#1 Most Important Rule - All safe canning (both water bath and pressure canning) comes down to two facts: Acidity and Temperature.
Most fruits are considered high acid (except for tomatoes which are on the acidity cusp) and can be processed in a boiling water bath which will bring the internal temperature of the jars to 212F (the temperature of boiling water). No matter how long you leave a jar in a boiling water bath the internal temp will never rise above 212F. The length of time in the bath specified in a recipe is how long it takes for the heat to penetrate the density of the canned product and raise it to 212F.
Most vegetables (and meats) are low acid and must be processed in a pressure canner where temperatures above 212F can be achieved. Steam under pressure becomes hotter than plain boiling water. For example, water boiling under 10 pounds of pressure (a typical poundage for many pressure canned foods) reaches a temperature of 240F. Again, the time specified in the recipe is how long it takes to raise the heat in the center of the jar to the desired temperature. In recipes for pressure canned food you will find both the pounds pressure and the length of time to be held at that pressure specified. Below is a chart showing the acidity of some foods.
| High Acidity Foods Using Boiling-Water Bath or | Low Acidity Foods Using Pressure Canning Only | ||
| Apples | Plums | Artichokes (Jerusalem) | Mushrooms |
| Apricots | Raspberries | Asparagus | Okra |
| Blackberries | Rhubarb | Beans (green or yellow) | Parsnips |
| Blueberries | Strawberries | Beets | Peas |
| Cranberries | Pineapple | Broccoli | Peas (snap) |
| Cherries | Tomatoes (with acid added) | Brussels Sprouts | Peppers |
| Cucumbers (pickled) | Cabbage | Potatoes | |
| Grapefruit | Carrots | Pumpkin | |
| Grapes | Cauliflower | Spinach | |
| Nectarines | Corn | Squash (summer) | |
| Oranges | Eggplant | Squash (winter) | |
| Peaches | Figs | Sweet Potatoes | |
| Pears | Lima Beans | ||
Chart courtesy of http://www.recipetips.com/kitchen-tips/t--1396/canning-temperatures-and-processing-times.asp
Please note on the chart that tomatoes are on the high acid list with "acid added". That's because it's been discovered that tomatoes vary greatly in the amount of acid they contain but in general hover at the cusp of low acid foods. Acidity varies not just between varieties but between individual tomatoes of the same variety. To safely water bath process them, it's necessary to acidulate the tomatoes with either bottled lemon juice or vinegar. We'll get into the details as tomato canning time approaches. Also, be aware that contrary to popular belief, sugar and salt are not preservatives when canning foods. They are flavor enhancers. You can preserve fruits in water but they will taste bland. In jellies, jams, and whole fruit preserves, sugar is used for flavor and for it's hygroscopic property, it's ability to attract water out of the fruit's cells and to itself. It is the action of pectin, either natural or added, that creates the gel in jellies. In whole fruit preserves the cell wall collapses after water is drawn out leaving less room for air in the cells and discouraging "fruit float," an annoying but purely cosmetic condition in canned fruits.
#1A - Altitude
For both boiling water bath and pressure canning, altitude comes into play. Recommended canning times and pressures are for sea level. As you rise above sea level, water boils at incrementally lower temperatures so you have to lengthen the recommended time or pressure to correspond to your altitude.
* Water Bath
For example, I'm 2200' above sea level so I add two minutes to the processing time in a recipe. If it calls for a jam to be boiling water bathed for 10 minutes, I process it for 12 minutes. Yeast, molds and enzymes that can affect high acid preserves are killed at temperatures between 160F and 212F. The extended time allows the heat to fully penetrate to the center of the jar.
* Pressure Canner
Botulism spores need heat higher than 212F to be inactivated which pressure canning does. For pressure canning you need to increase the pounds pressure. For a dial gauge pressure canner, process at 12 lbs pressure at altitudes of 2,000-4,000 feet, and 13 lbs pressure at 4,000-6,000 feet. If using a weighted gauge pressure canner, use 15 lbs rather than 10 lbs. Processing times remain the same. Take your pressure canner gauge, dial or weighted, to your local county cooperative extension agent on an annual basis for accuracy testing. It's free, only takes a few minutes, and can keep you and your loved ones safe.
