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UNSAFE CANNING (PROCESSING) METHODS

The following methods are all unsafe and not recommended methods for home food preservation. Why would we include a list then? Use this article as a reference to define what each method means, and most importantly, why they are no longer recommended for safe home food preservation. Our advice: always use a reliable recipe source. Water bath canning is recommended for all high acid preserved food (pH equal to or below 4.6). Pressure canning is recommended for all low acid preserved food (pH above 4.6). Check out our large range of water bath canning recipes (and more) on www.foodpreserving.org and check out our Food Preserving Guides for step-by-step instructions on safely preserving food. 
  
OUTDATED/UNTESTED RECIPES
This method is not recommended. 
We put this “method” first because it is the most commonly asked: i.e. using a family recipe from generations ago, making up (guessing) a recipe, changing a recipe, or using a recipe that doesn’t include the steps for processing correctly (if at all). There are many recipes we see online and in print that do not include the correct processing technique, include unsafe ingredients or not enough acidity for example. If a recipe doesn’t state processing, it will need to be replaced with a reputable recipe with the correct, up to date processing information. All high acid food (pH equal to or below 4.6) is processed in a water bath canner and all low acid preserved food (pH above 4.6) is processed in a pressure canner. There are changes to ingredients and steps over time, so older printed recipes may not be safe enough to prevent food spoilage. If it’s stored in the pantry, it must be processed accurately and safely.
 
WATER BATH CANNING LOW-ACID FOOD
This method is not recommended. 
Old-fashioned method that processed all food in a water bath. However, we know now with science that any food that is low in acid (pH above 4.6) must be processed in a pressure canner, as the higher temperature reached in a pressure canner destroys bacteria that can spoil the food and possibly make us sick. 
  
PRESSURE COOKERS
This method is not recommended. 
Pressure cookers are sealed vessels that cook food via high pressure steam in a water based cooking liquid (or water) over a short period of time (at a higher temperature). Pressure cookers can be manual (cooktop) or electric. Pressure cookers are not pressure canners – even if a pressure cooker states it can be a canner, we cannot recommend their use, because they often operate at a different pressure level. Dial gauges of pressure canners are tested annually (minimum) whereas pressure cookers are not. Weighted gauge pressure canners can have their weights adjusted for pressure canning – pressure cookers are commonly one pressure level only. Also, pressure cookers are usually too small for even heating of jars according to the tested pressure canning procedure. Pressure cookers are also thinner than pressure canners, which means they would heat jars at a different rate (not heat/cool down at the same rate as a pressure canner would), which means that the low-acid food in the jars could be underprocessed (not heated adequately, higher risk of spoilage). The safest method for pressure canning is to choose a pressure canner that can hold at least 4 x 1L (4 x quart) jars upright, including rack and secure lid – we recommend using a pressure canner 16 quarts (or larger) and following a pressure canning recipe. 
  
MULTI-COOKERS
This method is not recommended. 
Multi-cookers are electric kitchen appliances that cook food automatically (with a timer). They bake, boil, brown food, deep fry, fry, grill roast, steam, stew and/or simmer food. Multi-cookers may even have a “steam canning” button or state that you can can in them – however no testing has been done to ensure they heat jars of low-acid food adequately (temperature and/or processing time length) to prevent spoilage/contamination to ensure food safety, so we cannot recommend this equipment. 
  
SLOW-COOKERS (CROCKPOTS)
This method is not recommended. 
Slow-cookers retain more nutrients in food, as they cook food at a lower temperature. This is exactly why they cannot be used for canning: the food doesn’t reach the correct temperature for the processing time required in water bath canning; slow-cookers do not reach the temperature for pressure canning low-acid food either (the temperature could permit growth of microorganisms in food that was then stored in jars at room temperature instead of being processed in a canner). 
  
