We all use canned products in our daily lives, and manny of us enjoy making them our selves.
This instructable is intended as a basic introduction to canning methods, and to some of the science behind how it works.
The basic idea is of course to preserve food over time without having to refrigerate it. An added bonus is that you can prepare foods that are ready for eating, straight from the jar with no further preparation needed.
So how can we store food for years without going bad after only a few simple steps?
Why doesn't it rot?
Step 1: Causes of Decay and How to Stop Them.
So what is it that causes food to degrade
in the first place?
The main culprits are bacteria, fungi, enzymes in the food itself, oxygen and loss of water.
Lets have a look at each of them.
Loss of water is easily prevented by
canning. In an airtight container, there is simply nowhere for the water to go. Problem solved.
Oxygen is a very reactive chemical, and it readily binds to many compounds in our food. This is called oxidation and is what makes oils and fat go rancid. By boiling foods before canning, we effectively remove free oxygen by replacing it with water vapor. By canning food in airtight containers, we keep oxygen from getting to it. There will always be some oxygen left, unless you chemically remove it with additives that bind oxygen. In most cases the amount of oxygen available in a jar of canned produce doesn't noticeably degrade it.
Enzymes are active proteins that both build molecules and break molecules apart in all living tissue. Some of these enzymes remain active, even though the tissue is no longer alive. This will contribute to the degradation of some foods. There are several ways to stop enzymatic activity. All enzymes have a set of conditions required for them to function. This includes temperature, pH, salinity and other factors. Shift these conditions outside of the preferred range, and the enzyme will be slowed down or completely stop functioning. In canning we do this in several different ways. One is by boiling. The high temperatures during boiling causes the structure of many proteins to degrade, and kills the enzymatic activity. This change in protein structures is the main reason food changes texture after boiling. Another way to halt enzymatic activity is to lower the pH values in the food by adding vinegar, citric acid or lemon juice.
Microbes are the main problem when it comes to preserving food. Bacteria and fungi thrive in basically anything organic. They produce enzymes that degrade the food, and in some cases they produce compounds that are foul tasting or toxic in the process. In the canning process we aim to kill or deactivate all the microbes in our food, and prevent new ones from entering by making airtight seals.
So how do you kill microorganisms?
Boiling takes care of most of them, but many bacteria and fungi produce spores that are highly resistant to heat. To make sure these spores don't just start growing again when the food cools down, we can take additional measures. If living conditions are not ideal, the spores simply won't germinate, and the food stays fine, even though it contains viable spores. There are two main groups of bacteria producing spores. One is the Bacillus. All species in this group need oxygen to grow, so by removing oxygen in the canning process, we effectively halt their growth. The other group is the Clostridia. They thrive without oxygen, and removing it actually enhances their growth. To keep them from growing, we need to do more.
The most well known destroyer of canned goods is Clostridium botulinum. It produces an extremely potent neurotoxin that can kill you. This is the very same BoTox used in very low doses in medical treatment and cosmetics. One way to halt the growth of C. botulinum is to keep the pH low. A pH below 4.5 is generally considered safe. Most fruits are acidic enough to achieve this without adding more acid, but to be sure, you can measure pH with a pH-meter or pH strips. For foods with higher pH you need to actively lower it with something like vinegar, citric acid or lemon juice. If you don't want the food to be sour, you simply have to kill the spores as well as the bacteria. As mentioned earlier, normal boiling does not kill the spores. You need higher temperatures. Boiling at 121°C kills most known bacteria and spores. Keeping this temperature for 3 minutes is usually enough to kill the spores of C. botulinum, but remember that it takes a while for the entire contents of a jar to reach this temperature. Water temperatures this high can only be achieved in a pressure cooker, and pressure cookers usually come with a table to determine how long boiling times you need for different volumes. As most of us don't have pressure cookers available, acidifying or pickling is the most common way to preserve canned food.
Sugar and salt preserve food by lowering the water activity. What essentially happens is that the sugar or salt creates osmotic pressure that draws water out of the cells of microbes and renders them unable to grow.
Step 2: Containers
You can reuse jars from food you purchase, but there are a few things to be aware of. First of all, these jars might not be tempered to tolerate the pressure and temperature differences in canning. This could make the jars crack. To minimize the risk of this happening, you should not use jars with visible chips, cracks or scratches in them. Jars like these often have a relatively small sealing surface compared to purpose made canning jars, so the seal may more easily be corrupted. The seal on the lids could also be damaged, so you might have to buy new lids. Reused jars like these are most suitable for canning jams and jellies, where an airtight seal is not of vital importance. Reused jars lack one of the features of canning jars. Canning jar lids are designed to lift when the internal pressure gets to high, and release air. This reduces the risk of jar breakage.
Purpose made canning jars are naturally the best option, as they are tempered to take the normal heat differences in canning.
When cooling down warm jars, make sure to not place them on a cold surface like a stone or metal counter top. Also make sure they are not placed in a drafty location, as the air movement could cool down the outside of the glass to fast. One tip is to cover cooling jars with a towel. Do not attempt to cool down warm jars in the fridge.
Metal cans.This is where the term canning comes from in the first place, and we have all bought food in metal cans. Closing the cans however, requires equipment that the ordinary home canner is not going to invest in, so cans are primarily used in industrial production.