#2 - Cleanliness
Of course everything you can with should be sparkling clean from counters, towels and tools to your jars. But here's something I came across last year that took me by surprise: If the processing time for a product is 10 minutes or more it is no longer necessary to sterilize your jars prior to filling them! I came across this little fact on the National Center for Home Food Preservation website. It's an example of how home canning is constantly changing as more testing is done and discoveries are made. The NCHFP website carries the most up to the minute information and is a valuable tool for novice and experienced canners alike. Bookmark it!
Which brings us to:
#3 - "But that's how Mom/Grandma did it and we never got sick!"
Uhmmm.... you probably did. Or someone did and it just wasn't recognized as food poisoning. At least where high acid canned foods are concerned. In healthy teens and adults food poisoning can manifest itself as no more than a mild fever and/or feeling "kinda off." However, food poisoning from low acid foods is another, and far more dangerous, critter altogether. It most often involves Clostridium botulinum (botulism) and that can kill you. Quickly. Painfully. Not pretty. It thrives in an anaerobic, low acid environment. It is odorless and tasteless. You can't see it. But proper, up-to-date home canning methods, attention to detail, and cleanliness seriously reduces the risk.
My usual reply to the above statement is Mom/Grandma and food science didn't know then what we know now and if they had known what we know now they would have canned differently. The same goes for us now. Food science may discover something new to make home canning safer and/or easier by this time next year. Be willing to adapt and learn!
Some methods of canning that are NOT recommended are:
* Open-kettle canning
* Processing in conventional ovens, dishwashers and microwaves (microwaves? I have visions of exploding glass!)
* Steam canners. Don't confuse these with pressure canners. You'll see them advertised as more efficient, using less water (supposedly greener) but the fact is they don't heat effectively.
* Canning at pressures greater than 15 pounds.
* Use of glass caps or one piece zinc/porcelain-lined caps. These have a high seal failure rate.
* Paraffin seals for jams and jellies.
* Turning jars upside down after placing lids and rings on instead of processing in the water bath or pressure canner. Does not raise contents to a heat level to kill or inactivate enzymes and bacteria.
Signs of spoiled high acid foods
* Bulging lid
* Leaking jar
* Spurting liquid when seal is broken
* Off odor
* Mold
In addition to the above signs, low acid foods may be spoiled and show no signs of it. This is the result of improper canning. Discard jars of low acid foods without tasting if:
* The food was not processed in a pressure canner
* The canner's gauge was inaccurate
*Up-to-date researched processing times and pressures were not used for the size of the jar, the style of the pack, and the kind of food being processed.
* Ingredients were added that were not in an approved recipe.
* Proportions of ingredients were changed from the original approved recipe.
* The processing time and pressure were not correct for the altitude at which the food was processed.
If you've received a jar of low acid food as a gift and don't have confidence in the giver's canning abilities, you can either choose to interrogate the giver about his/her canning methods, or accept the gift in the spirit it was meant, dump the jar later and return it cleaned to the giver with more thanks. Sounds cold, I know, but better safe than sorry.
#4 - Always use tested recipes specifically created for home canning.
This brings us back to the acid content of a recipe. It is not possible to judge if a recipe has enough acid in it to safely water bath can by looking at the ingredients or using a strip of litmus paper. Lab equipment is required. Also, always use bottled lemon juice when lemon juice is called for by a recipe. Yes, fresh tastes better but like tomatoes, lemons are not consistently acid. Bottled lemon juice is. The same holds true for home made vinegars. Use commercial vinegars labeled 5% acidity in your recipes.
Here's an irony, at least when you consider that I'm blogging on food preservation. Be wary of canning recipes on the internet. I know of at least one blogger out there whose recipes I'm very wary of. She even admits to pushing the limits. In my judgement, I think she's occasionally gone over the limits. If you are a novice, stick to recognized tested recipes such as those provided by the NCHFP, the USDA, and the Ball Complete Book of Home Preserving. Once you are more experienced, you can use your own judgement when it comes to recipe sources.
