other maples can be tapped too. The maple tap count calculator helps you choose how many taps your trees and equipment can handle, and tree health limits how many taps each tree can carry. Collect sap in clean food-grade containers and keep it cold until you boil it, because warm sap spoils.

The classic ratio is about 40 gallons of sap to 1 gallon of syrup, but the real ratio depends on the sugar content. The Rule of 86 divides 86 by the sap's Brix to give the sap needed per gallon of syrup, so 2 percent sap needs about 43 gallons, and 3 percent needs about 29. The maple syrup yield estimator does this for your sap, and the Brix and density converter converts readings into density. Finished syrup is about 66 to 67 percent sugar, which occurs when the boiling liquid reaches about 7 degrees Fahrenheit above the boiling point of water at your location. Boil outdoors or in a well-ventilated space, because boiling releases large amounts of steam that can damage indoor finishes. Filter the syrup while it is hot and bottle it hot, at about 180 degrees Fahrenheit or above, into clean containers.

Cider and other fruit

Apples can be pressed into cider, and the cider press yield calculator estimates juice from a given weight of fruit. Unpasteurized cider can carry harmful bacteria, so heat-treat it or buy pasteurized if you serve it to children, older adults or anyone with a weakened immune system.

Common mistakes to avoid

Using a boiling water bath for low-acid foods.
Altering a tested recipe, especially ingredients, jar size or time.
Forgetting the altitude adjustment.
Using an untested family recipe or an old cookbook.
Skipping the dial gauge test.
Storing root crops too warm or too dry.

Read our disclaimer and safety notes before relying on any number here, and use the garden year guide to plan the harvest you will preserve.

Frequently asked questions

Can I use a boiling water bath for green beans?

No. Green beans are a low-acid food and must be processed in a pressure canner. A boiling water bath does not reach the temperature needed to destroy botulism spores.

Can I use an electric multi-cooker as a pressure canner?

Most have not been tested for home canning and are not recommended for low-acid foods. Check the National Center for Home Food Preservation for current guidance on any appliance before you rely on it.

How long does home-canned food keep?

For best quality, use it within about a year. Properly processed jars that remain sealed can stay safe longer, but quality gradually declines. Always inspect a jar before you open it.

Can I reuse canning lids?

Flat lids with a sealing compound are designed for one use. Bands and jars can be reused if they are in good condition.