On most days my house is a hive of activity. There's a steady stream of family and friends that regularly cross my threshold, not to mention the three men I live with and cook for on a daily basis. There's Hubby, the American and the Little Man, all of whom have different tastes, schedules and appetites. Here's how I stopped being a short order cook and turned my fridge into an open all hours self serve buffet.

Jar Safety

The question is asked continually, is it safe to cook in a jar?  The greatest concern I suppose is whether the jar will break.

The manufacturers of canning jars do not endorse the use of their jars for baking in hot ovens.  Canning machines can generate up to 120C (250F), and the jars are guaranteed to withstand the temperatures of canning machines or being placed in boiling water for the purposes of sterilization.

Most foods bake in an oven at about 160C - 180C (360F).  This is considerably hotter than what the manufacturers will guarantee.  That said there are dozens and dozens of anecdotal stories from bakers all over the world who have never had a canning jar break in the oven.

Canning jars are not glass bakeware.  Although most glass bakeware and mason jars are both made from soda-glass, so are milk bottles, window glass and just about everything else, it is the cheapest and most common form of glass.  The composition of the glass and how it is tempered determines how heat tolerant it is.

What does all this mean?  Basically, your glass may break or explode in the high heat of the oven, and if it does the manufacturers have no responsibility to compensate you for any damage or injury that results.  Even if it withstands one episode in the oven, or even a dozen, there is no telling how this could be structurally weakening the glass so that at some point it does break.  You put jars in the oven at your own risk.

For this reason all of my recipes that require cooking are done in a hot water bath in a slow cooker, or in a pressure cooker which create the same temperatures and pressures as canning machines, for which these jars are intended.  If recipes are required to go into an oven (eg for a meringue) then I ensure it goes into an oven at no higher than 120C (as a cheery co-incidence would have it that is the exact temperature meringues require!).  The Kerr jars are made to withstand 15psi, and all domestic pressure cookers generate 15psi or less, so this too is a completely safe way of cooking in jars.

With the issue of breakage addressed, cooking in jars is otherwise a completely safe method of cooking.  One blogger I read stated that "studies have shown" that cakes cooked in mason jars show signs of bacteria.  This is simply misread, misunderstood and misquoted information.

There is a practice of cooking a cake or bread in a jar and then sealing it immediately as a method of preserving it and storing it for up to a year.  Studies have shown that cakes sealed and stored in this manner have shown signs of bacteria, proving that the method of preservation is not sound, it has nothing to do with the manner in which it was cooked.

Here is the full article by the Penn State College of Agricultural Science.

Cooking in a jar is no more or less likely to cause bacteria than cooking in any other kind of glass bakeware.  It is the act of sealing the jar while it is still hot, and placing the jar in long term storage that can cause issue.

  • You would normally only store cakes and breads at room temperature for 4 - 7 days before mould starts to present itself.  The same rule must be followed for cakes in jars that you store at room temperature.
  • You would normally only store cheesecakes, custards, yoghurts etc for 7 - 10 days in the refrigerator, the same rule must be followed for dairy based products you cook in jars.
  • You would normally allow food to cool on the bench before wrapping and storing, the same rule must be followed for anything that is not a preserve.  Allow it to cool thoroughly before placing the lid on.

Despite being prepared in a canning jar you must not try and store the items you prepare long term as if they were canned goods, you must store and consume them as you would any other fresh product.