What is Pasteurization
Pasteurization originates with 19th Century scientist Louis Pasteur which is also the origin of the word to 'pasteurize'. Pasteurization is a process in which foods are subjected to just enough heat so as to kill off bacteria and pathogens but not so much as to drastically alter the composition, quality and taste of the product. Spoilage enzymes are also neutralized during pasteurization. Generally around 200 0 F, 212 0 or 100 0 Celsius maximum.
Pasteurization stabilizes foods by neutralizing organisms and enzymes that lead to spoilage, pasteurizations primary benefit is extending the storage and shelf life of foods, it is not the same as total sterilization. Pasteurization does not kill spores, and does not make it impossible for potentially harmful organisms to reactivate after the process is completed. A second "double" pasteurization is sometimes used to eliminate spores that may have germinated.
In most pasteurization processes excessive heat for extended periods reduces the quality of the product. It causes unnecessary softening - nobody likes a mushy pickle.
Tomatoes and other high acid foods for canning or pickling are generally pasteurized using a 'boiling water bath' . A Water bath canner is used, it is a deep kettle, usually with a wire insert to hold submersed canning jars [Image 1 ]. It's used to preserve foods high in acids , such as tomatoes, fruits and berries.
Mason jars, or similar suitable GLASS canning jars are used - metal is not advisable for home canning. Canned in glass jars they do not absorb the metallic taste and there is not the risk of chemical interactions between the metals and acidic foods.
See:Canning Drying Freezing Tomatoes
Pickling - Hot Water Bath
Pasteurizing Liquids in a Double Boiler
If you do not have a hot water bath canner, pasteurization can also be done with a 'double boiler'. A double boiler can be purchased or is easily jimmy rigged from items you should already have in your kitchen. A double boiler is 2 pots. The larger is a stockpot and the smaller one is a more shallow pan that will nest inside the other. The smaller pot or pan has to be small enough to rest in the top of the larger pot without touching the water. There should also be a seperation of 1 or 2 inches at least between the waters surface and the bottom of the small pan to prevent overflow.
Fill the larger, bottom pan, with about 2 inches of water, insert the shallow pan on top. When heated, the water from the bottom pan transfers a gentle but steady heat to the liquid in the upper pan.
Keep the double boiler over medium heat, not so hot where the water in the bottom pan reaches a full rolling boil, just enough to kepp the steam / heat flowing at a steady pace. You should see steam forcing its way out between the two pans. Turn the heat down a tad as need be in order to control the temperature in the top pan, it should never boil.
The temperature is vital - it should be 145 °F to 155 for at least 30 minutes. I prefer the 30 second window as there is less chance of accidentally over cooking the liquid. The same results can be achieved at higher temperature for a much shorter duratioon 161 °F (72 °C): for no mare than 15 to 20 seconds. This requires you constantly monitor the temperature and time - and as we all know a watched pot never boils.
Commercial operations sometimes use what is known as 'Flash pasteurization', instead of heating the food in a container, the batch of liquid is subjected to extreme temperatures while being poured into sterile containers. This method uses much higher temperatures than the traditional pasteurization process, but does not reduce food quality drastically as the heat is only applied for a very short period - about 15 seconds as opposed to 30 minutes or more for traditional pasteurization processes.
Steaming or Blanching
Steaming or blanching foods is not pasteurization, allthough it is useful in some instances. If drying your vegetable product, Steaming or blanching is sometimes used as opposed to a hot water bath to inactivate enzymes that cause produce to break down durring the drying process.
Some produce is also blanched or steamed prior to being frozen.