Heat sealable plastic bags is a different option. These bags are food safe, tolerate heat, and can be sealed airtight. They can be used for both water bath and pressure canning. For water bath canning, you can use bags intended for sous vide cooking. These bags are usually not made to withstand temperatures much higher than boiling, so for pressure canning, look for purpose made retort bags.
Step 3: Heating Methods
Now, lets look at different methods of heating the food. This is where we actually kill the microbes present in our food, so it is the most important step.
There are different ways to go about this, depending on what you are canning. The methods will be explained in detail in the following steps.
Open pot canning: Can be used on food with a pH value below 4.5. This includes a wide range of fruits. This method is usually used on jams and jellies which contain relatively high amounts of sugar in addition to the acid.
Water bath canning: Can be used for the same food as mentioned above, and it has the added benefit that the food is sterilized after the jar is closed.
Pressure canning: Is the only way to go for food with higher pH values than 4.5. This includes meat, fish, fruits like peppers, pees and zucchinis, and root vegetables like potatoes and carrots. Tomatoes are fruits with variable pH, and might need pressure canning.
Step 4: Open Pot Canning
Open pot canning is what was mostly done in previous times. It doesn't really refer to the pot where you prepare the food, but to the fact that the container is open after you have finished heat-treating your food. The principle is that you sterilize jars and lids by boiling them or leaving them in the oven. The food is boiled to kill microorganisms and poured directly into the hot jars. The jars have to be capped immediately to prevent microbes from entering. The residual heat from the food and the jar will in most cases kill any microbes that might enter before capping. This is not considered true canning, as the food is exposed to the environment after it is heat treated. This method should only be used on acidic foods where there is no danger of C. botulinum growth. The only advantages of this method are that it is quick, and reduces the chance of jars cracking if you are reusing old ones. The disadvantages are that there is a chance microbes will enter the container after heat treatment, and that these microbes are not killed by the residual heat of the food. You also have to handle very hot jars and food, and at the same time make sure everything remains sterile.
This method is typically used for jams and jellies.
Step 5: Water Bath Canning
Water bath canning can be done in two different ways. One is that you prepare the food by boiling, put it in cans, and then sterilize the cans in a water bath. The other is that the food is put into the jars raw, and the entire cooking process is done in the water bath.
The jars should be closed, and put into a water bath that covers them completely with about 5 cm of water above the lids. The boiling times for different food is something you need to check, but for pre boiled, acidic food, 10 minutes of boiling usually does the trick. Use a canning rack or towel in the bottom of the pot to make sure the jars are not in direct contact with the bottom of the pot. Direct contact can make the jar crack and the contents can get burned.
To avoid jars cracking, you can let them cool down in the water bath. Remove the pan from the heat, and let it sit until the water cools down. This only works for food where there is no danger of overcooking, like jams and jellies.
Jars should otherwise be cooled down in room temperature. Keep them away from draft by covering them with a blanket or similar. Make sure to not put hot jars down on heat conducting surfaces like stone and metal counters.
The main reason jars crack is differences in temperature between outside and inside that creates tension in the glass.
Step 6: Pressure Canning
When dealing with non acidic food, pressure canning is the only way to go. High pressure is the only way to achieve the temperatures needed to kill the spores of C. botulinum in food.
There are different types of pressure canners. Some have valves that can be set to open at a certain temperature or pressure. Others have valves that are controlled by adding weights to them to determine release pressure. These pressure canners are calibrated to work at sea level, so if you are using them at higher altitudes, you have to adjust for this by tuning or adding more weight to release valves. All of this is stated in the instruction manuals for the canners.
The magic happens at 121°C. No bacteria or spore harmfull to humans can survive temperatures above this for any length of time. At 1atmosphere pressure, water boils at 100 °C. To keep it in a liquid state at higher temperatures require higher pressure.
Pressure canners usually come with instructions regarding cooking times for different types and volumes of food.
Ordinary pressure cookers can not be used for canning. They are normally not equipped with adjustable pressure valves or any way of monitoring the internal temperature.
Step 7: Common Mistakes
There are some common mistakes in canning that are easy to prevent.
- Check your jars before use. If the edges have scratches or chips, they most probably will not seal well. The same goes for the lids if they are reused.
- Make sure that the rims of the jars are clean before closing them. Wipe of any food that was spilled on the rim.
- If your boiling times are to short, you are not going to kill all the microbes present.
- Make sure to remove all air bubbles when you fill the jars. This is easily done by carefully stirring with a glass or metal rod, or just a knife. Air is a poor heat conductor, and air bubbles can insulate parts of the contents of your jar from being properly heated.
- Check your jars for cracks and scratches before use.
- Don't expose them to sharp temperature gradients.
- Don't overfill jars. Some foods expand more than others when heated, and if the jar is to full, it may crack.
- Canning jar lids are designed to lift slightly when the air pressure inside gets to high, and release it. When the jars cool, the lids are sucked down and create an under-pressure seal. Not using propper canning jars increases the risk of jar breakage.
- Don't let the jars touch the bottom of your pot or canner. Use a canning rack or a towel underneath the jars.
-Yes, pressure canners can and do explode. The reason is usually a failing safety valve. Inspect your canner before and after use. Valves should be cleaned and on some canners lubricated. Working with high temperatures and high pressure is potentially dangerous, so don't be sloppy with the preparations.