Does this mean you cannot customize recipes to your own tastes? You absolutely can! You do this by tweaking the herbs, spices and other flavorings that don't affect the acidity balance. We'll get into this with specific recipes as the season progresses.
Tools
We'll cover the tools needed for water bath canning for now. Most of these are the same ones you'll need for pressure canning as well, the pressure canner itself being the major difference.
The Water Bath Canner
Water Bath canners come in a couple of sizes; big and really huge. I've had both sizes and when doing seriously large batches of product they are useful. Both sizes require sitting on top of two burners to operate and take forever to bring that amount of water up to boiling. They are more than incredibly heavy to move when filled and as I get older I have more difficulty working with them. The truth is you can use any pot large enough to accommodate a small rack to raise the jars off the bottom, the height of the jars, plus water to cover the jars by a minimum of two inches. For most of my water bath canning I use my 20 quart stock pot...
Towels
Dish towels, paper towels, and newspapers all come in handy. I use several thicknesses of newspapers on the counters under the jars when filling and cooling. The thickness of the newspapers helps to protect the jars from sudden temperature changes on cold counters and absorb any drips, then get tossed. Much cheaper and easier than using paper or cloth towels for this purpose. Paper towels are good for moistening and wiping the rims of jars before placing the lids. You don't have to worry about permanently staining your dish towels. Dish towels, especially those made of cotton sacking, are good for drying and buffing up your jars after they've been processed and washed for storage. The cotton sacking towels are super absorbent and leave no lint behind.
Utensils
Jars, Lids, Rings
Always use jars made specifically for canning. They have been through the Mason process which tempers glass to withstand repeated temperature changes and reuses. Kerr, Ball, Atlas, and Mason are the most commonly recognized names. Walmart has started offering their own MainStays line of canning jars and also Better Homes and Gardens canning jars. You can often come across jars at yard sales, flea markets and thrift stores and these are fine to use as long as there are no chips or cracks anywhere on them and you can get them sparkling clean. Lid rings are reuseable as long as they are rust free inside and out. Lids are one time use only. After one use the sealing compound is compromised. You can buy packages of just lids without the rings. There is also a reuseable lid that can be ordered online called Tattlers. They are an expensive initial investment but pays for itself quickly. I have yet to try them but they've received good reviews from those who have. I'm going to order a batch for testing shortly.
There are also Weck and Leifheit jars which use a glass lid, rubber gasket, and clamp system for sealing. These are not the same as the rubber gasket/wire bail jars you see in second hand shops and flea markets. Weck and Leifheit jars are very popular in Europe and are gaining favor with advanced canners here. They are gorgeous jars but not for the novice or less detail oriented person. They require close attention when sealing. They are also very expensive.
Finally, do not reuse supermarket jars for canning. Not even the ones that look like Mason jars. I don't care what Mom or Grandma did. These jars are tempered to withstand the commercial canning process one time only. I know people who reuse these jars and always say "Well, I've never had a problem with them!" Well, I never did either until all of a sudden I started losing entire batches to cracking jars in the canner. All that work gone to waste. I thought it was me. When going over the problem with a cooperative extension agent, she asked me about my jars and upon hearing that I was reusing supermarket jars, explained the Mason process to me versus jars tempered for one use. So yeah, you may not be having problems now, but you will. Bite the bullet and get real canning jars.
Clothing
Please do not can in sandals, flip-flops, or any other flimsy shoe. You are going to be on your feet for awhile working with boiling syrups and boiling water. A comfortable, supportive, closed toe shoe with good traction is what you want for comfort and safety.
Shorts and tank tops may be more comfortable in a hot kitchen but jeans and short sleeves will protect you from random splashes and blips. Apron optional.
Pull your hair back and up if it's long for comfort and to keep random loose hairs out of your food. I've found a bandana tied around my head, either as a band or gypsy style contains my hair, catches perspiration and keeps me cooler, and protects the food. Now that my hair is short, I still like the comfort of a bandana.
Now that you've got an idea of the basic science, and tools, we'll start off with canning strawberries!
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