OPEN KETTLE METHOD
This method is not recommended. 
Open kettle processing involved filling jars and adding the lids, then leaving them to self-seal on the bench (or even inverting/turning the jars upside down once they were on the bench to “create a better seal”). The open kettle method of preserving does not seal your jars via a water bath. Allowing lids to self-seal, or upturning them to seal better is actually not a true seal – it’s called a false seal (or weak seal) as the air does pull the lid down a little, but does not remove the air pocket from the headspace. Water bath canning does remove the air pocket from the headspace, resulting in a much stronger seal (as well as heating through the jar contents to reduce risk of contamination before and/or during storage). For an extra 10 or so minutes of your time (the processing time as per the water bath canning recipe) will help to prevent your canning work being ruined by spoilage (jars can be processing safely whilst cleaning up the kitchen after preparing your preserves so it isn’t much extra time at all, if any). 
  
OVEN CANNING
This method is not recommended. 
Oven canning is filling jars with food and then heating them in the oven (with or without lids on the jars) instead of using a water bath canner or pressure canner. This is dangerous as glass heats up to a higher temperature in the dry oven heat (even if on low). Jars are not manufactured to handle this dry heat and they don’t heat evenly either (also posing a safety risk: removing hot jars from the oven). Oven temperatures are not regulated, so there is no guarantee high acid food is heated correctly or retains the desired consistency (i.e. evaporation if jars do not have lids). Water bath canning high acid preserves is simple, efficient and safe! 
  
MICROWAVE CANNING
This method is not recommended. 
Processing jars of preserves via a microwave (including the use of a microwave pressure cooker) is also discredited. Microwaves do not heat food evenly, microwave models vary in power and operation, so jars would heat unevenly, putting preserves at risk of spoilage. Metal lids also pose a risk in the microwave. The safest, simplest method is to use a water bath canner for high acid preserving, as the temperature in boiling water is constant, timing is tested to ensure food is as safe as possible for long storage in the pantry. Microwaves can be used in some steps for preparation of ingredients, but not for the canning process itself. 
 
DISHWASHER CANNING
This method is not recommended. 
Steam processing jars of preserves in the heat of a dishwasher cycle was called dishwasher canning. However, the temperature reached during a dishwasher cycle is not adequate to remove microorganisms (which are removed in the boiling water canner method). Lower temperature dishwasher canning could result in bacteria, mould or yeast spoiling preserves. If the canning process does not heat food properly, to the correct temperature (and processing time) then we cannot recommend it. Boiling water bath canning is the safest option for high acid preserving, and pressure canning for low acid preserving, as per our recipes. 
  
DRY CANNING
This method is not recommended. 
There are two methods that describe dry canning. The first is canning vegetables or other food without a liquid. The second method of dry canning refers to filling jars with dry food, like flour, before heating the filled jars in an oven. Both dry canning methods are not recommended. If liquid is not added to vegetables when canned, they may be under-processed, as dry jars/ingredients heat at a different rate to filled jars of liquid/food. This poses a risk of bacterial growth or other undesirable microorganisms. Oven heating of jars is not recommended either because jars are not manufactured for oven canning (dry heating). Jars can be used for storing dry ingredients – we recommend using a vacuum sealer attachment to remove air from jars of dry food before storage in the pantry.
Note: tested recipes for pressure canning meat raw may not have liquid added. This is not dry canning as the meat releases liquid during processing in a pressure canner. 
 
VACUUM SEALER
This method is not recommended. 
Using a vacuum sealer to seal jars of canned food (instead of processing in a water bath canner or pressure canner) is not recommended, as the jar contents are not heated to the middle to the correct temperature, for the correct time length (which occurs when you use a water bath canner as per the recipe). This may result in spoiled/unsafe jars of preserves. We use a vacuum sealer jar attachment (pictured) to store dehydrated/dry food in the pantry. 
  
SEALING JARS WITH WAX
This method is not recommended. 
Using wax as a sealing agent on jars was sometimes used for high vinegar preserves - like pickles, chutney, relish, and salsa, as well as jam - to prevent oxidation (discolouration), crystallisation, retain “freshness” and extend the storage of these types of preserves. The water bath canning method has replaced this method, as it heats jar contents to remove microorganisms and remove the air pocket (headspace) that is responsible for oxidization (modern recipes also include the correct acidity to preserve colour and flavour better). Water bath canning also prevents crystallisation by creating a strong vacuum seal that stops moisture escaping (or entering) jars. 
 
JAR COVERS (NO LIDS)
This method is not recommended. 
Another old-fashioned method for sealing jars of preserves was using pieces of clear cellophane, waxed paper or aluminium foil – securing these covers on top of each jar with an elastic band. With more advanced testing, the temperature (and processing time) of water bath canning is essential to preserve high acid food for long term pantry storage. Acidic preserves could corrode foil covers, and paper/foil/cellophane covers are not airtight, increasing the risk of introduction/growth of spoilage microorganisms i.e. mould or bacteria. 
 
TWO+ DAYS OF WATER BATHING
This method is not recommended. 
Low-acid food is canned (“jarred”) over several days, i.e. boiled for x time on day #1, cooled down then boiled for x time on day #2. This technique originated somewhere in Europe for food (some mentions of high or low in pH) but this technique been replaced by scientific techniques of water bath canning (pH equal to or less than 4.6) and pressure canning (pH above 4.6) to ensure food preserving is safe for longer storage periods in different climates - especially important when we compare the temperatures we receive here in Australia! 
 
“SLOW” WATER BATH METHOD
This method is not recommended. 
In the 1960s, a USA magazine leaflet described how to process food via a slow water bath method. Jars were filled, put into a pot of cold water and heated to 55°C (130°F) for an hour or so, then heated up to a higher temperature over another 30 minutes (and held at this higher temperature for a certain time). Today’s method of water bath canning is much faster (and easier) and backed by science to ensure temperature, time and correct pH according to food safety for preserved food. 
 
This method is not recommended. 
Aspirin was mentioned in vintage preserving manuals, where one aspirin was placed into each jar before being sealed to prevent spoilage. This is not recommended as heating jars via a water bath canner (or pressure canner if pH is above 4.6) as per modern, tested recipes. 
 
SOLAR POWERED CANNING
This method is not recommended. 
Using a solar cooker to heat up jars to process was called solar canning. However, the temperature was not constant - not heating contents at the correct temperature and/or time length required – and also a dry, hot heat (similar to oven method above), which is not recommended by jar manufacturers. Solar powered cookers are ideal for using in slow-cooking recipes, but not safe for home food preservation (canning). 
 
THERMAL HEAT CANNING (VIA HOT COMPOST)
This method is not recommended. 
Large compost piles have large thermal mass – they create high heat that stay hot for a long time, i.e. heating water up for hot solar shower systems. However, for canning safely, specific temperatures and time length for processing is required, as well as the risk of microorganisms contaminating jars via compost (which naturally contain soil bacteria, fungi and other beneficial microorganisms for the garden). 
   
In summary: use a reliable recipe source. Water bath canning is recommended for all high acid preserved food (pH equal to or below 4.6). Pressure canning is recommended for all low acid preserved food (pH above 4.6).
  
Author: Megan Radaich           
Image Credit: Megan Radaich & Google Images
Publication: www.foodpreserving.org

Acknowledgement 
Kaya Wanjoo. Food Preserving kaditj kalyakoorl moondang-ak kaaradj midi boodjar-ak nyininy, yakka wer waabiny, Noongar moort. Ngala kaditj baalap kalyakoorl nidja boodjar wer kep kaaradjiny, baalap moorditj nidja yaakiny-ak wer moorditj moort wer kaditj Birdiya wer yeyi.
Hello and Welcome. Food Preserving acknowledges the Traditional Custodians of the land on which we live, work and play, the Nyoongar people. We recognise their connection to the land and local waterways, their resilience and commitment to community and pay our respect to Elders past and present.
 
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